5.basic aerodynamics 2

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Aircraft Component

DefinitionsFree Stream Flow or Relative Airflow (R.A.F.): The airflow in a region where pressure, temperature and relative velocity are unaffected by the passage of an aircraft through it. Total Reaction: The resultant of all aerodynamic forces acting on the wing or aerofoil section.

Drag: The component of the total reaction which is parallel to the to the flight path or relative airflow. Lift: The component of the total reaction which is perpendicular to the to the flight path or relative airflow. Wing Loading Weight per unit area of the wing W/S

Chord Line: A straight line joining the centres of curvature of the leading and trailing edges. Chord: The physical length of the Chord Line.

Wing Area (S): Area of the wing projected on a plane perpendicular to the normal axis. Mean Camber Line: A line joining all points equidistant from the upper and lower surfaces of the wing.

Camber: The max. distance between the chordchord-line and the mean camber line. Angle of Attack: The angle between the chord-line and chordthe R.A.F.

Angle of IncidenceAngle between Chord Line and longitudinal axis .Normally 3 -

Aspect Ratio (AR)Span/Chord or Span squared/Area. A measure of the slenderness of the wing

Sweep Angle The angle between the line of 25% chord and a perpendicular to the wing root.

U

Taper Ratio Is the ratio of the tip chord to the root chord, or = Ct / CrCr Ct

WashWash-out A reduction in the angle of incidence (design AoA) or camber of a wing section near the wing tip to prevent tip stalling.

Aerofoil TheoryBasic Properties of a Fluid The three properties of most importance to aerodynamics are: Pressure Density, and Viscosity.

Pressure Static and dynamic Measured in Force per Area (Newtons per square metre) the normal force per unit area exerted on a surface due to the time rate of change of momentum of the gas molecules impacting on that surface.

Static Pressure. The static pressure in a fluid acts equally in all directions and exerts a force which acts perpendicular to any surface it is in contact with.

Dynamic Pressure. Dynamic pressure is that pressure exerted upon an object by the moving air and is due to the kinetic energy of the air. Dynamic Pressure equation q = 1/2( V2)

The density of air is: the mass of air per cubic metre of volume. Density = m/v

Vicosity A measure of the resistance of air to fluid flow or the resistance of one layer of air to movement over the neighbouring layer measured in mass/length.time or kg/m.s

Viscosity

Caused by the internal friction between the fluids molecules. An ideal fluid is considered to be inviscid or it has no internal friction. Air is considered to be an ideal fluid below M 0.3

Types of Airflow: Steady streamline flow Unsteady flow 2 dimensional flow and 3 dimensional flow

Steady flow

Succeeding molecules follow the same steady path in a flow, each one following the one before it. The flow may be accurately represented by streamlines. At any point on the streamline, the velocity, static and dynamic pressures are constant w.r.t. time. Molecules do not swap from one streamline to another.

Unsteady Flow Succeeding molecules do not follow the same path. The flow cannot be represented by streamlines. At any point in the fluid, the velocities and pressures will vary w.r.t. time. The molecules mix freely.

2-dimensional airflow Theoretical tool that is relevant only to a wing of infinite span or a section which spans a wind tunnel from wall to wall. No induced flow at the wing tips.

3-dimensional airflow Occurs on wings of finite span where the pressure differential tries to equalise around the wing tip. Span-wise drift along with Spanpressure differential produces a wing tip vortex.

Vortex upsets the balance between the up-wash and updownwash of 2D flow. This causes: a reinforcing of the downwash, reduction of the effective , backwards tilting of the lift vector, and downwards tilt of the effective RAF.

Airfoil - AerofoilAn airfoil is a cross section of a wing. An Airfoil is a structure, piece, or body designed to obtain a useful reaction upon itself in its motion through the air.

Flat Plate

Cambered Plate

Airfoil terminology

The Chord Line (1) is a straight (1 line connecting the leading and trailing edges of the airfoil. The Chord (2) is the length of the (2 chord line from leading edge to trailing edge and is the characteristic longitudinal dimension of an airfoil.

The Mean Camber Line (3) is a line drawn halfway between the upper and lower surfaces. The chord line connects the ends of the mean camber line. Maximum Camber (4) (displacement of the mean camber line from the chord line) and where it is located (expressed as fractions or percentages of the basic chord) help to define the shape of the mean camber line.

The Maximum Thickness (5) of an (5 airfoil and where it is located (expressed as a percentage of the chord) help define the airfoil shape, and hence its performance. The Leading Edge Radius (6) of the (6 airfoil is the radius of curvature given the leading edge shape.

Types of AirfoilSymmetric Airfoil

Asymmetric Airfoil (Cambered Airfoil)

Asymmetric Airfoil (negative camber)

Reflex camber

Flat Bottom A Flat Bottom Wing is when the lower surface of the wing is primarily flat between the leading and trailing edges.

FullyFully-Symmetrical A Fully-Symmetrical Wing airfoil is curved on Fullythe bottom to the same degree as it is on the top.

A Fully-Symmetrical Wing airfoil is curved Fullyon the bottom to the same degree as it is on the top.

A Flat Bottom Wing airfoil is when the lower surface of the wing is primarily flat between the leading and trailing edges.

A Semi- symmetrical Wing airfoil has a curved Semibottom section but to a lesser degree than a symmetrical section. It is a compromise between the flat bottom and the symmetrical wing.

An Under-camber airfoil has the lower surface Underof the wing curved inwardly almost parallel to the upper surface.

Airfoil Subsonic Flow

Stream lines change well ahead of leading edge.

Airfoil Generate LiftAir flowing over the top of the wing flows even faster than the air flowing underneath. As the difference in the speed of the two airflows increases, the difference in pressure increases also. This difference in pressure generates the lift force. Angle of attack increases, the wing generates more lift.

Velocity

Pressure

Pressure Distribution around Airfoil

Pressure Distribution Around an Aerofoil Greatest positive pressures at stagnation points. Front stagnation point separates flow over and under wing.

The Lift Equation L = 1/2 v2SCL

L=Lift, CL = Lift Coefficient, = density, v = velocity, S = area (when projected onto a horizontal plane) The lift coefficient is a function of the shape and condition of the wing, the viscosity of the air and the angle of attack.

Lift Coefficient

The lift coefficient is a function of the shape and condition of the wing, the viscosity of the air and the angle of attack.

Lift Curve Slope

Airfoil Data

Lift Coefficient

Drag Coefficient

Classification of Airfoils - National Advisory Committee for Aeronautics (NACA) 1929 - National Aeronautics and Space Administration (NASA) - RAF - Goetingen

Factors Affecting Lift free stream velocity (V2) air density ( ) wing area (S) wing shape angle of attack (E) (E condition of surface viscosity of air (Re) Reynolds number speed of sound

Factors Affecting CL- angle of attack (E) (E - shape of wing section & plan-form plan- condition of wing surface - Reynolds number - speed of sound

CL - E curve: Effect of Camber

Effect of Leading Edge RadiusDetermines stall characteristics of the wing. Large radius produces well-rounded wellpeak on CL curve, i.e. gentle stall. Small radius will produce an abrupt stall.small leading edge radius CL large leading edge radius

AoA

Effect of Leading Edge Roughnesssmooth leading edge

CL

rough leading edge

E

Effect of Tickness Ratio (t /c) Lower t /c ratio results in less drag and a lower CL Mainly a consideration for supersonic wings, in an attempt to reduce drag, however it does effect CLMAX at lower IAS High Lift aerofoils employ a high t/c ratio, a pronounced camber, and a wellwellrounded leading edge

Effect of Thickness Ratio (T/C)

General purpose aerofoils employ a lower t/c ratio, less camber, and a sharper leading edge. High Speed aerofoils employ a very low t/c ratio, no camber, and a sharp leading edge. Looking for minimum drag, but also get low maximum lift coefficients @o V2

Effect of Point of Maximum Thickness

This effects the transition to a turbulent boundary layer, hence the adverse pressure gradient, and stall characteristics. Point of max thickness well aft of the LE can give an abrupt stall and therefore a small sharp peak to the graph

Effect of Aspect Ratio

Effect of Sweepstraight wing CL

swept wing

E

Effect of Taper Ratio on LiftEffects the local lift coefficients and thus the induced downwash. Elliptical Wing is ideal as equal local lift coefficients give equal downwash and minimum induced drag.

Effect of Wash-out on Lift WashWashWash-out is a reduction in camber or incidence at the tip of a wing to reduce tip stalling and simulate the elliptical ideal. Therefore one could expect similar effects to taper ratio.

Centre of Pressure: The point, usually on the chordchordline, through which the total reaction is considered to act. Aerodynamic Center: - The point, where pitching moment of an airfoil remain constant.

Centre of Pressure

Aerodynamic Center

For symmetric airfoils, the aerodynamic moment about the ac is zero for all angles of attack. attack. With camber, the moment is nonnonzero and constant for thin airfoils. airfoils. For a positive cambered airfoil, the moment is negative and results in a counter-clockwise counterrotation of the airfoil. airfoil.

With camber, an angle of attack can be determined for which the airfoil produces no lift, but the moment is still present. present.

Aerodynamic Center

For rectangular wings, the wing ac is the same as the airfoil ac. But for ac. wings with some other planform (triangular, trapezoidal, etc.) etc.)

Mean Aerodynamic Chord Mean Aerodynamic Center (MAC) (MAC) which is the average for the whole wing. wing. The computation of the mac depends on the shape of the planform. planform. Mean Aerodynamic Chord is the chord past through the MAC.

Mean Aerodynamic Chord

Swept Wing

Mean Aerodynamic Chord

Swept Wing

Mean Aerodynamic Chord

Trapezoidal Wing

Mean Aerodynamic Chord

Elliptical Wing

Stall

Wing Stall

Low AoA: No Stall

Wing StallHigh AoA : Stalls begin from trailing edge to leading egde.

Low AoA : No Stall

High AoA : Stall progress

Leading Edge Stall

Stall Progression

Low AoA

Critical AoA : max.CL

AoA higher than No more Lift

crit

Effect of Thickness Same Speed: more Lift

Low Speed: same Lift

Wing Planform Terminology

Wing Span ( b ) The Wingspan is the length of the wing as measured from wing tip to wing tip. Wing Area( S ) The Wing Area is the total surface area of the wing, usually calculated by the wing span times the wing chord.

Aspect Ratio (AR)Span/Chord or Span squared/Area. A measure of the slenderness of the wing

Taper Ratio Is the ratio of the tip chord to the root chord, or = Ct / CrCr Ct

Rectangular Straight Wing

Tapered Straight Wing

Rounded or Elliptical Straight Wing

Slight Sweepback Wing

Moderate Sweepback Wing

Forward Sweep Wing

Great Sweepback Wing Simple Delta Wing and Complex Delta Wing

Wing Lift Distribution

Wing Lift Distribution

Span Efficiency Factor

Components of Total DragTotal drag can be divided into two parts: Zero Lift Drag (parasite drag) Lift Dependent drag (induced drag)

Zero Lift Drag Consists of components of: Form drag (boundary layer normal pressure drag), Surface friction drag, and Interference drag

Lift Dependent Drag In producing lift the whole aircraft will produce additional drag composed of: Induced drag (vortex drag), and Increments of: form drag, surface friction drag, and interference drag.

Induced Drag The pressure difference between the upper and lower surface of a finite regular wing at a positive angle of attack will cause the following spanwise pressure distribution. CDi = CL2 / AR e

The effect of the induced downwash (due to the vortices) is to tilt downwards the effective relative airflow, thereby reducing the relative angle of attack. To regain the consequent loss of lift, the aerofoil must be raised until the original value of lift is restored.

The component of total reaction parallel to flight is now longer. The additional value of drag is known as induced or vortex drag.

Downwash Induce Angle of Attack Induce Drag

Infinite Wing

2 D Airfoil : no down wash at ac.

Finite Wing

3 D airfoil: Down wash at ac.= w

Lift curve slope of 2D and 3D Airfoil

Tip Wing Vortex

Tip Wing Vortex

The main factors affecting vortex formation and therefore induced drag are: Planform Aspect Ratio Lift and Weight Speed

Increments of ZLD resulting from Lift Production Surface friction & Form Drag As lift increases from zero forward movement of the peak of the low pressure envelope will cause earlier transition of the boundary layer to turbulent flow.

Increments of ZLD resulting from Lift Production Surface friction & Form Drag increasing adverse pressure gradient will cause earlier separation. earlier transition increases the surface friction drag and earlier separation increases the form drag.

Increments of ZLD resulting from Lift Production Interference Drag When the aircraft is producing lift, the boundary layers are thicker and more turbulent and therefore create greater energy losses when they mix.

Effect of T/C ratio on CD - E curveSimilarly a high T/C ratio increases drag due to earlier transition. High T/C used on slower lifting sections e.g. transport a/c, while the reduced drag of thinner lower T/C ratio wings is required for supersonic a/c

Effect of Tip wing Vortex

Tip wing vortex during take off

Tip wing vortex during take off

Effect of Tip wing Vortex

Effect of vortex after take off

Effect of vortex after take off

Effect of vortex after take off

Effect of vortex after take off

Effect of vortex before landing

Effect of vortex after landing

Wing Tip Design

Endplate

Winglet

Effect of Aspect Ratio

Effect of Aspect Ratio

Drag EquationTotal Drag = Parasite Drag + Induced DragFor straight and level flight

Lift = Weight

DTDT =

!

D p Di

1 1 2 2 VV S C D VV S C D 2 2 o i

L

=

W

=

1 2 VV S C L 2

CL !

W 1 2 VV S 2

CD =i

CL TARe

2

DT !

C2 1 1 2 L VS C D V VS TA 2 2 o 2

.V2 e

DT !

1 W 1 2 VS C D V . 2 1 2 o V STA e V 2

DT !

AV

2

B V2

A

!

1 V S CD 2 oW2

B

!

1 V S TA e 2

Drag Polar

CD !

CD o

CL TA Re2

2

CD !

C D kC Lo

CDt = Total Drag coefficient CDo = min. Drag coefficient at Zero Lift CL = Lift coefficient AR = Wing aspect ratio e = Wing span efficiency factor

Stall Pattern

Stall Pattern

Wing Stall Wing root stalls before wing tip

Why ? Stall warning : - Wing and fuselage shake - Control stick shake - Lift still present Aileron still effective

Wing CharacteristicsGood Stall Characteristics Low Design Cost Medium Span Efficiency High Stress at Root Chord

Wing CharacteristicsMedium Stall Characteristics Medium Design Cost Medium Span Efficiency Medium Stress at Root Chord

Wing CharacteristicsBad Stall Characteristics Medium Design Cost Medium Span Efficiency Medium Stress at Root Chord

Wing CharacteristicsBad Stall Characteristics High Design Cost Low Span Efficiency Low Stress at Root Chord Problem with Stability

Wing CharacteristicsMedium Stall Characteristics High Design Cost Good Span Efficiency Medium Stress at Root Chord

Stall Warning DevicesAerodynamical Stall Warning - Geometric Twist - Aerodynamic Twist - Stall Strip - Wing Fence

Stall Warning DevicesMechanic Electrical Stall Warning - Lift Transducer - Angle of Attack Indicating System

Aerodynamical Stall WarningGeometric Twist or Wash Out

Aerodynamic TwistUsed of Cambered Airfoil at Wing root and Symmetrical Airfoil at Wing tipCL

E

Stall Strip

Wing Fence

Mechanic Electrical Stall Warning

Lift Transducer

Angle of Attack Indicating System

High Lift Devices

FlapsPurpose: vary the camber of the wing section. Increase wing area Method: leading or trailing edge hinged sections.

Leading Edge Flaps

Trailing Edge Flaps

Types of Flaps

Plain Flaps

simple hinged design. increased in CLmax. Increased Drag. zero lift angle reduced. reduced.

Split Flaps

flat plate deflected from lower wing greater increase in CLmax than plain flap. Large increase in drag. Design good for steep approaches due to high drag

Slotted Flap

Similar to plain flap but with air slot. upper boundary layer is energised through slot. Separation is delayed Much greater increase in CLmax than previous types. Lower drag than previous types.

Slotted Flap

Slotted Fowler Flaps

Effect of Flaps on liftHigher CL max lower AOA

Effect of flaps on pitching moments.Trailing edge flaps produce nosenosedown pitching moment. Trailing edge flaps may produce increased downwash over the empanage.

Effect of flaps on pitching moments.Leading edge flaps create nose up pitching moment. Leading edge flaps reduce stability near the stall. Combined LE and TE flap pitching moments depend on individual aircraft.

Use of Flaps for Take OffShorter take-off roll. takeReduces best angle of climb performance. More than Takeoff flap results in high drag

Lift Augmentation devices on Landing.Slower approach speeds lower cockpit angles Increased drag

Used of flaps on landing

Slats/SlotsA small highly cambered auxiliary aerofoil section fixed to the LE of the wing. Types: Fixed, Automatic, Manual

Operation of SlatsThe slat generates a high lift coefficient because of its marked camber. The slot delays separation until higher AoA and lift coefficient than a plain wing.

Effect of Slot, Slats on Liftprolonged lift curve higher value of CL max.higher AOA.

Uses of SlatsIncrease CL max. Reduced landing speeds low speed handling Ideal on Delta wing aircraft Controls flow pattern and lift distribution

Effect of Slats on Pressure Distribution. Slats maintain higher energy boundary layer. Separation delayed by thin leading edge of slat separation delayed by energized slot boundary layer.

Effect of Slats on Pressure Distribution.

Fixed Slot

Effect of Flaps and Slats on lift

Higher CL max lower AOA

Boundary Layer ControlBlowing - high speed air is used to speed up retarded sub-layer and resubreenergize boundary layer Sucking - is used to reattach higher energy streamlined flow to the surface Vortex Generators - re-energize the reboundary layer by the use of vortices which move the faster moving air down into the boundary layer.

Blowing BLC

Sucking BLC

Vortex Generator

Vortex Generator

Spoilers