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Adequate airflow/cooling in cowl??

Old 03-10-2003, 06:31 PM
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NE0
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Default Adequate airflow/cooling in cowl??

Trying to get a better understanding of just how much airflow/cooling in a scale warbird cowl is "enough"...

I know that the rule of thumb is to have twice as much exit hole/area as you do for entry, but that doesn't answer the question as to how much "entry" is enough?

If your plane does not have any special "scoops" or "vents" for air entry, but has an 8 or 9 inch opening in the front of a radial cowl, it seems that you'd have plenty entry, but would be hard pressed to get 16-18" of air exit, so that's kind of confusing to me too.

I've heard some talk of building air "baffles" in your cowl to redirect the air over your cylinder, but never have seen any examples of how this is done.

If you have a dummy radial in your cowl, your 8" opening is significantly reduced... to the point that the openings between the dummy cylinders combine to make entry air much less. So much so that having a "doubled exit area" is much more possible. However, is that small amount of air passing through the dummy radial really enough? And, how do you then redirect it to your engine?

Does a gas engine need more air/cooling than glow or vice versa? I know I've had glow motors in 3D planes like an Extra that had very little entry air with no overheating problems. Nothing more than 2 small "nostril" holes and the cylinder head sticking out of the cowl maybe an 1/8 inch. (Plenty of exit area under the belly though)

If your gas cylinder head sticks out 1/2" or so, does that reduce the amount of your airflow/cooling requirements at all? Any advice or explanation of this concept would be appreciated.

Thanks,

Neo
Old 03-12-2003, 05:23 PM
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Default Adequate airflow/cooling in cowl??

Wow, no words of wisdom yet??? Surely someone has a take on this. ;-)

Neo
Old 03-12-2003, 06:40 PM
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G.F. Reid
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Default Adequate airflow/cooling in cowl??

I'll take a stab at it. The rule of thumb you mentioned is correct that you need at least a 2 to 1 exit to entry ratio. 3 to 1 is better if you can get it. As to the radial engined subject matter, you are also correct in the conclusion that you can't keep that ratio if you've got an 8 or 9 iinch opening.
The solution is that you have to close off the front off enough to acheive the ratio. It's best to close off all but the area directly in front of the cylinder. Now you've reduced the size of the inlet area and so have made the ratio more attainable. You've also forced all the incoming air to pass directly over the engine's cooling fins and induced a negative pressure in the cowl with the larger opening for the expanding hotter air to go out. Now you have a constant source of cool air passing over the engine.
If you can get ahold of a copy, there was a fantastic article by Pat McCurry in RC Report a few months ago explaining the proper methods of cooling a cowled engine
Old 12-19-2003, 11:09 AM
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Default RE: Adequate airflow/cooling in cowl??

I think I've got it worked out now. Thanks!

Tom

[link]http://www.renderwurx.com/rc/SBD/html/building/dummy.htm[/link]




Old 12-19-2003, 12:13 PM
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Default RE: Adequate airflow/cooling in cowl??

The issue is flow more than any thing else. One way of dealing with this is to block the entire air path except an area just left and right of your engine's top 30%. Create a smooth exit pathway out via cutting free cowl flaps and articulating them to move 180 degrees with throttle setting. High throttle would require less opening.,..etc. There have been recent postings showing articulated cowl flaps with photos. Keeping it simple is the difficult part This should really not be an issue on your SBD...

The full scale SBD has a very large airway gap between the firewall and the cowl which has a very smooth curve where the firewall meets the side of the fuselage.

The cowls on most SBD are never mounted at right angles to the thrust line to permit greater airflow.


This image shows just how much of a gap exists on the SBD. Its HUGE!


It would be really neat to see your cowl flaps open and close with throttle!
Old 12-19-2003, 02:34 PM
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Default RE: Adequate airflow/cooling in cowl??

Good points Ed! Yeah, I may have even added a tiny bit more gap (than scale) on mine. When you scale that gap down, it's not nearly as big. I wanted PLENTY of airflow and didn't want the cowl rattling against the firewall ring from the gas vibration.

Also, inverted gas engines mean your cylinder is at the bottom, so it's hard to really gain full benefit from the cowl flaps of the SBD. Since my cowl is cutout for the cylinder head on the bottom, I'm trying to force my air through the lower third of the cowl, across the cylinder and out the bottom. Of course, some will still exit around the ring, the cowl flaps and even the 4 cutouts in the firewall that the Bates design utilizes.

Tom

Old 12-20-2003, 02:13 AM
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Default RE: Adequate airflow/cooling in cowl??

Tom,
Here is an example of how I baffled the intake on this Fw190. The whole front of the cowl is blocked off except where the cilynder head is. Ratio of outlet is about 2.5 to 1 with the scale side outlets and one below.

JG
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Old 12-20-2003, 06:09 AM
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Default RE: Adequate airflow/cooling in cowl??

The ratio is the important thing not necessarily the volume. As long as your creating a low pressure within the cowling the hot air moves out. Regards G.

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