Spring Compressor for Landing Gear Struts

If you’re building or maintaining an airplane with die-spring landing gear, at some point you will have to compress the spring to assemble or service the spring strut. The force to compress the spring can be quite high (hundreds of pounds), with the potential for serious damage—or serious injury—if not handled correctly.

Owners of airplanes with bungee landing gear have a similar problem, but there the bungees are installed one at a time, so each individual bungee isn’t such a high force. But care is still required, and a variety of tools have evolved over the years to install bungees, with different aircraft requiring different tools. With a bungee gear, the gear is typically assembled first, and the bungees are then added.

But on a die-spring gear, the spring is slipped over the telescoping tubes and then has to be compressed far enough to insert the bolt that holds it together with whatever amount of preload is required.

A Rose Parrakeet, showing the external compression spring struts running from the axle to the side of the fuselage. This is a builder modification; the original Parrakeet design used bungees on the struts. (Photo: Mark Pasqualino)

A “die spring” is just a very heavy-duty compression spring. They get their name from being used in punch press die sets, but are used anywhere high spring forces are required. While they are commonly made with rectangular wire, round wire is sometimes used as well. They’re commonly color-coded according to the load rating, but there’s no standardization among different manufacturers on what the colors mean.

The spring struts on a landing gear can be set up to work in compression or tension. Tension is more common, with the struts under and between the gear legs as seen on Cubs and the like with spring struts replacing the original bungee struts. Compression struts, located above and outboard of the gear legs, are seen on some older designs such as Great Lakes biplanes and Fairchild 22/24 monoplanes, though in the case of the Great Lakes, at least, there is no preload when the aircraft’s weight is off the wheels (so this tool wouldn’t be needed), and both of these aircraft also incorporate hydraulic (oleo) damping.

My Rose Parrakeet also has compression spring struts, in this case with no oil damping but with significant preload. I needed a way to safely disassemble and reassemble the strut as part of a landing gear rebuild. The springs on my gear have a 440-pound-per-inch spring rate and an initial compression of 3/4 inch, meaning a preload of 330 pounds. The simple compressor tool presented here was made and sized for the compression struts on my airplane, but can be adapted for tension struts as described below.

The compressed strut, just after the bolt has been slipped in. Note the shallow holes in the blocks for the strut to locate in. For a tension strut, the holes would go through and the blocks would bear on the collars at the ends of the spring. Friction tape on the threaded rods protects the strut tube where the rods could scrape it, and dry lube sprayed where the rods pass through the tee nuts makes it work more easily.

For a compression strut, the strut needs to be compressed from the ends. The tool is simple; essentially two wooden blocks (2×4) with long threaded rods. The actual dimensions aren’t critical, and will be whatever your aircraft needs. The blocks have shallow holes for the ends of the strut to nest in so it doesn’t slip out. I used 5/16-18 threaded rod because it looked about right; 1/4 inch looked too light and 3/8 inch seemed like overkill. On the back of one of the blocks there are two tee nuts for the rods to thread into; the other block just has clearance holes, with washers against the block and a coupling nut and short bolt on the end of the threaded rods. The coupling nuts should be secured with red (permanent) Loctite; you don’t want them coming off the rods when under tension … ask me how I know! Some dry film lubricant on the tee nuts also makes it work more easily.

Tightening the screws with a cordless drill. Work back and forth, a few turns at a time on each side.

The threaded rods need to be just long enough for the tool to slip over the struts when the spring is completely relaxed. I left them full length as purchased as it didn’t hurt anything. To use, slip over the strut and start tightening the rods, alternating side to side every couple of turns. A cordless drill makes this easier, as does clamping the other end to the workbench. When the cross hole reaches the slot, the cross bolt can be slipped into place and the rods unscrewed again, letting the bolt take the load (it’s during the unscrewing operation that you really need the Loctite).

The back end of the tool, showing the tee nuts in the wooden block. I used the type that is secured with small nails, which hold better than the more common pronged type.

For tension struts, the principle is the same, except that the force needs to be applied to the collars at either end of the spring, not the strut ends. Instead of the shallow holes in the blocks, the hole needs to go all the way through, to let the strut tube pass through. In this case, the blocks should be larger to compensate for the strength lost to the through hole, perhaps 2×6 or 4×4. You may also need a clearance notch to allow the bolt to be slipped into place. The threaded rods can be a lot shorter (though you may need a socket with a universal joint if you’re using a drill to tighten them down).

Two assembled struts, ready for test fitting on the airplane. The plain nuts on the cross bolts will be replaced with lock nuts at final assembly.

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