Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/338

§ 215 assumption of a discontinuous system of flow, it is evident that the fluid in the dead-water region must be following the blade in its spiral path and must consequently be subject to centrifugal force. So much is this the case that it is hardly possible to conceive of the same fluid remaining in the dead-water region for any considerable length of time, so that the resistance on this basis will be very greatly augmented, and it will not be fairly represented by the figures deduced from rectilinear theory.

This objection does not apply to the calculations made on the pterygoid basis, or at the most only to a very small extent; the author is therefore disposed to think that the pterygoid basis is that indicated by the conditions as giving the correct data for propeller design, the alternative not having the importance that attaches to it in the main problem of flight.

Now the pressure values given in Table XIV. represent not only the pressures appropriate to different blades moving with the mean velocities given, but the pressures appropriate to different portions of the same blade according to its velocity at different points along its length. Moreover, the pressures are not definite plus values but represent the pressure difference