[NSRCA-discussion] Repairing Gear Block - Guidance Needed -Added Weight VS Strength

Fri Jun 14 09:00:07 AKDT 2013

I keep a stock of 1/8”  and 1/4” end grain balsa / Carbon Fiber laminate on hand for just such repairs.  That stuff is light and extremely strong . . .
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http://www.acpsales.com/Sandwich-Panels.html
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From: Ronald Van Putte 
Sent: Friday, June 14, 2013 11:45 AM
To: General pattern discussion 
Subject: Re: [NSRCA-discussion] Repairing Gear Block - Guidance Needed -Added Weight VS Strength

I think you are right to be concerned.  Finishing resin is not intended for structural use and I don't know whether your repair will hold together.  I also don't know what you used to replace the original gear blocks, but I would be concerned about the use of lite ply.  It is not nearly as strong as aircraft-grade plywood.   

I have intimate experience with landing gear plate replacement, having just completed a total replacement of the gear plate in my Vanquish.  For some reason, the manufacturer uses cheap three-ply plywood for the landing gear plate and a landing in a grass field that had a hole in the turf took out the left side of the gear plate.  The plywood opened up where the gear was bolted to it.  

I won't explain here what I did to replace the gear plate, other than to say that I removed a section of the bottom of the fuselage to access the area where the gear is bolted to the gear plate and used some ingenuity to manufacture a new gear plate and install it, using the existing gear plate tab holes in the adjacent bulkheads.  Contact me if interested in knowing what I did.

Ron Van Putte

On Jun 14, 2013, at 11:26 AM, Larry Diamond wrote:

  A bit of a delimma mostly because I don't know the Pro's / Con's of strength VS weight with various epoxy mixes.

  Issue... I damaged LG gear blocks of a 2M Monolog... It was my fault and I don't think it had anything to do with the design. Actually I believe the design saved me from a much more difficult task of rebuilding part of the fuse. The gear block did it's job mostly because I didn't do mine and I was stupid. Figured I could set a timer up later after a contest, flying electrics... Uhhh, not a good idea if you going to throw in any type of redos in 15 - 20mph winds... Just saying... Deadsticks are no fun... I was also lucky with the main batteries... Not too hot, didn't puff, and put 5100 and 5150 back into the two 5000 packs... Lesson learned and a small price to pay ...

  Here is what I'm thinking about trying and what I have done so far.

  I removed all the loose wood in the existing gear block area and dremeled out the wood that was shreaded due to being sheared. Everything fit back in place without being removed completely.

  If I removed it completely and replaced it, I would have created some damage to the surrounding area (fuse sides and formers) trying to cut it out or using a dremel to remove.

  I glued the main parts back in place with 30 minute epoxy and let cure.

  I then taped up any holes or gaps from missing wood to prevent the thin epoxy from flowing through.

  I mixed a 50/50 ratio (volume) of 20min Finishing Epoxy (extremely thin epoxy) and micro ballons. I used this very thin mixture (similar viscocity to motor oil) to fill voids and areas where the wood was removed so it would be flush with the existing wood. This provided for a nice level finish of the existing wood block and an unintetional outcome was a perfect fillet all the way around the block. I was very surprised with the outcome, much better than I thought it might turn out. This should also allow me to easily sand down high spots of the cured epoxy flush to the existing gear block for laminating light ply to the existing gear block for setting the t-nuts for the LG.

  This only added 3 grams of weight minus the weight of the wood that was removed. Measured the epoxy before and after use. 9 grams epoxy, 9 hardner, and 1 gram of microballons. 1 gram of microballons is a lot more than it sounds... After applying the epoxy, I had 16 grams left.

  My next step is to laminate a 3/16 light ply with 30 min epoxy to the top (inside of fuse) of the gear block spanning about 75% of the length (across the fuse) and the same width of the existing gear block. My theory is this should offer strength accross the gear block without over-reinforcing the gear block area and not add unnecessary weight.

  Does this sound sufficient or is the risk too great that the gear block will be too weak?

  My concern is that the Finishing Epoxy and microballoons will not be strong enough. My thought is...it isn't being used for strength and only for a filler to bond the existing gear block. It appears that the dried mixture is as strong as a filler/laminate than the original ply.

  It is not too late to cut it out. There is only cosmetic damage to the surrounding area. I'm concerned that if I try to cut out the gear plate, I will do more damage and it will be weaker and heavier in the end.

  Larry Diamond

  Yes, I have pondered on this since June 1st... Started putting it back together last night.
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