Control Surface Hinge Moments Questions and Answers

Answer: A

In a servo tab system the control run ends at the tab, and the aerodynamic moment it generates deflects the main surface, which floats free. It gives very light forces on a large aeroplane without hydraulics, at the cost of poor response at low speed where the tab has little authority. A fixed tab is a ground-adjustable trim device. Hydraulic drive would make it a powered control. And the tab works at all speeds, better as dynamic pressure rises.

Answer: B

A spring tab links the tab to the control run through a spring, so at low speed the spring is stiff relative to the hinge moment and the surface moves directly, while at high speed the spring deflects and the tab does more of the work. The result is balance that increases with dynamic pressure, which is exactly where it is needed. It is not a flutter device, not a trimmer, and not a gust lock, though a gust lock is a real and separate fitting.

Answer: C

Mass balance is weight forward of the hinge line, so the surface's centre of gravity moves onto or ahead of the hinge and inertia no longer drives it in response to structural bending. That breaks the coupling that produces control surface flutter. Weight behind the hinge would make matters worse. Matching left and right is a rigging and balance check rather than the definition. A bobweight is in the circuit and addresses stick force per g.

Answer: D

Flutter of this kind couples wing or tail motion with the surface's own inertia about its hinge: as the structure bends, an unbalanced surface lags behind, its deflection feeds energy into the motion, and the oscillation grows. Mass balance breaks the loop by moving the surface centre of gravity to the hinge. Pilot coupling is a pilot-induced oscillation, a distinct phenomenon at much lower frequency. Engine vibration and tab settings are not flutter mechanisms.

Answer: A

Flutter is a dynamic instability, so beyond the critical speed the amplitude grows and can destroy the structure within seconds. That is what makes it categorically different from a merely poorly damped mode, and why flutter clearance is demonstrated by careful expansion of the envelope rather than by exploration. A constant amplitude is the boundary case exactly at the flutter speed. Decay describes speeds below it. And it is self-excited, needing no continuing input.

Answer: B

Even a thin layer of filler or paint aft of the hinge moves the surface's centre of gravity aft and can reduce the flutter margin, which is why maintenance manuals specify balance checks after such work and set limits on permissible added mass. The aerodynamic derivatives are essentially unchanged by paint. Tab settings are checked separately as part of rigging. And the area change is negligible; it is the mass distribution that matters.

Answer: C

With hydraulic actuators carrying the load, an artificial feel unit provides the force the pilot feels, commonly scheduled with dynamic pressure so the aeroplane feels heavier as it goes faster. Requirements on force gradients and force per g still apply, since they exist for the pilot's benefit rather than as a property of the aerodynamics. And hinge moments still matter enormously, because they size the actuators and set the hydraulic system's power. That is why a surface carries several devices at once, each answering a different complaint.

Answer: D

It schedules with dynamic pressure, reproducing the behaviour a manual control would have had and giving the pilot a natural sense of speed through the control. Altitude alone is insufficient because dynamic pressure depends on speed too. Incidence scheduling is used in some feel systems as a supplementary input, particularly for stall warning through the stick, but it is not what q-feel names. Weight is not sensed by the feel unit.

Answer: A

The actuator must overcome the hinge moment at the worst combination of deflection, incidence and dynamic pressure, so hinge moment estimation sizes the jack and the hydraulic power. Stick force comes from the feel unit instead, which is the whole point of a powered system. Static margin is a whole-aeroplane property. Load factor limits come from the structure and the flight envelope, not from a hinge.

Answer: B

Both derivatives vary with incidence and with deflection, so a surface can be well behaved in the middle of the range and overbalanced at a large deflection combined with a high incidence. Since that corner is where a pilot might be recovering from an upset, it is exactly the case that must be cleared. Incidence certainly does affect hinge moments, through the local flow over the surface's nose. And certification covers the whole usable envelope.