# AILERON REVERSAL:
Table of Contents:
- What Is Aileron Reversal?
- Aileron Reversal Definition
- How To Prevent Aileron Reversal?
What Is Aileron Reversal?
A number of aircraft, when flying near their maximum speed, are subject to an important aeroelastic phenomenon. No real structure is ideally rigid, and it has static and dynamic flexibility. Wings are usually produced from aerospace materials such as aluminum and composite materials and have structures which are flexible. This flexibility causes the wing to be unable to maintain its geometry and integrity, especially in high-speed flight operations. The phenomenon, which is referred to as aileron reversal, negatively influences the aileron effectiveness.
Aileron Reversal Definition:
Consider the right section of a flexible wing with a downward-deflected aileron to create a negative rolling moment. At subsonic speeds, the increment in aerodynamic load due to aileron deflection has a centroid somewhere near the middle of the wing chord. At supersonic speeds, the control load acts mainly on the deflected aileron itself, and hence has its centroid even further to the rear. If this load centroid is behind the elastic axis of the wing structure, then a nose-down twist (αtwist ) of the main wing surface (about the y-axis) results.
The purpose of this deflection was to raise the right wing section. However, the wing twist reduces the wing angle of attack, and leads to a reduction of the lift on the right section of the wing as shown in the below figure. In extreme cases, the down-lift due to aeroelastic twist will exceed the commanded up-lift, so the net effect is reversed. This change in the lift direction will consequently generate a positive rolling moment.
This undesired rolling moment implies that the aileron has lost its effectiveness and the roll control derivative ClδA has changed its sign. Such a phenomenon is referred to as aileron reversal. This phenomenon poses a significant constraint on the aileron design. In addition, the structural design of the wing must examine this aeroelasticity effect of the aileron deflection. The aileron reversal often occurs at high speeds. Most high-performance aircraft has an aileron reversal speed beyond which the ailerons lose their effectiveness. The F-14 fighter aircraft experiences aileron reversal at high speed.
(a) An ideal and desired aileron; (b) An aileron with aileron reversal
How To Prevent Aileron Reversal?
Clearly, such aileron reversal is not acceptable within the flight envelope, and must be considered during the design process. A number of solutions for this problem are as follows,
- Make the wing stiffer.
- Limit the range of aileron deflections at high speed.
- Employ two sets of ailerons – one set at the inboard wing section for high-speed flight and one ste at the outboard wing section for high-speed flight.
- Reduce the aileron chord.
- Use a spoiler for roll control.
- Move the ailerons toward the wing inboard section.
The transport aircraft Boing 747 has three different types of roll control device: inboard ailerons, outboard ailerons and spoilers. The outboard ailerons are disabled except in low-speed flights when the flaps are also deflected. Spoilers are essentially flat plates of about 10-15% chord located just ahead of the flaps. When the spoilers are raised, they cause a flow separation and local loss of lift. Thus, to avoid roll reversal within the operational flight envelope, the wing structure must be designed with sufficient stiffness.
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