Aeroplane Construction and Operation

by John B. Rathbun

The 26 x 4 tires are used on the majority of training machines of the two-wheel type, while a 20 x 4 wheel is used for the front wheel of the three-wheel trainer. Two larger sizes, 30 x 4 and 34 x 4, have also been used to some extent, particularly on the Ackerman spring wheels.

CHAPTER XIV. ESTIMATION OF WEIGHT.

Effect of Weight. Weight is an all important consideration and is most difficult to estimate unless one has accurate data on existing machines of the same type. The total weight in flying order depends upon the useful load to be carried, and upon the weight of the power plant. The weight of the latter varies both with the useful load and with the speed, climb, and duration of flight. The type of aeroplane determines the relative head resistance which again reflects back to the weight of the power plant.

The only reason for the existence of an aeroplane is to carry a certain useful load for a given distance, and this useful load is the basis of our weight calculations. The basic useful load consists of the passengers and cargo, although in some specifications the live load may be construed as including the weight of the fuel, oil and instruments, and in the case of military aeroplanes, the weight of the armament, armor, ammunition, wireless and cameras. For comparison, the elements constituting the live load should always be specified.

For a given horsepower, speed and climb, it is obvious that the dead or structural weight should be at a minimum for a maximum live load capacity. The dead load carried in present aeroplanes will be undoubtedly reduced in the future by the adoption of lighter and stronger materials, better methods of bracing, and by reductions in the weight of the power plant. Just as the automobile industry developed light and powerful materials of construction, so will the aeroplane designer develop more suitable materials for the aircraft. While the present power plant has been refined to a remarkable extent when compared with the older types, it is still far from the lowest possible limit. At present the complete power unit—the motor, radiator, propeller, water, etc.—will weigh from 2 to 5 pounds per horsepower.

With a given aeroplane, the performance is determined by the total weight and power. The duration and flight range can be increased by increasing the fuel weight at the expense of the passenger or cargo weight. The power available for climbing is the excess of the total power of the motor over the power required for horizontal flight. Since the power for horizontal flight depends principally upon the weight, it is at once evident that the weight is a regulating factor in the climbing speed. In fact the climbing speed may be almost directly determined from the weight carried per horsepower at normal flight speed. A fast climbing scout may weigh from 8 to 12 pounds per horsepower, while the large low climbing machine will weigh from 16 to 20 pounds per horsepower, the respective climbing speeds being approximately 1,200 and 350 feet per minute.

Fuel Efficiency and Weight. The efficiency of the motor, or its fuel consumption for a given output, has a very marked effect upon the total weight of the aeroplane. Under certain conditions a very light motor with a high fuel consumption will often contribute more to the total weight than a heavier but more economical motor. In short flights, up to 3 hours, the very light rotating cylinder motor with its high fuel consumption probably gives the least total weight, but for longer flights the more efficient and heavier water-cooled type is preferable. For flights of over three hours the fuel weight is a considerable percentage of the total weight. The proper motor for any machine must be selected by computing the weight of the fuel and oil required for a given duration and then adding this to the total weight of the engine and its cooling system.

Distribution of Weight. Practically the only way to predict the weight of a proposed machine is to compare it with a similar existing type. After the ratio of the useful load to the total load has been determined, the useful load of the proposed machine can be divided by the ratio factor to obtain the total weight. It should be noted in this regard that if the proposed machine is much larger than the nearest existing example, a liberal allowance must be made to compensate for the increase in the proportional weight of the structural members. There have been many mathematical formulas advanced for predicting the weight, but these are very inaccurate in the majority of cases.

As a rough estimate, based on a number of successful machines, the weight of the actual aeroplane structure without power-plant, live load, fuel, oil, or tanks, is very nearly 32 per cent (0.32) of the total weight. The remaining 68 per cent is divided up among the power-plant, fuel and live load. Thus, the aeroplane structure proper of a machine weighing 2000 pounds total will be 2000 x 0.32 = 640 pounds. Taking the weight of the power plant, tanks and piping at 28 per cent, the total dead load of the bare machine without fuel or oil will be 60 per cent of the total. With a training aeroplane built for a 6-hour flight, the fuel and oil will approximate 16 per cent, so that the total percentage possible for the crew and cargo will be 24 per cent. With a given live load, the total load can now be calculated by dividing the live load by its percentage. Using the above value, for example, the total weight in order of flight with a live load of 720 pounds becomes: W = 720/0.24 = 3000 pounds.