The bare radiator will weigh from 0.48 to 0.56 pounds per horsepower, the average being safe at 0.52. The water contained in the radiator will average 0.35 pounds per horsepower. The weights of the piping and the water contained therein will be computed separately. The circular sheet metal cowl used over the rotary cylinder air-cooled motor is equal to twice the square root of the motor weight, according to Barnwell. Propeller weight varies considerably with the diameter, pitch, etc., but a safe rule will give the weight as 2.8 √H where H = horsepower. The tanks will weigh from 0.75 to 1.2 pounds per gallon of contents, or approximately 1/5 the weight of the contents when completely filled.
Chassis and Wheel Weight. The chassis of a two-wheel trainer will weigh about 90 pounds complete, although there are chassis of training machines that weigh as much as 140 pounds. The chassis of speed scouts will be from 22 to 40 pounds complete. Tail skids can be taken at from 6 to 8 pounds.
Tangent wire wheels complete with tires are about as follows: 26 x 4 = 21 pounds; 26 x 5 = 28 pounds; 26 x 3 = 14 pounds. Ackerman spring spoke wheels are estimated as follows: 20 x 4 = 17.5 pounds; 26 x 3 = 22 pounds; 26 x 4 = 32 pounds; 30 x 4 = 35 pounds; 34 x 4 = 45 pounds.
Military Loads. A 20-mile wireless outfit devised by Capt. Culver weighed 40 pounds with storage batteries, while the 120-mile outfit weighed 60 pounds with a 180-watt generator. The 140-mile U.S.A. mule-back wireless of 1912 weighs 45 pounds. The "Blimp" specifications allow 250 pounds.
The Lewis gun as mounted on the "11" Nieuport weighs 110 pounds, including mount, gun and ammunition. Lewis gun bare is 26 pounds. The Davis 6-pounder, Mark IV, weighs 103 pounds with mounting but without ammunition, while the same make of 3-inch 12-pounder weighs 238 pounds under the same conditions.
Controls and Instruments. The Deperdussin type controls used on the Curtiss JN-4B weigh 16 pounds per control, while those installed in the Standard H-3 weigh about 13 pounds. An average of 15 pounds per control is safe. An instrument board for the aviators' cock-pit, fully equipped, weighs from 20 to 24 pounds. The front, or students' instrument board will average 10 pounds. Pyrene extinguisher and brackets = 7 pounds; Speaking tube = 3 pounds; Oil pressure line and gage = 3 pounds; Side pockets = 3 pounds; Tool kit = 10 pounds.
Control Surfaces. The rudder, stabilizer, fin, and elevator can be made so that the weight will not exceed 0.60 to 0.65 pounds per square foot.
General Notes on Weight. Before starting on the weight estimates of the machine the reader should carefully examine the tables in Chapter II which give the weights, and general characteristics of a number of modern machines.
Weights and Wing Area. When the weight of the machine is once determined, the next step will be to determine the wing area. For speed scouts or very large heavy duty machines the choice of a wing section must be very carefully considered. For the speed scout several wings giving a minimum high speed resistance should be examined, such as the Eiffel 37 or the U. S. A-1 or U.S.A.-6. For the low-speed aeroplane to be designed for great lift, a number of sections such as the U.S.A.-4 or the R.A.F.-3 should be tried for a number of speeds and angles. For training machines a wing of the "All around" type such as the R.A.F.-6 should be adopted, the structural characteristics in the case of a trainer having an important bearing on the subject. If W = weight of the machine in pounds, V = low speed in miles per hour, A = total area in square feet, and Ky=lift coefficient, then the area becomes A=W/KyV². Compensation must be made for biplane interference for aspect ratio, and stagger as previously explained. For an ordinary training machine with the usual gap/chord ratio, and aspect ratio, the correction factor of 0.85 may be safely employed.