Davepar

Davepar

Well Known Member
I remember reading that someone rounded the corners between the hinge loops on the cowl hinges. I started doing that, but it's a tedious task. Is it really a recommended practice?

Dave
 
The hinge links are prone to breakage

The hinge links are prone to breakage which appears to start in this area. In a thread about it a few months ago one of the posts advised that this was a way to alleviate the problem. It sounds like very good advice. I wish I had it during my building time. I have lost one link in the upper aft left hinge where it curves down to the side (a likely place). The hinges at the bottom of the cowl at the aft end are simply a bad idea. I replaced mine before painting with a platenut and screw installation. During the AirVenture Cup race last year an RV-4 at race speed had the upper cowl collapse. The pilot was going to make an emergency landing but when she slowed down the cowl came back out. At Oshkosh she (A&P and veteran AirVenture Cup Racer #72) told me that 4 hinge links had broken out on one side at the back allowing the cowl to be pushed out on that side and it collapsed on the other.

I would continue the deburring and when I have to replace mine you can bet that if I replace them I will do it too.

Bob Axsom
 
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Different Situation

Flat hinge installations far removed from the engine area are not subject to the same stress and should not be a problem. I had to have one replaced on the Piper Archer II autoservo tab on the stabilator after around 20 years of the 22 I owned it.

Bob Axsom
 
Stress concentration

Yes it is tedious. Yes many people don't do it. Yes it helps lower stress concentration and fatigue cracking.

I have an aircraft structures background and the detail on the rolled AN hinges are pretty poor. Sharp corners and and the edges are rough with burrs.

I would do it and for me personally, buy the expensive extruded hinge for the aft bottom cowl sides. This location, even though they use the larger sized rolled hinge, never lasted for me, a few hundred hours. The extruded stuff is much stronger, better material and there are no sharp corners or burrs out the box.

Is it worth it. It can't hurt and hinges are sometimes known to fail. I think it has more to with pre-stress and poor installation and poor attachment than it does the burrs on the hinge. However you would not build your plane like these rolled hinges. Look at them, they don't follow the normal smooth, no burr, sharp corner rules. Than again they are meant for low stress applications. Again with air loads, vibration and pre-stress from installation can certainly cause them to fail. Since vibration and fatigue life is affected by stress concentrations you could improve life. However there are some tricks to keep the stress to a min. One is to off set the center line of the pin from being exactly aligned with the joint (halves of cowl). George

George
 
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Bob Axsom said:
The hinges at the bottom of the cowl at the aft end are simply a bad idea.
Click to expand...

Why is that? Does that area receive more vibration, since the exhaust pipes are right there?

George-

I see that Vans and $pruce sell extruded hinge, MS20001PX-X. Is that the stuff? Actually, the Vans catalog says "The MS20001-4 hinge is an extruded hinge. This is typically used in high vibration areas where loop fatigue is prevalent."

I agree that the hinge material in the kit looks like crud compared to all of my other parts that I've deburred and smoothed. Since Vans recommends epoxying the hinge to the cowl, I'd prefer it if the things didn't break.

Keeping the cowl hinges stress-free seems impossible to me. We're going to have a 200mph wind pushing back on the cowl. Every single hinge loop is going to feel that. Also, there is a fair amount of gap between the hinge loops and the pin has some slop. I can move the front of my cowl around about 1/8" in every direction. I hope that's normal.

Dave
 
Emperical rather than theoretical input

The bottom hinges are fairly short and the are at the end of a fairly long arm. I do not think it has anything to do with the exhaust. After I posted my experience others (most notably Mel) came forward with colaborating stories. I originally installed the hinges with AN426AD3 rivets and after mayby 50 hours of flight I saw the rivets at the nearest the exhaust port at bottom of the cowl "dishing". After a little worrying time I popped one off with my fingernail. I redid all of the holes to #30 with all of the associated hole prep and installed AD4 rivets. After a time they too started "working" on the fiberglass. After that I went to the platenut and screw approach. This required two plates extending forward from the lower fuselage position the platenuts to receive the #8 flathead screws and dimple washers. This has worked out very well.

Bob Axsom
 
There are a ton of people on the Camloc group buy, getting (I believe) a healthy discount:
http://www.vansairforce.com/community/showthread.php?t=4671

Filing the corners of the hinges isn't that big of a deal. I'm guessing it will take an hour or two for all of the cowl hinges. That's nothing compared to how long it's taken to get the cowl fitted. For me, I like the look of the hidden hinges quite a bit better than the Camlocs.

Bob- Did you use epoxy with the rivets to attach the hinge to the cowl? The instructions now recommend it, but I'm not sure if that's always been the case.
 
Yes I did

I did epoxy the hinge halfs to the cowl in addition to riveting and I like the look very much - perfect alignment over the full length of every seam.

Bob Axsom
 
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I filed the hinges on my cowl. I think it is necessary because the hinges in the upper cowl joints curve. If you don't file the edges, they will bind, making it difficult to install and remove the pins, and create stresses that will break the rolls off.

It was not that hard to do. A good, sharp file is all it takes. Kinda like sharpening a chainsaw blade.

Roberta
 
Correct. I was going to take a picture last night, but got lazy. My original question was about filing the corners between the hinge loops to relieve the sharp 90 angle. Filing a slant on the loop itself is absolutely necessary around the top curves.

I know what you mean about backing up. I just want the cowl to be done. It's a monotonous process. But hey, I have the baffling to look forward to. I hear that's real fun. :)

I just read in another thread that the milspec camloc group buy goes until Feb 10. But I think you're right. Both ways have their issues.
 
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