Contra rotating propellers

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davidjgall

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Why is that? I would have thought it would be the other way around.
Consider a gross oversimplification of the case of increasing from two blades to four blades, same diameter:

Twice the blades, half the load carried by each. Half the load, half the downwash angle and half the size of the "resultant vector" --> 1/4 the induced drag per blade. But there are now twice as many blades so 1/4 x 2 = 1/2 the induced drag.
 

Vigilant1

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Anyone have an idea how much of a typical GA prop's drag is due to induced drag? The tips are generally at some high fraction of Mach, that makes for a lot of parasite drag. I'd bet a nickel that, in typical climb or cruise, induced drag on the prop is a small fraction of all forms of parasite drag. Skin friction, the burble on the back side of that airfoil, turbulence due to uneven linear speeds/ pressures along the blade's length, etc. The drag we see in typical airfoil plots of Cl/Cd assumes an airfoil of unlimited span (no tip vortices), so that's all parasite drag, it ain't induced drag. A square test club has no induced drag at all (makes no lift), yet it effectively absorbs every bit of HP the engine can produce. Sure, a square test club is draggy, but it also absorbs the HP used by a real prop to do real work in producing thrust (that's where about 70-80% of the HP goes, after all).
We know that in many cases, a two-blade prop outperforms a prop with more blades. There are many other factors to work into the mix, and I suspect fixating on induced drag won't yield results that match what we see in the real world.
 
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davidjgall

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Anyone have an idea how much of a typical GA prop's drag is due to induced drag?

...

We know that in many cases, a two-blade prop outperforms a prop with more blades. There are many other factors to work into the mix, and I suspect fixating on induced drag won't yield results that match what we see in the real world.
There's nothing you can "do" about the induced drag on a prop anyway, despite some questionable claims over the years regarding different tip shapes and such. Minimization of the induced drag on propeller blades is best addressed by proper "spanwise" (radial) "lift" (thrust) distribution as proved by Prandtl, Betz, and Goldstein almost a century ago, and "computerized" for programmable calculator by Larrabee in the 1970s. By-and-large the customary design methods used for typical props have not subscribed to the Goldstein radial loading and have suffered efficiency as a result. Modern props designed using "computer" methods may or may not embody the Goldstein optimal radial load distribution, but changing to one that does can yield significant improvements in performance.

There was much hubbub over Paul Lipps' "Ellippse" propellers a few years back but, other than his own Lancair and a stunning success with the Phantom II, his design methodology did not yield similar results for other applications because it was fundamentally incorrect. He attempted to apply an elliptical load distribution instead of the Goldstein distribution; his Lancair and Phantom II props coincidentally matched the Goldstein distribution for those applications fairly well, but subsequent applications missed that mark and, thus, suffered substantially lower efficiency than anticipated.

The reason the two-bladed propeller seems to outperform props having higher numbers of blades on light aircraft can, as you suggest, be traced to "other factors," most notably to Reynolds' number effects. The required narrowing of the blade chord for >2 bladed props when using "standard" design methods leads to blade Reynolds' numbers falling below ~1,000,000. The propeller design community is a small one and has clung rigidly to airfoil sections that do not perform well at low Reynolds' numbers so the comparative inefficiency of the 3-blade as compared to the 2-blade is a predictable outcome and "popular wisdom" makes the "rule" absolute. It's just aerodynamics.
 

Vigilant1

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There's nothing you can "do" about the induced drag on a prop anyway, despite some questionable claims over the years regarding different tip shapes and such.
I agree, thanks. I must have misinterpreted your point below, I thought you were saying:
1) Induced drag produced by a prop can be effectively reduced by increasing the blade count (in and of itself) and
2) This reduction in induced drag has performance implications in real world applications
Consider a gross oversimplification of the case of increasing from two blades to four blades, same diameter:

Twice the blades, half the load carried by each. Half the load, half the downwash angle and half the size of the "resultant vector" --> 1/4 the induced drag per blade. But there are now twice as many blades so 1/4 x 2 = 1/2 the induced drag.
 

davidjgall

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I agree, thanks. I must have misinterpreted your point below, I thought you were saying:
1) Induced drag produced by a prop can be effectively reduced by increasing the blade count (in and of itself) and
2) This reduction in induced drag has performance implications in real world applications
You seem to be addressing two different questions as though they are one. Or maybe I am. Regardless....

Induced drag of "a prop" cannot be significantly changed. Changing to a more-bladed prop is not reducing the induced drag of "a prop;" it is replacing "a prop" with "a different prop." Does that clarify the seeming contradiction of my comments? I'm keying off of Swampyankee's post (Contra rotating propellers) as the root of your current questions.
 

Vigilant1

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You seem to be addressing two different questions as though they are one. Or maybe I am. Regardless....

Induced drag of "a prop" cannot be significantly changed. Changing to a more-bladed prop is not reducing the induced drag of "a prop;" it is replacing "a prop" with "a different prop." Does that clarify the seeming contradiction of my comments?
Maybe just to clear things up: Leaving aside the special case of counter- and contra-rotating props, are you contending that present GA aircraft would have lower induced drag from their props if they replaced their twin blade props with ones having more blades? And that this reduction in induced drag would be noticeable in practice?
 

Vigilant1

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what a difference ?
There's a lot of overlap in the common use of these two terms. Many people, though, make a distinction between contra-rotating (two props on the same axis/shaft turning in opposite directions, usually by a single engine) and counter- rotating (turning opposite, but not coaxial. A "conventional" twin engine plane with engines on the wings turning in opposite directions would be an example).
 

Dusan

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The reason the two-bladed propeller seems to outperform props having higher numbers of blades on light aircraft can, as you suggest, be traced to "other factors," most notably to Reynolds' number effects. The required narrowing of the blade chord for >2 bladed props when using "standard" design methods leads to blade Reynolds' numbers falling below ~1,000,000.
This is exactly my point in post #70, and the main reason I think contra-rotating are less efficient aerodynamically for light loaded props; even without counting gearing/transmission looses.
 

Sockmonkey

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Consider a gross oversimplification of the case of increasing from two blades to four blades, same diameter:

Twice the blades, half the load carried by each. Half the load, half the downwash angle and half the size of the "resultant vector" --> 1/4 the induced drag per blade. But there are now twice as many blades so 1/4 x 2 = 1/2 the induced drag.
Grossly simple is how I like it.
That makes it sounds like multiple little skinny wings would be more efficient than a single big pair.
 

davidjgall

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Maybe just to clear things up: Leaving aside the special case of counter- and contra-rotating props, are you contending that present GA aircraft would have lower induced drag from their props if they replaced their twin blade props with ones having more blades? And that this reduction in induced drag would be noticeable in practice?
No, I am not suggesting that.

The case of the GA prop is so close to the limits of "other factors" such as Reynolds' number effects that the change in theoretical efficiency from two blades to more blades is usually largely negated by the "other factors." Coupled with the industry's rigid adherence to "established" design practices which I won't expound on here, the case is largely moot for low-horsepower applications. Somewhere around 150hp and up, and with good design, there may be a possibility of actually observing a benefit of increasing the number of blades, and surely there are exceptions below that figure, but as a "rule of thumb" and positing that the other traditional rules of propeller design are followed, no, there's likely no benefit.

If you have a specific example you'd like to explore we can do a theoretical comparison if you'd like.
 

Sockmonkey

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No. The spinning propeller only interacts with any given bit of previously stationary air for a brief instant; there's insufficient time to take that air "along for the ride" long enough for any appreciable amount of spanwise flow to develop.
So it's that the prop is moving through the air at such a high speed then?
 
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