Re: Prop Efficiency

From: markdrela <drela_at_mit.edu>
Date: Tue, 29 Aug 2006 22:32:48 -0000

--- In Indoor_Construction_at_yahoogroups.com, "John Kagan"
<john_kagan_at_...> wrote:
>
> Which parameter did you adjust to match up with the indoor prop
> airfoils?

I used a simple cd(cl,Re) polar for the blade airfoil drag properties,
computed with Xfoil. But as I said before, the computed cl is fairly
insensitive to the cd values.

I specified the velocity and torque, and varied the overall blade
angle until the RPM came out right. The thrust and the blade cl's
then also came out as a by-product. In the end, everything seems to
be consistent, and the cl's look reasonable.

> I think this prop probably spends very little time at its lowest
> pitch. But it's good to see that the Cl etc. work out well for how it
> probably actually flies :)

If it spends most of its time in the high-pitch condition, then it
should probably be redesigned for that condition. Right now, it looks
like you're flying the blades on the edge of stall for most of the
flight. Increasing the chords and reducing the pitch slightly will
unstall the blades, which should increase the prop efficiency and
correspondingly drop the torque required for a given thrust. You then
translate this into more duration via a longer and thinner motor which
will take more turns.
Received on Tue Aug 29 2006 - 15:45:52 CEST