Principle of Flight

1. Forces

We live in a world full of forces. Lets consider a man running on a sports track asshown below. There are many types of forces that are acting on him as he runs.

He experiences a force between his shoes and the ground and this is called "Friction".Friction slows him down but it surely does prevent him from slipping. He provideshimself with some force to move his leg forward and backward. This is his internalforce and is gained from body fuel – food and water. And there is also his bodyWeight, which prevents him from moving quickly. Weight is also a force. If the manis extremely fat, he will experience great difficulty moving and vice-versa, if he isthin, he will be able to move quickly. Another hidden force that acts on him is the"Gravity". Gravity prevents the man from jumping too high and always ensures thatshould his feet leave the ground, they must touch the ground again. Gravity is a forcethat is exhibited by our planet on every body. We cannot escape gravity unless weleave this planet and enter space. In space, there is no 'gravity'. Let consider one moreexample before we start analysing forces. If the man in our example decided to run ona windy day, he will face another force. This force will prevent him from movingforward or it may even make him move faster. This very much depends on the winddirection. This force is exerted by Wind flow.

Now that we have seen many examples of force, let's further investigate themechanisms behind forces. Forces may be loosely regarded as 'energy created bythe movement of particles in some media'. We cannot touch force but we surely canfeel it. There are many types of forces in this world and depending on our view, some

FrictionWeight / Gravity

Wind Flow

are considered 'good' and others 'bad'.

Let's try our first experiment.

Experiment 1.

Cut a tiny rectangular strip of paper and paste some soap on of its end. Next place itin a vessel containing water and observe its movement. Can you explain why thepaper moves? Which direction does it move?

Let me now explain what happening to our tiny paper strip. When the strip is placedin water, you will observe some random movement. The soap breaks up the watersurface tension and causes the paper strip to move forward. Surface tension is animportant force and many insects in our world need it. For example, a dragonfly usessurface tension to stay afloat. A mosquito also uses this force to move on water.

But why random movement? To explain this, let's have a microscopic picture of ourexperiment. Water is full of molecules and these molecules move random. They

water molecule

have no order and definitely don't move " 2 by 2 " !! As you can see from the abovediagram, as soon as we place our paper strip in the water, it gets bombarded by watermolecules and as mentioned before, since the movement of these molecules is random,the paper strip also moves in a random order. One water molecule will hit anothermolecule and another molecule will hit the paper strip and eventually the paper stripwill move a random direction.

There are other places where you can similar phenomenon. When you go and watch amovie in you local cinema, take a look at the movie projector. You will see many dustparticles suspended in front of the projector's light. They all move in a random orderas they are bombarded with air molecules. These dust particles are also experiencingforces.

2. Aircraft Forces

In the previous action, you have seen some of the natural forces that exist in our world.Lets now turn our attention to the different type of forces that act upon an aircraft.When an aircraft is kept safely in a hangar or when it is taxied near a runway, it onlyexperiences one force - "Weight". No other force acts on it. However, when an aircraftis airborne i.e. flying, things become different - instead of one downward force, fourmain forces start to act upon the aircraft and they are:

Lift Lift is the force created by the interaction between the wings and the airflow.It always act upwards. It is considered to be the 'most important force' aswithout it, an aircraft cannot ascend from ground and maintain altitude.

Weight This force acts on an aircraft due to the interaction between the aircraft'sbody weight and Earth's gravity. Weight is a downward force.

Thrust This force is created by an aircraft's engine and is required for forwardmotion.

Drag This force acts in reverse direction to that of 'Thrust' and hinders forwardmotion. Drag is considered as a negative force and all engineers try their bestto reduce drag.

Now that you know that there are four main forces that act upon an aircraft, letsinvestigate each force and learn how these forces are created.

3. LIFT

Lift is a force that not only moves an aircraft upwards into the sky but also help anaircraft to maintain altitude. In order to achieve lift, aeroplanes are designed withspecial wings. These special wings are known as aerofoils.

Let's see what happens when air flows around an aerofoil.

When air hits the front edge of the aerofoil - 'Leading Edge', it separates into twolayers and flows both on the upper and lower surface. Air around the upper surfacetravels a longer distance while air around the lower surface travels a shorter distance.Aerofoils of such behaviour are knows as cambered aerofoils i.e. they havedifferences in lengths on both upper and lower surfaces. It is due to this camber effect,that air around the lower surface will experience 'high pressure' while air around theupper surface will experience 'low pressure'. Another way of proving this pressuredistribution is the Bernoulli's theorem (explained in the appendix section).

Let’s have a look at an aerofoil and some of terms associated with an aerofoil.

Leading edge The front edge of an aerofoil. This is the pointon an aerofoil where airflow starts to separate.

Trailing edge The trailing edge of an aerofoil where airflowleaves.

Chord line Imaginary line joining the leading edge andtrailing edge (c).

Chamber The thickness between the mean camber line tothe chord line of an aerofoil.

Angle ofattack

The angle between the chord line and theincoming air flow.

Aspect Ratio The length of the aerofoil multiplied by theaerofoil cross-sectional area.

leading edge

High pressureLow pressure

trailing edge

3.1 NACA

NACA stands for ‘National AdvisoryCommittee for Aeronautics’ - a committeewhich was based in USA. Their primary jobwas to issue standards for aerofoils. Allaerofoils are numbered by an internationalstandard known as ‘NACA Number’. Thereis a 4-digit standard, 5 and a 6 digit standard(out of our scope).

In a 4 digit NACA number such as NACA4138:

The digit ‘4’ represents the ‘Maximumcamber of an aerofoil’;The digit ‘1’ represents the ‘Maximumcamber position’;The last 2 digits ‘38’represent the‘Thickness of the aerofoil’.

Different aerofoils have different lift characteristics. Some provide greater lift thanothers and some less. Some operate at high speeds and some at low speeds.

NACA is now part of NASA and all aerofoil standards are issued issued by NACAdivision.

3.2 Wind Tunnel

A wind tunnel is an experimental tunnel where aerofoils of different shapes are tested.A wind tunnel plan view is shown below. An aerofoil is suspended in the test-bedsection of a wind tunnel and is then subjected to a windflow. In a typicalaerodynamics (study of air flow over aeroplanes), the windflow is kept constant andonly the angle of attack is varied. The angle of attack is varied from –2o to 32o and thepressure experienced over the upper and lower surfaces of the aerofoil are measured.In a NACA4212 foil pressure analysis, it is found that, as the angle of attack isincreased, lift increases. The lift continues to increases until a specific angle of attackis reached. Beyond this angle, no lift is produced, no matter how much the angle of

attack is increased.

Pressure measurements obtained from wind tunnel tests show that lift at various partof an aerofoil surface are unequal. All lift forces are perpendicular (90o) and two-thirds of the lift is obtained from the top surface of the aerofoil. Shown below is a liftdistribution round an aerofoil.

3.3 Centre of Pressure

When we add up all these lines of small lift forces together, we come up with onestraight line of force which represents all the lift the aerofoil is producing. Thisstraight line is drawn at a specific point where all forces balance. This average point atwhich all lift forces seem to act is know as the ‘centre of pressure’. CP

We did mention that lift increases with angle of attack and then begins to fall at someangle. Let’s have a closer look at this phenomenon. At –2 degrees angle of attack,the aircraft with the NACA412 moves in a straight line. The lift forces acting on theaircraft are equal on both upper and lower surfaces.

At 2 degrees angle of attack, the aircraftbegins to experience lift and the lift force ismainly on the upper surface.

At 5 degrees of angle of attack, the aircraftcontinues to experience lift, far greater thanthat at 2 degrees. The flow around theaerofoil is smooth. The lift forces increasesgreatly at 10o.

At 14 degrees of angle of attack, an interesting phenomenon takes place. A bubbleknown as ‘separation bubble’ forms on the upper surface of the aerofoil near theleading edge. The flow aft of this bubble starts to separate and turbulent flow begins

to show sign.

At 15 degrees of angle of attack, the bubble bursts and turbulent flow appears on theupper surface of the aerofoil. Lift decreases drastically and this is known as ‘Stalling’.

3.4 Factors affecting lift

Angle of attack is not the only factor which affects lift –there are other factors andthey are:

3.4.1 Air Density

Before understanding air density factor, we must first understand our atmosphere. Ouratmosphere appears as a blanket of air that covers us, however in reality, its structureis much more complex. There are in fact seven main layers in our atmosphere asshown here. The lowest layer is known as ‘Troposphere’ and the highest layer isknown as ‘Thermosphere’. As we move upwards from troposphere towards thefourth layer, stratosphere, air density falls, in other words, air gets thinner. We canexplain this phenomenon. At high altitude, there are not many air molecules present inthe atmosphere as compared in the troposphere.

Imagine a box of volume (1m3) filled with 10 small balloons (air molecules), each100grams and using the density formula,

Density = mass / volume Therefore, mass = 10 * 100grams = 1000grams = 1Kg

Thus density = 1000 / 1m3 = 1000grams/m3

Now imagine the same scenario with half the balloons, what we have now is,

Mass = 5 * 100grams = 500 grams

Thus density = 500/1m3 = 500grams/m3

The density has been halved

Lift greatly relies on air source and if air density decreases, lift decreases.

3.4.2 Airspeed

Lift is also affected by airspeed. At ground level, before take-off, an aircraftexperiences zero lift. As the throttle is opened, the aircraft experiences forwardmomentum and gather speed. This speed further increases as the engine powerincreases. An increase in airspeed of an aircraft results in an increment in lift and thislift continues to build up and eventually the aircraft is airborne. The pilot at this pointwill continue to ascend his or her aircraft before levelling off.

The relation between airspeed and lift is given the lift formula,

Where CL is the coefficient of lift (stated in NACA), ρ is the density of the air, V isthe airspeed and A, the aspect ratio

3.4.3 Wing Area and Shape (Aspect Ratio)

There are many types of aerofoils of various shapes as stated by NACA and of variouslength. The aspect ratio (length of the aerofoil multiplied by the cross-sectional area).The greater the aspect ratio, the greater the lift. The factors are determined and fixedby aeronautical engineers.

3.5 Stalling

As mentioned in section 3.3, lift continues to increase as the angle of attack increases.However there comes a point in every NACA aerofoil at which the lift no longer

( )Α= 22/1)( VCLLift L ρ

appears to increases and this point is known as the stalling angle. Different aerofoilshave different stalling angles and it very much depends on the aircraft’s role andaerodynamics needs. A commercial aircraft like an airborne 747 should not stall at anycost as loss of lift may cause severe structural damage, however a F-14 USAF (UnitedStates Air Force) jet may stall at tight manoeuvers. Manoeuver is a technical termused to describe the movement behaviour of an aircraft.

Most aircraft have built in alarms to tell the pilot if the aircraft is approaching thestalling angle. Some rely of pre-entered aeronautical values and while sophisticatedaircraft rely on the movement of the separation bubble. The separation bubblemoves towards the leading edge as the angle of attack is increased.

Most low speed aircrafts, stalling takes place at 15o and this angle does not vary.Airspeed does however affect the stall and this is known as the ‘Stalling speed’. Forexample, if a pilot in a level flight decides to switch off his aircraft’s engines whileflying with normal load - then continues to maintain the same lift by steadily easingthe control column back (increasing the angle of attack), the aircraft will eventuallystall.

3.6 Factors affecting stall

3.6.1 Weight

An increase in weight increases the stalling speed.

3.6.2 Engine Power

The higher the power used, the lower the stalling speed and vice-versa.

3.6.3 Flaps

With flaps lowered, the stalling speed lowers. Flaps increase the aerofoil camber.

3.6.4 Ice / Damaged Wings

Both ice and damaged wings reduce lift and increase the stalling speed.

3.6.5 Manoeuvers

In tight manoeuvers, stalling speed increases.

Do remember that stalling can occur at any attitude and it is the angle of attackthat determines the occurrence of a stall.

4. Thrust

Aircrafts have engines which provide enough power to lift them from the ground. Anaircraft will only take off if the lift is greater than its weight and once airborne, thepower generated by the engines is re-deployed for forward motion and othermanoeuvers. This forward momentum experienced during take-off, ascent flight, levelflight and other manoeuvers is known as ‘Thrust’. Thrust is well covered inaerodynamics topic ‘Powerplant’. Powerplant is the study of engines, enginesmaintenance/design and electrical/electronic equipment power distribution in anaeroplane.

5. Drag

With thrust, we get a ‘negative’ force – drag. Drag is created as a result of airresistance caused by the shape of aircraft. Various parts of an aircraft such as wings,fuselage, tail unit, undercarriage, engines and other parts contribute to drag.

The total drag of an aircraft is made up of three parts:

5.1 Form Drag

This type of drag is created by the shape of the aircraft. ‘Form’ means shape. If theshape of an aircraft is very flat and big, it will experience a lot of form drag. Formdrag reduces the fuel efficiency of an aircraft. Aeroplane designers, better known asaeronautical engineers reduce form drag by streamlining the aircraft’s body.Streamlining is a lengthy process which involves designing aeroplanes which allowfor smooth and fast airflows around its body. In streamlining, protruding parts areremoved or embedded in the aircraft to minimise air resistance that leads to formdrag.

A good streamlined shape has a fineness ratio of about 4 (length) to 1 (breadth).

Form drag increases with air speed - double the airspeed, four times the formdrag (approx.)

5.1.1 Skin Friction

This type of drag is caused by the roughness of aircraft surface and viscosity of air.When air flows over the surface or the ‘skin’ of an aircraft, there is always a narrowlayer of air close to the skin which flows beneath the main airflow. This layer can be

imagined to be composed of many very thin layers. The thin layer immediately nextto the skin is ‘stationary’, the thin layer above it has some movement, the next layermore, and so on until the top layers which travels at the same speed as the main flow.Air has viscosity (stickiness) and each layer slows up the layer above causing a dragknown as ‘Skin Friction’. The collective term for all the layers is ‘Boundary layer’. Ifthe flow in the boundary layer is smooth, we call it ‘Laminar flow’ and if the flow ischoppy, then it we call it ‘Turbulent flow’. When the flow is turbulent, thethickness of the boundary layer increases and causes more drag than a laminar flow.

Skin friction is reduced by polishing all the surface over which airflow passes. Thebetter the polish, the less the skin friction. In sophisticated aircrafts, there are suctiondevices and suck turbulent air in and out of an aircraft.

5.1.2 Induced Drag

An aircraft produces lift because of pressure difference between the upper and lowersurfaces. On the upper wing surface, there is a region of low pressure and on thelower wing surface, there is a region of high pressure and further away from the wings,we have the constant atmospheric air pressure. As the aircraft flies, surroundingatmospheric air rushes inwards over the upper wing surface and outwards beneath thelower wing surface. As a result, two streams of air moving in opposite directions areformed over the upper and lower surface of the wings. The two streams eventuallymeet and rotate at the trailing edge and weak ‘Vortices’ are formed. Vortices arecircular airflow, very similar to a wine corkscrew. The weak vortices get attracted tothe strong vortices and add up to form two main vortices at wing tips. These two mainvortices are known as ‘Wing tip vortices’. The wing tip vortices disturb the airflowaft of the wing and a lot of energy is waster in this air disturbance. The loss of energyresults in what we call “Induced Drag”. Induced drag is reduced by using taperedwings or elliptical wings.

6. Aircraft Controls

So far what we have learned are key forces that act upon an aircraft and now we shallfocus our attention to “Aircraft Controls”. Controls are either mechanical or electricaldevices that allow a pilot to alter an aircraft’s course by controlling the forces that acton it. Before we can understand how aircraft controls operate, we first need that toknow that there are six main movement co-ordinates in an aircraft.

An aircraft can move up and down - this movement is known as “Pitch”.An aircraft can roll from left to right – this movement is known as “Roll”An aircraft can move from left to right – this movement is known as “Yaw”

To control this six movements, we have various control surfaces on an aircraft.

6.1 Elevator

An elevator is situated in the tail section and it is used to create the pitch movement. Ithas two surfaces on the left and the right as shown below. The elevator is controlledby the pilot with the help of a control column known as ‘stick’. The ‘stick’ is usuallylocated between the legs of a pilot.

When the pilot moves the stick backwards, the elevator moves upwards. This reducesthe effective camber (less lift) and the aircraft nose begins to move upwards. Vice-versa, when the pilot moves the stick forward, the elevator moves downwards. Thisincreases the effective camber (more lift) and aircraftnose begins to move downwards. (Use two fingers andhold a pencil somewhere near its end and use the otherhand to move its rear end up and down. Notice whathappens to the front)

6.2 Rolling

Ailerons are situated in aircraft wings and are used to control the rolling movement.Like elevator, there have two moveable surfaces on the left (portside ) and the right(starboard) as shown below. However, the movement of the aileron is differentfrom that of elevator.

When the pilot moves the stick to left , the left aileron moves upwards and the rightaileron moves downwards. More camber (more lift) is created on the right wingcompared to the less cambered (less lift) left wing. The force on the right wing makesthe aircraft roll to its left.

When the pilot moves the stick to right, the left aileron moves downwards and theright aileron moves upwards. There is now less camber (less lift) on the right wingcompared to the more cambered (more lift) left wing. The force on the left wingmakes the aircraft roll to its right.

Do remember that in some aircrafts as in our example, both pitch and roll movementcan be controlled by a single control column - ‘stick’.

6.3 Yawing

Rudder is situated in the tail section and is used to controlthe yawing movement. There is only one moveablesurface and it is controlled by the pilot using the rudder‘foot’ pedals.

When the pilot moves his left feet forward, the ruddermoves to the left. There is an effective increase in camberon the left side (more sideways force) and this force,pushes ‘yaws’ the aircraft nose to the left about itsnormal axis. The aircraft continues to yaw in this

direction until the pilot returns the rudder pedals to their central position.

When the pilot moves his right feet forward, the rudder moves to the right. There isan effective increase in camber on the right side (more sideways force) and this force,pushes ‘yaws’ the aircraft nose to the right about its normal axis. The aircraftcontinues to yaw in this direction until the pilot returns the rudder pedals to theircentral position.

6.3 Trimming Tabs

6.3.1 Moveable tabs

Nearly every aircraft experience some sort of load changes and these happen due tomany reasons. One common reason is the consumption of aircraft fuel. For example,with a full tank (fuel), an aircraft will have a fixed load balance as the fuel is used up,its load balance will change. Eventually, the aircraft will start to experience forces onit due to these changes in the centre of gravity. Assuming that the fuel tanks arelocated near the rear end, and aircraft nose will begin to dip as the centre of gravitymoves forward. At this stage, the pilot will pull the stick backward to keep the noselevel. It can be extremely tiring for the pilot as he or she has to maintain the straightlevel flight by continuing holding the stick. This is where the trimming tabs come inuse. Trimming tabs are ‘mini control’ surfaces that are embedded into the maincontrol surfaces. By adjusting these controls, the pilot can relieve himself from themain controls and keep the aircraft in the desired path. Trimming tabs are only usedfor ‘fine-tuning’ aircraft movements and are not used to move the aircraft drastically.

Trimming tabs are found in the rudder and these are called ‘Rudder trimming tab’and they are adjusted by the pilot in event of minor offset yaw movements. Trimming

tabs are also found in the elevator – “Elevator trimming tab’ - and this is adjusted bythe pilot for offset pitch movements. Last but not least the ailerons are also equippedwith trimming tabs for preventive offset rolling movements. Aileron trimming tabs areknown as ‘Aileron trimming tab’. The three aforementioned tabs are adjusted by threeseparate trimming controls which are mounted directly on the cockpit panel. Thepilot adjusts can use any three controls in flight to ‘trim out’ extra forces. 6.3.2 Fixed tabs

Some light aircrafts especially old ones have tendency to fly “one wing low” and thistendency can be cancelled out by ‘fixed tabs’. Unlike moveable tabs, fixed tabs mustbe adjusted on the ground prior to flying. The exact setting is determined by trial anderror during one or more test flights.

6.3.3 Balance tabs

These tabs are not under the direct control of a pilot and are deployed automaticallyduring flight. When a control is moved one way, the balance tab moves in the otherdirection. The airflow acting on the tab assists the movement of the control surface,thereby reducing the effort required from the pilot.

6.4 Flaps

Do you remember when we talked about camber effect andlift in the earlier sections? If yes, you will know how thiscamber effect can affect an aircraft landing and taking-offcharacteristics. No fixed length wings can provide a samehigh performance while landing and taking-off. Usuallywings of this type provide good performance while take offand an average performance while landing or vice-versa.Performance measurements taken during landing and take-off are extremely important as they can greatly affect thefuel efficiency of an aircraft. To combat this problem offixed length wings, aeronautical engineers came up withthe ‘flaps’ solution.

Flaps are hinged surfaces which are fitted to the trailingedges of the wings – inboard of the ailerons. During take

off, flaps are project outwards to increase the overall camber of the wings. Thisincrement provides extra lift to an aircraft and in turn reduces the take off distance.During pre-landing, flaps are employed to reduce the stalling angle and landingdistance. There are many types of flaps and each flaps have different landing andtake-off characteristics. In general, a flap deployed at 15o degree angle shortens thetake-off run due to lift increment. A flap deployed at 30o degree angle provides anincrement in lift while take-off and equally an increment in drag while landing. A 60o

degree deployed flap does not produce much lift but does provide a great deal of drag,which is ideal for steeper and slower landing conditions. Lastly and rarely, 90o

degree deployed flap will only induce large amount of drag with little lift. Doremember that drag is not always evil as imagined.

Drag provides braking effect while landing and reduces the touch-down speed.

6.5 Slats

Like flaps, they are small auxiliary hinged surfaces. They are however located at theleading edge of an aerofoil unlike flaps. They are automatically deployed with flapsbut in some aircrafts, they may be controlled by a pilot. Slats are used to delay tall andgain extra lift at low speeds. Stalling takes place at a fixed angle and soon after it isexceeded, turbulent flow forms on the upper layer of the wing. Prior to stall angle, aslat is deployed automatically to delay the oncoming stalling. Lets look this in detail.

As we approach stall, the centre of pressure builds up near the upper leading edge ofthe wing. This pressure automatically moves the slat forward. When the slat opensvia a spring loaded mechanism, ‘imminent turbulent’ airflow rushes through the gutterto form smooth airflow. This effectively increasesthe stalling angle. When the aircraft reverts to thenormal attitude, the centre of pressure movesbackwards pulling the slat inwards along with it toits original position – closed slat.

Slats and flaps are usually interconnected and areoperated from the cockpit. In interconnected slatsand flaps, both the flap and the slat movesimultaneously. During lift-off, the leading edgeslats are operated to the fully open position and atthe same time, the flaps are deployed to 20o

degrees. The end result is a shorter take-offdistance. While airborne, both slats and flaps areclosed and trimming tabs are used. During lowspeed and steep glide landing, both slats and flapsare fully extended.