stunthanger.com
Speed,Combat,Scale,Racing => Carrier => Topic started by: john vlna on May 23, 2016, 03:00:20 PM
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Last year at the NATS I entered my all foam electric Seafire AMA profile. You might say the construction is somewhat experimental, but I have been building these type planes for some time now. However, the AMA carrier has tougher pull test requirements than sport flying. It pulls about 63 lbs and although it had been pulled before I felt something give. I didn’t examine it closely at the time. I just assumed the mount had given way. I came home, put it on the shelf and sort of forgot about it. I have a new profile for AMA E profile, but with my field closed by the FAA I have been unable to find a place to fly carrier. (CL models are such a threat to full size aviation) so I am behind in testing and trimming.
So this last week I thought I had better repair and have the Seafire available in case things don’t work out on the other model. When I got into the BC issue I found there wasn’t any structural failure. The wing has two ply plates (Top and Bottom of the wing) , the BC is mounted by simply bolting it through the plates. What happened was the 6-32 bolt holding the BC bent. I had three different 6-32’s so I tested them, two bent the socket head broke all this with minimal force. The 4th is a #8 (see picture). I simply chucked the screws in a vice and bent them with a pair of pliers. The # 8 requires a bit more force but not a lot.
The BC is a Sig nylon and I think the problem may be that the thickness puts too much force on the bolt. I will test with a #8 bolt, but I think it will be best to switch to a GS, LR, Brodak crank.
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Try these, made to a MIL spec, at least you will know the quality.
http://www.aircraftspruce.com/catalog/hapages/ms27039.php
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What about a piece of music wire with a collar on both ends?
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- What Russel said -- hang a piece of music wire or a bolt between two pieces of plywood bedded into the structure, with the bellcrank in between
- All the bolts you show are all thread, and the strength of such a screw is pretty close to that of a rod the inner diameter of the threads. See if you can find a smooth-shank bolt -- it should be much stronger. Your local hardware store should have such a critter. A grade-8 bolt with a smooth shank would be better, but would probably need to come from a specialty house. Jim's link shows smooth-shank screws, but you'd want to make sure (normal not-from-hardware-store screws have an inch of thread and the rest is shank; I dunno about aviation screws).
- Get a shoulder screw with the right OD of the shoulder -- this means you need to know the OD of the bushing in the Sig bellcrank, and you need to hope it's a standard size
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Thanks for the comments, I'll let you know which option works best.
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At the carrier contest this weekend, I heard from some Contest Board insider that the DESTRUCTIVE pull test numbers have been reduced somewhat.
The VERY heavy pull tests in Carrier were leftovers of the old days when the planes were a lot faster. If you fly in a contest be sure to download and print a copy of the latest pull test numbers.
And double shear mount your bellcranks.
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The 2015-2016 rules show no changes. Don't know what the '17-'18 book will show. There is a proposal to reduce the limits, but final voting won't take place until September, if I read the schedule correctly.
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You've got to support the bellcrank from top and bottom. Nobody in Stunt mounts a bellcrank like that for contest work. Too little control of the bellcrank rocking. The way you show yours looks like a Sterling Jr. Ringmaster, not so strong. The strength of the thick plastic bellcrank is from mounting top and bottom. Stunters don't pull too hard, the reason that bellcrank is like that is because Stunters fly hundreds of practice flights. The thick plastic is a natural bushing. The way to go is to mount it top and bottom with a through-bolt. Even all thread is okay as long as you double nut it top and bottom. Oh, and make sure it's supported from both top and bottom... ;)
Chris...
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Maybe a length of 3/8" dowel all the way through the wing to support the middle of the bolt will stop the bending. A 6-32 should be plenty strong enough in shear.
BB