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Author Topic: Help with cooling  (Read 2202 times)

Offline Robert Redmon

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Help with cooling
« on: July 18, 2012, 06:05:46 PM »
I have been fighting a motor cooling issue on a 12 year old, 64 oz (w/battery) Legacy that I converted from 4S to electric last year about this time. The motor (E-flite 25) has always run hot (160-185 degrees F according to my remote sensing thermometer). The motor is front mounted and large cooling holes are cut between the mounting lugs to allow air to flow through the motor. The gap between fuse top and (now obsolete) motor bearers is about 1/8". I am using a TT "Turbo-cool" spinner with an APC 12/6.5epw prop. I have fitted a small baffle just below the spinner at the beginning of the cowl opening to direct air up toward the motor case. The bottom of the cowl is completely open to allow access to the battery. The combination flies the airplane very well, but the motor gets very hot. The motor is not drawing excessive current as I am using less than 2000 mah per 5:30 m. flight. 

I have two other electrics that run very cool (different motors/planes, etc.).

What am I missing?

Bob
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Offline Wynn Robins

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Re: Help with cooling
« Reply #1 on: July 18, 2012, 06:17:13 PM »
directing the hot air OUT sounds like an issue.

the open cowl at the bottom does not mean it is addign cooling - in some instances it will be creating a vortex above it that my be holding the hot air IN.

I would add vents that direct the air directly out from the back of the motor and then see how it goes.

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Offline Tim Wescott

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Re: Help with cooling
« Reply #2 on: July 18, 2012, 06:39:56 PM »
Quote from: Wynn Robins on July 18, 2012, 06:17:13 PM
directing the hot air OUT sounds like an issue.

That's certainly what I thought was missing.  The air can't go through the motor unless it can get out the other side.

You want inlets where the air gets stopped by the airframe (i.e., in the front), and outlets where the air tends to be whooshing by.
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Offline Dennis Adamisin

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Re: Help with cooling
« Reply #3 on: July 18, 2012, 07:16:23 PM »
Yes, definitely sounds like you could use some help on the air exit side.  Di you reomve or ventilate the firewall when you did your conversion?  Do you have any holes in the top block, say at about the firewall location?

What about the other components, how do the tems compare for the battery & ESC versus your other birds?



That motor and that prop should not be running that hot...
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Offline Robert Redmon

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Re: Help with cooling
« Reply #4 on: July 19, 2012, 06:02:38 AM »
Dennis and Wynn,

Thank you both for your thoughtful replies. The esc is mounted in a space that would have been been below the tank compartment. The heat sink on the esc is flush with the bottom of the fuse and remains very cool. Battery slides in behind the motor in what was the tank compartment. It is always warm (maybe 10 degrees above ambient temperature) after a flight, but never exceedingly so. When I did the conversion, I created a path for the air below and above the battery to exit through the hole in the bottom where the esc is located, but after all the gear was mounted, the area available to ventilation ended up being rather small.

You may be correct in that the open bottom may be acting as a scoop forcing air into the cavity where the motor and battery are located and prohibiting the exit of hot air moving through the motor. I may try adding a baffle just below the motor to see what that does for engine temp. I suspect it may help the motor but increase battery temp somewhat, but at this point I am willing to try anything.

Bob
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Offline Andrew Borgogna

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Re: Help with cooling
« Reply #5 on: July 19, 2012, 10:29:14 AM »
Something my friend Larry Renger told me, "the air exit hole/holes must must have twice the area of the intake".  I have been using that theory and it seems to work.  I too had a heat build up in my Continental, I added extra exit holes per Larry's suggestion and I added the modifications to the spinner that Dennis Adamisin suggested.  The combination reduced the heat to a safe level.
Andy
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Offline Robert Redmon

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Re: Help with cooling
« Reply #6 on: July 19, 2012, 11:30:56 AM »
Thanks, Andrew. However, my problem is somewhat different. The entire bottom of the nose of my plane is open back about 5" from the spinner ring. I am thinking now that this may be creating a high pressure bubble in this area that prohibits air flow through the motor from the front...essentially stagnating the air in the forward upper nose section where the motor resides. I am working on a scheme to facilitate a more traditional cowling sans the chin vent (relying upon the air coming in through the spinner for input air) that will still have a large bottom exit vent (3 or 4 square inches) behind the motor. The problem is that it will have to be removable to facilitate battery removal and installation....but we do what we must.

Bob
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Offline Wynn Robins

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Re: Help with cooling
« Reply #7 on: July 19, 2012, 02:13:20 PM »
I have a legacy that has an open cowl also - all you need to do is put som holes behind the motor - through the fuse to let air out  it will work....

I have been flying my legacy for 2 years with no issues.
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Offline Robert Redmon

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Re: Help with cooling
« Reply #8 on: July 19, 2012, 06:03:41 PM »
Wynn,
Do you mean in the fuselage sides? I had not even considered that.

Bob
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Offline Wynn Robins

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Re: Help with cooling
« Reply #9 on: July 19, 2012, 07:54:16 PM »
yes - exactly.
You can make them look cool too - like sharks gill slots etc- they dont have to be "holes" 
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Offline bob branch

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Re: Help with cooling
« Reply #10 on: July 20, 2012, 07:02:26 AM »
No one has mentioned it, but there is a way to check the cooling flow for that bubble you mentioned. Just take some short pieces of thread and tape them to the fuse around your air openings. They should terminate just in front of the opening and extend maybe half way the length of the opening. Then just flip the plane so you can see em and start the motor. If the threads go into the exhaust holes you are not getting the flow you want. I don't particularly like using thread, but on models our size we cannot use other materials as they are too big.  btw, the primary airflow the airplane is seeing in the nose area is the prop wash. Its velocity is markedly higher than anything from the aircraft's speed thru the air.

One thing that was hit on earlier is the flow of air thru the plane. Fluids flowing thru something want to flow in straight lines. You can deflect and bend the path a little, but you cannot reverse the path or make a 90 degree angle without causing turbulence. Also given narrow spaces and wide spaces, air will want to fly thru wide spaces. 1/8 inch around the battery isn't much. Air starts to deflect around an object before it gets to it... quite a bit before. The big block of a battery will certainly cause a lot of turbulence. Look to create a fairly smooth flow without constrictions that gets larger as it moves aft towards the air exit. A small lip sticking up at the front of an opening will create more draw of air out of the opening as well. Its used very often in 100% scale aircraft cowls.

bob branch
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Offline Robert Redmon

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Re: Help with cooling
« Reply #11 on: July 20, 2012, 04:22:55 PM »
Thanks, guys.

I tried adding a second, larger baffle directing air down toward the motor, and that reduced the temperature by 15-20 degrees, but I still don't think the air is moving through the motor like I want. I will experiment with "gills" and let you know how that works out. Wonder about the effect of putting the motor in a duct routed directly outside the fuselage?

Bob
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Offline Howard Rush

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Re: Help with cooling
« Reply #12 on: July 20, 2012, 08:03:51 PM »
Quote from: bob branch on July 20, 2012, 07:02:26 AM
I don't particularly like using thread, but on models our size we cannot use other materials as they are too big.
 

I used a Post-it flag, http://www.post-it.com/wps/portal/3M/en_US/Post_It/Global/Products/Catalog/?PC_7_RJH9U523000P60II85TCFL1863000000_nid=L1DDHX69VJbeQF4PQ1K69Mgl .    I actually found some with my name on them in an office supply store. 

 
Quote
btw, the primary airflow the airplane is seeing in the nose area is the prop wash. Its velocity is markedly higher than anything from the aircraft's speed thru the air.

Maybe for a heavy airplane going straight up.
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Offline Dennis Adamisin

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Re: Help with cooling
« Reply #13 on: July 21, 2012, 11:38:09 AM »
Quote from: Howard Rush on July 20, 2012, 08:03:51 PM
I used a Post-it flag, http://www.post-it.com/wps/portal/3M/en_US/Post_It/Global/Products/Catalog/?PC_7_RJH9U523000P60II85TCFL1863000000_nid=L1DDHX69VJbeQF4PQ1K69Mgl .    I actually found some with my name on them in an office supply store.  


You found Post-Its with HOWARD printed on them?  Oh wait, you meant...... (never mind)


About the second part of your post - can you elaborate a little?  I thought the prop stream in cruise was 10%-15% faster than airspeed???
« Last Edit: July 25, 2012, 06:28:02 PM by Dennis Adamisin »
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Offline Mike Scholtes

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Re: Help with cooling
« Reply #14 on: July 21, 2012, 07:06:23 PM »
Hi Robert:

I have a 64-ounce E-Legacy built as electric from the beginning. AXi 2826-12 power, 3200 ma 4S batteries, Phoenix 45 ESC. Motor is rear mounted. I added a NACA vent to the top of the cowl, above the motor, to try to suck air in and over the motor and into the battery compartment. I added a baffle in the front cowl opening (where normal IC engine opening would be) after first flights indicated hotter than desired temps on the motor. It also directs air over the ESC and battery. The baffle diverts air up at the front of the motor, in an attempt to get more cool air into the can. Seems to work, as I have had no worrisome temp issues. The cowl has air exit holes about three times the area of the entry opening all the way to the back of what would have been the tank compartment. Getting the air out is every bit as important as allowing it in, according to what I have learned.
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Offline Robert Redmon

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Re: Help with cooling
« Reply #15 on: July 24, 2012, 10:04:55 AM »
Thanks, Mike.

I have had no heat issues related to the battery or ESC. They both run 10 -15 degrees or so hotter than ambient. The battery is where the tank would have been, but the upper "floor" has been moved a bit to try to center the weight of the battery on the wing cl. The ESC is mounted so that its heat sink is flush with the fuselage bottom. Its compartment is vented through the battery box and air exits around the ESC (about 1/8" clearance on all sides. The battery just slides in from the bottom and is secured with a Velcro strap. Recently, I added a second baffle (likely similar to yours) near the front of the cowl (aimed at the center of the motor) and opened the area around the motor up a bit so that I now have about 3/16 minimum clearance around the top and upper sides of the motor). I had added a small "scoop" baffle at the very front of the cowl opening last fall, but that had no measurable effect on motor temperature. The daytime air temperature here has been over 100 degrees for the past several weeks, so I have refrained from flying the setup very much. My best measurement thus far has been about 40 degrees above ambient.

I have two other electrics flying without issue running Aerowind 2820/07 motors, and both run only 10-20 degrees above ambient. One has a cowl similar to the Legacy, but has a bulkhead behind the motor (and is rear mounted). It has a regular spinner with lightening holes in the backplate, and has no baffles or vents to enhance cooling. Granted, it is smaller and lighter than the Legacy, and uses 1500-1600 mah per 5:30 flight compared to the Legacy's 1950-2050 mah, so the motor will be dissipating more heat. 

Bob
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Offline Howard Rush

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Re: Help with cooling
« Reply #16 on: July 24, 2012, 08:47:17 PM »
Quote from: Dennis Adamisin on July 21, 2012, 11:38:09 AM
About the second part of your post - can you elaborate a little?  I thought the props stream in curise was 10%-15% faster than airspeed???

I regret that I don't have my prop program with me.  I might do the ciphering when I get home, except my wife has a backlog of chores for me.  10%-15% sounds about right.
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