§ 3. This resistance is made up of—
- 1. Aerodynamic resistance.
- 2. Head resistance.
- 3. Skin-friction (surface resistance).
The first results from the necessity of air supporting the model during flight.
The second is the resistance offered by the framework, wires, edges of aerofoils, etc.
The third, skin-friction or surface resistance, is very small at low velocities, but increases as the square of the velocity. To reduce the resistance which it sets up, all surfaces used should be as smooth as possible. To reduce the second, contours of ichthyoid, or fish-like, form should be used, so that the resultant stream-line flow of the medium shall keep in touch with the surface of the body.
§ 4. As long ago as 1894 a series of experiments were made by the writer[6] to solve the following problem: given a certain length and breadth, to find the shape which will offer the least resistance. The experiments were made with a whirling table 40 ft. in diameter, which could be rotated so that the extremity of the arm rotated up to a speed of 45 miles an hour. The method of experimenting was as follows: The bodies (diam. 4 in.) were balanced against one another at the extremity of the arm, being so balanced that their motions forward and backward were parallel. Provision was made for accurately balancing the parallel scales on which the bodies were suspended without altering the resistance offered by the apparatus to the air. Two experiments at least (to avoid error) were made in each case, the bodies being reversed in the second experiment, the top one being put at the bottom, and vice versa. The conclusions arrived at were:—
For minimum (head) resistance a body should have—
1. Its greatest diameter two-fifths of its entire length from its head.
2. Its breadth and its depth in the proportion of four to three.
3. Its length at least from five to nine times its greatest breadth (nine being better than five).