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Help with James Cowl

Hi folks. I have an RV-7, IO-360-M1B engine, James cowl, Skybolt fasteners.

Getting the James Cowl to fit has been an excruciating process -- I speak from the experience of successfully installing a Van's cowling on a different engine, same airframe.

Anyway, finally got to it where it looked like it fit nicely, and took it flying. Holy chit, as soon as I get above 110 knots, the back edge of the top cowling pulls upwards, and causes a godawful vibration/noise that's just not going to work for me. The Skybolt fasteners are holding it down, obviously, but the fiberglass is flexing upwards in the 3.5" gaps between the fasteners. At 140 kts, the peaks are seriously .5" or more from their at-rest positions. At this point, the noise is painful through my Bose A20s.

I was able to verify this as the noise source by duct-taping the aft edge of the cowling to the aluminum airframe behind it. Blessed normal airplane noise.

Let's see, more details: I'm using Van's baffle system around the back and sides of the engine. I made fiberglass diffusers to join the baffles to the aluminum inlet rings. I'm using this system because the James plenum fit exactly as any fiberglass molded by a 4 year-old would. I'm using the James ram-air inlet.

Any thoughts are welcome before I scrap this troublesome system and send Van's more money.
 
Operating without the plenum lid means the upper cowl is subjected to a significant percentage of available dynamic pressure. With a plenum, it only sees the same pressure as the lower cowl.

For the no-plenum case, at 140 KTAS and 3000 feet, the dynamic pressure would be about 60 lbs per square foot. Bad systems convert about 60% to static pressure, good ones as much as 85%. Let's use 70%; the force lifting the upper cowl would thus be the area enclosed by the baffle walls x 60 x 0.70. If we assume roughly 30" x 30" for the baffle, the force is 263 pounds. Not all of it is resisted by the fasteners at the firewall, but it's probably safe to assume 25%, as it's one side of a box. So, 65 pounds at this low speed. In reality, the design point should be at least VNE plus 20% at 1000 feet, where dynamic pressure would be 190 lbs/sq ft rather than 60.

Point is, lots of folks underestimate the how much force is applied to a cowl. And there is a very large difference between plenum and no plenum.

You should be able to reinforce the cowl edge enough to mitigate the puckering without too much trouble. I assume it's not finish painted yet. Two plies of 3" wide uni carbon tape on the outside, and two more on the inside should do wonders for stiffness. If not enough, well, call it a cheap and easy experiment.

Note the placement using the existing fiberglass as a core to separate the inner and outer carbon. Four plies on one side would not be anywhere near as stiff.

https://www.aircraftspruce.com/cata...MIiqmhrM7_7AIVhYbACh3QtQbmEAQYASABEgLKH_D_BwE
 
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Operating without the plenum lid means the upper cowl is subjected to a significant percentage of available dynamic pressure. With a plenum, it only sees the same pressure as the lower cowl.[snip]

That is by far the best explanation for what's going on that I've received. Thank you for the time and effort it took to write this.

Brooks
 
I wanted the James cowl and Skybolts on my airplane, but I was worried about exactly what is happening to you - so I put the Jame plenum on my airplane to keep the pressure over the engine instead of on the cowl. That part worked great - but I'll tell you that if you thought the James cowl was bad, then don't even think about the James plenum. Do it from scratch.

My experience with the James cowl was great - fit right up and life was good. The plenum experience is pretty much unprintable here. Just say no.
 
Bill Lane Plenum

If I recall correctly, Bill Lane (username BillL) had the same frustration with the Sam James plenum - so he designed his own.

I was able to buy one from him and it fit beautifully and works well. I was able to install the plenum in about a day and a half.

Hopefully Bill still has the molds and might be willing to sell them to someone (James Aircraft?) to put the plenum into production. Downside is it was only designed for a parallel valve 360 engine, I believe.

 
I also was able to talk Bill into doing a plenum for my 0-320. Bill had to make a few mods and I reworked the round inlets but the main body fit perfect. I have seen some nice plenums and mine is one of them! I will find a picture and upload it when I get to my laptop with the pictures.
 
Here are a few pics of my plenum. PM me your email me if you have any questions. Have fun.
 

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I'm building a "7" and with James cowl and just received my extended hub Hartzell prop to match my I(O)360-M1B. James is under new ownership and talked to the new owner, really nice guy and pilot with lots of fiberglass production skills. I should have my cowl in ~Feb and engine April/May if all goes according to plan. I would love a plenum that fits as I understand the James version does not and reached out to Bill L via PM as I'm not looking forward to a scratch build.
 
I had lots of issues with the James cowl. Everything, cowl halves and plenum must have come out of the mold very green. Lost of fiberglass work on the cowl to get it to fit, I kept just the snorkels from the front of the plenum and threw the remainder in the trash, made my own plenum using piano hinge on the laterals, cam locks on the front and aft. I wish I had stayed with the stock Vans cowl. Having said that, I see an honest 190 mph trud at 8500' on 8 gph.
Lots of mods, so I don't know how much the James "Holy" Cowl reduced drag.
PS use a lot of care when building not to glue the cowl halves together!

DAR Gary
 
I wanted the James cowl and Skybolts on my airplane, but I was worried about exactly what is happening to you - so I put the Jame plenum on my airplane to keep the pressure over the engine instead of on the cowl. That part worked great - but I'll tell you that if you thought the James cowl was bad, then don't even think about the James plenum. Do it from scratch.

My experience with the James cowl was great - fit right up and life was good. The plenum experience is pretty much unprintable here. Just say no.

+1, I returned the Plenum and built own... cowl installed with piano hinges for me with custom plenum.
 
I installed a James plenum and cowl on my -7, while it was definitely not a bolt on job it did not seem to be harder than the rest of the plane. I would say I spent about half the time making the baffles. As far as I can tell it does a good job. I also have the wheel pants.
 
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