WHILE much experimental work has been done on the controllable-pitch propeller, complexity of existing devices has prevented their being placed on the market. After reviewing briefly the difficulties encountered, due to propeller and engine characteristics, the author discusses the effect of camber ratio and of angle of attack on the speed at which burble occurs, following this with comments on the efficiency of propellers as static-thrust producers, the use of the method of momentum to compute thrust and the application of adjustable-pitch propellers to supercharged engines.
The causes of the forces required to operate the control adjustments are given as (a) friction, (b) twisting moments produced by centrifugal force and (c) twisting moments produced by air pressure. The second of these is taken up in considerable detail and a mathematical expression is derived, by which this quantity can be evaluated at various stations along the blade and plotted, after which the integration can be carried out graphically.
A method of elastic-stress analysis is given which involves determining the direct centrifugal stress; plotting thrust, torque and bending-moment curves; determining the most highly stressed fibers; and finding the deflection at the various stations by a process of double integration, the mean ratio of the deflection necesary to balance the air pressure by centrifugal force to the deflection that would occur if no centrifugal force were present giving an approximate idea of the relative magnitudes of the elastic and static stresses. While this method is only rough, it serves very well for purposes of comparison and in the author's opinion gives a much better picture of the strength of the propeller than the stress analysis alone.
In conclusion the author points out that with the increase of propeller-tip speeds and engine horsepower the aerodynamic advantages of the controllable-pitch propeller become more important and with improved materials and design, the practical requirements of the problem can soon be met.