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I have an RV and an ES. When RV8 guys put the 200hp Lycoming in their planes, they had cooling problems. One interesting way they dealt with it was to install
a louver vent on the bottom of the cowl similar to your heating vent at home. This vent had a sliding plate on the inside to open and close it. It produced a nice low pressure pull when open and very little drag. It was also easy to install. Just a thought.
Chat Daniel
Super ES N891AC
RV8 891AC
From: Lancair Mailing List [mailto:lml@lancaironline.net]
On Behalf Of Terrence O'Neill
Sent: Monday, May 31, 2010 10:51 AM
To: lml@lancaironline.net
Subject: [LML] Re: FW: [LML] Re: cowl flap cooling drag reduction
Many thanks for the info. Another vote for retaining the inlet area, and modifying the cowl to closeable cowl flaps.
Again reminds me of watching the cowl flaps close down to not more than a crack around the P2V's big CW 3350s when going to cruise... and marvelling at how such a small amount of exit air could cool those big engines... Also intend to
try to radius the bottom of the firewall in the cowl exit area... trickey geometry at the nosewheel strut... and maybe make a sealing fairing around the two exhaust stacks on the left side of the cowl.
On May 30, 2010, at 8:54 AM, Frederick Moreno wrote:
I put cowl flaps on my Lancair IV along with a top cooling air plenum and I also went to fairly extreme measures to eliminate cooling leakage around the engine. Inlet area remained
unchanged (6 inch diameter) although the inlets were raised 1.5 inches for a straight shot in and moved outboard 1.0 inch so that I could insert a boundary layer dam between spinner and inlets. This removes the boundary layer coming off the spinner as well
as the slower-moving separated wake flow at the shank of the propeller where it enters the spinner. I drastically cut the exit area with cowl flaps closed compared to the stock Lancair outlets which are meant for turbo engines at 25,000 feet. (I believe
turbo engines would gain no benefit from closable cowl flaps – they need all that air flow at altitude.)
Net result was that overall flat plate drag area went down (compared to stock) perhaps 7-10% with the cowl flaps closed, and it runs cold at 65% lean of peak, CHTs below 300F, lower
than 250F at low altitudes. With cowl flaps open (and lots of exit area) I can climb unrestricted at Vy (135 knots, best power) starting at near sea level on a 90F day. Difference between cowl flaps closed and faired and wide open and deflected outward about
three inches is about ten knots. That does NOT mean a ten knot speed improvement over stock. It means that when the cowl flaps are open, they are draggy.
I have measured pressure drop across the engine and even with cowl flaps closed, it is too high (too much cooling air, confirming temperature data). So last week I made modifications
to reduce the closed cowl flap exit area another 25%. No testing yet.
Would I do it again? Maybe. It was a lot of work. Estimate of the overall speed benefit: perhaps 3.0-3.5%. It sure is nice to climb unrestricted on a hot day.
Okay, so I think I would keep the L235 cowl inlet area as-is, and be generous with the flappable outlet area.
Actual experience is very reassuring.
I also reduced the inlet area about 20%, adding a diffuser section. The outlet area was reduced to about half of the original, depending on what kind of bulges on the bottom are assumed to be "standard." The cooling is marginal, but adequate - on hot days
climb speeds have to be increased and cruise is done with the flaps open. This is to keep the CHT's below 400 and oil below 220. Closing the relatively small cowl flaps increases the speed by 2 to 4 knots.
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