Upper cowl deformation inflight

[Johnnybgoode]

Looking for ideas on how to stiffen a couple areas on my upper cowling. Inflight, at higher speeds, I see a couple of depressions aft of the rear baffle line. One is right on the top and one is just over the oil door.
1. Anyone else notice these?
2. What have you done about it?
I'm thinking there should not be a low pressure area back there unless air is getting past my baffles. I do have one segment that often turns back, so I think this is the problem but I'm not sure whether the not-so-rigid upper cowl is the issue or whether longer baffling is needed.
I'm considering stiffening the cowl with either strips of AL angle fluted to curvature or laying up some fiberglass inside the cowl. AL strips obviously require riveting, which I really don't want to do to my cowl but I'm seeing signs of cracking just aft of the oil fill door so need to find some way to alleviate the flex back there.
Thanks for reading,
Patrick

[Kyle Boatright]

A friend's RV-4 had the same issue. The geometry of the air exit on the -4 must be amazing, because they cool very well and (as you noticed) suck the cowling in, which is incredible.

His solution was to lay a couple of pieces of rope on the inside of the upper cowl and lay fiberglass over the rope to make stiffeners. I'm sure there are a hundred other solutions, but I'd choose a fiberglass solution to avoid knocking holes in the cowl...

[David Paule]

A bit of carbon would increase stiffness much more than fiberglass, for the same thickness. And if you put some inside and some outside, making a sandwich, it would be very stiff.

Its density is low so it wouldn't add much weight, either.

Dave

[Tony Spicer]

Yep on the carbon fiber:

https://picasaweb.google.com/tonyboy...COTm7Iul0szAew

Tony

[BillL]

Note the difference in stiffness. The flexible one has 5 layers of glass, the stiff one uses the same glass with a 2mm core w/two layers on each side. The big difference is the thickness, that is what you need.

This sample uses Soric, specifically made for this, but you could use some 2-3mm thick balsa wood. I used Soric because of its ability to match a contour.

Don't waste your time and money on just adding layers of glass or carbon, use some core material.

Edit:Apologies for all the junk on the bench, the picts were not for a beauty shot.
Edit 2: There was no difference of weight between the two samples. Added Soric and one less glass layer balanced out, even though the cored sample had more resin to eliminate the weave texture on the shiny side. Two layers of 8.9 oz glass would probably be enough as strength is not the issue.

[glenn654]

That is a common problem with the -4, I believe.

I made stiffening ribs for the top cowl and oil door using 1/4" foam covered with four layers of fiberglass that was extended about 2" around all the edges to give added strength.

On the oil door I used two short ribs but probably didn't use but two layers of glass.

At the rear corners of the top cowl which will flex out under pressure, I made two alum strips that I attached to the inner surface of the cowl extension.

These were placed so that when the cowl pin between the top and bottom halves was installed the end was captured and pinned the rear corners in place.

Hope you can visualize what I mean. These mods fixed the problem for me.

Glenn Wilkinson

[fixnflyguy]

I have the newer style cowl with the honeycomb reinforcement. They do not flex. If you have the old style, polyester solid type, I would do a honeycomb or foam and glass layer inside to stiffen it.

[wjb]

Stiffness goes up with the 3rd power of thickness for the same material ... a lightweight core plus some thin top and bottom caps is just as effective as the solid equivalent at a much lighter weight. Think "I-beam" They are shaped that way for a reason!

[DanH]

Originally Posted by Kyle Boatright

A friend's RV-4 had the same issue. The geometry of the air exit on the -4 must be amazing, because they cool very well and (as you noticed) suck the cowling in, which is incredible.

That would indeed be...

[Infidel]

Originally Posted by DanH

That would indeed be...

I guess that's why Marvel Comics named him the Incredible Hulk instead of the "amazing hulk."