Rudy,
If you look at the AXI web site, they mention that you need to allow cooling air to enter the front facing (in the direction of airflow) side of the motor. The airflow is necessary to use the maximum power ratings for the motor (at least according to AXI). Of course the air needs to flow out the back and out of the plane (passing by the ESC wouldn't be a bad idea!
Remember in an outrunner motor, the coil windings are not rotating, and that is where the heat is being generated.
I say this because for my Brodak Super Clown, the mounting scheme covers the holes completely. I am considering drilling some holes in the nose ring to allow at least some air to enter from the front. Probably doesn't matter a lot in cool winter/spring flying temperatures, but might be important in summer.
One issue with heat is that the neodymium permanent magnets begin to lose their magnetization when they get too hot. That may  mean (for an outrunnner at least since they are mounted on the outer rotating cylinder shell), that the internal cooling flow might not be that important---but I don't know for sure about that.

I haven't tried to draw air through a spinner yet (haven't had the need to), but I have used scoops/baffles to direct the air. It has worked very well for me.

Some motors (mainly heli motors) have fans to draw air through the motor. In a heli application there is almost no airflow.

Neodymium magnets are safe up to about 100C (212 deg F). The case and copper wire can handle much higher temperatures. If you take a temperature reading try to get a reading directly off the magnets.

This is a general statement, because different magnets (ferrite or cobalt) can handle higher temperatures. Go by the manufacturers specifications to be safe (or at least see what they reccomend).   

Hi Mike,

Thanks for your post. I was hoping you had already experimented with this. (using the extra 5 min. per week you have left over in your busy schedule. ;-) The reason I asked about the "cooling holes in the spinner" is that our outrunner elec. motors need most of their cooling inside, which is totally blocked by our spinners. With our IC engines the spinner actually helped us by directing some of the airflow into the cylinder cooling fins. As you, more than most, know, many of the "old ways" of doing things with our airplanes no longer apply to elec. power! The standard spinner is sure one of them.

I envision the perfect Elec. CL (and elec. R/C) spinner to have large openings with some form of scops in the spinner and or in the backplate that forces air through the motor for the best possible cooling. We are already part way there with the excellent Tru Turn Electric spinners that have most of the backplate cut out, and they are light weight. If someone could just make the backplate do double duty as an impeller it might be the answer? ...... With our very small customer base it may take awhile before we have a real "cooling spinner", made for us. We may get help from the R/C elec. world, ........ their customer base is probably ours to the fourth power +? ;-)

I realize that we can cool our motors with good baffles, 2X+ air exhaust ports, and good cowl designs, etc. but I was just hoping someone had experimented successfully with an elegant spinner solution. ..... Bob Hunt has tried using small holes, but he said this did not work out. I'm sure someone will come up with one someday, I just hoped it would be sooner than later. ;-)

MOTOR TEMPS: Here are some good comments from the tech dept. at MEGA motors:

QUESTION: "What is the maximum temperature a Mega Motors can reach?

ANSWER: MEGA Motor recommends that your motor not exceed 140 degrees F. As a rule of thumb, you should be able to hold your finger on the motor after operation.  If the motor burns your finger or is uncomfortable to hold, it is too hot. Motors that are continuously operated at high temperatures will suffer from demagnetization of the permanent magnets and lose power. Models such as airplanes should be designed with air flow around and through the motor. For helicopters an impeller or fan blade can be added to the motor shaft to create cooling and increase air flow.  Heat sinks added to the case are also a good option for cooling.  Cool motors such as MEGA Motors run better and last longer......"   Tech Dept.

I have talked to several experts about our motors and they all seem to agree on this 140 degree F as a safe top  limit. The possible motor failure temp, and/or very short life span, seems to start around 165 to 175 F, and like you said total failure somewhere above 200 F. They all agreed that we should shoot for < 125 F to get good performance and longer life.

The research also says that our motors want to run as cool as possible, (I'm not sure about below freezing? ;-) to get MAX power, but our batteries want to run warm (90 to 110) for max power. This is why I was looking to maximize our motor cooling, not just to stay under the too hot limit, but to get closer to the MAX power lower temps. IMHO we should be working harder to get our motors cooler than we are running them now. There is some free power being missed, and as pilots often say: "you can never have too much power!" 

Alan mentioned this "cooler motor is better" fact, and I agree with him that a cooling fan forcing air through our motors is a good start. In R/C elec. we seldom run very long at full power, and we often have a power off downhill line to cool the motor back down after the full power up line. In CL we are full power for 6+ min., with NO cooling period.

In the future we may end up using the shaft that sticks out the back of our motors to drive a small ducted-fan unit that doubles as a cooling fan sucking air through the back of the motor AND providing some extra thrust. ...... So many projects, So little time!  ;-)

Alan, thank you for your informative post. I would like to make one suggestion re: the exhaust of the motor's hot air. We need to be careful directing this heated air over the battery, or the controller. If possible it should exit the plane through it's own exhaust system. The batt and controller, etc., should have their own "fresh" air and exhaust system. If I could post photos to this forum I could show you how I accomplished this on my elec. Vector. After more than 30 years as a commercial pilot, I have been fortuneate to have had the opportunity to see the many methods that have been used to move air in and around airplanes. The engineering used for intake and cooling systems, and jet fan systems, is amazing. I have always been impressed with the genius of engineers who thought through and solved the many problems in order to make air flow exactly where it was needed to get the job done. ............  We can learn from full scale A/C and hopefully be able to improve our much less sophisticated, but still challenging aircraft systems. ;-) 

I feel that, like props, cooling is an area where we can make some easy (almost cost free, and weight free) improvements to our electric CL systems.  Our systems are very good now, but there is still room for improvement. ....... I'm sure innovators like Mike, Dean, Bob, Will, etc. will continue to improve our systems and make our elec. CL planes even better than they are today. My hats off to them for their past and future help!   

Regards,

Hi Gang!
Oddly enough a fair number of the Pletty in-the-spinner motors have been cooked in Pattern. Oh well ...
Cooling is important, and while it is best to keep the motor under 150F, the cooler the better. The magnets work better at even extremely cold temps, and even the copper windings get less resistive! It is also a good (but not critical) idea not to poison the battery cooling air with warm motor air.

Spinner cooling is a tricky issue. If you just took a normal spinner and poked the front out of it, the air would tend to come IN the spinner at the back near the center of the lightened backplate, accellerate to the same rotational speed as the spinner itself, and then flung OUT through the holes around the prop blades. Add the big hole in front, and this inadvertant locked vortex-centrifugal air blower would be throwing the cooling air away before it gets to the motor.

Fans ... they use horsepower, and any horsepower I have should be used to fly the plane. We fly at a constant 55 MPH or so, so keeping a constant flow should not be an issue. Fans are a band-aid for crummy cooling path design. They have a place in helicopters and 3-D RC ships: they hover a lot.

I like the following ... but there is definitely more than one way to skin this cat! ... The spinner is a good heatsink that conducts heat out of the motor through the propshaft. Don't use a plastic spinner.

Put the motor in a simple duct with maybe 1/8" air gap all around, and duct the air from the chin scoop into the front of that duct. Yes, it's a tight turn! (Scale cheek inlets work well!!) Some air will cool the bell by passing outside the motor, and some will go through the motor and cool the windings directly. If you can't put a full duct in the cowl, put a vertical wall in the motor bay with a big hole that goes completely around the motor. The lower half will probably have to be part of the chin cowl. Now there is a front inlet air chamber and a rear outlet chamber. Force the air from the top 1/3 of the chin scoop into the front chamber and some of it will go through the motor and some will at least hug the outside of the bell as it goes through the ring-shaped gap in the vertical wall. Then dump that warm air out of the plane. As it turns out, this air isn't that warm, so if it joins the battery cooling air then  it will work. That's how Hunt's Genesis was set up. The careful ductwork allows my 2500W pattern motor to run only 20F above ambient!

Remember it is more important to get the hot air out than it is to get the cool air in. A rear facing scoop will suck the warm air out of the exit air chamber. It could be disguised to look cool, too.

While it is a styling challenge, using a spinner with a diameter that is substantially smaller than the nosering is another option. Making the gap small for appearance  gets sticky because of that same centrifugal outflow. The gap has to be large.

I apologize up front if I sound brusque or too authoritative: like I said there is more than one way to skin this cat, but there are schemes out there that may seem reasonable but have fundamental problems. ALL this airflow stuff is black magic

All the best,
Dean

Dean
that is excactly the direction I was thinking,ON my bf109 I have been looking at possibly building it electric, full fuse, there is a really cool cooler scoop under the cowling about midway between the prop and the leading edge, I have two thoughts, on is to use it as a flow through venturi to draw air from inside the cowling, using the stock exhaust stack locations as well as a couple prototypical openings to bring air in. I was contemplating using an open spinner that would be prototypically correct as the 109 had a blunt spinner for the through crank cannon. However after reading your Rant, I have reconsidered that. The other thought I had wa s to use that belly scoop to draw air in, and exhast it through the exhaust stack openings, but not sure there is enough area to do the job. If designed right you could create negative pressure at the end of the exhaust tubes to help accelerate the air out of the opening.

Hi Mark,

As Dean hinted, YES, we can make the air move forward. But it will take a little planning and maybe a few attempts to get it right.

A good example is the Pratt & Whitney PT6 Turbo Prop engine. They bring air in from a scoop in the front, direct it all the way to the back of the engine, then direct it through a series of compressor fans, hot box, more fans driving the prop shaft, then out the FRONT of the motor. They are helped greatly by the fact that the air is REALLY hot when exiting the engine. This hot, rapidly expanding gas/air helps "suck" the air forward through the system.

Our exit gas is not as hot (at least I hope not!;-) but it is hotter than the intake air, and it is expanding causing a rush to escape. This can help you in routing it through your ME109 lower intake, then forward into some form of intake ducting in front of the AXI motor, then through and around the motor, then out an exhaust exit.

As Dean and others have said in the past: getting the hot air out is CRITICAL. The rule of thumb has always been:  "the exhaust exit area should be 2 to 3 times the area of the intake." The other issue that can be a big help to us is using NASA shaped inlet and outlets for our cooling systems. We can also use a simple cowl flap and or a louver exhaust outlet. Anything that curves the air outward just before the exhaust opening will help cause a low pressure area outside the exhaust outlet (hole?). This will really help "suck" the heated air out of our motor.

We have to be careful to remember KIS. Moving air efficiently while bending it around many turns is not an easy task. Full size A/C have very smooth, and complex systems for moving air inside an engine, and or cowling. The PT6 system is a work of ART as well as science.

Cooling your system to keep it below safe limits (<130 F) will be relatively easy. Cooling it to ambient + 20 F like Dean does will take more work and an understanding of a pressure cowl system. Like Dean says, keep your tolerances tight. Don't let the air "wander" off where it wants to go (it ALWAYS takes the easiest escape route), you need to direct it to where YOU want it to go to keep your motor cool! ;-)

Now, cooling your system to the level my original post was shooting for, that will be a real challenge!!!!! :-) The motors are more efficient at cooler temps than we are running them. Like Dean said, we should be able to come up with some clever ideas. I agree with him.

IMHO, we need to think outside the box. ..... For example, because we are not confined to long run times like full size A/C, or even R/C A/C, we may be able to use something like a simple, small frozen swamp cooler device?

Imagine one of those squares we put in the freezer to use in our coolers to keep our soft drinks cold in the summer. Now imagine one smaller than a postage stamp that is put in a slot in your air ducting on the way to the motor. We would be putting refrigerated air into our motor. Because the frozen (glycol like solution enclosed in a plastic case?) piece is in ambient air, not in the hot exhaust air, it should stay cool for at least a six min flt. ? It would be simple to take them to the field in our coolers that already have large ones keeping our soft drinks cold.

This may help us for now, or at least until Dean and the Skunkworks crew give us "Berilium Spheres" to power our planes? ;-)

Or Imagine: You fill in the blanks     ....... ? :-)

BTW: Most of the Full scale Aerobatic planes, Extra 300L, Edge, etc. and all the Round cowl planes are excellent candidates for electric and will be easier to cool because of their cowl arrangements.

Mark, please keep us posted on your ME109 project. ...... It sounds like it will be a great CL plane! :-)

Thanks again for posting my planes photos on the Stooge thread in the open forum. 

Regards,