Mount wings using differential screws

Model Airplane News, Aug 1998 by Ehrenfels, Al

HE WING-MOUNTING method shown on the plans of my 1/4-scale Cub left me cold. After considering turnbuckles and finding only some rather crude and bulky examples on the market, I developed an alternate approach that works surprisingly well and may just be what you need for your next project.

The method I use employs a differential screw in place of a turnbuckle. The differential screw is an old engineering concept, and this is merely a new application. Before you snort "So what!" and move on, consider this: it requires no special taps; it uses readily available material; it needs no fancy machining; and it can develop an incredible amount of retaining force.

The accompanying sketch shows an aluminum rod with lengths of threaded rod fitted to each end. Different threads are used on each end, hence the name "differential screw." It can be any combination of threads, providing they are different on each end of the assembly. In my application, 8-32 and 10-24 threads were used, since that's the threaded rod the local hardware store had on hand. The matching blind nuts and the aluminum rod were available at the same place. The blind nuts are sold as "T-nuts" in the local hardware store, so try that moniker if "blind nuts" draws blank stares.

Looking at the drawing of the installation in my Cub, you will see that I used two screws passing through, but floating in, the fuselage. Blind nuts are fitted to the root ribs of the wing, and locater dowels in the root ribs fit matching holes in the fuselage.

To assemble the plane, I hold the left wing against the side of the fuselage and turn the 8-32 screws into the blind nuts. I screw them in until they are almost flush with the fuselage on the right side. Then I hold the right wing against the fuselage and proceed to turn the screws into the 10-24 blind nuts on the right wing. You might think this would result in my unscrewing the left wing at the same time, but note that the screw is moving into the right wing faster than it is moving out of the left one. That's the secret to this approach.

The reason it works so well is clear if we do a little arithmetic. An 8-32 thread has a pitch (advance per turn) of 0.03125 inch, and a 10-24 thread has a pitch of 0.04166 inch. Thus, as I am screwing into the right wing, both wings are being pulled together by 0.0104 inch (0.04166 minus 0.03125 inch) on every turn of the screw, and that's the pitch of a 96-threads-per-inch (tpi) screw. A few turns of the "turnbuckle," and the wings are pulled to the fuselage with the mechanical advantage of a 96tpi screw, and that amounts to about 100 pounds, using only fingertip pressure to tighten the screws.

For the occasions when my hands were slippery with oil, I drilled two cross-holes in the rod so that I could use a short piece of music wire to turn the screws. Go easy, or you'll have a crushed fuselage.

Does it work? My 9-foot Cub can be picked up by the wingtips, struts removed, with virtually no sag. When I recently walked away from a skirmish with a runway marker light (I swear the thing was magnetic!) with only minor cuts and bruises to the Cub, I knew for sure that I had a good thing going with this design.

I recommend it for a simple, but very effective, solution to the problem of attaching wings to your aircraft's fuselage.