GM4 Bracket Improvement

The GM4 bracket attaches the forward half of the main gear truss to the wing spar on the Lancair 235, 320, and 360. It also serves as a mounting point for the over-center linkage and for the microswitches used to detect the gear down position. This bracket is therefore one of the most critical structural components on the main gear. Both of my brackets were damaged which required that I source, or build, new ones.

Lancair Design

From what I had been able to determine Lancair made two iterations of these brackets. Early planes were shipped with GM4 brackets having holes to reduce weight.

While these holes look comically large, this design does not rely solely on the bracket to support side loads transferred from the over-center linkage. During construction the builder also fabricates a phenolic block, GM13, which is subsequently bonded and glassed to the inside of the bottom spar cap. This block also helps support the over-center linkage and is directly attached to the GM4 bracket using a bolt. I think the additional strength from the block attachment is why Lancair was initially comfortable with the large holes.

At some point it was found (refer to service bulletin SB050) that this phenolic block could de-bond from the spar and that the bracket could not support the side load alone and would fail. To prevent this issue Lancair made two changes to the design: 1. They asked owners to add a brace between this phenolic block and the top (inside) of the spar so that the block would have nowhere to go, and 2. They removed the holes from the brace design. From what I have been able to tell Lancair never publicly instructed owners to swap the brackets with holes to those without. Subsequently, I have seen many aircraft, even today, with the lightweight brackets.

N320ED’s gear collapse appeared to have been caused by the failure of this owner fabricated brace and the lightweight bracket. The brace installed was not fabricated using the correct material, resin, nor proper technique to bond and glass it in. As a result, the phenolic block became loose and the bracket subsequently failed at one of the holes, allowing the gear leg to fold. While the poorly fabricated brace was the real issue, its quite possible that a stronger bracket could have held on long enough for the issue to be spotted during an inspection.

Another fairly common issue with these brackets is cracking, and subsequent separation, of the over-center mounting boss from the main bracket weldment. To save costs these brackets were machined from 3/8″ thick, 4″ wide, 6061 aluminum extrusion with the boss welded on. This welded joint is a weak spot because the bending loads and weakened material properties due to the weld. These bosses also tend to stretch due to the thin wall thickness and yield strength of the material.

New Design

N320ED’s new oleo gear is slightly different from the original, using COM8 bearings instead of COM6. These allow a larger 1/2″ diameter hardened steel pivot pin to be used (instead of 3/8″) which has better margins for landing loads. Originally, my focus on the GM4 bracket design was to simply replicate the bracket in one piece (no weldment), using better materials, and a COM8 bearing. The design for that version is shown below.

This bracket is lighter then the original, but also does not have a microswitch bracket as this plane’s gear indication is being made by proximity sensors mounted directly on the over-center linkages.

Subsequently I had a number of other owners ask me to make brackets for them, so I decided to take this idea and further refine it into an even a better, stronger, part, but with COM6 bearings and a microswitch bracket for all the normal people.

The new design, part number 700-0061L and 700-0061R, offer a number of improvements over the original GM4 brackets from Lancair. The first of these is the material. These brackets are made of a 7000 series aluminum which offers far better strength then the original 6061 parts. With this improved strength I was able to use FEA to size some lightening features which are less drastic then the early hole design without sacrificing strength. In fact, these brackets weight exactly the same as the brackets from Lancair without holes, yet have much higher strength throughout, having the material better distributed to parts of the bracket which are under more load.

Another improvement to the bracket’s strength was made by milling the boss as one piece with the rest of the part. While I had done this before on the custom set for N320ED, I found I could afford to add additional material, making the mounting boss a much more solid part of the bracket.

One area of the original design that bugged me was how thick the bracket was in the area for mounting the microswitch. This was left thick simply because that was the lowest cost way to make the parts. I elected to thin this bracket considerably, and I also made the slot for the microswitch adjustment into a proper arc, which is funny because it was much easier to do then to make some fixture to produce the parts like were were originally made (square cut slot). As a result, the microswitch hardware is easier to get to and adjust with this design as the fasteners are spaced further away from the spar and cannot interfere with the GM13 bracket, regardless of how the builder shapes or positions it.

For corrosion protection these brackets are anodized and dyed in a really neat gunmetal grey. I will likely use this color for all parts going forward.

The 700-0061R and 700-0061L brackets for the stock gear are now available on the store and come preinstalled with genuine Aurora COM6KH bearings as well as the shorted microswitch hardware required. The COM6KH bearings have a higher load rating then the COM6 (What originally came on the planes) but are otherwise identical. 700-0061 (GM4) brackets may be purchased here.

Also new are GM5 brackets for the rear of the main gear, and GM15 brackets for the nose gear. These are undrilled and ready for either a fresh installation or to correct alignment of an existing installation and are made of a 7000 series aluminum giving them much more strength then the originals. These also come preinstalled with the stronger COM6KH or COM8KH bearings.