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Author Topic: Props with narrow versus wide airfoils. What difference would you expect?  (Read 5984 times)

Offline RknRusty

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Just a curiosity; how much difference would I expect to see between props with airfoils of different widths? For example, Master Airscrew used to make props of a given length and pitch with wider blades than the ones they sell now. I have two of their 10x4 GF series props, both are 10" x 4" but the older one's blades' airfoils are significantly wider. If I mount them on a .35 2-stroke glow engine, what difference in performance should I expect to see?
I'm guessing the wide airfoil blade would pull the plane faster at a given RPM... more thrust. I suppose in reality, you would needle the engine at a different RPM with one than the other.
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Offline phil c

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The wider blades put more load on the motor, so depending on how much excess hp it has you could see almost anything.

The more load on the motor the hotter it tends to run and the fussier getting all the variables right- needle setting, correct plug(trial and error), head clearance, combustion chamber size, valve timings, etc.

If the motor isn't already straining it should run fine and pull the plane a bit better.  You'd get a similar effect from a larger diameter prop with narrower blades.  Similar power absorption but better transfer of power to the plane during maneuvers.
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Offline RknRusty

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Thanks Phil. That's about what I expected.

Rusty
DON'T PANIC!
Rusty Knowlton
... and never Ever think about how good you are at something...
while you're doing it!

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Online Brett Buck

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Thanks Phil. That's about what I expected.

   Another "feature" is that, for whatever reason, you tend to get more "whip up" in the wind with wide-blade props. That was mostly an issue back in 4-2 break days but it still a semi-real problem. I am not entirely sure why but I expect that with more load in the first place, the effect of unloading it is enhanced.

   Brett

Offline Igor Burger

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It is just like a wing. We have wings of large span and short chord and long chord with short span. The difference is in aspect ratio. So you can expect that you can trade diameter to blade like Phil wrote. However it will make difference in efficiency:

1/ wide blade with smaller diameter will make stronger tip induceg drag (tip vortices) ... simply when prop act with slippage, it will load motor more then the prop with larger diameter (can explain what Brett wrote -ovespeeded model will unload prop and allow too high rpm)

2/ wide blade with smaller diameter and with the same pitch will loose thrust in slippage (or static thrust) earlier then larger prop because larger part of prop will be stalled

3/ wide blade with smaller diameter in high rpm application reaching sound speed on tips will be better, because tip speed is lower

4/ wide blade with smaller diameter has usually better RE number, because RE is linear to blade width, while diameter pushes it to higher RE number only slowly (because of higher tip speed)

Offline Steve Helmick

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Master Airscrew did a redesign on their props in an attempt to improve the efficiency and market share. They still have not got the pitches correct...a 10-4 would be closer to 3" pitch, and a typical .35 will likely not rev high enough to get the model up to a comfortable airspeed.

I would like to ask why people often say a prop is "stalled" when the airplane is not moving? Some air is being pushed aft and some other air refills the low pressure area ahead of the propeller, no? I can't envision the prop stalling at the airspeed at which they rotate. It's not like a stall of a wing at high speed, which would be caused by a radical increase in angle of attack. Wouldn't there be a radical increase in thrust when the prop is no longer stalled?  H^^ Steve   
In 1944 18-20 year old's stormed beaches, and parachuted behind enemy lines to almost certain death.

In 2015 18-20 year old's need safe zones so people don't hurt their feelings.

"Be polite, be professional, but have a plan to kill everybody you meet." General Mattis.

Offline Igor Burger

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Master Airscrew did a redesign on their props in an attempt to improve the efficiency and market share. They still have not got the pitches correct...a 10-4 would be closer to 3" pitch, and a typical .35 will likely not rev high enough to get the model up to a comfortable airspeed.

I would like to ask why people often say a prop is "stalled" when the airplane is not moving? Some air is being pushed aft and some other air refills the low pressure area ahead of the propeller, no? I can't envision the prop stalling at the airspeed at which they rotate. It's not like a stall of a wing at high speed, which would be caused by a radical increase in angle of attack. Wouldn't there be a radical increase in thrust when the prop is no longer stalled?  H^^ Steve   

Because every airfoil has its particular AoA called critical angle of attack. Over that angle prop become stalled. So if you rotate prop with helical pitch distribution, some part of blade is stalled (close to center) as the angle of incidence goes to 90 degrees at hub. If you let model move forward, whole blade goes to 0 AoA at pitch speed. Somewhere in between become "unstalled" and working fine

"Unstalled" prop not needs to have better thrust, but it has better efficiency as the airfoil of stalled blade loses only fraction of its lift, but drag of such airfoil could be may be 10 or even more times higher then "unstalled". So such prop does not neeed so high power to rotate at the same RPM. That is why it os not so good idea to let electric motor run on ground more then few seconds.

Offline Tim Wescott

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I would like to ask why people often say a prop is "stalled" when the airplane is not moving? Some air is being pushed aft and some other air refills the low pressure area ahead of the propeller, no? I can't envision the prop stalling at the airspeed at which they rotate. It's not like a stall of a wing at high speed, which would be caused by a radical increase in angle of attack. Wouldn't there be a radical increase in thrust when the prop is no longer stalled?  H^^ Steve   

Steve, I'm not sure how right people are when they talk about our stunt propellers being stalled, because the angles of attack really aren't that severe.  However, the phenomenon of a stalled prop being just like a wing at a too-high angle of attack makes perfect sense to me.

If you look at race planes from the 1920's and the 1930's, when engines turned slowly and variable-pitch props hadn't been invented, it'll be obvious that yes, that prop just about has to be stalled when the airplane is at a stand-still.  Indeed, you also hear stories about long take-off runs until the aircraft got up to flying speed and could finally use the prop.

Maybe a speed guy will weigh in here -- those have the same super-high pitch props that you see on the old race planes, and they certainly unload a hell of a lot when they come up to speed.

I did a quick web search hoping to find some smoke pictures or movies showing propeller stall, but found nothing.  I did find a page by Martin Hepperle that mentioned the phenomenon.  He's a competition modeler and a consulting aerodynamics expert, so he ought to know: http://www.mh-aerotools.de/airfoils/prpstati.htm
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Offline Igor Burger

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If you look at race planes from the 1920's and the 1930's, when engines turned slowly and variable-pitch props hadn't been invented, it'll be obvious that yes, that prop just about has to be stalled when the airplane is at a stand-still.  Indeed, you also hear stories about long take-off runs until the aircraft got up to flying speed and could finally use the prop.

Stalled could be only fraction of prop or whole prop. All depends on P/D ratio.

P/D=0.5 is somehwre between good properties at static thust and wrong. Means prop 10x8 will not work well in static regime, 10x3 will work very well. You can compare thrust of motor with 12x6 prop and piped 11x4. It makes big difference. And if you compare it to speed props P/D=1 which make only little static thrust, difference is clear. You can also sometimes hear a strange sound when P/D=1 prop "gets" air. When tips of such prop (otherwise stalled) little accelerate air, it can become "working" because of incomming air and move air even more, so  more of prop will be "unstalled" ... so you can hear and feel abrupt change of sound and thrust. That was case when whole prop was stalled.

Offline Steve Helmick

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Has anybody tested an IC stunter with an audio tachometer to see how much change there is from launch rpm to flight rpm?  H^^ Steve

In 1944 18-20 year old's stormed beaches, and parachuted behind enemy lines to almost certain death.

In 2015 18-20 year old's need safe zones so people don't hurt their feelings.

"Be polite, be professional, but have a plan to kill everybody you meet." General Mattis.

Offline Dennis Toth

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Re: Props with narrow versus wide airfoils. What difference would you expect?
« Reply #10 on: January 22, 2014, 06:04:47 PM »
Some years back I carved up a 12 x 4 prop with narrow thin blades and tried this on a Enya 35 powered Tutor. I had gotten into reading a lot of prop articles and this was the way they seemed to think would provide higher efficiency without loading the motor. Well, the thing worked great, even thought it was still at an rpm close to what a 10 x 6 would be (around 9200 rpm ground). It did take more control to get the corners but not much. I still have that prop, I had gotten side tracked for a few years after trying this and never worked with it any more. This thread makes me think about pulling it out and finish the tests.

Best,           DennisT

Offline Howard Rush

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Re: Props with narrow versus wide airfoils. What difference would you expect?
« Reply #11 on: January 22, 2014, 07:16:10 PM »
Has anybody tested an IC stunter with an audio tachometer to see how much change there is from launch rpm to flight rpm?

Yep.  We recorded signals from all four engines on the Bomber.  They were about 1,500 RPM difference from launch to cruise.  Same for an F2D.  We did it with a .40VF, too, but I forgot the number. 
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Offline Mark Scarborough

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Re: Props with narrow versus wide airfoils. What difference would you expect?
« Reply #12 on: January 22, 2014, 07:31:21 PM »
Steve, I'm not sure how right people are when they talk about our stunt propellers being stalled, because the angles of attack really aren't that severe.  However, the phenomenon of a stalled prop being just like a wing at a too-high angle of attack makes perfect sense to me.

If you look at race planes from the 1920's and the 1930's, when engines turned slowly and variable-pitch props hadn't been invented, it'll be obvious that yes, that prop just about has to be stalled when the airplane is at a stand-still.  Indeed, you also hear stories about long take-off runs until the aircraft got up to flying speed and could finally use the prop.

Maybe a speed guy will weigh in here -- those have the same super-high pitch props that you see on the old race planes, and they certainly unload a hell of a lot when they come up to speed.

I did a quick web search hoping to find some smoke pictures or movies showing propeller stall, but found nothing.  I did find a page by Martin Hepperle that mentioned the phenomenon.  He's a competition modeler and a consulting aerodynamics expert, so he ought to know: http://www.mh-aerotools.de/airfoils/prpstati.htm
as I recall reading, this phenomenum is why speed record airplanes, supermarine and Macci come to mind, are all seaplanes,, nice long runways to work with,, otherwise the loss afforded by big ugle floats would be a detriment not worth it,,
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Offline Matt Spencer

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Re: Props with narrow versus wide airfoils. What difference would you expect?
« Reply #13 on: February 09, 2014, 05:29:49 AM »
 %^@

What was that about floats .  n1 :-X

Online Brett Buck

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Re: Props with narrow versus wide airfoils. What difference would you expect?
« Reply #14 on: February 16, 2014, 04:01:04 PM »
Yep.  We recorded signals from all four engines on the Bomber.  They were about 1,500 RPM difference from launch to cruise.  Same for an F2D.  We did it with a .40VF, too, but I forgot the number. 

   Around 800-1000 on mine.

     Brett

Offline Steve Helmick

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Re: Props with narrow versus wide airfoils. What difference would you expect?
« Reply #15 on: February 22, 2014, 09:21:25 PM »
Hey, isn't that David Fitzgerald standing there behind that Gee Bee? Probably a replica...the Gee Bee, I mean!  :o Steve

%^@

What was that about floats .  n1 :-X

« Last Edit: July 01, 2014, 02:59:19 PM by Steve Helmick »
In 1944 18-20 year old's stormed beaches, and parachuted behind enemy lines to almost certain death.

In 2015 18-20 year old's need safe zones so people don't hurt their feelings.

"Be polite, be professional, but have a plan to kill everybody you meet." General Mattis.

Offline Fredvon4

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This and other Gee Bee photos, way back when I was a kid, confused me about what I THOUGHT I KNEW about propellers

Sorry to drag up this old thread

But I am trying to re-educate myself all about propellers now that I have many different 2 and 3 bladed props to play with for all the various .20 /.25 /.36 / 40/ .46 / .52 I am going to use on a few bucket list models

Way back as a kid... there was a nut case slightly older than me, who proved to all of us unbelievers, that a brick could fly with a big enough motor.... then came a lot of real  stupid experiments making all sorts of crap zoom around in a semi controlled flight circular path ( My belief being centripetal force was more at play than any true aerodynamics)

Just for discussion ...as a kid I got the idea from the Gee Bee that ONLY the props TIPS did ANY work

Side note.. The only man carrying aircraft I ever owned, was a Rally 2 B Sport Ultra Light with a Rotec pusher engine & unknown  prop ----and I don't remember a darned thing about the prop except TBO...actually Time Before Replacement (TBR)

And now it occurs to me that I really should be keeping a log book on every plane, engine, and prop like I did with the, sort of real, airplane

Thus the question... is there really a "time", AND "in use", TBO for model propellers?

I see MAS has some Finite life and "in use" notes within ALL of the "these products will Kill you" cautions

For example--- IN MAS states that we must use (the product)  within 3 years of purchase and discard after.....(very subjective as I have no idea HOW long it sat on the shelf)

They also state that the Max service life is 500 hours...   BUT how can I get 500 hours of service (for my $12 product) if I put it in service on the 34th month after production... hummmmmm?... Of course there is NO BORN ON DATE OF ANY Prop I ever bought....

OK OK, I am being deliberately obtuse.... I like to think the WC and NATs champs have a readily source for new, precise, and current perfect props.
BUT I do get the impression many of the champs also cannot source great props and have to fiddle lot within the High zoot versions

For my sport use, $3 to $15 props...I just see if they are close to balanced, trim some flash (if needed), and fly it until I break it...

I have wood props, in use, manufactured in the 60s..... That said, I dd put all the OLD TF Nylon props into the Wing Fillet bag

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Offline phil c

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Fred- the TBO for the ultra light was an ultra conservative estimate of fatigue life.  The engines, especially the 2 cylinder in lines, have a lot of torsional vibration.  Any prop has a finite life with vibration.
Wood props are nice because wood is a very complex composite  that gives it excellent fatigue life.
Unreinforced plastic is a blade waiting to let loose.  Reinforced plastic is similar but gives more life and usually gives signs, such as micro cracks on the front of the blade.
Fully reinforced carbon fiber tow props are practically unlimited life and rarely throw a blade because of all the fibers and the tremendous strength.
phil Cartier

Online RandySmith

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Has anybody tested an IC stunter with an audio tachometer to see how much change there is from launch rpm to flight rpm?  H^^ Steve

Yes  and  on typical  4 cycle stunt run on 45 to 88 stunt engines and  12  to 16 inch props  , we  got 400  to 1000  RPMs  difference

Randy

Offline bob whitney

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Re: Props with narrow versus wide airfoils. What difference would you expect?
« Reply #19 on: September 28, 2017, 03:45:09 PM »
 a few years back I got a narrow blade 12/6 carbon prop from Randy and made a carbon copy .we found that it worked much better than the APC 11/5 most were using. it drives through the maneuver without speeding up. /we have used it on LA 46, ST 46, and a PA open face 40
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Offline RknRusty

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Re: Props with narrow versus wide airfoils. What difference would you expect?
« Reply #20 on: September 29, 2017, 10:31:03 PM »
How about an engine that's built to run a 4-2 break, is there any significant RPM change between 4 and 2?
I had always assumed there was a big change, but I think I recall someone, maybe Brett, say there was very little RPM difference when it breaks.

I started this thread when I was a beginner in 2013, flying a 31oz. Goldberg Shoestring and struggling with an overpowered TT Pro .25(not much of a 4-2 breaker). At the time I was mostly using Master Airscrew props and they narrowed their width.

Now I think that neither that plane nor MA props are very good for flying PAMPA stunt. Except for one of my flying friends who loves MA props. You just have to keep in mind their claimed pitch number is exaggerated. Now I usually use APC because they're easy to buy or borrow in a pinch, have fractional sizes, and run well on my 25 through 46 FP and LA engines... though they are butt ugly. I've thought of dying them black.
Rusty
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Offline pmackenzie

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Re: Props with narrow versus wide airfoils. What difference would you expect?
« Reply #21 on: September 30, 2017, 06:31:51 AM »
Now I usually use APC because they're easy to buy or borrow in a pinch, have fractional sizes, and run well on my 25 through 46 FP and LA engines... though they are butt ugly. I've thought of dying them black.
Rusty

They take RIT dye well, and do look much better in black.

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Offline Paul Smith

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Re: Props with narrow versus wide airfoils. What difference would you expect?
« Reply #22 on: September 30, 2017, 07:06:40 AM »
Hey, isn't that David Fitzgerald standing there behind that Gee Bee? Probably a replica...the Gee Bee, I mean!  :o Steve

These racers were just like today's models.  In the absence of GEAR REDUCTION, the props ran at engine RPM.  They, like us, knew that the props were too small, but they couldn't do anything about it.  Later on, props were geared-down 3 or 4-to-1.  The last two major production planes without gears were the Beechcraft 17 & 18, which look like models because the tiny props.  They Bearcat & Corsair are examples of what planes look like with high power and geared props.
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Online Brett Buck

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Re: Props with narrow versus wide airfoils. What difference would you expect?
« Reply #23 on: October 12, 2017, 10:00:43 PM »
These racers were just like today's models.  In the absence of GEAR REDUCTION, the props ran at engine RPM.  They, like us, knew that the props were too small, but they couldn't do anything about it.

  Huh?  I *could* run a prop 25% larger if I wanted, and PA75 will turn a *16"* prop with no real problem if you wanted to.  You don't want to because large props hurt the performance.

   Larger props at low RPM are much more efficient than small props, and that is *poison* if you are putting a PA75 on a 625 square inch airplane.

    Brett

Online Brett Buck

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Re: Props with narrow versus wide airfoils. What difference would you expect?
« Reply #24 on: October 13, 2017, 11:21:38 AM »
Maybe by "us", Paul meant combat flyers (and racers, etc).

Preston

    Does anyone genuinely believe that they could use a reduction drive on a combat plane - the fastest of which are far out of the ability for most people to handle even with direct drive? Racing, when, again, at the end of fast rat they were to the point that almost no one could successfully fly the airplanes in traffic because of the speed?

   As far as I can tell, there is no one stopping anybody from doing it in speed, the weight doesn't matter and no maneuvering. You might go from 30% efficiency to 50%, sounds like about 18% increase in speed to me, which (if successful) would immediately beat everyone comprehensively. You might need variable pitch, too, because most people's arms are up to the task of whipping it up to speed with 12" of fixed pitch. So, sounds like a plan.

      It is ABSOLUTELY unnecessary in stunt (where people have certainly tried it), we are running at 10% of capacity in most of our engines and the biggest problem by far is how to dump most of it most of the time without too much trouble.

    Brett

Offline RknRusty

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Re: Props with narrow versus wide airfoils. What difference would you expect?
« Reply #25 on: October 14, 2017, 12:27:53 AM »
  ...You might go from 30% efficiency to 50%, sounds like about 18% increase in speed to me, which (if successful) would immediately beat everyone comprehensively. You might need variable pitch, too, because most people's arms are up to the task of whipping it up to speed with 12" of fixed pitch. So, sounds like a plan.

Brett
:! Yeah, shifter paddles on the handle, how cool is that. I'll get my people right on it!  :)! And maybe one of them micro superchargers I hear the mountain guys use... might get kinda parasitic on the top end though. But id'd sure look cool.
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Online Preston Briggs

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Re: Props with narrow versus wide airfoils. What difference would you expect?
« Reply #26 on: October 14, 2017, 10:42:55 PM »
Some free flighters have built/used geared .15s, so maybe...

I think Paul was just talking history of old racers like the Gee Bees, not suggesting any modelers do anything.

And while few people could keep up with a faster Fast combat model (as if more than a few people could keep up with them now), more power could also be used to pull along larger wings at the same speed, perhaps yielding quicker turns.

Preston

Offline Ken Burdick

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