Grumman Sto-Wing Redux

Click HERE to see a summary of Grumman's innovative wing-fold concept that it called the sto-wing.

Pat Donahue wrote to ask me how the aileron control mechanism spanned the large gap created when the wing was folded alongside the fuselage. He had looked at the wing-fold area of the F6F at the National Naval Aviation Museum at Pensacola without being able to determine how it was done.

 Uhh - that's a good question. As it turns out, Grumman created three different ways to do it, although the original one, on the F4F Wildcat, appears to be the only one that was used on more than one Grumman design. (Click on the images for a bigger picture.)

In the closeup of the area encircled in green on the first picture, you'll see a rod coming out of the wing stub that pushes/pulls on a bellcrank that has two contact points on it on either side of its pivot point. If you then look at the inboard end of the outboard wing section in about the same location, you'll see another bellcrank that has an upper and lower arm connected by two small flat plates that correspond to the contact points on the bellcrank mounted on the wing stub. It's hard to see, but the left side (aft side when the wing is in the flight position) of this bellcrank is attached to a rod that disappears into the outer wing panel.

In other words, the ailerons are disconnected at the fold joint when the wings fold down and aft. When the wing is in the flight position, the two bellcranks have come together as a unit so the push/pull tube in the wing stub is pushing and pulling on the push/pull tube in the outer wing panel.

This would seem 1) difficult to rig without loss motion or overload of the bellcranks in compression and 2) to require some means of restraining the ailerons when the wings were folded. Perhaps in recognition of these drawbacks, Grumman used a different concept on each of its next two designs.

The TBF took me a while to figure out, although the mechanism is hidden in plain sight, right where Pat thought it should be, at the wing panel pivot axis.

The Grumman Archives came to my rescue with a maintenance manual. The TBF aileron control system changed from a push-pull mechanism to a cable system in vicinity of the fold joint. Two pairs of pulleys located at the pivot axis were used to transfer the motion across the fold joint This illustration depicts the pulleys on the right wing when the wings are spread:

One pair was mounted on the outer wing panel and the other on the wing fold actuator link, with the cables crossing between the two pairs so as to not introduce any slack or tension in the cables in the folding process. All that is visible are the pulleys and cables under the link that the two wing-fold hydraulic actuators are attached to. The following illustration shows the right wing pivot axis with the wings folded, looking inboard. Note that the transition between the two sets of pulleys is on the wing pivot axis.

For the F6F Hellcat, a link between two bellcranks was used to span the gap between wing stub and outer wing panel when the latter was folded back. The bellcrank in the wing stub and one in the outer wing panel were cleverly shaped and positioned so that when the wings were folded, the control stick imparted little or no motion to the ailerons. (Note that the outboard end of the gap-spanning link was positioned on the wing pivot axis.) Left wing folded:

 

Wing Spread:

The two concepts that did not break the connection between control stick and the ailerons when the wings were folded would appear to be more desirable, particularly the approach used on the F6F, but Grumman and/or the Navy thought otherwise for some reason. Grumman reverted to the original concept used on the F4F for its subsequent designs incorporating the sto-wing: the AF Guardian, WF (E-1) Tracer, and E-2 Hawkeye.