Aerodynamics PDF - FIS Book 1

FIS Book 1: Aerodynamics 198 Additional effects due to aero-elasticity and compressibility may be present and will modify the roll response. 35. Effect of Altitude. In a steady state of roll the rolling force and the damping in roll force are balanced. The rollin...

FIS Book 1: Aerodynamics 198 Additional effects due to aero-elasticity and compressibility may be present and will modify the roll response. 35. Effect of Altitude. In a steady state of roll the rolling force and the damping in roll force are balanced. The rolling force is caused by the change in lift due to aileron deflection and is proportional to the amount of aileron deflection and to EAS. The damping force Fig 12-6: Damping in Roll Effect On Down going Wing is due to the change in lift caused by the increase in angle of attack of the downgoing wing and the decrease in angle of attack of the upgoing wing. The value of the damping angle of attack can be found by the vector addition of the TAS and the rolling velocity as illustrated in Fig 12-6. It can be seen that for a constant damping angle of attack, the rolling force, and therefore the rate of roll, will increase in direct proportion to TAS. When an aircraft is climbed at a constant EAS the rate of roll for a given aileron deflection therefore increases because TAS increases. 36. Effect of Forward Speed. It has been shown that the rolling force is proportional to the amount of aileron deflection and to EAS. It follows that rate of roll increases in proportion to EAS. Aero-Elastic Distortion 37. Aileron Reversal. Ailerons are located towards the wing tips by the necessity in most aircraft to utilize trailing edge flaps for landing and take-off. This may cause the wing to twist when the ailerons are deflected and lead to an effect known as “aileron reversal”. It must be recognized that aero-elastic distortion of the airframe may affect stability and control in pitch and yaw as well as in roll. As the wings are usually the least rigid part of the airframe, aileron reversal assumes importance as it reduces the ultimate rate of roll available at high forward speeds. 38. In Fig 12-7 it can be seen that downward deflection of the aileron produces a twisting moment about the torsional axis of the wing. The torsional rigidity of the wing depends on the wing structure but will normally be strong enough to prevent any distortion at low speeds. Aileron power, however, increases as the square of the forward speed, whereas the torsional stiffness in the wing structure is constant with speed. At high speeds therefore, Fig 12-7: Aileron Reversal the twisting moment due to aileron deflection