Stress Measurement
A New British Technique for Determining Stresses in Airscrew Blades
By K. R. Boydell, B.Sc., Tech., Vibration Engineer, Rotol Airscrews, Ltd.

In the design of any engineering structure it is essential that the stresses in its various members are known if there is to be any assurance as to its safety. These stresses are often calculated, but in many cases the problem is far too complex for such treatment, and it is easier and quicker to carry out an experimental determination rather than attempt an involved computation.

In the particular case of airscrew blades, the stresses arising from centrifugal and aerodynamic loads could, even in the early days of the development of metal blades, be calculated with an accuracy sufficient for all practical purposes. The stresses arising from vibration, however, were indeterminable, although many unsuccessful attempts were made to design blades whose vibration stresses would be low throughout the engine-running range.

It should be noted that there was no difficulty in this respect with the design of wooden blades, as these are not susceptible to vibration, their damping qualities being greatly superior to those of metal blades, and, consequently, more energy is required to cause excessive vibration.

At first this difficulty with metal blades appeared insurmountable. Calculation had proved to be inadequate, and so attention was focused on the alternative method, that of direct measurement of these unknown stresses, and, working along these lines, a solution was finally found.