Single and Dual-Rotating Propeller Question

[Aun Muhammad]

TL;DR 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.

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."

 

[DaveC426913]

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:

 

[Aun Muhammad]

Thanks for your reply.
The props are counter-rotating.

 

[DaveC426913]

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.

 

[DaveC426913]

Taking a shot in the dark, the only other angle I see that could be relevant is this one:

But that doesn't jive with the description.

 

[Aun Muhammad]

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.

 

[DaveC426913]

Any chance you could link to the article, so we might gather some further context?

 

[Aun Muhammad]

Sure. This paper is for four- and six-bladed props but with identical test conditions.
https://ntrs.nasa.gov/search.jsp?R=19930091825

 

[DaveC426913]

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.

 

[Baluncore]

The aim was to reduce diameter and tip velocity, to transfer more energy from more powerful engines being developed.
Here is a link to similar study. Note; fig. 4. photo shows they are NOT counter-rotating.
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090014884.pdf
Also;
http://naca.central.cranfield.ac.uk/reports/1942/naca-report-747.pdf

 

[DaveC426913]

Note; fig. 4. photo shows they are NOT counter-rotating.

Yes. I was about to draw attention to that.

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.

 

[Aun Muhammad]

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.

 

[DaveC426913]

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?

 

[Aun Muhammad]

Thanks a lot. Much appreciated.

 

[Baluncore]

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.

 

[Aun Muhammad]

The aim was to reduce diameter and tip velocity, to transfer more energy from more powerful engines being developed.
Here is a link to similar study. Note; fig. 4. photo shows they are NOT counter-rotating.
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090014884.pdf
Also;
http://naca.central.cranfield.ac.uk/reports/1942/naca-report-747.pdf

Hello @Baluncore. How can I determine the incoming velocity and the rotational speed for the case highlighted as a blue spot in the diagram below. The results for the experiment have been presented in a non-dimensionalized format as you are already aware perhaps. Some facts mentioned in the paper for the experiment are that the tunnel speed ranged from 0 to 110 mph, max propeller speed was 550 rpm. I need to determine the rotational velocity and incoming airspeed for the maximum efficiency condition. How can I go about this?

 

[Baluncore]

I need to determine the rotational velocity and incoming airspeed for the maximum efficiency condition. How can I go about this?

Regarding; https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090014884.pdf
You have plotted a point on;
“Figure 10.- Efficiency curves for four-blade dual-rotation propeller without wing”,

You selected the curve for angle of attack = 50.0° front, and 48.4° rear;
The point is at; Efficiency = 0.875; and V/nD = 2.5;

V appears to be undefined. Can we assume it is the air velocity in the wind tunnel ?
It might be the velocity in feet/second or miles/hour.

n is the angular velocity of the propeller in revolutions per second.

D is the diameter of the propeller, probably in feet. ( 10 ft ).

You want values for V; and n; but there are many values of V and n that can satisfy V/nD = 2.5

For the experiment the propeller was driven by an induction motor that would fix n near 0.95 * synchronous speed; The speed of the wind tunnel could then be changed to plot the curve on the graph.

 

[Aun Muhammad]

All that you have pointed out is correct.

For the plotted graph which one is true?
1. The air velocity in the tunnel is constant while the angular velocity of the blades is changed
2. The angular velocity is fixed while the air velocity is changed
3. Both are changed?

I assumed point 1 to be true and proceeded as follows:

There is an excerpt from the paper just before conclusions that needs a mention here:

"Relative thrust curves are given for several airplane categories, defined by the blade angle settings for high speed. Thus blade angles of 30° , 45° , 50° , 55° , 60° and 65° correspond roughly to speeds of 250, 375, 425, 450, 500, 525 miles per hour respectively if a tip speed of 900 to 1000 feet/second is assumed. In as much as the engine speed and diameter are constant, the V/nD is directly proportional to the airspeed."

Since the test conditions state that the maximum air velocity in the tunnel was 110 miles per hour, I assumed that this must be for the 65° angle setting. For 50° angle, the air velocity is 89 miles per hour. This gives me a rotational velocity of 313.44 rev/min (from test conditions maximum rpm was 550). Although this does give me a value of Advance Ratio = 2.5, however the fact that this is the correct condition for the Maximum Efficiency point is questionable.

What is your view on this?

 

[Baluncore]

I can only report my progressive analysis of the details from the paper so far.

At the top of page 4; D is defined as feet; and n as rps = rev/sec.
Earlier, on page 3;
“Test conditions.- The tunnel speed ranged from 0 to about 110 miles per hour, The maximum propeller speed was about 550 rpm, which corresponds to 287 feet per second rotational tip speed.”

The drive from the front induction motor to the front prop was direct, while the speed of the rear prop was synchronised by using a frequency converter.

The number of poles for an induction motor is always even.
The highest speed is stated to be 550 RPM.
For synchronous motors at 60 Hz (in the USA); we have RPM = 7200 / poles.
Computing backwards; 7200 / 550 RPM = 13.1 poles. Which suggests the motors were 12 pole.
The synchronous speed would then be 7200 / 12 = 600 RPM.
550 RPM would indicate a realistic minimum slip under load of 8.3%.
That seems to suggest the experiment was carried out between maybe 500 and 550 RPM.
The equivalent n value would range from n = 500 / 60 = 8.333 rps and n = 550 / 60 = 9.167 rps.

The graphs show V/nD values up to 6.0 for D = 10 ft.

If the V of V/nD is in mph, then the maximum 110 mph implies;
Maximum V/nD = 110 / ( 8.333 * 10) = 1.32

If the V of V/nD is in ft/sec. 110 mph = 161.33 ft/sec.
Maximum V/nD = 161.333 / 83.33 = 1.936

Neither is near 6.0