FLYTECH AVIATION ACADEMYAERODYNAMICS-SEMESTER I

FLYTECH AVIATION ACADEMYIndias Leading ISO 9001 Aviation Training School

Compiled by Mr. Rushav Samant & Mr. J.K. Pandey

Revised on: 31st MAY 2014

Head office: Nagam Towers 3rd & 4th Floors NTR Circle Minister Road Secunderabad-03PH: 040-6620 000, FAX: 040- 30604493

Reference books:Introduction to flight James AndersonAerodynamics James AndersonMechanics of Flight A.C. KermodeHelicopter Aerodynamics R.W ProcetyAutomatic Flight Control E.H.J PelletWebsites Referred:Wikipediawww.av8n.comEdited by:Rushav Samant

INDEXTopic No.TopicTotal Hrs.Page No.

P 3.1.1Physics of the AtmosphereInternational Standard Atmosphere(ISA),application to aerodynamics.0205-09

P 3.1.2Aerodynamics Airflow around a body; Boundary layer, laminar and turbulent flow, freestream flow, relative airflow, up wash and downwash, vortices, stagnation; The terms: camber, chord, mean aerodynamic chord, profile (parasite) drag, induced drag,centre of pressure, Angle of attack, wash in and wash out, fineness ratio, wing shape and aspect ratio; Thrust, Weight, Aerodynamic Resultant; Generation of Lift and Drag: Angle of Attack, Lift coefficient, Drag coefficient, polar curve, stall; Aerofoil contamination including ice, snow, frost.0710-31

P3.1.3Theory of Flight Relationship between lift, weight, thrust and drag; Glide ratio; Steady state flights, performance; Theory of the turn; Influence of load factor: stall, flight envelope and structural limitations; Lift augmentation.0632-34

P3.1.4 Flight Stability and Dynamics Longitudinal, lateral and directional stability (active and passive) 0235-37

P3.2.1.1Aero plane Aerodynamics and flight Controls Operation and effect of: 10 Hrs Roll control: Ailerons and spoilers; various types of Ailerons: Differential aileron movement. Pitch control: elevators, stabilizers, variable incidence stabilizers and canards; Yaw control, rudder limiters; Roll causes Yaw and Yaw causes Roll; Dutch Roll. Control using elevons, ruddervators; High lift devices, slots, slats, flaps, flaprons; Drag inducing devices, spoilers, lift dampers, speed brakes; Effects of wing fences, saw tooth leading edges; Boundary layer control using, vortex generators, stall wedges or leading edge devices; Operation and effect of trim tabs, balance and anti balance (leading) tabs, servo tabs, spring tabs, mass balance Control surface bias, aerodynamic balance panels.1038-47

P 3.2.1.2 High Speed Flight Speed of sound, subsonic flight, transonic flight, supersonic flight, Mach number, critical Mach number, compressibility effect, shock wave, aerodynamic heating, area rule; Sonic Bubble, Mach Cone, Sonic Bang, Aileron Reversal Factors affecting airflow in engine intakes of high speed aircraft; Effects of sweepback on critical Mach number.0448-57

P 3.1.2Polar Curves

0158-59

P3.RA.2.1 Theory of Flight Rotary Wing Aerodynamics Terminology; Effects of gyroscopic precession; Torque reaction and directional control; Dissymmetry of lift, Blade tip stall; Translating tendency and its correction; Coriolis effect and compensation; Vortex ring state, power settling, over pitching; Auto-rotation; Ground effect.0360-84

P3.RA.2.2 Flight Control Systems Cyclic control; Collective control; Swash plate; Yaw control: Anti-Torque Control, Tail rotor, bleed air; Main Rotor Head: Design and Operation features; Blade Dampers: Function and construction; Rotor Blades: Main and tail rotor blade construction and attachment Trim control, fixed and adjustable stabilizers; System operation manual Hydraulic, electrical and fly-by wire; Artificial feel; Balancing and Rigging.0585-98

P.3.1.1 Physics of the Atmosphere

Atmosphere is the ocean of air surrounding the earth extending up to 500 miles. Air is a fluid having very low internal friction between the molecules with in limits as an ideal fluid. Air is a physical mixture of many gases comprisisng of following atmospheric content:

The entire atmosphere is divided into four main gaseous layers. They are

Troposphere extends up to 38,000 feet (11km) from the surface of the earth and stratosphere extends from 11km to 25km these two layers are of main consideration when it comes to aero plane navigation.

With the increase in altitude the atmosphere air properties such as temperature, pressure, density and humidity changes continuously. The study of these changes plays an important role in studying the performance of aircraft.

The International Standard Atmosphere (ISA)

Because the pressure, temperature and density in the real atmosphere are continually changing, a theoretical international standard atmosphere [the ISA or ICAO (International Civil Aviation Organization) standard atmosphere] has been defined as, a measuring stick by the ICAO.

The international standard atmosphere has a mean sea level (MSL) pressure of 1013.20 mb (milli bar) 760 mm of HG/14.7 PSI/29.92 of HG/101.325 KPa.The mean sea level temperature is taken as 15 C/288k/59F.

Adiabatic Lapse Rate :

This is the increase or decrease in the temperature of air for a given change in altitude. Some times the temperature of air about 1000 feet is higher than it is at earth surface. This condition is called inversion generally in troposphere the temperature decreases with increase in altitude. Mountains, clouds, surface winds, water bodies and sun shine all effect the air temperature.

In troposphere, under standard conditions temperature decreases at 1.98C, for each increase of 1000 feet of altitude until a altitude of 38,000 feet (11km).

Adiabatic lapse rate varies form 3F per 1000 feet for moist air and more than 5F for dry air. But standard rate shown in SCAD chart is 3.5F per 1000 feet.

Pressure Change with Altitude :

The atmospheric pressure varies with height or altitude. The more the height the lesser the pressure, the standard ISA mean sea level pressure is 1013.2mb and this decrease at about 1mb per 30 feet increase in height.

At 600 feet in the ISA, the pressure will have decreased by approximately (600/30) = 20mb i.e., from 1013mb. It will have decreased to 993mb. If the point where your aeroplane is situated has a pressure of 993mb, then we say it has a pressure altitude of 600 ft i.e., it is the equivalent of 600 ft above the 1013mb pressure level in the ISA.

Pressure altitude is the height in the international standard atmosphere above the 1013.2mb pressure level at which the pressure equals tat of the aircraft or point under consideration

Density :

As air is compressible when it is compressed it becomes denser. Density directly varies with pressure while temperature is constant. Unit for density in SI system is Kg/m3. It is also expressed in slug/cubic feet.1 slug = 32.175 pounds = 14.59 kg. Air density is presented by Greek letter(rho)

The density is directly proportional to pressure and inversely proportional to temperature.

With same thrust an a/c files faster at high altitude that at a low altitude because air offers less resistance to a/c at high altitudes.

Effect of Humidity :

Humidity is the condition of moisture of dampness in the atmosphere. The amount of water vapor that air can hold depends on temperature. Higher the temperature of air, more vapor it absorbs.Weight of water vapor is 5/8 of dry air. The vapor is directly proportional to temperature. As water vapor is less in weight, when the humidity is more the air density is lesser. So density is inversely proportionate to the humidity.If the temperature is more, the air gets heated up through conduction and radiation. The heated air rises and expands. This expansion causes drop in temperature and density also.

In humid conditions (rainy season / damped atmosphere) the aircraft requires longer runways for landing and take off.

Relative humidity is defined as the amount of water vapor present in parcel of air compared to the maximum amount than it can support (i.e. when it is saturated) at the same temperature.

P.3.1.2 Aerodynamics

Airflow around a body :

Before going to the main topic we will learn different kinds of fluid flow, which will make us to understand the main topic easily.

Types of fluid flow :

Steady and unsteady flows.Uniform and non-uniform flows.Laminar / viscous and turbulent flow.

Steady and unsteady flow :

Steady flow is defined as that type of flow in which the fluid characteristics like velocity, pressure, density etc, at a point do not change with time.

Unsteady flow is that type of flow in which the velocity, pressure or density at a point changes with respect to time.

Uniform and non-uniform flows :

Uniform flow is defined as that types of flow in which the velocity at any given time doesnt change with respect to space (i.e. length of direction of the flow).

Non-uniform flow is that type if flow in which the velocity at any given time changes with respect to space.

Laminar and Turbulent Flows :

Laminar flow is defined as that of flow in which the fluid particles move along well-defined paths or streamline and all the streamlines are straight and parallel. Thus the particles move in laminas or layers gliding smoothly over the adjacent layer. This type of flow is also called streamline flow or viscous flow.

Turbulent flow is that type of flow in which the fluid particle moves in zigzag way. Due to the movement of fluid particles in a zigzag way, eddies formation takes place, which are responsible for high-energy loss.

Boundary Layer :

The behavior of the airflow nearest the surface through which it is flowing (cab be any solid surface, including airfoil) is most important. A layer of fluid close to the surface of a moving body, through which the velocity of the fluid changes from zero on the surface of the body to the velocity of the free stream, is known as boundary layer.

Friction between a surface and the air flowing over it slows down the layers of air nearest to the surface. The air actually in contact with the surface may in fact have a relative velocity of zero. The thickness of this boundary layer, in which the relative velocity is reduced, is typically several millimeters.

Laminar and turbulent boundary layerThickness of the layer greatly exaggerated.Transition point :

At some point of the surface the airflow with in the laminar boundary layer becomes turbulent and the layer thickness significantly i.e. the flow changes form laminar to turbulent. This is known as transition point. At this point the flow no longer flows in streamlines and the flow gets separated. So this is also known as point of separation.

Transition Point

It is always desired to make the flow streamline as long as possible. So the separation of the flow should be avoided. The methods that are employed to avoid boundary layer separation are discussed later in this book.

Note : With the increase in angle of attack the transition point moves forward towards the leading edge and flow separation takes place very near the leading edge.

Free stream airflow :

There is not much difference between relative airflow and free stream airflow. Free stream airflow is defined, as the airflow may exist over the airfoil at a distance away from the surface or airfoil. Where as relative airflow is defined only before the leading edge of the airfoil.

Relative airflow :

It refers to the relative motion between a body and the remote airflow, i.e. the airflow far enough away from the body not to be distributed by it.

Airflow can lift a flat plate

Up wash :

The upward deflection of air before reaching the airfoil is known as up wash.

Down wash :

The downward deflection of current due to air by an airfoil or part of the aircraft at the trailing edge of the reacting part.

Stagnation Point :

The point at which the airflow is not in motion is known as stagnation point. Before reacting, the aerofoil the airflow stops momentarily in front of the leading edge. This point is known as stagnation point

Camber (The cause for lift) :

Increasing the camber on the upper surface causes the airfoil over it to accelerate more and to generate more lift at the same angle of attack.Wings with a large chamber, give a good lift, making them suitable for low spend flight and carrying heavy loads.It is quite evident that due to the camber there is a increase in velocity on the top surface, this in turn decreases the pressure on the top surface, due to the decreases of pressure on bottom surface makes the wing to rise up hence creating lift.

Low well cambered wing :

At the small positive angles of attack common in normal flight, the static pressure over much of the top surface of the aerofoil is slightly reduced when compared to the normal static pressure of the free air stream well away from the aerofoil. The static pressure beneath much of the lower surface of the aerofoil is slightly greater than that on the upper surface.

From the pressure distribution it is estimated that there is maximum pressure difference at an angle of attack around 15 - 16.

So the maximum lift is obtained at an angle of around 15 for general aerofoil shape.The angle of attack (AOA) at which maximum lift is obtained is called as critical angle of attack or stalling angle.

Up to critical angle of attack the lift increases, beyond which the lift decreases tremendously and stalling takes place.

Stalling is a condition in flight where there is tremendous decrease of lift or tremendous increase of drag or both.

Chord Line :

The line joining the leading edge and trailing edge of the airfoil is known as chord line, and the distance between the leading edge and trailing edge is known as chord.

Chord / Chord length :

The length of the chord from leading edge to trailing edge is known as chord length or simply chord.

Mean Aerodynamic Chord :

The imaginary line that divides the airfoil into two equal halves is known as mean camber line. OrThe line drawn half way between the airfoil (between upper surface and lower surface) is known as mean chamber line. In tapered or trapezoidal wings the length the chord changes gradually from wing root to wing tip.Mean aerodynamic chord is the chord, which divides the entire wing area into two equal halves i.e., equal amount of wing area will be present on both sides of mean aerodynamic chord.

Bernoullis principles :The production of the lift force by on aerofoil is explained by Bernoullis principle. Daniel Bernoulli (1700-1782) was a Swiss scientist discovered this effect. It can be expressed as

P/ + V2 + gZ = ConstantOr

P/ g + V2/2g + z = Constant.

Or

P + V2 + gZ = Constant.

When considering the flow of air, the potential energy can be ignored for all aerodynamic practical purposes.

Therefore, Kinetic Energy + Pressure Energy of a smooth flow is always constant. Therefore if one energy raises other energy drops proportionately to keep the total energy constant.

In a simplified from Bernoullis theorem can be explained as fluid velocity increases pressure decreases and when pressure increases velocity decreases. This is the basic principle involved in the production of lift in aircraft.

:

GENERATION OF LIFT:MAGNUS EFFECT

The explanation of lift can best be explained by looking at a cylinder rotating in an air stream. The local velocity near the cylinder is composed of the air stream velocity and the cylinder's rotational velocity, which decreases with distance from the cylinder. On a cylinder, which is rotating in such a way that the top surface area is rotating in the same direction as the airflow, the local velocity at the surface is high on top and low on the bottom.

As shown in following figure, at point "A," a stagnation point exists where the air stream line that impinges on the surface splits; some air goes over and some under. Another stagnation point exists at "B," where the two air streams rejoin and resume at identical velocity's. We now have up wash ahead of the rotating cylinder and down wash at the rear.

The difference in surface velocity accounts for a difference in pressure, with the pressure being lower on the top than the bottom. This low pressure area produces an upward force known as the "Magnus Effect." This mechanically induced circulation illustrates the relationship between circulation and lift. An airfoil with a positive angle of attack develops air circulation as its sharp trailing edge forces the rear stagnation point to be aft of the trailing edge, while the front stagnation point is below the leading edge.

Magnus Effect is a lifting force produced when a rotating cylinder produces a pressure differential. This is the same effect that makes a baseball curve or a golf ball slice.

Air circulation around an airfoil occurs when the front stagnation point is below the leading edge and the aft stagnation point is beyond the trailing edge.

DOWNWASH EFFECT:The downwash created by an aerofoil acting as the bound vortex slipstream creates a reaction lift as per the Newtons 3rd law .

BERNOULLIS PRINCIPLE: The differential pressure created due to the Bernoullis principle on an aerofoil surface help achieve lift.

Total Drag

Parasite DragWing Drag

Interference DragInduced DragProfile Drag

This is the total drag of an a/c from all sources. It will be increases with use of flaps, dive breakers, towering or under carriage, denoted cowlings, bad repairs, chipped or roughed surface, accumulation to dirt and mud on fuselage and wing.

Wing Drag :

This drag is caused by the wings and includes profile and induces drag. The profile drag further splits into two types :

Form dragSkin friction drag.

Form drag/pressure drag :

When an object is placed in moving airflow the direction of airflow changes to pass around the object. If this change takes place smoothly, the drag will be less. If the object shape is such that (non-streamlined shape) the airflow becomes turbulent and smooth airflow changes and eddies are formed.

Form drag is reduced by, streamlining the body.

Skin Friction Drag :

It is the drag caused due to air passing over aero-plane skin surface. This type of drag is more where air density to more where air density and velocity is more.

The layer of air near the surface retards the layer further away due to the friction between them. Therefore there is general increase in velocity in as the distance from the surface form the surface increases therefore all surface of a/c should be made as smooth as possible. Skin friction drag depends on area of surface over which air flows the speed of airflow and viscosity of air.

Induced Drag :

The drag that is included due to aerofoil surface because of pressure difference on aerofoil, which makes the airflow turbulent, is known as induced drag.

In other words it is the penalty that we have to pay for getting the lift as the pressure difference is the main reason due to which we are getting lift.

How induced drag is created :

Experiments with smoke show quite clearly that the air flowing over the top surface of a wing tends to flow inwards. This is because the decreased pressure over the top surface is less than the pressure outside the wing tip. On the other hand the airflow below the wing tends to flow outwards as the pressure below the wing is greater than that outside the wing tip. Thus there is a continual spilling of the air around its wing tip, from the bottom surface to the top.

When the two airflows, from the top and bottom surface, meet at the trailing edge they are flowing at an angle to each other and cause vortices rotating clockwise (viewed form the rear) from the left wing, and anti-clockwise from the right wing all the vortices on one side tend to join up and form on large vortex which is shed from each wing tip. Hence these are called wing-tip vortices.

In a sense, induced drag is part of the lift, so as we have lift we must have induced drag, and we can never eliminate it altogether however cleverly the wings are designed. But the grater the aspect ratio, the less violent are the wing-tip vortices and the less the induced drag.

Parasite drag :

It is the drag or air resistance which is produced by any part of aero plane that does not produce lift (non-lift producing surfaces).

E.g. : Landing gear, engine cowlings open inspection panels etc.

Form drag and skin friction are components of the parasite drag.

Interference Drag :

This is the drag caused due to interference of airflow between the adjacent parts of aero plane.

E.g. : Intersection of wing to the fuselage this drag prevented by fitting fairing to attachment.

Thrust :

The force that is produced by engines (power plant) to make the aircraft move forward is known as thrust.

Weight :

Every aircraft will have its own weight, which acts vertically downwards.

The detailed explanation of these forces will be discussed later.

More about lift and drag :

Experiments show that with in certain limitations the lift, drag of an aerofoil depends on :

The shape of the aerofoilThe plan area of the aerofoilThe square of the velocityThe density of air

When measuring drag we consider the frontal area of the body concerned, on aerofoil we take the plan area.

With the above conclusion, we can express the lift and drag as

Lift L = CL V2Where CL = Coefficient of lift = Density of air v = velocity s = surface area/plain area.

Where CP = Co-efficient of drag

Drag D = CP V2 = density of air V = velocity S = surface area / plan area

Coefficient of lift and drag :

The easiest way of setting out the results of experiments on aerofoil sections is to draw curves showing how

The lift coefficient The drag coefficientThe ratio of lift to drag --- after as the angle of attack is increased over the ordinary angles of flight.

It is much more satisfactory to plot the coefficient of lift, drag and pitching moment rather than the total lift, drag and pitching moment, because the coefficients are practically independent of the air density,

the scale of the aerofoil and the velocity sued in the experiment, where as the total lift, drag depend on the actual conditions at the time of the experiment.

The coefficients of lift and drag are the direct measure of total lift and drag.

Drag Curve :

Here we find much what we might expect the drag is lest at about 0, or even a small negative angle, and increase on both sides of this angle, up to about 6, however the increase in drag is not very rapid then itgradually becomes more and more rapid, especially after the stalling angle when the airflow separates.

The Lift / Drag ratio curve :

While designing an aeroplane, it is taken care that we get much lift, but as little drag as possible from the lift curve. We find that we shall get most lift at about 15, from the drag curve least drag at about 0, but both of these are at the extreme range of possible angles, and at neither of them do we really get the best conditions for flight i.e., the best lift in comparison to drag, the best lift/drag ratio.

We find that the lift/drag ratio increases very rapidly up to about 3 or 4, at which angles the lift is nearly 24 times the drag, the ratio gradually fails off because, although the lift is still increasing, the drag is increasing ever more rapidly, until at the stalling angle the lift may be only 10 to 12 times as great as the drag, after the stalling angle the ratio fails still further until it reaches 0 to 90.

The chief point of interest about the lift/drag curve is the fast that this ratio is greatest at an angle attack of about 2 or 4, on other words it is at this angle that the aerofoil gives its best all round results. It is most able to do what we chiefly require of it, namely o give as much lift as possible consistent with a small drag.

Aerodynamic resultant: The net resultant aerodynamic force R acting through the centre of pressure on the aerofoil represents mechanically the same effect as that due to the actual pressure and shear stress loads distributed over the body surface.

P.3.1.3 Theory of Flight

Relation between Lift, Drag, Thrust and Weight :

Lift, Thrust, Weight, and DragIt is better to be on the ground wishing you were flying, rather than up in the air wishing you were on the ground. Aviation proverb.4.1DefinitionsThe main purpose of this chapter is to clarify the concepts of lift, drag, thrust, and weight. Pilot books call these the four forces.It is not necessary for pilots to have a super-precise understanding of the four forces. The concept of energy (discussed in chapter 1) is considerably more important. In the cockpit (especially in critical situations like final approach) I think about the energy budget a lot, and think about forces hardly at all. Still, there are a few situations that can be usefully discussed in terms of forces, so we might as well learn the terminology.The relative wind acting on the airplane produces a certain amount of force which is called (unsurprisingly) the total aerodynamic force. This force can be resolved into components, called lift and drag, as shown in figure 4.1.

Figure 4.1: Total Aerodynamic Force = Lift + DragHere are the official, conventional definitions of the so-called four forces: Lift is the component of aerodynamic force perpendicular to the relative wind. Drag is the component of aerodynamic force parallel to the relative wind. Weight is the force directed downward from the center of mass of the airplane towards the center of the earth. It is proportional to the mass of the airplane times the strength of the gravitationalfield. Thrust is the force produced by the engine. It is directed forward along the axis of the engine.It is ironic that according to convention, the total aerodynamic force is not listed among the four forces.

Figure 4.2: The Four Forces Low Speed DescentFigure 4.2 shows the orientation of the four forces when the airplane is in slow flight, i.e. descending with a nose-high attitude, with the engine producing some power. Similarly, figure 4.3 shows the four forces when airplane in a high-speed descent. The angle of attack is much lower, which is consistent with the higher airspeed. Finally, figure 4.4 shows the four forces when the airplane is in a climb. I have chosen the angle of attack, the lift, and the drag to have the same magnitude as in figure 4.3.

Figure 4.3: The Four Forces High Speed Dive

Figure 4.4: The Four Forces ClimbNote that the four forces are defined with respect to three different coordinate systems: lift and drag are defined relative to the wind, gravity is defined relative to the earth, and thrust is defined relative to the orientation of the engine. This makes things complicated. For example, in figure 4.2 you can see that thrust, lift and drag all have vertical components that combine to oppose the weight. Meanwhile the thrust and lift both have forward horizontal components.Glide Ratio :

Steady state flights Performance :i. Climbing performance :

The forces acting on an a/c during a climb are shown in the figure.The thrust must equal the sum of drag plus the opposing component of weight.The lift must equal to the opposing component of the weight.i.e. T = D + W sin L = W cos ii. Straight and level :

For straight and level flight the opposing force must be equal and opposite i.e. lift = weight, thrust = drag.Although the opposing forces are equal, there is a considerable difference between each pair of forces. The lift and weight will much higher than the thrust or drag.If the a/c files an uniform speed in straight and level flight then the a/c is said to be in equilibrium.Theory Of Turn:Forces in turns : if an airplane were viewed in straight and level flight from the rear (fig.3-20) , and if the forces acting on the airplane actually could be seen , two forces ( lift & weight ) would be apparent , and if the airplane were in a bank it would be apparent that lift did not act directly opposite to the weight it now avts in the direction of the bank. The fact that when the airplane bank lift acts ,

Theory of Turning :

The wings of the aero plane are banked through the angle . Hence the lift vector is inclined at an angle to the vertical and W = LCOS

Therefore the resultant force F, equals

Fr =

=

=

Fr =

LIFT AUGMENTATION:Although any airfoil surface moving through air will be creating lift of its own bound by its nature , there are many devices installed on an aircraft for enhancement of the lift produced there on. Those are as follows : Flaps /Slots/Slats Movable stabilizers Vortex generators Flaps /Slots/Slats:These increases the camber of the wing , increasing the wing plan thereby increasing the overall wing area but on the other hand decreasing the aspect ratio.Hence they create both lift and drag when deployed but the amount of reaction lift produced dominates on the induced drag created. The best lift augmentation achieved is by slat & double slotted fowler flaps which increases the angle of attack as high as up to 270 .

MOVEABLE STABLIZERS :These have got the privilege of increasing the angle of attack there by increasing the lift without any considerable change in attitude of the aircraft. Hence they do not affect the stability of the aircraft while increasing the lift to that extent as is achieved by other lift augmenting devices.

VORTEX GENERATORS:They help obtaining more of the lift by downwash effect by inducing more energized air mass into the low energized trailing edge bound vortices.

P.3.1.4 Flight Stability and Dynamics

Stabilities :

There are three types of stabilities

Longitudinal StabilityLateral StabilityDirectional Stability

Longitudinal Stability :

The stability of an a/c about lateral axis is known as longitudinal stability. If the a/c go far a live (descending) or climb (pitching up) then the control is released the a/c should return to its original level flight automatically if the a/c has good longitudinal stability.

Factors effecting longitudinal stability :

The position of C.G must not be too far back. The pitching moment of main plane movement of center of pressure. The pitching moment fuselage. The angle at which tall plane fixed of fuselage, design of tall plane.Lateral stability :

The ability of an a/c to achieve leveled wing altitude after being displaced from a level altitude by some force (i.e. turbulent air) is known as lateral stability. Lateral stability is achieved about longitudinal axis.

Factors governing lateral stability :

Dihedral angle Sweep hack of wing High wing and low C.G arrangement Placing most keel surface above C.G.

Note :Keel surface : The complete surface which is seen in side elevation.

Directional Stability:

Stability about the vertical axis or normal axis is known as directional stability.The a/c should be designed so that when it is in level flight the it must remain on its heading even though the pilot takes his hands off from the control stick.If an a/c recover or return automatically a skid, it has been well designed will and have good directional stability. The vertical stabilizer of fin in the primary surface which controls directional stability.Some times sweep back wing also provides directional stability. In some of the a/c directional stability is added by using a dorsal fin and a long fuselage.

Factors effecting directional stability :

More keel surface behind C.G. Fin area.

Flaps :Flaps are high lift devices or lift-augmenting devices that are used in increases lift during landing or take off. When flaps are selected (lowered) the camber of the wing increases resulting in increase in lift.Most high-speed aerofoil however has a mean camber line that is fairly straight and hardly curved at all. If the trailing edge can be hinged downwards, then a more highly cambered aerofoil section results, which mean it, can produce the required lift at a lower airspeed, i.e. it has become more of high-lift wing.Virtually all aircraft have trailing edge flaps. Larger aircraft especially those with swept back wings, often have leading edge flaps as well. These have a similar function to trailing edge flaps in that they increase the camber of the wing and thus increase its effectiveness in producing lift.There are different types of flaps being used on todays commercial airplanes some of them are :

More about Slats and slots :

If a small auxiliary aerofoil, called a slat, is placed in front of the main aerofoil, with a suitable gap or slot in between the two, the maximum lift coefficient of the aerofoil may be increased by as much as 60 percent. Moreover the stalling angle may be increased from 15 to 22 or more.

Stalling is caused by the breakdown of the steady streamline airflow. On a slotted wing the air flows through the gap in such a way as to keep the airflow smooth. Following the shape of the surface or the aerofoil, and continuing to provide lift until a much greater angle is reached. This is one of the forms of boundary layer control.

There are two types of slats generally used.

Controlled slat.Automatic slat.

Controlled slat (Permanent / fixed slat) :

This slat is moved backwards or forward by a control mechanism, so that it can be closed at high speed and can be opened at low speeds.Automatic slats :

This slat is moved by the action of air pressure, i.e. by making the use of forward and upward suction near the leading edge.

Drag inducing devices

Drive brakes spoilers, lift dampers, airbrakes (speed breaks) :

The main purpose of these devices is to increase drag or to density lift or both. They are used to steepen the gliding angle on some planes to check the speed before turning or maneuvering. They prevent the speed from reaching critical value during dive. Their position may differ from plane to plane. They may be on main wings or fuselage top depending upon the design.Wing fences :

These are vanes of similar height to vortex generators, but running from leading edge to trailing edge across the top surface of the wing. On heavily sweptback wings fences helps in arresting the span wise flow of air along the wing which way cause a breakaway of the flow near the wing tips and so lead to tip stalling.

Saw-tooth or dog-tooth :

These serve the same purpose as that of wing fences. In highly swept wings the tendency of the flow to separate in the tip region first. This can cause problems like large changes in pitching moment. This effect may be reduced by, introducing a notch on saw-tooth in the leading edge. The notch also generates a strong vortex, which controls the boundary layer in the tip region.

Boundary layer control :

The usual tendency of boundary layer is start being laminar near the leading edge of a body, but there comes a point, called the transition point, when the layer tends to become turbulent and thicker. As the speed increases the transition point tends to move further, so more of the boundary layer is turbulent and the skin friction greater.So it is quite necessary to control the boundary lay separation.

Suction method :One of the methods of controlling the boundary layer is to provide a sourced of suction near the trailing edge, with the object of sucking the boundary layer away.Blowing method :An alternative and one easier method is to provide a source of blowing air from a similar position, thus blowing away the boundary layer.

Vortex generators :These are small plates or wedges projecting an inch or so from the top surface of the wing. Fast moving air will be slowed down by, these vortex generators and help in delaying the flow separation. This circulating air provides energy to the slow moving sluggish layer.Stall wedges or leading edge device : high-lift devices can also be applied to the leading edge of the aerofoil the most common types of which are fixed or movable slots and leading edge flaps (Krueger flaps)used in BOEING AIRCRAFT.

Balancing the Control Surfaces :

The forces required to operate control surfaces would be excessive in case of huge and fast moving a/c . To assist the pilot to move the controls in the absence of powered or power assisted controls, some degree of balance is required this balance is called Aero dynamic balance.Aerodynamics balance is achieved in several ways all of which decreases the force required to move the control in flight. The flight effort required to move any control surface is determined by the aerodynamic force acting through the center of pressure of the aerofoil multiplied by its distance from the hinge line. This force is known as the hinge moment of the control surface; The smaller the hinge moment lesser is the effort required to move the surface through a given angle at a given speed at a given speed at a speed. The types of aerodynamic balanced used are :

Horn Balance :In this a proportion of the total area lies ahead of the hinge line. This is mostly given to rudders and elevators.

Insert Hinge Balance :

This is generally done by hinging the control surface about a line set back from the leading edge of the control surface.

In the above sold two methods usually not more than one-fifth of the surface may be in front of the hinge. In each some part of the surface is 9in front of the hinge and each of it has its advantage.

Trimming and Balance Tabs :

A tab is the small hinged forming part of the trailing edge of primary control surface. If an a/c is out of trim and requires, say a constant pulling force on the control column to maintain level flight would soon tire the pilot. A tab can be used to trim out the holding force and ease the task of the pilot. Besides their use for trimming, tabs have been adopted to a large variety of shapes, sizes, applications and method of operation, giving the designer a broad range of solution to a particular problem.

Fixed tabs :

Fixed tabs can only be adjusted on the ground and their setting is determined by one or more test flight. An early form of fixed tab, sometimes still used on light aircraft, consists of small strips of cord doped above or below along the trailing edge of the control surface. A strip of cord above the trailing edge deflected the surface downwards, the amount of deflection on the length of the cord used.

Trimming tabs :

These are used to trim out any holding forces encountered in flight such as those occurring after a change of power or speed, or when the C.G. position change owing to fuel consumption, or after dropping bombs, expending ammunition. These are pilot operated usually by hand wheels in the cockpit which operate in the neutral sense. Some of the trimming tabs may be electrically operated by small switches.

Control reversal

Control reversal is an adverse affect on the controllability of aircraft. To the pilot it appears that the controls have reversed themselves; in order to roll to the left; for instance, they have to push the control stick to the right, opposite of the normal direction.

There are several causes for this problem : pilot error, effects of high speed flight, incorrectly connected controls, and various coupling forces on the aircraft.

Pilot error is the most common cause of control reversal. In unusual attitudes it is not uncommon for the pilot to become disoriented and start feeding in incorrect control movements in order to regain level flight. This is particularly common when using helmet mounted display systems, which introduce graphics that remain steady in the pilots view, notably when using a particular form of attitude display known as an inside-out display.

Incorrectly connected controls is another common cause of this problem. It is a recurring problem after maintenance on aircraft, notably home built designs that are being flown, for the first time after some minor work. However it is not entirely uncommon on commercial aircraft, and has been the cause of several near-accidents.

P.3.2.1.2High Speed Flight

Speed of Sound

Speed of sound plays and important role in the field of aeronautics. The comparison of speeds of different a/c is made taking speed sound as reference.

At sea level conditions (NTP/STP)

Speed of sound is = 340.17 m/s = 760 mi/hr = 1118 ft/sec = 1216 KMPH

Speed of sound is generally denoted by a.

Subsonic Flight :

If the speed of an a/c is up to 0.78 mach then it is said to be subsonic flight.

Transonic Flight :

If the speed of an a/c is between 0.78 and 1.2 mach then it is said to be transonic flight.

Supersonic Flight :

If the speed of an a/c is between 1.2 and 5 mach then it is said to be transonic flight.

Mach Number :

It is the ratio of speed of any moving object to the speed of sound.

Mach number is denoted by M.

M =

Critical mach number :

The free stream Mach number at which for the first time the local mach number will reach the sonic is called critical mach number.

Explanation :

We all know that when air flow pasts a aerofoil there is an increase in velocity on the top surface of aerofoil. Hence the velocity on the top of the aerofoil will be more than the free stream velocity for each aerofoil there will be a free stream velocity for which the velocity on top of aerofoil (local velocity) will reach sonic speed. Hence we have to remember that the free stream mach number will be less than the local mach number.

For e.g. : The free stream mach number for a particular aerofoil be 0.78 for which the local mach number reaches sonic speed (mach 1) then 0.78 is the critical mach number for that particular aerofoil.

Compressibility effect :

It was emphasized in the early chapters that air is compressible; it was also emphasized, though perhaps not emphatically enough, that though it is compressible it does, in fact behave at ordinary speeds almost as though it were incompressible. Of course such an assumption is not true, air is really compressible or what is sometimes more important, expandable at all speeds and the density does change, i.e. increases or decreases, as the wings and bodies of aero planes move through air at quite ordinary speeds, but the point is that the error in making the assumption is so small as to be negligible, while the simplification that the assumption gives to the whole subject is by no means negligible.

As we approach the aped of sound the error in making this assumption of incompressibility can no longer be justified, the air is definitely compressed, or expanded. We are now dealing with compressible and expandable fluid.

It should be clear from this that the change is gradual, not sudden; it is all a question of deciding when the error becomes appreciable, and a rough idea of the error involved may be obtained from the following figures which the error in assuming the ordinary laws of aerodynamics when estimating the drag of a body moving through air at the speeds mentioned.

SpeedError in assuming incompressibility

m/sKnotsKm/h

4587161About %

90175322Less than

1342604834 %

1793476447 %

22443680511 %

26852296616 %

It will be sufficiently clear from this that we must begin to change our ideas at speeds considerably lower than 340 m/s.

Shock waves :Let us see if we can find out a little more of what actually happens during the change from incompressible flow to compressible flow, and so discover the cause of the mounting error in making the assumption of incompressibility . Let us also investigate the shock, together with its cause and effects.As the speed of airflow over say streamline body increases, the first indication that a change in the nature of the flow is taking place would seem to be a breakaway of the airflow from the surface of the body usually some way back, setting up a turbulent wake (figure below). This may occur at speeds less than half that of sound and has already been dealt over and above that which is expected at the particular speed as reckoned on the speed-squared law,

As the speed increases still further, the point of breakaway, or separation point tends to creep forward, resulting in thicker turbulent wake starting forward of the trailing edge. When we reach about three-quarters of the speed of sound a new phenomenon appears in the form of an incipient shock wave (Figures below). This can be represented by a line approximately at tight angles to the surface of the body and signifying a sudden rise in pressure and density of the air, thus holding up the airflow and causing a decrease of speed of flow.

Incipient shock wave

Incipient shock wave(By courtesy of the shell Petroleum Co Ltd)An incipient shock wave (taken by schlieren photography) has formed on the upper surface; the light areas near the leading edge are expansion regions, separated by the stagnation area which appears as a dark biob at the nose.

Aero Dynamic Heating :

We all know that friction increases temperature, an example of deterioration of energy from the highest to the lowest form (from mechanical energy to thermal energy), natural process and skin friction in the flow of fluids is no exception. We all know, too, that an increase of pressure, as in a pump, raises temperature; another example of the same process and the stagnation pressure on the nose of a body or wing is no exception. So when an aero plane move through the air it gets hot; some parts more than others, some owing to the temperature increase created by skin friction, some owing to that created by pressure.

Many device have been tried, and no doubt many more will be tried, in an effort to counter this heating problem. These devices may be classified under the following headings.

How the surface temperature rises with the Mach Number.

The graph relates to a height of 28000 ft (8500m) where the local temperature of the surrounding air is -40 C.

To insulate the structure from the heat.To use materials which can stand the high temperatures without serious loss of strength.To encourage radiation from the surface and so reduce the temperatures.To circulate a cooling fluid below the surface.Refrigeration by any of the normal methods.

As regards = materials for the aircraft structure light alloys are suitable for Mach numbers up to 2, or even higher for short periods. Between M2 and M4 titanium alloy may be the answer, but above 3 or 3 stainless steel is probably better as being more readily available.

It must be remembered that the crew, the equipment and the fuel must be protected as well as the structure itself, so there is no point in using materials which will stand the high temperatures, unless there is also refrigeration to keep the interior of the aircraft cool.An interesting aspect of surface heating is the effect of shape. It is the speed of flow adjacent to the boundary layer which is the deciding factor in the temperature rise and to some extent, of course, the nature and thickness of the boundary layer itself and the speed of flow depends on the shape of the body. But there is more in it than that. A rise in temperature is created owing to skin friction.

There is a tendency foe the breakaway and turbulent wake to start from the point where the shock wave meets the surface which is usually at or near the point of maximum camber, i.e. where the speed of airflow is greatest.

Area rule :

We should by now realize that if the drag is to be kept to a minimum at (word incomplete) speeds, bodies must be slim and smooth, and have clean lines. What is the significance of clean lines Well, it is often said to be in the eye of the beholder, what looks right is right yes, but it depends in who looks at it; and a little calculation, a little rule, formula, or what ever it may be will often aid our eyes in designing the best shapes for definite purposes.

The area rule (figure below) is simply one of these rules, and put in its simplest form it means that the area of cross-section should increase gradually to a maximum, then decrease for transonic speeds, and in deed for high subsonic speeds, the maximum cross-sectional area should be about half-way, rather than one-third of the way back, this giving more gradual increase of cross-sectional area with an equally gradual decrease.

Transonic area rule

The transonic British Aerospace Buccaneer which finally saw action in the Gulf War shortly before retirement. The bulge in the rear fuselage is for purposes of area rule.

The body in figure below obeys the area rule-but hasnt got any wings. If we add a projection to a body, such as the wings to a fuselage, we shall get sudden jump in the cross-sectional area and that means that the area rule is not being obeyed. What then can we do the answer is that we must decrease the cross-sectional area of the fuselage as we add the cross-sectional area it is the total cross-sectional area that must be gradually decreased.

Hypersonic flight :

The speed of an a/c above 5 mach then it is said to the hypersonic flight.

Subsonic = up to 0.78 machTransonic = 0.78 to 1.2 machSupersonic = 1.2 to 5 machHypersonic = above 5 machSonic speed = 1 mach

Characteristics of sub-sonic aerofoil :High or max lift coefficient.Thickness to chord ratio is more.Small or stable movement C.P.Sufficient depth to accommodate spars.High lift to drag ratio.

Characteristics of Supersonic Aerofoil :Low lift to drag ratio.Less camber and sharp leading edge.Maximum thickness of aerofoil is about 50 % of chord.Very less space to accommodate spars.

Note : Flat plate is most efficient section for supersonic flight and it is used on certain guided missiles.

Mach Cone :

It was illustrated the piling up of the air in front of a body moving at the speed of around, and explained how the incipient shock wave is formed. This incipient shock wave is at right angles to the direction of the airflow, and this means as near as matters at right angles to the surface of a body such as a wing.

Now suppose a point is moving at a velocity V (which is greater than the speed of sound) in the direction A to D (Figure below). A pressure wave sent out when the point is at A will travel outwards in all directions at the speed of sound; but the point will move faster than this, and by the time it has reached D, the wave from A and other pressure waves sent out when it was at B and C will have formed circles as shown in the figure, and it will be possible to draw a common tangent DE to these circles this tangent represents the limit to which all these pressure waves will have got when the point has reached D.

Now AE, the radius of the first circle, represents the distance that sound has traveled while the point has traveled from A to D, or, expressing it in velocities. AS represents the velocity of sound usually denoted by a and Ad represents the velocity of the point V.

So the Mach Number M = =

(as illustrated in the figure this is about 2).

The angel ADE, or a, is called the Mach angle and by simple trigonometry it will be clear that

Sin a = =

In other words, the greater the Mach Number the more acute the angle a. At a Mach Number of 1, a of course is 90.

Mach Angle

If the moving point is a solid 3 dimensional body, such as a bullet, a complete cone called the Mach Cone will be formed, the angle at the apex being 2 a. If the moving point represents a straight line such as the leading edge of a wing, a wedge will be formed, again with an angle 2 a at the leading edge.

The tangent line DE is called a Mach Line, and it clearly represents the angle at which small wavelets are formed; the velocity of the airflow can even be calculated by measuring the angle on photographs of the wavelets.

Again the hydraulic analogy may be useful, since similar effects are seen when a ship passes through water or a thin stick is placed in a fast moving stream of water. Only the region within the wedge formed by the bow waves is affected by the stick; the water outside this region flows on as if nothing was there. And the faster the flow, the sharper is the angle of the wedge.

It might be thought the Mach Line represented the inclination of the shock waves but this is not so. Disturbances of small amplitude travel at the speed of sound, but shock waves, which are waves of larger amplitude, actually travel slightly faster than sound, and therefore they form at a rather larger angle to the surface. This fact is difficult to explain without going into the mathematics of fluid flow, which is quite beyond the scope of this book, but the following explanation of how shock waves are formed may help us to understand how their slope is determined.

Imagine a supersonic flow of air over a flat surface, This surface can never be perfectly smooth, and may be considered as consisting of a very large number of particles or slight bumps. At each of these bumps a Mach Line will be formed; its angle to the surface depending on the speed of the flow in accordance with the formula sin a = .

If the speed of flow is accelerating, the Mach Lines will diverge as the angle becomes more acute with increasing speed.

But if the speed of flow is decelerating the Mach Lines will converge, add up as it were, and form a more intense disturbance or wave, one of greater amplitude.

[In Short : When the object files at speed of sound the pressure disturbances would accumulate immediately in-front of the a/c in the form of a continuous line or waves. At even greater speed the infinite number of pressure waves would produce a continuous line diverging backwards in the form of a cone called mach cone. This cone marks the boundary of the sphere of the influence of the body.]

Effect of Sweepback :

The angular swept back of main planes relative to the fuselage or hull is known as sweep back.

The sweep angle is usually measured as the angle b/w the line of 25% of chord and a perpendicular to the root chord. Low speed a/c have slightly swept back wings in order to shift the center of lift point on the wing shaft and move it close to C.G in order to improve stability.

Advantage :

When the a/c is banked by a disturbance, the airflow over the lower wing passes with a greater effective camber than that of raised wing, therefore relative greater amount of lift from the lower wing is achieved which restores the a/c laterally.

Disadvantage :

Viewed from the front the smaller is the frontal area of the flaps and ailerons. These are less exposed to the airflow thus decreasing the effectiveness of flaps and ailerons.

Sonic Bangs :

We are now all too familiar with the noises made by aircraft breaking the sound barrier. These so-called sonic bangs, or booms, are of course, caused by shock waves, generated by an aircraft, and striking the ears of an observer on the ground, or his glass house, but there has been considerable argument as to the exact circumstances which result in the shock waves being heard, why there are often two or more distinct bangs, whether the second one came first, and so on.

An aircraft diving towards the earth at supersonic speed, and at an angle of say 45, then suddenly slowing up and changing direction, will shed it shock waves, which will travel on towards the earth and strike any observers which may happen to be in their path. It is certainly quite clear from schlieren photographs that a bow wave approaches from the front as the speed of sound is approached and conversely goes ahead of the aircraft when it decelerates below the speed of sound.

As far effects at ground level are concerned, we know that these become less intense with the height of the aircraft; more intense with Mach Number, though not anything like in proportion; that they affected by the dimensions of the aircraft, increasing with its weight and volume, and being of longer duration according to its length; that they are more intense during accelerated flight.

are angle rearwards , like the bow waves on a ship or boat.

P.3.1.2 Polar curve (aviation)

A polar curve is a graph of, the rate of sink of an aircraft, often a glider, versus its horizontal speed.Contents1. Measuring a gliders performance 2. Glide ratio 3. Plotting the curve4. External links

Measuring a gliders performance

Knowing the best speed to fly is important in exploiting the performance of a glider. Two of the key measures of a gliders performance are its minimum sink rate and its best glide ratio, also known as the best glide angle. These occur at different speeds. Knowing these speeds is important for efficient cross-country flying. In still air the polar curve shows that flying. In still air the polar curve shows that flying at the minimum sink speed enables the pilot to stay airborne for as long as possible and to climb as quickly as possible, but at this speed the glider will not travel as far as if it flew at the speed for the best glide. When in sinking air, the polar curve shows that best speed to fly depends on the rate that the air is descending. Using may often be considerably in excess of the speed for the best glide angle to get out of the sinking air as quickly as possible.

Polar Curve Showing Angle for Minimum Sink

Polar Curve Showing Glide Angle for Best Glide

Glide ratio

The glide ratio is expresses as the ratio of the distance traveled to height lost in the same time. The ratio of the horizontal speed versus the vertical speed gives the same answer. If the glider flies at 40 knots for an hour and experiences a 2-knot (4 km/h) sink rate, it will travel 40 nautical miles and descend 2 nautical miles (4 km). The guide ratio is 20 using both methods.

Plotting the curve

By measuring the rate of sink at various air speeds a set of data can be accumulated and plotted on a graph. The points can be connected by, a line known as the polar curve. Each type of glider has unique polar curve.

The origin for a polar curve is where the air speed is zero and the sink rate is zero. In the first diagram a line has been drawn from the origin to the point with minimum sink. The slope of the line from the origin gives the glide angle, because it is the ratio of the distance along the air speed axis to the distance along the sink rate axis.

A whole series of lines could be drawn from the origin to each of the data points, each line showing the glide angle for that speed. However the best glide angle is the line with the least slope. In the second diagram, the line has been drawn from the origin to the point representing the best glide ratio. The air speed and sink rate at the best glide ratio can be read off the graph. Note that the best glide ratio is shallower than the glide ratio for minimum sink. All the other lines from the origin to the various data points would be steeper than the line of the best glide angle. Consequently, the line for the best glide angle will only just graze the polar curve, i.e. it is a tangent.

P3.RA.2.1 Theory of Flight Terminology Advancing Blade: Any rotor blade or wing on rotary wing aircraft, in horizontal motion, moving into the relative wind is called advancing blade. Aerofoil: An aerodynamic surface designed to obtain a reaction from the air through which it moves e.g. aileron, win^ rudder, elevator, rotor blade etc. Angle of Attack: An angle between the relative wind and the chord line which is represented by the symbol Pitch Angle: An angle between a fixed reference surface and the chord of an airfoil. Articulated Rotor: In a rotor craft wing design, each blade is joined at its root to allow the blade to change pitch, lead and lag, and flap, either individually or collectively, is known an articulated rotor. Autorotation: A rotorcraft flight condition, in which the lifting rotor is driven entirely by action of the air flowing upward through the rotor disk. Blade Stall :The condition occurring when a rotor blade operates at an angle of attack greater than the angle of maximum lift. Blade Twist :The twist, or variation in the blade angle, built into a blade usually so that the blade angle decreases toward the tip. A deformation of a rotor blade by unequal air forces causing variation in the built-in blade angle from root to tip. Coning Angle : The average angle between the span axis of a blade of a rotary wing system and a plane perpendicular to the axis of rotation. Disk Area : The area of the circle described by the blade tips of a rotating propeller or rotor.

Feathering : Change in the pitch angle of the rotor blades periodically by rotating them around their feathering axis. Feathering Axis : The axis about which the pitch angle of a rotor blade is varied. Flapping : The vertical movement of a blade about the flapping hinge is known as flapping. Flapping Hinge : The Hinge with its axis parallel to the rotor plane of rotation, which permits the rotor blades to flap to equalize lift between the advancing and retreating blades of the rotor disc. It is also known as Delta hinge. Hovering : A condition of rotorcraft flight where there is zero horizontal and vertical motion of the rotorcraft. Retreating Blade : On a rotary-wing aircraft in horizontal motion, any rotor blade or wing moving with the relative wind. Rotor Disk : The area of a circular plane described by the path swept by the rotor blades. Rotor Lift : The lift component, parallel to the plane of symmetry and perpendicular to the line of flight, acting on a rotor. Rotor Mast : A column or structure supporting a rotor on a rotary-wing aircraft; usually called a mast or pylon. Swash plate : The element used to transmit input from non-rotating helicopter control system to the rotating rotor control system.

Axis of rotation : The line through the rotor head at right angles to the plane of rotation (POR). The blades actually rotate around this axis. Tip path plane : The plane within which the tips of rotor blades travel. It is parallel to the plane of rotation which acts through the rotor head. A pilot may alter this plane through movement of the cyclic control. Tip path : The circular path described by the tips of rotor blades. Span : The span of the blade is the distance from root of the blade to the tip of the blade, measured.

Aerodynamics and Theory of Flight of the Helicopter Symmetrical airfoils :The airfoils which are used for helicopters are usually referred to as symmetrical airfoils, which mean the airfoil section has the same shape above and below the chord line. This curvature of the airfoil is referred to as the camber. Some successful designs have been built with an unsymmetrical airfoil, meaning that the top and bottom camber are not the same shape. Relative wind :As the rotor blade moves, it is subjected to relative wind. The relative wind is the direction of the airflow with respect to the blade. This is always opposite the flight path of the blade. For example, if the blade moves forward horizontally, the relative wind moves backward horizontally. If the blade moves horizontally, the relative wind moves forward horizontally. If the blade moves forward and upward, the relative wind moves backward and downward. If the blade moves backward and downward, the relative wind moves forward and upward. Relationship between the flight path of an airfoil and the relative wind.Relative wind is parallel and in the opposite direction to the flight path. The forward moving blade is referred to as the advancing blade, while the backward blade is called the retreating blade. The relative wind may be affected by several factors such as movement of the rotor blades, horizontal movement of the helicopter, flapping of the rotor blade, wind speed, and direction. The relative wind of the helicopter is the flow of air with respect to the rotor blade. For example : when the rotor is stopped , the wind blowing over the rotor blades creates a relative wind. When the helicopter is hovering in a no-wind condition, the relative wind is created by the motion of the rotor blades. If the helicopter is hovering in a wind, the relative wind is a combination of the/wind and the rotor blade movement. When the helicopter is in (forward flight, ) relative wind created by the rotor blades, the movement of the helicopter, and possibly a wind factor. Pitch angle :Pitch angle is the acute angle between the rotor blade chord and a reference plane. The reference plane of the helicopter will be determined by the main rotor hub. The pitch angle is varied by movement of the collective control which will rotate the blade about the hub axis, increasing or decreasing the pitch. The pitch angle may also be varied by movement of the cyclic control which will be discussed in detail later in this section. Often the pitch angle is confused with the angle of attack. The pitch angle of a rotor blade is the angle between the control line and a reference plane determined by the rotor hub or the plane of rotation.Coning : The action of rotating blades slanted or lifted upward at the tips to form a cone-shaped pattern.Coning Angle : The average angle between the span axis of a blade of a rotary wing system and a plane perpendicular to the axis of rotation. The angle of attack in relation to the relative wind. Angle of attack :The angle of attack is the acute angle between the chord line of the airfoil and the relative wind. The angle of attack may be equal of attack may be equal to the pitch angle. How-ever, it may also be greater or less than the angle of attack. The pilot can increase or decrease the angle of attack by moving the pitch angle of the rotor. When the pitch angle is increased. Then the angle of attack is increased and vice versa. Since the angle of attack is dependent upon the relative wind, the same factors that affect the relative wind also affect the angle of attack. Lift :Lift is the force produced by the airfoil that is perpendicular to the relative wind and opposes gravity. The lift is developed by the rotor blade according the Bernoullis Principle, which simply states that as velocity is increased, the pressure is decreased., This principle creates a low pressure at the top of the rotor blade, while the bottom of the blade has an increased pressure. This applies to both symmetrical and unsymmetrical airfoils. Whenever lift is produced; drag is also produced. Drag :Drag is the force which tends to resist the airfoils passage through the air. Drag is always parallel to the relative wind and perpendicular to lift. It is this force that tends to slow down the rotor when the angle of attack is increased in order to produce more lift. In fact, drag varies as a square of velocity. Center of pressure :The center of pressure is an imaginary point where the results of all the aerodynamic forces of the airfoil are considered to be concentrated. This center of pressure can move as forces change. On some unsymmetrical airfoils, this movement can cover a great distance of the chord of the airfoil. As the angle of attack is increased the center of pressure moves forward, along the airfoil surface and as the angle of attack is decreased the center of pressure moves forward along the airfoil surface. This is of little consequence in fixed wing aircraft because longitudinal stability may be achieved in several other ways. On helicopters, because the rotor blades are moved from a fixed axis (the hub), this situation could lead to instability in the rotor, with the rotor blades constantly changing pitch. For this reason, the preferred airfoil is symmetrical where the center of pressure has very little movement. Accompanying lift and drag is stall. This is the reason that power must also be added in order to maintain the velocity of the rotor when the pitch is added to the rotor system. This also means that the lift of the rotor could be controlled by varying speed increasing or decreasing the relative wind. However, this situation is avoided because of the slow reaction time, in favor of keeping the velocity constant and changing the angle of attack. Effects on lift :Lift will also vary with the density of the air, Air density is affected by temperature, altitude, and humidity. On a hot day the air is less dense than on a cold day. Because of this, the rotor system will require a higher angle of attack to produce the same lift. This will require more power to maintain blade velocity. The same situation is true when changes in altitude occur. Often a helicopter may be able to hover at sea level with a certain load but not at altitude because air is only two-thirds as dense at 10,000 feet of altitude as it is at sea level. Humidity will have the same effect since humid air is less dense than dry air. Forces on the rotor. The lift developed by the helicopter has to be sufficient to overcome the weight. The heavier the weight of the helicopter is, the greater the pitch angle and power requirement to overcome this weight vs. lift action. Also acting on the helicopter will be thrust and drag. Thrust is the force moving the helicopter in the desired direction, while drag is the force which tends to resist thrust. Therefore, before any movement may take place thrust must overcome drag. Thus far the principles of flight have been much the same as that of the fixed wing airplane. However, remember that the actual movements that govern flight will be accomplished by driving the rotor blades in a circle rather than wings being flown in a straight line. Considering this situation, number of forces are applied to the rotor system that are not present with the fixed wing. Rotor droop occurs when the rotor is at rest. The rotor consists of a hub which is driven by the shaft (mast). Attached to the hub are the blades. The blades are somewhat flexible, when at rest will droop due to the weight and span of the blade. This is referred to a blade droop. When the rotor is turned, this droop is overcome by centrifugal force, which will straighten the blade. This centrifugal force will be dependent, upon the weight of the blade and the velocity. On small rotor systems this could be approximately 20000 pounds. Larger system may approach 100.000 pounds of centrifugal force per blade. With forces of this magnitude, the utmost care must be taken in maintenance procedures. In addition to centrifugal force, lift will react perpendicular to the rotor as pitch is applied to the rotor. This will result in the blade seeking a new position which will be the result of centrifugal force and lift. Disc area :The area contained within the tip path plane. In flight , this area is not a constant since it is affected by the coning angle of the blades. Tip path, tip path plane, shaft axis and plane or rotation. Axis of rotation acts through the rotor head at right tingles to the plane of rotation while the shaft axis remains in line with the rotor mast. Shaft Axis :The line consistent with the rotor shaft (mast). Only when the plane of rotation is exactly perpendicular to the shaft axis will the axis of rotation coincide with the shaft axis. Lead - lagging (dragging) :Movement of a blade forward or aft in the plane of rotation. Coning angle : Blade angle can be altered by rotating the blade around its feathering axis. This movement of the blades is referred to as coning of the rotor. The amount of coning is dependent upon the amount of lift and the weight of the helicopter. A helicopter with a light load will have less coning than a heavily loaded one. Note : The blade tips will pass through a circular surface formed by the rotor blades. This circular plane is referred to as the rotor disc or the tip path plane. The satisfactory relationship of the rotor blades to each other in flight is referred to as track. If this relationship is incorrect, it is referred to as being out of track. Such a condition will result in vibrations in the rotor system. Thrust :Thus far we have discussed the flight for the helicopter only in regards to obtaining lift, with little mention of thrust. Since the rotor will produce lift force and at the same time propel the helicopter directionally, thrust is most important. It is thrust that gives this directional movement. Directional flight :The rotor disc will be tilted in the direction of the movement desired. This will result in lift and thrust being perpendicular to each other, giving the helicopter the ability to maintain flight and move directionally. Thrust is obtained by movement of the tip path plane of the rotor or rotor disc. If the helicopter is ascending vertically or at a hover, lift and thrust are both in the same direction, vertical. However, in order to obtain forward, backward, or sideward directional flight, the rotor disc will be tilted in the direction of movement desired. This will result in lift and thrust being perpendicular to each other, giving the helicopter the ability to maintain flight and move directionally. Movement of the tip path plane to change the direction of the helicopter is accomplished by changing the angle of attack of the individual blades as they pass along the disc. In order to accomplish this, the hub have provisions for a feathering axis, which simply allows the pitch to be moved.The feathering axis or pitch axis of the rotor. Basic principles of the swash plate : Tilts with cyclic control it does not rotate. Horizontal Movement :Two conditions have to be satisfied for the helicopter to fly horizontally. First, there must be force acting vertically upwards to support the helicopters weight; secondly, another force must act in the appropriate; horizontal direction. Since the helicopter has no separate or horizontal thrust, both forces must be supplied by the main rotor. This can only be achieved by tilting the axis of rotation of the rotor blades so that the total rotor thrust is inclined from the vertical; one component of the thrust force supports the helicopter, and the other component produces horizontal movement. When the helicopter is hovering in still air, the pitch angle is equal on all rotor blades and the thrust being developed by each blade caused the blade to rise until balanced by the centrifugal force. The rotor axis and the total rotor thrust direction are then vertical. Now let us consider what would happen to a blade if its pitch was gradually increased during half a revolution and then gradually decreased to its original value during remainder of the revolution. For half a revolution the increasing thrust would lift the blade more and more above its normal height. After that would gradually sink as its thrust decreased until, at the end of the revolution, it would be back at its original height. If the remaining rotor blades were subjected to similar changes of pitch, at exactly at the same point in rotation, the blade would rise, one after other, and reach their maximum height as they passed the same rotational point. The blade would be then describing a conical path with the cone axis and the total thrust force inclined from the vertical. The continuous alteration in the pitch angle of the rotor blades as they rotate is known as a cyclic pitch change, and the same amount of change and also the position in rotation at which it occurs are controlled by what is called the cycle stick. Effects of Gyroscopic Precision Gyroscopic Precession : The spinning main rotor of a helicopter acts like a gyroscope. As such, it has the properties of gyroscopic action, one of which is precession. Gyroscopic precession is the resultant action or deflection of a spinning object when a force is applied to this object. This action occurs approximately 90 in the direction of rotation from the point where the force is applied (fig below). Through the use of this principle, the tip-path plane of the main rotor may be tilted from the horizontal. The movement of the cyclic pitch control in a two-bladed rotor system increases the angle of attack of one rotor blade with the result that a greater lifting force is applied at this point in the plane of rotation. This same control movement simultaneously decreases the angle of attack of the other blade like amount, thus decreasing the lifting force applied at this point in the plane of rotation. The blade with the increased angle of attack tends to rise; the blade with the decreased angle, of attack tends to lower. However, because of the gyroscopic precession property, the blades do not rise or lower to maximum deflection until a point approximately 90 later in the plane of rotation. Gyroscopic Precession Principle : When a force is applied to a spinning gyro, the maximum reaction occurs 90 later in the direction of rotation. In this illustration (fig. above) the retreating blade angle of attack is increased and the advancing blade angle of attack is decreased resulting in a tipping forward of the tip-path plane, since maximum deflection takes place 90 later when the blades are at the rear and front respectively. In a three-bladed rotor, the movement of the cyclic pitch control changes the angle of attack of each blade an appropriate amount so that the end result is the same a tipping forward of the tip-path plane when the maximum change in angle of attack is made as each blade passes the same points at which the maximum increase and decrease are made in the illustration (fig. below). For the two-bladed rotor. As each blade passes the 90 position to the right, the maximum decrease in angle of attack occurs. Maximum deflection takes place 90 later maximum upward deflection at the rear and maximum downward deflection at the front and the tip-path plane tips forward. Rotor disc acts like a gyro. When a rotor blade pitchchange is made, maximum reaction occurs approximately 90 later in the direction of rotation.Torque Reaction and Directional Control Tail rotor thrust compensates for the torqueeffect of the main rotor. Torque Reaction and Directional ControlNewtons third law states that for every action there is an opposite and equal reaction. Therefore when power is applied to the rotor system the fuselage of the helicopter will tend to move in the opposite direction of the rotor. This tendency is referred to as torque. The torque problem plagued designers since the inception of the helicopter. Several designs of rotor systems were tried to eliminate this problem. Compensation of Torque ReactionSome means must be used to prevent the helicopter fuselage being turned. Several solutions are employed to compensate the torque reaction. They are :An off set propeller facing forward.A propeller directing air flow over tail surfaces. A controlled jet of gas at the tail. NOTARAnti-torque tail rotor (Used in Chetak/Cheetah)Dual or multi-rotor system. Coaxial, TANDOM, side by side.Jet powered rotor blades. (Torque reaction is eliminated) Anti Torque Tail RotorThe tail rotor is a small propeller rotating in a vertical plane. The tail rotor is driven by a take off drive from the main gear box and will always rotate whenever the main rotor is rotating. The tail rotor by exerting a side thrust on a tail arm (Tail boom) neutralizing the torque reaction and preventing the fuselage form turning. For maximum efficiency this torque compensating rotor must be placed as for as possible from the main rotor axis and its distance from the axis. This way anti-torque rotor is installed at the tail of helicopter. To balance varying torque moments and to change the helicopter heading the anti-torque moments and to change the helicopter heading the anti-torque must itself be variable. For this purpose the tail rotor is provided with a pitch change control system. The tail rotor pitch variation is controlled by the pilot through rudder pedals. By actuating the rudder pedals the tail rotor pitch can be increased or decreased collectively. This in turn increases or decreases the side thrust produced by the tail rotors. By increasing or decreasing the thrust of the tail rotors, the pilot will be able to move the fuselage in any direction under control and thereby change the helicopters heading in flight as well as while taxying. One such design was the coaxial helicopter which two main rotors were placed on top of each other rotating in opposite directions. Another design requires two main rotors placed side by side. Some of these designs actually used intermeshing rotors turning in opposite directions. Still other designs have used single rotors powered at the tip by ramjets or hot air passing through the blade and ejected through nozzles at the tip. The disadvantages of these systems seem to outweigh the advantages to the point that most helicopters use one main rotor with an auxiliary rotor on the tail to counteract torque. Anti-torque control is applied by the tail rotor. Many of the conventional helicopters using tail rotors have found methods to help reduce this power requirement in flight. One of these methods is a vertical fin, which is offset in order to keep the fuselage straight during forward flight. This in turn unloads the tail rotor. Auxiliary Rotor :The force that compensates for torque and keeps the fuselage from turning in the direction opposite to the main rotor is produced by means of an auxiliary rotor located on the end of the tail boom. This auxiliary rotor, generally referred to as a tail rotor, or anti-torque rotor, produces thrust in the direction opposite in the cockpit, permit the pilot to increase or decrease tail-rotor thrust, as needed, to neutralize torque effect. Dissymmetry of LiftThe area within the tip-path plane of the main rotor is known as the disc area or rotor disc. When hovering in still air, lift created by the rotor blades at all corresponding positions around the rotor disc is equal. Dissymmetry of lift is created by horizontal flight or by wind during hovering flight, and is the difference in lift that exists between the advancing blade half of the disc area and the retreating blade half. At normal rotor operating RPM and zero airspeed, the rotating blade-tip speed of most helicopter main rotors is approximately 400 miles per hour. When hovering in a no-wind condition, the speed of the relative wind at the blade tips is the same throughout the tip-path plane (bottom fig. in the figures below). The speed of the relative wind at any specific point along the rotor blade will be the same throughout the tip-path plane; however, the speed is reduced as this point moves closer to the rotor hub as indicated by the two inner circles. As the helicopter moves into forward flight, the relative wind moving over each rotor blade becomes a combination of the rotational speed of the rotor and the forward movement of the helicopter (top fig. in the figures below). At the 90 position on the right side, the advancing blade has the combined speed of the blade velocity plus the speed of the helicopter. At the 90 position on the left side, the retreating blade speed is the blade velocity less the speed of the helicopter. (In the illustration, the helicopter is assumed to have a forward airspeed of 100 miles per hour). In other words, the relative wind speed is at a maximum at the 90 position on the right side and at a minimum at the 90 position on the left side. For any given angle of attack, lift increases as the velocity of the airflow over the airfoil increases. It is apparent that the lift over the advancing blade half of the rotor disc will be greater than the lift over the retreating blade half during horizontal flight, or when hovering will roll to the left unless some compensation is made. It is equally apparent that the helicopter will roll to the left unless some compensation is made. The compensation made to equalize the lift over the two halves of the rotor disc is blade flapping and cyclic feathering. It was Juan De Cierva who incorporated the flapping hinge into each blade, eliminating this problem. Dissymmetry of Lift: Comparison of rotor blade speed for the advancing blade during hovering and forward flight. Flapping hingeThis system is still used today in most multi-bladed systems. This flapping hinge allows each blade to move freely about its vertical axis or to move up and down. This movement is referred to as flapping. Since more lift is created by the advancing blade, the blade has a tendency to move up. This decreases the amount of lift on the advancing side of the disc. At the same time, the retreating blade takes a more horizontal position, which creates more lift because less lift is being created by the retreating half of the disc (fig below). The flapping hinge is used to control dissymmetry of lift. Seesaw systemAnother method which is quite widely used for the correction of dissymmetry of lift is the seesaw system. This system utilizes two blades. System one blade is advancing while the other is retreating. Since the advancing blade moves up, and because the two blades are connected, the retreating blades moves down a like amount, thus creating the seesaw action (fig below). This seesaw action is used on semi rigid rotors.Blade Tip StallThe helicopter rotor blades, likely any airfoil, are subject to stall. However, a stall of the rotor is quite different from that of the fixed wing. As a brief review, it was learned that in forward speed the advancing blade is moving at a faster speed than the retreating blade. As the speed of the helicopter increases, this speed differential becomes greater. Because of the dissymmetry of lift, the retreating blade will be seeking a higher angle of attack than the advancing blade. This, couples with the low airspeed of the retreating blade, can lead to blade tip stall. An airfoil may stall due to any of the following reasons:Insufficient airspeedToo great an angle of attackHeavy wing loading In a helicopter flying at 200 miles per hour, the advancing blade will have a tip speed of approximately 600 miles per hour, while the retreating blade tip speed is reduced to 200 miles per hour (Fig. drawn below). At this point, the root areas are producing no lift. The retreating blade must continue to seek a higher angle of attack in order to maintain lift. Even though the blade has twist built attack at the tips. This is due to the tilting of the rotor and its relationship to the inflow of air to the rotor. It is not possible, however, to predict at what point the rotor will stall each time due to the forward speed because several other factors must also be considered. One of these is wing loading. It is more likely for the blade to the stall under heavy loads than under light loads. Heavy loading will only decrease the speed at which the stall will occur. Other factors, such as temperature, altitude, and maneuvers must also be considered. For these reasons a stall may occur at rather low operating speeds. In Fig. drawn below, a rotor system is shown with the stall area marked. If can be seen that as the tip enters the stall condition, only a few inches are involved; but as the blade continues, several feet towards the middle of the blade travel in the stall area, and then it will move out toward the tip. Stall occurs first on the retreating half of the disc. The indication of a stall condition will first be a vibration as each blade passes through the stall region. The beat could be 2:1, 3:1, or 4:1 depending upon the number of blades in the rotor system. If the stall continues, the helicopter will pitch up. Although the stall will occur on the left side of the helicopter, due to gyroscopic precession, the result will be at the tail of the helicopter, which will pitch the nose up. When a stall is experienced, the corrective action is to reduce forward speed, reduce forward speed, reduce and increase rotor speed if possible, but the important factor is always to unload the rotor system. Blade Sailing This undesirable feature of the helicopter can occur in windy conditions when the blades are moving slowly as they start to revolve or are stopping. The blade advancing into wind generates too much thrust for the low centrifugal force and is able to sail upwards with little restraint. As the blade moves round to retreat with the wind it loses thrust and sails down from its abnormally high position. The added momentum can bend the blade against its downward stops to such and extent that it strikes, the helicopters rear fuselage or even the ground. In gusty conditions blade sailing cannot be avoide