Re: Prop Efficiency

From: markdrela <drela_at_mit.edu>
Date: Tue, 29 Aug 2006 22:59:52 -0000

--- In Indoor_Construction_at_yahoogroups.com, "Bill Gowen" <b.gowen@...>
wrote:
>
> But if you have to use a heavier motor (less turns, faster run-down)
> or a lower prop pitch (higher than desired RPM for the bulk of the
> flight) then it seems you are losing turns at a faster rate than
> optimum. I realize that the trade-off is that with a lower initial
> pitch you'll burn more turns in the climb.

It's best not to think of turns and RPM, but rather in terms of rubber
energy, prop efficiency, and airframe power requirement. The duration
can be in principle computed by

 duration = rubber_energy * avg_prop_efficiency / avg_airframe_power

and nothing else matters. The torque and turns will get sorted out
during motor selection.

If you have an F1D in a very high ceiling where you launch with max
winds, the rubber energy is fixed and no longer a consideration. This
is exactly the same as for an F1B with no thermals. It then boils
down to trading off prop efficiency versus airframe power. A
fixed-pitch prop favors prop efficiency at the expense of a high-speed
blastoff and hence high average airframe power. A VP favors the
airframe more via a gentler climbout, at the expense of average prop
efficiency. It's not obvious which one is better, but I'd wager that
a VP which stalls the prop blades over much of the flight is not optimal.

In low ceilings, the VP has the secondary effect of allowing more
rubber energy at launch (in the duration equation above). So it can
still be beneficial even if it really clobbers the prop efficiency.
Received on Tue Aug 29 2006 - 16:06:57 CEST