Zero-thrust glide testing for drag and propulsive efficiency of propeller aircraft

Abstract

The authors have developed a new method, zero-thrust glide testing, that provides the first really practical, accurate, and inexpensive method for measuring the drag of propeller aircraft in gliding flight. Zero-thrust conditions were sensed on a classic Luscombe 8£ by monitoring the fore and aft clearance motion of the engine crankshaft as the engine rpm or power was changed. With zero engine thrust conditions, gliding flight measurements were made to find the drag polar and propeller airfoil zero-lift angles. The glide polar and level flight measurements have been used to find the propulsive efficiency. The airfoil zero-lift angles were used to accurately calculate the propeller thrust, and therefore, the drag in powered level flight. Drag in level flight cruise is approximately 30% larger than in glide, resulting in a propulsive efficiency equal to 0.62. Propeller efficiency has been calculated using standard methods corrected for the interference between the propeller and the airframe flowfields. The interference changed the propeller thrust, created an inviscid buoyancy drag on the fuselage, and generated viscous-related drag forces. The viscous forces and engine cooling duct losses are the cause for the poor propulsive efficiency and are believed to be typical of many propeller-driven aircraft.