Aeroplane Construction and Operation

by John B. Rathbun

Effects of Altitude. At high altitudes the density is less, and consequently the thrust is less with a given number of revolutions per minute. The thrust can be maintained either by increasing the speed, or by increasing the pitch. For correct service at high altitudes the propeller should undoubtedly be of the variable pitch type, in which the pitch can be controlled manually, or by some automatic means such as proportional blade flexure.

Effects of Pitch. Driven at a constant speed, both the thrust and horsepower increase with the pitch up to a certain limiting angle.

For a given horsepower the static thrust depends both on the diameter and the pitch. If the pitch is increased the diameter must be decreased in proportion to maintain a constant speed. As the pitch is regulated by the translational speed and revolutions, the static thrust of a high speed machine is very small. As the translational speed increases, the pitch relative to the wind is less, and consequently the thrust will pick up until a certain limiting speed is reached.

Thrust and Horsepower. The calculation for thrust and power are very complicated, but the primary conditions can be given by the following: Let V = the pitch velocity in feet per minute, T = thrust in pounds, and H = horsepower, then H = TV/33000E from which T = 33000HE/V, the efficiency being designated by E. Since the pitch velocity is NP, where N = revs. per minute and P = pitch in feet, then T = 33000HE/PN. Assuming a 5-foot pitch, 1200 revs., the efficiency = 0.75, and the horsepower 100, the thrust will be:

T = 33000 x 100 x 0.75/5 x 1200 = 412.5 pounds. The pitch in this case is the blade pitch, and the great uncertainty lies in selecting a proper value for E. This may vary from 0.70 to 0.85. The diameter is also an unknown factor in this primitive equation.

Materials and Construction. The woods used for propeller construction are spruce, ash, mahogany, birch, white oak, walnut, and maple. Up to 50 H. P. spruce is suitable, as it is light, and strong enough for this power. In Europe walnut and mahogany are the most commonly used, although they are very expensive. Birch is very strong and comparatively light for its strength, and can be used successfully up to 125 horsepower. Ash is strong, light and fibrous, but has the objectionable feature of warping and cannot withstand moisture. Maple is too heavy for its strength. White oak, quarter-sawed, is the best of propeller woods and is used with the very largest engines. It is strong for its weight and is hard, but is very difficult to work and glue. For tropical climates, Southern poplar is frequently used as it has the property of resisting heat and humidity.

One-inch boards are rough dressed to 7/8 inch and then finished down to 13/16 or 3/4 inch. After a thorough tooth planing to roughen the surface for the glue, they are thoroughly coated with hot hide glue, piled together in blocks of from 5 to 10 laminations, and then thoroughly squeezed for 18 hours in a press or by clamps until the glue has thoroughly set. Only the best of hide glue is used, applied at a temperature of 140°F. and at a room temperature of 100°. The glue must never be hotter, nor the boards cooler than the temperatures stated. The propeller after being roughed out is left to dry for ten days so that all of the glue stresses are adjusted. If less time is taken, the propeller will warp out of shape. The propeller is worked down within a small fraction of the finished size and is again allowed to rest. After a few days it is finished down to size by hand, is scraped, and tested for pitch, tracking and hub dimensions.

The finish is glossy, and may be accomplished by several coats of spar varnish or by repeated applications of hot boiled linseed oil well rubbed in, finishing with three or four coats of wax polish. There should be at least 5 applications of linseed oil, the third coat being sandpapered with No. 0 paper. The wood should be scraped to dimension and must not be touched with sandpaper until at least two coats of varnish or oil have been applied.

The wood must be absolutely clear and straight grained, and without discolorations. The boards must be piled so that the edge of the grain is on the face of the blades, and the direction of the annular rings must be alternated in the adjacent boards.