I like aluminum landing gear struts greatly. Their advantages considerably outweigh their disadvantages; and with Sig carrying such a wide range of preformed sizes, blanks, and tempered hard aluminum sheet, just about anything anyone could want in the way of a "main gear strut" can be easily achieved.
The only problem occurs in bending hard aluminum. It's very easy to weaken or even crack an aluminum LG blank by using the wrong method -- and unfortunately the instructions Sig supplies are based on a misconception.
Aluminum hardens in a totally different way from steel -- and is "annealed" differently too. To harden steel you heat it and quench it quickly. That's how you ANNEAL aluminum! Tempered aluminum's springiness comes mostly from its alloying elements. After annealing, the hardness will gradually return of its own accord.
Hardened aluminum can be temporarily annealed by heating it to about 500 degrees F, and quickly quenching it with cold water. It will then remain soft for between a few hours and a couple of days (depending on how hot the weather is); gradually regaining its original hardness. (To speed up the re-hardening, you can re-heat the aluminum in boiling water or a 250-degree oven for a half hour or so.)
Here's how to bend tempered aluminum landing gears:
- With a gas stove burner or a propane torch, carefully heat the local area you wish to bend. Test the metal's temper- ature often, by removing it from the heat and firmly pressing a piece of scrap medium balsa on the area where the bend will be. When the wood turns noticeably brown after holding it on the hot area for a second or two, briefly re-heat, then quench the metal, with a sopping wet cloth or by plunging the hot area into ice water. The faster and colder the quench, the softer the aluminum will become.
- For bending the aluminum blank to shape you'll need a full- size layout drawing such as is used when forming landing gear wire. Mark the bend locations accurately on the metal, with a grease pencil or masking tape.
DON'T use anything that will scratch the aluminum!
- Make a pair of "soft jaw inserts" for your vise, using strips of birch plywood held in place with tape. Radius the top inner edges of the plywood "jaws" about 3/32". This will ease the bending action, and reduce stress concentration at the bend.
- Bending the annealed areas of the landing gear blank will tend to re-harden them. For this reason, try to make your bends with as few "re-adjustments" as possible. If you have trouble and the metal resists re-bending, DON'T FORCE IT ! Go back to Step 1 and re-anneal the problem area.
- To make neat, professional-looking bends, place the aluminum blank between the plywood "soft jaws" in the vise, with the bend line exactly even with the top edge of the plywood. Tighten the vise; then use a hardwood "pusher block" (held flat against the blank above the vise jaws) to make the bend.
Doing the job this way insures that the landing gear "legs" will remain straight during and after the bending procedure.
This same technique is also useful in re-shaping preformed aluminum landing gears. Sometimes a preform doesn't have quite the right dimensions for a particular model. But "spot annealing" allows easy re-bending of "tempered aluminum" to whatever size and shape you need.
Now for attaching the strut to the airplane. The "usual method" is to install a plywood plate in the fuselage bottom, and bolt the aluminum LG to it. But that's not the best way to do the job!
Instead, glue 3/16" X 1" birch plywood "doublers" inside the fuselage sides at the landing gear location. Drill and tap two # 4-40 threaded holes vertically in each doubler. These are for nylon screws, which attach the LG strut via matching holes close to the bend line. Leaving the center area of the strut unattached allows it to flex there under heavy loads, thus absorbing impact.
(This basic technique is standard practice in homebuilt & "kitplane" man-carrying aircraft.)
