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Old Jul 08, 2014, 11:37 AM
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LI, New York, USA
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Cooling for E-glider motors

Looked around but have not seen much on this subject in a dedicated thread.

For those of use running ALES or F5J a typical motor run is 30 seconds then soaring so the built up heat has time to dissipate, even without much in the way of cooling. Or does it?

What are your thoughts on the need for cooling vents?

What are you doing?

Anyone tried some kind of vented spinners?

If you are cutting vents, what advice can you offer?

Are you using cowls or scoops for intake or exhaust vents?

Inquiring minds want to know.
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Old Jul 08, 2014, 12:15 PM
Red Merle ALES
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United States, Mt, Helena
Joined Apr 2002
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I don't add drag to my models. If the heat is high, the motor, gearbox, prop, ESC or battery are not optimized.

Just my .02 cents worth.
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Old Jul 08, 2014, 12:36 PM
Flying = Falling (Slowly)
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Tulsa, OK
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Quote:
Originally Posted by Curtis Suter View Post
I don't add drag to my models. If the heat is high, the motor, gearbox, prop, ESC or battery are not optimized.

Just my .02 cents worth.
+1

At some point in time pilots will discover that it is very difficult to comply with the ALES rules pertaining to properly installing your switch AND providing for ventilation.

Lower powered planes (100 watts/ pound or less) won't have big problems, but even a stock RADIAN will experience distortions of as much as ten or fifteen meters (overlaunches) under some circumstances.

A 150 watt/lb+ plane can achieve horizontal speeds in excess of eighty mph which can result in MATERIAL distortions.

Size your power system right and forget fiddling with vent holes.

Happy Landings,

Don
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Old Jul 08, 2014, 12:59 PM
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In no particular order.

It seems like most designs have cooling intakes but the outlets are often too small or entirely lacking. Adequate exit area, especially if one is using scoops or a "turbo spinner" to ram air into the motor, is very important.

I enjoy having good climb performance and that means using props at the top of the range for a given motor. Both of my current gliders will climb vertically, as high as I want, in 10-15 seconds. If the motor gets uncomfortably hot after 10 seconds on the ground but the current draw is within the motor's range? I look towards improved cooling.

The drag from adding small scoops is probably less than the drag of having one prop blade hanging down.

I have not used a flow-through spinner but I do drill holes in the motor mount so that air can flow through the motor. I am still working off a fairly small sample but it seems to make more difference than increasing the amount of air than can move past the motor.

Whether it's with a glow engine or a motor I am a big believer in using props that have a relatively large blade area and relatively low pitch. Let's assume a simple system with a direct drive outrunner that's around 1,000kv. If one wants optimum climb performance there is little reason to use a prop with more than a 6 inch pitch and increased pitch will really increase the load on the motor and increase the operating temperature.
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Old Jul 08, 2014, 01:27 PM
Thermal Junkie
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No cooling provisions on any of my stuff either. 5 or 6 years running like this and to date no problems.
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Old Jul 08, 2014, 02:12 PM
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I have a few points...

#1. Air always moves from high pressure to low pressure. If you have a low pressure area behind the spinner and most (non turbo) spinners will... air will move forward through the fuse and be sucked out the spinner.

#2. When the motor is running, the spinner acts as a centripetal air pump, again drawing air forward rather than allowing air to pass through nose to tail.

#3 NACA scoops work very well, but be sure to off set them. I vented a canopy with 2 scoops inline, above the esc. My heat problems increased due to the smooth flow of air not being directed at the hot parts, rather a smooth flow of air that bypassed the issue. I built up a diversion inside the canopy to force the air to take a circuitous path rather than a straight line.

My current ALES ship is an Elegant 700 and it has no cooling provisions. The only time I abused the motor and battery was when I was setting my flap configuration. I spent a whole battery climbing to cut off and diving back to land with flaps immediately thereafter.

I do have excess motor, esc and battery capacity compared to the competitive pilots. I am running a 6s capable scorpion motor and a 4000 mah battery pulling less than 50 amps throu th an 85 amp esc. Under normal ALES flight condition I see zero need to add cooling to this plane, even flying in 100+ degree ambient temperatures.
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Old Jul 08, 2014, 02:30 PM
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Thanks guys.

I was wondering about heat build-up. I did not even consider the ALES/F5j unit in my question.

So you are suggesting that if I were to vent the fuse I might distort the altitude cut-off due to pressurization of the equipment area of the fuselage? Jeeze, things can get complicated.
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Old Jul 08, 2014, 03:27 PM
turn, turn, turn.
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from what I understand, if you want to keep the altimeter reading properly, no venting at all.

pretty much what Don said, I think.
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Old Jul 08, 2014, 03:40 PM
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Hmmm. I wasn't thinking of throwing off a sensor either. Perhaps one could set up a "static port"?
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Old Jul 10, 2014, 06:47 AM
tender
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Europe - Slovakia
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Best solution - use the power unit or the motors with build in cooling fun :-)
This one in the sketch is RedFox 500W with planetary gear...

Palo
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Old Jul 10, 2014, 10:08 PM
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As Arte Johnson (laugh in) would say, "Very Interesting".

I had never considered that air would be sucked out the front of the fuselage from behind the spinner.

Would it be worth trying to take advantage of this phenomenum and run a larger spinner, maybe 1/4" oversize, to increase the amount of airflow?
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Old Jul 11, 2014, 03:32 AM
turn, turn, turn.
Athol, Massachusetts
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Quote:
Originally Posted by peterlngh View Post
Hmmm. I wasn't thinking of throwing off a sensor either. Perhaps one could set up a "static port"?
this is what I suggested in the sailplane forum... Simply encapsulate the unit, and put a hollow tube through it, to the outside of the fuselage, perpendicular to the air flow.
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Old Jul 11, 2014, 08:45 AM
Flying = Falling (Slowly)
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Quote:
Originally Posted by JimNM View Post
I have a few points...

#1. Air always moves from high pressure to low pressure. If you have a low pressure area behind the spinner and most (non turbo) spinners will... air will move forward through the fuse and be sucked out the spinner.

#2. When the motor is running, the spinner acts as a centripetal air pump, again drawing air forward rather than allowing air to pass through nose to tail.
I have seen this argument before and it ignores other factors which are at play. It may be true with respect to the pressure profile BEHIND the propeller, but it is not true for the difference between ambient air pressure and the pressure at the hub behind the propeller. If we first look at the pressure profile BEHIND the propeller it is not unreasonable to observe that centripetal force will result in a pressure profile where the pressure at the hub COULD BE lower than the pressure away from the hub. But that does not mean that the pressure at the hub BEHIND the propeller is lower than the ambient pressure away from the plane. In fact, what makes the plane go is the fact that the propeller is able to create a pressure that is higher than the ambient pressure BEHIND the propeller. So whatever pressure differential that MAY be developed by centripetal forces will be more than offset by the pressure differential between the front and back of the propeller itself. In our applications the spinner will not act as an air pump.

Quote:
#3 NACA scoops work very well, but be sure to off set them. I vented a canopy with 2 scoops inline, above the esc. My heat problems increased due to the smooth flow of air not being directed at the hot parts, rather a smooth flow of air that bypassed the issue. I built up a diversion inside the canopy to force the air to take a circuitous path rather than a straight line.
What you are suggesting is nice in theory, but nearly impossible in practice. It requires that exits more or less exactly balance the pressure produced by the forward facing openings. As I mentioned earlier, I ran some experiments with a stock AVA fuselage using an RF turbo-type spinner, and four 1/4" holes in the motor bulkhead and the normal opening in the end of the boom. I mounted this on top of the car and measured the "pitot" distortion that might be realized. And even with openings that seemed to me to be more or less in balance I measured "pitot" distortions that were in the range of half of the distortion that would be expected if the fuselage had had no exits at all.

I have attached a graph of the maximum distortion that can be expected from a fuselage that is configured to take full advantage of the pitot effect -- inlets facing forward, no exits. As you can see this is a very significant issue. Even half of it is a significant issue. And it is very easy for a composite glider flying at an angle of 30 or so degrees above horizontal to exceed 70 or 80 mph.

As to suggestions that one can offset combinations of turbo spinners, holes in the bulkhead and NACA scoops, I would suggest that the experience I have had with the lowly Radian, might give a person reason to pause. I have attached a log plot of a TYPICAL Radian launch (I have dozens of these and they are all alike). Upon releasing the plane, instead of seeing a steady climb to 4 or 5 meters immediately after release, the log plots show the plane diving a few meters below ground -- something I can assure you did not happen. This is from a plane with VERY modest performance capabilities. And while I can look at the plane now and explain why its particular combination of inlets and outlets might produce that result, I would never have guessed it without actually measuring it.

Happy Landings,

Don
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Old Jul 11, 2014, 09:54 AM
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Thanks for the analysis. I am sure that there are a multitude of factors that are in play, and the unique designs make a blanket statement impossible.

I do not have a Radian, and never will. My anecdotal statements are for entertainment purposes only Everything i know, i freely stole from the internet. I would be happy to have you run similar tests on my planes that are not Radians, if only to add more data points to your galaxy of experience.

My first point is the crux... air can only flow from high to low pressure. The body of air in the fuse is under entirely different forces that the air outside the fuse. Propwash is minimal at prop hub, as the greatest amount of presssure is generated at the prop tips.

Oftentime i see where people posit that the exit area must be much larger than the inlet area to account for expansion due to heating. I do not accept that the brief time the air is exposed to the hot components allow for sufficient heat transfer to cause significant expansion of the air.
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Old Jul 12, 2014, 12:06 PM
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Just some interesting data..I am diving my Flash 2 hotliner, and the air in the fuselage becomes "compressed". So these alt readouts should show a dive to the lowest altitude, but you see a "bump" in altitude, at the bottom of the dive, then it takes a second or so for this pressure to drop off, after the dive, and high speed low level pass.
Just before I start the next climb, and the Flash has slowed back a bit, the true altitude of the aircraft is displayed.

This Flash 2 has cooling holes in the nose and a "turbo" RFM spinner. Motor is only switched on for a second or two during the vertical climb, then off.
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