Lateral Stability Questions and Answers

Answer: C

The weathercock turns the nose into a developing turn faster than the wings can be rolled level. The resulting spiral is slow and easily controlled, which is why designers often accept it in preference to a bad Dutch roll.

Answer: D

Correct answer: Extra induced drag when lowered

The down-going aileron increases the lift on its wing, and induced drag rises with lift. The up-going aileron reduces lift and induced drag on the other side, so the aeroplane is left with more drag on the wing that is rising.

That drag differential yaws the nose away from the direction of roll - adverse yaw. It is greatest at low speed and high lift coefficient, where induced drag dominates, which is why the effect is most obvious in a slow, steep turn entry and is countered with rudder.

Answer: A

Correct answer: Deflecting further up than down

Differential ailerons are rigged so that the up-going surface travels further than the down-going one. The larger upward deflection adds profile drag on the rising wing, while the smaller downward deflection limits the induced drag added on the other side.

The two changes together reduce the drag differential that causes adverse yaw. The mechanism is purely geometric, arranged in the control linkage, which makes it a cheap fix - and it costs a little roll authority, since the down-going aileron does less than it might.

Answer: B

Correct answer: Projecting its nose into the flow

A Frise aileron is hinged behind its leading edge, so when it deflects upward the nose of the surface projects below the wing's lower surface into the airflow.

That projection adds profile drag on the up-going wing, offsetting the induced drag added on the down-going one, and so reduces adverse yaw. It also lightens the stick force by helping to balance the hinge moment. The mechanism is quite different from differential rigging, which changes the travel; the Frise changes the flow around the surface itself.

Answer: C

It is defined as the roll rate times the semi-span divided by the true airspeed and is the standard non-dimensional measure of roll capability. Certification requirements are written in terms of it rather than of roll rate alone.

Answer: D

Raising a spoiler destroys part of the lift on that wing and rolls the aeroplane towards it. Because spoilers add drag on the inside of the turn they produce favourable rather than adverse yaw, which is a useful secondary benefit.

Answer: A

Correct answer: Wing twist overcomes the aileron

A down-going aileron produces its lift well aft on the section, which twists the wing leading-edge-down. That twist reduces the wing's own angle of attack, and the lift it loses opposes the lift the aileron gained.

Both effects grow with dynamic pressure, but the twist grows faster because the wing's torsional stiffness is fixed. Above the reversal speed the twist wins outright and the aeroplane rolls the wrong way. The remedy is stiffness or geometry - inboard ailerons and spoilers - not a change to the control run, which is correctly rigged throughout.

Answer: B

Correct answer: Avoid the wing twist that causes reversal at high speed

An inboard aileron applies its load near the wing root, where the structure is deep and torsionally stiff and the moment arm to the torsion box is short. Very little twist results, so the aileron's own lift change is not cancelled.

An outboard aileron works at the flexible tip, where the same load twists the wing appreciably. Large transports therefore use outboard ailerons at low speed for their long moment arm, and lock them out at high speed, rolling on inboard ailerons and spoilers instead.

Answer: C

Lowering an aileron raises the local incidence on a wing already near the stall, so the section may stall rather than gaining lift. The resulting roll reversal near the stall is one reason spoilers and rudder are preferred there.

Answer: D

The dihedral effect tries to roll the aeroplane away from the sideslip, so the pilot must hold aileron into it to maintain a steady slip. The amount required is a direct flight test measure of the dihedral effect's strength.