Flight Controls Flashcards
3 a/c axis
- Lateral (wingtip to wingtip)
- Longitudinal (nose to tail)
- Normal (top to bottom)
Rotation about the normal axis
Yaw
Rotation about the lateral axis
Pitch
How is control in pitch achieved
Forward and back movements of the control column move the elevator (hinged to the trailing edge of the tailplane), this alters the aerodynamic force produced by the tailplane which rotates the the a/c about its CG to change pitch attitude
Rotation about the longitudinal axis
Roll
Control column back =
Elevator up = increased aerodynamic force on the tail =nose up
Control column forward =
Elevator down = decreased aerodynamic force on the tail =nose down
Secondary effect of the elevator
None
How is control in roll achieved
By rotating the control column left / right, this moves the ailerons (which are hinged to the outer trailing edge of the wings) to roll the a/c
Control column left =
Left aileron up + right aileron down = decreased lift in left wing and increased lift in right wing = roll to the left
Control column right =
Right aileron up + Left aileron down = decreased lift in right wing and increased lift in left wing = roll to the right
Secondary effect of roll
Yaw - when the a/c is banked the lift vector is tilted in the direction of the bank, this tilted lift vector combined with the weight of the a/c produce a resultant force that will cause the a/c to slip sideways toward the lower wingtip. Once this slip occurs the airflow impinging on the greater area of fin surface behind the CG will cause the a/c to weathercock in the direction of the bank
Adverse yaw
It is another secondary effect of roll, when the aileron is applied to roll the a/c, the camber of the outer section of the up going wing is increased and so lift and drag of that wing is increased. conversely the camber of the down going wing is decreased and so the lift and drag of that wing is also decreased. This difference in aileron drag between each wing manifests itself as adverse yaw and the yaw is in the opposite direction to the application of aileron
Counteracting adverse yaw
- Using rudder in the direction roll
- Frise - type aileron
- Differential ailerons
- Coupling of controls
- Differential spoilers
Frise - type aileron - counteracting adverse yaw
Aileron nose protrudes below the wing when deflected upward thereby increasing drag at the same time lift is reduced resulting in the drag of each wing being more evenly balanced
Differential ailerons - counteracting adverse yaw
For a given movement of the control column the down going aileron is deflected through a smaller angle then the up going aileron thereby reducing the difference in drag and adverse yaw
Coupling of controls - counteracting adverse yaw
On some a/c the rudder is coupled to the ailerons so that when the aileron is moved by the control column the rudder moves automatically to counteract adverse yaw
Differential spoilers - counteracting adverse yaw
Deployed on the down going wing decreasing lift and increasing drag which offsets the tendency for adverse yaw
How is control in yaw achieved
Through the rudder which is operated by the pilot using the rudder pedals
Secondary effect of the yaw
Is to cause roll, this is because of the following:
- The strong yawing of the nose to one side will speed up the outer wing which will then produce more lift
- Once the a/c begins to skid the wing which is to the rear is slightly shielded from the oncoming airflow resulting in less lift
- In a/c with dihedral the forward wing in the skid has a slightly higher effective AoA resulting in more lift
What effects control effectiveness
Airspeed and slipstream
Airspeed effect on control effectiveness
Increasing airspeed over the flight control surfaces makes them more effective
Slipstream effect on control effectiveness
The slipstream is the body of faster moving air that is accelerated rearward by the propeller, as power increases the stronger slipstream increases the effectiveness of the elevator and rudder (particular at slow speed), it tends to make the nose rise and cause yaw to the left (vice versa as power is decreased)
Trim controls purpose
Relieve the pilot from having to hold constant pressure on the control column / rudder
Types on trim control
Handles, trim wheel, electrical buttons
Operation of trim wheel / handle
It varies the angle at which the trim tab on the trailing edge of the control surface is set, it creates a small aerodynamic force that acts near the trailing edge of a control surface, the control surface will maintain its angle of deflection when the moment created by the trim tab is equal and opposite to the moment of the control surface itself
Correct method of trimming
Hold the a/c in the required attitude with steady pressure on the controls and then trim this pressure off
Trimming if steady elevator back pressure is needed
Rotate the top of the trim wheel / move the trim control backward
Trimming if steady right rudder pressure is needed
Move / rotate the rudder trim to the right
Trimming using an electrical button / switch trim control
It will be spring loaded in the central OFF position, if forward control pressure is required then the trim switch is held in the forward direction and once control pressure is relived the trim is released and it should return to the OFF position
Types of control balancing
Aerodynamic and mass balancing
Aerodynamic balancing - stick force
When a control surface is deflected the aerodynamic force produced by the control surface itself opposes its deflection, this causes a moment to act on the control surface about its hinge line which tries to return it to its original faired position, the pilot must overcome this moment to maintain the desired control position and feels this as stick force
Main reason for aerodynamic balancing
Is to ease the difficulty with which the control can be moved. By altering the design of the control surface and the positioning of its hinge linei t is possible to adjust the control force moment so that the stick forces aren’t too heavy or too light
Methods for achieving aerodynamic balance
- Hinges
- Horn Balance
- Balance tabs
Inset hinges
If the hinge line of the control surface is inset the distance between the hinge line and the CP of the control in decreased therefore the control surface moment and stick force moment will also be decreased. In addition this means the nose of the control surface protrudes into the airflow when the control surface is defected and the acceleration of the airflow around the nose of the control surface causes a decrease in pressure in that area co CP of the control surface moves closer to the hinge line
Horn balance
Is when a control surface is designed with a portion that protrudes ahead of the hinge line resulting the overall CP of the control surface moving closer to the hinge line
