NACA Report No. 101

NACA Report No. 101 - (The Calculated Performance of Airplanes Equipped with Supercharged Engines) was issued by the United States National Advisory Committee for Aeronautics in 1921. It covers the theoretical discussion of the performance of an airplane as affected by the use of a supercharging engine, and includes a thorough discussion of the respective merits of different supercharger types.

Summary
The power developed by an aircraft engine under any given external conditions can be computed approximately if the normal power at the given speed is multiplied by appropriate temperature and pressure correction factors. The temperature correction factor is given by an equation taken from NACA Report No. 45. When the intake and exhaust pressures are equal, it is best to use an equation based on the work of NACA Report No. 46 for the pressure correction factor. For unequal intake and exhaust pressures, the correction factor for a small range of values may be taken from a graph in Report No. 45. The temperature rise in the compressor, which has an important part in determining the power output of the engine, can be computed when the pressure ratio, the shaft efficiency, and the heat radiated per pound of air by the compressor and discharge pipe are known. For typical conditions, with the compressor exposed to the full force of the propeller slipstream, the computed value of the ratio of the actual temperature rise to the theoretical rise without heat radiation is 0.864.

The efficiency of the compressor and the power which it absorbs depend on the quantity of air handled per unit time. It therefore becomes necessary to discuss the variation of the volumetric efficiency of the engine with the intake temperature and the exhaust back pressure. It is presumed that the compressor is designed for operation at a certain normal altitude and normal speed. The calculation of the net horsepower available at the propeller under these normal conditions is particularly simple. In a numerical example it is assumed that the Liberty engine is fitted with a gear-driven compressor designed to furnish sea-level carburetor pressure at I8,000 feet altitude and an engine speed of 1,700 revolutions per minute. The shaft efficiency of the compressor is assumed to be 64%. The computed horsepower is 371. In calculating the power of an engine equipped with a turbine-driven compressor, it is assumed that the back pressure created by the turbine is equal to the increase in the carburetor pressure produced by the blower. The computed power to be expected from a Liberty enbtie fitted with a turbine-driven supercharger under the conditions of the preceding problem is 394.

In laying out performance curves showing the power to be expected from an_engine-compressor unit at various speeds and altitudes, the variation in the efficiency of the compressor should be taken into account. The computation is somewhat involved, but can be carried through graphically.

This report is the outgrowth of a set of calculations made during the war on the probable performance characteristics of an airplane whose engine is equipped with a supercharging compressor of the gear-driven type. The discussion is here extended to the case of the turbine-driven type of compressor on the basis of the rough empirical ruIe that the exhaust back pressure created by the turbine is equal to the rise in the intake pressure due to the compressor. The purpose of the report is twofold. It aims, in the first place, to outline method of predicting the probable performance curves of an airplane fitted with a supercharging centrifugal compressor, and in the second place to apply this method to the case of a typical modern airplane in order to determine, as nearly as possible with the somewhat meager data now available, the gains which the use of a supercharger may be expected to bring in the near future.

Conclusions
If the heat leak from the gas turbine and exhaust pipes to the water jackets is prevented, and if the cooling system is kept under a constant pressure independent of that of the atmosphere, no additional radiator equipment should be required when a supercharging compressor is fitted to an airplane engine.

The total additional weight of the propelling plant due to the use of a supercharger is estimated at about 120 pounds.

A method of estimating airplane performance at altitudes with the aid of curves for the reduced thrust horsepower available and required, is developed. This method simplifies the graphs of the thrust horsepower required at altitudes, and is particularly useful in comparing the performance of planes of different sizes, wing loadings, and propelling plant characteristics, which have the same lift and drag coefficients.