To Plenum, Or Not........

[DanH]

Originally Posted by Jamie Aust

I removed top ( white ) plenum, note that the ramps at the front are part of the plenum. I then fitted the standard black baffle seal around the edges as per the Vans FWF install plans.

So what sealing measures were in place around the plenum inlet?

[RV8RIVETER]

Originally Posted by DanH

So what sealing measures were in place around the plenum inlet?

Also, how is the air being controlled around the cylinders?

[Jamie Aust]

Originally Posted by DanH

So what sealing measures were in place around the plenum inlet?

you can see in this photo, the black seal for the bottom, the same was used for the top cowl.

Also, how is the air being controlled around the cylinders?
No other control around the cylinders fitted or removed.

[Weasel]

I considered to plenum or not to plenum when starting the RV-10. I had some experience by helping a friend finish a -10 with a homebuilt plenum and also converting a -7a from stock to plenum that resulted in a 30 deg drop in CHT's at the same conditions and retained the same TAS.

The goal with the RV-10 was to spend as much time as necessary to produce maximum airspeed at cruise without overheating in the climb.

The louvers on the bottom of the -10 that most people are using looked less than optimal to me so I set out to eliminate them and still cool the engine.

The engine is a IO-540 C4B5 that was fitted with 9.1:1 CR pistons and new flow matched tapered barrel ECI Cyl's.

Here is some of the baffle and plenum pics.





here is a pic of the cowling to inlet intersection. The gap that you can see is sealed completely around the inlet with a rubber seal.


below the engine we tried to help smooth the air flow as it went out the exit.



I flew the first 25 hrs or so and determined that about the only way to keep the CHT below 400 F in the climb was to reduce climb power to 25" MAP and 2500 RPM and keep forward speed up around 120 KIAS.

I still was against putting louver on the bottom but felt I needed a slight increase in outlet area so I opened it up 3/4" like this.



After these modifications the cooling system will keep the cyls below 400 F when the OAT is 100 and plane is at gross weight taking off with a heat saturated engine with the Air Conditioner eating up 70 amps on the 24 volt alternator and climbing at 110 KIAS.

As far as efficient cooling flow goes. I have no way to quantify the amount of cooling drag reduction, if any, that was gained because this was not a modification but rather incorporated into the initial build. However, It will consistently fly 179-181 KTAS on 12.0-12.4 GPH so I have to believe we gained something for the effort.

I think there are gains to be had by utilizing a plenum but its not gonna work to just smack a lid on the stock baffles and expect good results......it takes work and work and lots of it!!!!!!!!!!!

[RV8RIVETER]

Jamie

Hard to have a good opinion from the web. You are using a parallel valve engine and a stock cowl, which should not be an issue. There are ways to improve the exit, see Vetterman's fairing and Bob Axom's work. I would say that using stock size inlets that you should have way more than enough air available. I would suspicious of how well the upper is sealed. Once pressurized there will be high pressure at the leading edge of the inlet all the way back. The seal needs to be pressure tight, like a balloon. And it needs to make the air go through the cylinder fins to get out.

Your rear cylinders are hottest, so they are not seeing the same amount of mass air flow as the front. Therefore, leaks are robbing them, the pressure below them is higher, and or the flow is being diverted elsewhere.

Once you have everything sealed well you want to make sure you have the air going where you want it. To increase your efficiency and get the maximum amount of heat transfer, the air should stay inside the fins all the way around. Borrowing the NACA radial engine research I made cylinder baffles like those suggested for radial engine cylinders. The barrel shrouds were made with high temp epoxy and they are band clamped to the cylinder. The plenum hugs against the head fins and does not let it out until it gets to the bottom. Air wants to follow a curved surface so that is why the entry edges are curved.

Sorry, no really detailed pictures yet. I made a "half engine" mock up with red tagged cylinders. Here is a simple quick sketch and the pictures I have handy.



Early first try. Ended up extending to the base to be able to clamp them.

[Jamie Aust]

Wade,

Thanks for the info and pictures.
On the weekend we changed from a plenum to STD Vans Baffle system, I had some help from a friend that has made many baffles on Rvs and all of them have very low temps.
We did everything that we could to seal all gaps, knowing that a small leak can cause problems, and yet I still have problems.
I might look at re-fitting the dam wall in front of #1 to see if that pushes air back to #3.
All the other local Rvs are around 20 - 30 F lower than mine, I guess I just have to keep looking for the problem.