--- In Indoor_Construction_at_yahoogroups.com, "hermbarb" <hermanna@...>
wrote:
>
> Blade element analysis will show the maximum
> efficiency will occur where the ADVANCE is at 45 deg.
The maximum profile efficiency occurs where the advance angle is
45 - arctan(0.5*cd/cl)
where cd/cl is the blade airfoil's 2D drag/lift ratio. For typical
indoor prop airfoils, cd/cl = 0.1, so the optimum advance angle is
about 42 degrees. Close enough.
> 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.
This assumes that the blade chords are the same for all P/D's. In
general, it is necessary to increase the blade chords pretty fast as
P/D is increased. The simple reason is that as the blades slow down,
they must be given more area to generate the same thrust without stalling.
> 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.
I wouldn't go all the way to a square tip, but it's true that if the
profile losses dominate, as they do on an indoor prop, the outer parts
of the prop should carry relatively more load.
By far and away the most important design variable on an indoor prop
is matching the average radius-weighted blade chord or area to the
required thrust and airspeed (or torque and airspeed). The objective
is to minimize the blade airfoil cd/cl and hence minimize the profile
losses which dominate on indoor props.
If blade area is too big, cl will be too small, and cd/cl will be too
large (the analogy is flying a wing too fast). If the blade area is
too small, cl will be too big, and cd/cl will again be too large (the
analogy is flying a wing too close or even past stall).
An typical arc-type indoor airfoil has a minimum cd/cl in the range cl
= 0.4 - 0.5, so you want to use the "just right" blade area which will
produce this blade cl range. One advantage of the Larrabee method,
and other related methods (Xrotor, Liebeck/Adkins, JavaProp, etc), is
that they will determine the correct blade area needed to achieve a
specified cl value, subject to other inputs such as speed, RPM, and
torque.
Received on Tue Aug 22 2006 - 11:41:32 CEST
This archive was generated by Yannick on Sat Dec 14 2019 - 19:13:44 CET