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Advice on ducting/baffling

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Advice on ducting/baffling

Old 09-10-2008, 03:41 PM
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chris-s
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Default Advice on ducting/baffling

I've done a lot of reading on baffles and ducting, but since this is my first attempt at this I feel I need some pointers over installing some cooling ducting for the engine on my latest build. It's a Pitts S12 with a ZDZ40 motor in the radial cowl and an in-fusalage silencer can. The acquired the model part completed with the engine and silencer already mounted, and the chin scoop in the cowl already cut.








The cylinder head is below the lower edge of the opening so I'm having to direct the air down and then over the fins. Using some liteply, this is what I came up with so far (I dug out a scrappy 80mm spinner to see how much hole it covers up)...











I think I might extend the lower edges down a little so they can be glassed into the insides of the scoop, but should I do anything to direct the air out of the cowl?

Would it be worth putting a 'lip' at the front edge of the scoop hole? I actually wondered whether it would be worth extending the sides of what remains of the scoop forward to encompas the section which has been cut infront of it?

What about directing any air around the back of the engine or closer to the fins on the very top of the engine?

You will see that the exhaust manifold goes thru a large hole in the firewall, it does not need to be this large, I can only assume it was cut like this to allow air to pass into the fus and exit the rear (thru non-existant holes) to remove heat from the exhaust, does that sound right?

Do you think I need any ducting behind the engine? Looking at the photos now, maybe I should extend the ducting to encompas the exit thru the lower scoop and also the hole into the fus?





When it comes to finishing the ducting, do you think it would be sufficient to glass it with a light weight cloth?

Thanks!

Chris
Old 09-10-2008, 04:06 PM
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da Rock
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Default RE: Advice on ducting/baffling

Looks good for a start. Will probably work without any more effort.

I'm not experienced with gasoline engines but the intake would be enough for a glow of that frontal area. Can't get a feeling for how much area you'll have for the exhaust. If you've read anything here, you know you need at least 1.5X the intake area for the exhaust. Without that, the engine will be in trouble on hot days.

If all that woodwork is going to be attached to the cowl, I'd suggest reinforcing the cowl and it's attachment design.
Old 09-10-2008, 04:15 PM
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da Rock
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Default RE: Advice on ducting/baffling

The crankcase will need air too. I don't know how heat soaked a gasser will get down there however.

It won't hurt your lower intake to provide some intake holes in the round front baffle to let in some air for the crankcase. But they'll increase the need for sufficient heated air outlet area.
Old 09-10-2008, 05:57 PM
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Default RE: Advice on ducting/baffling

I am in the process of baffling my Yak 54 50cc.
What you have done is about what I am trying to accomplish.
I planed on putting some (4-5) louvers behind the head of the engine. In my Yak 54, that would be in the radiator box of the cowl. The air passing by the louvers will create a vacuum in the cowl and should remove heat from the crankcase also.
How are you going to attach the ducting?

Greg
Old 09-11-2008, 02:26 AM
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chris-s
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Default RE: Advice on ducting/baffling

Thanks for the replies.

Currently there is probably about four or five times the exhaust area in the cowl plus there will be an exit thru the fus for removing the hot air generated by the in-fus muffler. Good point about cooling the crankcase, a few slots in the front should take care of that I presume.

To secure it, I was planning on glassing it all into the cowl, glassing around the front and extending the sides down and glassing them into the 'sccop' area. Being liteply it is very light and I can't see it being an issue, but, I'm going to glass the woodwork first, so I'll re-assess the weight once thats done.

Cheers

Chris
Old 09-11-2008, 04:48 AM
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Default RE: Advice on ducting/baffling

Remember, some of the greatest problems is caused by crank shaft/bearings over heating!
Your method directs ALL airflow over the cylinder head and removes the secondary ambient air flow.
From what I'm seeing, it's just too good. Too contained and directed.
Just blocking of the front of the radial cowl and leaving open an area infront of the Cylinder is a good start.
See how it goes before going over the top... or at least build in some Crank cooling.
Also the canister need some serious airflow as mentioned. It's hot in there.
The equation for in-out sizes tends to be 1 in and 3 out.
Stay there and you'll be OK.
But remember the crank... and those poor torchured bearings!
Old 09-11-2008, 06:56 AM
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chris-s
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Default RE: Advice on ducting/baffling

Thanks David, I'll be sure to allow for cooling the crankcase.

Chris
Old 09-11-2008, 07:28 AM
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Default RE: Advice on ducting/baffling

I am no expert but there is a lot of engine there that isn't being cooled like the exhaust header....I don't understand the baffle at all.....why not just leave the front of the engine open and direct the airflow with the exits?
Old 09-11-2008, 09:54 AM
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chris-s
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Default RE: Advice on ducting/baffling


ORIGINAL: jetmech05

I am no expert but there is a lot of engine there that isn't being cooled like the exhaust header....I don't understand the baffle at all.....why not just leave the front of the engine open and direct the airflow with the exits?

Lots of reasons, have a read of this thread and check some of the links it contains....

[link=http://www.rcuniverse.com/forum/m_6147421/mpage_1/key_ducting/tm.htm]http://www.rcuniverse.com/forum/m_6147421/mpage_1/key_ducting/tm.htm[/link]

Chris

Old 09-11-2008, 09:56 AM
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Default RE: Advice on ducting/baffling

The biggest problem people get with a radial cowl is leaving it all open.
It all down to the ol' 1-in 3-out rule. If you have a massive opening at the front, you need a extra massive opening at the back.
The simple way around this is to restrict the opening and direct the air where it's most needed.
Chris-s is on the right track, he's just gone over the top!
That tunelling is going to be forcing the airflow through the cylinder fins... which is good. But totally missing everything else... which is bad!
Just don't make it so directional, change the baffle so it let's some air flow around the engine.
I don't mean flow around the whole maissive cowl... just the engine.
Old 09-11-2008, 12:16 PM
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da Rock
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Default RE: Advice on ducting/baffling

ORIGINAL: jetmech05

I am no expert but there is a lot of engine there that isn't being cooled like the exhaust header....I don't understand the baffle at all.....why not just leave the front of the engine open and direct the airflow with the exits?

Because cool air pushes warm air aside. Cool air has more momentum too.

What happens if you try to direct the airflow from the exits is fairly simple. The air coming in the front is cool. As the engine heats the air around it, that air becomes stagnant. It's warmer than the cool coming in and has less momentum. It's also easier to move cool air than it is to move warm air, so the air coming in will move the cool and not move the warm. So it winds up following the cool wherever it's going. And the warm ain't going nowhere unless it's lucky and there are no restrictions to speak of that would back up all the air coming in. Because if there is any restriction or backup, the cool air quite easily takes up all the room available to the exits.

When the area behind an engine is wide open, there is no problem. When it isn't open, then you've got to direct cool air onto the parts you need to cool. It won't go there unless it has to. It follows however much cool air is filling the exit area and moves around the warm air (stagnating it) on the way.


.......... You don't HAVE to route the air all the way to the exit. This H9 P47 Thunderbolt cowling is only baffled. The baffling blocks any air from bypassing around the engine and muffler. Any air that comes into that cowling has to pass within about a half-inch of the engine and muffler. After that, the exits are in negative pressure areas and have greater area than the intake area. You don't have to route the whole way, but that works great. I'm just comfortable with my baffling material. It's 1" polystyrene insulating foam sheet and is simple to work and very light weight.
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Old 09-11-2008, 12:30 PM
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da Rock
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Default RE: Advice on ducting/baffling

Blocking works great for more than just big wideopen cowls.

Lots of our models have twin intakes on either side of a spinner. We usually mount our engines to fill one of the two intakes. Yup, that cylinder is going to see air coming in for sure. And more than enough that it ought to be cooled pretty good. But it doesn't always work that way. All the air coming in the other hole has absolutely nothing standing in it's way. And it sees no heated air at all. So it heads straight to the exit and fills that sucker right up. And more cool keeps right on coming from that open intake. And as the warm air from the engine waits it's turn (a turn that it won't get) it warms up even more. And that makes it harder for it to push into the line of cool air that's hogging the exits.

This Cap 232 got a baffle in it's open intake. I figured to deflect that intake flow to the crankcase of the engine. That baffle also reduces the intake's area quite a bit. It's slap up against the opening and sealed top, side, and more than half the bottom. I wanted it to also direct air at the Pitts muffler. It took me maybe 15-20 minutes to cut and fit the piece of foam insulation that does the job. And it is pink foam too. It did get some red paint because the pink looked funny.
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