The non-technical reason why the support of the machine becomes easier as the speed increases is that the sustaining power of the atmosphere increases with the resistance, and the speed with which the object is moving increases this resistance. With a velocity of 12 miles an hour the weight of the machine is practically reduced by 230 pounds. Thus, if under a condition of absolute calm it were possible to sustain a weight of 770 pounds, the same atmosphere would sustain a weight of 1,000pounds moving at a speed of 12 miles an hour. This sustaining power increases rapidly as the speed increases.
While at 12 miles the sustaining power is figured at 230 pounds, at 24 miles it is four times as great, or 920pounds.
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:
Supporting Weight Surface Horse area Bird in lbs. in sq. feet power per lb.
Pigeon 1.00 0.7 0.012 0.7Wild Goose 9.00 2.65 0.026 0.2833Buzzard 5.00 5.03 0.015 1.06Condor 17.00 9.85 0.043 0.57So 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 17pounds. 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:
Supporting Weight Surface Horse area Machine in lbs. in sq. feet power per lb.
Santos-Dumont . . 350 110.00 30 0.314Bleriot . . . . . 700 150.00 25 0.214Antoinette. . . . 1,200 538.00 50 0.448Curtiss . . . . . 700 258.00 60 0.368Wright. . . . .[4]1,100 538.00 25 0.489Farman. . . . . . 1,200 430.00 50 0.358Voisin. . . . . . 1,200 538.00 50 0.448[4] The Wrights' new machine weighs only 900 pounds.
While the average supporting surface is in favor of the aeroplane, this is more than overbalanced by the greater amount of horsepower required for the weight lifted. The average supporting surface in birds is about three-quarters of a square foot per pound. In the average aeroplane it is about one-half square foot per pound.
On the other hand the average aeroplane has a lifting capacity of 24 pounds per horsepower, while the buzzard, for instance, lifts 5 pounds with 15-100 of a horsepower.
If the Wright machine--which has a lifting power of 50pounds per horsepower--should be alone considered the showing would be much more favorable to the aeroplane, but it would not be a fair comparison.
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 258square feet of surface, finds an engine of 50 horsepower is needed. Other things, such as frame, etc., being equal, it stands to reason that a reduction in the area of supporting surface will correspondingly reduce the weight of the machine. Thus we have the Curtiss machine with its 258 square feet of surface, weighing only 600 pounds (without operator), but requiring double the horsepower of the Wright machine with 538 square feet of surface and weighing 1,100 pounds. This demonstrates in a forceful way the proposition that the larger the surface the less power will be needed.
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 wind is apparently offset by the propulsive force of the wind when the machine is travelling with it. This, however, is called faulty reasoning. The increase in fuel consumption, as figured by Mr. F. W. Lanchester, of the Royal Society of Arts, will be fully 30 per cent over the amount required for a similar operation of the machine in still air. If the journey should be made at right angles to the wind under the same conditions the increase would be 15 per cent.
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.
