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Rules of thumb for cooling

How big to make the holes?

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Nigel R10/08/2017 08:55:24
2985 forum posts
471 photos


Do you have any rules of thumb regarding cooling holes for our electric setups?

I'm aware of the "out hole = 4 x in hole" one, but is there anything to relate the size of the in hole to the power of the electric system?

My context is a 350W (ish) electric conversion on a small (36" biplane (Precedent Bi Fly), I've experienced several thermal cutouts after a few consecutive flights which I can only pin on the ESC getting too hot (buried in the "under tank" area with little through flow). Obviously some mod to the cooling airflow is in order but I'd like to get it about right first time!

Thanks in advance

Denis Watkins10/08/2017 09:01:48
3812 forum posts
52 photos

When I started electric flight, we fitted the ESC outside the model, with the wires routed inside where necessary. So the ESC cooling had more priority than a pretty model.

I had a BiFly for years before it warped due to storage, but the power plant, an OS25 IC, was out in the open?

Cannot your installation be up there amongst the bearers?

Engine Doctor10/08/2017 09:12:07
2271 forum posts
25 photos

Had a similar problem with a Nigel Hawes  Tucano I built some years ago. Motor not a problem cooling with small air inlet in front of cowling but ESC kept getting too hot as tucked away beside battery The cure was a small air scoop mounted on the fuz just in front of the ESC . It doesn't look out of place and works well. The ESC still gets warm but cooler than before and keeps going even on the hottest days.I could have changed the set up,ESC,prop,motor etc for cooler running but the Tucano is small and wanted a hot/powerful set up for speed. The small scoop is in the prop wash so really drives the air in . Air exits in a hole at rear / underside of fuz where angle of fuz causes depression in pressure that draws air through the fuz. A small lip fitted in front of exit hopes can increase air draw and improve cooling.

Edited By Engine Doctor on 10/08/2017 09:19:55

Chris Walby10/08/2017 09:26:35
954 forum posts
228 photos

Plastic spoons...make nice air scoops be it either tea spoon or the larger versions. this jus leave you to make a suitable exit hole.

Just cut soon at highest point while lying flat, then mark suitable point on fuselage. Make hole in fuselage/make good covering & glue scoop in place. Paint to suit

I cannot take any credit, but they are cheap, work well and don't requite a lot of modifications + look way better that a straight hole!

KiwiKid10/08/2017 10:06:16
472 forum posts
465 photos

Yip spoon scoops work well and can also be used to cover up outlet holes to make them less noticeable.


Commercially moulded scoops can also look fairly unobtrusive.


And NACA scoops can be cut into canopies on non-scale models.


Addressing the OPs initial post, I don't think a 4X outlet hole is really necessary and you could get into some structural problems with some models. The aim is principally to create indoor/outdoor flow and 2X should do this adequately. The efficiency of cooling will be markedly affected by the outside air temperature, which is a variable, and I usually go with the biggest inlet that can be accommodated given the 2X exit hole needed. Then it's just a case of "suck it and see".

Nigel R10/08/2017 11:45:24
2985 forum posts
471 photos

Spoons, brilliant idea.

Denis the motor is a mega inrunner, quite long, it takes up most of the engine bay, I think realistically I'm stuck with the under-tank location unless I do quite literally dangle the ESC out in the breeze.

Brian Spearing10/08/2017 14:28:18
46 forum posts

Would a switch mode ESC produce less heat?

Olly P10/08/2017 14:54:28
3215 forum posts
181 photos
Posted by Brian Spearing on 10/08/2017 14:28:18:

Would a switch mode ESC produce less heat?


I'd look at if you can mount the ESC so the cooling fins/heat sink sit in open air. one of my long term projects has a very high wattage requirement and I plan the ESC to sit with the fins under the fuse belly, protetected by the U/C.

Old Geezer10/08/2017 20:05:01
605 forum posts

Scale's one thing, sports flying's another. 20 feet up and 30 feet out, doing more than 10 or 15 mph neither you nor anyone else will see, let alone notice, or bother about your 'al fresco' ESC.

Erfolg10/08/2017 20:56:00
11322 forum posts
1122 photos

Is cooling to the ESC necessary?

IMO not always. I have gliders with no cooling to either the motor or the ESC. With a motor run of less than 30s, if things get warm, they cool during the next 5-10 minutes.

On the other had some models really need cooling. In some cases I cool both motor and the ESC. As has been suggested, in my TH CS, the ESC sits over a tunnel that is 1:1. In that case I also cool the motor. The Lipos, well they just have to tough it out in the body.

My trainer has the ESC sitting on the outside, as does the motor. However the LIPOs again tough it out in a cramped nose, with no cooling. The Lipos do not seem to like it.

In my opinion it is all about judgement and how much full power running you do, relative to the rating of each piece of kit.

Nigel R11/08/2017 09:16:27
2985 forum posts
471 photos

"In my opinion it is all about judgement and how much full power running you do, relative to the rating of each piece of kit."

There's the thing. The model is quite draggy (as an aside, I'm trying to streamline the cowling and add a bubble canopy to cut drag a bit), and spends much of the flight at reasonably high throttle. Add in a breeze and you have to drive it quite hard to make headway.

As for ratings, the ESC is a 40A and it runs at around 25A full power, so not underrated by any means.

Perhaps a stick on heatsink might aid matters.

Engine Doctor11/08/2017 10:03:51
2271 forum posts
25 photos
Posted by Nigel R on 11/08/2017 09:16:27:

"In my opinion it is all about judgement and how much full power running you do, relative to the rating of each piece of kit."


There's the thing. The model is quite draggy (as an aside, I'm trying to streamline the cowling and add a bubble canopy to cut drag a bit), and spends much of the flight at reasonably high throttle. Add in a breeze and you have to drive it quite hard to make headway.

As for ratings, the ESC is a 40A and it runs at around 25A full power, so not underrated by any means.

Perhaps a stick on heatsink might aid matters.


Overheating will only be delayed by fitting a stick on heat sink unless you can increase air flow over the ESC.

Erfolg11/08/2017 10:26:52
11322 forum posts
1122 photos

On most of my questionable set ups, and occasionally on those where ducted cooling is used, after the initial flights, I do a finger tip test. Does anything feel warm or hot. It does seem habitual, i do this judgemental check quite regularly, particularly on those things I can get at easily.

I general most things turn out to be pretty cool. This could partly be down to conservative selection of ESC relative to current draw by the motor, I rarely run my motors at full throttle for long, and then tend to prop down for current draw. The same goes for the Lipo size and "C" rating.

In my case it is Lipos that seem to suffer the most, due to heating, even though I do try to be conservative.

Edited By Erfolg on 11/08/2017 10:27:23

Edited By Erfolg on 11/08/2017 10:27:53

PatMc11/08/2017 10:39:11
4191 forum posts
520 photos

If running at full throttle is within the ESC's current range it creates less heat in the ESC than running at part throttle.

Nigel R11/08/2017 10:51:16
2985 forum posts
471 photos

for more context, it's a fairly conservative setup all round:

lipo of 35C 4S 2200mAh; at full throttle of 25A that is about 12C, and the lipo is quite cool when removed after flight

the motor is a 200g (ish) mega inrunner (4/22/30) - like the ESC a max rating of 40A, also comes down very cool (but it is as noted out in the breeze)

Erfolg11/08/2017 12:21:38
11322 forum posts
1122 photos

So where is the problem?

From here, everything in the hen coupe seems well

Nigel R11/08/2017 12:25:07
2985 forum posts
471 photos

The problem is really the lack of airflow to ESC, it simply sits and gradually warms up. At three consecutive flights it reaches cutout point...

The original question was more about ducting than the system, I guess.

Frank Skilbeck11/08/2017 12:59:38
4430 forum posts
101 photos

On electric models it's really somewhere for the heated up air to go, if it can't get out of the model it just gets warmer and warmer, so it's not really like trying to cool an IC motor.

I did have one model where the inlet was bigger than the outlet and it worked just fine.

I also on another model put a temperature sensor on the ESC and the temperature went up more at part throttle than full throttle, but I can't be sure whether that was because the model was flying faster or the ESC generating less heat.

Simon Chaddock11/08/2017 12:59:56
5418 forum posts
2833 photos

It always seems odd to duct air inside when there is a lot of it traveling fast just outside the fuselage skin and when coupled with the fact that most of the heat is generated by the power MOSFETS which usually has a flat heat sink on them then why not mount the heat sink flush with the outer skin of the fuselage?.

Of course if the insulating shrink wrap around the ESC can be removed then the heat dissipation capacity is whole orders better.

These are the ESCs of an EDF that are mounted flush with the wing underside just ahead of the air intakes.

Forward ESCs under

Really good 'drag free' cooling.wink 2

In principle an ESC produces nearly as much heat as a motor so its cooling has to be seriously considered.

Erfolg11/08/2017 13:35:25
11322 forum posts
1122 photos

Simon, we both know particularly on full size it is the opportunity to control the velocity and potentially to gain some thrust, using ducted radiators, to alleviate some of the ensuing drag increases. On our models it is generally about aesthetics. A bit of protection is also achieved.

Radiators sticking out vastly increase drag.

With our models, surface heat sinks will generally work, in principal in a similar way that the evaporative systems that never really worked on Full size.

My own thoughts are it is only those that are buried and well insulated, in high relative current applications that can be an issue. I have had one.

We have all seen some very powerful motorised Gliders, such as Pike Perfections,Hitch Hikers and other F5J,100s electric types, where everything is squeezed and cajoled into something that looks like 25mm dia (obviously a bit bigger). With not a hint of trouble.

However there are other extremes, as with my trainer.



As can be seen, an IC conversion from a 35 to a +400w set up. No cooling eventually fried the ESC, initially spotted by the heatshrink having shrunk to the extent it had mostly split, no longer covering the heat shrink. The biggest clue was that the set up stopped delivering full power. This meant I struggled to do a "A" award schedule. It (HK Blue Series) never stopped working, after all the abuse

This was modified to lets stick the thing outside, hanging in the breeze, to Hell with the aesthetics and aerodynamics (if it has that much effect anyway).

I do have a few other models, a profile and indoor model with the ESC stuck on outside.

Yet I think you really do have to Push the ESC hard, relative to its rating, for these measures to be necessary.

Edited By Erfolg on 11/08/2017 13:36:16

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