9

Aeroelasticity

125

Control Reversal

It is a loss due to the deformation of the main lifting surface, which is also considered

to be an expected response of a control surface. Generally, control reversal speeds

are derived analytically according to torsional divergence. In some aerodynamic

applications, control reversal can be used as an advantage and forms some part of the

Kaman Servo-Flap Rotor design. Servo flap is a small airfoil located at about 75%

span of the rotor blade, situated on the trailing edge of each rotor blade. Their function

is similar to that of an elevator on fixed-wing aeroplanes. The control reversal can

be used to aerodynamic advantage (Kana-Servo-flap rotator design).

This problem of control reversal with flexible wings of a high aspect ratio seems

to be an important area of study. With a high aspect ratio, the performance and flight

speeds tend to increase while compared with low aspect ratio wings. However, in

modern aircrafts with flexible wings at very high speeds, additional elastic deforma-

tions are introduced in the wing structure, which influences spanwise aerodynamic

load distribution [3]. This in turn affects the aerodynamic performance of the flier.

The study of control reversal and torsional divergence speed as discussed above is

helpful to design a prescribed flight envelope that helps for a safe flight with a guar-

antee. There are many engineering tools available to analyse control surface reversal

such as non-linear multi-body dynamic analysis, DYMORE, cross-sectional analysis,

two-cell analysis, VABS and two-dimensional aerodynamic coefficient analysis [3].

Dynamic Aeroelasticity

ThisisduetotheinteractionsbetweenallthethreeforcesshownintheCollarDiagram

(Fig. 9.1). The interaction between the inertial and elastic forces causes structural

vibrations. Typical examples of dynamic aeroelasticity are flutter and buffeting.

Flutter

It is a dynamic instability of an elastic structure in a fluid flow. This is caused by a

positive feedback mechanism between the deflection of the body and the fluid flow,

as it happens in the functioning of heart valves at low frequencies. Such flutter is

typically noticed in the functioning of heart valves in higher animals because of the

interaction of the blood flow with semi-elastic heart valves. The action of the heart

and its components are controlled by the Sino-Auricular node. Atria beat faster than

ventricles. Atrial flutter is a type of common abnormal heart rate rhythm similar to

atrial fibrillation. This is the most common abnormal heart rhythm. It is a kind of rapid

heartbeat supra-ventricular tachycardia. The flutter can be explained through a linear

oscillator system executing Simple Harmonic Motion (SHM) and can be thought of