Air flow behind the cylinder head

[Haase]

I am mounting a 3w-106 on a katana and was wanting to know how much room I need between the head and motor box for cooling. If I mount the motor to the box Ill have a 1/4 or should I cut another1" off the motor box and go with 1" standoffs. Brian

[Vic3D]

1/4'' is way too close to the firewall..... at least an inch, inch and a half.

[Kiwi]

Brian.

If you baffle your engine so you force the air over the top of it and then force it down the back between the firewall and the rear of the cylinder you will be doing your engine a great favor.

If you look at real aero engines you will see they are cowled really tight and the reason for that is to get air velocity over the fins. In short the faster you can move air over an aircooled engine the better it will cool. Just sitting your engine in a big open hole does very little to help cool your engine. Yaks and round cowls are a perfect example. Have a look at the build threads on the Dalton 260's here and you will see Tony has gone to great lengths to get the air to hit the upper front of his engine with little or no leakage around the rest of the motor.

Anything that keeps the air moving close to the fins and moving fast is all good. Wide open spaces equals lower air velocity and less effective cooling. A 1/4 inch gap is likely a little bit small at the rear as it could act as an air dam. That will stop the flow way to much and be detrimental. 1/2 and inch will be plenty.

Also remeber the air outlet under your cowl should be greater than the inlet by a factor of two from memory. That ensures you have a low pressure area under the engine and that helps suck air through as well.

Hope this helps a bit.

Kiwi

[Haase]

Thanks Kiwi, I thought a 1/4 would would be close, I have to cut two inches off my motor now, I didnt want to cut three inches off. SWB has 1/2 standoffs that will give me 3/4 of an inch behind the closest head, hope thats enough. Brian

[dick hanson]

I have chewed this over for years and finally did a tight baffling - ALL the inlet air passes thu the ducts shown
The engine runs hard with never a sag on 30x14 prop pulling 39 lbs vertically easily from a stop. my point is that the engine is really loaded -not revving fast - 5850 on the ground and 6300 in the air
so cooling is really important
after checking pistons after 3 weekends of flying, they are shiny nice and no marking
In taking after flight temps with a little infra red gun the temps are balanced -typically within 5 degrees -the best I ever have had
temps read quite cool and the engine never shows full heat soak - no hot spinner or case . The lower side of the baffling is attached to the cowl and cowl has nice baffle lip on discharge opening to induce low pressure internally . The big plate on the top slips into the rolled inlet edge of the top of the cowl. Never forget --ther air is NOT blown into the cowl - it is simply flowing thru due to low pressure at outlet hole
I am a believer
praise be !

[SleepyC]

Thats Awesome.
Looks simple to do, and i bet even the curvature helps the flow of the wind.
In fact I bet that the curve completely stops the "Pick up truck bed" effect of the wind in the back of the motor box area.

DAMN.... nice!!
Thanks for the picture!

[Temptation]

Now that is what I am talking about. I have been planning a very similar air duct for my Cap 232 with a DA 150. Awesome.

Mike Darr

[Thunderbird]

Excellent Dick.

I spent a number of years developing cooling for computer capstan motors.

You know, aluminum wire hollow basket (coreless) 0 to 3000 rpm in 1 millisecond while moving tape type motors with huge Alnico magnets.

Two methods were developed: high differential pressure, low volume that let the case get hot and the low differential pressure high volume that kept the case cool and cooled primarily by radiation.

There was very little difference in cooling capacity between the methods, but the high pressure method cooled the rotor more uniformaly (the end turn areas that
were not under the magnets in particular). Air flow in this method as kept to .015 of the surface to be cooled.

This is obviously not practical on our engines but the baffleing on a full scale aircraft engine (Contintal 0-200) is very tight, virtually a spring tensioned wrap on the vertical surfaces of the cylinder fins.

Good Job.

[Haase]

Dick, very nice job, I see alot of engines were they are mounted tight against the motor box and no baffles or standoffs and it doesnt seem like enough air flows around the head for cooling, thanks again.

[Kiwi]

Dick.

Danged nice job there mate. I dont think you could do any better than that no matter how you try.

By rolling that baffle over the cylinder you have eliminated that dead zone that must occur when you just baffle up to a square fire wall.

As you know a lot better than I ever will, cooling an aircooled cylinder in such a manner as to have an even and equal airflow around as much of the cylinder as possible is critical to the amount of power the engine is able to produce.

Overcooling the front of a cylinder and passing no air around the back of it leads to oval cylinders with inherent gas bypass, crankcase gas recirculation and thus lower efficiency. Thats means low power, loaded engines and rough running.

Dick you have my vote for Mr two cycle guru. Its always a learning experience when you come on here.

Thanks mate.

Kiwi

[aerografixs]

	Quote: Originally Posted by dick hanson
	I have chewed this over for years and finally did a tight baffling - ALL the inlet air passes thu the ducts shown The engine runs hard with never a sag on 30x14 prop pulling 39 lbs vertically easily from a stop. my point is that the engine is really loaded -not revving fast - 5850 on the ground and 6300 in the air so cooling is really important after checking pistons after 3 weekends of flying, they are shiny nice and no marking In taking after flight temps with a little infra red gun the temps are balanced -typically within 5 degrees -the best I ever have had temps read quite cool and the engine never shows full heat soak - no hot spinner or case . The lower side of the baffling is attached to the cowl and cowl has nice baffle lip on discharge opening to induce low pressure internally . The big plate on the top slips into the rolled inlet edge of the top of the cowl. Never forget --ther air is NOT blown into the cowl - it is simply flowing thru due to low pressure at outlet hole I am a believer praise be !

Cool stuff Dick,
I love it, now what a 4 cylinder Boxer, any good suggestions for that
Roger

[dick hanson]

yes - This is a Cassut Racer setup and strange a it looks the idea is the same
one inlet and a very complete duct to an exit - the Cassut is going like stink so the actual inlet/outlet are much smaller but idea is the same