§ 30. The Horse-power of Model Petrol Motors.—Formula for rating of the above.
(R.P.M. = revolutions per minute.)
H.P. = (Bore)2 × stroke × no. of cylinders × R.P.M./12,000
If the right-hand side of the equation gives a less h.p. than that stated for some particular motor, then it follows that the h.p. of the motor has been over-estimated.
Fig. 56.
§ 30A. Relation between Static Thrust of Propeller and Total Weight of Model.—The thrust should be approx. = ¼ of the weight.
§ 31. How to find the Height of an Inaccessible Object by Means of Three Observations taken on the Ground (supposed flat) in the same Straight Line.—Let A, C, B be the angular elevations of the object D, as seen from these points, taken in the same straight line. Let the distances B C, C A and A B be a, b, c respectively. And let required height P D = h; then by trigonometry we have (see Fig. 56)
h2 = abc/(a cot2A - c cot2C + b cot2B).
§ 32. Formula for calculating the I.H.P. (indicated horse-power) of a single-cylinder double-acting steam-engine.
Indicated h.p. means the h.p. actually exerted by the steam in the cylinder without taking into account engine friction. Brake h.p. or effective h.p. is the actual h.p. delivered by the crank shaft of the engine.