Static Directional Stability Questions and Answers

Answer: A

The weathercock turns the nose into the developing turn faster than the dihedral effect can roll the wings back level. The result is the spiral mode, which diverges slowly and is easily controlled but must not be ignored.

Answer: B

The disc's side force acts further ahead of the centre of gravity, so its destabilising moment grows. The fin may need enlarging to restore the derivative, which partly offsets whatever was gained on the loading range.

Answer: C

Control is lost before the wing stops flying, so the speed schedule must be built around the higher figure. Certification requires the relationship to be established because it determines which limitation the crew must respect.

Answer: D

The aeroplane behaves like a weather vane, turning to align itself with the relative wind. The property concerns the yawing axis alone, though it interacts strongly with the rolling behaviour through the sideslip both share.

Answer: A

Correct answer: Sideslip

Sideslip angle \( \beta \) is measured between the relative wind and the plane of symmetry - the vertical plane through the fuselage centreline. It is the yawing-plane counterpart of angle of attack.

Angle of attack is measured between the chord and the relative wind, in the pitching plane. Bank is an attitude about the longitudinal axis relative to the horizon, and track is the path over the ground. Only sideslip drives the fin's restoring moment, which is why every directional stability derivative is taken with respect to it.

Answer: B

Correct answer: dorsal

A dorsal fin is the fillet extending the fin forward along the top of the fuselage. Its very low aspect ratio and highly swept leading edge make it shed a stable vortex in sideslip, and that vortex keeps the flow attached to the main fin well beyond its natural stall angle.

The result is directional stability retained at large sideslip, and rudder lock avoided. A ventral fin does a related job below the fuselage, staying clear of the wake at high angle of attack; a strake is a smaller vortex-generating surface; and a canard is a forward lifting surface in the pitching plane entirely.

Answer: C

Correct answer: air

Minimum control speed in the air, \( V_{MCA} \), is the lowest airspeed at which directional control can be maintained after the critical engine fails, with the remaining engines at take-off power and a bank of up to five degrees permitted.

Its ground counterpart, \( V_{MCG} \), applies to the take-off roll and allows rudder only. The reason there is a speed at all is that the asymmetric thrust moment does not fall with airspeed while the rudder's opposing moment falls with dynamic pressure, so below some speed the rudder simply runs out of authority.

Answer: D

Correct answer: Sidewash

Sidewash is the lateral deflection of the flow by the wing and fuselage before it reaches the fin, so that the fin meets a local sideslip angle smaller than the free-stream value.

It is the yawing-plane analogue of downwash, which alters the tailplane's angle of attack in the pitching plane, and it is written into the fin's contribution as a factor \( (1 - d\sigma/d\beta) \). Because the deflection depends on the wing and fuselage flow field, it changes with angle of attack - one reason directional stability is not constant across the envelope. Upwash and backwash describe different flow fields entirely.

Answer: A

The two forward contributions together push towards instability, and only the fin's larger positive value leaves a stable total. Anything that increases those forward contributions demands a larger fin in compensation.

Answer: B

Correct answer: Large sideslip and high angle of attack

Each device answers a different row of the table. The dorsal fin addresses the falling effectiveness at large sideslip, where the fin is approaching its own stall, by shedding a vortex that keeps the flow attached well beyond the natural stall angle.

The ventral fin addresses the reduced effectiveness at high angle of attack, where the main fin is blanketed by the wing and fuselage wake; mounted below the fuselage, it remains in clean air. Neither is needed in cruise at small sideslip, where the fin already works at full effectiveness with attached flow.