Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

Don't forget, your prop is an airfoil

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Old 12-16-2009, 10:55 AM
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LGM Graphix
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Default Don't forget, your prop is an airfoil

Reading over and over again how many people say they are going XXXXmph with XX engine at XX RPM and then reading others saying it's impossible, the theoretical speed will only allow for..... Prompted me to dig up this article that was printed in MAAC magazine a couple years back. I posted this once before, but thought it would be interesting discussion again now that we have some other people in the speed forum who have a very good understanding of aerodynamics as well. Here is the excerpt from the article:



People make claims about speed s far greater than what the theoretical speed would be, well, here is a very plausable explanation and some experimenting that was published in MAAC magazine a few months back.



Aircraft.....................Engine............RPM ........Pitch................. Theoretical mph............Actual mph
6ft Chipmunk............90 4 Stroke.....9540.......14x6..................54.20. .........................84.7
Hanger 9 mustang.....100 4 stroke...9240........14x7..................61.25.. ........................72.43
1/A wing...................061 Norvel.....21,990.....5x3....................62.5. ...........................96.4
Quicky.....................40 2 stroke.....18,500.....9x8....................141.. ...........................139
Diamond Dust..........40 2 stroke.....24,400.....7x8....................184.8 ...........................165

So go figure........
For interest, the article says:

Each competitor must fly straight and level over a 528 foot course, both ways and is timed from two pylons at each end. The timing system used is manual timing to a single stop watch, hard wired from each end. This method has proven more accurate than a radar gun that was used in the past. The flying mile is comprised of five timed laps around the pylons.

So yes, there is room for error etc, but I just found it quite interesting when I read it

Another short excerpt from the article that actually made great sense to me when explaining why an aircraft would move faster than the theoretical pitch would allow was

" 'it's perfectly understandable, it's a rotating wing with an airfoil .' As I had a very good understanding of boat propellers of which always had slip factors of from 12% to 30% it became very apparent that air and airfoil s were very different"

I guess when you consider that a wing in forward motion creates lift, it stands to reason that a forward facing rotating wing will create more pull than just what the pitch of that prop is giving as it's creating lift as well.
I don't understand the physics behind it, nor do I want to, all I know is, I've had airplanes that were radar clocked faster than what they "should" have been going and never understood why, this sort of tells me why it COULD be anyway
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Old 12-16-2009, 11:35 AM
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Default RE: Don't forget, your prop is an airfoil

How was the inflight rpm obtained?
Some of these results are so far off that I wonder if the advertised pitch is really so?
So far, I've never been the beneficiary of any results approaching these figures.
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Old 12-16-2009, 06:29 PM
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LGM Graphix
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Default RE: Don't forget, your prop is an airfoil

Hi Combat pig, I can't answer any questions regarding that article for you, I didn't write it, just retyping it here. However, I have owned more than one airplane that flew significantly faster than the theoretical speed of the prop said it should be able to. It's not hard to see that though when you consider what an airfoil does, it is creating lift, or in the case of a propeller, thrust. It stands to reason that a propeller SHOULD move forward more than just it's pitch will allow when it is generating lift, same way a wing on an airplane allows it to fly. It would be cool to have somebody who is a professional aerodynamicist be able to explain prop efficiency to us and why they don't follow the "theory". The "theory" does not ever take drag into consideration, yet I don't think there would be anyone denying that a prop that should do 100mph according to theory would need a 0 drag airframe to acheive that (impossible) yet there are plenty of 100mph airframes out there that are flying that speed with props that in theory should only be doing 100mph.
A typical sport engine doesn't unload that much in the air, still not enough in most cases to match a real airspeed with a theoretical prop speed, but there are many airplanes out there that are flying faster than the prop should allow. There is more going on with a prop than a simple screw thread working it's way through the air. Unfortunately I don't understand any of it, I just know what real world experience has shown me.
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Old 12-16-2009, 06:46 PM
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Default RE: Don't forget, your prop is an airfoil

Unless you have a way of knowing inflight rpm, it's hard to draw conclusions. The diameter of the prop as well as the drag of the airframe has influence on unloading, even a sport engine can give you 10% unloading.
The chart looks like a pretty representative sample of combos you would see out there...no doubt there is something to this phenomena if they did 2 way passes over a 1/10th mile long.
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Old 12-16-2009, 08:36 PM
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Default RE: Don't forget, your prop is an airfoil

i am with MJD anD CP on this issue, its math theory for one which is a great starting point and a good solid basis for tuning basis, but i can refer to my experience with cross country flight of models both power and sailplane types, we competitors consistently found that the airplanes flown for more than a half mile were still accelerating... our driver/helpes and our spotters alerted us to cases of flutter in level flight, this isnt ever really apparent fliying froma fixed base as we do at our local flying sites, but it points to actual facts not easily perceived, with the advent of gps and our different telemetry systems we get some insight, but we consistently get scoffers here, who have little if no actual experience with this area of modeling, and they still scoff or dismiss gps or telemetry data, they remind of a professor of science who stood on his superiour education, emphaticly stated that top fuel dragsters couldnt exceed 240mph in the quarter mile, had math and physics equations to prove it as well, this was in the late 70's FWIW
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Old 12-16-2009, 09:11 PM
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Default RE: Don't forget, your prop is an airfoil


ORIGINAL: lfinney

they remind of a professor of science who stood on his superiour education, emphaticly stated that top fuel dragsters couldnt exceed 240mph in the quarter mile, had math and physics equations to prove it as well, this was in the late 70's FWIW
Funny, I have a Popular Hot Rodding from 1955 (I think that's it..somewhere here..) that shows it was not possible to exceed 156mph, all math related too!
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Old 12-16-2009, 09:51 PM
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Default RE: Don't forget, your prop is an airfoil

I cheat on my speed rally engines.

never ever never for a race engine though. My experiments are limited to race engines that have broken down and are no longer suitable for race day. NEVER EVER
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Old 12-16-2009, 10:22 PM
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Default RE: Don't forget, your prop is an airfoil


ORIGINAL: LGM Graphix

...Prompted me to dig up this article that was printed in MAAC magazine a couple years back...
I posted this information in a different forum just a few day ago (because you your post months ago). Do you happen to know what issue (date) of MAAC magazine this article was in?
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Old 12-17-2009, 12:22 AM
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Default RE: Don't forget, your prop is an airfoil


ORIGINAL: Mike Connor


ORIGINAL: LGM Graphix

...Prompted me to dig up this article that was printed in MAAC magazine a couple years back...
I posted this information in a different forum just a few day ago (because you your post months ago). Do you happen to know what issue (date) of MAAC magazine this article was in?

I have no idea Mike, I wish that I had noted the mag issue in my original post a long while back. I don't keep many of my MAAC mags either, never enough useful info in there, though I do have the odd one that had something of interest in them, I'll check to see if one of the few I still have has this article but I wouldn't hold my breath, I'm pretty sure it's a couple years past now.
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Old 12-17-2009, 12:48 AM
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Default RE: Don't forget, your prop is an airfoil

I know 1 thing for certain.....22,000 with a 5x3 is nothing for a Norvel AME, they'll routinely peak at 26,000 on the ground.
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Old 12-17-2009, 01:25 PM
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Default RE: Don't forget, your prop is an airfoil

It does seem that unloading is not considered in the numbers. That may explain why the differences are so large between actual and theoretical speed of the below 100 mph aircraft. Outrunning the prop pitch speed at slower speeds is no more a mystery then flying at the prop pitch speed at higher speeds, when you consider that drag quadruples every time the speed doubles.

I posted this in an earlier post:
The NACA 4412 airfoil (used by APC) has a camber of 4% and if we put that in the foil simulator on the NASA site we can see that it has a lift factor equal to an AOA of about 4 degrees. We then add 1 degree because of the altered zero-lift angle as stated on the APC site. We then have an airfoil lift factor equivalent to 5 degrees pitch. A 10 x 8 prop is twisted 15 degrees at the tip to give it an 8” pitch. Therefore, because of the airfoil shape, we have a pitch equivalent to 1.33 times 8 (15 degrees for pitch and 5 degrees for airfoil) or a 10 x 10.64 prop.
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Old 12-17-2009, 01:41 PM
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Default RE: Don't forget, your prop is an airfoil

Mike, so how to you want to look at it?

Call it variable effective pitch? Variable efficiency?
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Old 12-17-2009, 02:10 PM
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Default RE: Don't forget, your prop is an airfoil

And when you put a pitch gauge on a prop, everyone I've seen do it uses the lower surface.. that's not an aerodynamic datum of any sort.

So.. does APC label a prop 7" pitch based on center datum line, lower surface datum, or zero lift angle? As the designers and CNC mold fabricators, I would figure it was based on zero lift angle.

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Old 12-17-2009, 06:32 PM
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Default RE: Don't forget, your prop is an airfoil


ORIGINAL: combatpigg

Mike, so how to you want to look at it?

Call it variable effective pitch? Variable efficiency?
I would think variable effective pitch or variable slippage, since efficiency has to do with engine HP making it to the prop thrust.
As long a thrust exceeds drag your speed will increase. As you have pointed out many times, prop selection is very important.
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Old 12-17-2009, 07:01 PM
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Default RE: Don't forget, your prop is an airfoil


ORIGINAL: MJD

And when you put a pitch gauge on a prop, everyone I've seen do it uses the lower surface.. that's not an aerodynamic datum of any sort.

So.. does APC label a prop 7'' pitch based on center datum line, lower surface datum, or zero lift angle? As the designers and CNC mold fabricators, I would figure it was based on zero lift angle.

MJD, good point. The gauge was useful for pylon racers and C/L speed freaks back in the day...I'm not sure how useful they are now with molded props.
The January issue of Flying Models is supposed to have an article on carving props, making a pitch gauge, etc. My annual budget for model mags is about $5 nowadays...
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