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Author Topic: Cowl clearance around engine  (Read 1155 times)

Offline kevin king

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Cowl clearance around engine
« on: March 28, 2022, 08:50:38 AM »
Was just wondering what the minimum safe cowl clearance around the muffler and head of the engine should be? Its for a vector 40 with a wood cowl to be fiberglassed and sprayed in 2 part automotive clear. Engine is an OS LA 46 with either a stock or tongue muffler.

Offline Dan McEntee

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Re: Cowl clearance around engine
« Reply #1 on: March 28, 2022, 11:12:55 AM »
  I would make it about 1/8" around the cylinder and head and a bit more around the exhaust stack exit. This is just to give room taking the cowling n and off so it doesn't get beat up. It's close enough to look good but room enough for some 'wiggle " room when needed. Just make sure the air exhaust opening befind the cylinder is about 2 times the size if the air inlet in front for proper cooling.
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Offline kevin king

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Re: Cowl clearance around engine
« Reply #2 on: March 28, 2022, 02:07:27 PM »
👍 Thanks Gentlemen.

Offline john e. holliday

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Re: Cowl clearance around engine
« Reply #3 on: March 28, 2022, 06:44:56 PM »
Just make sure it doesn't touch any part of the power system.  The engine will get hot. D>K
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Offline Ken Culbertson

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Re: Cowl clearance around engine
« Reply #4 on: March 28, 2022, 07:00:26 PM »
I know it is an IC question, but I have an electric profile where the clearance for the spinning can on the cheek cowl side is only 1/16".  The other side is completely open.  Is this a problem?

Ken
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Offline Dan McEntee

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Re: Cowl clearance around engine
« Reply #5 on: March 28, 2022, 07:27:41 PM »
I know it is an IC question, but I have an electric profile where the clearance for the spinning can on the cheek cowl side is only 1/16".  The other side is completely open.  Is this a problem?

Ken

   Is it an operational airplane? If so, check for signs of rubbing. It's not hard to imagine enough flex in the mount and everything related to it flexing in hard corners and such. I don't know for sure that they do but that would be my first concern when thinking about it.
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Offline Ken Culbertson

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Re: Cowl clearance around engine
« Reply #6 on: March 28, 2022, 08:08:03 PM »
   Is it an operational airplane? If so, check for signs of rubbing. It's not hard to imagine enough flex in the mount and everything related to it flexing in hard corners and such. I don't know for sure that they do but that would be my first concern when thinking about it.
    Type at you later,
    Dan McEntee
No rubbing, check that often.  My concern was uneven cooling.  I still don't really understand how these outriders cool themselves. 

Ken
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Offline Dave Hull

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Re: Cowl clearance around engine
« Reply #7 on: March 29, 2022, 01:46:14 AM »
The closer the fit of the cowl to the fins (assuming the engine design is finned correctly) the more efficiently you can cool it. But efficiency in a stunt plane isn't super important since extra cooling drag is overcome by our typically huge power margin. For contrast, compare to an F2C model running a diesel engine. The clearance is set around two or three thicknesses of masking tape when you finish the fuselage. And the opening is designed to a minimum so that you never have excess air (excess drag) being force thru the cowling.

Make sure that you get rid of sharp edges in the cowl where you want the air to flow around something. Deadheaded flow is not good. The way to think of this is that aerodynamics are in play inside the cowling as well. Cooling the back side of the cylinder should be a priority, not just hammering the front and leaving dead air in the back.

As far as the outrunner motors, putting half the outer housing in direct airflow as the housing rotates will give you uniform circumferential cooling of the magnets since the heat flow is much, much slower than the rotation rate. (ie. you won't be able to measure a temp rise in the magnet while it is in the "shadow" of the profile fuselage.) Something that I have done on profile race planes is to core a circular duct thru the fuse into the recess where the bottom end of the engine sits in otherwise dead air. Just use a sharpened 1/4" brass tube and put it in at a shallow angle to the fuselage axis. No reason you can't do something like that on electrics, too.

And, since the magnet array is also exposed to the internal airflow, that is a primary cooling mechanism, too. The real heat being generated is in the stator windings. The cooling for that would seem to be almost entirely via airflow in thru the front of the outer can. Again, dead air is a killer, so having a smooth path behind the motor for the air to exit is important.

For motors, they usually are spec'd to survive a certain max winding temperature. It is a situation where time-at-temperature determines when the winding insulation will break down and things short out. The modern "magnet wire" typically uses some kind of epoxy "varnish," which is way, way better than the old kinds of varnish insulation. Still, it breaks down over time. The other heat issue in the motor is the demag due to high temps. The strongest magnets often have a lower Curie point, meaning if you overheat a motor beyond the working temperature of a neodymium magnet, it "recovers" to a much lower energy product rating. Samarium cobalt has a significantly higher temperature rating, but most will have a lower energy product value.

From a theory standpoint, for convective heat transfer the variables are essentially the delta-T between the hot surface of your component and the incoming air, the heat transfer coefficient, the area of the hot component exposed to cooling flow, and the air mass flow rate. Improve any of these and you improve cooling. So having a huge clearance inside the cowling where the air can most easily flow thru the center of this gap--and very little of it thru the fins, think boundary layer here--is really inefficient. We get away with it in many cases because we have inlet and exhaust apertures that are far larger than needed. And if it is working, no one really complains on a model. But aerospace companies spend huge bucks to make their designs efficient in this regard.

Dave

Offline Ken Culbertson

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Re: Cowl clearance around engine
« Reply #8 on: March 29, 2022, 07:47:17 AM »
... Something that I have done on profile race planes is to core a circular duct thru the fuse into the recess where the bottom end of the engine sits in otherwise dead air. Just use a sharpened 1/4" brass tube and put it in at a shallow angle to the fuselage axis. No reason you can't do something like that on electrics, too.

And, since the magnet array is also exposed to the internal airflow, that is a primary cooling mechanism, too. The real heat being generated is in the stator windings. The cooling for that would seem to be almost entirely via airflow in thru the front of the outer can. Again, dead air is a killer, so having a smooth path behind the motor for the air to exit is important.


Dave, this is most helpful.  I have read that the modern outrunners are designed to draw air in through the motor for cooling the stator and that is why we don't use fans anymore.  Since we mount them facing both directions, which way does the air go?   I have used the cooling hole on all of my IC angling it to the rear to create suction in flight but mainly as a vent to keep the crankcase cooler after the flight.  Is the same logic true for electric - a vent for after flight cooling?  I have been underwhelmed by the effectiveness of those spinners with cutouts.  They don't seem to create much airflow and if what I read about airflow around the stator they may actually hurt.  The major source of cooling air seems to be the spinner gap.  Brave new world, in which, common knowledge isn't so common yet!

Ken
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Offline Dave Hull

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Re: Cowl clearance around engine
« Reply #9 on: March 29, 2022, 07:30:40 PM »
Kenno,

I'm not an electric guy, so I don't have the details of a "best practices" installation in hand. Still, a lot of it is going to be similar enough to IC if you understand those principles--not rules of thumb, principles--and apply or extrapolate them.

I've seen the "vented" spinners and wondered how well that works. Are there any features in these spinners that promote flow, or do they just have holes in them? Seems that you get a slight positive pressure due to the inlet location, but with all the thrashing going on, I wonder how you get any organized flow thru it. Might be an interesting experiment to remove the prop, then run the motor/fuse/spinner in front of a fan and apply smoke to see if you are getting any significant flow thru that pathway. I'd start the test off at very low rpm not only to validate the test setup, but because if there are no "fan-like" features in the spinner, there might be better flow at slow speeds than fast?

If the spinner diameter was small enough for the air to flow directly into the holes in the rotor housing I can see that working. But expecting air to get sucked into a transverse gap between a normal spinner and the fuse nose ring seems really limiting. This is almost like a boundary layer control feature, and that always requires input work, to establish suction. And in an electric, where is that coming from? An internal fan?

I really was surprised by your comment that the motors internally have features to actively promote flow. Did I understand that correctly? If it has internal ribs or a fan blade, then it will move air from the front of the installation to the back, regardless of whether you turn the motor around. You just have to make sure that your direction of rotation (ie. lefthand) stays the same.  Go flip a prop around and convince yourself of this....   Conversely, if these motors do have internal ribbing or ??? to promote axial flow, then they are set up for one direction of rotation only, else it would be fighting any ram air and local flow ....

I attached another way of eliminating dead air in a tight cowling, or a pocket in a profile. This one is tightly fitted to a K&B .40 run as hard as it'll go. So a little case cooling was thought to help. The air "inlet" was a 1/16" gap between the nose ring and the engine drive hub. The exhaust is the cored duct leading out of the fuse. Best if the exhaust is in a low pressure region of flow. This sounds exactly like what you described in your last post.

Dave

Offline Ken Culbertson

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Re: Cowl clearance around engine
« Reply #10 on: March 29, 2022, 08:14:47 PM »
It does get confusing.  I read about the internal flow while researching fans and why nobody sells them anymore.  Mostly on the RC sites.  From what I came away with was that the spinner gap was where most of the cooling occurred, and these slotted spinners actually hurt the flow.  BS?  Maybe, maybe not.
One of these days I am going to mount a motor in the open, take off the prop and get some smoke going like you suggested.  With Electric I am pretty sure that we are not "cooling" much of anything.  What we *are* or should be doing is carrying off as much of the heat generated as we can.  My approach is to have a tunnel from the nose to as far back past the electronics as I can and make it so that there is suction from the airflow over the outside of the fuselage pulling out the air trapped in the fuselage.  With IC you are generating heat from combustion.  Heat is what is driving the engine.  With Electric heat is the byproduct and it is considerably more controllable by proper balancing of components.   Hopefully someone that actually knows how these motors cool themselves will chime in. 

Ken
AMA 15382
If it is not broke you are not trying hard enough.
USAF 1968-1974 TAC


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