Hey T: I've been told your vents are suppose to be twice the size as your intake holes. I use a dremel sanding drum to make mine.

Yep, cowls are a form of art. That's certain. And Fancher is a good one to model yourself after. He's very good at it.

The above suggestion are great.

I believe Ted cuts a lot of his holes with sharpened brass tubing. I also use them a lot as they cut clean and really easy. Like 114 holes in the flaps of my SBD.

Don

I tried something brand new for me and it worked well enough that I plan on doing it again.
I used 1/64 lite ply and made a louvered outlet for a 1/2A Fierce Arrow I'm building. First I built the outlet which was rectangular in shape. I used the outlet for a tracing template and cut out the cowling to fit it. When I sanded it to shape I had what I think is a very cool outlet. And it doesn't weigh enough to get a reading on my scale.

Your right Frank that is cool. Good idea

The air blows right through it too. I think of it as a hole with embellishments.

I tried something brand new for me and it worked well enough that I plan on doing it again.
I used 1/64 lite ply and made a louvered outlet for a 1/2A Fierce Arrow I'm building. First I built the outlet which was rectangular in shape. I used the outlet for a tracing template and cut out the cowling to fit it. When I sanded it to shape I had what I think is a very cool outlet. And it doesn't weigh enough to get a reading on my scale.

Frank, are you sure that's 1/64th ply. you used?  It appears thicker than that, maybe just some camera distortion. 1/64th is almost paper.

--Ray

OK I'll buy 1/32".  Truth in advertising, you know.

I'm of the opinion that the 2 to 1 outlet to inlet area  rule is not the best for optimum cooling. In my mind and how I've done it for years is to make the outlet only about 10% to 20% larger than the inlet. The best cooling is achieved when the cooling air flow is relatively low volume ( adequate to dissipate the heat) and high pressure around the engine.  The increased pressure in the cowl helps to promote even flow and  eliminate some stagnant areas. In order to have high pressure in the cowl, the outlet has to be of an area that will account for inlet pressure and pressure rise from the heating of the air in the cowl. This means maybe 10% to 20% more area than the inlet. Good ducting of air around the engine goes a long way to improve cooling too .

We don't really wring out our stunt engines so the cooling requirements are modest compared to some other events. One thing that jumped out at me was the ratios of cowl inlet to cowl flap areas on most general aviation airplanes. In most cases the cowl flap area is at best equal to the inlet area. Location of the cowl flap does influence the pressure that the outlet sees. An interesting note is that the exit area of the Mustang's radiator cooling pod on the underbelly is smaller than the inlet too.

I'm with Mark on this. Repeated experiments had optimum cooling at something between 2 to 1 outlet to intake and 3 to 1, depending on the shape and expansion area of the the inlet

Mark..."With all due respect, the pressure is inverse to the velocity. Therefore the more you slow the volume of air down the greater the pressure. think in terms of Bernoullies principle and wing flow/lift."

Very true and that's what I said in my previous post. I stated that it is of benefit to increase the pressure within the cowl to improve cooling. The only easy method to achieve this is to create some restriction at the outlet and in this case it can be accomplished by reducing the area of the cowl outlet and thus reduce the through flow volume. The maximum pressure available in the cowl is dependent on airplane speed, current atmospheric conditions, and a bunch of aerodynamic considerations of the front end of the airplane and cowl along with the internal resistance of the cowl and engine.

Here is what I see wrong with the current 2 to 1 outlet inlet ratio theory. If the cowl was a perfectly straight tube, nice and clean and no clutter inside like an engine and such, this represents the minimum resistance and thus maximum possible flow through the tube. Flaring (increasing the area) of the end of this tube would not increase the amount of airflow through the tube. With resistance to flow fixed, the maximum volume is determined by the incoming air velocity  and area at the inlet (Q = area X Velocity)… not the outlet if the outlet is the same or larger. So increasing the outlet size will not increase the flow through the cowl. The flow is limited by the intake conditions, not the outlet in this case so it’s a waste to make the outlet any larger. There are some aerodynamic things that can be done to reduce the pressure at the cowl outlet thus increasing the delta p in the cowl but huge exit area isn’t a fix.

 As I see, it the biggest problem we have with the enclosed engine in a cowl is usually the engine is in very close proximity to the opening creating a big restriction. This big chunk of metal is the biggest restriction to airflow in the whole cowl/engine scheme. Consider the flow around the engine is not laminar and the eddies and vortices at the back of the engine creates a dead zone so that in my opinion about half of the cooling area of the engine isn’t being properly washed by cooling air. If we can increase the pressure in the cowl we can help to increase the pressure of the air against the backside of the engine (which in theory and from my empirical experience) should improve cooling. Proper ducting around the engine within the cowl is the real fix in my opinion. (See attached picture for the flow around the engine head I was trying to describe.)

Just in passing, I don’t think Bernoulli does a really good job of explaining lift. I prefer the concept of circulation, in my mind it’s a better explanation… but then again my degree is in Physics… not Aeronautical Engineering so what do I know… airplanes are rocks on strings to me!

Mark…” If (in a perfect world) you were to move 1 slug of air into the intake at a crossection of 1 sqaure inch, then increase the crossection to two sqaure inchs, the velocity would decrease to half, the pressure would increase a proportional amount.”

Well not really true in our case. If we confine the discussion to the intake, the velocity of the incoming air is relatively constant as is the pressure (see my comments above) and so in this situation the area is increased by two so the quantity of air would increase by two. Q = area X velocity. The pressure remains constant. ( Pressure = (velocity/constant based on fluid)^2. You have to agree that the outlet controls the pressure in the cowl. If I continue to reduce the area of the outlet, I would expect the pressure in the cowl to rise until I close off the outlet completely. At this point the pressure in the cowl should be equal to the incoming air pressure and no flow. Drag would also reach max level at this point.

 Mark…” pressure is NOT built by restricting the outlet, that causes more in terms of spillage around the front of the intake. My basis for this approach, I designed the intake for a turbine engine where pressure at the compressor inlet was critical to ensure  proper engine performance. If you are by sum chance  interested, here is a link for the page showing the conversion,,,  http://www.cascadeflying.com/index.htm ...  This was verified in real world tests using a manometer to verify the pressure changes within the intake. The other benefit of slowing the air down is that it creates less turbulance internally at a lower velocity, as well as the stabilizing effects of the increase pressure on the irflow.

I think we have mismatch in applications here. Your design is pretty cool (pun intended), by the way… the cowl would look great on a stunt ship, but it appears to be a situation where air is being accelerated internally which adds considerable complexity to the problem. Our cowls are a bit different in that no additional energy (Other than thermal) is being added to accelerate the flow. Our cowling is a less complex case.


Mark…” No I must disagree, an outlet of 10% to 20 % IMHO does not even allow for expansion from heat transfer into the air, I must go along with the premise of a 2 to 1 outlet same MASS of air in double the volume will show a decrease in velocity which is what is needed.”

In my earlier post, I believe I said the outlet is 10% to 20% larger than the inlet. So the outlet area is 1.1. to 1.2 large than the inlet which allows for thermal expansion. I would agree that 10% to 20% of the inlet area would be a problem.

Mark…” Watch the Nascar boys, how much inlet do they have to cool 800 horsepower?  My theoory is to restrict the intake size and maximize the outlet size.
The mustang used a different phenomenum for its radiator, and it involved pressure on the outlet side to draw air out, plus if you ever watch one take off, you would note that the radiator door is open much wider, more drag, but greatly increased outlet size compared to inlet”

NASCAR guys are like airplane designers… always looking to minimize “cooling drag” Big frontal area is a killer. Again, I’m not an Aeronautical guy but I agree with you. I think the delta p in the pod is enhanced by some aero tricks. Most aircraft have the cowl flaps open during high power use periods like take off and climb but I don’t know about more drag… maybe, maybe not.

I find this discussion of cowlings and air flow extremely interesting,back in the
earlier seventies my Rats had a modified speed cowling that wrapped behind the cylinder of the engine thereby forcing the air into the dead space or the hottest part of the engine.
The whole point of this comment is that I got by with an intake of only 3/16 x 3/4
of an inch and an outlet of 5/16 x 7/8 that tapered back to 1/2 x7/8 of an inch.
Of course this was on 60% nitro with 18% oil,most of the colling was accomplished by the massive amounts of fuel running thru the forty K& Bs and OS maxs.,5 1/2 ozs in about 1 1/2 minute.
Absoulely no problems with cooling.
Busby

Yes ,it was the speed planes that rang my bell ,some how or another my Rats all looked like Pink Ladies, including the cowling, except they were all light blue. Now everyone knows why all of my stunt ships are light blue.
Busby