I am building a model from a very old set of plans. The engine called out is difficult to find, and so am planning on installing a common K&B .40 glow engine.

Now, the plans are not that good, but it called out for the nose cone to be carved from a solid chunk of balsa, and gave the diameter for air cooling around the cylinder. It was about 1/4" but have spots of 1/8" clearance and some larger.

The question is just how much should I hog out for air cooling the cylinder and crankcase? Instead of using one big block of wood, am doing this with laminated 1/4" balsa now. Would an overall clearance of 1/4" be acceptable to pass and circulate air? Thing is if I get too much clearance, the shell gets too thin, and then subject to damage.



Wm.

Less clearance is better.

Force the air to flow through the fins and the cylinder head. A classic cooling problem is the WW1 radial cowled engine with lots of air going in through the front and out the hole cut in the bottom of the cowl. However the poor old upright engine is sitting in stagnant hot air with little flow.

The image shows a typical pylon race set up. These engines run at 30,000 rpm and get HOT. Rule of thumb for all set ups, the outlet must be at least 3 times the inlet.

Less clearance is a good thing, but only if the both the air intake and the exit are large enough. If you are using a more powerfull engine then you may want to make both larger than called for on the plans.

Hi!
On modern pylonracing airplanes (.40 size) an opening of about 6-8mm and 20mm in height is all it takes. Then we talk high performance engines like IR and Nelson. Quadrouple the outlet area and everything would work out nicely . It's very important that the area surounding the engine cylinder and head is really tight, 1/4 inch is nearly too much distance, better to keep it as tight as possible

Jan....


Now you got me worried here. The cylinder head sticks out on this A/C a good 1/4 to 1/2" (on slope). There is a way that I could move the engine up in the mounts, but this would offset teh indicated thrust lines.

Your mention of 6-8MM clearance is much more than 1/4 inch also. Or, am I missing something in your dimensioning?

Than for everyone, how many square inches of intake area is best? 1/2 inch square OK or must I go bigger?



Wm.

The 6-8mm x 20mm describes the cooling inlet size not clearance. Just let the head hang out. In most speed design it seems to be flush.

Greg....

For you and others, I scanned an image off original plan to helpt you better visualize the situation. This ought to help some, and let you get the feeling.

It is a middle 1960's racer, had a K&B .40 up front. As you may see it gets rather tight around the cooling fins. Nothing in the notes, nor on the plan as to suggested clearances. Makes it look as though the intended balsa block cowling was to fit tightly around the engine.

I am going to install a slot at the firewall to let air out. Nothing decided on inlet yet.


Wm.

Since the hottest part of the engine is in the breeze, I don't think you need to change a thing. Just increase the outlet area a bit to be sure.

Hey Coos,

Check this!

http://www.supercoolprops.com/articl...ed_cooling.php

or page 8 of this PDF

http://dkd.net/clmodels/acln/acln64.pdf

Stop worrying. The cyl. head is out in the breeze. That is the key bit.

Add a hole to feed the carb and bias/duct the air around the EXHAUST SIDE of the engine and make a hole somewhaere at the back to let the hot air out.

That should do it.