Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/339

Rh between the face and back of the blade. But we know that under real conditions a fluid is only competent to bear a certain maximum negative pressure (that is, a certain absolute minimum) without giving rise to physical discontinuity; that is to say, the formation of a void. Consequently there will be some critical pressure which cannot be exceeded without destroying the peripteral system of flow. The velocity at which this critical pressure is reached marks the limit of speed at which full propeller efficiency can be obtained; for speeds involving any higher velocity the design of a screw propeller becomes a compromise.

The production of a physical discontinuity by the screw propeller was discovered by Messrs. Thornycroft, and is termed “cavitation”; the phenomenon is one that has given considerable trouble to naval architects where high speeds have to be developed. We will endeavour to estimate the critical value of $$V$$ and show what modifications of design are indicated when the said critical value is exceeded.

We do not know definitely how much of the total reaction is carried as pressure on the face, and how much as vacuum on the back of the blade, but assuming pterygoid form and neglecting the effect of thickness, it is probable that the reaction is equally divided. The influence of thickness will be to superpose a streamline system of flow on the peripteral system which will result in a general diminution of pressure on both faces, so that the reaction will be more than half borne by the vacuum on the back of the blade. If the peripteral system comprises any discontinuity it is probable that this will tend in the opposite direction.

On the whole, it is perhaps best to assume the equal division of the reaction; and in making this assumption, to bear in mind that if the blade is of heavy section, cavitation will probably commence at a lower velocity than that which theory leads us to expect.

Let us assume that the propeller is working under a head of 2 feet of water in addition to the atmospheric pressure, that is, let us take the total pressure to be 16 pounds per square inch.