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RE: Props-More Myths Busted - 12/18/2006 6:01:02 PM   
DarZeelon



Posts: 7104
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From: Rosh-Ha'Ayin, ISRAEL
Status: offline
quote:

ORIGINAL: MyWay

Lets see. A 7.4 inch dia. prop used on a good Jett, or Nelson unloaded to about 27,000 RPM in the air, has a tip speed of about 600 mph. Sure don't look to be inefficient to me.



They use a blade tip profile optimized for such speeds - i.e. not a Clark-Y... Planes do fly at such speeds, you know...

And, would you say for sure it is not noisy!? That was my point with this type of tip-speed.



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RE: Props-More Myths Busted - 12/18/2006 6:38:11 PM   
MyWay


 

Posts: 451
Joined: 2/21/2004
From: Benicia, CA, USA
Status: offline 		Your point about noise and safety are correct. My point about thrust and tip speed I believe are also correct. I have been testing different props and rpms for over a month trying to get something with good throttle response and good thrust. I found out the hard way you can have to much thrust. Still looking for the right combination.

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RE: Props-More Myths Busted - 12/18/2006 7:20:18 PM   
DarZeelon



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From: Rosh-Ha'Ayin, ISRAEL
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quote:

ORIGINAL: MyWay

...My point about thrust and tip speed, I believe, is also correct....I found out the hard way you can have too much thrust.



YourWay,


I am still not sure how you determined that maximum thrust from a prop would be available if it is spun so blade-tip speed is 600 mph. Can you give me a pointer?

Can you elaborate on how, exactly, you found out that you can have too much thrust?


< Message edited by DarZeelon -- 12/18/2006 7:22:01 PM >

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RE: Props-More Myths Busted - 12/18/2006 7:41:35 PM   
MyWay


 

Posts: 451
Joined: 2/21/2004
From: Benicia, CA, USA
Status: offline 		In general, thrust at static conditions seem to be only propeller dia. and rpm dependent. Pitch for what ever reason does not seem to provide any more thrust at any given rpm. As to to much thrust, on a 40 size fun fly the rudder was blown off in a flat spin. On a 25 size fun fly the servo gears stripped out in flight on the elevator. The modified OS 28 was putting out 6 1/2 lbs thrust with a 10/3 apc prop spinning 17000 rpm with a jett tuned muffler. The plane was a little over 2 1/2 lbs.

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RE: Props-More Myths Busted - 12/18/2006 9:04:28 PM   
DarZeelon



Posts: 7104
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From: Rosh-Ha'Ayin, ISRAEL
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quote:

ORIGINAL: MyWay

In general, thrust at static conditions seem to be only propeller dia. and rpm dependent. Pitch for what ever reason does not seem to provide any more thrust at any given rpm.



If you trust the ThrustHP it wouldn't , but if you are interested in the truth, a high pitch prop will give you much more thrust than a flat one.

See the PropPower calculator from Pé Reivers.

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RE: Props-More Myths Busted - 12/18/2006 9:21:32 PM   
aresti1963


 

Posts: 43
Joined: 11/25/2006
From: glenrothes, UNITED KINGDOM
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quote:

ORIGINAL: MyWay

In general, thrust at static conditions seem to be only propeller dia. and rpm dependent. Pitch for what ever reason does not seem to provide any more thrust at any given rpm.



So, what you're saying, MYWAY, is that a 10x0 (if you could get one) produces as much thrust as a 10x6??

I dont think so Buddy!!!!

quote:


Original : MyWay

As to to much thrust, on a 40 size fun fly the rudder was blown off in a flat spin. On a 25 size fun fly the servo gears stripped out in flight on the elevator. The modified OS 28 was putting out 6 1/2 lbs thrust with a 10/3 apc prop spinning 17000 rpm with a jett tuned muffler. The plane was a little over 2 1/2 lbs.



That doesn't show too much thrust..... it shows inadequate hinging methods and incorrect understanding of the leverage forces resulting from using the wrong servo geometory. ie, using a hole in the servo disc that is too far from the servo center. (or it could just be a faulty servo)

< Message edited by aresti1963 -- 12/18/2006 9:27:10 PM >

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RE: Props-More Myths Busted - 12/18/2006 9:37:38 PM   
jaka


 

Posts: 4536
Joined: 12/11/2001
From: Upplands Vasby, SWEDEN
Status: offline 		Hi!
What is it you try to explain???
Measuring static trust says nothing! Sorry! This is a well known fact among experienced fliers world wide. It's what happen in the air that counts!
Why 3 or more blade props aren't popular among experienced fliers is a well known fact too, have a look what's used in the competitive circle in both pylonracing and in aerobatics. It ain't a 3 blade Zinger or Graupner or MAS. and the reason for this is that they (the props) don't give the same performance (pulling power and speed) as 2 blade prop.

Why APC props are so common or let's be frank, dominating most competitive events, is just because they are good! People who are into pylonracing or aerobatics have tried different props and found out the hard way that APC is mostly the best prop. I know that there are other props too that are equally good but in pylonracing and in aerobatics APC's are hard to beat.

Regards!
Jan Karlsson
Sweden
MVVS distributor

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RE: Props-More Myths Busted - 12/18/2006 9:53:14 PM   
RVM


 

Posts: 1975
Joined: 3/12/2006
From: Sacramento, CA, USA
Status: offline 		Actually, I think what he is saying is that using a lower pitch allows you to use a larger diameter, thus allowing for more thrust. Obviously, there are diminishing returns, but there is a reason large diameter, low pitch props are used, especially for 3d.

quote:

ORIGINAL: aresti1963

So, what you're saying, MYWAY, is that a 10x0 (if you could get one) produces as much thrust as a 10x6??

I dont think so Buddy!!!!




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RE: Props-More Myths Busted - 12/18/2006 9:56:33 PM   
MJD


 

Posts: 1701
Joined: 5/27/2003
From: Orangeville, ON, CANADA
Status: offline
quote:

ORIGINAL: MyWay

In general, thrust at static conditions seem to be only propeller dia. and rpm dependent. Pitch for what ever reason does not seem to provide any more thrust at any given rpm. As to to much thrust, on a 40 size fun fly the rudder was blown off in a flat spin. On a 25 size fun fly the servo gears stripped out in flight on the elevator. The modified OS 28 was putting out 6 1/2 lbs thrust with a 10/3 apc prop spinning 17000 rpm with a jett tuned muffler. The plane was a little over 2 1/2 lbs.


That sounds like too much throttle, not too much thrust..

Static thrust doesn't blow off rudders or strip gears, airspeed does. And inadequate airframes/servos for the flight regime intended.

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RE: Props-More Myths Busted - 12/18/2006 10:16:39 PM   
MJD


 

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From: Orangeville, ON, CANADA
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quote:

ORIGINAL: RVM

Actually, I think what he is saying is that using a lower pitch allows you to use a larger diameter, thus allowing for more thrust. Obviously, there are diminishing returns, but there is a reason large diameter, low pitch props are used, especially for 3d.


Now that I can accept if that is what is being said.

On a related subject, I scratched my head a bit recently at a comment made by a well known engine reviewer, who said that for maximum static thrust, prop the engine for maximum torque, not maximum HP. "Static" thrust is a misleading term, in that the only static part is the speed of the engine relative to the surface of the earth. The air sure ain't static. To accelerate a larger mass per second of air to the same speed, or the same mass per second to a higher speed per second takes more power, not more torque. So for a particular engine to produce maximum static thrust it needs to run at it's maximum power output AND be coupled to the right prop for the job (ah, there's the rub!).

IOW, if he and I were squared off on a test stand with the same .60 engine, pipe/muffler, and fuel, and if prop choices were anything you want - I'd choose running the engine at maximum HP rpm with a large diameter fine pitch prop.

But I wouldn't be flying with that same prop unless all I was going to do was hover or fly slowly.

I liken the prop situation to a bell curve - there is an optimum air speed for a particular prop/engine combo, and on either side of that you start to lose some pull but it doesn't drop off like a rock. But if you go too fine on the pitch, the peak of that curve is sitting at or close to zereo airspeed, so you lose the "left" side of the curve and therefore your total flight speed regime is narrowed. But yeah, for 3D flying I guess that is pretty much where you want to be, or close to it depending on Vne for your airframe.

MJD

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RE: Props-More Myths Busted - 12/18/2006 10:44:41 PM   
pe reivers



Posts: 2782
Joined: 1/23/2002
From: Arcen, , NETHERLANDS
Status: offline 		They make up for that by wasting more power and don't care about that, as long as the end result is more speed. The plane would still be faster if a high efficiency could be maintained under these adverse conditions.

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RE: Props-More Myths Busted - 12/19/2006 12:02:39 AM   
speedster 1919



Posts: 1467
Joined: 12/17/2004
From: Martinsville, IN, USA
Status: offline 		JAKA My club has 2 pattern flyers that compete at the AMA Nationals. One uses a 4 blade and the other uses a 3 blade. I guess I better tell them to knock it off and put 2 blades on their planes because JAKA said so. Have you priced a 4 blade lately ? The price of 3 and 4 blades keep alot of flyers from using them.

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RE: Props-More Myths Busted - 12/19/2006 1:26:04 AM   
B.L.E.


 

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Joined: 8/27/2005
From: Austin, TX, USA
Status: offline
quote:

ORIGINAL: speedster 1919

JAKA My club has 2 pattern flyers that compete at the AMA Nationals. One uses a 4 blade and the other uses a 3 blade. I guess I better tell them to knock it off and put 2 blades on their planes because JAKA said so. Have you priced a 4 blade lately ? The price of 3 and 4 blades keep alot of flyers from using them.


I believe in pattern competition, there is a strict noise limit that must be met and a small diameter 3 or 4 blade prop has a lower tip speed than a large diameter 2 blade prop and thus lower noise. The extra blades allow a smaller diameter prop to absorb the power of the engine. It's not always about maximum efficiency.
Another thing you have to consider about competition in general. There is a lot of monkey-see-monkey-do in competition. If the person who won next year's F3A worlds did it with a biplane that was painted pink, you would see a lot of pink biplanes the next year.

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RE: Props-More Myths Busted - 12/19/2006 3:25: