• Version
• 197 Downloads
• 390.00 KB File Size
• 1 File Count
• November 3, 2016 Create Date
• November 3, 2016 Last Updated

National Advisory Committee for Aeronautics, Technical Notes - Notes on Propeller Design - II - The Distribution of Thrust Over a Propeller Blade

The best distribution of the thrust over the length of the
propeller blade is investigated, taking into account chiefly the
slip stream loss and the friction between the blades and the air.

The energy losses of the propeller depend noticeably on the
distribution of the thrust over thalength of the blades, and the
losses can be diminished by a favorable distribution. The two in—
duced losses of smaller importance, the loss due to the finite
number of blades and the loss due to the rotation of the slip
stream, call for a gradual decrease of the thrust per unit of pro—
peller disc area towards the inner and outer end of the blades.
Near the center the thrust is naturally less dense, and hence the
loss from rotation is generally kept reasonably small without"
special effort of the designer and no further improvement is here
possible.

The breadth of the blades, howeVer, is not always tap;
ered towards the tips as much as would be desirable in order to
keep small the loss due to the finite number of blades. It is
true, the actual thrust distribution is almost as favorable
in spite of it and the density of thrust always decreases proper~
ly, because it is physically impossible for a wing to produce a
finite density of lift quite close to its end. Still the Wing.
works then under less favorable conditions and with smaller effi—
ciency, and the weight of the propeller and the centrifugal force
is unnecessarily great too. Now, the following investigation will
show that a gradual decrease of the density of thrust towards the
ends is also desirable for other reasons. It is sufficient therem
fore to keep in mind that the wing tips have to be round, and to
consider in the following investigation only the two chief energy
losses, the energy absorbed by the air friction and the slip
stream loss.

A small variation of the distribution of the thrust hardly
changes noticeably the entire loss, especially if the distribution
is already close to the best distribution. Hence the problem is
less the exact determination of the best distribution of thrust
than the derivation of a simple expression which gives quickly an
idea as to how the thrust has to be arranged.