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FLUTTER PREVENTION OR FLUTTER CONTROL
The following measures may be used to secure stability in the design airspeed range:
1. Provide sufficient stiffness, so that the critical speeds of aeroelastic instabilities are inherently high.
2. Furnish good aerodynamic design, so that the flow remains unseparated in service conditions. If, on the other hand, the aerodynamic force is undesirable, as in a suspension bridge, attempt should be made to render the structure aerodynamically ineffective, to reduce the lift and drag. Drag reduction is especially beneficial in the case of stall flutter.
3. Break the inertia and aerodynamic couplings:
(a) By a suitable arrangement of mass and elasticity distribution so that the elastic axis, the inertia axis, and the line of aerodynamic centers are as close to each other as possible.
(b) By addition of masses to achieve dynamic mass balancing.
(c) By arrangement of mass and elasticity so that the lower modes of free oscillation of the structure do not have a nodal line close to the 3/4-chord point.
4. Provide servomechanisms to control the phase relationship between various components of motion.
To put any of these measures on a quantitative basis, a detailed analysis of the special type of structure under consideration is necessary.
A successful aeroelastic design secures stability without adding much material to the structure in excess of what is required to carry the live and
dead loads for which the structure is intended. Proper mass distribution is of supreme importance. For example, in a particular transport design, it is found that the mass of the fuel in the outer panel of the wing (near the wing tip) has a very strong destabilizing effect; and a more economical design results if the fuselage or the inner panel of the wing is made larger to carry the excess fuel so that the wing tip region will be relieved of heavy masses. Considerations of this nature indicate clearly that a comprehensive flutter analysis which is made at the early stages of an airplane design, and which takes into account a wide range of variations of structural parameters, can be of vital importance.
Aircraft design has advanced to a stage where the safeguarding against aeroelastic instabilities demands as much attention as providing sufficient strength for flight and ground loads. One wants an airplane of minimum weight that has structural integrity for prescribed design requirements (loads, geometry, etc.). Although in the early stages of avidtion history the structural integrity against flutter could be achieved by minor changes in the design, with little cost in weight, the new trend toward optimum design of high-performance airplanes creates a very different picture.
We have seen that, other things remaining equal, the rigidity of the structure is the ultimate safeguard against flutter. Although the rigidity depends on the manner in which the structure is fabricated, the ultimate limitation always lies with the materials of construction.
In comparing different materials for aircraft construction, an interesting criterion is the speed of propagation of sound in the material. To realize this let us consider two airplanes identical in geometry and construction, but differing in material. Let the density of the two materials be a and a’ and their Young’s moduli be E and E’, respectively. For dynamic similarity the dimensionless parameters alp, Ef(pU2) must have the same values for both airplanes. Hence,
In other words, the higher the Eja ratio of a material, the higher will be the critical speed. Since the sound speed in a material is proportional to VHfo, we may say that the critical speed is directly proportional to the speed of sound in the material of construction.
The speeds of sound VE/a at room temperature in several materials are approximately as given in Table 7.1. The speeds of sound in most
structural metals and wood are surprisingly close to each other. Plastics have lower E/a ratios.
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Table 7.1
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