Aeroplane Construction and Operation

by John B. Rathbun

While the weight of the radiator, water and piping increase the weight of the water-cooled motor very considerably, the total weight is not excessive. When the fuel is considered, the total weight is below that of the rotary when long flights are attempted. The radiator and water add complication and are a source of danger. The radiators increase the head resistance and add very considerably to the maintenance cost.

A Motor Installation in a Pusher Type Biplane, Showing the Motor at the Rear and the Double Radiator Sections Over the Body.

Each type of cooling has its limitations, and it is hoped that an improvement in cooling may be had in the near future. This system should primarily reduce the size and resistance of the power plant, and if possible the weight, although the latter is a secondary consideration. At present the cooling system prevents even an approach to the true streamline form of the body.

Propeller Speed. For the best results, the propeller speed should not exceed 1200 revolutions per minute, and for structural reasons this is generally limited to 1500 R. P. M. This at once puts a limiting value on the output of a given size engine unless a gear down arrangement is used. It should be understood, between certain limits, that the power output increases roughly as the speed. With direct drive arrangements in which the propeller is mounted directly on the end of the engine shaft, the motor revs. are necessarily the propeller revs., and the only way of increasing the speed is by increasing the length of the stroke or by gearing down. An increase in stroke adds rapidly to the weight by increasing the cylinder length, length of connecting rod, length of crank throws, etc.

Horsepower Rating. At present there are many methods of calculating the horsepower of gasoline engines. Formula applying to auto or boat motors does not apply to flight conditions, for the aero motor is essentially a high compression type and has a greater output per unit of displacement. It is not practical to give the rated horsepower as the maximum output possible under ideal conditions, for this would give no idea as to the practical capabilities except by long tedious calculation. The brake horsepower would give no overload capacity at a fixed propeller speed, and the conditions are entirely different from those regulating the rating of auto motors. The latter can be forced up to the wrecking speed, or many times the normal automobile speed of 30 miles per hour.

As aero engines are generally well kept up, and well tuned at all times, the rated horsepower may be taken from 15 to 20 per cent below that of the maximum brake horsepower. In geared-down motors, the gear efficiency is still to be considered. The question of the quality of the mixture, and barometric pressure, also enter into the problem whether the power is rated on the maximum obtained with a rich mixture, or is calculated from the output at the maximum efficiency. A writer in "Aviation" suggests that the rated horsepower be taken as 95 per cent of the power developed at a point midway between the maximum output, and the output at the greatest efficiency. Barometric pressure to be 30 inches and the revolutions 1200.

Owing to the great diversity in the bore-stroke ratio, a power formula must include the bore and stroke. This makes the S.A.E. formula for auto motors impossible. A formula is proposed by a writer in "Aviation." The writer has checked this up with the published performance of several well-known aeronautical motors.

H = B²SNR/12,500 Where B = bore in inches, S = stroke in inches, N = number of cylinders, R = Crankshaft revolutions per minute, and H = rated horsepower. This applies only to the four-stroke cycle type.

Power and Altitude. The power drops off rapidly with an increase in altitude unless corrections are made for compression and mixture. With constant volume, the decreased density causes decreased compression. As the weight of air taken in per stroke is reduced, this also reduces the amount of fuel that can be burned per stroke. By holding the compression constant through adjustment of the clearance or valve motion, a fairly constant output can be had through a wide range of altitudes.