McFly

McFly

Well Known Member
I am in the process of making an all metal plenum for an O-360 on an RV-7. I am using Vans baffle kit as a starting point. I have a few questions that are all over the map so please bear with me.

1) The baffle kit directions state .5? as the baffle to cowl distance. Is .5? really sufficient for the side baffles to upper cowl distance (along the rocker boxes)? I don?t want the plenum smashing into the top of the cowl.
2) Is a close fitting plenum top generally more efficient than a roomy one? Is there a magic engine/plenum distance that provides the best cooling?
3) Can I assume the fiberglass inlet ramps provided in the finish kit are non-structural?

Thanks.

Hugh "McFly"
 
Yes

Yes, but if you are using that to size your metal plenum clearance that's a different question and I don't know the answer. You are doing the development here and you are not only eliminating the rubber interface but you are also reducing the volume of the plenum.
Don't think so. Don't know.
Yes.

Sit back and put on a rain coat because this will probably open the flood gates on the subject once again.

Bob Axsom
 
Last edited:
McFly said:
2) Is a close fitting plenum top generally more efficient than a roomy one? Is there a magic engine/plenum distance that provides the best cooling?
Click to expand...

From what little I've read about this (the odd NACA report or two), I think "efficient" in this context means "provides the most cooling with the least drag" so a bigger plenum would be best. The intent is to bring the air close to stagnation (i.e. almost stop it), then through the fins at a lowish speed for lowest pressure loss, then accelerate it again before it exhausts from the cowl.

That suggests that a big cross-sectional area between the engine and the plenum is what you want.

Will you be able to measure difference in top speed? I don't know...but I do know that cooling drag is a big fraction of the drag budget for a clean little airplane like an RV. Reducing it even a little is worth some work.
 
Metal Plenum

I also fabricated a metal plenum on my -8 using Van's baffle kit as a starting point. I tried to maintain the 1/2" clearance, but in several places I believe it's probably closer, really kinda hard to tell. I have not seen any signs of interference though. It gets real tight on the top of the engine case!
This is the same engine, oil cooler setup that came off my -6, but it had the rubber seal on it. I can tell you that the current setup is producing much lower oil temps than I saw in the -6.
I did leave off the ramps, as I believe they are for directing air at lower speeds and high angles of attach in an open cowl. Not necessary with the plenum.
Did I see any real advantages to building the plenum? I can't really say, but I pretty sure it is helping with my oil temps. That alone was worth the effort to me. I only have one EGT, CHT probe, and it's running about the same as the temps were on the -6. I have no reference to say whether I picked up any speed.
Good luck with your plenum. I don't think you'll regret the little bit of extra effort it will require.
Bill Waters
N217BW
RV-8
 
Don't overlook a bit of a time saver and probably money saver too in that the Sam James Plenum has already been engineered for proper cooling and less drag.
And if I understand correctly, it fits under the standard Van's cowl except that you'd want to customize ducts to match up the Vans inlet to the round inlet of the SJ Plenum.

I've seen a couple of the home-made aluminum plenums and thought seriously of going that route until I became less clear about how the cooling issues have to be handled other than to just put a box on top the engine.

So for those of us who don't have the confidence to insure that the airflow physics happens as it should, the SJ option is pretty tempting.
 
A great Plenum

Maybe he is on the list but tracking him down will save you a bunch of work. Mark Phillips (51PW) has one of the neatest plenums I've ever seen. It looked as simple as a plenum can be and if I were going to do one I'd copy his model. Sorry I don't have any other contact information.

Darwin n. Barrie
RV-7 N717EE
 
Another great one is Bob Watson's award winning -9A. He is out of California. Glenn Brasch, Tucson.
 
Plenum Design Ponderings

Hi Hugh,

Like you I looked at building a plenum, and after speaking with a friend who was a big wheel at Lycoming, I was encouraged to go ahead with the idea.
I simply used the Vans baffle kit and added an alloy sheet to the top to form a closed plenum. It it important to get a smooth transition from the fibreglass inlets into the chamber, and try to keep the internal volume up by retaining as much of the vertical height of the baffle sides.
My 4 CHT's run around 300 - 320degF in cruise, around 360-380 in climb - and that is on 85 degF days in summer.
Go for it !

Martin in Oz
 
RV7Guy said:
Maybe he is on the list but tracking him down will save you a bunch of work. Mark Phillips (51PW) has one of the neatest plenums I've ever seen. It looked as simple as a plenum can be and if I were going to do one I'd copy his model.
Click to expand...
Ditto, I saw Mark's plenum at OSH and it is the pick of the litter design as far as I'm concerned. I've installed a Sam James plenum before and if I were to do it again Mark's would be the one I'd copy.
 
You must log in or register to reply here.