In government specifications the total weight of the pilot and passenger are taken at 330 pounds, or 165 pounds per man. Gasoline and oil are for a 4-hour flight. A safer average figure will be 170 pounds per man, and a fuel allowance of 6 hours. The floats of a seaplane or flying boat bring the percentage of the dead load much higher than with the land type of chassis.
The following table will give an idea as to the weight distribution expressed both in pounds, and as a percentage of the total weight. It covers a wide range of types, varying from the training types Curtiss JN-4B and the Standard H-3, to the Handley-Page Giant bomber and the Nieuport speed scout. The average values found by Hunsaker for a number of machines weighing in the neighborhood of 2500 pounds is given in the fourth column. Under each heading are the actual weights and the percentages of the total weight for each item. Items marked (*) include both gasoline and oil. Mark (C) is the power plant complete, and (@) includes radiator.
Weight Per Horsepower. As already explained, the weight carried per horsepower varies with the type of machine. When the total weight is determined for any aeroplane, the power requirements can be calculated by dividing the total weight by the weight per horsepower ratio. A fair value for a training or exhibition machine is from 18 to 20 pounds per horsepower, while for a very high speed machine, such as a chaser, the weight will be taken at 10 pounds per horsepower. For two-seater fighters 16 to 18 pounds is fair practice. For a comparison of the horsepower-weight ratios used on different well-known machines see tables in Chapter II. Thus, if our total weight is found to be 2400 pounds as determined from the above table, and if this is a training machine, the horsepower will be: 2400/20 = 120 horsepower. Using the same total weight, but powered for two-seater fighter conditions, the power will be increased to 2400/16= 150 horsepower. As a scout the power will be increased still further to 2400/10=240 horsepower.
As a problem in solving the weight and horsepower from the data, we will assume that we are to design a two-seater fighter with a total useful load of 1200 pounds. This load consists of the following items: Personnel (2) = 330 pounds; gas and oil = 500 pounds; guns and ammunition = 370 pounds. The nearest example that we have to this live load is that of the Standard H-3, which carries 744 pounds and in which the percentage of live load is 28.1 per cent. As our machine will be somewhat larger, we will not be far from the truth if we take the percentage as 0.27 instead of 0.281. The total weight, in flying order, will now be 1200/0.27 = 4440 pounds. At 16 pounds per horsepower the motor will be: 4440/16=277 horsepower.
An empirical formula for a high-speed scout was set forth in "Aviation and Aeronautical Engineering" by D. W. Douglas. This is based on the horsepower unit. A unit wing loading of 8.45 pounds per square foot, and a low speed of 55 miles per hour was assumed. The wing section chosen was the U.S.A.-1. In the formula, H = horsepower:
- Power plant weight = 3 H.
- Chassis weight = 0.7 H.
- Tail weight = 0.25 H.
- Fuel for 2.25 hours = 1.4 H.
- Military load = 250 pounds.
- Tanks and piping = 0.42 H.
- Fuselage weight = 1.84 H.
- Wing weight = 1 lb. sq. ft.
- Propeller = 2.8/H.
- (Total) = (7.61 H + 2.5/H + 250)/7.45 = Weight of aeroplane fully loaded in the order of flight.
Weight of Wings. The weight of the wings depends upon the span, very small machines having wings that weigh only 0.38 pounds per square foot, while the wings of very large machines may run as high as 1.1 pounds per square foot. For average size biplanes from 0.75 to 0.80 pounds per square foot would probably be safe—that is, for areas ranging from 450 to 550 square feet. The weight of the upper wing of the Nieuport is 0.815 pounds per square foot, while the lower wing (short chord) is 0.646 pounds per square foot. The wings of the Standard H-3 trainer will average 0.77 pounds per square foot, the lower wing and center section being heavier than the upper wing. The wings of the Curtiss JN-4B will average 0.75 pounds per square foot. These weights do not include the interplane wires or struts, nor the fittings. The total weight of the interplane struts of the JN-4B, the Aviatic, and machines of similar size will average from 28 to 30 pounds. The ailerons will weigh about 12 pounds each.
Weight of Motors. There is a considerable difference in the weight of air-cooled and water-cooled motors. The water, water piping, radiators and jackets of the water-cooled motors adds considerably to the weight of the complete power plant. The mountings are heavier for the water-cooled motors, and because of the tandem arrangement of the cylinders, the crankshaft and crankcase weigh more. In taking the bare weight of the power plant all of the accessories must be included. In the following table, the "bare engine" includes the carbureter, magneto, and necessary integral accessories, but does not include the jacket water, mounting, radiator, oil in base, water piping, nor controls. Water-cooled motors are marked by (W) and air-cooled by (A). Rotary air-cooled are (RA), and gallons (G).