WHILE virtually all aircraft builders agree in placing reliability as the most important factor in gaging engine performance, from there on agreement is lacking. The author believes that all factors exclusive of reliability can be evaluated so as to provide a good basis for choosing an engine. These factors include durability, which despite the opinion of some aeronautic engineers is not synonymous with reliability; weight per horsepower of the complete powerplant, including radiators and cowling; head resistance; fuel consumption; and first cost.
The effect of changes in engine weight on operating cost are discussed, the text being supplemented by tables showing the effect of increased engine-weight, operating cost and the operation-expense items that are affected. The effect of increasing the engine weight on the durability is gone into, and the statement is made that increasing the weight of a 200-hp. engine from 400 to 494 lb. would be justified provided increases in the time between partial and major overhauls and the total life could be correspondingly increased.
In discussing engine resistance, the author makes the point that the engine manufacturers should be in an excellent position to develop the cowling and select the propeller, and this change would effect a saving to the industry as a whole, since cowling was designed primarily for the engine rather than for the airplane.
In the discussion* following the paper one speaker points out that the mean effective pressures will be increased rather than decreased and at the same time the interval between overhauls has been lengthened. The effect of accessibility on the overhaul cost is mentioned by another speaker, while two take exception respectively to the savings claimed by the author and the pay-load that he assumes can be carried. The designing of the cowling is discussed, one speaker arguing that an engine manufacturer would find difficulties in going into production on a cowl that would fit various types of airplane, while another feels that the problem would resolve itself into how far back the engine builder could carry his cowling or how far forward the airplane designer should carry his.