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Bully, the formula's correct. However, you must put
in the rpm of the prop - not the engine which in our case means we need
to simply divide whatever answer you got with the formula by
2.17
I get 772 ft/sec for a 64" dia prop with engine turning at
6000 rpm with a 2.17
So formula for us should be tip speed = (Dia" *3.1416*
rpm/(12*60))/2.17 or whatever your gear ratio is.
Ed
----- Original Message -----
Sent: Monday, December 19, 2005 6:44
PM
Subject: [FlyRotary] Re: Props
Thanks Dave. it makes me feel better now that I know I was
wrong. My problem now is: I can spin my "monster prop" (static) up-to 4000RPM
and 50" MP I don't think I can get much more in motion and may not get enough
rpm for flight. I'll see first how much difference it will make on fast taxi.
John Slade also will bring his Performance prop one day, so we can compare
them.
BTW the prop tip speed formula given to me was:
Prop D" x 3.1416 x RPM divided by 720
Buly
On Dec 19, 2005, at 6:06 PM, David Leonard wrote:
Oh, BTW Bully. You will also note that at 6000 RPM the prop is
doing 2750 which a lot of certified props will turn, even if 74"long.
Also, short prop are not necessarily less efficient, its just that the
speed of peak efficiency is shifted to a higher airspeed, and probably that
peak is a little more narrow.
Dave Leonard
On 12/19/05, Buly
<atlasyts@bellsouth.net>
wrote:
Yesterday
somebody gave me the formulas to calculate the prop tip
speed. I did
some calculations and looks like us with the 2.17
redrives are kind of
painted in the corner. Hope I'm wrong, but if I
want to keep the tips
from going supersonic, I have to cut down the
prop to 64" or keep the
engine below 5500 RPM. The choice is either
inefficient prop and
adequate power, or efficient long prop and not
getting all the power
from the engine? Am I wrong?
Buly
atlasyts@bellsouth.net
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Dave Leonard
Turbo Rotary RV-6 N4VY
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