Single and Dual-Rotating Propeller Question

  • #1

Summary:

Hey Physics Community, I'm reading a paper on Wind-Tunnel Tests carried out on a set of Dual-Rotating Propellers carried out by NACA in the 1940's. There is a paragraph I do not quite understand hope someone can explain it to me. It concerns single-rotation propellers. Thanks.

Main Question or Discussion Point

Hoping that someone can explain what the optimum angular displacement between the front and rear propellers actually means here in the following paragraph. It concerns the single-rotation case of the experiment:
"Both the eight-blade single- and dual-rotating propellers were mounted in four-way hubs spaced 9 15/16 inches apart, thereby providing identical blade shank and spinner conditions. Preliminary tests were made to determine the optimum angular displacement between the front and rear propeller blades for the single-rotation test; the blades of the front propeller were set to lead the blades of the rear-propeller by 75°, 52.5° and 30°. Although the results indicated little difference between these three spacings, the 52.5° spacing was considered the best. Equal spacing of 45° was not possible owing to a limitation imposed by the shaft spline."
 

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  • #2
DaveC426913
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the blades of the front propeller were set to lead the blades of the rear-propeller by 75°, 52.5° and 30°.
This doesn't make sense if the props are counter-rotating - which is what one would normally expect.

But it doesn't actually say anywhere that they're counter-rotating.

If they are not counter-rotating then surely it simply means this:

1577992885561.png
 
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  • #3
Thanks for your reply.
The props are counter-rotating.
 
  • #4
DaveC426913
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The props are counter-rotating.
In that case, the description makes no sense - at least to me.

If "the front propeller were set to lead the blades of the rear-propeller by (x)°" they would only stay that way when parked.
 
  • #5
DaveC426913
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Taking a shot in the dark, the only other angle I see that could be relevant is this one:
1577993470596.png

But that doesn't jive with the description.
 
  • #6
I agree. Also, in the description, it says that the front leads the rear for the single-rotation test.
P.S. The excerpt from the article has been added as it is.
 
  • #7
DaveC426913
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Any chance you could link to the article, so we might gather some further context?
 
  • #9
DaveC426913
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Ah.
Excerpt from text body just prior to that diagram:
The propeller shaft were locked together for single-rotation operating conditions.
That seems to suggest a configuration where, for some of the tests, the two props are co-rotating.

I would have though that would require a bit more description, such as what they did to reverse the prop pitch, so I'm still not sure.
 
  • #10
Baluncore
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  • #11
DaveC426913
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Note; fig. 4. photo shows they are NOT counter-rotating.
Yes. I was about to draw attention to that.
1578000763732.png

Top two blades (1 front/ 1 rear) are edge-on; bottom two blades (1 front/ 1 rear) are flat-on. i.e. both have same pitch and thus same rotation direction.
 
  • #12
Friends, I'm not investigating the single-rotation case. I'm more interested in the dual-rotation cases.
For the dual-rotation case, the fore and aft props are left and right handed as stated in the report. Which implies that they are counter-rotating.

Hence, what I have been able to understand is that for the single-rotation case of let's say a 6-Bladed Configuration, there are 3 blades each in the front and back and they're not counter-rotating. The lead angle is the same as initially.

The 6-Blade dual rotation implies 3 blades fore and aft rotating counter to each other. The lead angle is not for this case as it doesn't make sense.

Please confirm that I understood correctly.
Thanks a lot.
 

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  • #13
DaveC426913
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Friends, I'm not investigating the single-rotation case. I'm more interested in the dual-rotation cases.
Then you can safely ignore the text you quoted and asked about in the OP.

Reformulate your question perhaps?
 
  • #14
Thanks a lot. Much appreciated.
 
  • #15
Baluncore
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I believe we now understand the experiment.
Dual-rotation is synonymous with counter-rotation. Hubs rotate in opposite directions, with opposite hand blades.
Single-rotation is the experiment control. Hubs rotate in the same direction, with the same hand blades.
 

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