13. CHAPTER XIII.
PECULIARITIES OF AIRSHIP POWER.
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 1,200 pounds, and equipped with a 50-horsepower engine, 50 miles 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,
Miles per Hour | Horse Power per sq. foot |
10 | 0.013 |
15 | 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
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
One of the Early Multiplane Gliders.
[Description: Black and white photograph: Man in glider with multiple rows
of wings.]
The flying machine must move faster than the air to
Take another case. An aeroplane, capable of making 50 miles an hour in a calm, is met by a head wind of 25 miles 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.
The non-technical reason why the support of the machine becomes easier as the speed increases is that the
Huffaker's Model Bird for Soaring Experiments.
[Description: Black and white illustration: Diagram of model bird.]Supporting Area of Birds.
One of the things which all producing aviators seek to copy is the motive power of birds, particularly in their relation to the area of support. Close investigation has established the fact that the larger the bird the less is the relative area of support required to secure a given result. This is shown in the following table:
Bird | Weight in lbs. | Surface in sq. feet | Horse power | Supporting area per lb. |
Pigeon | 1.00 | 0.7 | 0.012 | 0.7 |
Wild Goose | 9.00 | 2.65 | 0.026 | 0.2833 |
Buzzard | 5.00 | 5.03 | 0.015 | 1.06 |
Condor | 17.00 | 9.85 | 0.043 | 0.57 |
So far as known the condor is the largest of modern birds. It has a wing stretch of 10 feet from tip to tip, a supporting area of about 10 square feet, and weighs 17 pounds. It. is capable of exerting perhaps 1-30 horsepower. (These figures are, of course, approximate.) Comparing the condor with the buzzard with a wing stretch of 6 feet, supporting area of 5 square feet, and a little over 1-100 horsepower, it may be seen that, broadly speaking, the larger the bird the less surface area (relatively) is needed for its support in the air.
Comparison With Aeroplanes.
If we compare the bird figures with those made possible by the development of the aeroplane it will be readily seen that man has made a wonderful advance in imitating the results produced by nature. Here are the figures:
Machine | Weight in lbs. | Surface in sq. feet | Horse power | Supporting area per lb. |
Santos-Dumont | 350 | 110.00 | 30 | 0.314 |
Bleriot | 700 | 150.00 | 25 | 0.214 |
Antoinette | 1,200 | 538.00 | 50 | 0.448 |
Curtiss | 700 | 258.00 | 60 | 0.368 |
Wright | [1]1,100 | 538.00 | 25 | 0.489 |
Farman | 1,200 | 430.00 | 50 | 0.358 |
Voisin | 1,200 | 538.00 | 50 | 0.448 |
While the average supporting surface is in favor of
Other Parts of Huffaker's Bird Model.
[Description: Black and white illustration: Parts of model bird.]More Surface, Less Power.
Broadly speaking, the larger the supporting area the less will be the power required. Wright, by the use of 538 square feet of supporting surface, gets along with an engine of 25 horsepower. Curtiss, who uses only 258 square feet of surface, finds an engine of 50 horsepower
But there is a limit, on account of its bulk and awkwardness in handling, beyond which the surface area cannot be enlarged. Otherwise it might be possible to equip and operate aeroplanes satisfactorily with engines of 15 horsepower, or even less.
The Fuel Consumption Problem.
Fuel consumption is a prime factor in the production of engine power. The veriest mechanical tyro knows in a general way that the more power is secured the more fuel must be consumed, allowing that there is no difference in the power-producing qualities of the material used. But few of us understand just what the ratio of increase is, or how it is caused. This proposition is one of keen interest in connection with aviation.
Let us cite a problem which will illustrate the point quoted: Allowing that it takes a given amount of gasolene to propel a flying machine a given distance, half the way with the wind, and half against it, the wind blowing at one-half the speed of the machine, what will be the increase in fuel consumption?
Increase of Thirty Per Cent.
On the face of it there would seem to be no call for an increase as the resistance met when going against the
From Fly.
Diagram of An Entirely New Aerodrome.
51—single surface; 52—framework; 53-54—wheels and spring runners;
55—operator's seat; 56—motor; 57—crankshaft to propeller; 60—helm,
operated by handle 71, which is within of aviator; 72—movable soaring
blade; 75-76—balancing planes. The other figures are self-explanatory.
[Description: Black and white illustration: Diagram of aerodrome.]
In other words Mr. Lanchester maintains that the work done by the motor in making headway against the wind for a certain distance calls for more engine energy, and consequently more fuel by 30 per cent, than is saved by the helping force of the wind on the return journey.