cooling problem

[vmcurry]

has anyone used the Sonex laser cut baffling on a Sonerai. Can the fiberglass cowl handle the heat above the engine or must it be boxed in with aluminum. I did not build this aircraft. trying to fix a number of issues. rear cylinders too hot. current baffle sits right down on top of cyl heads. air coming in goes thru front cyl fins then to rear cyl fins. point me in right direction

 

[ThePipster]

Yes and no.......

So I bought the Sonex baffle kit and used only one part, the rear baffle face, I ended up needing to make every other piece from scratch and the rear baffle is the bit that is the worst fit of all of them!

OK, some basics, How big are your inlet holes in the front of the cowling and how big is the outlet? Mine started out with 32sq inches of inlet and 24 inches of outlet, courtesy of the original builder. This would never work, the rule of thumb is an oulet 1.3 times bigger than the inlet. Not an absolute ratio but a good place to start.

Next, baffles have to fit. Leaving a 1/4" gap around the baffles just means that the cooling air has a route to escape before doing its job, Tight as possible please and good baffle sealing tapes are essential.

A lip can make all the difference. A small angled lip on the exit can help with an airflow draw through the cowling setup that can will drag temps down.

Finally, attention to detail and really trying to ensure that all the available air passes through the cooling fins can mean the diffrence between great engine temps and constantly worrying about cooking the heads......

ATB Pipster

P.S. This note from Bob Hoover (not the test pilot!) seems to show good results .....

http://bobhooversblog.blogspot.com/2006/12/vw-head-baffles.html

 

[eschrom]

I don't know anything about the Sonex kit but do have a few other thoughts that might help. Yes, the cowl can handle the heat. I've used a fence type baffle with the cowl as the top of the plenum with no adverse effects. My latest arrangement though is tight fitting boxes and man, do they work! With the fence deal I saw 375 F in cruise and now it's 275, measured in exactly the same place. A whopping 100 degree reduction. Maybe too cool but that's easy to fix. My inlets are 6 sq. in. per side. Offhand I'm not sure what the outlet is, probably 15-20 sq. in.

The tops of my boxes are about a half inch above the fins, not right down on them, but on the back side of the rear cylinders they wrap tight to the fins all the way to the pushrod tubes on the bottom. In front of the front cylinders there's a wrap so the air can't short circuit by that route. The wraps are made of plain old fiberglass with PE resin and are withstanding the heat.

My oil cooler, a 72 row plate type, is under the engine with about 3/4" clearance to the case. The air comes in the smile and into the gap, then down through the cooler. I have a summer smile that's 5 or 6 sq. in. and a cool weather one that's half as much. The oil stays below 210 in the summer and in the fall I have to partially plug the smile to get it up to 180.

I'm not trying to steer you toward boxes, only letting you know that they do work. But so does a fence and it has the advantage of easier maintenance.

One other thing to look at is the gaps between the fins. If they are clogged with flashing, take some time and clean it out. It makes a difference.

Ed

 

[cosmic]

My inlets are 6 sq. in. per side. Offhand I'm not sure what the outlet is, probably 15-20 sq. in.

Did I read that right? 6 square inches seems very small.

 

[eschrom]

Six is correct. 6" x 1".

 

[eschrom]

I estimate the area of the standard 'D' shaped opening to be about the same.

 

[johnbouyea1]

I don't know anything about the Sonex kit but do have a few other thoughts that might help. Yes, the cowl can handle the heat. I've used a fence type baffle with the cowl as the top of the plenum with no adverse effects. My latest arrangement though is tight fitting boxes and man, do they work! With the fence deal I saw 375 F in cruise and now it's 275, measured in exactly the same place. A whopping 100 degree reduction. Maybe too cool but that's easy to fix. My inlets are 6 sq. in. per side. Offhand I'm not sure what the outlet is, probably 15-20 sq. in.

The tops of my boxes are about a half inch above the fins, not right down on them, but on the back side of the rear cylinders they wrap tight to the fins all the way to the pushrod tubes on the bottom. In front of the front cylinders there's a wrap so the air can't short circuit by that route. The wraps are made of plain old fiberglass with PE resin and are withstanding the heat.

My oil cooler, a 72 row plate type, is under the engine with about 3/4" clearance to the case. The air comes in the smile and into the gap, then down through the cooler. I have a summer smile that's 5 or 6 sq. in. and a cool weather one that's half as much. The oil stays below 210 in the summer and in the fall I have to partially plug the smile to get it up to 180.

I'm not trying to steer you toward boxes, only letting you know that they do work. But so does a fence and it has the advantage of easier maintenance.

One other thing to look at is the gaps between the fins. If they are clogged with flashing, take some time and clean it out. It makes a difference.

Ed

Are the details of your installation with photos available anywhere? I know I would like my numbers to be like yours!

 

[Steve Stephenson]

My theory is "big air" for the inlet and as much as you can over the cylinder heads. You need to maintain high pressure on the top of the engine and low pressure below to draw air through the fins. Surprisingly, you only need about 0.25 PSI positive pressure difference on top. Whatever system gets that done is great. My inlets are opened up as much as possible. Remember also the exhaust stacks are a huge heat dump and so are the valve covers. Believe it or not, valve covers are part of the overall cooling system too. I enclosed a photo of the boxes I'm using. Fins are to assist with directing air to the top of the cylinders. I will eventually experiment those becoming airfoils.

 

[eschrom]

Jonbouyea: sorry, I'm a lazy picture taker but next time the cowl is off I'll try to remember to snap a few. I can tell you this, though - mine aren't nearly as nice looking as Steve's.

 

[cosmic]

I estimate the area of the standard 'D' shaped opening to be about the same.

I have a Jabiru engine in my S1 and the Jab inlets are about 16 square inches. Maybe 16 is too big.

 

[Steve Stephenson]

Only a small percentage of overall drag is "cooling drag." It's something like 7 percent. To reduce this has a negligible speed benefit but has big impact on cooling. Unless you're looking for 200 mph, I would leave it. As a disclaimer at this point, I fully acknowledge that the pressure difference (for cooling) increases exponentially with airspeed. Basically, if you reduce the size of your inlets, you are going to need the extra speed and you may not be able to fly your climb-out at the speed you are used to. You'll have to stay "hot" to keep the temps down.

 

[vmcurry]

thanks to all, read Hoovers article as well as each post here. any other pis welcomed. mulling over how best to accomplish pressure diff. and sealing to cowl. I also may have too much air coming in below engine thru smile
and carb hole too large. holes around frontand rear exhaust pipes too large.

 

[vmcurry]

one more thought. my intake manifold is greatplains cast and sits right over and close on head at the point i need most airflow down over rear exhaust valve area

 

[Steve Stephenson]

The holes around the exhaust nearest the firewall actually provide suction. Just make sure you have a big enough exit for that air plus the hot air coming down from the cylinders. I really wouldn't mess with the openings in the front just yet. Get your baffles installed and see where you're at.

 

[Steve Stephenson]

one more thought. my intake manifold is greatplains cast and sits right over and close on head at the point i need most airflow down over rear exhaust valve area

Mine are flipped to the outside over the valve covers. I know I get air slipping along side the cowling. I also have louvers on the underside of the forward cowl. Not my doing, but it's genius and I'm keeping it. The NACA inlet on the underside feeds the carb. Another on the top splashes the oil cooler.

 

[Steve Stephenson]

If you are interested in flipping your intakes, I designed a set of intake tubes that provide smooth flow from the carb up to the intake. I had VR3 in Stratford fab them up. Same people who make really accurate frame kits. They clear all the business behind the engine by snaking back toward the firewall A little. Pics enclosed. Pay no attention to the aluminum valve covers in the photo. Stick with the steelies.

 

[vmcurry]

thanks again. louvers are intriguing. going outboard on cast intakes might work also. vacation for 2 weeks before i can get back to it

 

[johnbouyea1]

Jonbouyea: sorry, I'm a lazy picture taker but next time the cowl is off I'll try to remember to snap a few. I can tell you this, though - mine aren't nearly as nice looking as Steve's.

From follows function and looks are definitely secondary to performance. My KR2S is in the 3rd iteration for cooling and I want lower temps. Any chance you've snapped photos yet? Thanks.