Aeroplane Construction and Operation

by John B. Rathbun

Chart Showing Forms of 7 Typical Aeroplane Fuselage

Problem. Find the resistance of a Curtiss Tractor Type JN body with a breadth of 2' 6" and a depth of 3’ 3", the speed being 90 miles per hour. The slipstream is assumed to be 25 per cent, with an additional 10 per cent for added fabric loss, etc.

Typical Stream Line Strut Construction.

Solution. The cross-sectional area = 2' 6" x 3’ 3" = A = 8.13 square feet. The velocity of translation is 90 M. P. H., or V² = 8100. The value of the resistance coefficient is taken from the table, Ko-0.00273. The total resistance R = KxAV² = 0.00273 x 8.13 x 8100 = 178.2 pounds. Since a slipstream of 25 per cent increases the resistance by 40 per cent, the resistance in the slipstream is 1782 x 1.4 = 249.48 pounds. The addition of the 10 per cent for extra friction makes the total resistance = 249.48 x 1.1 = 274.43 pounds. The resistance of this body, used with "twin" motors, would be 178.2 x 1.1 = 196.02, but as a tractor with the body in the slipstream, the resistance would be equal to 274.43 pounds as calculated above.

CHAPTER XVII. POWER CALCULATIONS.

Power Units. Power is the rate of doing work. If a force of 10 pounds is applied to a body moving at the rate of 300 feet per minute, the power will be expressed by 10 x 300 = 3000 foot-pounds per minute. As the figures obtained by the foot and pound units are usually inconveniently large, the "Horsepower" unit has been adopted. A horsepower is a unit that represents work done at the rate of 33,000 foot-pounds per minute, or 550 foot-pounds per second. Thus if a certain aeroplane offers a resistance of 200 pounds, and flies at the rate of 6000 feet per minute, then the work done per minute will be equal to 200 x 6000 = 1,200,000 foot-pounds. Since there are 33,000 foot-pounds of work per minute for each horsepower, the horsepower will be: 1200000/33000 = 36.3.

As aeroplane speeds are usually given in terms of miles per hour, it will be convenient to convert the foot-minute unit into the mile per hour unit. If H = horsepower, R = resistance of aeroplane, and V = miles per hour, then H = RV/375, the theoretical horsepower, without loss. If an aeroplane flies at 100 miles per hour and requires a propeller thrust of 300 pounds, then the horsepower becomes:

H = RV/375 = 300 x 100/375 = 80 horsepower. This is the actual power required to drive the machine, but is not the engine power, as the engine must also supply the losses due to the propeller. The propeller losses are generally expressed as a percentage of the total power supplied. The percentage of useful power is known as the "Efficiency."