Aero Engines - the Special Problem Altitude

AERO ENGINES - THE SPECIAL PROBLEM: ALTITUDE Effect on Engine and Air-screw (Air-screw termed pro peller in the U.S.).—Under comparable conditions the pressure developed in the cylinder of an internal combustion engine will be proportional to the weight, and therefore to the density, of the combustible mixture drawn in. At heights above ground level the atmospheric pressure falls off at a rate of roughly 'in. of mercury per i,000 feet. At the same time-the average tempera ture of the air becomes lower at a rate and in a manner which varies from day to day. The density of the air will therefore fall off somewhat less rapidly with height than the pressure, and the relation between density and height may be very variable.

It is now possible to measure the indicated power of an engine in flight, and experiments on these lines have shown that the indicated power is very nearly proportional to atmospheric density, provided a proper air-fuel ratio is maintained. This, however, is a matter of some difficulty. The suction produced in the choke tube of a carburetter—and hence the quantity of fuel normally supplied through the jets—will, for the same engine speed, be proportional to the square root of the air density. The weight of fuel supplied by a carburetter will therefore fall off less rapidly, as an aircraft rises, than the weight of air drawn in, and the mixture will become too rich. To compensate for this a "mixture control," operated either manually or automatically, must be fitted which reduces the normal flow of petrol at high altitudes.

Engine and Air-screw.

The useful performance of an air craft engine cannot be considered apart from its air-screw. These two must be regarded together as the power unit, for the power and efficiency of an engine may be largely thrown away if com bined with an inefficient air-screw. Since both indicated engine power and air-screw resistance are proportional to air density, at constant speed, it follows that they both fall off at the same rate as the aircraft climbs. The b.h.p. of the engine, friction losses being approximately constant, will fall slightly more rapidly; but the difference is not large, except at great heights, and since air-screw resistance is also proportional to the square of the revo lutions the difference between available engine torque and air screw resistance will produce only a slight drop in the speed of rotation. A fixed bladed air-screw, therefore, combined with a normal engine will, as the machine climbs, continue to work at nearly constant revolutions and efficiency. When a controllable pitch is used the engine speed may be maintained at any desired value.

Variation of b.h.p. with Altitude.

Although the indicated power falls off, the mechanical losses in the engine as a whole will not vary much as a machine rises, so that b.h.p. may be expected to fall off more rapidly than indicated horse-power and air density. To determine by direct observation just how fast the b.h.p. falls off with increasing altitude in any given case would be a research of the highest interest. Its accurate determination would involve measurement of the torque actually delivered to the air-screw. Several attempts have been made to design a torque meter for use in the air, but the mechanical difficulties are so great that so far these attempts at direct measurement have been only par tially successful.

The alternative is to deduce how the b.h.p. varies from observa tions of the performance of the aircraft. This again is a difficult calculation to make with certainty from figures for speed and rate of climb, since the necessary reduction from actual observa tions must rest upon an answer to the question we are asking : how the altitude has affected engine b.h.p. and air-screw efficiency. It is probably impossible, by tests in the air, to arrive at any satisfac tory general law for diminution of engine b.h.p. with height, ap plicable to all engines, for the simple reason that the mechanical efficiency, which must always vary from engine to engine, and may even vary from day to day with the same engine, has so large an effect on an aircraft's performance as to produce very discordant results. Take, for example, an engine giving Soo i.h.p. on the ground, and assume that the i.h.p. will fall off in proportion to air density and the friction losses remain nearly constant. If we calculate the b.h.p. at 2o,o00ft. where the i.h.p. is halved, this will be 200 or 175, a difference of 121%, according to whether the mechanical efficiency is 90% or 85% on the ground. Even i 5, variation of mechanical efficiency would have an appreciable effect on performance.

Examination of the results of a large number of aircraft per formance tests has led to the conclusion that the best approxima tion to a law of variation of b.h.p. with height is to take this as being a function of (pressure)i X (density) 3. For a full consid eration of this problem reference should be made to the Reports and Memoranda series of the Aeronautic Research Committee. Low pressure refrigerating engine test chambers at the Bureau of Standards in Washington have gone far to determine these unknown factors.

Maintenance of Ground Power at Altitude.

Since the power delivered by an engine will be roughly halved at a height of 2o,000ft., any plan for counteracting this effect of reduced air pressure holds out great opportunities for improved performance. But here the air-screw problem comes in. If the ground power of the engine should be delivered with the air-screw in low density air, then the engine will race away and burst, for unless the inclination of the air-screw blades can be altered the resistance to rotation must fall off with the air density. The usefulness of maintaining ground power at altitude is therefore dependent upon adapting the pitch of the air-screw blades so as to prevent ex cessive engine speed and at the same time retain a good air-screw efficiency. This problem of providing a variable pitch air-screw is one which in recent years has been extremely satisfactorily solved, notably in the instance of the Hamilton Standard "con stant speed" controllable pitch propeller. This and similar types of variable blade angle air-screws may be credited with more than a little of the percentage of the remarkably improved per formance of aircraft.

Supercharging for Maintaining Ground Power.—To maintain ground power, combustible mixture must be compressed so as to be delivered to the cylinders always at normal atmos pheric density whatever the height. This would mean, at 25,000ft., compressing in the ratio of 2.7 to I and then cooling through about 60° C. The necessary cooler involves additional head resistance and, if a piston compressor is used, the displacement will have to be about three times that of the main engine. All this means considerable additional weight, and every added pound is a dead loss at low altitudes and a serious handicap in getting off the ground. For this reason very high-speed rotary blowers offer the best chance of success. Such a blower might either be gear-driven from the engine shaft, or some of the energy of the ex haust gases might be used through the medium of an exhaust-gas turbine. The difficulty attending a blower, if gear-driven from the main shaft, is that, with the exceedingly high gear ratio necessary, about io to 1, the inertia of the blower rotor is enor mous, and unless some flexible drive is provided the gears will probably be stripped on the very sudden starting-up of the en gine. The gear-driven blower is employed not only to maintain ground pressure at altitude, but also to maintain pressure through out. With "ground boost," an engine of given size may be made to deliver an increased power output, but such an engine must be strengthened correspondingly and so is heavier than an engine which is supercharged only at altitude. The weight per horse power, however, of the engine with "ground boost" is no greater than that of the engine without this provision.

The exhaust turbine drive is very attractive, for, whereas with a gear-driven blower the entire horse-power needed for com pressing the air has to be supplied by the main engine, this type makes use of what would otherwise be largely wasted energy. The difficulties are chiefly those of providing turbine rotor blades which will stand the constant exposure to the hot exhaust gases and at these temperatures will retain a strength adequate for a speed of rotation of 25,000-30,000 revolutions per minute.