Prop pitch cooling - the final word  
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All Forums >> Glow Engines, Gas Engines, Fuel & Mfg Support Forums >> Glow Engines >> Prop pitch cooling - the final word
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Prop pitch cooling - the final word - 2/14/2007 6:53:58 AM   
downunder



Posts: 3191
Joined: 10/10/2002
From: Adelaide, AUSTRALIA
Status: offline 		It's taken me a while to get around to doing this final test but I've tried to meet everyone's ideas as to what's needed for a valid test. For those who missed it before, a comment was made that a high pitch prop is necessary for good engine cooling on the ground. Someone had been running an engine with an 11x4 prop and he was told to use a 10x6. I disagreed as did a few others so I did an intial test using both those size props and didn't find any noticeable difference. Of course, I was soundly rounded on because I mentioned the 11x4 had a lighter load than the 10x6 .

Obviously to meet the criteria of one person I had to not only use different pitches but also match the load. So I decided to use a fairly high pitch prop (APC 10x8) and compare it with the lowest pitch prop I could buy which was an APC 12.25x3.75, the typical 3D prop. On initial runs the 3D prop had much more load so I trimmed 1/4" from each tip. This went slightly too far because now I was getting 11,300 revs compared to 11,000 with the 10x8 with no change to mixture setting (simply swapping props and restarting). I didn't want to alter the 10x8 because it's a good size to use for comparisons between engines and with other people's engines using that size. Instead I slightly repitched the outer 2" of the 3D prop and by sheer good luck it now matched exactly the 10x8 with 11,000 revs on both.

To do the test I'd start the engine, go to full throttle and give the temps time to settle then check revs and immediately check the temperature. Stop the engine, change props and restart and repeat the checking process without touching the needle valve. The tank was set up as a uniflow to avoid any leaning out due to head change of the fuel level between runs. Engine tune was set by ear at a level that I would fly with and not peaked out.

So here are the results as best I can possibly do them.
10x8...11,000 and 270F
11.75x3.75...11,000 and 270F

I think the findings speak for themselves .


       Post #: 1

RE: Prop pitch cooling - the final word - 2/14/2007 9:08:40 AM   
wjvail



Posts: 375
Joined: 9/5/2002
From: Meridian, MS, USA
Status: offline 		Can you post a link to the original thread?

I don't find your results at all surprising but we've had discussions on the sources of cooling for alcohol, two stroke engines before.

Bill Vail
www.RCScreenProtectors.com

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RE: Prop pitch cooling - the final word - 2/14/2007 11:03:21 AM   
speedster 1919



Posts: 1467
Joined: 12/17/2004
From: Martinsville, IN, USA
Status: offline 		Excellant Downunder---I'm not sorry I put you thur that . Now ,no one can say it was amount of fuel or one prop setting was too rich or too lean. and with an 8 pitch to a 3.75 pitch that is a big enough spread.
chaulk up 1 prop myth busted

Again good job -Who knows that trimmed prop might really scream now.

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RE: Prop pitch cooling - the final word - 2/14/2007 1:11:32 PM   
proptop



Posts: 4495
Joined: 10/18/2002
From: Rome, NY, USA
Status: offline 		"Final word"...good one D-U!
Thanks

I wonder how many final words there are gonna be?

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RE: Prop pitch cooling - the final word - 2/14/2007 2:00:55 PM   
downunder



Posts: 3191
Joined: 10/10/2002
From: Adelaide, AUSTRALIA
Status: offline
quote:

ORIGINAL: wjvail
Can you post a link to the original thread?

It took a bit of digging to find them but the rot started in post #9 in
http://www.rcuniverse.com/forum/m_5016672/tm.htm

I then started a thread at http://www.rcuniverse.com/forum/m_5025858/tm.htm which went wayyyyyyyy longer than it had any right to .

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       Post #: 5

RE: Prop pitch cooling - the final word - 2/14/2007 2:44:26 PM   
pe reivers



Posts: 2768
Joined: 1/23/2002
From: Arcen, , NETHERLANDS
Status: offline 		Chapeau DU.
The outcome surprises me to say the least! Indeed the myth seems busted, and thinking needs re-adjusting.

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RE: Prop pitch cooling - the final word - 2/14/2007 8:39:02 PM   
JNorton



Posts: 3135
Joined: 1/29/2002
From: Montague, MI, USA
Status: offline 		The results surprise me too as it is contrary to what I was taught. Like the old saying goes live and learn. Thanks for doing the test.
John

EDIT I don't know where the attachments came from as I did not add any?????

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< Message edited by JNorton -- 2/14/2007 9:39:45 PM >

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RE: Prop pitch cooling - the final word - 2/14/2007 8:52:41 PM   
DarZeelon



Posts: 7081
Joined: 4/9/2003
From: Rosh-Ha'Ayin, ISRAEL
Status: offline 		Brian,


Now you an try a flat prop, i.e. large diameter with just enough drag.

...No pitch.


We may find it is only radiation that cools the engine, not air-blast.

Your results are contrary to the experience of many 3-D fliers, who do suffer from over-heating, with the 3.75-4" pitch props.

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MVVS - Jett - Nelson - Bolly - Mejzlik

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RE: Prop pitch cooling - the final word - 2/14/2007 9:31:02 PM   
pe reivers



Posts: 2768
Joined: 1/23/2002
From: Arcen, , NETHERLANDS
Status: offline 		Hey Dar, be a sport and admit defeat. I know it is not easy, but it is something you gotta do!
Who will ever fit a zero pitch prop, or 90 degree angled blades. These are extremes that will prove you right for the zero pitch. The infinity pitch however also will not cool, and just stir some air. That is why testers who use these props also use an external blower.
Heck, I am mighty amused, even if I too was wrong.

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There is sanctuary in analisys.

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RE: Prop pitch cooling - the final word - 2/14/2007 9:39:52 PM   
Hobbsy



Posts: 10677
Joined: 12/2/2001
From: Colonial Beach, VA, USA
Status: offline 		Pe, as my Dad used to say, there ain't much in this mans creation that's set in stone, it makes life interesting.

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RE: Prop pitch cooling - the final word - 2/14/2007 9:42:25 PM   
Sport_Pilot



Posts: 7715
Joined: 1/21/2002
From: Acworth, GA, USA
Status: offline 		Was this measured with infrared? Wondering because there should have been some differance if only 5 or 10 degrees. I don't trust infrared thermometers because they measure infrared heat not temperature.

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RE: Prop pitch cooling - the final word - 2/14/2007 10:18:42 PM   
HighPlains


 

Posts: 2663
Joined: 3/2/2003
From: Over da rainbow, KS, USA
Status: offline 		I think that the fuel is what is really cooling these engines. Excess fuel and the oil carry out much of the heat.

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RE: Prop pitch cooling - the final word - 2/14/2007 10:32:04 PM   
pe reivers



Posts: 2768
Joined: 1/23/2002
From: Arcen, , NETHERLANDS
Status: offline 		Dave, DU, Dar, and all of you.
After pondering a while, this pitch related cooling seems to follow a bathtub kind of curve with engine on test stand. There seems to be a quite broad range of pitches that cool the engine well enough, even to about the same level.
@ HighPlains :
Oil cooling is another myth. The oil contributes extremely little to cooling. There is just too little of it, and it does not evaporate. Very little heat is required to bring that amount of oil to Exhaust gas temperature. Methanol OTOH has a very high heat of evaporation, and there is lots of it. All must evaporate for the engine to reach best power, or the engine will start fourstroking. That evaporation, and air movement through the cooling fins is what keeps the engine cool.
@ sport_pilot.
IR measurement is good enough for our purposes, because it measures temperature (a value), and not heat which is a quantity. I take it the cylinder head was not shiny on that old ST DU used for the test. Shiny surfaces and IR temperature measurements do not agree so well, but in the hands of the same tester can still provide comparable results.

< Message edited by pe reivers -- 2/14/2007 10:38:46 PM >

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Pe, Dealer for MVVS, MTW and Mejzlik; www.mvvs.nl
There is sanctuary in analisys.

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RE: Prop pitch cooling - the final word - 2/15/2007 12:04:57 AM   
gkamysz


 

Posts: 1575
Joined: 6/24/2003
From: Crystal Lake, IL, USA
Status: online 		I did some math and it seems that the cooling from evaporation of methanol is fairly small, but not insignificant. For a .40 burning 1 liter per hour of FAI fuel 80/20 it comes out to 240 watts of heat to evaporate the methanol. Compare this to the 450W in output at the shaft and roughly 4500W of heat made when the fuel burns. A good portion of the 4500W goes right out the exhaust. When you run rich, you increase the cooling effect without increasing the potential heating beause you can only burn so much fuel with the air available.

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Greg

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       Post #: 14

RE: Prop pitch cooling - the final word - 2/15/2007 12:26:05 AM   
TimC



Posts: 2257
Joined: 1/7/2003
From: Lone Pine, CA, USA
Status: offline 		Thanks for the test downunder. I wonder how a head button effects cooling? It would seem the heat would have a harder time passing through the lamination. Much as with a sleeved cylinder.

(in reply to gkamysz)