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Rules of thumb for cooling

How big to make the holes?

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John Emms 111/08/2017 13:37:42
233 forum posts

Hi Nigel, there is little that I can add to what has been written so far.

Controller issues. Your 22/30/4 wants zero to around 6 degrees of timing advance, though the Foxy's tended to work best with the Megas at 22 degrees (the same as outrunners). If the timing or frequency is not at optimum, that could result in a build up of heat. I found optimum prop on the 22/30/4 to be around 11x5,5 running at maximum rating of 450W on 4S.

On the gliders, the top of the fin being open, with holes in the motor mount to allow cooling air through the motor, and to be drawn between the front of the fus, and the spinner always worked well.

On power type models, we always opened up all the formers in the fus, and generally cut a large hole underneath the rear of the fus for cooling, and that allows air in or out as required. Of course, air needs to be able to flow through the motor. For your ESC cooling, I would simply open up a hole in the bottom of the firewall that will allow air to flow through the fus, cooling both the ESC and the batteries.

For a general rule, with glow models, I always sealed them as much as possible, and with an electric model I open up the inside of the fus to cooling air as much as possible - and yes, that still feels strange! Using this strategy, we (as a test and display team of 5) never had an issue with overheating.

Hope that helps,

John

Erfolg11/08/2017 14:27:06
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11366 forum posts
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John

I certainly agree with the general thrust, of what you do and advice.

I am coming around to one of your implied concepts, that is to get some air moving around the Lipos. Particularly where the Lipos are pretty much squeezed in. The drag increase will be next to nothing in practical terms, as the air bleed in will be a very slow moving body of air in the fuz, a largish hole towards the rear, will effectively have no drag consequence.

I have more issues with short live Lipos than anything else. It will always be the same models. The ratings will always be conservative. I am pretty sure it is the lack of effective cooling. Like the ESC, it does not seem to be much that is ever needed (perhaps other than DF).

So more often than not, I have started to look to get some air moving past the Lipos, if the cabin area has little free volume. I am a long way from all of my models using this concept. Having moved from gliders to sport electric models, I am far more aware that it is the long motor runs at the limit that can be an issue.

John Emms 111/08/2017 22:03:38
233 forum posts

HI Erfolg, sorry, I don't have a real name.

You got me thinking. I or we got used to the idea of really opening the inside of the fus up to air freely moving through, which was a real shock after the idea that everything is packed inside and wrapped in foam! The big fear was things breaking loose, so batteries always have been stuck to a light ply plate with self adhesive Velcro, and then have a Velcro strap to make sure the batteries stay in place. I said we never had a problem - but I remember one:

The LN Model Accord 2200 2,2m was a freestyle or 3D model designed for 40cc. Libor of LN then produced a lighter version intended for 10S electric - I bought two as demonstrators. In hindsight, Libor had allowed for cooling air around the cowl that contained the speed controller, but had not allowed for cooling air through the fus that contained the batteries. We had the best 3700 cells available at the time (25C), and peak power was conservative at around 65A. At the end of every flight I was aware that the batteries were warmer than I was comfortable with, and became very slightly puffed. The same power system with the very same batteries was used in my Rascal 110 conversion. The batteries were in a ply box or tube that was in the tank bay, and slid in under the cowl. The battery box had openings at the end, and cooling air could escape through a hole cut in one of the side windows. The batteries were hardly warm at the end of the flight, and I was never concerned about them - other than knowing they had slightly puffed during use in the Accord.

I had always thought that people suffering short battery life were being sold cheap batteries, using them at high currents, and not balancing regularly, but I think you could well be right, that good levels of battery cooling are a large part of long battery life. Yes, I also agree with you that a small amount of air that is allowed to flow freely through the fus does make a huge difference.

Regards,

John

Nigel R15/08/2017 14:23:48
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3046 forum posts
475 photos

Some good points all, many thanks. John thanks for the specifics on the mega motor. Must admit power wise it is about right on the 10x5, freshly charged that is giving about 450W down to around 400W at empty, and I'm getting a 6min flight in with about 25% left in the battery.

Today I flew with the ESC hanging outside in the breeze - 4 consecutive flights, no hint of a problem.

The battery is coming out just warm, so I think that is OK as is. The old fuel line hole serves that compartment quite well.

I think the answer is simple at this point, I will open up the former at the rear of the wing saddle, and open a hole in the fuselage at the same point. And then sort a small scoop to get some air into the ESC compartment (or a hole in the firewall if I can get access with a drill).

Erfolg15/08/2017 16:05:17
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11366 forum posts
1188 photos

I guess that many of us have wrestled with the issue of Lipo life and cost that John Emms has raised.

As with so much a lot has changed and possibly will change significantly in the future.

On returning to model making, the switch to brushless motors, was starting, in earnest. The advent of Lipos presented far more viable options for the ordinary modeller such as myself. My first Lipos were I believe called Energens?. These were followed by Overlander products including the Supreme version. They were OK, not brilliant, unfortunately the lives of these units was very variable. Probably the first so called budget brand was Loonmax (again, or something like that), these Lipos were certainly as good and arguably better than the earlier Lipos. Since then i have used loads of different brands, Rhino, Turnigy.

All I can really say, there does not appear to be one brand that appears to be consistently good, because so far most if not all the brands have had a shorter life than expected. I now have a internal resistance meter, that loads the cell, to provide a reading. I do not treat the output as definitive, providing an indicator on a numeric basis on the condition of each of the cells. Often right from the word go, the resistance are often different by an amount that is noticeable. With usage, it becomes clearer that a cell within the Lipo has such high resistance, it is time to bin it. I do have a spread sheet which i have entered the initial values and the values when it becomes apparent when the pack seems to be failing.

But at the end of the day, no definitive answers, although a suspicion, that even a Lipo pack becoming warm, is not the best of news. Although I think this seems to be born out, by it seems more often than not, it is the middle cell on a 3s exhibiting a notably higher resistance than the outer cells. Which may be implying that a warm outer wrapper, indicates that the central cell is much warmer, probably down to less effective cooling. It is just a contention that it is heat that degrades the pack life. I am sure that pulling heavy currents is also a factor. I certainly do not have enough results, nor undertaken with sufficient controls, to say anything beyond I suspect.

Edited By Erfolg on 15/08/2017 16:08:47

John Emms 116/08/2017 06:48:23
233 forum posts

Erfolg makes a really good point, that if the outer two cells in a 3S pack are warm, then the inner cell is going to be warmer - and it would appear, at greater risk of short life (though I have no evidence to support this).

On reliablity of different brands: A handful of brands can be from the same manufacturer, but I have not been able to recognise which brands share a common manufacturer - and there are also more than a couple of manufacturers....

I have been using the Pelikan Ray and Foxy brands, partly for obvious reasons, and partly because Daniel Pelikan can choose from any manufacturer (and Daniel would never knowingly sell me anything that was less than good).

I also have an internal resistance meter, and although the Ray are more than capable of supplying the current I need, I will probably stick with the lower resistance (and heavier) Foxy in future. I suspect that the same cells as the Foxy are Overlander's premium range of batteries - the surface mount LEDs on the end of the pack help with identification.

Nigel R16/08/2017 08:45:20
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3046 forum posts
475 photos

John - intrigued to know - how does the internal resistance meter take its reading?

Also the elevated cell temperature, >50deg, I have read this leads to shorter cell life. My yardstick for temperature on most electricals is, if it hurts to hold it, it has got too hot.

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