Page:Popular Science Monthly Volume 28.djvu/22

 impressed upon the air, downward or in any other direction, it becomes an easy matter to determine the power.

For example, in the practical case just considered, to lift the machine from the ground would require an expenditure of at least eighteen horse-power. This is the least power that would do the work—the actual power would depend entirely upon the efficiency of the propeller.

Having at last succeeded in getting away from the ground, we wish to fly in any direction—to set the birds an example of how the thing ought really to be done.

Here, again, we must apply the principles just announced. To go forward, the air must be driven aft. Knowing the speed proposed, our table will give us at once the resistance for each square foot; and knowing the size or bulk of our machine, we can readily estimate the power required.

The management of the wind unquestionably will be a very important factor in the construction of a flying-machine; indeed, it may be considered the most troublesome part of all. Properly handled, the wind might be made a useful servant, otherwise a dangerous master.

The only plan that suggests itself is through the use of an inclined plane. Here, at any rate, we must imitate the birds.

My attention was not long ago called to an article on Aëronautics, in the Proceedings of the New Zealand Institute for 1878, and in it was a table from experiments by Mr. Skye, giving the lifting power of the wind, blowing at the rate of twenty-three miles an hour upon a plane surface, one square foot in area, inclined at various angles. These figures lead to some very surprising and interesting results:

It will be seen from the second column that while the greatest lifting effect occurs at about an angle of 40°, even at so small an angle as 5° it is still considerable. The third column gives values for the corresponding horizontal pressures; that is, the force which tends to move the plane in the direction of the wind. The fourth column gives the ratio between the two.

It will be seen that the drifting force diminishes at a much faster rate than the lifting force, as the angle of inclination of the plane becomes less.

Consider again the flying-machine weighing 600 pounds, and