Principles of Flight

Tim FreegardeJim Crawford

www.uskgc.co.uk

Principles of Flight

• what you need for the Bronze exam• useful knowledge for flying

• terminology• principles• how it all works

Q. Questions from Bronze & Beyond

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Terminology• acceleration• aerofoil• aileron• airspeed• angle of attack• attitude• bank• centre of mass/gravity• centre of pressure• chord• control deflection• drag (induced, profile, …)• elevator• fin• flutter• fuselage• glide angle/slope• laminar flow• lift• load factor

• minimum sink• pitch• polar curve• roll• rudder• skid• slip• span• spin• stability• stall• tailplane• turbulence• Va• Vne• vortices• washout• wing• yaw• yaw string

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Principles of Flight

1. aerofoil lift & drag depend upon Angle of Attack2. aerofoil lift & drag vary as airspeed2

3. Newton was right

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• an object’s velocity remains steady, unless a force acts upon the object…

• …in which case the object’s acceleration is proportional to the force

• if object A exerts a force on object B, object B exerts an equal but opposite reaction upon object A

Principles of Flight

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• balanced flight wings levelsteady turn

• stability in

pitch

bank

the whole glider

forces on

bits of the glider

yaw

• changing

pitch

bank

yaw

elevator

ailerons

rudder

tailplane dihedral

fin

SITUATIONS

Forces in level flight

• aerodynamic force balances weight• Newton: no acceleration

aerodynamic force produced by glider’s

wings

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WEIGHT

LIFT

WEIGHT

DRAG

• split into LIFT and DRAG:• LIFT: perpendicular to airflow• DRAG: parallel to airflow

TRIANGLE OF FORCES

• glide angle L:D

DL

L

D

1. Name the forces on a glider when it is flying straight at a steady speed.

Forces in a steady turn

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2. Name the forces on a glider when it is in a well banked turn at a steady speed.

3. Why does a glider increase its speed in a turn unless you move the stick back?

• greater lift needed to balance weight

• need higher speed or angle of attack

• horizontal part for centripetal force

Aerofoil

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Aerofoil

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Aerofoil

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• at given AoA, lift ∝ airspeed2

• at given airspeed, lift and drag varywith AoA

• L/D varies with AoA (D ∝ L2)AIRSPEED

LIFT

or D

RAG

31 kt

43 kt

108 kt

Aerofoil

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AIRSPEED

LIFT

or D

RAG

• at given AoA, lift ∝ airspeed2

• at given airspeed, lift and drag varywith AoA

• L/D varies with AoA (D ∝ L2)

31 kt

43 kt

108 kt

Aerofoil

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• in steady flight, for every angle of attack there is an airspeed at which the lift will support the weight

• at other speeds, the glider will climb or fall, changing the AoA

Force distribution

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• glider weight acts through Centre of Gravity/Mass

• wing lift acts through Centre of Pressure

• pitching torque if displaced• tailplane provides balance

Centre of Gravity/Mass

12. What is the purpose of a tailplane?

Control surfaces

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• downward deflection increases angle of attack

• lift is increased

• upward deflection decreases angle of attack

• lift is reduced

Pitch control – effect of elevator

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Pitch control – effect of elevator

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Stick is moved forward which deflects the elevator down

Pitch control – effect of elevator

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Pitch control – effect of elevator

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Pitch control – effect of elevator

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Pitch control – effect of elevator

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13. How does an elevator work?

Roll control – effect of ailerons

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Aircraft is trimmed and flying wings level. Stick is moved to the right.

Lift increased from this part of the wing.

Roll control – effect of ailerons

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Yaw control – effect of rudder

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Yaw control – effect of rudder

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Effects of controls

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AEROFOILS• aerodynamic force depends upon • airspeed

• angle of attackPRIMARY EFFECTS• elevator pitch• aileron roll• rudder yaw

FURTHER EFFECTS• aileron adverse yaw• rudder roll

Further effect of ailerons

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Aircraft is trimmed and flying wings level. Stick is moved to the right.

Further effect of ailerons

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8. Why do you normally apply rudder when applying aileron?

Secondary effect of rudder

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Yaw stability – effect of fin

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STABILITY• tendency of aircraft to

recover when upset

• tail fin (vertical stabilizer) has angle of attack to airflow

• lateral aerodynamic force creates torque

• glider weathercocks back to point into airflow

11. What is the purpose of a glider's fin?

Bank stability – effect of dihedral

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STABILITY• tendency of aircraft to

recover when upset

• bank without yaw results in sideslip

• sideslip steepens AoA of lower wing

• lower wing produces more lift

• torque tends to return glider to wings level

• slip will cause weathercock into turn

LIFT

WEIGHT

Dihedral angle

Roll stability (damping)

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STABILITY• tendency of aircraft to

recover when upset

• downgoing wing experiences higher AoA

• downgoing wing produces more lift

• torque reduces rate of roll• will not level wings

Pitch stability – effect of tailplane

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STABILITY• tendency of aircraft to

recover when upset

• tailplane AoA increases more significantly than wing AoA

• tailplane moment increases more than wing moment

• torque pitches nose down

12. What is the purpose of a tailplane?

Pitch stability – C of G

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STABILITY• tendency of aircraft to

recover when upset

CG behind aft limit• light/unstable in pitch• prone to spin

CG ahead of forward limit• heavy/too stable in pitch• difficulty rounding out

14. What is the effect on stability of reducing the cockpit load?

Stall

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• lift no longer increases with AoA• airflow separation and turbulence• drag increases• ailerons less effective

Stall

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• lift no longer increases with AoA• airflow separation and turbulence• drag increases• ailerons less effective

• a glider can stall at any speedin any attitude

• high AoA: slow flightsteep turnshigh g manoeuvrestaut winch cablerapid pitch rotation

Stall

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• lift no longer increases with AoA• airflow separation and turbulence• drag increases• ailerons less effective

• a glider can stall at any speedin any attitude

• high AoA: slow flightsteep turnshigh g manoeuvrestaut winch cablerapid pitch rotation

9. How can gliders stall at higher speeds?

Spin

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• one wing stalled• less lift, so descends ⇒ higher AoA• ⇒ wing stall sustained

• a glider can stall at any speedin any attitude

• high AoA: slow flightsteep turnshigh g manoeuvrestaut winch cablerapid pitch rotation

10. Why does a glider spin?

Limiting speeds

• Vs 1g wings-level stall speed• Va no single control can overstress aircraft*• Vne never exceed (CS22: 1/3 elevator, aileron

or rudder)

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DG300-17www.dg-flugzeugbau.de

SB9Akaflieg Braunschweig

17. What name is given to the maximum speed at which it is safe to use full deflection of any one control without damaging the glider?

18. What is the maximum airspeed and maximum manoeuvring speed of the glider that you normally fly?

* used non-dynamically – see AA587: www.ntsb.gov/investigations/AccidentReports/Reports/AAR0404.pdf

Drag

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• induced drag

• profile drag • form drag• pushing air aside

• interference drag• airflows meet

• skin drag• boundary friction

• leakage drag• high-low pressure

• depends on AoA

4. How does the profile drag change with airspeed between the stall and VNE?

5. How does the lift-induced drag change with airspeed between the stall and VNE?

Drag

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• induced drag

• profile drag • form drag• pushing air aside

• interference drag• airflows meet

• skin drag• boundary friction

• leakage drag• high-low pressure

• depends on AoA

7. Why put sealing tape between the wings and fuselage?

Drag

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• induced drag

• profile drag • form drag• pushing air aside

• interference drag• airflows meet

• skin drag• boundary friction

• leakage drag• high-low pressure

• depends on AoA

The glider polar

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• sink vs airspeed for given loading

15. Why is the best glide angle given by the tangent to the polar curve?

6. At what speed is total drag at a minimum?

Winch launch: rotation to full climb

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• lift must provide centripetal acceleration = v ωspeed (m s-1)

rotation rate (rads s-1)

= 25 (2π/40)

= 4 m s-2 (0.4g)

• requires higher angle of attack

5 s

Winch launch: rotation to full climb

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• lift must provide centripetal acceleration = v ωspeed (m s-1)

rotation rate (rads s-1)

= 25 (2π/40)

= 4 m s-2 (0.4g)

• requires higher angle of attack

5 s

https://members.gliding.co.uk/bga-safety-management/safe-winching/

• phyweb.phys.soton.ac.uk/quantum/lectures/gliding/principles.pptx• phyweb.phys.soton.ac.uk/quantum/lectures/gliding/principles.ppt• phyweb.phys.soton.ac.uk/quantum/lectures/gliding/principles.pdf• Richard Lancaster www.rjplancaster.net

tim.freegarde@soton.ac.uk

Books and websites

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Principles of FlightPrinciples of FlightTerminologyPrinciples of FlightPrinciples of FlightForces in level flightForces in a steady turnAerofoilAerofoilAerofoilAerofoilAerofoilForce distributionControl surfacesPitch control – effect of elevatorPitch control – effect of elevatorPitch control – effect of elevatorPitch control – effect of elevatorPitch control – effect of elevatorPitch control – effect of elevatorRoll control – effect of aileronsRoll control – effect of aileronsYaw control – effect of rudderYaw control – effect of rudderEffects of controlsFurther effect of aileronsFurther effect of aileronsSecondary effect of rudderYaw stability – effect of finBank stability – effect of dihedralRoll stability (damping)Pitch stability – effect of tailplanePitch stability – C of GStallStallStallSpinLimiting speedsDragDragDragThe glider polarWinch launch: rotation to full climbWinch launch: rotation to full climbBooks and websites