Reply to Discussions of the Wright Brothers Lecture

Abstract

A NUMBER OF THE POINTS raised in the discussion are concerned with the inherent restrictions on the operation of turbine-powered aircraft. As Mr. Kelly comments, if for any reason it becomes necessary to depart from the flight plan, there is, by comparison with piston-engined aeroplanes, much reduced flexibility of choice of operating speed and altitude. Mr. Hazen points out that some alleviation of this handicap is possible on a multiengined aeroplane by shutting down engines and, that with a coupled installation, this can be done without stopping propellers; so the need for an apology to the passengers would not arise. For an aeroplane with eight engines, for each engine shut down the best altitude falls by about 4,000 ft. at the expense of about 25 m.p.h. in cruising speed and an increase of about 4 per cent of fuel consumed. In any case, reserves must be carried to provide for the possibility of inadvertent engine failure. Whereas at first sight the procedure of selecting the best combination may appear to place an added burden on the flight crew, if ambient conditions are known, there is no ambiguity as to the right course of action. The argument Mr. Kelly used in his comparison of the net performance of aeroplanes powered with piston, turboprop, and turbojet engines is appropriate and can be extended to the case of greater ranges. I t emphasises the danger of too general a conclusion and the increasing need for tailoring an aeroplane for specific routes. For example, if intermediate stops are compulsory for refuelling, then, in addition to the effect on block speed, time is lost on the ground and the cruising speed at altitude is a quite unreliable measure for journey time. Also, the daily and seasonal variation of wind speeds, known to increase in velocity with altitude, may in some cases disproportionately penalise turbine aircraft with fuel reserves. The less flexible aeroplane suffers by its reduced ability to overcome adverse atmospheric conditions. In making a case for the use of turboprops for shortrange medium-altitude operation, Mr. Fink calls for a low-compression engine. I t appears likely that, in selecting the best engine characteristics, the value of the compression ratio should bear some direct relationship