Post on 15-Oct-2020
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