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Tuesday, June 04, 2013

Landing brake - part 4

Landing brake hell! (6.5 hrs)

Once the brake finally cured I removed the 2x4s with a mallet, and trimmed it down to its semi-final shape, confident in the tight fit to come.

Wood frame removed from landing brake

But that’s not what the Long EZ Gods had in store for me this time. Upon closer look, I discovered that all was not well in paradise, and I cringed at the thought of having to remake this part.

While the foam was indeed a perfect fit, maybe too much of one, it turned out that the hinge would have its own say about how things ought to be. So, while the back side fit like a glove, the front side would stick up disrespectfully, with the hinge riding up on the fuselage bottom skin.

Hinge riding up on the fuselage bottom

Of course, the first thing I tried was to slide the panel back so that the hinge slid back in the proper position within the depression, but then the rear end would stick up like a “skateboard ramp”.

Rear edge of the brake not fitting

Same detail viewed from the opposite side

The urge to take the panel out back and shoot it was nearly irresistible, but tried to remain analytical about it (difficult thing for a hot blooded Italian to do under these circumstances), and figure out what in the world had happened to my landing brake.

You see, there are no directions in the plans helping you place the hinge in the proper position, and no warnings about how important this step is. All they say is: “The hinge is now bonded in place with flox”, with not a hint of what “in place” really means.

This lack of detailed explanations, belied the critical nature of the hinge positioning, and this is how mine ended up about 1/4” (6.4 mm) too far forward.

With the hinge further forward that it needed to be, the front end of the brake ended up being lifted out of the depression at the hinge. Vice versa, when the hinge was moved backward and into the depression, the rear end of the brake would ride up on the sloped edges of the depression, and out of it.

The issue!

Well, I really thought I’d have to remake this part, but one of the reasons I didn’t want to, was that in the absence of a method for properly positioning the hinge, I had no assurance that the next brake would turn out any better.

The only fix I could think of was to remove some foam out of the back of the brake, and re-glass, but I hadn’t done that type of a fix before, and I was a bit unsure on how to best accomplish it.

With RAF (Rutan Aircraft Factory) having shut down decades ago, there is really no one else to turn to, beside other builders. As luck would have it, Mike had a great idea on how to take care of this issue.

Mike easy fix solution 

What I ended up doing, is pretty much what he described. 

I cut the extra foam off, sanded the 3 BID plies (staggered) out to about 3” (7.6 cm), or about 1” (2.5 cm) per ply, exposing ply 1 for 1”, and ply 2 for 1”. Ply 3, being the top one, got prepped for about 1” itself. 

Cutting the back side

Offending foam removed

Brake sanded and ready to go

Once I was ready to glass again, I used 3 staggered plies of BID to replace the original ones, plus one more ply over the whole thing, as explained in chapter 3 page 22, section 4.2 “Large defects - Repair”. 

RAF guidance on repair

Cross section view of a repaired section

While I was at it, I also took some time to lower the wood profile in correspondence of the hinge, since that was interfering with the hinge movement, and create a flat surface for the washers to fit.

These are not the correct washers, I used slightly bigger ones so that I could create larger flat sections

With everything ready to go, I started glassing the brake once again.

Fiberglass in place

Glassing done, and peel-ply applied.

Brake curing, again.

The next day, I cut and sanded the brake to its final shape, and tested it again for fit.

1" (2.5 cm) radiuses are required at all the corners

Sanding and vacuuming at the same time

Brake finally fitting

Same thing from the other side

Finally, it was starting to look like the tide had turned in my favor, and I was moving in the right direction once again. If I were to build another brake today, I'd probably trim the rear end of the foam 1/2" (1.3 cm) shorter to begin with, so that I would have some room to play with, to account for the inherently inaccurate positioning of the hinge section.

Before getting any further though, I wanted to open a small parenthesis on how I was able to recover from a prior misstep, and drill bolt holes in the hinge that lined up with the holes on the fuselage.

Basically, I took some small pieces modeling clay, stuck them to the hinge section near where I thought the holes should go, and pressed it against the fuselage. When I removed the hinge, the modeling clay had 4 formed dome heads right where the bolts needed to go. I marked the center of the domes with a scribe, by hitting it with a hammer, then I removed the clay, and center-punched it with an automatic center-punch. I checked these marks with a paper rubbing of the fuselage holes before drilling them. 

Although I did not take pictures of the process at the time, I recreated the steps for the purpose of demonstration, and took the following pictures.

Modeling clay blobs (recovering from a previous screw up)

Domes formed in the clay by pressing the hinge in position

Very carefully marking the domes center position

Center-punching the scribed marks

Taking an impression of the holes pattern

Double checking the center-punched marks with the pattern, before drilling

The resulting holes lined up perfectly with those on the fuselage, and the bolts slipped right in with no trouble. I was relieved to say the least, and this ended the previous screw up in a satisfactorily manner (see Landing brake - part 1 for what not to do).

Excellent fit! :-)

Close parenthesis.

Here’s what the brake looked like after trimming the excess.

Brake cured and trimmed to final size and shape

Rear section close up

The fiberglass ended up rounding the flats sections I had made for the washers a bit, so I sanded them again.

Recreating washers flat sections

Washers flats close up

After it was all said and done, I was aghast at what I saw when I bolted it all together again...

"Son of a b....! What now!?"

This brake job was starting to feel more and more like a nightmare! After all that work I was back to square one with a brake that didn’t fit!

Investigating the problem further revealed that I had contact in a few places that a bit more sanding might cure. So, I trimmed a little more glass in a few strategic locations, and the fit turned out nearly perfect.

Now, “nearly perfect” is not nearly good enough when talking about aerodynamically active components. A gap here would increase drag, perhaps not greatly, but over the entire life of the plane fuel would be consumed unnecessarily to compensate for this flaw. 

Determined not to have this happen, I looked more closely and found that the nuts I used were making contact, preventing a good seal. 

Hard to see from this angle, but the bottom end of the top bolts are touching the hinge flat section.

These nuts were the self locking MS21044N3 type, a model of nut with a nylon insert whose thickness is necessarily greater than that of a regular nut.

Locking nuts with nylon insert

The obvious fix in this case was using nuts with a lower profile, but not having any on hand, the work on the landing brake ground to a screeching halt.

The brake saga will resume after Aircraft Spruce delivers the goods.