As a general proposition it takes much more power to propel an airship a given number of miles in a certain time than it does an automobile carrying a far heavier load. Automobiles with a gross load of 4,000 pounds, and equipped with engines of 30 horsepower, have travelled considerable distances at the rate of 50 miles an hour. This is an equivalent of about 134 pounds per horsepower. For an average modern flying machine, with a total load, machine and passengers, of 1,200pounds, and equipped with a 50-horsepower engine, 50miles an hour is the maximum. Here we have the equivalent of exactly 24 pounds per horsepower. Why this great difference?
No less an authority than Mr. Octave Chanute answers the question in a plain, easily understood manner. He says:
"In the case of an automobile the ground furnishes a stable support; in the case of a flying machine the engine must furnish the support and also velocity by which the apparatus is sustained in the air."Pressure of the Wind.
Air pressure is a big factor in the matter of aeroplane horsepower. Allowing that a dead calm exists, a body moving in the atmosphere creates more or less resistance.
The faster it moves, the greater is this resistance.
Moving at the rate of 60 miles an hour the resistance, or wind pressure, is approximately 50 pounds to the square foot of surface presented. If the moving object is advancing at a right angle to the wind the following table will give the horsepower effect of the resistance per square foot of surface at various speeds.
Horse Power Miles per Hour per sq. foot 10 0.01315 0 044
20 0.105
25 0.205
30 0.354
40 0.84
50 1.64
60 2.83
80 6.72
100 13.12
While the pressure per square foot at 60 miles an hour, is only 1.64 horsepower, at 100 miles, less than double the speed, it has increased to 13.12 horsepower, or exactly eight times as much. In other words the pressure of the wind increases with the square of the velocity.
Wind at 10 miles an hour has four times more pressure than wind at 5 miles an hour.
How to Determine Upon Power.
This element of air resistance must be taken into consideration in determining the engine horsepower required.
When the machine is under headway sufficient to raise it from the ground (about 20 miles an hour), each square foot of surface resistance, will require nearly nine-tenths of a horsepower to overcome the wind pressure, and propel the machine through the air. As shown in the table the ratio of power required increases rapidly as the speed increases until at 60 miles an hour approximately 3 horsepower is needed.
In a machine like the Curtiss the area of wind-exposed surface is about 15 square feet. On the basis of this resistance moving the machine at 40 miles an hour would require 12 horsepower. This computation covers only the machine's power to overcome resistance. It does not cover the power exerted in propelling the machine forward after the air pressure is overcome. To meet this important requirement Mr. Curtiss finds it necessary to use a 50-horsepower engine. Of this power, as has been already stated, 12 horsepower is consumed in meeting the wind pressure, leaving 38 horsepower for the purpose of making progress.
The flying machine must move faster than the air to which it is opposed. Unless it does this there can be no direct progress. If the two forces are equal there is no straight-ahead advancement. Take, for sake of illustration, a case in which an aeroplane, which has developed a speed of 30 miles an hour, meets a wind velocity of equal force moving in an opposite direction. What is the result? There can be no advance because it is a contest between two evenly matched forces. The aeroplane stands still. The only way to get out of the difficulty is for the operator to wait for more favorable conditions, or bring his machine to the ground in the usual manner by manipulation of the control system.
Take another case. An aeroplane, capable of making 50 miles an hour in a calm, is met by a head wind of 25miles an hour. How much progress does the aeroplane make? Obviously it is 25 miles an hour over the ground.
Put the proposition in still another way. If the wind is blowing harder than it is possible for the engine power to overcome, the machine will be forced backward.
Wind Pressure a Necessity.
While all this is true, the fact remains that wind pressure, up to a certain stage, is an absolute necessity in aerial navigation. The atmosphere itself has very little real supporting power, especially if inactive. If a body heavier than air is to remain afloat it must move rapidly while in suspension.
One of the best illustrations of this is to be found in skating over thin ice. Every school boy knows that if he moves with speed he may skate or glide in safety across a thin sheet of ice that would not begin to bear his weight if he were standing still. Exactly the same proposition obtains in the case of the flying machine.
