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Props with narrow versus wide airfoils. What difference would you expect?

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RknRusty:
Just a curiosity; how much difference would I expect to see between props with airfoils of different widths? For example, Master Airscrew used to make props of a given length and pitch with wider blades than the ones they sell now. I have two of their 10x4 GF series props, both are 10" x 4" but the older one's blades' airfoils are significantly wider. If I mount them on a .35 2-stroke glow engine, what difference in performance should I expect to see?
I'm guessing the wide airfoil blade would pull the plane faster at a given RPM... more thrust. I suppose in reality, you would needle the engine at a different RPM with one than the other.
Rusty

phil c:
The wider blades put more load on the motor, so depending on how much excess hp it has you could see almost anything.

The more load on the motor the hotter it tends to run and the fussier getting all the variables right- needle setting, correct plug(trial and error), head clearance, combustion chamber size, valve timings, etc.

If the motor isn't already straining it should run fine and pull the plane a bit better.  You'd get a similar effect from a larger diameter prop with narrower blades.  Similar power absorption but better transfer of power to the plane during maneuvers.

RknRusty:
Thanks Phil. That's about what I expected.

Rusty

Brett Buck:

--- Quote from: RknRusty on October 14, 2013, 01:10:55 PM ---Thanks Phil. That's about what I expected.

--- End quote ---

   Another "feature" is that, for whatever reason, you tend to get more "whip up" in the wind with wide-blade props. That was mostly an issue back in 4-2 break days but it still a semi-real problem. I am not entirely sure why but I expect that with more load in the first place, the effect of unloading it is enhanced.

   Brett

Igor Burger:
It is just like a wing. We have wings of large span and short chord and long chord with short span. The difference is in aspect ratio. So you can expect that you can trade diameter to blade like Phil wrote. However it will make difference in efficiency:

1/ wide blade with smaller diameter will make stronger tip induceg drag (tip vortices) ... simply when prop act with slippage, it will load motor more then the prop with larger diameter (can explain what Brett wrote -ovespeeded model will unload prop and allow too high rpm)

2/ wide blade with smaller diameter and with the same pitch will loose thrust in slippage (or static thrust) earlier then larger prop because larger part of prop will be stalled

3/ wide blade with smaller diameter in high rpm application reaching sound speed on tips will be better, because tip speed is lower

4/ wide blade with smaller diameter has usually better RE number, because RE is linear to blade width, while diameter pushes it to higher RE number only slowly (because of higher tip speed)

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