CRUISING FLIGHT OF LIGHT AIRCRAFT AND MOTORGLIDERS

Abstract

After recalling the traditional cruising conditions for maximum endurance and range, the concept of cruise for maximum average-speed/fuel-consumption (V/f) is introduced. A different type of cruise, altemating climbs at full power with glides with engine off or at low engine rating, is then taken into consideration. A simple mathematical model, assuming constant engine specific fuel consumption and propeller efficiency, yields simple mathematical equations. Numerical solutions for minimum f and maximum V/f show that a substantial gain in performance is obtained whenever the aircraft aerodynamic polar for the gliding configuration is better than for the climbing configuration. This is typically the case for the motorgliders retracting the engine and the propeller during the glide. Preliminary evaluations show that a sensible advantage should be obtained also in the case of aircraft having the same aerodynamic polar in both climbing and gliding configurations, provided that the variation of engine specific fuel consumption and propeller efficiency are accounted for.