Aerodynamic Flutter on Aircraft Control Surfaces: Testing and Measurement Analysis

Aerodynamic Flutter on Aircraft Control Surfaces: Testing and Measurement Analysis

Flutter is of great concern to any pilot, since excessive flutter has caused a number of aircraft to lose control and crash. Although any surface on an aircraft which is exposed to air flow can experience aerodynamic flutter, the most common type of flutter involves the control surfaces such as ailerons, elevators, and rudders. The mass properties of these control surfaces are critical and have to be measured with great care to make certain that flutter is minimized. Many mass properties engineers ignore product of inertia when measuring control surfaces. We suspect that these engineers will be surprised to discover that the product of inertia unbalance of the control surface can be the key element in understanding and eliminating aerodynamic flutter, and that it is vital to measure this quantity.

Control Surface CG

If the center of gravity of a control surface assembly is not located exactly on the hinge line, then a torque will be applied to the control surface whenever the wing (or other mounting surface) accelerates in a vertical direction. If the inertial forces acting through the CG of a control surface amplify the vibration of a wing, then the flutter amplitude will rapidly increase and can result in destruction of the aircraft. Small changes in control surface CG location can have a dramatic effect on flutter instability.

Control Surface Moment of Inertia

The moment of inertia of the control surface is also a critical parameter. If the wing (or other mounting surface) accelerates in a rotational sense due to twist, the position of the control surface will lag behind the wing due to its moment of inertia. In other words, the orientation of the control surface will change relative to the orientation of the wing during wing twist, even if the control surface is statically balanced about the hinge line. Generally, it is necessary for control surface MOI to fall within a narrow range of values, in order to avoid flutter problems. It is not sufficient to simply minimize control surface MOI, since it is important to avoid mechanical resonant modes whose frequencies are harmonics of each other. Note: It is possible to neutralize this MOI effect by moving the CG ahead of the hingeline. There is also another consideration involving moment of inertia: the frequency of the notch filter in the actuator circuitry which drives the control surface is based on the particular moment of inertia being driven. If this moment of inertia deviates significantly from the nominal value, the control circuit can become unstable.

Control Surface Product of Inertia

When a wing suddenly bends upward as the result of a gust of wind, mass on the outer end of the wing will experience a greater acceleration than mass near the fuselage. An aileron can be balanced statically but have a concentration of mass near the trailing edge on the outboard end and a corresponding concentration of mass near the leading edge on the inboard end. This will create an unstable condition when dynamic forces are applied.

POI can also aggravate twist of the airfoil. Although the control surface does not rotate, its rapid vibration creates inertial forces that can distort the shape of the wing.

Summary

Performance requirements for aircraft and other aerodynamically controlled products have become ever more demanding with less and less margin for error. Materials, especially composites, have become more complex and are inherently less homogeneous than the materials previously used. This has led to a situation where the mass properties of control surfaces may not always be within the test tolerances required for safe and stable aircraft control over the full range of anticipated operating conditions. There is a growing trend among experienced aerospace and mass properties engineers to require the measurement of the mass properties of aircraft control surfaces to ensure that they meet prescribed tolerances in a flutter analysis.

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