Static Directional Stability Questions and Answers

Answer: C

Correct answer: The critical combination must be examined, not each case alone

Read the rows against the available travel of 25 degrees. Engine failure alone needs 20 degrees and a limiting crosswind de-crab needs 8, so each is satisfied on its own. Together they need 28 degrees, which exceeds the stop.

That is the lesson: a design can pass every case examined singly and still be unable to meet a combination that can actually occur. Certification therefore considers the critical combination, and where a combination exceeds the available travel the answer is a limitation - a reduced crosswind limit with an engine inoperative, for instance - rather than a claim that the design passes.

Answer: D

Correct answer: Aft area stabilises, forward destabilises

All four rows are the same statement applied in turn. Enlarging the fin and lengthening its arm both increase side force or moment acting aft of the centre of gravity, and both raise the derivative.

Moving a tractor propeller forward and lengthening the forward fuselage both increase side force or moment acting ahead of it, and both lower the derivative. Which side of the centre of gravity a side force acts on decides the sign of the moment it produces, and how far from it decides the size. Larger aeroplanes are not automatically more stable, and the third and fourth rows show plainly that the fin is not the only contributor.

Answer: A

Correct answer: The runway may be contaminated

\( V_{MCG} \) is established with aerodynamic controls alone because the nosewheel's cornering force cannot be relied upon: a wet, icy or otherwise contaminated runway may offer very little, and the published speed must remain valid on the day it is most needed.

The reasoning is the same conservatism that assumes the critical engine fails and the remaining engines are at take-off power. Nosewheel steering is fitted to essentially all transport aeroplanes and is used well above taxi speed, so neither of those options is the reason; and allowing it would give a lower speed that could not be relied on when the surface was poor.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both statements are true and the second is the reason for the first. The fuselage carries most of its side area ahead of the centre of gravity, and a tractor propeller's side force in sideslip acts at the disc, also ahead of it.

A side force forward of the centre of gravity produces a moment that increases sideslip, so both are destabilising and the fin must be large enough to overcome their sum and still leave \( C_{n\beta} \) positive. That surplus is the stability margin. It is why fin sizing begins with the negative contributions rather than with the fin itself.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true, and the reason identifies exactly the mechanism the dorsal fin interrupts. Rudder lock begins with the fin stalling at large sideslip; the separated flow reverses the rudder's hinge moment, so the surface floats hard over instead of tending to centre.

A dorsal fin prevents this by preventing the stall - its swept, low aspect ratio leading edge sheds a vortex that keeps the fin's flow attached to a far larger sideslip angle. Because it removes the cause rather than treating the symptom, it cures the loss of directional stability at large sideslip in the same stroke.

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason is precisely why the case is critical. Asymmetric thrust produces a moment set by thrust and engine offset, and neither falls as the aeroplane slows.

The rudder's opposing moment, by contrast, is proportional to dynamic pressure and therefore to the square of speed. So the demand stays constant while the authority falls away, and the binding condition is the lowest speed at which flight must continue - which is what defines \( V_{MCA} \). Rudder size on a multi-engine aeroplane is set by this balance rather than by any manoeuvring requirement.

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. At very high angle of attack the wing and fuselage shed a broad low-energy wake, and a conventional centreline fin sits within it, losing much of its effectiveness just where directional control matters most.

A ventral fin mounted below the fuselage remains in clean air at those attitudes, so its contribution holds up as the main fin's falls away. That is why the device appears on aeroplanes expected to reach extreme incidence - fighters, and types cleared for spinning - rather than on transports that never go there.

Answer: B

Both statements are true and the second is the reason. The balance between the two determines whether the aeroplane exhibits a badly damped Dutch roll or a divergent spiral, which is a matter of handling rather than of either axis alone.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason explains the preference. A propeller in sideslip meets the flow obliquely and generates a side force at the disc; on a pusher installation that disc is behind the centre of gravity.

A side force acting aft produces a moment that reduces sideslip, so the pusher contributes in the stabilising sense while a tractor contributes in the destabilising one. It is the same geometric argument that makes the fin stabilising and the forward fuselage destabilising. Directional stability is only one consideration among many, of course - ground clearance, cooling and blade noise all argue the other way.

Answer: D

Correct answer: A is false but R is true

The reason is sound and it is exactly why the assertion fails. On a contaminated runway the nosewheel may have little cornering force, leaving the rudder as the only directional control that can be depended on.

\( V_{MCG} \) is therefore established using aerodynamic controls alone, deliberately excluding nosewheel steering, so that the published speed remains valid in the conditions where it matters most. The assertion states the opposite of the certification requirement. This is the pattern where a true reason contradicts rather than supports the assertion, so each must be assessed on its own before they are compared.