View Full Version : Max RPM for composite props??
04-08-2004, 07:03 PM
What are the design limitations for composite props, like powerfin warp drive etc. Can one be spun over 3000 rpm without coming apart, My suzuki 530 is redlined at 8750 rpm, What will this do to the prop! currently my Arrow spins the 60" prop at about 2370 with a 2.73 reduction drive. I know a bensen will spin a wooden 52" prop close to 3800 rpm with no problem.
What would happen if you put a warp drive prop on a direct drive VW at 3800 RPM???
:confused: :confused: :confused:
04-09-2004, 06:10 AM
I have seen warps in lycoming at 2700 and at mazdas but dont know what is their rpm. Just do the math of tip speed. We use 50-30 in vw. you get a bit more rpm and reduce rpm in cruse for a cooler engine.
04-09-2004, 04:17 PM
What type is your Suzuki engine? I had a Black Max a few years ago.Redline was 7k RPM.
Thanks, Gordon Gibson.
The only real way to know for sure is to drop the manufacturer a line. Any opinions you might hear here are just that - opinions.
But your post begs the question: Why would you want to? Tip-speed would be well into the supersonic at those revs, and you're certain to anger all those within earshot, surely?
04-09-2004, 07:44 PM
Well the real quandry for me is that everytime I see a propeller speed calculator and type in my numbers, I am WAY short of The desired .mach speed. So I guess my question is why don't composite props spin at tip speeds that make them more efficient?
I understand that 2 strokes need to be able to get into thier powerband, and etc. If a guy can spin a 52" wooden prop with a VW at 3800 RPM at max power, Why can't I spin a 52" composite prop with a ROTAX at the same rpm on the same gyro with out having to go to a bigger prop, taller mast, offset thrustline, etc.
Just asking that's all.
My Suzuki is a 530, the same setup that is on the aircommand in the "for sale" section. It is rated a 86hp at 8750 rpm with tuned exhaust, Probably less HP with single Ex.
My Arrow is spinning a 60" warp drive through a 2.73 gear reduction. works out to be 2490rpm at max engine rpm of 6800. So If I spin that prop Faster it would make more thrust? right? If I reduced the diameter of the prop a few inches and spun it faster with a different engine, would I still get the amount of thrust that I need??
Then I could shorten my mast and get the thrust line back in to where it should be with a Bensen, instead of a KB3.
My Arrow seems to run OK, I had 2 engine outs in the first 2 flights with 2 different people. I now have it running well, and have flown it for about 1 1/2 hr with no problems. I don't trust it, and there is no parts available to rebuid it when it gives up the ghost. So I will be changing to the suzuki this fall, requires major redo on the gyro, so I thought if I could get all this prop stuff figured out before I start the rebuild, I would only have to do it once.
I could always just go with the suzuki and a 2.58 box, and a 60" prop and be done with it. It just seems that 60" is the magic number for props, and I don't know why. Thanks for listening to my rant.
04-10-2004, 05:22 AM
You have to rev the engine to get to max torque/hp this in the rotax is 6800 . This will put the tip above mach speed. It dont work backward. You have to develop max HP and then adjust prop accept this load.
The math is from steba props
RPM X Diameter Inches X .00436 = tip speed (Ft/Sec)
The normal maximum for a wood prop is 900 Ft/Sec
2700 X 68 X .00436 = 800 Ft/Sec
RPM X Pitch Inches X .000947 = Speed (MPH)
This approximates the aircraft forward speed.
2700 X 60 X .000947 = 153 mph
Pitch/Circ. = the arc. tang. = angle (Degrees)
To find the angle needed at any blade station for a desired pitch.
Example: 60 pitch, 68 diameter (214" Circ.)
60/214 = .2804, then arc. tang of .2804 = 15.70 Degrees
and last rule
ALSO! When handling large snakes, always use one adult for every 4 feet of snake. This is one of the most important rules of all.
04-10-2004, 05:55 AM
use this link. it take you to a calculator and plot the numbers. you will see that youyr tip is 206% of mach speed.
04-10-2004, 06:34 AM
Cool! That helps. Thanks for the info.
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