Control Surface Hinge Moments Questions and Answers

Answer: A

Hinge moment is proportional to dynamic pressure, so doubling the design q roughly doubles the moment the jack must overcome and therefore its size and the hydraulic power behind it. Nothing about the change leaves the requirement alone, and it certainly does not fall. The forty per cent figure is the square root of two, which is the way frequencies scale with dynamic pressure rather than the way forces do - a useful reminder that not everything scales the same way.

Answer: B

The control must have a positive gradient, so it self-centres and cues the pilot, and absolute forces inside the specified band, neither too heavy to manoeuvre nor so light that the aeroplane can be overstressed. Zero breakout is neither achievable nor desirable, and infinite gearing is meaningless. Overbalance is a fault, not a requirement. And maximum effectiveness with minimum area is a design aspiration rather than an acceptability criterion.

Answer: C

A plain surface is heaviest; a set-back hinge lightens it by a useful fraction; and a servo tab, where the pilot flies only the tab, is lightest of all because the main surface is aerodynamically driven. So the order is plain, set-back, servo tab. The reversed sequences invert the physics. The progression is also roughly historical, and the next step beyond a servo tab is full hydraulic power, which removes the pilot from the load path altogether.

Answer: D

A large breakout force swamps the gradient around neutral, so the pilot cannot feel small displacements and tends to over-control; too little leaves the control feeling loose and the aeroplane vulnerable to inadvertent inputs. It is not a flutter mechanism, which depends on mass and stiffness rather than friction, and it does not change the surface's aerodynamic effectiveness at all. Handling is precisely what it affects, which is why both bounds exist.

Answer: A

The two moments are taken about different points, and that is the whole of it. Balance rearranges the load either side of the hinge line, which changes the moment about the hinge while leaving the total force and its moment about the aeroplane's centre of gravity essentially as it was. The surface still deflects the same air by the same amount, so the pitching or rolling moment it delivers is nearly unchanged. Area is not removed; it is redistributed relative to the hinge. The effect operates at all deflections, although its magnitude varies with them, which is why the whole range must be checked for overbalance. And the hinge line moves aft only within the surface itself, by a fraction of the surface chord, which is far too small to matter to the arm from the centre of gravity.

Answer: B

It sets a ceiling on manual flight controls. A force that is comfortable at 150 knots is nearly three times as large at 250 and more than six times as large at 400, so beyond some design speed no amount of gearing or balance keeps the forces inside the certification limits. That is the engineering history of the subject in one sentence: aerodynamic balance bought the first factor, tabs of various kinds bought the next, and hydraulic power removed the pilot from the load path altogether. It is worth naming the quantity before naming the device, because the devices overlap and the quantities do not. Effectiveness actually improves with dynamic pressure rather than falling. Static stability is unrelated to hinge moments. And trim tabs are adjusted in flight by the pilot, precisely so that they need not be re-rigged.

Answer: C

The hinge moment is a product, and both factors peak in that corner: the coefficient grows with deflection while dynamic pressure grows with the square of speed, so the two multiply into the design case that sizes the actuator or defines the manual force limit. Recovery from an upset at high speed is the realistic scenario, which is why it is examined rather than merely computed. Temperature affects material properties, not air loads. Gearing is mechanical and constant. And what the pilot can apply is the limit being tested against, not a cause of the moment.

Answer: D

In a power-assisted, or boosted, system the actuator carries most of the hinge moment but a defined fraction is fed back to the pilot, so the natural feel and its variation with speed survive and no separate feel unit is needed. A fully powered irreversible system takes the whole load, which is why artificial feel becomes mandatory there. The power source may be hydraulic or electric in either arrangement. Both act in both directions. And mass balance addresses flutter, so it can never substitute for a feel system, whatever the actuation.

Answer: A

For a vertical surface, sideslip is precisely what incidence is for a horizontal one: the angle at which the fin meets the flow, so the rudder's floating tendency and its hinge moment vary with it in exactly the way an elevator's vary with angle of attack. Large sideslip is also where the fin can stall and the rudder float hard over, the rudder lock condition, so the corner has to be measured rather than extrapolated. Dynamic pressure at the fin changes only slightly. Mass balance is a property of the structure. And the requirement exists because the physics demands it.

Answer: B

It gives a tactile cue: as the aeroplane speeds up from trim the pilot feels a push, and as it slows down a pull, so speed is held almost without conscious effort and an inadvertent departure announces itself through the hand rather than the instruments. That is why a minimum gradient is required and why a downspring is fitted when aft loading makes it too shallow. The control gets heavier rather than lighter at speed. Flutter is a structural dynamics matter untouched by force gradients. And bank recovery is a lateral property with no connection to the elevator circuit.