Flight Controls

Question 1

It is more convenient to study the coefficients of the various moments of each control, rather than their total value.

The abbreviation for pitch movement is …

Answer 1

Cm

Question 2

It is more convenient to study the coefficients of the various moments of each control, rather than their total value.

The abbreviation for roll movement is …

Answer 2

Cl

Question 3

It is more convenient to study the coefficients of the various moments of each control, rather than their total value.

The abbreviation for yaw movement is …

Answer 3

Cn

Question 4

what is the secondary effect of elevator input?

Answer 4

none

Question 5

what is the secondary effect of rudder input?

Answer 5

roll

Question 6

what is the secondary effects of aileron input?

Answer 6

adverse yaw

sideslip with bank

Question 7

what is the purpose of a trim tab?

how does its position change when the main control moves?

Answer 7

to hold the main control in a position in flight so that the aircraft will maintain its attitude if flown hands off

Does not change its set position relative to the main control surface as the main control moves

Question 8

what is the purpose of balance tabs?

how does it position change when the main control moves?

Answer 8

assists the pilot in moving the controls

move in the opposite direction to the main control surface do assist its movement

Question 9

what is a servo tab?

Answer 9

Pilot’s control input goes directly to the tab, which moves the main control through aerodynamic forces – no connection at all between the control column and the control surface

Question 10

what is a spring tab?

Answer 10

A spring is introduced to the control linkage to change stick forces at high speed. The only connection between the linkage and the control surface is a spring, which at low speed will move the control surface without any tab movement

But when the force required increases at high IAS, the spring stretches and deflection of the tab begins, reducing stick forces at high speed

Question 11

what must you be careful of with spring tab control locks?

Answer 11

must be externally locked as the control surface would still be able to move due to the spring stretching

also movement of the control stick does not confirm that the control lock is out because the spring will allow movement of the stick while the control surface is locked in position

Question 12

what are anti-balance tabs used for?

how do they move in relation to the main control surface?

Answer 12

Used if control forces are too light

Deflect in the same direction as the main control to increase the control forces

Question 13

what causes adverse yaw?

Answer 13

Induced drag increases on upgoing wing, and reduces on down going wing, thus creating a yaw in the wrong direction

Question 14

what are differential ailerons and what are they used for?

Answer 14

down going aileron is deflected less than the up going aileron re-balancing the drag to couteract adverse yaw?

Question 15

what are frise ailerons and what are they used for?

Answer 15

have a projection on their leading edge that extends down below the wing when the aileron deflects upwards, increasing drag on the down going wing to counteract adverse yaw

Question 16

when are spoliers most efffective?

Answer 16

Drag increase is with the square of the speed so most effective at high speed

Question 17

what effect do spoilers have on stalling Vs?

Answer 17

Loss of lift means an increased stalling speed – use of speed brakes not permitted below 1000ft on approach

Question 18

what is roll damping and how does it vary with altitude?

Answer 18

Because alpha is increased on the down going wing it will generate more lift than the upgoing wing, and these forces will balance out the applied rolling force at a specific rate of roll. This is reached very quickly, so effectively a control column displacement brings a rate responseor steady state responsein roll – so long as the control column displacement is held constant the aircraft will continue to roll at the same rate

As TAS increases with altitude for a constant IAS, the alpha changes become less so the damping will become less and roll rate will be higher for any given control position

Question 19

what input does an artificial feel system require?

Answer 19

Needs an input of dynamic pressure (IAS or ‘Q’) and the size of the control deflection in order to create normal stick forces

Question 20

what is flexural aileron flutter?

Answer 20

May occur when the aileron’s CG is behind the hinge line

If a disturbance causes the wing to bend upwards, the inertia of the aileron’s CG causes it to lag behind and aileron deflection is created which increases the moments that are both bending and lifting the wing up

As the wing’s elasticity stops the upward flexing, the inertia of the aileron’s CG causes it to continue its upward movement and it overshoots into an upwards deflection

The process then repeats the other way and an oscillation develops

Ailerons fitted with any of the aerodynamic balance tabs are more prone to flutter and any backlash in the control system will increase chance of flutter

Question 21

what is torsional aileron flutter?

Answer 21

Most likely with long, thin wings that are prone to twisting at the tip

Again, a disturbance of the wing causes it to move upwards and the inertia of the aileron gives it a downward deflection

But now, instead of lifting the wing up more, the moment created by the down going aileron twists the wing around it torsional axis, reducing or even reversing its angle of attack

The loss of lift causes a rapid reversal in the direction of the wings movement, changing it into a downward movement and an oscillation develops

Question 22

what is high speed aileron reversal?

Answer 22

(do not confuse with low speed aileron reversal)

The aileron deflection may have been a pilot input, trying to raise the wing, but the resulting wing twist will cause the wing to drop and the aircraft rolls in the opposite direction to the control input – this is why it is called aileron reversal

Flutter may develop but control reversal can occur without the oscillation developing

When the twisting is only slight, the pilot may just experience loss of control effectiveness

Most likely to occur at high speed when the highest aileron forces are created

Question 23

what is torsional flexural wing flutter?

Answer 23

The wing is bending and twisting at the same time, on its own without the help of the ailerons. The result of the CG of the wing aft of the torsional axis

If the wing bends upwards from a gust, inertia of the CG creates a force that twists the wing in the direction that increases angle of attack, with the resulting increase in lift promoting further bending

The spring of the wing eventually causes the wing to move downward, the inertia of the CG lags behind and twists it the other way to decrease alpha, rapidly lowering the wing and developing an oscillation