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Replica Aircraft Topics related to the construction of WWI replica aircraft

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Old 29 July 2009, 09:04 PM   #1 (permalink)
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Cooling issues Rotary vs. Radial

Sorry if this has been asked before -

are there any cooling issues involved if someone stuffs a radial engine behind a, let's say Fokker Dr.I cowling that was meant to have a self-cooling rotary in it?
I would think that the bottom cylinders get all the fresh air and the top ones none. Can't be good.
Any real world experiences?
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Old 30 July 2009, 07:02 PM   #2 (permalink)
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Boy, am I going to get some flames on this, but that is how we compare theory to reality.
I am pretty sure that the spinning of the rotary helping cool it is a myth (donning asbestos skivvies now). A cylinder spinning at the speed of a rotary would end up with each cylinder moving through the wake of the previous one. This is NOT good for cooling and basically would be like having the cylinders spinning in a mass of turbulent air with little heat transfer over what they would get if the engine wasn't turning. To get good heat transfer you need to reduce the speed of the air over the hot parts, which increases the pressure. Increasing pressure will increase heat transfer. Reducing turbulence will also improve heat transfer. That is one of the reasons you usually PULL air through a radiator and don't PUSH it (notice you never see a radiator fan in front of a radiator, you DON'T want turbulence!) and in a flat 4 engine the designers do everything possible to smooth the flow of air over the fins with NO LEAKAGE (cooling air will take the easiest path, which is NOT through the fins). Just the opposite of what is happening in a cowl with a rotary.
The uneven heating of a radial in a rotary cowl IS a problem, but not because the engine isn't spinning, it's because some cylinders are getting clear flow and others aren't. The solution is to close off all the spaces around the fins so all of the air must pass through the fins, not past them. The cowl in front of the engine becomes a pressurized (by the air flow) plenum pushing the cooling air through the fins to the low pressure plenum behind the engine.
OK, I'm ready. Does it make sense?
Hank
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Old 31 July 2009, 05:05 AM   #3 (permalink)
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Hank,
I would like to agree with on this as it makes sense. The GWFM round engine aircraft all work well with fully enveloping cowls (N28, 1-1/2 Strutter) and all engines have the inter-cylinder baffels in place. When originally built the 165 Warner powered triplane had a 'traditional' closed upper section cowl and experienced cooling problems because of the lack of even flow through the fins. (Conjecture at the time. None of the original build team is still with the museum to validate how this was determined) The upper blanking plate on the cowl was removed and everything deemed OK. That was great until I saw the 7 Dr1s at Omaka, all with 'traditional cowls' and 6 with Warners! Apparently none of these aircraft have cooling issues. What I didn't ask is whether they go on 1 to 2 hour cross-countries like our aircraft do. We are planning on testing one of the DR1s with a blanked upper cowl later this fall.
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John
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Old 31 July 2009, 05:29 AM   #4 (permalink)
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Quote:
Originally Posted by hank jarrett View Post
Boy, am I going to get some flames on this, but that is how we compare theory to reality.
I am pretty sure that the spinning of the rotary helping cool it is a myth (donning asbestos skivvies now). A cylinder spinning at the speed of a rotary would end up with each cylinder moving through the wake of the previous one. ....
OK, I'm ready. Does it make sense?
Hank
Yes, but ...
The engine is mounted behind the prop. Axial component of the air velocity is about 40 to 60 m/s. Radial component is about Pi * 1.0 * 1200 / 60 = 63 m/s so the wake is shifted a bit.
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Yavor
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Old 31 July 2009, 05:31 AM   #5 (permalink)
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rotary cooling

I had supposed that one of the components of rotary cooling was the rich mixtures they seem to burn, as well as the total loss oil systems. It might be that the benefits of washing unburned fuel and oil through the cylinders doesn't help that much OR I am off-base on the rich mixture idea with regard to rotaries.
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Old 31 July 2009, 06:15 AM   #6 (permalink)
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Oil that doesn't burn, but does get hot and then gets thrown away would have a significant cooling effect. That is a good reason for Castor Oil. It gets VERY hot before it burns and all of that heat is passed out of the exhaust. Rich mixture does pretty much the same thing. I had a friend who flew P-51s in WW-II. He took a hit to his cooling system that caused a total loss of coolant. He flew all the way back to England by pumping extra fuel into the engine with the primer making it VERY rich, but not so much as to kill the engine or set the plane on fire. He said his palm was an open wound where his flight glove wore through by the time he landed and the engine was shot, but it never stopped and he brought it home, with NO COOLANT in the normal system. Half way home I think he was more worried about running out of fuel than engine failure.
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Old 31 July 2009, 06:29 AM   #7 (permalink)
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Hi , FWIW. (Who said "Not much " ? )....First , just a small point is that the cooling fan bit on cars is more to do with the need to cool when stationary , which of course doesn't apply to an aeroplane in flight ......Re. the cooling of rotaries , there will exist ( I think ) a swirl flow behind the cylinders due to their motion , which assists in constantly evacuating the hot air (which has passed through the rotating cylinders ) out behind and from under , more to one side , whilest assisting a smaller incoming ballencing flow from the other ...The entry flow would seem to be adaquate .? ..Often the "key " to good cooling is making sure that the hot and expanded air is adequately exhausted after it has passed through the cooling fins...Often a hot collector ring lies behind the cylinders on a radial and all this hot air has to be dispelled ..The outlets need to be carefully placed (for "looks" as well as "cooling" )to get rid of all the hot air ,just as /more importantly than/as itis to achieve the inflow....Inter-cylinder baffle plates (usually a std. fitment on radials ? )would ,as Maxim sugests best be retained......At the end of the day , it's all down to experimentation ....How say you all ! ?...Regards John
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Old 31 July 2009, 06:32 AM   #8 (permalink)
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Quote:
Originally Posted by hank jarrett View Post
...
I had a friend who flew P-51s in WW-II. He took a hit to his cooling system that caused a total loss of coolant. He flew all the way back to England by pumping extra fuel into the engine with the primer making it VERY rich, but not so much as to kill the engine or set the plane on fire......
Hank
Boy, what awesome presence of mind. It pays off to be a mechanic as well as a pilot. Does a guy in that situation figure it out on the spot or is the solution sort of thought out beforehand based on a conscious knowledge of principles? I find this fascinating and ingenious!
marc
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Old 31 July 2009, 06:48 AM   #9 (permalink)
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I have no idea when he came up with the idea or if someone had told him, but he still had the scars on his palm and he was a good 25 years older than me. He WAS an actual tool user (and a good one) as well as a great pilot, but I don't know when he started being interested in building airplanes. I would imagine if he didn't know a lot about engines before, he saw the value after!
He is very old now and I haven't talked to him in years. I need to.
Hank

P.S.
He also is documented to have been in actual combat with an ME-262, and lived to tell it. Came back to the states with an English bride and flew for the airlines. Really neat guy.
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Old 31 July 2009, 07:23 AM   #10 (permalink)
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My quick 2 cents. Hank's position regarding the wake of the cylinders has some validity, but lets look at it differently. The rotary cowlings were frequently cut away on the bottom and this helped them dump waste oil, fuel, etc out the bottom of the airframe away from the pilot's face. The cut out also has the effect of supplying fresh air into the cylinder environment which is then taken for a whirl around the engine and then dumped out when it gets to the cut away again. So you do get cooling with this cowling configuration, just it has a little longer contact time to transfer heat throughout the swirling mass of air around the cylinders. Have I confused anyone yet?
As I understand it, with a radial engine masquerading as a rotary, you need to have some sort of a gap at the aft end of the cowl to ensure air exchange or otherwise, you will have cooling problems.
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