A question on cooling. Does heated air rise or fall in the cowling?

[riobison]

A question on cooling. Does heated air rise or fall in the cowling?

As the majority if not all of the planes that we are flying, we ram the air in from the top of the cylinders? through and down out the bottom of the cowl.

Now let?s reverse this.

As heat rises naturally would it make any difference or be more efficient to cool from the bottom up and exhaust the heated air out the top of the cowl?

Yes, a negative pressure or vacuum will draw the air out but how much more if any negative pressure is there on the belly of the plane versus on the top of the cowl?

Tim

[Morraine]

Hi Tim,
Good thought re the air path but one problem with aircooled aircraft engines is that the exhausts are underneath so any air travelling up thru the cylinder cooling fins has already been heated by the exhaust. Also the flow of air over the spinner then front cowling tends to cause a bigger neg pressure area beneath the cowling as has been shown in many tests. There is a good explanation on a web site which at present eludes me but I will try to find the link. Get onto Ross Farnhams SDS web site and look at the cooling section. I think it may be there???? I suspect trying to fit plenums and dams around exhaust and intake plumbing may also be a negative point.
Cheers

[riobison]

I was only thinking of moving the existing intake for the cooling to be only a few inches lower from where they are now. Nowhere near the exhaust.

Just low enough to supply air in on the bottom of the cylinder head and then exhaust it out the top of the cowl.

This might work better for when the plane is on the ground but may not work at all when in flight.

Someone may have tried this and it doesn?t work that?s why we don?t see it.

Then again maybe not?

Tim

[pierre smith]

Many years ago, a Formula One racer, named Bill Falck, built "Rivets"...a very fast F-1 racer and often won. His air intake was roughly where our carb intake is and his cheek cowls were closed.

An incredibly complex updraft cooling air ductwork guided the air up, through the cylinders and heads and then, methodically, downward and out right behind where it had entered.

IIRC, he stalled it in a turn over a lake, during the start of a race and he and Rivets was lost.

Best,

[videobobk]

If you want to see an example of this type cooling, look no farther than the nearest Globe Swift...

Bob Kelly

[DanH]

Originally Posted by riobison

As heat rises naturally would it make any difference or be more efficient to cool from the bottom up and exhaust the heated air out the top of the cowl?

Peter Garrison (of Flying) has written extensively on the subject, as he used an updraft scheme in the design of his Melmoth II.

http://www.melmoth2.com/texts/CoolingFlow.htm

Yes, a negative pressure or vacuum will draw the air out but how much more if any negative pressure is there on the belly of the plane versus on the top of the cowl?

Not quite as simple as "belly" or "top", but rather a question of exactly where on the upper or lower surface.

Much interesting stuff in the above writings. Would I do an updraft system for an RV? No.

[Kyle Boatright]

Anyone remember the prototype RV-6 cowling with a semicircular cooling air intake wrapped around the bottom of the spinner? Extra points if someone can find a picture.

Heck, my plans (#243 of the RV-6 family), even show the prototype cowl on the 3 views. Too bad the plans are at the airport.

[apkp777]

There's a fair amount of compressed air pumnped through the cowl by the prop. You'd have to take that into account.

[AlexPeterson]

Tim, to answer your question, the normal forces of convection (hot air rising, cool air falling) are completely overwhelmed by the pressure forces involved. A little like trying to watch a candle flame in a hurricane. I wouldn't guess that convection forces are a factor in updraft designs like Garrison's. Things like angle of attack vs speed are probably big factors.

[David-aviator]

Originally Posted by riobison

A question on cooling. Does heated air rise or fall in the cowling?

As the majority if not all of the planes that we are flying, we ram the air in from the top of the cylinders? through and down out the bottom of the cowl.

Now let?s reverse this.

As heat rises naturally would it make any difference or be more efficient to cool from the bottom up and exhaust the heated air out the top of the cowl?

Yes, a negative pressure or vacuum will draw the air out but how much more if any negative pressure is there on the belly of the plane versus on the top of the cowl?

Tim

Why not? It would make a good windshield anti-ice system.

The canards with the rear mount engine have up draft cooling. It works but I'm not sure it works better or worse that what we have.

Down draft cooling might help keep a carb from icing up.