Static Directional Stability Questions and Answers

Answer: C

Correct answer: False, a slender body tends to increase its own yaw

A slender body in sideslip develops a side force that acts well forward, near the nose, and the resulting moment turns it further from the relative wind - the classic destabilising behaviour of a fuselage.

The effect follows from geometry rather than from the fin's size, so it is present whether the fin is large or small and cannot be made stabilising by any choice of fin. Nor is the fuselage neutral: its contribution to \( C_{n\beta} \) is a definite negative term, and overcoming it is one of the reasons the fin has to be as large as it is.

Answer: D

Correct answer: It increases the rudder's control power

A dorsal fin is a fixed fillet carrying no movable surface, so it adds no control authority at all. It changes the moment produced by sideslip, not the moment produced by a pilot input.

The other three statements are true. It maintains stability at large sideslip and prevents rudder lock, both by delaying the fin stall; and it has a very low aspect ratio with a highly swept leading edge, which is exactly what lets it shed the stable vortex that keeps the flow attached. Stability and control are sized by different criteria, and conflating them is the trap here.

Answer: A

The engine failure may occur on a contaminated surface where tyre friction cannot be relied on. Excluding steering makes the resulting speed valid whatever the runway condition, which is what allows it into the take-off calculation.

Answer: B

Correct answer: Moving a tractor propeller further forward

Moving a tractor propeller forward makes matters worse. Its side force in sideslip acts at the disc, ahead of the centre of gravity, so lengthening that arm increases an already destabilising moment.

The other three all help, and all for the same reason: they add side force, or moment arm, aft of the centre of gravity. Enlarging the fin increases the force available, lengthening the fin arm increases the moment that force produces, and a ventral fin adds area aft and below where it stays clear of the wake at high incidence.

Answer: C

Correct answer: False, the surface floats over after the fin stalls

Rudder lock is aerodynamic, not mechanical. At large sideslip the fin stalls, and the separated flow reverses the rudder's hinge moment so that the surface is driven towards its stop rather than towards centre, floating hard over.

The linkage is intact and nothing has seized; the pilot simply cannot generate enough force to overcome the reversed aerodynamic moment. That is why the cure is a dorsal fin, which prevents the fin stall, rather than any change to the control run. Balance and hydraulics affect the forces involved but neither causes the condition.

Answer: D

Correct answer: Cruise in still air at altitude

Cruise in still air makes no demand on the rudder: thrust is symmetric, there is no crosswind, and the aeroplane flies in balance with the surface near neutral.

The other three share a shape - a large yawing moment to be balanced at low airspeed, where dynamic pressure and therefore rudder moment are least. Engine failure just after take-off and continuing a take-off after a failure are the airborne and ground versions of the asymmetric thrust case, giving \( V_{MCA} \) and \( V_{MCG} \); de-crabbing in a limiting crosswind is the other classic case.

Answer: A

Correct answer: No, sideslip excites both axes

The two cannot be separated because a single variable drives both. A sideslip disturbance produces a yawing moment through the fin and, through dihedral, sweep and the fin's height above the rolling axis, a rolling moment as well.

That coupling is what gives rise to the lateral-directional modes: Dutch roll, in which yaw and roll alternate, and the spiral mode, whose stability turns on the relative strength of the weathercock and dihedral effects. Removing dihedral does not uncouple them, since the fin's own side force still rolls the aeroplane, and the elevator acts in the pitching plane and is irrelevant to either.

Answer: B

The other three are fixed area improving the yawing moment derivative. The rudder is a movable control surface whose purpose is directional control rather than stability, and the two requirements are sized quite separately.

Answer: C

Correct answer: False, the derivative collapses when the fin stalls

Beyond the stall the flow over the fin separates and its side force falls away, so the fin's contribution to \( C_{n\beta} \) collapses. The derivative can approach zero or go negative, since the destabilising fuselage and propeller terms remain unchanged.

That is the essence of the problem at large sideslip: the aeroplane loses the very restoring moment it needs at the moment it needs it most, and rudder lock may follow from the same separation. A dorsal fin does not make a stalled fin effective - it delays the stall, so the derivative never collapses in the first place.

Answer: D

Correct answer: An increase in the fin's side force

Blanketing reduces the fin's side force; it cannot increase it. The wake is low-energy air, so the local dynamic pressure at the fin falls and with it the force generated for a given sideslip.

The other three all follow. Low-energy air over the fin is the blanketing itself; reduced directional stability is its immediate consequence, since \( C_{n\beta} \) depends on that side force; and the case for a ventral fin follows in turn, a surface below the fuselage remaining in clean air at the attitudes where the main fin is shadowed.