Cooling inlet area math?

[n3480h]

Ivan, thanks for the very relevant information. Since I am not likely to get a doctorate in fluid dynamics any time soon, experimenting is my practical option. What I have seen so far indicates that the exit IS the throttle for cooling, and smoothing exit airflow with a shaped form is the way to go. This seems to indicate that an adjustable cowl exit flap may be a solution for my desire to provide sufficient cooling at taxi, climb, and cruise. Now Bingelis's suggestion of the VW heat activated control diaphragm mechanism makes sense, though a lightweight Bowden cable control would also work.

Paser's book arrived yesterday. Its hard to put down. Adding 64 mph to a Mustang II, while significantly reducing fuel burn, is very impressive.

Tom

[dlynd]

Dr. Raymer's book lists this equation:

Cooling intake area = BHP/2.2*Vclimb

Results are in square feet. Vclimb is in feet per second (=kts*1.689 or =mph*1.467)

This is a general formula, and obviously is sizing for the worst case climb condition.

He also suggests the .8 size for the exit compared to the intake. As others have mentioned exit area is the restrictor in the system and his book suggests an adjustable exit as the way to fine tune the system for various flight conditions. Other drag reduction tips:

Have the exit in a low pressure area, which is difficult as that would put it top of the cowl or over the wings. Not great if oil or anything else is coming out with your cooling air.

Dr. Raymer also mentions not putting the cooling inlets too close to the spinner: 3-6 inch clearance

I definitely suggest throwing his books on the reading list with the others. "Aircraft Design: a Conceptual Approach" is more of a text book. "Simplified Aircraft Design for Homebuilders" is just what the title says and is where this equation was pulled from.

Derrick

[lqbanotxano]

The centers of my inlets are 5-1/2" away from the spinner edge. This is as far away from the spinner as physically possible. The back of the propeller blade passes less than an inch away from my round inlets. I get plenty of cooling during taxi. During taxi, the propeller is pushing air into the inlets & the exhaust augmentation is helping draw air out of the lower plenum (not in the Sam James cowl). It all works…these are not strictly all my ideas. I read a lot on this subject plus….just copying the Sam James cowling & cutting the numbers in half. This has been tried on many RV’s. The exhaust augmentation idea came from Pacer & a Gary Hertzler Oshkosh seminar. The carb air intake snorkel idea I got from the RV cowling. I had to modify it to use a K&N coned filter. The oil cooler snorkel idea was mine but…I don’t believe it is working efficiently yet.
My cowling & incorporated systems have been successfully flying on my plane for over 1-1/2 years. My plane usually flies in 100 degree weather all summer.

[oahupilot]

Derrick.
Another section of lower pressure would be the sides of the fuselage.

Juergen

.7 to .8 is for air cooled motors not liquid like the p51.

I see what your getting at in your plane. Every plane is probably set up different so I imagine every on would have to do their own trail and error, but I would start from .8 and see where I needed to go.

I think if you really wanted to make a difference in the cooling drag I would replace the bottom tin of the vw motor with a collector and duct the air to naca vents on the side of the nose. Ducting the air would probably be better then letting it wander around the engine bay before it finds it way out. I was actually planning on doing this to my vw motor but then I bought another motor of the liquid cooled variety.

[daddo2]

You can realize how critical it is to have at least a large enough cooling area and efficient baffle system after reading this.....
I had noticed on a few flights that my cyl head temps were a little higher than normal, and getting higher it seemed with every flight. I couldn't isolate the problem untill I noticed a piece of aluminum tape used to create a cover over the back of my dipstick had come loose, essentially creating a hole in the baffle the width and length of my thumb. This was enough to allow my cyl head temp on climb to go from under 400F to about 420. Taping up the hole cured the problem and the temps came back down to normal. This small leak in the cooling baffle was all it took to have an moderate impact on my engine temp.

[bil438]

I have about the same cooling inlet area, My engine is 2200 CC but not VW. It's more demanding to cool. So I have cowl flaps. The outlet area is about equal to the inlet when closed, but 4 times the inlet area when full open. A cowl flap does affect the CHT's but not as much as you'd think.
The big benefit is oil temp control. My oil cooler is on the firewall, with inlet air from a NACA duct and scat hose. Opening the cowl flaps will drop the oil temp by 40 degrees F, say from 220F to 180F. It's nice to have choices in a long climb.