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Giant Scale Great Planes Yak 55M repair

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Old 11-15-2011, 05:07 PM
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Default Giant Scale Great Planes Yak 55M repair

Hi folks, I recently repaired the landing gear mounting system and surrounding woodwork on a friend's giant-scale GP Yak 55M and thought you guys might like to see the blow-by-blow.

First the background, my friend had a very heavy set up for the plane due to the original engine selection (O.S. FR7-420 Sirius7)
http://www3.towerhobbies.com/cgi-bin...&I=LXZVB2&P=ML (it's OK, go ahead and drool, I do all the time).
This engine, though lovely, weighs in at 111 ozs, which is easily more than 4 lbs over similar power gas jobs. The all up weight ended up about 7lbs over recommended, because over 2lbs of weight were required in the tail to balance the whole thing.

So picture a blustery day, strong cross-wind landing situation, overweight airplane. On touch down one wheel digs into the soft ground and SNAP out comes the RHS landing gear, bring with it about half of the 9mm plywood mounting plate.

My friend wisely decided to replace the O.S. FR7-420 Sirius7 with a more suitable 50cc class gasser, but got a little lazy and decided to "repair" the landing gear mount by pouring an ounce or two of epoxy and clamping it all back together. Needless to say, 2 flights latter, while taxiing back to the pits, the RHS LG just collapsed...

I offerred to do a proper repair and that's how it ended up in my workshop.

The hardest part was actually cleaning out the results of the the previous "repair" - removing gobs of epoxy and splintered wood, was required before I could assess what I was dealing with.

This is the LG area AFTER clean up. notice half the LG mounting plate is missing.



Some of the side sheeting had been ripped out when the LG collapsed and I need to see what was going on under the LG mounting plate, so I removed a porting of the sheeting which was set aside for reinstallation later.

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Old 11-15-2011, 05:21 PM
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Default RE: Giant Scale Great Planes Yak 55M repair

Unfortunately, when the LG plate was ripped out, it left behind the "tabs" that spread the torsional loads length-wise along the fuselage, It was not practical to try to replace these, so I had to find an alternative means to spread the torsional loads. My solution was to fabricate a 1/4" plywood upright that would "clamp over the new LG plate and be firmly epoxied to the fuse side structure.

First though, I had to fabricate and test fit a new LG plate and make a template for the new upright from cardboard. Below is the old LG plate, its replacement and the template for the former.



Trial-fitting new LG plate and cardboard upright template. I also installed 3/32" ply doublers along the existing cross-fuselage formers because my new upright former will depend on them for part of its load bearing.

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Old 11-15-2011, 05:30 PM
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Default RE: Giant Scale Great Planes Yak 55M repair

Photo of fuse upside down. There is a sturdy fuselage member that runs lengthwise under the LG plate, so to this I epoxied a 1/2" section of hardwood. This increased the surface available for gluing considerably.



It also provided a good spot to drive a couple of 1" wood screws into from the top.
The above pic also shows tabs installed inside the edges of the sheeting which will make installing the patch much easier later.

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Old 11-15-2011, 05:36 PM
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Default RE: Giant Scale Great Planes Yak 55M repair

New LG plate and upright firmly epoxied in place.



The old fuselage floor to mount the tank, batteries, etc was all busted up, so a replacement was fitted, including a new spot for the throttle servo.



A tip, for non-structural functions like the above, plywood from mandarine crates make a great source of FREE wood!!

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Old 11-15-2011, 05:41 PM
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Default RE: Giant Scale Great Planes Yak 55M repair

All that was left at this point was to close up the fuselage by replacing the missing sheeting. I could use part of the surviving sheeting, but a good chunk needed to be made from scratch.

Again, note the use of tabs glued to inside of existing sheeting to support the template.



A thin cardboard template was used again to get a good fit.





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Old 11-15-2011, 05:47 PM
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Default RE: Giant Scale Great Planes Yak 55M repair

Here's the completed repair, all remaining imperfections taken care of with light weight wood filler and sanding smooth.



I had the required colors of Monokote in my scrap box, so my friend didn't have to go out and spend extra dough on 3 rolls of covering.


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Old 11-16-2011, 03:06 AM
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Default RE: Giant Scale Great Planes Yak 55M repair

Oooooh, nice. Looks like it never broke.


Such a shame he couldn't make the radial work properly though. I'd have found a way to make it work somehow, it's infinitely cooler than 50cc gassers. What's he gonna do with it? Fee-bay it? Save it for a more suitable aircraft? Make it into an outdoor fan/running display model?
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Old 11-16-2011, 05:39 AM
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Default RE: Giant Scale Great Planes Yak 55M repair

Thanks for the kind words, 378. My friend was also happy with the results.

Re the OS Sirius 7 cyl radial, no worries, he's keeping it, but wisely decided to get a plane more suited for it: http://www.aero-works.net/store/detail.aspx?ID=232

I can't wait to see THAT combo in action!
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