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New Cowl and Plenum

pitts12driver

Registered User
San Francisco, CA
As some of you know, we had a devil of time trying to get the cylinder head temperatures below 400 with our O-375 and the the standard nose bowl. We did our best with baffles and it looked tight, but we were never able to lean the engine at all.

So, we bit the bullet, and built a new plenum and nose bowl. Today was the first test flight (apologies for the mismatched primer) but it was successful. Temps were down across the board by 30-40 degrees depending on scenario. Normal operations had the cylinders uniformly in the mid to high 300s. Success! We are likely to still add some louvers on the lower cowl but this is a major step forward! It took 3 months, but it was worth it.

I've enclosed some pictures of the plenum, the old "stock Piper" nose bowl, and the newly fabricated pieces.

Adam & Doug
 

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Man that's beautiful. could you guys tell us how you made it, some of us want one
 
love to get more clarification on the "lip on the bottom". Not exactly sure what you mean - a picture would be enormously helpful. thank you!

For those who wondered how it was made: the plenum was foam carefully laid on the engine, and shaped. The nose bowl began with turned aluminum rings, a spare nose bowl, foam, and a lot of patience. The entire messy 3-month process can be viewed here:

https://drive.google.com/folderview?id=0B9oOX1NzeeLjTUJvMVlaaW1DRGs&usp=sharing
 
Have you taken any pressure measurements on the high (inlet)and low side (firewall)of the cowl?
 
love to get more clarification on the "lip on the bottom". Not exactly sure what you mean - a picture would be enormously helpful. thank you!
This is a picture of a cowl lip. The airflow passing under the lip swirls behind the lip generating a lower pressure area behind itself at the cowl opening. This pulls the hot air out from under the engine. These are sometimes controllable from the cockpit and are called cowl flaps.

IMG_39161.JPG
 

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How long does it take to pull one of the upper spark plugs for inspection?

IMG_3881.JPG
 
Its tricky, but you can actually pull the plugs through the air inlets
IMG_7183.JPG
 

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Very nice...especially nice and rewarding when the hard work fixes the problem. Did you make the nose bowl too?
 
The nose bowl started life as a stock unit. Not much left of it besides the mounting flange. We are going to add louvers on the bottom of the boot cowl and call it a day.

Love to take some pressure measurements @fobjob - suggestions on how to accomplish that?
 
Love to take some pressure measurements @fobjob - suggestions on how to accomplish that?

First, That's a particularly nice job. You clearly have mastered the building techniques.

You should be able to find techniques on measuring pressure at various points inside the plenum in back issues of Sport Aviation. If memory serves me, a fellow built a plenum for his Lancair and used piccolo tubes and a manometer to measure the pressure at multiple points. Although he didn't give step-by-step instructions, the photos that accompanied his article showed enough detail to noodle out the particulars.

When I looked through the photo album that you linked above a couple of questions came to mind. First, how did you select the inlet diameter? Second, I saw the photos showing the spacing options you considered. I think you used 34" as your final choice. How did you arrive at 34"?

Again, that's a great job.
 
The horizontal spacing was easy. We had noticed that there was a ton of turbulent flow with the original Cub nose bowl interacting with the giant MT spinner so we needed to be further outboard. We then lined up the openings to be as directly in-line with the cylinders as we could while not violating the external mold line of the nose bowl because we didn't want to add cheeks.

Sadly, the diameter choice was completely empirical. I wish I could tell you that there was a quantitative metric backed up by rigorous CFD analysis but it just isn't the case. We held up rings of 5, 6, and 8" to see how they looked visually. We were running so hot we wanted to get as much air through the engine as we could. We didn't think the drag penalty on a Super Cub destined for amphibious floats would make a real-world difference in cruise speed, so we just built it with the big ones. I'm happy with how it looks, and how it cools, so it appears to have been an acceptable - if not optimal - choice.
 
[Have you taken any pressure measurements on the high (inlet)and low side (firewall)of the cowl?]
Nice job getting the air into the cylinders and keeping it in contact with the fins, but the measure of efficiency is to measure the pressure drop across the engine. Getting 4 inches on the pressure side(at 100 smph indicated) is easy, but getting rid of the volume expanded hot air is the requirement. You can test this easily by measuring the pressure on the exhaust(suction) side. If it's down around zero with respect to static pressure, then you have succeeded. If it's greater than zero, then your job isn't done yet.

*sorry to be a parade precipitator....
 
Excellent job! Glad to see your hard work paid off. Thank you for the Google Drive pictures of the whole process as well.
 
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