§ 245. The Method of the Ballasted Aeroplane.—The method of the ballasted aeroplane not only permits of the determination of the coefficient of skin-friction but simultaneously provides data from which the constant and the relation between the angle and centre of pressure of the aeroplane may be calculated.
The following examples will serve for the purposes of illustration. A standard form of aeroplane has been employed throughout (Fig. 109), measuring 8 in. by 2 in., and ballasted by a lead shot presenting a resistance taken as equivalent to .025 sq. in. of normal plane. The weights of different planes employed for any given series of experiments are all brought up to the same amount by gumming lead-foil in the region of the centre of gravity, the only difference between the different planes of a series being the position of the centre of gravity, and therefore the position of the centre of pressure, and consequently the angle of equilibrium.
The launching of the planes was in all cases effected by means of a launching stick, the aeroplane being placed on a small platten on the top of a straight stick, the lower end of which is held about shoulder high, the act of launching being accomplished by swaying the body so as to give an approximately parallel motion. A certain degree of skill is easily acquired, and a reasonable percentage of good straight flights may be obtained without difficulty.
Example,
Two planes, weight 5 grams (.372 poundals).
Launching data.
| Velocity. | 7.5 ft. Altitude. | |
| No. 1 No. 2 |
15 ft./sec. 12.5 ft./sec. |
47 ft. mean glide. 35 ft. mean glide. |
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