OTHER FLUTTER PROBLEMS

In the flutter problems discussed so far, the chordwise deformation (distortion of the airfoil cross section as distinguished from its rigid body motion) is of minor importance, and can be neglected entirely except for wings of small aspect ratio. However, there is another type of flutter in

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Подпись: Fig. 7.1. Deformation of a two-dimensional panel.

which the chordwise deformation is of paramount importance. A typical example is a flat plate which spans two rigidly supported edges, as shown in Fig. 7.1. Let the air flow over one side of the plate and remain stagnant on the other side. In a supersonic flow, a type of self-excited oscillation may occur in certain ranges of critical dynamic pressure, whose value depends on the initial curvature and the stiffness of the plate, the ratio of the density of air to that of the plate, the dimensions of the plate, and the thrust exerted by the supports at the edges of the plate. This is called panel flutter.

Aircraft wings of high-speed aircraft are often of multi-spar multi-rib construction. The spars and ribs support the skin against flexural deflec­tion. In some cases the resistance of the spars and ribs to displacements in the plane of the skin is small, then panel flutter can be treated by a line­arized theory.7,120"7,122 In other cases the spars and ribs are of such rigid construction that they resist displacements in the plane of the skin; then the thrust offered by the spars and ribs is a nonlinear function of the deflection of the skin. This nonlinearity may induce a “relaxation” type
of oscillation which is associated with the “oil canning” or the Durchschlag of the skin.7-119

One of the practical causes of panel flutter is the thermal stress induced in the skin due to aerodynamic heating in flight at high speed. If the skin is hotter than the supporting structures, compressive stress may be induced in the skin. If the temperature difference between the supporting structures and the skin is sufficiently large, the skin may become buckled (one can easily verify that often it takes only a few degrees of temperature difference to buckle a plate). A buckled skin has a much lower critical dynamic pressure than an unbuckled one.

The most practical method of preventing panel flutter is to introduce tension into the skin, for example, by internally pressurizing the wing or the fuselage.

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