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Monday, March 21, 2016

Main landing gear - part 11

Flox doughnuts (14.0 hrs)

It might seem a little strange at first, but building an airplane does not always progress in a linear fashion going from step 1 to say… step 2. This might happen for a number of reasons, such as parts availability for example, but in my case certain building procedures needed to be discussed with “the elders” of this trade, and thoroughly understood before jumping in. It is not unusual thereby that one might occasionally have to go back and take care of unfinished business. 

With that in mind, I had saved a few things for a later date, like the installation of the big washers (AN960-1016) on the fiberglass landing gear tabs, and the flox doughnuts that take up the remaining open space between them and the aluminum gear mounts.

This latter item is not covered in the plans, but is the core of LPC#128 in CP#48.

Plans update in Canard Pusher #48

Not wanting to ever have to crawl back up the “hell hole” after the plane is finished, I decided this LPC would be complied with ASAP while the fuselage is still upside down.

First on the list though, the washers needed to be buried beneath two layers of BID.

I used scissors to cut the hole matching the AN960-1018 washers

If you have ever handled the landing gear without assistance, you will know that it is almost impossible to reinstall it on your own due to its top heavy nature, and the super tight fit of the gear mounts.

Because I mostly work solo, I decided not to remove the landing gear, and to work on one side at a time, with the other side still attached loosely to the plane. That meant I couldn’t just do all four washers at the same time, since one side would have to cure before I could use the same trick on the opposite side, a slight building delay I could live with.

Airplane-right landing gear disconnected 

Landing gear pivoting on opposite side bolt, allowing work to be done on without complete gear removal.

This job was pretty easy, but it took three days to complete, one day per side, and one to clean things up after the second side cured.

AN960-1016 washer floxed to the gear tab

2 plies of BID going over the washer

BID wetted with pure epoxy...

... and peel-plied.

Next morning, aircraft-right rear tab before trim...

... and aircraft-left rear after trim.

Aircraft-left front tab with washer

The second item on the list, the flox doughnuts, were what I was unsure of. 

I did get the general idea... build four dams and pour some wet flox in it, but I had some unresolved questions. How could I make sure that the flox would not stick to the metal tabs, preventing me from being able to remove the gear in the future? What material would make a suitable dam? How tall should the dam be? How could I prevent trapped air within the doughnuts? 

The longer I thought of it, the more questions I’d come up with, but eventually the time came when I realized I’d just have to answer those questions for myself by doing it, and finding out what worked and what didn’t.

One example of something that didn't, was covering the inner part of the brackets with clear tape. This way of preventing flox from sticking to surfaces is pretty much standard operating procedure, but it couldn’t possibly have worked in this scenario. The clearance between the bushing within the LGMA steel tube embedded in the fiberglass tabs, and the aluminum tabs is 0.000”. This meant that LMGA's bushing would just scrape the tape off as the gear was lowered into its normal position.

This did not work at all!

However, that didn't stop me from trying anyway, and I was able to confirm what a terrible idea that was.


Back to the drawing table.

About this time Gina walked by, and asked what I was so upset about. After my somewhat complicated explanation, she blurted out “Why don’t you just spray it?”

This of course sounded to me like the most ridiculous thing I had ever heard, and only served to increase my blood pressure, and with it my frustration. But I shouldn’t have been so rash in my judgement.

After exploring all other ideas, I eventually became desperate enough to give the spray theory a second thought. I wasn’t keen on spraying WD-40 (or PAM as per her suggestion) on the tabs for many reasons, the most important of which was that I did not want to take a chance on my gear getting stuck forever by using some random product.

"If there only were something designed for this purpose…" then I remembered about the urethane bumper YouTube video I had just filmed a week before. 

You see, in order to prevent liquid urethane from sticking to the aluminum mold, I had to spray it with a release agent (Pol-Ease® 2300), basically a silicone based lubricant, and if it was good enough for urethane, perhaps I could abuse it in my situation.

One look at the label, and I was shocked to see that it was actually the prescribed release agent for urethane AND epoxy. 

I totally lucked out!


My wife was right again! 


So, the new plan of action became… 

  1. detach the gear on one side
  2. cover the gear tabs to protect the fiberglass from the overspray
  3. spray the metal mounts with release agent (without getting any on the fiberglass of course) 
  4. lower that side of the gear and reattach it to the metal tabs 
  5. raise the other side of the gear and cover it again
  6. spray the other tabs with the release agent
  7. lower and reattach the gear
  8. build the dams with suitable material
  9. fill the dams with wet flox

I finally had an actionable plan, rather than just hopes and fears, though one last question lingered. What material should I use to make the dams?

My choice could have easily been better had I only lined the paper strips I chose to use with tape, but it all worked out well in the end anyway.

Don't do it Marco!

Oh well! You'll pay for this later during cleanup.

Paper cup strips used to create the bottom of the dam

A different view of the dams

Aluminum tape used to seal the dam, and build it up above the the level of the doughnut.

I sealed it as much as possible to prevent flox from escaping below and creating a void

I also fashioned a bit of a lip to the top of the tape to help when pouring the flox

All four dams ready to go

Another look at the finished dams

I made the flox as dense as I could while keeping it flowing, and filled the cavities above the level of the metal tabs. I figured I could always sand it down later, but not add to it.

Flox going in!

The lips that helped with pouring could now be pressed back in, before the flox hardens.

Curing time

I made sure no flox was leaking below the dams, thereby reducing the height of my doughnuts, then went to bed with high hopes for an half way decent outcome.

The next day I could hardly wait to see the gear. Would I be able to raise it off the metal tabs? Would I be able to trim it as needed? Etc, etc, etc.

After removing one of the two gear bolts, I found the gear very easy to pivot off the opposite bolt and out of the metal tabs which were still wet with silicone. 


Looking rough early in the morning (left rear)

Left front

BizMan (aka Beasley) had warned me the tabs would look pretty rough at this stage, but that I would be able to clean them up nicely, and that’s exactly what I intended to do.

Dremel to the rescue! (left rear)

Left rear cleanup completed

Left rear detail

Left front tab after ripping the dam off

Right front tab needing some TLC

Right rear cleaned up as much as possible by hand

Allright, here goes the Dremel again! (left front)

Removing the paper cup strip from the left front tab the hard way. "Told you so!" 

Left front tab cleaned up

Looking pretty good!

Left front tab in place on the aluminum gear mount

Same tab seen from the opposite side of the fuselage

I only found one air pocket in the right rear tab that I had to fill.

Rear right tab developed a void

Right rear tab all cleaned up, void getting repaired.

Nothing a little flox couldn't fix

Overall the process worked better than I had expected. I attribute 90% of my success to the release agent, and to the very tall dams I created on the gear tabs. 

If I were to do it again, I’d just spray some release agent on the paper strips before attaching them to the landing gear, or perhaps just attach clear tape to their inner surfaces to make clean up quicker and easier.

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