1. CGS Ground School Principles Of Flight Controls © Crown Copyright 2012
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The views expressed in this presentation do not necessarily reflect the views or policy of the MOD.

2. Flight controls can be divided into:
Primary Flight Controls:
Elevators.
Ailerons.
Rudder.
Secondary Flight Controls:
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Tabs.
Airbrakes.
Flaps.
Slats.

3. Primary Flight Controls
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4. An aircraft can be manoeuvred about its 3 axes.
Flight Controls
An aircraft can be manoeuvred about its 3 axes.
Axis - Lateral.
Movement - Pitch.
Control - Elevator.
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5. An aircraft can be manoeuvred about its 3 axes.
Flight Controls
An aircraft can be manoeuvred about its 3 axes.
Axis - Longitudinal.
Movement - Roll.
Control - Ailerons.
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6. Axis - Normal (vertical).
Flight Controls
An aircraft can be manoeuvred about its 3 axes.
Axis - Normal (vertical).
Movement - Yaw.
Control - Rudder.
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7. The three axes cross at one point, the centre of gravity (C of G).
Flight Controls
The three axes cross at one point, the centre of gravity (C of G).
The aircraft therefore
manoeuvres about its
C of G.
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8. Elevators
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9. Elevators The elevators are located on the rear of the tailplane.
They alter the lift produced by the tailplane.
If the control column is moved
forwards, the elevator moves down.
The angle of attack of the tailplane
and therefore the lift at the tailplane
is increased.
The tail moves up,
the aircraft rotates about its C of G, and the nose pitches down.
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C of G

10. Elevators The elevators are located on the rear of the tailplane.
They alter the lift produced by the tailplane.
If the control column is moved
rearwards, the elevator moves up.
The angle of attack of the tailplane
and therefore the lift at the tailplane
is now negative.
The tail moves down,
the aircraft rotates about its C of G, and the nose pitches up.
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C of G

11. Ailerons
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12. Ailerons
The ailerons are located on the trailing edge of the wings, near the wing tips. They alter the lift produced by each wing.
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13. Ailerons
If the control column is moved to the right, the right aileron moves up, the left aileron moves down.
The angle of attack and therefore lift of the left wing is increased. The angle of attack and therefore lift of the right wing is decreased.
The left wing goes up, the right wing goes down.
The aircraft rotates about its C of G and rolls right.
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C of G

14. Ailerons
If the control column is moved to the left, the left aileron moves up, the right aileron moves down.
The angle of attack and therefore lift of the right wing is increased. The angle of attack and therefore lift of the left wing is decreased.
The right wing goes up, the left wing goes down.
The aircraft rotates about its C of G and rolls left.
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C of G

15. Adverse Yaw
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16. Ailerons - adverse yaw
Unfortunately an increase in lift on the up-going wing produces a corresponding increase in lift dependent drag.
This is known as aileron drag and is the cause of adverse yaw.
As lift increases on the up-going wing,
so lift dependent drag also increases.
The aircraft rolls left but
the increased drag causes it to yaw right.
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17. Ailerons - adverse yaw
There are 4 common methods of reducing adverse yaw:
1. Differential Ailerons.
2. Frise Ailerons.
3. Coupling of Controls.
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4. Spoilers.

18. Ailerons - adverse yaw 1. Differential ailerons.
An up-going aileron causes a smaller change in drag than a down-going aileron.
With differential ailerons the up-going aileron is deflected further than the down-going aileron for any given stick deflection.
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This reduces the difference in drag between the two ailerons, and thus reduces the adverse yaw.

19. Ailerons - adverse yaw 2. Frise ailerons.
Frise ailerons are different from normal ailerons due to the ‘nose’ on the leading edge.
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When the Frise ailerons are deflected, the nose of the up-going aileron protrudes into the airflow.
This creates drag on the up-going aileron to help compensate for the extra drag of the down-going aileron.

20. Ailerons - adverse yaw 3. Coupling of Controls.
In this technique the ailerons are geared to the rudder so that when the ailerons are deflected the rudder moves to produce an appropriate yawing moment to oppose the adverse yaw.
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21. Ailerons - adverse yaw 4. Spoilers.
Some larger aircraft have spoilers in the form of flat plates which can be raised into the airflow to increase the drag of the down-going wing.
This extra drag compensates for the extra drag from the up-going wing.
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22. Ailerons - further effect
The ailerons also have a further effect.
Once the aircraft has rolled away from wings level, the aircraft's weight acting downwards induces a side slip.
As the aircraft side slips the airflow strikes the side of the aircraft.
The aircraft has a larger side area to the rear of the C of G
than ahead of it.
The larger side area to the rear of the C of G therefore resists the sideslip motion more than the front. The aircraft therefore yaws in the direction of the applied roll.
The roll and yaw in turn lead to a spiral descent.
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Side slip
Weight

23. Rudder
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24. Rudder
The rudder is located on the trailing edge of the tail fin. It alters the lift produced by the tail fin.
If the left rudder pedal is moved forwards, the rudder moves to the left.
The angle of attack of the tail fin is altered, and it moves right.
The aircraft rotates about its C of G, and yaws left.
C of G
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25. Rudder
The rudder is located on the trailing edge of the tail fin. It alters the lift produced by the tail fin.
If the right rudder pedal is moved forwards, the rudder moves to the right.
The angle of attack of the tail fin is altered, and it moves left.
The aircraft rotates about its C of G, and yaws right.
C of G
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26. Rudder - further effect
The rudder also has a further effect.
As an aircraft with dihedral wings yaws, the outer wing will have a greater angle of attack than the inner wing.
A greater angle of attack creates more lift and so the outer wing begins to rise.
This motion is roll and occurs in the same direction as the yaw.
The combination of yaw and roll again leads to a spiral descent.
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27. Balancing
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28. Primary Flight Controls - Balancing
If the control surfaces were simply hinged at their leading edge and allowed to trail from this position then the force required to move them, at all but very low airspeeds, would be prohibitive.
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In the absence of powered or power assisted controls some form of aerodynamic balancing is required to overcome this force.

29. Primary Flight Controls - Balancing
There are 4 common methods of aerodynamically balancing flying controls:
a. Inset hinge.
b. Horn balance.
c. Internal balance.
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d. Balance tabs.

30. Balancing - Inset hinge
The force required to move a control surface is proportional to the hinge moment (FX).
Where
F = the aerodynamic force
and
X= the distance between the hinge and the aerodynamic force. F
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The inset hinge reduces the distance X, and therefore the force required to move the control.

31. Hinged at trailing edge
Balancing - Inset hinge
The force required to move a control surface is proportional to the hinge moment (FX).
Hinged at trailing edge
Inset hinge
Where
F = the aerodynamic force
and
X= the distance between the hinge and the aerodynamic force. F X F X
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The amount of inset is usually limited to 20-25% of the control's chord length to prevent the control's C of P moving in front of the hinge at large control deflections.
The inset hinge reduces the distance X, and therefore the force required to move the control.

32. Balancing - Horn balance
This is similar to the inset hinge, except that the area ahead of the hinge is concentrated on one part of the control surface.
There are two types of horn balance
The shielded horn
The unshielded horn
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Shielded horn
Unshielded horn

33. Balancing - Horn balance
As the control surface is moved against the airflow, the horn is exposed to the airflow.
The airflow striking the horn, acting in front of the pivot point, helps the control surface to move in the required direction.
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34. Balancing - Internal balance
Although this is a common method of balancing controls, it is not visually obvious because it is contained within the contours of the controls.
Control
Control hinge
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Aerofoil
Flexible seal
Control beak

35. Balancing - Internal balance
When the control is deflected up a pressure differential occurs with the top of the control surface at an increased pressure.
The increased pressure above the beak,
and reduced pressure beneath the beak,
Higher pressure
Produce a force ahead of the hinge point which assists the movement of the control.
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Lower pressure

36. Balancing - Internal balance
The reverse is true when the control is deflected downwards.
Lower pressure
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Higher pressure

37. Balancing - Balance tabs
Balance tabs change their deflection automatically as the main control surface is deflected.
They are geared to work in the opposite direction to the main control.
When the control surface is deflected up, the balance tab moves down.
This creates an extra "lift" force on the control, helping to move it in the required direction.
When the control surface is deflected down, the balance tab moves up.
This creates a negative "lift" force on the control, helping to move it in the required direction.
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38. Secondary Flight Controls
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39. They are usually divided into:
Tabs
Tabs are small hinged surfaces forming part of the trailing edge of the control surface.
They are usually divided into:
a. Fixed Tabs.
b. Trim Tabs.
c. Balance Tabs.
Fixed tabs are adjusted on the ground, the required position being determined by one or more test flights.
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Trim tabs are adjustable by the pilot in the air.
Balance tabs have already been covered earlier in this presentation.

40. Tabs
Consider an aircraft that is slightly tail heavy and requires a constant forward pressure on the control column to maintain its attitude.
The force required to hold the elevator in the required position is given by (CP1 x d1).
The position of the tab is therefore adjusted to produce a downward force.
This force is given by (CP2 x d2 ).
When :
CP1 x d1 = CP2 x d2
the tab will hold the elevator in the desired position and the forward pressure on the stick will have been removed.
CP1
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CP2

41. Tabs Other trimming techniques include:
a. Combined trimming tab & balance tab - as in the Vigilant.
b. Adjusting the angle of incidence on all moving tailplanes.
c. Variable incidence wings.
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d. Spring bias trimming - as in the Viking.

42. Airbrakes
Airbrakes are an integral part of the airframe, which when extended increase the aircraft's form drag.
On gliders, where they are mounted on the wing, they also cause a significant reduction in lift.
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43. Airbrakes
The change of lift alters the centre of pressure, moving it slightly aft. CP
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44. Airbrakes
The change of lift alters the centre of pressure, moving it slightly aft.
This causes the nose to pitch down when airbrakes are opened. CP
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45. Airbrakes The effect of the airbrakes on rate of sink is non linear:
Opening the airbrakes to half causes a large increase in sink rate.
Opening the airbrakes further only increases the sink rate slightly.
The drag produced by the airbrakes varies as the square of the speed.
Hence the airbrakes create 4 times more
drag at 100 kts than at 50 kts.
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46. Flaps Flaps are used to vary the camber of the wing section.
High lift wings have a curved mean camber line.
High speed wings have a straighter mean camber line.
Flaps (both leading edge and trailing edge) are used to increase the camber of the wing section and therefore increase the lift coefficient.
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47. Flaps
The increase in coefficient of lift will vary with the design of flap used:
LIFT
Simple flap
50% increase in CL.
Fowler flap
90% increase in CL.
LIFT
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48. Flaps Lowering flaps also increases drag. For typical flap settings:
30° - steady rise in CL, small increase in drag.
60° - smaller rise in CL, large increase in drag.
90° - very small rise in CL, very large increase in drag.
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49. Slats
Slats are fitted to the wing leading edge to increase the stalling angle of the wing, and hence delay the onset of the stall.
They work by smoothing the airflow over the top surface of the wing at
high angles
of attack.
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16°

50. THE END
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