Re: Prop Efficiency

From: markdrela <drela_at_mit.edu>
Date: Fri, 25 Aug 2006 00:36:36 -0000

--- In Indoor_Construction_at_yahoogroups.com, "John Kagan"
<john_kagan_at_...> wrote:
>
> Is the next step to try to increase efficiency by trying different
> parameters (diameter, rpm, etc)?

There are too many uncertainties in the JavaProp calculation for it to
predict efficiency accurately. But it should be able to accurately
predict the blade cl's of an existing prop if given the correct flight
data (don't know if JavaProp is set up for this, though).

Specifically, a suitable analysis method would be given:

1) an estimated blade cd ~ 3.5 / sqrt(Re)
2) the measured velocity
3) the measured RPM

You can accurately measure the flight velocity in cruise by tracing
out the flight path on the gym floor using coins, and measuring the
distance flown over an elapsed time.

During the calulations, the collective pitch would also need to be
iteratively tweaked to get the measured torque, since you really don't
know the true aerodynamic pitch of the prop in the presence of
aeroelastic twisting. But even with the estimated cd and the pitch
fudging, the blade cl's will be very accurately predicted if the
velocity, RPM, and the torque match the measurements (based on a
simple torque-matching argument).

A good target average blade cl is about 0.4 or so. If the predicted
cl is significantly different, then I'd scale all the blade chords by
the factor

  cl_predicted / 0.4

and build this new prop. The blade cl's of the new prop will be much
closer to 0.4, and it should have better cruise efficiency as a result.

Of course, there's also the climb portion to consider...
Received on Thu Aug 24 2006 - 17:46:06 CEST