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Typical rpm of a propellor? - 6/26/2012 11:22 AM   
kaylol


 

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Status: offline 		What would be the typical range of rpm when using a 0.28m propellor like this one : http://www.modelflight.com.au/graupner_propellers_nylon.htm ?
I need to know the rpm to do some calulcations since I need to know a typical reynolds number which apparently varies with rpm.


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RE: Typical rpm of a propellor? - 6/26/2012 12:47 PM   
HighPlains


 

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Status: offline 		Really difficult to say what the RPM would be, since most set-ups unload in the air to higher RPM than what a ground reading would be. But 11,000 to 13,000 would be pretty close. But you also have to include the actual path of the blade though the air which adds in the effects from the forward motion of the aircraft.

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RE: Typical rpm of a propellor? - 6/26/2012 1:04 PM   
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Status: offline 		That makes sense. Thank you.


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RE: Typical rpm of a propellor? - 6/26/2012 2:57 PM   
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Status: offline 		For an approx. 11" diameter composite prop I would assume an upper rev limit of 15,000 rpm.

Karol

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RE: Typical rpm of a propellor? - 6/26/2012 4:49 PM   
iron eagel



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Status: online 		The upper limit will vary quite a bit depending upon the amount of horse power supplied to it and the drag of the airframe it is pulling.

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RE: Typical rpm of a propellor? - 6/27/2012 6:51 PM   
Sport_Pilot



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Status: offline 		I thought the upper limit was either the maximum rating by the prop manufacture or the speed where the tips are about to break the sound barrier?

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RE: Typical rpm of a propellor? - 6/27/2012 7:05 PM   
iron eagel



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But what you actually can get the prop to do in flight is dependent on the horsepower applied especially once you get into the mach critical region of operation.
I know several times I have had a prop which was operating well beyond the maximum rating of the manufacturer. The downside of this is that the lifetime of the prop is decreased dramatically when they are subject to high stress.

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RE: Typical rpm of a propellor? - 6/27/2012 7:39 PM   
airraptor


 

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Status: offline 		a 11" prop will start making noise at 17,000RPM in level flight. This is around 600MPH tip speed. mach one is 760 at sea level and 29.92 baro.

i would say 15,500-16,000 would be a good limit on the ground for a APC 11" prop 14,500-15,500 for a zinger type wood prop.

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RE: Typical rpm of a propellor? - 6/27/2012 11:33 PM  1 votes
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Status: offline 		The 15 and up size of props you linked to are typically used on the lower revving gas engines used on big models. As such there's only a few rare examples which hit 10K.

More importantly what will YOU be powering the prop with? If you're using a spark gas engine look up the manufacturers specs. If you're using these props on a geared down electic power system you need to figure out at what RPM the unit will spin any particular prop.

In short you're putting the cart before the horse. You need to know what RPM the power package will turn the prop at before you can go ahead and do the rest of the stuff.

Looking at the prop itself you need to know what radii of the prop is being considered for Reynolds number. Reynolds numbers are based not on RPM but instead on airspeed and size of the object traveling through the air. On a prop the tips cover more distance per rev so the tip is moving a LOT faster in terms of linear feet per second than the root area. But typically the root is seen as a drag making strut that supports the outer working 2/3 of a prop. If you think about it a proper helical pitch would see the pitch angle of the blade at the root go to infinity right on the axis. Even a good half inch away where the hub transitions to the blade the angle would be very extreme. We simply don't see that since the velocity at that point is so low it would do little but create drag. Hence the rather liberal "de-pitching" for the first 15 to 20% of the blade which turns it into a supporting strut more than an actual propulsive device.

For the outer portion that actually does the work you're going to find that the Reynolds number changes dramatically as you move out from the center axis. For the most part it will increase rapidly as the width and linear speed rises and then fall away towards the tips for shapes such as the APC prop. The speed is still rising but the blade chord falls with the shape they use. With other more traditional shapes you'll see the Rn rise all the way to the tip.

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RE: Typical rpm of a propellor? - 6/27/2012 11:38 PM   
BMatthews



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Status: offline 		The following has been quoted from a different thread by kaylol as it so closely relates to this thread instead of being on its own.

quote:

ORIGINAL: kaylol

I found a plot of the lift and drag coefficients on an airfoil that I'm interested in, but the plots differ with reynolds' number. How do I decide which reynolds number to take it at?

I am working with a 0.28m diameter propeller.



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RE: Typical rpm of a propellor? - 6/28/2012 9:33 AM   
kaylol


 

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Status: offline 		Thanks BMatthews for combining the threads.

So, any ideas on the Reynolds' number?

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RE: Typical rpm of a propellor? - 6/28/2012 9:49 AM  1 votes
da Rock



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quote:

ORIGINAL: kaylol

Thanks BMatthews for combining the threads.

So, any ideas on the Reynolds' number?



His post three up from this one covers the issue fairly well. Post #9 above.

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RE: Typical rpm of a propellor? - 6/28/2012 7:13 PM   
BMatthews



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quote:

ORIGINAL: kaylol

Thanks BMatthews for combining the threads.

So, any ideas on the Reynolds' number?



It generally makes sense to keep all the questions on a related idea all in the one thread.

By now I would expect that you've found the equation for calculating Rn for free air which is for wings, props, fuselages and such.

A little googling found me this handy online calculator;

http://aero.stanford.edu/StdAtm.html

You will need to calculate the distance traveled by a given radii station and blade chord to fill in the blanks. For example at 10 inches from the root and a pitch of 10 inches and assuming an 70% advance ratio such as would be found during a climb of some angle this radial station would rotate through 31.4 inches of circular distance and advance 7 inches ahead. So the real distance traveled by this station would be the square root of (10^2 + 7^2) = 13.0 inches.

For an RPM of 5000 this would then be 5000/60 x 13/12 to get the feet per second. Which happens to be 90.3 fps.

If we assume a blade chord of 1.5 inches, or 0.125 feet, at the 10 inch radial station we can now plug our numbers into the calculator and we get a Rn of 71, 790.

You would have to repeat this same exercise for your own prop at a number of radial stations. And likely you would want to run the calculation for each station at something like one or two inch increments to get a feel for how the Reynolds numbers change.

As you can see this is why I mentioned that it is so important to know what prop you want to use and how fast it will turn based on using it on your own power system. There is no one value you can simply pop in and use for what you're after.

This is always the case. For example there is no one Rn for a wing in flight. As the speed of the wing changes during landing and takoff to higher speeds during the thrilling diving passes along the field the Rn of the wing is constantly changing. It is part of what makes designing a plane or prop or rotor so challenging.

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RE: Typical rpm of a propellor? - 6/28/2012 7:31 PM   
iron eagel



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Status: online 		Bruce,
I hope this is what you feel is a related question as far as prop design.
Is there any rule of thumb as far as what part of the prop is generating the most amount of thrust, and does that change with RPM and speed?

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RE: Typical rpm of a propellor? - 6/28/2012 9:07 PM   
rmh



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Status: offline 		each different task requires a different prop design-
there is no perfect prop design
Example
look at the blades used on the man operated helicopter - also note they are on booms to keed the HUGE blades in ground effect
also note . NO curved top airfoil
The only reason I ever found for a curved front surface was strength .
For most of our apps - it must be stiff enough for the rpm and load -the curves -pretty much a streamlining thing
The actual efficiency is of value only as it relates to the work required .

A perfect pitch ( pitch at any given station relative to diameter )- is great on paper but may not be a good prop

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RE: Typical rpm of a propellor? - 6/30/2012 4:40 AM   
chuckk2


 

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Status: offline 		There is a "rule of thumb" formula or three about propeller size, type, and safe RPM used by one of the MFRs. Google Master Airscrew, and poke around.

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RE: Typical rpm of a propellor? - 7/10/2012 6:53 AM   
BFoote


 

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From Memory

Clark Y Re 120,000
CL 1.3 @ 12.5 degrees
Decreases 0.1 CL/AoA

Otherwise look up and download the Low speed Reynolds number airfoil data from University of Urbana Illinois provided by Micheal Selig. who is a professor there.

http://www.ae.illinois.edu/m-selig/pd.html

Hundreds of airfoils. Clark Y is in there as well for different Re #'s.

Cheers.

PS. Check out his propeller data as well.

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RE: Typical rpm of a propellor? - 7/10/2012 11:02 AM   
kaylol


 

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Status: offline 		Thanks for all the responses. Problem solved.


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RE: Typical rpm of a propellor? - 11/14/2012 4:43 PM   
PA441914


 

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Status: offline 		Alright here's the thing. All yourself this question. How long is a piece of string? There's no correct answer right? Same thing that's going on with your question. But, there is a general rule that you have to obey. Prop efficiency decreases dramatically as the top speed approaches the speed of sound. Find a prop that gives you the most rpm without breaking mach 1. Another thing is that sometimes you might have to lower your rpm in the winter because the speed of sound is slower in the winter.

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