R

RV8R999

Well Known Member
I used Skybolt Fastners to secure my cowling entirely. I really like them for ease of cowl removal and how they look (personal preference).

What I do notice however, during flight, the upper cowl edge at the fwd fuse junction becomes highly scalloped due to the pressure inside the cowl. I will take a picture next time but for now I'll describe this as a very distinct wave between each of the fastners. Has anyone had any trouble with cowling cracks near the fastner locations due to this effect?

The other obvious issue is a bunch of air must be escaping through these "gaps" vice out the lower exit. Even the oil door raises up about 1/2" during flight. Other than the standard ignition cooling hoses, I don't believe air is escaping from above the cyl as the baffling is very tight and conforms very well to the top cowl, ot Plus my CHTs are low 300's.

I imagine those who used the hinge method do not see this "scalloping" as the hinge provides continuous support.

Mostly I'm concerned with long term integrity of the upper cowl at those fastner locations. I used the Skybolt standard spacing per instructions.

thanks,

Ken
 
Ken, I have the same setup as you and am seeing the same exact thing on my cowling. I can't remember exactly what the spacing is, but I am thinking it was no more than 2 inches. I do remember that I am less then what Skybolt recommends.
 
Yes, I'm less than 3" between fastners at the top where the greatest scalloping is. I think Skybolt recommended 3-4".
 
My 7a does the same thing

and I have skybolts around the firewall. About 2.5" spacing I think. I was planning to add a fiberglass "rib" to the underside of the cowl just forward of the Skybolt flange. I may still do so, but you got me thinking...

The area we are talking about is not in the high pressure section of the cowl. That is forward of the aft baffle and above the cylinders. I'm wondering if this a symptom of too little exit area. Obviously the pressure below and aft of the engine has to be higher than the free stream pressure outside the outlet(s), but the cowl deformation makes me think maybe it's too high.

On the other hand, my Mooneys did the same thing in the same areas. Just not quite as bad...
 
Same thing on my -4

I recently switched from hinges to Skybolt fasteners along the upper portion of the firewall and cowl. I have the same puckering described by many. Never had it with the hinges. I plan to add a fiberglass stiffener of some design. I am just about to buy a vacuum bagging set-up for another project and will likely need it for this as well. (I would like to cure the stiffener under vacuum while the upper cowl is in place).

Regards,
 
My airplane has screws across the top cowl and doesn't exhibit any puckering. When I was installing the cowl, I added 2-3 layers of carbon fiber (in 1" strips) along the edges of the cowl, which really stiffened things up.
 
I have fixed a few 6 and 7 cowlings with the this problem. What worked for me was adding two layers of glass to the underside of the cowling at the rear edge about 1-2 inches wide. Spacing of the camlocks were 2 1/2 inches apart. Larry
 
Skybolts

I was an early user of Skybolts and they included a strip of very heavy fiberglass matting that I was to place around the top cowl rear edge.

Make sure you have adjusted each fastener properly so it is a tight fit. Use the small black plastic pins or a small pick to hold the circular spring out of its notch and use a flat screwdriver to gradually tighten each fastener so it pulls the cowl down tight and flush.
 
my skybolt fasteners are all as tight as they can be and still allow them to latch upon installing the upper cowl. No such instructions nor material provided to reinforce the cowling with my Skybolt kit. I have 2 3/4" between fasteners.

It is too late to add stiffener for me as I worked hard to ensure my cowl and fuse were flush with each other. Adding anything under the cowl edge will ruin the flush fit.
 
Stiffeners

Add the carbon fiber or fiberglass stiffening band just ahead of the fasteners . a few inches toward the prop)
 
I had the same issue. Spacing was similar. After the first couple of flights I added more glass just ahead of the fasteners as suggested by others. I still get a bit of "pillowing" but much less. When I get a chance I plan to add some more glass and maybe shape it into a "rib" to give even more rigidity to the area.

While it is not in the high pressure area of the cowl I believe there is sufficient pressure forward of the baffle to distort the aft portion of the cowling without having the cooling suffer. While it is a bit concerning to look at from the cockpit the pilot is the only one that can see it and I have not heard any reports of cracks forming.

I used spacing as suggested by Skybolt but ended up with two extra fasteners. I used them on the oil door. With perfect hindsight I would do it again using less spacing and reinforce the cowl. I believe the top of the cowl is the problem so maybe spacing that is less there and increasing down the sides --- as long as there was no appearance issue.

The only total solution is the hinge but I wouldn't trade the ease of the Skybolt fasteners.
 
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