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Baffling a Round-Cowled Plane

Old 02-23-2006, 12:00 PM
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Default Baffling a Round-Cowled Plane

Any suggestions on how to baffle a round-cowled plane. I'm considering building a 30% Yak and I want to construct baffling to get the most out of my motor and motor life. Most baffling construction seems to get done with some thin balsa and cut at right angles.

I was thinking about starting with a 1 liter soda bottle, cutting off the top, and cutting an opening that is about where the cylinder position is. Does this seem doable and/or too flimsy and or too complicated. I can't think of an efficient way to take a round opening and funnel all the air to one side using "square" balsa.

Anybody figured out how to do this efficiently have pictures to prove it?
Old 02-23-2006, 03:55 PM
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Richard L.
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Default RE: Baffling a Round-Cowled Plane

Most people would just construct the baffle out of 1/8" ply to cover up most of the front cowl opening, leaving enough inlet area directly in front of the cylinder head for cooling. I suppose you could cut out one or two more slots and install vanes to direct the incoming air toward the cylinder head. The idea is to force cool air to flow through the cylinder head cooling fins and not elsewhere. A dummy radial engine with one cylinder removed would serve the same purpose.





Old 02-23-2006, 04:03 PM
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Default RE: Baffling a Round-Cowled Plane

Isn't this approach just blocking "available cooling" air? If you can funnel all of the existing air across the entire frontward surface area of the cowl, you can create positive pressure for cooling.
However, thats the only way I could come up using square wood in a round hole as well....block some of it.
Old 02-23-2006, 04:17 PM
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Default RE: Baffling a Round-Cowled Plane

You are not blocking "all" available cooling air. There is still a area directly in front of the cylinder head and just slightly larger than the frontal area of the cylinder head for cooling. You do not want to funnel "all" of the existing air across the entire frontal surface area of the cowl inside the cowl. For one thing, hot air expands and needs a large exit area to escape. If you funnel all the existing air into the cowl, there won't be enough exit area and the air will just stagnate inside the cowl, preventing new cool air from coming in. As a result, your engine will overheat.
Old 02-23-2006, 04:23 PM
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Default RE: Baffling a Round-Cowled Plane

The general rule of thumb is that the air exit hole must be at least twice the area of the air inlet hole. If you can cut enough of your cowling away so that your exit area is at least twice as large as your entire cowl opening, then yes, you can funnel all of the available air inside the cowl. Otherwise, you must restrict the amount of incoming air in order to achieve the 1:2 ratio between the incoming cool air and the outgoing hot air.
Old 02-23-2006, 04:25 PM
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Default RE: Baffling a Round-Cowled Plane

Well theres plenty of exit area in my application.
Old 02-23-2006, 05:21 PM
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Default RE: Baffling a Round-Cowled Plane


ORIGINAL: Richard L.

The general rule of thumb is that the air exit hole must be at least twice the area of the air inlet hole.
Yeh, possibly at equal pressures. However, the final size will be about the same size as the inlet seen in the picture....so your general rule of thumb will still be true. However, it be at positive pressure. A positive inlet pressure will reduce the likely of stagination.
Old 02-23-2006, 07:30 PM
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Default RE: Baffling a Round-Cowled Plane

The purpose of a baffle is to direct cooling air through the fins of an engine, and exhaust the hotter air as efficiently out of the cowl as possible. Simply restricting the inlet air to an area roughly corresponding to the engine location is not adaquate with most twins. If there is a way for the inlet air to flow around the engine, rather than through, it will always do so. That's why a true baffle both directs the flow of air through, and prevents the flow around.

Pat
Old 02-23-2006, 09:45 PM
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Default RE: Baffling a Round-Cowled Plane

With a 30% model, I assume you are powered with a single cylinder engine. Silversurfer is correct insofar as the very best cooling method, which is to leave no other place for the cooling airstream to go than through the cooling fins on cylinder, head and close to the rest of the crankcase. But coming up with that configuration is an awful lot of work, largely of the cut-and-fit variety, which is onerous as heck on a one-off prototype. And very possibly not necessary.

I'm flying a Sukhoi at the moment, round cowl, single cylinder engine, sidemounted. I did what RichardL suggests, which was to cut a piece of 1/8" lite ply in a circle just bigger than the backside of the cowl opening. From the circle I cut out a center circle just smaller than the diameter of the spinner. Then I cut an opening tangent to the top & bottom of my center cutout and extending horizontally to the outer circumference, giving me a slot maybe 2" wide and 3+" long, positioned right in front of my engine cylinder. So my incoming area is maybe 6-7 sq in. My exit area is more like 4"x5", or 20 sq in. So I have about 3x exit area relative to entry area. Engine is very happy, even at static full rpm the head shoots ~250˚ on my IR gun.

As silversurfer says, the incoming air will take the easiest route to the exit area, avoiding as much as it can the "friction" of rubbing up against a hot engine. But if my simple installation works OK from a practical point of view, then I save myself a lot of work by leaving it as it is. If I were getting high temp readings, sagging power, deadsticking, or bearing failure, then I would lay in a couple more pieces of lite ply at right angles to my cooling slot, to carry the incoming air right up against the cylinder (with maybe 1/4" clearance). This would force the incoming air to stream directly at the engine, rather than giving it room to flow around and away from.

A superefficient baffle would be a vertical plate blocking all flow through the cowl, with an opening cut out matching the front view of the engine/mount, leaving maybe 1/4" all the way around the outline, the plate positioned maybe just in front of the sparkplug. Getting the outline would not be so hard, and getting the outer shape to fit inside the cowl at the selected location might be a little harder; but getting the cutout positioned properly on the baffle plate before gluing it in, that would take some cutting & fitting. Work it out with some cardboard stock, and it's doable.

But to be realistic, a simple air dam is a good start, and may be all you need. If you need more, then make a small chute from the dam opening to the engine, to keep the incoming air from simply bouncing off the engine and heading directly for the exit. If that's still not enough, or if you just like to take on a challenge, do the tight outline.

As to how much air you need--surprisingly little: I have a flying buddy master modeler, who used to make test models for a full scale aircraft maker. He spent years in pylon racing, where they developed something called the "compression cowl," which was super slippery on the outside, and encased the engine cylinder very closely on the inside. The cooling air intake was a tiny slot, maybe 1/2" wide x 1-1/2" tall, with a flared intake (which served to compress the incoming air). The slot led to a cylindrical chamber that fit around the engine with the typical 1/4" clearance. The exit was a narrow vertical slot on the opposite side of the chamber from the incoming, which led off in two directions to exit slots on either side of the cowl. This was for a .60 size glow engine. It looked like not much air at all, but it was compressed to some extent, and none of it was wasted.
Old 02-23-2006, 11:36 PM
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Default RE: Baffling a Round-Cowled Plane

Simply blocking the total air flow, without directing it, so that its 3/4 less than it would be if you did nothing, is not my understanding of baffling at all and not what I read on the FAQs of the engine websites. So I would say now, I'm more confused than I was. I understand what baffling is from reading the FAQ's. However, none of them deal with a round cowl...and none of them describe a process where flat pieces are set perpendicular to the travel of the plane to "stop" air flow....all the Faqs describe a process where the flow is channelled right up to within 1/4" of the fins of the engine and air can but go one place --- though the engine fins. All of the Faqs describe a process where for the most part wood is placed parallel to the direction of the plane but directed and angled from the opening towards the engine.
Old 02-24-2006, 01:21 AM
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Default RE: Baffling a Round-Cowled Plane

Click on my name and check out the Bearcat the P-47 and the KI-84 in my gallery pictures. I have a DA-150 in the cat and a Brison 4.8 twin in the KI. All have dummy engines in front and in fact the openings are less than half of the cylinder fin width. They all fly well in the Texas summer heat but the air that goes in is forced to pass thru the cylinders by baffles that fit closely around the cylinders.
The primary flow thru the cowling on the ground and in flight is from differential air pressure caused by the accelerated airflow over the outside of the cowling.
If the pressure in the cowl was more than a few inches Hg. difference than the outside air pressure, cowls and cowl flaps would have been blown all over the country side when the big round engines roamed the skys.
The birds that I flew with cowl flaps did not require any great amount of force to open and close the flaps and they were all manual linkages from the cockpit to the cowl flaps.
Basically if you just block off the excess area that is not directly in front of the cylinders things will work fine and you won't even have to get into mass flow rate calculations.
ORIGINAL: 3d-aholic

Simply blocking the total air flow, without directing it, so that its 3/4 less than it would be if you did nothing, is not my understanding of baffling at all and not what I read on the FAQs of the engine websites. So I would say now, I'm more confused than I was. I understand what baffling is from reading the FAQ's. However, none of them deal with a round cowl...and none of them describe a process where flat pieces are set perpendicular to the travel of the plane to "stop" air flow....all the Faqs describe a process where the flow is channelled right up to within 1/4" of the fins of the engine and air can but go one place --- though the engine fins. All of the Faqs describe a process where for the most part wood is placed parallel to the direction of the plane but directed and angled from the opening towards the engine.

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