AEROELASTIC EFFECTS

The requirement for structural stiffness and rigidity is the consideration given to the inter­action of aerodynamic forces and deflections of the structure. The aircraft and its components must have sufficient stiffness to prevent or minimize aeroelastic influences in the normal flight range. Aileron reversal, divergence, flutter, and vibration should not occur in the range of flight speeds which will be normal operation for the aircraft.

It is important to distinguish between strength and stiffness. Strength is simply the resistance to load while stiffness is the resist­ance to deflection or deformation. While strength and stiffness are related, it is necessary to appreciate that adequate structural strength does not automatically provide adequate stiff­ness. Thus, special consideration is necessary to provide the structural components with specific stiffness characteristics to prevent un­desirable aeroelastic effects during normal operation.

An obvious solution to the apparent prob­lems of static strength, fatigue strength, stiffness and rigidity would be to build the airplane like a product of an anvil works, capable of withstanding all conceivable loads. However, high performance airplane con­figurations cannot be developed with ineffi­cient, lowly stressed structures. The effect of additional weight is best illustrated by pre­liminary design studies of a very long range, high altitude bomber. In the preliminary phases of design, each additional pound of any weight would necessitate a 25-pound increase in gross weight to maintain the same performance. An increase in the weight of any item produced a chain reaction—more fuel, larger tanks, bigger engines, more fuel, heavier landing gear, more fuel, etc. In the competitive sense of design, no additional structural weight can be tolerated to provide more strength than is specified as necessary for the design mission requirement.