659. Weight Considerations in Dynamically Similar Model Rotor Design

Paper

Abstract

This paper presents a generalized discussion of scaling for dynamic similarity and of the influence of weight in the design consideration of dynamically similar model rotors, A brief review of some of the basic concepts of dynamic scaling is pre-sented showing how all of the parameters involved are considered, and how the problem is simplified by combining the many parameters into a lesser number of dimensionless ratios, An example of scaling one dimensionless ratio illustrates the mathematics involved, The results of scaling other ratios are shown to emphasize the importance of weight in the scaled relationships.
Testing in a medium heavier than air is shown to be a way of increasing the allowable weight while still maintaining dynamic similarity, This permits a less efficient structural design to obtain the required stiffness characteristics, and thus simplifies the design and/or fabrication requirements of the model, An example of a heavier-than-air test medium is Freon-12. Some of the physical properties of Freon-12 are presented along with a discussion of some of the advantages and disadvantages of model testing in Freon.
The effect of testingin Freon is emphasized by comparing the ratio, model value/full-scale value, of several familiar relationships(Locknumber,Reynoldsnumber,Froudenumber, stress level,etc,) in both Freonandair, The model values used in this comparison are for a one-quarter scale dynamically similar model of the U,S, Army AH-1G (HueyCobra)helicopter rotor.