Correlation of Propeller and Engine Power with Supercharging 330005
THE primary purpose of this paper is to discuss some of the most pressing problems involved in choosing the propeller that is most suitable for use on a particular airplane. Propeller design is not dealt with, the discussion being limited to the selection of metal propellers of established design. Questions of noise, efficiency and diameter limitation are merely mentioned, and the emphasis is placed upon the choosing of propellers which will transmit the most engine power for the most needed condition of airplane performance; maximum and cruising speeds at altitude, or take-off and climb. Airplane performance enters only inasmuch as it is used to illustrate a case of power absorption.
The proper choice of a propeller is becoming increasingly difficult to determine because of the current design trends of both airplanes and engines. Especially important is the fact that many of the supercharged engines now in use cannot be operated at full throttle below their critical altitudes. To discuss the selection of propellers for these engines, it has been necessary to explain at some length the restrictions placed by the engine manufacturer upon their operation.
The methods of analysis used are short-cut approximations. In most cases the logic of their selection and the extent of their accuracy are evaluated. The validity of the propeller-load curve is discussed in detail.
As illustrations of the methods, two problems are investigated. By analytical processes, an estimate is made of the loss in engine power resulting with a fixed-pitch propeller, from limiting the brake mean effective pressure of a supercharged engine at sea level to the same value as held at critical altitude. The other problem is discussed graphically. It is an estimate of the engine powers and cruising speeds obtained at five altitudes with seven different propellers on an unsupercharged engine.
It was found that one of the propellers yielded an increase in cruising speed of 8 per cent over the best obtainable from a propeller chosen in the normal manner. The disadvantages resulting from the use of such a propeller are also discussed, and it is shown how many of these can be overcome by a propeller the pitch of which can be adjusted in flight to the definite values most needed for certain performance characteristics. Much of the analysis is applicable to the study of multiple-pitch propellers.
In conclusion, short-cut methods are outlined for estimating the engine power available for take-off and climb with different propellers. A new chart, Fig. 10, is given for estimating the change in propeller speed which results from a change in airplane speed. This curve may be used when there are changes in density altitude and engine brake mean effective pressure.