pose that, in addition to the propeller, we furnish it with an inclined plane having the same area, or, perhaps after the manner of birds, make the propeller act also as an inclined plane; and let it be inclined five degrees, with the wind blowing at the rate of twenty-three miles an hour. Then the table shows us that the total lifting force due to the wind would be 278 pounds, leaving 322 pounds to be supported in some other way. The horizontal or drifting force would be 023 pounds on each square foot, or only 56 pounds altogether. To counteract this, let us make our propeller act as a kite-string by sending backward the air at an increased velocity. Our other table tells us how great this velocity should be, and makes the necessary power amount to only about half a horse-power. To support the balance of the weight, we should need also to send downward a current of air, involving an additional expenditure of about seven horse-power.
Combining the two, we get this extraordinary result, that while nearly nineteen horse-power was necessary to lift our machine from the ground, it could hold its own in a breeze of twenty-three miles an hour with an expenditure of only seven and a half horse-power.
No account has been taken of the wind blowing against dead surfaces, such as the body of the bird or machine. This, of course, would depend upon the shape. A bird's body is long and narrow, cleaving the air without great resistance, and a flying-machine should be fashioned similarly.
Other losses have not been considered, but still the broad result holds that it is possible in this way to utilize part of the energy stored up in the wind. The accuracy of the results will depend upon that of Mr. Skye's table; but if future experiment should verify it, we can understand why it is that the albatross, and wild-duck, and heavy birds generally, while rising with great difficulty, when once up keep on the wing with so much apparent ease.
However, there is still the necessity for a kite-string of some sort. There is a force tending to carry the bird along with the wind which must be overcome somehow, and I still fail to understand how the albatross can sail in the air indefinitely without some muscular effort.
From Mr. Skye's table, in connection with the other, we get this important practical result—that in a flying-machine, properly constructed, the greatest power required will be that necessary to lift it from the ground; and that once off, up to a certain limit, the stiffer the breeze the better.
The efficiency of a propeller of any sort will depend not only upon its area, but also upon its ability to send the air away in parallel streams. If we wish to go forward, the air must be driven aft, and a forced current in any other direction will at best give us back but a fraction of its energy. Ordinary screw-propellers have not proved very effective, for the reason, probably, that revolving at great speed, they send off a large amount of air tangentially.