Advances in airplane performance during the last few years may be ascribed mainly to advances in aerodynamics and to improvements in powerplants. The latter have resulted in producing more power for the same weight of engine and smaller over-all dimensions for engines of the same power-rating. The accompanying paper describes two engines of 500 and 800 hp. respectively that have been recently developed by the Packard Motor Car Co. for aircraft service. When these engines are compared with previous types they are found to be more compact and to produce more power per pound of weight. When each is operated at its rated speed, the Model 1500 engine develops 100 hp. more than the Liberty while weighing 140 lb. less, and the Model 2500 engine develops 250 hp. more than its predecessor, the Model 2025, with a decrease in weight of 75 lb.
In applying these facts to commercial aviation, these comparative performances mean that the new engines can carry double the pay-load over the same distance or the same pay-load 2¼ times as far as could their progenitors.
These improvements have been made possible largely because of a new type of cylinder construction, original studies with regard to the loads that can be carried by bearings, reduction of the weight of the crankshaft while at the same time strengthening it, and the compacting and lightening of the timing and accessory-drive layout. Other improvements were made in the lubricating system and in the design of the valve-gear and springs.
Nothing was taken for granted and every detail was given the closest scrutiny. It was decided that if a distinct advance in the art was to be made, it could be accomplished only by disregarding precedent and starting at the ground. As a result of investigating the relation of bearing materials to allowable speeds and loads, it was ascertained that failures of aircraft bearings rarely occur because of lack of lubrication or of wear but are caused by fatigue of the babbitt lining produced by minute flexing of the back of the bearing. Tests showed that the limitations of the bearings could be raised provided they could be prevented from flexing under load and ample force-feed lubrication were provided. The PV values of the bearing loads adopted, as compared with those of the Liberty engine, are: for the crankpin, 18,520 lb. per sq. in. as against 13,200; for the center bearing, 35,000 as against 22,650; and for the intermediate bearing, 27,000 as against 14,000.
The critical speed of vibration of the Packard crankshaft is 64 per cent higher than that of the Liberty; it is also twice as stiff as well as weighing 30 per cent less, a feature accomplished by the use of journals having comparatively large outside diameters but bored out through their centers.
Novel cylinder construction enables the cylinders to be spaced closely together and the weight of the whole engine to be diminished. Other advantages incorporated into the design include water circulation in close contact with the heated surfaces, the use of a steel cylinder-barrel as a wearing surface that carries the explosion loads down to the crankcase, the locating of the hold-down flange some distance from the end of the cylinder barrel so that the ends of the barrels of the cylinders of the two banks can practically be allowed to touch inside the crankcase and the engine can be run in an inverted position.
Still other features comprise improved types of valve-housing and valve-gear layout; positive cooling of the exhaust-valve by oil pumped through it; a special type of multiple-cluster small-diameter piano-wire valve-spring; simplicity in the grouping of the accessories; a special type of magneto having a single magnetic circuit and two independent electrical circuits, either one of which will fire all 12 cylinders; the possibility of replacing magneto ignition with battery ignition by substituting a generator for the magneto but without other change to the engine or to the wiring between the distributors and the spark-plugs; the use of very short comparatively light rugged slipper-type pistons; and the ability to use either direct drive or gear reductions.
As each improvement in engine design means an immediate improvement in airplane design, recent developments in engine design have already made possible maneuvers that would have been impossible a few years ago. Although experimental work is continually being directed along conventional lines, such as the
barrel and cam types of engine and those of the Diesel or the semi-Diesel type, the most important advances, in the opinion of the author, are to be made by conventional 12-cylinder water-cooled engines and by 9-cylinder fixed-radial air-cooled engines.
When the weight of airplane engines is reduced to 1 lb. per hp., as seems likely in the near future, the engine will consume its weight of fuel every 2 hr. It is important, therefore, that, as the carrying capacity of the airplane is reduced by the amount of fuel that must be carried, efforts to reduce the fuel consumption should run concurrently with those to improve the engine.