Cooling an RCV 91 in Seagull edge 540 (68")

[Shaunie]

I am converting this plane from electric to glow using an RCV91CD, perhaps not the best choice but I was given the plane as an (underpowered) electric and was also given the engine, so it's a case of "use what yer got".

When it comes to calculating exit areas one rule of thumb seems to be an exit of 2 1/2 to 3 times the entry but I recently read another of four times.

Currently I have an exit about equal to the entry and the engine seems to get quite hot after a longish period of full throttle ground running.

What I don't want is to have the exit like a barn door and spoil the look of the plane, neither do I want to damage the engine by overheating it or crash due to a deadstick at a bad moment.

Here's what I have now

Would it be better to close the additional circular hole at the bottom (was already there) and possibly close the left intake just leaving the one by the cylinder barrel open? This would give me about 2:1 then.

What do you guys think?

AUW is 4.1Kg is it going to be too heavy for this engine? I think it may be ok but that I will have to fly it properly, not just point and squirt .

[Martin Harris - Moderator]

I would block both the circular hole and the unused inlet. The important thing is to ensure that any air going in is doing cooling work - any other air is wasted and competing with the hot air trying to escape from the cowling exit(s)

[Shaunie]

Thanks Martin, I was thinking that way but also wondering whether to duct the left inlet over the crank case with some litho plate.

Most of my I.C stuff has the engine or at least the barrel out in the breeze! The nice thing visually is that the engine fits entirely inside the cowl.

I was hoping for the Bernoulli effect to draw air out of the exit due to the step, perhaps I was being optimistic.

Should I enlarge the exit anyway, say about 50%?

[TheFlyingCrust]

Shaunie, The Wizard of Oz (AKA Brian Winch) did an article in the December issue of the Mag specifically on this topic. Available on the Subs Extra tab if you don't have the Mag. Page 71 onwards. Should tell you all you need to know.

Ian

[Martin Harris - Moderator]

Even a small lip can give significant extraction. If you leave a hole in the air it needs to be filled and it will do so from the nearest and easiest source - your hot air will love the free ride!

You should have plenty of inlet area in front of the cylinder without letting any more in - the more air you allow in the bigger the exit area needed. Take a look at my posting of 28/1 at 23.59 in this thread to see an example of how little inlet area can provide effective cooling.

Ducting can be very useful if the design of the model doesn't lend itself to directing air straight past the finning - in your case with the cylinder close to and aligned with the inlet I would imagine that the cooling may work well enough without it but if in doubt you could provide a simple duct to and around the cylinder.

Edited By Martin Harris on 23/03/2013 00:49:17

[Solly]

Shaunie, the ratio of 3 to 1 has always worked for me, - if I were you I would only have an open intake directly in front of the engine, block the rest off, and calculate it from there.

[Myron Beaumont]

Nobody has mentioned the (torturous maybe) path of high pressure air in and around the inside of an average un-aerodynamic engine cowl .Far from being capable of having a smooth in/out configuration ,you couldn't imagine anything much worse .My experience on my Puppeteer with a huge opening and small exit (SC52 4st fully enclosed ) is simply ----No problem!

[Martin Harris - Moderator]

Sounds like you were lucky, Myron.

My Puppeteer's 40FS used to lose power during flight, only recovering after some throttled back running (not that easy to arrange for any length of time with a rather elderly 40!) and was transformed by blocking off most of the huge opening with a dummy engine.

A clubmate's Flair Harvard had much the same symptoms except I don't think he ever landed it with the engine still running - eventually, I persuaded him to blank off the cowl - instant transformation...

And recently, a friend's Zenoah 62 powered Yak54 needed a blanking plate in the big hole to cool it adequately.

[Myron Beaumont]

Martin

I don't think luck comes into it .It was designed that way and works for me .The only restriction in the cowl is between the rocker cover and the ali round cowl at the front where I had to "panel beat" a bump in it for clearance . Maybe this has forced air over heat sensitive areas ie Cylinder ?( certainly not the crankcase) than specified for a smaller 2-st .

[Shaunie]

Superfluous inlets now blocked with black foamex sheet.

Looks ok and seems to have done the trick!

While I had the cowling of I took the inlet manifold off and found the matching of carb to manifold and manifold to inlet port was awful. 15 minutes of fettling later it was back on. An engine run later on shows I'm now getting 9200 rpm where I had about 8900 before. every little helps!

Shaunie