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The Usual Cooling Issue

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Old 04-09-2012, 07:11 AM
  #1
hoggle65
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Default The Usual Cooling Issue

I have a 90SP in a H9 Hellcat. I cutout between the top two dummy cylinders in the dummy radial face. Then built two scoops in the inside of the cowl to redirect down across the engine. Then two big cutouts in the bottom rear to exit. The dimesions are roughly the 1 sq inch in and two out as recommended. Yet it still overheated and quit on the first flight.

I could tweak the inlets and stuff more but I have seen references in forums about big radial engine planes not being an issue like the tightly cowled ones (Spits and Mustangs). I haven't found any details as to why that's the case. Now I really don't care that much about a truly scale appearance and so I'm wondering if just removing the dummy engine (or a portion of) would solve the problem.
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Old 04-09-2012, 07:45 AM
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Default RE: The Usual Cooling Issue

For efficient cooling you must have air coming in but you must also provide a way for it to exit too. Where can the air exhaust from? Usually the exiting air from the cowl needs aprox 3 times the area of the inlet to flow efficiently. Can the air exit around the cowl along the gill flaps on your model?
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Old 04-09-2012, 08:11 AM
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Default RE: The Usual Cooling Issue

Probably from the "two big cutouts in the bottom rear to exit.". [:-]

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Old 04-09-2012, 08:16 AM
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Default RE: The Usual Cooling Issue

Hoggle can you take and post pics?

Try flying it 1 time without the dummy engine cover and see if the 90SP performs correctly; it may have died for other reasons than overheating.

ps. I thought the ratio was 1:3 not 1:2? I've always read to give 3X the exit area as compared to intake area for proper cowled engine cooling. But try flying it 1st without the dummy engine cover and make sure that airflow is really the problem... actually you shouldn't even have to fly it just running it up on the ground for a few minutes standing with your legs in front of the hoizontal tail fin to hold the plane back should tell you alot. And post pics!

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Old 04-09-2012, 12:55 PM
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Default RE: The Usual Cooling Issue

Keeping the SP cool is a bit of an art form.

The exit ratio is from 2 to 3 times the inlet, and the SP's run hot, so you had better calculate on 3.

You might have to build in a funnel for the motor, so that ALL the incoming air is forced down over the motor.
Just consider that any path past the motor is the path the air will take in preference to going over the motor, so your ducting has to give the air no option but to flow over the motor with no way to go straight through to the exit.
Removing the dummy engine might actually make things worse.

On your exit, consider adding small lip to the front edge as the high pressure flow over the cowl could be causing a blockage of the exiting air.
The lip will cause a low pressure trough which will aid in extracting the hot air from the cowl.

I had a 120SP in a TF Corsair with these problems, and currently have a 60SP in a scratch built Sea Fury with a funnel duct that works. The bottom half of the front is blocked by a baffle, the air flowing into a funnel shape tightly fitted around the motor with the lower section open. Took 3 tries to get the baffle shape working but now it's fine.
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Old 04-09-2012, 02:44 PM
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Default RE: The Usual Cooling Issue

Sure would like to see pictures of those setups!!!!!

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Old 04-09-2012, 04:07 PM
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Default RE: The Usual Cooling Issue

Here is the Corsair setup.
The ABS cowl did not stand up well to the vibration over time, and the baffle was not as efficient as it should have been, due to it not being close or tight enough around the motor.
You can see the lip created in front of the eflux hole which was just heated with a heat gun and bent. It did make a difference but I was having other issues with the 120SP and the Corsair now has a Saito 125 in it.
The 120SP was a loverly engine and pulled the heavy Corsair like a freight train. Not fast but with heaps thrust.
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Old 04-09-2012, 04:13 PM
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Default RE: The Usual Cooling Issue

The Sea Fury is a more recent build with the 61SP in it. Just enough power but good scale flight.

It took 3 goes at the baffle to get the right shape and get it close enough, although I could still probably improve it.
What I ended up doing was carving a piece of foam I had shoved into the cowl until it was the shape I wanted, then glassed it, dissolved the foam, then bonded the baffle to the inside of the cowl. Apart from the carving time it was pretty quick and easy.

Air doesn't like going around corners so you have to think of it as forcing the air over the fins, cause it won't do it on its own.

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Old 04-09-2012, 04:57 PM
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Default RE: The Usual Cooling Issue

Wow, thanks for all the feedback and pics! It looks like I didn't have nearly enough airflow. here are the pics of my cowl.
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Old 04-10-2012, 07:45 AM
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Default RE: The Usual Cooling Issue

I agree! You need to open up a few more slots in your dummy raidal engine plate around the top! Your exit areas look ok, but not enough inlet area there.

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Old 04-10-2012, 07:49 AM
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Default RE: The Usual Cooling Issue

ps. Wanted to throw this out there... to cover up the inlet/outlet holes you can use wire window screen glued to the inside and painted to match. Optically it'll make the cutouts disappear, but still be transparant to air flow just like window screens in your house when the windows are opened. My Century Airwolf fuselage has air holes in it all covered with screen and painted to match the fuselage, I presume it should work well for airplane cowlings as well. Check your local hardware store for screen. It helps to maintain the scale look we're all after here! [>:]

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Old 04-10-2012, 12:50 PM
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Default RE: The Usual Cooling Issue

I am not sure that the air scoops will keep the air flow in the cylinder, the Weston UK site has some pictures of the recomended air scoop and this shows how th4e air is forced to flow through the fins as the side plates capture the whole engine.

[link=http://www.westonuk.co.uk/westonuk2_036.htm]WestonUK RCV page[/link]

On yours do the side plates but right up to the engine fins, if they do not then the air can escape around the outside of the fins. Additionally, if the end of the surve does not fit well against the fins then the air can escape backwards. Finally, with the V formation some of the air could short circuit out of the top of the fins, I would consider a blanking plate between the two air scoops to prevent this.

I have a pair of RCV60SPs in my Mosquito (I know it is not radial cowl but it is another example), I go in through the scale air scoop on the cowl chin, have a curved air scoop with side plates that fit closely to the fins so all the air flows upwards through the fins, it then turns around in the top of the cowl and goes down around the crankcase, some of it spills out around the scale exhaust slots and some carries on down around the WestonUK in cowl silencer and through a slots by the model exhausts. This works well and as the Mossie is 18-19lbs in flying condition the engines have to work hard.

The photos show

1. The air scoops in the cowl along with the mounts for the exhaust stacks
2. The cowls in place showing the tight fit between the fins and the air scoops so all incoming air has to flow through the fins
3. The air inlet into the scoop in the cowl chin
4. The air outlets at the back of the exhaust stacks which are by the side of the crankcase, as these holes are at the bottom of the exhaust stack the air has to flow through the fins and past the crankcase before exiting
5. The slots in the bottom of the cowl to ensure some air flows over the in cowl exhuast.
6. Shows the engine in the cowl without the scoop showing the in cowl exhausts.

Peter.

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Old 04-10-2012, 01:06 PM
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Default RE: The Usual Cooling Issue

I may have to try cutting out the chin intakes and making them functional.  One problem with trying to come in the top is the needle valve and extension run right through the middle top.  So working around that is tricky.  The bottom is wide open.  I just had my doubts about forcing the air up and then having it find its way back out the bottom again.  I really liked the idea of using foam to form the intake and glassing it.
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