I believe Jerry is correct regarding the 3 blader being better at low speeds.
I went through this in college for my Aero. Eng. degree. It's all in the
math, and the difference is fairly significant, although I don't remember it
being more than 10% in static thrust. It's been awhile, so this is all off
the top of my head. I think it's due to the lower disc loading on a 3 blade
prop at low airspeeds giving more efficiency, but at high airspeeds, the
increased parasitic drag of the 3rd blade starts to dominate.
I should have asked Bob yesterday, as he, Michele, and Ed were so kind as to
show up at our house yesterday on the way to Oshkosh with a big yellow truck
and drop of stage 2 and 3 of our kit. He did mention to me that the Whirlwind
was a really slick prop, when the time came for us to go prop shopping. Not
cheap, tho.
5 days 'til we go to Oshkosh...it's nice being a 45 minute drive away..
Chris Beck
A159
Graham Singleton <grasingleton@avnet.co.uk> wrote:
> > I am not convinced that two blades are more efficient than three.
> > Especially on the Rotax 912S which has a higher reduction ratio. It may
> > be more efficient to have two blades at high speeds but my money is on
> > three blades in the takeoff and climb phase. For us in the UK that's a
> > major consideration. I'll put my three blades up against two any day of
> > the week.
> >Jerry
>
> I agree. As I understand it 2 blades will give more theoretical efficiency
> but unfortunately also more vibration. When the blades are moving
> vertically one has a higher AoA than the other because of the pitch
> attitude of the aircraft. With more blades it evens out and is smoother. I
> suspect the vibration in the yaw direction might well degrade the laminar
> flow a bit, so more drag.
>
> Graham
>
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