INTRODUCTION
The gas-turbine engine driving a conventional or supersonic propeller appears to be an attractive propulsion system for certain classes of long-range airplanes designed to fly at moderately high subsonic speeds. In the aerodynamic design of such a turboprop air-plane, consideration must be given to the direct effects of the propellers on the airplane stability.
A propeller produces forces normal to the thrust axis whose verification with angle of attack is destabilizing for a conventional tractor airplane having the propellers ahead of the center of gravity. The magnitude of these propeller forces must be determined either by experiment or by a satisfactory theoretical method. The experimental normal force characteristics of propellers designed to operate at high section mach numbers and having large power absorption capabilities have been virtually nonexistent. Among the several existing methods of calculating propeller normal force, Ribmer's method (ref.1) has perhaps been most widely accepted. This acceptance has been fostered by the fact that it affords a solution which has, heretofore, been satisfactory and requires little more than a knowledge of blade shape. However, the method does not completely account for the no uniformity of the flow field due to the present of a nacelle, fuselage, or wing, nor does it adequately account for compressibility effects, particularly when the helical mach numbers are high.
The present investigation includes measurements of normal force for two, three-blade propellers of high solidity, differing only in thickness, at forward mach numbers up to 0.90 and a comparison of measured and calculated results. The theoretical method used for the calculations is developed herein and is based on the concept of oscillating aerodynamic forces associated with propeller blades operating in an inclined flow filed. The authors are indebted to Messer's. Vernon L. Rogallo and John L. Mc cloud III, of the Ames Aeronauticl Laboratory, for their contribution to the theory.