Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/340

§ 215 Converting this into square foot units, we have permissible vacuum 2,300 pounds per square foot, or maximum total reaction = 4,600, which corresponds to a velocity of approximately 108 feet per second. If we take the pitch as equal to 1$$\tfrac{1}{4}$$ times the diameter, this velocity, at the extremity of the blade, corresponds to 39.7 feet per second for speed of vessel, that is, approximately, 27 miles per hour or 23$$\tfrac{1}{2}$$ knots. In practice, the blade of a propeller is never brought to a point as we are now supposing; the end of the blade is always rounded and the pressure consequently less than contemplated by our theory. It is probable that from this cause cavitation does not commence to give trouble till a somewhat higher speed is reached.

It is evident that at speeds above the cavitation limit the design will need modification. It will be necessary to give an area to the blades in excess of that proper to greatest economy, the additional area being required first at the tip of the blade, and as the speed becomes higher the blade will become affected over a greater portion of its length. The secondary consequences of this will be that it will no longer pay to employ the outer blade extremities, and the diameter of the propeller in terms of its pitch will have to be diminished; in ordinary parlance, the screw will become of quicker pitch. Beyond this the aspect ratio of the blades will cease to be of the same importance, since we are unable to employ the higher pressures to which the greater values of n give rise. We may therefore expect to find the blade form becoming of more compact outline as higher speeds come into vogue.

It is manifest that for marine work at high speeds it is impossible to construct a fine pitch propeller to give any reasonable economy, for at a comparatively low vessel speed the velocity of the blades will begin to exceed the limiting value, and it will be necessary to add so much extra surface to reduce the pressure that the efficiency will be poor.

In aerial propellers we are fortunately not concerned with the phenomenon of cavitation.