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

Fri Jun 14 10:35:00 AKDT 2013

Can you provide a pointer to the balsa/cf panels you used?  Are these the
ones from ACP or something else?

Thanks,
Peter+

On Fri, Jun 14, 2013 at 11:14 AM, Larry Diamond <ldiamond at diamondrc.com>wrote:

> Thanks for all the advice...
>
> Just ordered the end grain balsa / CF panels.
>
> I'll go this route...
>
>   *From:* Keith Hoard <klhoard at hotmail.com>
>
> *To:* General pattern discussion <nsrca-discussion at lists.nsrca.org>
> *Sent:* Friday, June 14, 2013 11:59 AM
>
> *Subject:* Re: [NSRCA-discussion] Repairing Gear Block - Guidance Needed
> -Added Weight VS Strength
>
>   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 . . .
> .
> http://www.acpsales.com/Sandwich-Panels.html
> .
>
>  *From:* Ronald Van Putte <vanputter at gmail.com>
> *Sent:* Friday, June 14, 2013 11:45 AM
> *To:* General pattern discussion <nsrca-discussion at lists.nsrca.org>
> *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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-- 
Director, Fixed Wing Flight Training
Santa Clara County Model Aircraft Skypark
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