Static Directional Stability Questions and Answers

Answer: C

Correct answer: Only three

The first three are correctly matched. Engine failure after take-off tests rudder power against asymmetric thrust; a crosswind landing tests rudder power to de-crab at low speed; and high angle of attack tests fin effectiveness when the surface is blanketed by the wing and fuselage wake.

The fourth pairs engine failure with blanketing, which confuses two different critical cases. Engine failure is a control-authority problem at low dynamic pressure, not a wake problem - the aeroplane is at a normal take-off attitude, not at the high incidence that puts the fin in the wake.

Answer: D

Correct answer: Only three

Three definitions are right. \( V_{MCA} \) is the lowest speed for directional control after an engine failure; \( V_{MCG} \) is the lowest speed at which the take-off may be continued using aerodynamic controls alone; and the stalling speed is the lowest speed at which lift can equal weight.

The fourth pair gives \( V_{MCG} \) the stalling speed's definition, and the two are quite different in kind: one is a control limit set by rudder authority against asymmetric thrust on the ground, the other a lift limit set by the wing. The repetition of \( V_{MCG} \) with two definitions again marks the false pair.

Answer: A

Correct answer: The fin

The fin is the stabilising surface, and the only one in the list that is. Its side force acts well aft of the centre of gravity, so in sideslip it yaws the nose back towards the relative wind.

The other three are destabilising for a single shared reason: their side force acts forward of the centre of gravity, so the moment they produce increases the sideslip rather than reducing it. The fuselage and a tractor propeller are simply particular instances of the general statement in the last option. Whether a contribution is stabilising or not is decided by which side of the centre of gravity the force acts.

Answer: B

Correct answer: The rudder

The rudder is a movable control surface, hinged to the fin and deflected by the pilot to produce yawing moments on demand. It provides control, not stability.

The other three are fixed surfaces added to improve directional stability: the dorsal fin, which delays the fin stall at large sideslip; the ventral fin, which works below the wake at high angle of attack; and a strake ahead of the fin, which sheds a vortex to keep the fin's flow attached. Keeping the two functions distinct matters because they are sized by different criteria - stability by the derivative, control by the largest moment to be balanced.

Answer: C

Correct answer: Cruising at high altitude in still air

Cruise in still air makes no demand on the rudder at all: thrust is symmetric, there is no crosswind, and the aeroplane is in balanced flight with the rudder near neutral.

The other three are the cases that size the surface, and they share a shape - a large yawing moment to be balanced at low airspeed, where dynamic pressure and hence rudder moment are least. Engine failure and a large asymmetric thrust moment are the same case seen twice, and de-crabbing in a strong crosswind is the other classic one. Rudder power is always sized by the worst combination of demand and dynamic pressure.

Answer: D

Correct answer: Fitting a tractor propeller further forward

Moving a tractor propeller further forward makes directional stability worse, not better. Its side force in sideslip acts at the disc, ahead of the centre of gravity, so lengthening that moment arm increases an already destabilising moment.

The other three all help. Enlarging the fin increases the side force available; lengthening the fin's moment arm increases the moment that force produces; and a ventral fin adds side area aft and below, where it stays clear of the wake at high angle of attack. Area and arm aft of the centre of gravity help; area and arm forward of it hurt.

Answer: A

Correct answer: An increase in the yawing moment derivative

A fin stall does the opposite. Beyond the stall the flow separates and the side force collapses, so the fin's contribution to \( C_{n\beta} \) falls away rather than rising - the derivative can approach zero or go negative.

The other three are all genuine consequences of the same event. The collapse in side force is the immediate effect; loss of directional stability follows from it, since the restoring moment depends on that force; and rudder lock follows too, because the separated flow reverses the rudder's hinge moment and the surface floats hard over.

Answer: B

Below that speed the dynamic pressure is too low for the rudder to generate the required moment. The speed is established by flight test precisely so that the take-off can be planned to keep above it before the aeroplane is committed.

Answer: C

Correct answer: Rudder lock following a fin stall, prevented by a dorsal fin

The symptom is diagnostic. At large sideslip the fin stalls, the separated flow reverses the rudder's hinge moment, and instead of tending to centre the surface is driven against its stop and cannot be recovered by normal pilot effort.

A dorsal fin is the standard cure because it attacks the cause: its low aspect ratio and swept leading edge shed a vortex that keeps the fin's flow attached to a far larger sideslip angle, so the stall never occurs. A jam or hydraulic failure would show at any sideslip, not specifically at large ones, and overbalance is a hinge-line design fault rather than a stall phenomenon.

Answer: D

Correct answer: The fin is blanketed by the wake

The geometry is unchanged, so the loss must be aerodynamic. At very high angle of attack the wing and fuselage shed a broad wake of low-energy air, and a conventional centreline fin sits inside it.

Reduced local dynamic pressure means far less side force for a given sideslip, so a large fin can behave as though it were a small one. That is the point of the question: fin area is a fixed quantity and does not "effectively reduce", but the dynamic pressure acting on it does. Sideslip certainly occurs at high incidence - departure and spin entry are precisely such cases - which is why ventral fins and strakes exist.