Static Directional Stability Questions and Answers

Answer: A

It is often called weathercock stability because the aeroplane behaves like a weather vane, turning to align itself with the flow. The property concerns the yawing axis alone, though it interacts strongly with the rolling behaviour.

Answer: B

The sign convention makes a positive derivative the stable one, with a right sideslip producing a yawing moment to the right. That moment turns the nose into the relative wind and removes the sideslip that produced it.

Answer: C

Correct answer: The wind and the plane of symmetry

Sideslip angle \( \beta \) is measured between the relative wind and the aeroplane's plane of symmetry - the vertical plane through the fuselage centreline. It is the yaw-axis counterpart of angle of attack, which is measured in the pitching plane.

The last option in fact defines angle of attack, between the wing chord and the relative wind. The angle between heading and track is drift, an outcome of wind rather than of the aeroplane's attitude to the airflow, and the fuselage against the horizon is a pitch attitude. Only sideslip drives the fin's restoring moment, so it is the quantity every directional stability derivative is taken with respect to.

Answer: D

The vertical surface sits well behind the centre of gravity, so a small side force there generates a large restoring yawing moment. Its area and moment arm are combined into a fin volume ratio in the same way as for the tailplane.

Answer: A

Correct answer: A destabilising yawing moment in sideslip

A conventional fuselage is a slender body with most of its side area ahead of the centre of gravity. In sideslip the resulting side force therefore acts forward of the centre of gravity and yaws the nose further from the relative wind - a divergent, destabilising moment.

The effect is present at every speed, since it depends on geometry and on sideslip rather than on dynamic pressure ratios. This is exactly why a fin is needed: its job is to overcome the fuselage's negative contribution and leave the total yawing moment derivative positive.

Answer: B

Span is used as the reference length rather than chord, because yawing moments are naturally non-dimensionalised on the lateral dimension. The grouping captures the fin's stabilising power in a single number just as tail volume ratio does in pitch.

Answer: C

Correct answer: Flow deflection altering the fin's sideslip

The wing and fuselage deflect the flow before it reaches the tail, so the fin does not meet the free-stream sideslip angle but a modified one. That deflection is sidewash, the lateral analogue of the downwash that alters the tailplane's angle of attack.

It generally reduces the local sideslip at the fin, so the fin's contribution is less than its area and arm alone would suggest, and the effect is written into the fin's contribution as a factor \( (1 - d\sigma/d\beta) \). Since sidewash depends on the wing and fuselage flow field, it varies with angle of attack - one reason directional stability is not constant through the flight envelope.

Answer: D

The disc develops a side force when the flow meets it at an angle, and forward of the centre of gravity that force yaws the nose further from the wind. A pusher propeller behind the centre of gravity is stabilising for the same reason reversed.

Answer: A

The main fin can stall at large sideslip, at which point its restoring moment collapses. The dorsal extension has a very low aspect ratio and keeps working to much larger angles, filling in exactly the region where the fin gives up.

Answer: B

It remains in relatively clean air when the main fin is blanketed at high incidence. The penalty is ground clearance, which is why such fins are often made to fold or are confined to aeroplanes with long undercarriages.