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Skybolt Pillowing

SeanB

Well Known Member
Thinking ahead.....I plan to install Skybolts in my engine cowl.. I believe their directions describe adding additional plies of fiberglass (maybe carbon?) just forward of the fastener line to alleviate pillowing. Anyone do this and did it help? Any details on placement, width, number of plies? Pics?

Thank you!!!
 
Yes, I did this...I added several layers of glass, about 2" in width or so, along the entire edge, top and bottom, right up to the edge. I had planned ahead for this, so I knew it would mean adding a shim between the receptacle mounting surface and the firewall/skin flange. I added a .050" shim, and then built up the fiberglass until the cowling matched the skins perfectly.

It greatly reduced the pillowing from what I've seen via pics, but even then it did not *entirely* eliminate it. However, it's very, very minor and essentially unnoticeable unless you're looking for it. I can get a pic next flight and post it if you'd like.
 
Thinking ahead.....I plan to install Skybolts in my engine cowl.. I believe their directions describe adding additional plies of fiberglass (maybe carbon?) just forward of the fastener line to alleviate pillowing. Anyone do this and did it help? Any details on placement, width, number of plies? Pics?

Thank you!!!

Been a while, but I think I added five plies of E-glass tape, with attention to shimming as previously noted. Here are the plies with peel ply over the top to fair the tape edges:

Cowl%20Shim.jpg


I have higher internal cowl pressures than most, and it doesn't pillow too badly. I think the next one would get 1 carbon, 2 glass, 1 carbon, 1 glass.

Cowl%20Edge%20Lifting.jpg


Oil door got 10 plies of 9 oz, and it still bulges a little when I close the cowl exit. Definitely a call for carbon.

Oil%20Door%20Bulging%20800w.jpg
 
....I think the next one would get 1 carbon, 2 glass, 1 carbon, 1 glass.....

Note that Dan is describing putting the carbon on the outside plies, although one is covered by glass as a corrosion barrier to the airframe. The reason for using it on the outside plies is to have the stiffer material be akin to the caps on an I-beam. The further out from the neutral axis, the more effective it is for stiffness.

At least I think that's why he would choose that approach. He might have other reasons too.

Dave
 
Thanks for the replies everyone.

Dan, your results look good. I may follow your thought and install alternating strips of fiberglass tape and carbon. It appears you installed strips where the fastener line lies, correct? That is discussed in Skybolt's installation instructions, and then they say, "We highly recommend bonding a support strip adjacent to the fastener line around the firewall." So...not sure which way to go. In line with fasteners, or adjacent?

Any suggestions on the best weight of fiberglass tape and carbon tape? Install carbon just as fiberglass? Do this technique only at the firewall line of fasteners, or also on the longitudinal ones?

BTW....this is a stock Van's Cowling (pink).

Thanks for all the help!
 
At least I think that's why he would choose that approach. He might have other reasons too. Dave

Nope, that's the whole list.

Dan, your results look good. I may follow your thought and install alternating strips of fiberglass tape and carbon.

Not quite "alternating". As Dave noted, what I described was a carbon sandwich with two plies of fiberglass in the middle.

Have some fun; do a little experiment. Buy some carbon and e-glass tape. Do three layups over plastic sheet on the bench top, each 6" long. The first is 4 plies of glass. The second is carbon, glass, glass, carbon. The third is glass, carbon, carbon, glass. When cured, bend them to get an idea of relative stiffness. Tell us what you observe.

It appears you installed strips where the fastener line lies, correct? That is discussed in Skybolt's installation instructions, and then they say, "We highly recommend bonding a support strip adjacent to the fastener line around the firewall." So...not sure which way to go. In line with fasteners, or adjacent?

Inline if you're willing to shim the socket strip to accommodate the new cowl thickness. Adjacent if you're not. Inline and a shimmed strip is superior.

Any suggestions on the best weight of fiberglass tape and carbon tape?

Straight from the TLAR School of Design...Fiberglast has 2" wide carbon tape at 0.012" thick. Ordinary e-glass tape is probably around 0.010".

Do this technique only at the firewall line of fasteners, or also on the longitudinal ones?

Very few folks have observed the longitudinal seam in flight. And a lot depends on how much internal pressure you plan to generate. The carbon is a total waste of money if you're not going to pump it up. Use glass plies.
 
Very few folks have observed the longitudinal seam in flight. And a lot depends on how much internal pressure you plan to generate.

Also, there is no "resistance" trying to hold it in place. I would guess very little deformation there. Just shear forces on the fasteners.

I am jealous of lower cowl pressure.
 
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Options for stiffness

I did not realize the Skybolts pillowed with good lower cowl pressure. I am using a plenum so never gave this issue any thought. So now it is too late to add more glass/carbon as described.
However, one option for significant stiffness is to add material at 90 degrees to the suface that flexes. This is easily done adjacent to riveted receptacles on the cowl horizontal seam as well as firewall and oil door.
I have molded carbon and glass ? angle iron? onto aluminum angle covered with packing tape and it makes an impressive structure. I used this material to fab a custom canopy frame in conjuction with Lasta-foam.
 
Cowl pressure can add up. Stock cowls with typical baffles will ballpark around 5" H2O at 200 KTAS in a basic VNE descent from cruise. That's about 26 lbs per square foot. Close the exit and it can reach double that figure. We have many square feet.

The cowl is roughly rectangular at the firewall. Internal pressure attempts to blow it into a round profile:

Cowl%20Pressure%20Shape.jpg


The worst pillowing will be located at A and C. Even cowls attached with hinges can pillow at A, between the ends of the two upper hinge sections. A rib added across the upper span (as Larry described, here in red) can help by transferring load to the ends, just like a bridge. However, the thin cowl edge will still deform to some degree. I'd add the rib with a hinge cowl, and thicken the flange with Skybolts.

Given time, the pressure cycles break hinges at B. With enough pressure, even flanges riveted into the firewall seam for Skybolt sockets or nutplates will develop cracking or loose rivets. With higher than average pressures, I needed bracing for the Skybolt flange at B.

Brace.jpg
 
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Correct?

Wow! Great information everyone. Thanks for helping me learn.

Not even close to a good Dan Horton Drawing....but is this about correct?

 
pillow

I placed my Skybolts at 2 7/8 inch center to center . No pillowing . Did not use the skybolt brackets, used a continuous strip with holes between the retainers. When you put the cowl on you can't get the edge under under the socket on the other side and scratch your paint. I think the brackets give you about a 3 5/8 inch spacing.If they are still there, some pics of our plane can be found in the Skybolt instructions for RV or experimental.
 
Just a reminder...given moderate cowl pressure, the carbon adds little. And it brings a potential hardware corrosion problem.
 
I placed my Skybolts at 2 7/8 inch center to center . No pillowing . Did not use the skybolt brackets, used a continuous strip with holes between the retainers. When you put the cowl on you can't get the edge under under the socket on the other side and scratch your paint. I think the brackets give you about a 3 5/8 inch spacing.If they are still there, some pics of our plane can be found in the Skybolt instructions for RV or experimental.

I'm doing the same, with ~3" spacing center/center and am using a straight mounting strip. I'm hoping for similar performance!
 
As for the oil door. I made mine from 0.063 aluminum. It was actually pretty easy. Probably easier than adding a bunch of glass and foam or whatever else to the stock glass door.
 
door

Another vote for .063 al. door. I used the hidden hinge modified and four skybolts one in each corner. Fits very tight. In 500 + hours cowl has no wear points, cracks, or signs of stress fatigue areas.Most cowls that I had dealt with that did not have enough fasteners looked a bit rough when they got very many hours on them. More work and cost but appears to be worth it in the long run.
 
I used the same pictures from Don when doing my lay out!!

I did use the .063 oil door, but the SJ cowl did not curve like expected, so getting it to lay flat around the perimeter needed a few minutes on an english wheel - The EAA chapter has one, luckily.

Now I have to shrink my exit and generate some pressure. (and SPEED);)
 
Unidirectional carbon fiber would be the ideal choice for this application.

https://compositeenvisions.com/carb...50k-50-127cm-11-8oz-400gsm-zoltek-px35-fiber/

A good friend and neighbor is building a very unique homebuilt aircraft that uses carbon fiber and aluminum for its primary structure. In consultation with a PhD chemist and by testing various coupons left outdoors, he has proven he has no concerns with galvanic corrosion.
 
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