RE: YS is the only choice for racing  
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RE: YS is the only choice for racing - 11/1/2003 2:39:59 AM   
Red Baron Mike



Posts: 295
Joined: 1/24/2003
From: Rocklin, CA, USA
Status: offline 		Stick with a 2-stroke unless you get into the 1.20 size 4-strokes and can turn a 14" pitch prop and run 50% or higher nitro.

The "layman's" formula is to first pick the largest and most powerful 2-stroke engine allowed.
Next pick the smallest diameter prop that will fly the plane. Finally you pick the greatest pitch in the prop diameter you already decided on. I run a 10x10 APC prop on a World P-51 with a .61 on mini-pipe turning 14,500 RPM at about 160 MPH.

Good Luck,

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Red Baron Mike

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RE: YS is the only choice for racing - 11/1/2003 10:15:47 AM   
Ed Smith


 

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quote:

I run a 10x10 APC prop on a World P-51 with a .61 on mini-pipe turning 14,500 RPM at about 160 MPH.


Red Baron Mike,

Do the arithmetic. 10" pitch @ 14500rpm will give you 136.3 mph max.

Ed S

(in reply to Red Baron Mike)
       Post #: 27

RE: YS is the only choice for racing - 11/1/2003 4:31:41 PM   
PylonWorld



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Joined: 12/29/2001
From: Monroe, NC, USA
Status: offline 		Ed,

That doesn't account for unloading in the air.

An engine on a minipipe will unload a lot in the air. An engine not on a pipe still unloads at least 750-1500 rpm.

There is some software on an Aussie site that lets you use a computer with a microphone to determine in-air rpm. I tried it on the F3D/30 with a .36 on a muffled tuned pipe. The prop was an APC 7.4x8 and I left the ground at 16,000. In-air rpm read about 19,500. It actually read over 20K at times.

So, with a highly loaded engine with the prop cavitating on the ground, you can't predict in-air speed.

< Message edited by PylonWorld -- 11/1/2003 11:18:11 PM >

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Don Stegall
RCPRO Chairman of the Board

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RE: YS is the only choice for racing - 11/1/2003 5:08:31 PM   
js3



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From: Arvada, CO, USA
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quote:

There is some software on an Aussie site that lets you use a computer with a microphone to determine in-air rpm.


Don,

Do you have the URL handy for the AU site? Or failing that, do you have the program and instructions that you could email me? This program would be a good thing to make available on your web site. I don't know about copyright permissions etc but it would be nice.

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John
I feel a lot more like I do now than I did earlier!

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RE: YS is the only choice for racing - 11/1/2003 5:21:41 PM   
PylonWorld



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Joined: 12/29/2001
From: Monroe, NC, USA
Status: offline 		John,

The web site is http://www.alphalink.com.au/~hughan/

I looked on the "Low Cost Speed Measurement by Doppler" but I don't see the links for program or the Excel Spreadsheet.

He says that he has a Quick Basic program for the calculations ... I used the spreadsheet.

Try emailing John Hughan from the "Send me an E-Mail" page.

I had this stuff on my wife's laptop and its hard drive died. So I had to get her a new laptop for the Kitchen and around the house use.

I may or may not have the programs on my main computer.

It is an interesting experience. Basically you have to average the plane coming toward you and going away from you, because of the doppler effect. But it's real easy to pick out the turns and the straights.

< Message edited by PylonWorld -- 11/1/2003 11:17:52 PM >

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Don Stegall
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RE: YS is the only choice for racing - 11/1/2003 8:31:25 PM   
Ed Smith


 

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quote:

That doesn't account for unloading in the air




I used the figures given. Which ignores all of the drag and prop inefficiences. Maybe I am too skeptical of some of the mph figures posted.

Ed S

< Message edited by Ed Smith -- 11/1/2003 3:35:04 PM >

(in reply to PylonWorld)
       Post #: 31

RE: YS is the only choice for racing - 11/1/2003 8:42:47 PM   
PylonWorld



Posts: 1346
Joined: 12/29/2001
From: Monroe, NC, USA
Status: offline 		Ed,

I've seen the formula, and I could work it out, but can you post it?

With a 10x10 prop at 14,500 on the ground, and unloading to 17,000 in the air, what would the mph be (disregarding inefficiencies and drag)?

I understand the skepticism. I had an old F1 guy tell me my 424 plane was doing 170 mph or better (125 max) ... When I mentioned that people often over-estimated the speed of planes, I got one serious lecture.

< Message edited by PylonWorld -- 11/1/2003 8:43:26 PM >

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Don Stegall
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Pitch Speed - 11/2/2003 3:57:48 AM   
PylonWorld



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Status: offline 		Ed,

I did the math and I think I figured out the reduced formula.

Is it:

Max Speed = ( (pitch inches * rpm) / 1056 )

If Red Baron Mike's engine unloads 2500 rpm in the air, the max speed would be 161 mph. That does not account for drag and prop inefficiency. However, with a 10" pitch on that horsepower of engine, it may even unload to 17,500 which would yield a theoretical max of 166 mph.

I should add a max speed calculator page to www.PylonWorld.com

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RE: Pitch Speed - 11/2/2003 10:04:08 AM   
Ed Smith


 

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quote:

I did the math and I think I figured out the reduced formula.

Is it:

Max Speed = ( (pitch inches * rpm) / 1056


Or,

Theoretical MPH = Pitch x rpm x 0.00094

The danger of inflated speed claims is it does harm to our hobby. A few years ago, about the time the off course worker came into being, there were claims of 200mph Q40s. The AMA got scared. Then there was the usual clamour to slow everything down. This led to all sorts of arguments about detuning engines, different airframes, changing the rules, all our equipment will be obsolete, on and on ad nauseum. Then somebody actually checked the speeds accurately. The actual speeds were found to be much lower than the wild claims. Too late, the damage was done. The perception of Pylon Racers now is, and always will be "Oh, those dangerously fast noisy things"

Ed S

(in reply to PylonWorld)
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RE: Pitch Speed - 11/2/2003 2:58:47 PM   
Stand



Posts: 94
Joined: 1/1/2003
From: Austin, TX, USA
Status: offline 		There is a fairly good write up on propellers for powered model airplanes at:
Aeronotes

The write up is mostly concerned with control line speed, but the same principles apply. There is some info on efficiency as a result of wind tunnel tests. Big aircraft obtain up to 90% and most small propellers range from 60% to 80%.

There's also a comment there about the airfoil in a model propeller being so small that it is almost impossible to reproduce exactly. We see this from experience, take a bag of 9x6 APC's which presumably come from the same mold, tack them all and you will see a variation of up to 800 RPM from one to the other. This is why I have a specific prop that I use only for test tacking engines. This way I get a more accurate comparison of engines.

S Douglas

(in reply to Ed Smith)
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RE: Pitch Speed - 11/2/2003 10:27:53 PM   
Ed Smith


 

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quote:

Big aircraft obtain up to 90% and most small propellers range from 60% to 80%.


Now that is interesting. So using 70% some of the speeds claimed with pitch and rpm quoted seem even less likely.

Ed S

(in reply to Stand)
       Post #: 36

RE: YS is the only choice for racing - 11/6/2003 2:03:47 AM   
Red Baron Mike



Posts: 295
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From: Rocklin, CA, USA
Status: offline 		Ed,

I "did" the math. The plane was clocked with a radar gun.

Cheers

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Red Baron Mike

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Air Speed Formula - 11/6/2003 2:12:04 AM   
Red Baron Mike



Posts: 295
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From: Rocklin, CA, USA
Status: offline 		Ed,

The plane I referenced was clocked by radar gun at 150, not 160 (type-O). I am editing the RCU Post.

As regards the formula you use in your comment to me "do the arithmetic", please advise what formula you use.

Tx,

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Red Baron Mike

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RE: Air Speed Formula - 11/6/2003 1:35:32 PM   
Ed Smith


 

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Joined: 12/29/2001
From: Brantford, ON, CANADA
Status: offline 		Theoretical MPH=Pitch x RPM x 0.00094

Pitch x rpm will give you how many inches traveled in a minute. The 0.00094 figure converts that figure to MPH.

It must be emphasised that the formula ignores drag and all of the inefficiences associated with propellers. By manipulating the formula the required pitch/rpm combination can be determined to reach a required speed. Remember, in straight and level, no wind flight the airplane can go no faster than the prop can pull it. So using the figures from your original post, factor in say 80% efficiency from another post it can be seen that in straight and level ,no wind flight, your reading of 150 mph is questionable. It can also be seen that unless accurate figures are used some speed claims are optimistic at best.

Ed S

(in reply to Red Baron Mike)
       Post #: 39

RE: Saito FA-40a engine for a pylon racer? What do ya t... - 11/25/2003 6:34:06 PM   
PylonWorld



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From: Monroe, NC, USA
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quote:

ORIGINAL: PylonWorld

quote:

Is it really that much easier? I'd still have to mask. I've just painted small areas of monokote before (like around the canopy). I've never tried a significant amount of