Static Directional Stability Questions and Answers

Answer: A

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

Both are true and the reason explains the assertion. The crab angle held on approach must be removed at touchdown so the wheels align with the runway, and that de-crab is a yaw demanded at the lowest speed of the whole approach.

Rudder moment falls with the square of speed, so the demand peaks where the authority is least. The crosswind at which the rudder can just achieve it sets the demonstrated crosswind for the type. Note that the figure is a demonstrated value rather than a structural limit - but where rudder power is what limits it, the number is a genuine aerodynamic boundary.

Answer: B

Correct answer: 1 and 2 only

The first two are correct. A positive \( C_{n\beta} \) means a sideslip produces a moment yawing the nose back into the relative wind, which is the definition of directional stability; and the fin is by far the largest stabilising contribution, sized precisely for that role.

The third is wrong. A conventional fuselage carries most of its side area ahead of the centre of gravity, so its side force in sideslip produces a divergent moment - destabilising. Overcoming it is one of the reasons the fin has to be as large as it is.

Answer: C

Correct answer: 1 and 2 only

The first two are correct. A dorsal fin maintains stability at large sideslip by delaying the fin stall, and a ventral fin helps at high angle of attack by remaining clear of the wing and fuselage wake.

The third confuses stability with control. Both are fixed surfaces carrying no movable element, so neither adds any control power; they change the yawing moment produced by sideslip, not the moment produced by a pilot input. Rudder power is increased by enlarging the rudder or its moment arm, which is a separate design decision sized by a separate criterion.

Answer: D

Correct answer: 1 and 2 only

The first two state the certification conditions correctly. \( V_{MCG} \) is established without nosewheel steering, since a contaminated runway may offer little cornering force; and \( V_{MCA} \) may be established with a small bank towards the live engine, typically up to five degrees, which lets a component of weight assist the rudder.

The third is wrong and confuses two different limits. Minimum control speeds are control limits set by rudder authority against asymmetric thrust; the stalling speed is a lift limit set by the wing. Certification in fact requires the control speeds and the stall to be separated by a margin.

Answer: A

Correct answer: 1 and 2 only

The first two describe the chain correctly. At large sideslip the fin can stall, its side force collapsing; and the separated flow reverses the rudder's hinge moment, so the surface floats hard over - rudder lock.

The third reverses the dorsal fin's effect. It is fitted precisely to cure both problems, its swept low aspect ratio leading edge shedding a vortex that keeps the fin's flow attached far beyond the natural stall angle. Because it removes the stall it removes both consequences at once, which is why one small fillet answers two apparently different faults.

Answer: B

Correct answer: A-1, B-2, C-3, D-4

The fin is strongly stabilising, its side force acting well aft of the centre of gravity. The fuselage is destabilising, tending to increase the yaw, since most of its side area lies forward. A tractor propeller is destabilising for the same geometric reason, its side force acting at the disc ahead of the centre of gravity.

The swept wing is modestly stabilising through differential drag: in sideslip the forward wing meets the flow at a lower effective sweep, presents more area and produces more drag, yawing the nose back into the wind. Sign is decided by which side of the centre of gravity the force acts.

Answer: C

Correct answer: A-1, B-2, C-3, D-4

Three of the four are fixed surfaces and one is a control. The dorsal fin maintains stability at large sideslip by delaying the fin stall; the ventral fin adds stability at high angle of attack by working below the wake; twin fins are placed outboard in cleaner air and reduce overall height.

The rudder alone is movable, and it provides directional control rather than stability. Keeping the two functions apart matters because they are sized by different criteria - stability by the yawing moment derivative, control by the largest moment that must be balanced.

Answer: D

Correct answer: A-1, B-2, C-3, D-4

The four cases divide into two pairs. Engine failure after take-off and a crosswind landing both test rudder power - control authority against a known moment at low dynamic pressure, asymmetric thrust in the first case and the de-crab in the second.

High angle of attack and large sideslip both test fin effectiveness - whether the stabilising surface still works, the fin being blanketed by the wake in the first and approaching its own stall in the second. Recognising which of the two quantities a case examines is usually enough to place it without recalling the pairing.

Answer: A

Correct answer: A-1, B-2, C-3, D-4

Sideslip angle is the angle between the relative wind and the plane of symmetry. Fin volume ratio is \( S_f l_f / (S b) \) - fin area times arm, over wing area times span - the non-dimensional measure of how much fin an aeroplane has.

Sidewash is the lateral flow deflection that alters the fin's local sideslip, reducing its contribution below what geometry alone would give. Rudder lock is the surface floating hard over after a fin stall reverses its hinge moment. The first three are quantities; the fourth is a failure condition, which is enough to place it.

Answer: B

Correct answer: The fin

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

The other three are destabilising for one shared reason: their side force acts ahead of the centre of gravity, producing a moment that increases the sideslip instead of reducing it. The fuselage and a tractor propeller are simply instances of the general statement in the first option. The sign of any contribution follows from which side of the centre of gravity the force acts on.