Static Longitudinal Stability Questions and Answers

Answer: A

Correct answer: Both Statement I and Statement II are correct

Both are true. \( V_H = S_t l_t / (S \bar{c}) \) is dimensionless and is precisely the grouping in which the tail enters the neutral point expression, so it is the parameter that matters rather than area or arm separately. A small tail on a long arm and a larger tail on a shorter arm with the same \( V_H \) contribute the same to stability, which is why the ratio is the quantity quoted when comparing layouts.

Answer: B

The second statement is correct and it explains why the first is false. Reflex or washout provides only a small restoring moment, so such aeroplanes are demanding to balance and tolerate very little centre of gravity movement.

Answer: C

The first four steps are static stability, concerning only the direction of the initial response. Whether the final oscillation damps out is a dynamic question governed by pitch damping rather than by the restoring moment alone.

Answer: D

Static margin falls steadily as the centre of gravity moves aft, reaching zero at the neutral point and becoming negative beyond it. The last position is statically unstable and would require artificial stability to be flyable.

Answer: A

Correct answer: 1, 2, 3, 4

The moment slope is built up component by component and then totalled. The wing is taken first, destabilising when the centre of gravity is aft of its aerodynamic centre; the fuselage is added next, generally destabilising; the tailplane follows with its large stabilising contribution; and the sum must come out negative for the aeroplane to be statically stable. The order reflects both the physical arrangement from nose to tail and the way the estimate is assembled in practice.

Answer: B

The two limits arise from entirely different physics, one from stability and the other from control authority. Both must be respected throughout the flight, which is why fuel burn is considered in the loading schedule.

Answer: C

The first two obtain their restoring moment quite differently from the last two. A canard's foreplane is destabilising in itself, so the centre of gravity must be placed well forward for the arrangement to work at all.

Answer: D

Correct answer: Only three

The tailplane is strongly stabilising, the fuselage is generally destabilising, and a wing with the centre of gravity aft of its aerodynamic centre is destabilising - three correct pairs. The fourth, which calls the tailplane destabilising, contradicts the first: the tail is the component that makes the conventional layout stable, since it is the only one supplying a large negative contribution.

Answer: A

Correct answer: Only three

The first three definitions are exact: the neutral point is the centre of gravity position at which \( \mathrm{d}C_m/\mathrm{d}C_L \) is zero, the static margin is \( h_n - h \), and the tail volume ratio is \( S_t l_t/(S\bar{c}) \). The fourth is wrong - the point through which the lift acts is the centre of pressure, which moves with incidence, whereas the neutral point is a fixed property of the layout.

Answer: B

Correct answer: Only three

The aft limit is set by the minimum acceptable static margin, the forward limit by the elevator power available at low speed, and the neutral point is where the moment slope is zero - three correct pairs. The fourth attributes the forward limit to the static margin, which is exactly the wrong way round: at the forward limit the margin is at its largest, and it is control authority, not stability, that runs out.