Re: Prop Efficiency

From: markdrela <drela_at_mit.edu>
Date: Wed, 23 Aug 2006 13:48:43 -0000

--- In Indoor_Construction_at_yahoogroups.com, "John Kagan"
<john_kagan_at_...> wrote:
>
> Regarding the speed variable (forward speed) for an Indoor prop in
> Javaprop, etc., is it reasonable to use a value somewhat higher than
> the actual forward speed of the model? And, if so, do you have any
> recommendations for what value to use?
>
> I'm envisioning the extreme case of a hovering helicopter. The
> vertical speed is 0, but there must be some airflow since the rotor
> isn't stalled (right?).

The standard Larrabee method starts to crap out when the "thrust
coefficient", or

  Tc = disk_loading / flight_dynamic_pressure

gets too big. As you approach the hovering case, the flight dynamic
pressure goes to zero, and Tc goes to infinity.

Anyway, it's possible to add a modification to the Larrabee method so
it works OK for the hover case. I don't know if JavaProp includes
this modification. Try specifing a speed of 0.1 m/s, and then 0.01
m/s. If it gives nearly the same thrust/power ratio and nearly the
same prop geometry, then it probably has the modification. The
efficiency will be nearly zero in hover, so ignore that.



> It seems that a nose up F1D would be part
> way toward that situation, with a prop speed relative to the
> surrounding air higher than the speed of the plane.

I doubt it. Any indoor model flies noticably faster during the power
burst, even if it's climbing fast. So although the disk loading is
larger then, so is the flight dynamic pressure. So the Tc ratio
probably isn't too different. That's what matters.
Received on Wed Aug 23 2006 - 06:54:25 CEST