Static Directional Stability Questions and Answers

Answer: A

Correct answer: Both Statement I and Statement II are correct

Both are true and the second describes the mechanism of the first. In a developing turn a sideslip appears; a strong fin yaws the nose into it quickly, which steepens the turn and increases the bank still further.

Weak dihedral effect cannot generate the rolling moment needed to level the wings against that, so bank continues to increase and the aeroplane enters a slowly tightening descending spiral. The balance is a genuine design trade: strengthening the fin to cure Dutch roll worsens the spiral mode, which is why yaw dampers are fitted rather than the fin simply being enlarged.

Answer: B

Correct answer: Both Statement I and Statement II are correct

Both are true and the second explains the first. In sideslip the forward wing meets the airflow at a lower effective sweep angle than the rear one, so it presents a greater effective span and frontal area to the flow and generates more drag.

That drag differential, acting at a distance from the centre of gravity, produces a yawing moment towards the relative wind - a stabilising contribution. It is modest compared with the fin's but it is genuinely useful, and it grows with sweep and with lift coefficient. The same sideslip on a swept wing also produces a strong rolling moment, which is why sweep contributes an effective dihedral as well.

Answer: C

Correct answer: 1, 2, 3, 4, 5

The chain is the definition of directional stability, and each step is the cause of the next. The gust yaws the nose, so the relative wind no longer lies in the plane of symmetry and a sideslip angle appears.

The fin, now meeting the flow at an angle, generates a side force. Because the fin is aft of the centre of gravity, that force produces a yawing moment; had it acted forward of the centre of gravity the moment would have been divergent instead. The moment turns the nose back towards the relative wind, and the sideslip that produced it decays to zero - the disturbance erases its own cause.

Answer: D

Correct answer: 1, 2, 3, 4, 5

The order follows the physics and then the pilot's response. The engine fails, leaving thrust on one side only; the resulting moment yaws the nose towards the dead engine.

The pilot's first action is rudder, which balances that moment directly. A small bank of a few degrees towards the live engine follows, letting a component of weight assist the rudder so that less deflection - and less rudder drag - is needed for the same balance; this is why the certified minimum control speed assumes a bank of up to five degrees. With yaw balanced and drag minimised, the aeroplane climbs away.

Answer: A

Correct answer: 1, 2, 3, 4, 5

The order runs through the airframe from nose to tail, ending with the sum. The fuselage comes first, its side area largely ahead of the centre of gravity and therefore destabilising. A tractor propeller is destabilising for the same geometric reason, its side force acting at the disc.

The wing contributes modestly, usefully so if swept, through the drag differential between forward and rear panels. The fin comes last and is by far the largest term, sized precisely to overcome everything before it. The total \( C_{n\beta} \) must be positive for stability, which is what the whole exercise is for.

Answer: B

Correct answer: 1, 2, 3, 4, 5

Each step causes the next. A large sideslip develops - typically after an engine failure or a control input at low speed. Beyond the fin's stall angle the flow separates and the side force collapses, so the restoring moment vanishes.

The separated flow over the rudder reverses its hinge moment, so instead of tending to centre, the surface is driven further over and floats hard against the stop. With the aerodynamic moment now acting the wrong way, the pilot cannot recentre it by normal effort, and the aeroplane is locked in the sideslip. A dorsal fin breaks the chain at step two by delaying the stall.

Answer: C

Correct answer: 1, 2, 3, 4, 5

The sequence is the standard de-crab technique. The aeroplane approaches crabbed into wind, tracking the centreline with the nose offset by the drift angle.

Shortly before touchdown the pilot applies rudder to yaw the fuselage into line with the runway, since the wheels must be aligned with the direction of travel. Removing the crab lets the crosswind push the aeroplane sideways, so aileron into wind is applied to prevent the drift - which banks the aeroplane slightly and brings the into-wind wheel down first. The rollout then continues on the centreline with rudder holding the heading.

Answer: D

Correct answer: Only three

The first three are right. The fin is strongly stabilising in yaw, since its side force acts well aft of the centre of gravity. The fuselage is destabilising, its side area lying mostly forward of it. A tractor propeller is destabilising for the same geometric reason.

The fourth pair contradicts the first: the fin cannot be both stabilising and destabilising. That contradiction is the quickest route to the answer - where a list repeats a component with opposite contributions, one of the two must be the false pair, so three correct is the most that can be claimed.

Answer: A

Correct answer: Only three

The first three are correct. Sideslip angle is measured between the relative wind and the plane of symmetry; the fin volume ratio is \( S_f l_f / (S b) \), fin area times arm over wing area times span; and rudder power is the yawing moment per degree of rudder deflection.

The fourth defines angle of attack, not sideslip - the angle between the chord and the relative wind, measured in the pitching plane rather than the yawing one. It also contradicts the first pair, which is the signal that one of the two must be false.

Answer: B

Correct answer: Only three

The first three pairs hold. A dorsal fin maintains stability at large sideslip by delaying the fin stall. A ventral fin adds stability at high angle of attack because, mounted below the fuselage, it remains clear of the wake that blankets the main fin. Twin fins are placed outboard in cleaner air and reduce overall height.

The fourth is wrong. A dorsal fin is a fixed surface with no rudder on it and adds no control authority; it improves stability, not rudder power. Rudder power is increased by enlarging the rudder or its arm.