Engine cooling

[John Minchell]

What's the "rule of thumb" for cooling a glow ic engine (OS 90 FS) in a scale installeation. 

The air inlet is small in the front of the cowling (44mm x 20mm).

Does the exit area need to be 2 or 3 times the intake area or even more?

Any help appreciated.

John M

[Martin Harris - Moderator]

Opinions vary - 3 times is often advised but the important thing is that the air passes close to the cylinder finning and head.  This usually means providing baffling and ensuring that the exit is in a low (preferably) or neutral pressure area.

 

I've quoted the example of my 1/12 scale Hellcats in the past - a small 3/4" x 1/4" curved slot in the scale oil cooler position directly in front of the cylinder with the front of the cowling just painted black where the cylinders would be seen instead of the more usual cut out - just a tight fitting hole for the prop driver to fit through.  The head lay flush with the cowling and I made a slightly egg shaped hole below it (engine inverted) of around the same size so that the air exited at the back of the cylinder/head with a small lip formed at the leading edge of the hole.  7 minute bouts of combat at almost exclusively high/full power settings proved the effectiveness of the arrangement which was poo-pooed by various "club experts".

[Frank Skilbeck]

John, as Martin says, the important thing is to make sure that the air doesn't bypass the engine without taking any of the heat away. Flying friend has a Pup with an OS120 in it, first flights where with an open cowl and we had severe overheating issues, put a baffle in so small air inlet was directly in front of the cylinder and problem solved. My Jungmeister with an ASP 90 FS is basically the same.

[Martin Harris - Moderator]

Yes - took me weeks to convince a clubmate with a Flair Harvard to block off the open cowl except for the area around the cylinder.  He kept saying that there was loads of air around the engine in the cavernous cowl so it couldn't possibly be overheating.  Instantly cured the problem that had led to dead sticks on every flight...

[Christopher Wolfe]

I found this article quite useful for understanding cowlings and cooling. Cooling Large Model Engines

 

I also fail to understand the 2-3 times intake area for an exit hole as heated air does not expand by 2 - 3 times.

 

As Martin said above, a strategically positioned lip in a low pressure area is  invaluable.

 

Chris

[Jon H]

1:1 in and out is fine if you get the pressure gradient right. As has been said already a lip on exit and things like that are all you need to make sure there is a little suction on the exit to help draw the air out. 

 

Do you have a photo of the model and the engine in the front? Might be able to offer more accurate help

Edited December 31, 2021 by Jon - Laser Engines

[John Minchell]

Jon pics of motor in cowl.  Model is inverted on stand.

You can see that there is only a small inlet (44x20 = 880mm2) and I guessed I would need internal baffles to direct the air through the fins. 

But the exits on each side are very narrow (60x3 =180 x2off =360mm2) and add up to less than half the inlet area.

Hence my questions.

I can cut holes in the lower firewall and add some tubes to route the exit air into the wheel wells where it can exit around the periphery of the wheels which will add sufficient area, but is a lot of work which I don't want to do unless necessary. 

[John Minchell]

Thanks Chris - really useful article on cooling so I've saved it.

John M

[Martin Harris - Moderator]

Looks very similar to the arrangement on the Airsail Chipmunk which was my first build on returning to the hobby a couple of decades ago. I didn’t baffle it and with only the scale outlets at the rear of the cowling (ideally situated by the full size designer) despite being advised by the club brains trust that it would overheat, it didn’t and continues to perform consistently on its original engine to this day. 
 

I can’t make out where the exits are but if there’s a decent path into the area in front of the wheel well then I’d expect it to work in a similar fashion.

[Jon H]

i think it will be fine. Add a baffle from the intake to the head and have it more or less touching the engine. The only air that can then enter the cowl has to go through the fins and this reduces the air in figure when it comes to area. 

 

The outlet side is fine in terms of design, but you could probably remove a little from the fuselage so the gap between it and the cowl is a bit bigger. Dont go mad, but see if you can mirror the shape of the cowl on the fuselage side to give a little more flow. 

[John Minchell]

Martin

The outlet exits are shown on pic 2 & 3 at the rear of the yellow cowling - exactly the same as on a Tiger Moth or Chipmunk - a slight bulge at the rear edge away from the flat fuz side is the air outlet.  It's only 3mm wide but 60mm deep top to bottom.

 

Jon

I can't cut away more of the flat side of the fuz because it would not be scale and cutting away inside the cowl on the side cheeks from the firewall back will not acheive any larger opening - just a sort of plenum chamber which is still restricted by the small side exit area.  So I will cut extra holes in the lower firewall and tunnel them through to the wheel wells to get sufficient area/flow for extraction

 

One last thought -  do I need to provide a separate cold air feed to the carb instead of it using the warmed up cooling air from the front inlet?

Edited December 31, 2021 by John Minchell
spelling

[Martin Harris - Moderator]

Ah yes, I can see it on closer examination. Even a slight deepening of the woodwork and thinning of the (much thicker than scale?) cowling would give significant additional area - and it must have worked on the full size!

 

Jon may have more thoughts on this but if tuned correctly, the warmer air shouldn’t make too much difference - for example, full sized Chipmunks normally have the “hot” air control wired open anyway as the air isn’t anywhere near as hot as intake air deliberately heated by an exhaust manifold heat exchanger such as used by Lycomings and other similar aircraft engines. They then use only (slightly) filtered instead of totally unfiltered air.  At least you’re unlikely to suffer from carb icing!

[Jon H]

1 hour ago, John Minchell said:

 

I can't cut away more of the flat side of the fuz because it would not be scale

 

I dont wish to be a buzzkil, but it has to work. With the best will in the world and the highest attention to scale detail we are still playing with toy aeroplanes and the laws of physics are not always compatible with perfect scale appearance. It would be easy to nibble away material from the fuselage side to allow greater area. It would be hidden by the cowl and only a close up examination down the hole would reveal any scale issues. Certainly it would not be visible in flight. Even on the ground i really doubt anyone would spot that the gap is 6mm wide and not the scale 3mm. 

 

While i understand the desire for perfect scale compromises need to be made if the model is to fly reliably. It will be of no enjoyment at all if you spend your flights worried about the engine roasting itself and i am not completely convinced venting into the gear bay will be effective as you may find you end up with a back flow due to the gear bay acting as an intake and not an exhaust. 

 

 

[Jon H]

56 minutes ago, Martin Harris - Moderator said:

 

 

Jon may have more thoughts on this but if tuned correctly, the warmer air shouldn’t make too much difference - for example, full sized Chipmunks normally have the “hot” air control wired open anyway as the air isn’t anywhere near as hot as intake air deliberately heated by an exhaust manifold heat exchanger such as used by Lycomings and other similar aircraft engines. They then use only (slightly) filtered instead of totally unfiltered air.  At least you’re unlikely to suffer from carb icing!

 

Within reason warm induction air will not matter. Cooler is better, but it will not make a huge difference unless it starts getting really hot. By then the engine would already be overheating anyway so its a non issue in real terms.