|Jul 29, 2013, 08:19 AM|
Joined Oct 2004
1,2,3 or 4 bladed head????
(1 was a joke before you answer).........
Is there a ball park figure regarding the how much more area blade will be needed if reduced from 3 to 2 blades?? I have used a 3 bladed prop on several aircraft as opposed to a 2 bladed one (same thing but going backwards) and defiantly had to drop an inch or two in diameter although changing the pitch did not seem to make much difference. I know that blades are effectively turning in ‘dirty air caused by each others turbulence but I would imagine that at the RPM’s which my props were turning that it would be far more pronounced than at the lower speed of AG rotor blades.
Assuming that the same profile is retained and the same shim angle theoretically you would need to increase your blade area by 50% to keep it the same if you ‘loose’ a blade, but how much will you gain in efficiency due to the cleaner air??
In the same vain, if you increase the blade length to retain the aspect ratio you would obviously have to increase the chord proportionally which will in turn increase the chord thickness (drag!!!). Long skinny blades or short fat ones??? I know that Clark Y produces pretty much the same amount of lift proportionally with a thickness of 8 – 13% with most people going for 10 – 12% but drag and goes up disproportionally with both an increased Angle of attack and increased thickness.
At our low Reynolds numbers is there an advantage in going one way or another?
Asking because both Mickey has mentioned low drag blades to keep the rotor speed up several times and Tom has been experimenting.
Also, following all the good theory being offered here (with my limited understanding) it is obvious that as well as knowing what is happening to each blade per revolution you have to consider what is happening to the opposite one at the same time and the effects that they have on each other. With 2 blades you need a pretty much equal and opposite interaction 180 degrees apart so that the whole shooting match stays ‘in balance’ and stable but with 3 blades is it ’60 degrees’ less critical (180 minus 120 degrees)????
Or should I just make a 2 bladed head and fly the thing to find out for myself
|Jul 30, 2013, 05:58 AM|
Joined Oct 2009
Yep-works for model Helicopters;
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