There have been a number of references too the maximum efficiency
occuring at 2:1 P/D. Blade element analysis will show the maximum
efficiency will occur where the ADVANCE is at 45 deg. Placing this at
the half area radius (70.7%) gives about 2:1 P/D.
There are 2 things wrong with this. First, the element effiency drops
off much quicker at the higher angles inboard than at lower angles.
Secondly, it is impossible to generate the huge lift coefficients
required to generate the required thrust. This is most true when the
diameter is limited as in SO. Here the high P/D is totally stalled
and the only way to generate the thrust is with much slippage and
burning up excessive torque.
Because lift of any blade element depends on velocity squared, most
props generate most thrust in the outter 80 or 90% of their radius.
This causes greater induced losses, similar to using a smaller diameter.
The Larabee designs with much area near the hub, with a P/D from 1 to
1.5 as used by Aerovironment in their Micro Air Vehicles etc are much
more efficient than more conventional designs. For most model
applications squared off tips as opposed to rounded should be even better.
My opinion,
H
Received on Mon Aug 21 2006 - 11:35:54 CEST
This archive was generated by Yannick on Sat Dec 14 2019 - 19:13:44 CET