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Engine cooling with cowl aircraft

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Old 04-20-2010, 11:48 AM
  #1  
wjcalhoun
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Default Engine cooling with cowl aircraft

Hi guys
I have a Wild Hare baby extra; OS AX 1.20 up front with 16x8 APC. The plane flies great, but I have a problem with cooling, and don't know if I need more inlet air, or more outlet area, or outlets in a different place. After I land, the engine smokes very lightly for about 15 seconds. It is not the cowl, not tubing, and not debris that is smoking. Obviously something is too hot. Never had this problem with my other cowled aircraft.

The mixtures seem about right - I am 3-4 clicks rich of peak (150-200 rpm down from peak), and the pinch test still gives me a burst of speed, so I think the hi speed needle is ok. It idles well, and takes throttle without stumbling or cut out, so I think the lo speed needle is about right also.

I know the rule-of-thumb ratio of 3:1 exit area to inlet area. The intakes are about 3 square inches surrounding the spinner, but part of that area is blocked by the spinner. I have about 17 square inches of outlet area, along the bottom of the cowl, so I am better than 5:1.

The OS AX 1.20 muffler can be mounted transversely, so it would almost fit into the cowl, but I opened the cowl forward of the muffler, to about 1 inch forward of the cowl ring. It is open on the bottom about 4.5 x 4", so there is a lot of exit area.

So... the question is, should I cut a few slots in the top and front of the cowl for more cooling air inlet over the head, or do I somehow need more outlet area. Is there any reliable method to tell which is the problem?

Thanks in advance




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Old 04-20-2010, 11:58 AM
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Default RE: Engine cooling with cowl aircraft

What fuel and what plug are you using? Also, where is your tank?

I have my 120AX under a Yak cowl as well; didn't pay much attention to the "3:1" ratio, but I do have a hole the size of the engine head cut out over the engine so most of the air should escape there.

I had problems with over heating on another engine - the problem was that the engine was leaning out in flight due to problems with the fuel system and the engine not pulling enough fuel through the lines. It checked out fine on the ground and was difficult to diagnose because of that.

I use 10% nitro most of the time, and a #8 plug, sometimes a cooler plug on hot days. It makes a difference.
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Old 04-20-2010, 11:59 AM
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Default RE: Engine cooling with cowl aircraft

Can you post a pic of the engine mounted inside the cowl?
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Old 04-20-2010, 12:11 PM
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Default RE: Engine cooling with cowl aircraft

wjcalhoun,

I had that same setup a couple years ago. Never had an issue with over heating. I was running an APC 17X6 prop and Byrons 10% fuel with 20% oil. I think I have some pics at home of the plane and setup. I'll take a look and post some pics if I still have them.
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Old 04-20-2010, 12:21 PM
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Default RE: Engine cooling with cowl aircraft

3 to 4 clicks off of peak seems a little lean to me......I usually go 1/4 turn rich off of peak as a minimum......

How's your glow plugs holding up? One of the signs of a lean mixture is going through plugs....

You have an Extra...are both cooling holes open on both sides of the spinner? If so close the side away from the head......

You may have to build a baffle to port cooling air to the head.....Lastly take a good look at how cooling air would escape the cowl....and where the muffler is as heat rises.....

Let us know what you find
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Old 04-20-2010, 12:23 PM
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Default RE: Engine cooling with cowl aircraft

You can try closing the front opening in the cowl opposite the engine head. This lets less air in, so less has to go out, but it still lets in the air that's cooling the head. This is a fairly common proceedure
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Old 04-20-2010, 01:25 PM
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Default RE: Engine cooling with cowl aircraft

Thanks for the suggestions, gentlemen.

Fuel - 10% NM; 18% oil (syn:castor blend); plug is an OS #8; original glow plug with probably 4-5 running hours on it. I have not had to change out the glow plug. The fuel tank is about 6" aft of the firewall. I have actually held the plane vertically (worried about just what gaRCfield implies) to be sure that it was not leaning out too much in flight.

I tried to keep the cowl clean externally, so only have a 1/2" hole over the cylinder head for glow driver access. I could open the cowl over the head more, but was trying to avoid that if I could.

I put in a small baffle plate below the starboard air inlet to keep cooling air flowing up over the cylinder fins; prior to that, air COULD have moved directly from the starboard inlet, below the crankcase, and out the vent. A small baffle now forces the air up over the cylinder. Several have suggested that I close off the port inlet (away from the cylinder), and that is an easy mod I can make. Might try opening the HS needle a bit more also, but I have found that this engine/prop combination slows quite a bit if the mixture is too rich.

I have the muffler mounted using the 90* adapter, so it sits transversely, partially in the open airstream, and at the front of the vent opening on the bottom of the cowl (I know, a pic would be way better). The muffler is essentially directly below the crankcase.

I'll take some pics tonight and post them later.

I appreciate the advice. Thanks again.
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Old 04-20-2010, 01:47 PM
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Default RE: Engine cooling with cowl aircraft

I am running 15% and a OS #8 plug. I do have the muffler converted to a pitts style mount. With a 17x6 prop I have no trouble, and I only have one half of the cowl open like Minn said. Like others said I would look at the fuel system. Sounds like your are getting lean. Have you checked the clunk and filter?

David
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Old 04-20-2010, 02:45 PM
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Default RE: Engine cooling with cowl aircraft

Thanks David
I had not, but will, check the fuel clunk and filters
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Old 04-20-2010, 09:56 PM
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Default RE: Engine cooling with cowl aircraft

Here are four pics of the cowl setup; bottom view showing the large exit area and transverse muffler; view in starboard inlet - a small baffle is visible which is set angled up at 45* from the cowl interior; view in port inlet shoing bottom of crankcase; side view of cowl; small hole is the glow driver port

If any of you see something I should consider fixing, I would be happy to hear about it.

Thanks again


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Old 04-20-2010, 10:37 PM
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Default RE: Engine cooling with cowl aircraft

Well, there are some suggestions above. You can try them. I personally have no experience with an engine without a large area around the cylinder head exposed for the escape of cooling air. I know it's a difficult decision to start cutting away a cowl, and hopefully some of the other suggestions will work. I personally believe that you need to open up the cowl over the cylinder head, but I would wait to hear what others had to say before you begin cutting. It looks to me like you will just get a big zone of stagnant air (which in itself creates permits higher temperatures) in the side of the cowl where the motor head is.

It may be a combination of everything - nearly maxed out needle position, lack of exit for cooling air, and tank being a few inches from the firewall. I have all the same in my Yak, with the exception of increased ventilation, and while the engine doesn't get hot it does sound pretty lean once I get airborne and start wringing it out.

Just my late night opinion
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Old 04-20-2010, 11:34 PM
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Default RE: Engine cooling with cowl aircraft

Build a baffle. As already suggested, block off the left side opening in the cowl to force the air over the cylinder. Block off everything inside the cowl so the air MUST travel over the cylinder head before it can exit the cowl. It's a bunch of work, but it ........ works.
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Old 04-21-2010, 12:49 AM
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Default RE: Engine cooling with cowl aircraft


ORIGINAL: Rcpilot

Build a baffle. As already suggested, block off the left side opening in the cowl to force the air over the cylinder. Block off everything inside the cowl so the air MUST travel over the cylinder head before it can exit the cowl. It's a bunch of work, but it ........ works.
Hi wjcalhoun,

Couldn't agree more with Rcpilot. If I were you I would block off the left hand side of the cowl first and see if that is sufficient. Alloy from soft drink cans makes easy to form and fasten baffles if you need to go down that road.

Good Luck,

Colin
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Old 04-21-2010, 08:21 AM
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Default RE: Engine cooling with cowl aircraft

Gentlemen
THanks again

As noted, I'll check and clean the fuel filter and clunk, richen up a little, and install a port side baffle. If that does not solve the problem, I'll consider opening up the cowl a bit more over the cylinder head if those things don't completely solve the problem. One last question - do you generally use an infrared temperature gauge to assess cylinder head temps on your birds?
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Old 04-21-2010, 09:38 AM
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Default RE: Engine cooling with cowl aircraft

I never have. After 20 years of this a touch of the head after a flight gives me a good idea if it's to hot. One other thing before you do all that work. Fly it without the cowl. If the problem is still there you will know where to look. It is possible you have a bad bearing or a muffler baffle problem. These things would be last on the list of causes, but they do happen.

david
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Old 04-21-2010, 10:03 AM
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Default RE: Engine cooling with cowl aircraft


ORIGINAL: daveopam

I never have. After 20 years of this a touch of the head after a flight gives me a good idea if it's to hot. One other thing before you do all that work. Fly it without the cowl. If the problem is still there you will know where to look. It is possible you have a bad bearing or a muffler baffle problem. These things would be last on the list of causes, but they do happen.

david
Thats a good suggestion.

I might be wrong, but I remember reading that your outlet area should be about 3 times the size of your inlet in order to have good airflow. Can anyone confirm this? Seems to make sense.
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Old 04-21-2010, 11:25 AM
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Default RE: Engine cooling with cowl aircraft


I might be wrong, but I remember reading that your outlet area should be about 3 times the size of your inlet in order to have good airflow. Can anyone confirm this? Seems to make sense.
There seems to be a consensus on that; the OP even states so in his first post.
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Old 04-21-2010, 12:31 PM
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Default RE: Engine cooling with cowl aircraft

The 3x exit air is just a general rule of thumb.

What you should be more concerned with is making sure the exit hole has a little lip or air dam to disrupt the air passing over the outside of the cowl.

By disturbing the air as it passes over the bottom part of the cowl where the air is SUPPOSED to exit - you can create a low pressure air dam to help SUCK the air out of the cowl. Air entering the cowl doesn't mean it's necessarily cooling the engine. Air can go into the cowl and become pressurized. It can become stagnant.

The first picture is a rough idea of how air can enter the cowl and become stagnant. The air slipping past the bottom of the cowl does nothing to help EXTRACT the hot air from inside the cowl.

The 2nd pic shows how a small lip or air dam can help create turbulence or low pressure right near the cowl exit. This helps pull air out of the cowl and improves flow of air through the cowl.

Air don't do ya any good if it ain't moving through the cowl.

So do you really need 3x the exit air? Maybe. But you'd be better off, regardless of the exit opening, if you put a small air dam on the cowl to create that turbulence and low pressure spot.


EDIT:
I'm not saying a cowl without an air dam doesn't work. You'll get SOME cooling, regardless.

I'm saying an air dam HELPS to extract the air. It helps keep air moving through the cowl. Cool air in. Hot air out.
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Old 04-21-2010, 12:54 PM
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Default RE: Engine cooling with cowl aircraft

haha, I was going to get into the fluid mechanics of the problem...

The air dam is a cool idea and theoretically should really help. It also doesn't have to be very big.

Air is not compressible at the velocities/conditions we are experiencing, so I don't believe the 3:1 ratio is scientifically founded. By increasing the outlet area you shouldn't get any more flow going through the inlet, and you can't draw out what hasn't gone in.
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Old 04-21-2010, 01:10 PM
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Default RE: Engine cooling with cowl aircraft

I use an air dam on my gasoline powered planes. Gassers run a lot hotter, so cooling is CRITICAL if you want your engine to survive more than a few gallons. It works. Check out this cowl on my 75cc Extra. The lip goes around 3 sides. I didn't make it - came from the factory like this. But it's not just for looks. It's a functional air dam to draw air out of the cowl. The louvers on the side are cut and functional as well.
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Old 04-21-2010, 04:55 PM
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Default RE: Engine cooling with cowl aircraft

Looking at his pictures. The fins on the muffler should do the job.

david
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Old 04-21-2010, 06:36 PM
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Default RE: Engine cooling with cowl aircraft

Gentlemen:

I appreciate very much your outstanding and very helpful suggestions. RCPilot - I like the air dam idea, and will incorporate that also. Looks like a small lip - perhaps 1/4 - 3/8 might be enough to increase outflow draft.

I am thoroughly embarrassed to say that I identified and corrected the problem today. Those of you who said my mixture was too lean were right on the money (in my specific case - the general suggestions are most helpful in a more general sense, and will inform how I build and configure planes in the future).

I found a tiny split in my pressure line between the muffler nipple and the tank; it was small, but clearly enough to release muffler pressure. Hence, the tank was not pressurized, and my engine had to pull fuel, and therefore experienced very lean conditions in flight. I found the split today at the field, replaced that small segment of line (between the muffler and my pressure line fuel filter), and immediately had to lean my mixture screw by 1 full turn! Obviously tank pressure enhances fuel flow, and lack thereof starves the engine.

With that sole change, the problem went away. I did fly the plane first without cowl, and engine temps seemed normal [spit test evaporated, but did not sizzle :-) ]. I replaced the cowl, and flew again, once again without any hint of overheating.

I will still install the air dam to enhance outflow, and will still block my port side air intake to encourage airflow over the cylinder fins. Just make sense to me to protect my investment in my engine. It does remind me of the basics - a hot engine is either lean or starved of cooling air (or maybe has a bad bearing).

Thanks to all for your very helpful suggestions! This is a great hobby.

Bill


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Old 04-21-2010, 07:41 PM
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Default RE: Engine cooling with cowl aircraft

Hi Bill,

Glad to hear the problem is solved. Isn't it amazing how often the simplest solution is the correct one, (OCCAM'S RAZOR).

Cheers,

Colin
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Old 04-21-2010, 08:19 PM
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Default RE: Engine cooling with cowl aircraft

wjcalhoun,

Here is the setup I ran with the OS 120AX and pitts style muffler. Hope you get it figured out. Nice flying plane with this combo.

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Old 04-21-2010, 08:53 PM
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Default RE: Engine cooling with cowl aircraft

Nice setup Zippi
Looks like I have it solved!
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