A liability of retractable landing gear is the potential repairs they can need if we
mess up a landing. Not being perfect, I was able to bring you this How-To.
Text, photos and video by Tom Hintz
Posted 2-25-2016
If you have been following FlyingRC.net you know that I am working with a brand new Top-Flite Giant Scale P47 Razorback that I recently maidened on a Saturday. After four flights that day I took the plane home and got up the next day to do a thorough nuts and bolts check before continuing the maiden sequence that day. That all sounds so correct and non-threatening to the plane but I was about to find out that reality can trump the best of intentions.
While building speed down the runway the first time that day a 10/32 setscrew I forgot to check for tightness showed its level of disenchantment by spitting off the right side wheel and dropping the now-bare retract into the grass. The resulting leverage applied to the retract mount ripped it from the wing. Poof! Instant How-To story!
There is something to the near automatic “pick up the pieces” quotation in a situation like this. The retract applied a huge amount of torque that snapped the mount into several pieces that were laying around the plane where it skidded to a stop. Because we took the time to find and retrieve those pieces repairing the Top-Flite Giant Scale P47 Razorback was easier than I anticipated.
When the plywood retract mounts break they usually produce jagged edges that can actually help align them with each other to restore the structure with near spot on accuracy. Some of the jagged edges can get bent over or smooshed altogether which makes it necessary to carefully trim some of those little points/edges to make fitting them back together easier. This can also help regain the proper alignment as well as simplify applying the epoxy evenly to produce a strong repair. I used a relatively thick, glass-filled Epo-Grip Matrix epoxy on these joints to maximize the strength of the joint and its resistance to vibration.
After fitting and gluing the major pieces back into place with the epoxy I let them cure overnight because I needed them to be solidly bound to the airframe for the next step. Most retract mounts incorporate heavy plywood ribs on either side and I was happy to see that the Top-Flite Giant Scale P47 Razorback had those oversized ribs.
Carefully fitting the damaged pieces
back together takes time but results in
a better, more durable repair.
Using a 1/8” drill I opened the original screw holes so I could plug them with pieces of a hardwood dowel. The process of installing these retracts is simple enough that plugging the screw holes makes re installing the retract mechanism easier. I also drilled diagonal holes through the mounting rails and the heavy ribs on both sides of the mount. After filling all of the holes with 1/8” hardwood dowels secured by a generous dose of thin CA to each one the assembly was set aside to dry.
Because Top Flite designed the Top-Flite Giant Scale P47 Razorback around the Robart retracts the task of installing the unit is very easy. I just had to lay the retract in place, center the wheel in the well and then drill holes for one of the screws. Then I run the retracts up and down to be sure there are no interferences before installing the remaining screws.
The outer door on this retract had also been broken in the crash but because it was made from good quality fiberglass I could fix it easily. After bending it back into shape I used thin CA to “freeze” that shape while I spread a coat of the Epo-Grip Matrix epoxy over the broken area and set it aside to cure. This repair will live behind the wheel so I was more focused on a strong repair than a pretty one.
The wings, like most of the Top-Flite Giant Scale P47 Razorback are sheeted with balsa. There was surprisingly little damage to the sheeting and none that could not be fixed by fitting the cracked sections back together and securing with CA. These repairs expanded the area that would have to be recovered slightly but that was a small addition to the overall repair job.
The Top-Flite Giant Scale P47 Razorback has a 1/16”-thick plywood cover over the retract mechanism and that was broken in a couple places in the crash. The cover on the remaining retract is identical to this one, just reversed. I used that cover as a template to trace out the needed piece on some fresh plywood and cut it out using a powered scroll saw that is probably overkill in a big way but that is the guy’s way of doing it. After giving the new cover a coat of Balsarite I covered it and drilled the screw holes in the corners.
I also fixed a couple of “dinged” spots of balsa sheeting with CA and then sanded them smooth. Finally, I wiped all of the areas needing recovering down with alcohol before applying the covering. Repairs like this make me happy that Monokote colors always match each other so well. All that remained was checking the function of the retract one more time to be sure it opened and closed cleanly and that the gear door aligned with the opening properly.
Altogether I had maybe 4 hours of labor in this repair and just a few dollars in materials. I expect this repair to be as strong as the original structure so the Top-Flite Giant Scale P47 Razorback can have a long life at the field. Unless of course I somehow mess up again…...
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