Stick Fixed and Stick Free Stability Questions and Answers

Answer: B

Correct answer: The floating elevator relieving incidence

Every row traces back to one thing. A released elevator floats towards the local flow when the tail's incidence changes, partly cancelling the lift change that would otherwise occur.

That is what the free elevator factor of 0.65 records, and multiplying the tail's slope by it gives 2.6 per radian. The weaker tail contributes less to stability, so the neutral point moves forward from 0.44 to 0.38, and the static margin at a fixed loading falls from 0.16 to 0.10. Nothing about the wing, the tail area or the loading has changed.

Answer: C

It is fitted deliberately to raise stick force per g where the aerodynamic value falls below the certification minimum. Because it responds to acceleration alone, it leaves the trim force gradient with speed untouched.

Answer: D

Correct answer: Each identifies a different point

The four curves locate four distinct positions - the two neutral points and the two manoeuvre points - and they do not coincide. Speed sweeps are flown at 1 g and reveal the neutral points; pull-ups introduce a pitch rate and reveal the manoeuvre points.

Since the aft centre of gravity limit must respect all four, and the most forward of them governs, none can be inferred from another. The instrumentation is largely common and no regulation asks for redundancy; it is the physics that requires four measurements.

Answer: A

Correct answer: Certification force limits translate directly into centre of gravity limits

Read the table as a chain. Stick force per g is proportional to the manoeuvre margin, and the manoeuvre margin is fixed by where the centre of gravity sits. So a force requirement expressed in newtons becomes a permitted range of margins, and hence a permitted range of loadings.

Note that both ends bind: too small a margin gives a control light enough to overstress the aeroplane inadvertently, too large gives one too heavy to manoeuvre. Stability alone is therefore not sufficient, which is what the second option overlooks.

Answer: B

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

Both are true and the second is precisely why the first must be so. A floating elevator weakens the tail's contribution, so the stick free neutral point lies forward of the stick fixed one.

The centre of gravity must stay forward of both, and the aft limit is therefore drawn against whichever comes first. Using the stick fixed figure would permit a loading at which the aeroplane was stable with the column held but not with it released - the very condition hands-off flight presents.

Answer: C

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

Both are true and the second gives the mechanism. When the tail's angle of attack rises, the hinge moment on the free elevator becomes non-zero and the surface floats towards the local flow.

That movement is in the direction that reduces the tail's effective camber, so part of the lift change is cancelled before it can occur. The tail therefore behaves as though its lift curve slope were smaller, by the free elevator factor - typically 0.6 to 0.8 - and contributes proportionately less to stability.

Answer: D

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

Both are true and the second explains the first. In a steady pull-up the aeroplane is rotating in pitch, so the tail, being aft of the centre of gravity, meets an additional downward velocity component from that rotation.

The resulting extra tail lift is a restoring moment present only when pitching, so the tail is effectively more powerful in a manoeuvre than in steady flight. The manoeuvre point therefore lies aft of the neutral point, and the manoeuvre margin exceeds the static margin at any loading.

Answer: A

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

Both are true and the second is why the device suits the requirement so exactly. Stick force per g is a manoeuvring quantity, and a bob weight responds to normal acceleration.

In steady trimmed flight the acceleration is 1 g at every speed, so its contribution is constant and trimming removes it - the stick force gradient against speed is untouched. In a pull-up the acceleration rises and the force rises with it. The device therefore raises exactly the quantity certification sets a minimum for, and nothing else.

Answer: B

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

Both are true and the second is the failure being avoided. Balance area ahead of the hinge carries airload that opposes the hinge moment, lightening the control.

Taken too far, that airload exceeds what it was meant to offset and begins to assist the deflection instead: the surface tends to move further of its own accord and the stick force reverses, leaving the pilot without usable feedback. Since manufacturing tolerance and flow variation both shift the balance point, the design is set conservatively short of it.

Answer: C

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

Both are true and the second explains the first. A servo tab is an aerodynamic device: the pilot moves the tab, and the tab's airload deflects the main surface.

Its authority therefore falls with dynamic pressure, so at low speed the tab does less and the pilot must move the surface more directly - the control feels heavy relative to what it achieves. The characteristic is a known drawback of the arrangement, and it is one reason spring tabs, whose assistance grows with load rather than with speed, are often preferred.