hi all

i was wondering how big should the cut outs in the front of the cowl and the out let in the back of the cowl be to provide adequate cooling for an engine ... cos i was running my saito 82 the other day in the flip 3d ... did a few flat spins at constant hight ... at about 3/4 throttle ... and when i tried pulling out .... it had not power what so ever it had overheated in the proccess ... i ve seen guys hover for like 10 minutes with no problems what is the secret ???

please help

G'day Mate, whatever size opening you have at the front, you must make the outlet at least double the inlet size, or larger if you can. This has helped me here in Australia where we often fly in 35 degree C days in summer.
Good Flyin Mate.
Allan.

Only cut the area directly in front of the cylinder cooling fins to force air to flow across the cylinder only and not anywhere else. On the rear, make the air exit hole at least twice as large in area so hot air can expel out.

Not that it's totally applicable to sports models but some might find it interesting the extent that one guy went to with a pylon engine to get decent cooling.
http://www.supercoolprops.com/articl...ed_cooling.php
Some of the ideas could be a help with even our basic type cowling, especially the bit about a plenum chamber and intake shape.

thanx guys ... been a real help ... i have the gian u can do now .. and i will have the outlet double the size of the inlet ... how ever i will also have the exhaust stiking out that area ... is that crutial ?

I have started a rant about cooling at http://prme.nl/forum/viewtopic.php?t=37 . Experimental baffling pictures of my 35cc test engine included.
So far, it works very well, as could be expected.

Nice Pe but have you taken any temp readings to show exactly what affect the baffles have made? With all my cowled models I've always tried to make them a close fit around the cylinders but left the head out in the airstream. It seems that may not be the best way to do it.

One simple trick that I've read about is to have a triangular shaped extension from the firewall up to the back of the cylinder. This normally has stagnant air in that area because of the flow around each side meeting up at that point and just swirling around against the flat firewall instead of escaping and carrying away the heat. The extension keeps the flow seperated and directs it out each side.

I have done measurements before, and not yet after. Before, the rear plug boss temperature in this particular engine ranged from 120 - 145 degrees C , rich 6000 rpm >> max 6100 rpm, on verge of power loss, using a 3W 20x8 prop and MVVS 3271 in cowl muffler (82 dB(A) @ 7 meters)
I was aware of the air stagnation at the rear. For that reason the baffles closely shroud the fins way past the cylinder center line, so they converge like in full size radial engines. The wedge idea may be very good. Anything that causes stagnant airflow swirl is to be taken care of. Maybe have different exit angles in the baffles, so any stagnation occurs after the cooling section.
To check cooling in a lean set engine for best power, I did take out the cylinder lining to check heat coloring. There was no liner discoloration at all, and the liner outside surface was oily/slippery without any signs of oil baking in the hottests section above the exhaust port. The piston was clean in the colder parts, and showed slight carbon in the hot parts. Piston liner wet with oil and slippery. Ring land above ring slightly black with soft soot.
Also, the engine did not have the typical very slight rpm drop that hot gas engines normally show after a few laps in flight.

I cannor repeat the same measurements, because I had to change my setup to comply with our club's 80 dB(A) requirements.
New data: 7100 rpm using Mejzlik 20x8 and 3W smallest canister (stinger reduced to 6mm ID) 79 - 80.5 dB(A)@7 meters. Thrust is in excess of 9 kilos (20 lbs) .my 9 kilo tow plane goes straight up with this setup.
I will take some more measurements asap and post.

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Every time my eye grabs your avatar, I keep expecting a massive "pock" on the end of my nose! <G>

It makes me flinch.

I finally did my measurements with the modified setup with engine running full lean and tuned at best rpm. Idle needle 1 3/4 turns open, High needle 1 turn open. Muffler with stinger outlet reduced to 6 !! mm to meet noise restrictions.

With the restricted muffler, I expected the temperatures to rise way above 150 degrees C, but this was not the case. Both cylinder head and back of cylinder cooling ribs were at a stable 145 degrees with engine at prolonged full bore and plane kept stationary.
Reducing throttle to 1/2, it took less than two seconds for the engine to drop in temperature to below 110 degrees.
No measurements of the front ribbing was made out of respect for the spinning 20x8 prop.

It seems the baffling certainly didn't hurt anything but it would be interesting to try it again on the new muffler set up without the baffling to get a comparison.

Artisan.....*pock pock pock*