by Rex Sydney
Few people — even in aviation — know what they are talking about when they discuss the modern aircraft engine. Here are some helpful facts.

Widespread and frequent criticism of American fighting planes has generated considerable heat from time to time since the war began — but not much light.

Recently, the Allison Division of General Motors Corporation, which produces the Allison liquid-cooled aircraft engine used in several types of fighter planes, decided that something should be done to furnish the missing illumination on the subject. Accordingly, it prepared a paper in clear non-technical language on some of the fundamentals of aircraft engine design and showed how they affect the performance of airplanes.

Herewith is a condensed version of the 48-page document:

An example of the inaccuracies which frequently reach the public with regard to aviation is the statement that more horsepower is required to fly at high altitudes. Actually the reverse is true because of fundamental physical laws which assert themselves as soon as the airplane leaves the surface of the earth. These same fundamentals, however, affect engine horsepower, resulting in a decrease in power which, unless something is done to counteract it, occurs more rapidly than the decrease in resistance to the plane flight.

Perhaps the most influential single factor among the fundamentals governing aircraft engine operation is that of the density of air. This decreases, of course, as the aircraft ascends and because the power developed by such engines is "directly proportional to the amount of air we can get into the cylinders" — it takes about 14 pounds of air mixed with a pound of gasoline for so-called complete combustion — airplane engines must be equipped with superchargers. These are simply centrifugal fan-type air pumps run at very high speed which are geared to force air into the engine cylinders constantly at sea-level pressure.

The Allison engineers pointed out that in addition to the fundamentals affecting aircraft engines and the flight characteristics of any given airplane, the special service which a military plane is intended to perform governs to a large extent the specifications upon which the plane is built. For example, the primary mission of the plane determines how much armor it will carry, how many guns, its cruising range, speed, climb and maneuverability.

"No single design can be superior in all the required qualities of the military airplane," the paper said, "and consequently the ship's design starts only after the military strategists have determined what particular type of military mission the airplane is to perform."

Because the factors affecting airplane performance at varying altitudes seemed to have been one of the principal stumbling blocks of persons honestly seeking to evaluate American combat planes in comparison to those of our allies and enemies, the Allison paper dealt particularly with the auxiliary engine equipment such as superchargers and characteristics involved in the altitude performance of planes.

Illustrating what happens to engine performance if the sea level weight of air is not supplied to the cylinders as the plane rises, the paper showed that the engine which delivers 1,000 hp at sea level will produce 35 hp at 50,000 feet.

As a result of the temperature change and the decrease in back pressure on exhaust fumes which come from increased altitude, the engine would produce more power if it could get air at sea-level pressure plus the proper proportion of fuel. That's where superchargers come in.

There are eight methods of supercharging which can be said to have important differences, and each of them has its particular advantages. The methods are:

  1. sea level;
  2. single-speed, single-stage;
  3. two-speed, single-stage, mechanical clutch;
  4. variable-speed, single-stage, hydraulic clutch;
  5. two-stage with mechanical clutch;
  6. two-stage with hydraulic clutch;
  7. two-stage with hydraulic clutch and intercooling, and
  8. turbosupercharger with intercooling.

All high horsepower aircraft engines, whether sea level or altitude, have superchargers. The sea level engine, however, has a supercharger with only sufficient capacity to provide enough air for the engine to develop the horsepower rating determined for it at sea level. While it involves the minimum sacrifice of weight and space, its use results in lower horsepower as the plane climbs above sea level.

The same supercharger as was used for the sea level engine can be speeded up to deliver sufficient air for an engine whose maximum horsepower is required at 17,500 feet altitude. However, to do so would take more horsepower than was permitted in determining the sea level rating and this must be deducted from the power that was previously available for driving the plane. On the other hand, the engine thus supercharged (single-speed, single-stage) would develop 1,600 to 1,700 hp if the throttle is opened fully at sea level, instead of the 1,000 hp for the sea level type of supercharger. Full use of that power at sea level might easily damage the engine. Furthermore, at sea level, the temperature of the air being fed into the engine would be increased by compression (due to supercharging) with resulting adverse effects.

Use of the single-speed, single-stage supercharger as outlined would result in loss of power for takeoff and for the first few thousand feet of climb. There would then be a gradual increase in the power above the net power permitted at sea level. This form of supercharger is the simplest, lightest and most compact form of supercharger. It can be effectively used for light airplanes in which loss of power at takeoff is not too important up to critical altitudes of approximately 20,000 feet.

The two-speed, single-stage supercharger with mechanical clutch may be driven at low speed at low levels so as not to create detrimental temperatures and use up excess horsepower and at high speed at high altitudes where the need for supercharging increases. The pilot manually changes the speed when he deems it advisable. It may also be automatically changed by aneroid control. This type gives an advantage at takeoff and early climb but is heavier and more complex.

By variations in supercharging, the same basic engine can be adapted to use in as many different plane models. A ground attack airplane, for instance, with no altitude performance, becomes the best highest altitude engine when fitted with turbosupercharger and intercoolers.

Some of the other types of superchargers include the two-stage supercharger with mechanical clutch — two separate superchargers with their airflow lines connected in series, one of which is positively connected to the engine and is always in operation, and the other one which can be put into operation by engaging the clutch — and the aforementioned turbosupercharger with intercooling.

The use of intercooling to lower the temperature of the air and air-fuel mixture is necessary because of the heat generated by the greater use of supercharging. A heat exchanger between the first and second stages of supercharging provides the "intercooling" and contributes to "volumetric and thermal efficiency" and permits the use of maximum compression ratios without danger of "detonation" or "knock."

Turbosupercharging is one of the outstanding developments of American aviation and dates back to 1917. Instead of being driven directly by power from the engine's crankshaft, the turbo drives the second-stage supercharger by the exhaust blast of the engine and thus does not subtract from the engine's power.

With the variety of methods of supercharging, a plane designer can fix the "critical altitude" — that altitude at which the plane attains its maximum speed in level flight — about where he desires. Thus, to say that any particular aircraft engine has a critical altitude of, say, 15,000 feet might be grossly inaccurate for, by varying its supercharging, that engine might be given a dozen different critical altitudes ranging anywhere between 1,000 feet and 30,000 feet.

While the different types of supercharging give maximum engine performance all the way from sea level to critical altitude, it should be remembered that the effect on the airplane performance of the additional equipment can more than offset the advantages because of weight or drag.

"It is therefore axiomatic that in planes of carefully balanced design," the paper said, "those planes that perform best at the high altitudes cannot compete in the lower altitudes with those equipped with the simpler lighter supercharging designed for low altitudes, or vice versa.

"For planes designed to operate predominantly in the relatively low altitudes the advantage of the simpler types of supercharging, with less weight and less bulk and fewer pieces to fit into the plane, becomes obvious. This weight and space can be used for guns, armor, ammunition or more gasoline."

The paper pointed out that one of the commonest errors in lay evaluation of American and European planes has been to compare the critical altitudes of the American craft with the "service ceilings" of the latest European models. Actually that is like comparing apples and pears, because the service ceiling of a plane is the highest altitude at which it is practical to fly that plane. Usually it is defined as the maximum altitude at which the plane still can maintain a rate of climb of 100 feet per minute. The true measure of superiority, therefore, should be the performance of a plane at any altitude, compared to that of the enemy's plane at the same altitude.

"Another common misconception is that the critical altitude is the highest point to which a plane can fly or at which it can effectively fight," the paper went on. "This, of course, is simply not true. A sea level high-horsepower engine will take a plane into the stratosphere if the plane is sufficiently light and has enough wing area. The point is that it will not take it there as fast as will a higher supercharged engine, nor can it operate the plane at as high speeds as can the higher supercharged engine when high altitudes are reached.

"As a matter of fact, the service ceilings of all American fighter planes, supercharged or unsupercharged, are surprisingly high over the critical altitude …"

Stressing the importance of the effect of the weight of a plane upon certain features of its performance — particularly takeoff, maneuverability and climb — the Allison engineers showed that a fighter plane weighing 5,000 pounds (roughly the weight of a Jap Zero) needs 500 feet in which to take off, whereas a plane of otherwise identical aerodynamics but weighing 8,000 pounds requires 1,275 feet for takeoff. Similarly, the 5,000-pound ship can turn in less than half the distance required by the 8,000-pounder at the same speed.

Using a hypothetical bombing attack to illustrate the effect of weight upon a fighter plane's rate of climb, the paper showed that if the 5,000-pound fighter intercepted an enemy bomber, flying at 25,000 feet altitude at a speed of 350 mph, 55 miles from the target city, a 6,000-pound fighter would intercept two minutes later when the bomber was only 43 miles from the city. The 8,000-pound fighter, however, would not make contact with the bomber until nine minutes later, at which time the bomber would be directly over the city and, because of the forward thrust imparted to the bombs by the speed of the plane, would already have released its deadly cargo.

Superficially, the Allison men wrote, it would appear that for the business of protecting a city from bombers the lightest fighting plane would win hands down.

"It can get off the ground much quicker, it can turn much quicker and it can get where it has to go much quicker. But the plane designer with that given amount of horsepower and supercharging to work with must remember something equally vital in protection against bombers:

"What is going to happen when the plane gets there? Is the fighter going to have sufficient armor to enable the pilot to face the bomber's murderous machine gun fire at sufficiently close range to get in effective shots of his own`?

"Is he going to have sufficiently large guns to really blast the bomber out of the sky?

"Is he able to carry sufficient ammunition to stay in there and fight until the job is done?

"Can he have leakproof tanks so that a well-directed shot from the bomber will not instantly make his fighter a flaming coffin, or reduce his gas supply to the point that he has to abandon the scrap and get to the ground?

"Can he carry enough gasoline to stay in the fight until he has accomplished his mission and still be able to get back home even though the chase has carried him a long way off?

"Can he have sufficient of the ingenious instrumentation devised by American engineers to enable him in split-second glances to know the things about his engine, plane and guns that supplement his natural and acquired skill in their greatest utilization in a hot fight?

"Is his plane sturdy enough to take some enemy hits, and undergo the strains of dives and sharp turns without falling apart?

"It becomes obvious that the lightest plane cannot have much of these advantages. It is equally obvious that the heaviest plane many times will 'get there' too late to prevent the bomber from carrying out its mission. Once more compromise becomes the effective answer. The 7,000-pound plane could carry approximately a ton more equipment and supplies for effective fighting than the 5,000-pound plane and still meet the bomber 25 miles from the city in our theoretical charted attack.

"Maneuverability and climbing ability are important, but they are by no means the whole story of successful air fighting."

In view of the effect of weight upon plane performance, it becomes clear that the plane designer is limited constantly as to the types of supercharging and other auxiliary equipment he may put in a fighter.

"Some of the more complicated types of supercharging equipment are so bulky with relation to the trim outlines of small fighter planes," wrote the Allison men, "that their installation would necessitate changing the shape of the fuselage, bringing up new problems in aerodynamics, and possibly resulting in loss or reduction of the original aerodynamic advantages of the particular plane.

"Sudden changes in the requirements for pursuit performance such as changing the high altitude fighter to a low altitude fighter, or vice versa, require not only airplane modifications but also delicate rebalancing of reduction gear ratio and propeller designs for optimum performance. Tuning the gear ratio and propeller to the particular job of the airplane is imperative. For instance, larger diameter, wider and heavier propeller blades are required for high altitude work than are most suitable for low altitude. These factors are of particular importance with regard to climb and maneuverability."

In summation, the Allison paper pointed out the global warfare involves a wide variety of climatic and tactical situations, as a result of which this country "dares not be without a complete line of planes that can meet these varying conditions. It must have the lowfighters for attack (in cooperation with ground troops, etc) as well as high fighters for defense and they must be able to operate in heat or cold, dust or damp."

In all those planes, it was explained the basic engines can be the same, although the complete power plants will vary because of the necessity of varying the auxiliaries, such as superchargers, in order to preserve the greatest possible amount of power up to the altitude at which maximum performance is desired.

"This country has fully developed the basic engines and the auxiliaries necessary to provide this power plant performance at whatever altitude and for whatever duty the Army or Navy specifies," the paper said, "and they are good the world around.

"Care must be taken in attempting to evaluate fighter aircraft to insure that the basis of comparison with enemy or allied craft is the same. No fair comparison can be made unless it is of planes designed for the same mission under the same world-wide conditions."

By the same token, it is unsound to compare the latest model planes, which the enemy happens to put into action, with America's last-year's models because of the likelihood that at that very moment, the US may have already in production a plane far superior to the enemy's but which is being kept secret for obvious military reasons.

This article was published in the March, 1943. issue of Flying magazine, vol 62, no 3, pp 30-32, 80, 82.
The original article includes photos of P-38 and P-51, and five charts and graphs.
Photos are not credited.