Stick Fixed and Stick Free Stability Questions and Answers

Answer: A

Holding the surface makes the elevator angle a controlled variable rather than something the aerodynamics determine. It is the simpler of the two cases and the one usually analysed first, since the tail's full lift curve slope is then available.

Answer: B

Correct answer: Float to whatever angle gives zero hinge moment

With no hand on the column the elevator is free to rotate about its hinge, and it settles wherever the aerodynamic hinge moment vanishes - generally trailing towards the local airflow.

The consequence is what makes the stick free case distinct. When a disturbance changes the tail's angle of attack, the elevator floats with it and partly cancels the lift change the tail would otherwise produce, so the tail contributes less to stability. That is why the stick free neutral point lies forward of the stick fixed one.

Answer: C

A surface with its hinge line ahead of the centre of pressure floats upward as incidence increases. That behaviour is what makes the stick free case differ from the stick fixed one and is governed by the hinge moment derivatives.

Answer: D

Correct answer: How strongly the elevator floats

The hinge moment derivative with respect to tail incidence, \( C_{h\alpha} \), says how much moment a change in the tail's angle of attack applies to the free surface, and hence how far it floats before the moment returns to zero.

Taken with \( C_{h\delta} \), the derivative with respect to elevator angle, it fixes the float angle per degree of incidence as \( -C_{h\alpha}/C_{h\delta} \). Those two numbers are what separate stick free stability from stick fixed; neither has anything to do with area or maximum deflection.

Answer: A

Correct answer: The tail lift curve slope remaining

The free elevator factor is the fraction of the tailplane's lift curve slope that survives when the elevator is allowed to float - typically around 0.6 to 0.8.

The floating surface partly cancels the lift change that a shift in tail incidence would otherwise produce, so the tail behaves as though it had a shallower lift curve. Multiplying the slope by the factor gives the effective value used in the stick free calculations, which is why the stick free neutral point comes out forward of the stick fixed one.

Answer: B

Releasing the elevator reduces the tail's effective contribution, so the aeroplane becomes neutrally stable at a more forward centre of gravity. The stick free case therefore sets the more restrictive aft limit of the two.

Answer: C

Both margins are measured from the same centre of gravity to their respective neutral points, and the stick free neutral point is further forward. The difference between them is a direct measure of how much stability the floating elevator costs.

Answer: D

Correct answer: Requiring progressively more up elevator as speed falls

Slower flight needs a higher angle of attack, and on a statically stable aeroplane the elevator must move progressively further up to hold each lower speed.

The slope of that curve is the measure: a steeper gradient means a larger stick fixed static margin, and the gradient reaches zero at the stick fixed neutral point. A curve requiring the same elevator at all speeds would indicate neutral stability, and one requiring down elevator as speed falls would indicate instability.

Answer: A

Correct answer: Requiring a pull force that increases as speed falls below trim

The stick force gradient is the stick free counterpart of the elevator angle gradient. Below the trimmed speed a pull is needed, and it grows as the speed falls further; above the trimmed speed a push is needed.

The gradient's slope measures the stick free static margin and reaches zero at the stick free neutral point. The trim tab setting moves the speed at which the force is zero - the curve slides along the axis - but does not change its slope, which is why the fourth option is wrong.

Answer: B

It relates directly to the manoeuvre margin, which is the manoeuvring counterpart of the static margin. Certification sets both upper and lower bounds so that the aeroplane is neither exhausting nor dangerously easy to overstress.