2. How to calculate the power applied.
3. Its mounting.
WHAT AMOUNT OF POWER IS NECESSARY.—In the consideration of any power plant certain calculations must be made to determine what is required. A horse power means the lifting of a certain weight, a definite distance, within a specified time.
If the weight of the vehicle, with its load, are known, and its resistance, or the character of the roadway is understood, it is a comparatively easy matter to calculate just how much power must be exerted to overcome that resistance, and move the vehicle a certain speed.
In a flying machine the same thing is true, but while these problems may be known in a general way, the aviator has several unknown elements ever present, which make estimates difficult to solve.
THE PULL OF THE PROPELLER.—Two such factors are ever present. The first is the propeller pull. The energy of a motor, when put into a propeller, gives a pull of less than eight pounds for every horse power exerted.
FOOT POUNDS.—The work produced by a motor is calculated in Foot Pounds. If 550 pounds should be lifted, or pulled, one foot in one second of time, it would be equal to one horse power.
But here we have a case where one horse power pulls only eight pounds, a distance of one foot within one second of time, and we have utilized less than one sixty-fifth of the actual energy produced.
SMALL AMOUNT OF POWER AVAILABLE.—This is due to two things: First, the exceeding lightness of the air, and its great elasticity; and, second, the difficulty of making a surface which, when it strikes the air, will get a sufficient grip to effect a proper pull.
Now it must be obvious, that where only such a small amount of energy can be made available, in a medium as elusive as air, the least change, or form, of the propeller, must have an important bearing in the general results.