On Naval aircraft carriers, real estate is at a premium in spite of their huge size. Consequently most naval aircraft have folding wings which reduce the area required to park them, whether on the flight deck, the hangar deck, or the elevator which runs between decks. The tiny Douglas A4D Skyhawk was an exception to this rule.
One of my early assignments was to find out why the eyebolts of the hydraulic wing-fold actuators of the Douglas AD (A-1) Skyraider were sometimes failing in service during the Vietnam war.
An interesting bit of information came to light. There were six bomb-rack stations outboard of the wing-fold hinge line on each side of the airplane. It was planned that the wings would be in their spread position when the aircraft was armed by hanging bombs on those outer station. However, the Marines added their own twist to the procedure. At their leisure, they would hang the bombs with the wings spread, then fold the wings, with bombs attached, so the plane was ready for rapid deployment. Of course, this greatly increased the loads on the folding mechanism.
The eyebolt, or end attachment fitting, would sometimes break at a transitional area between the “eye” and the threaded shank which attached to the piston rod. Lead engineer Bob Cole suggested that the rough as-forged surface finish of the steel might be responsible, causing stress concentrations which could initiate cracks..
I reviewed the original design calculations, and recalculated the stresses with those bombs hanging on the outboard wing panels. Sure enough, the eyebolt was calculated to be strong enough. So I had a test eyebolt polished up in the failure area so there was no stress concentration, I believe the surface roughness was defined as 32 microinch, or possibly 16 microinch finish. Then I installed the test actuator in a fixture which allowed loading the actuator to the new higher loads while cycling it through a life-cycle test (I believe it was 20,000 load cycles.) It passed, and subsequently all those eyebolts were reworked to have the smooth finish.
We got word one day that a navy pilot, probably hung over, had climbed into a Skyraider, and neglecting to spread the wings, took off. He managed to get some altitude but couldn’t control the airplane, which crashed. I was asked to calculate whether in such a situation the wings could be spread while in flight. The answer was no, the wing-fold hydraulic cylinder had nowhere near the power to spread the wings in flight. Sorry fellows, use your checklist (and/or common sense).
The Douglas A3D Skywarrior was a greater challenge. This was the largest carrier-based airplane up to that time. The original designers had come up with a simple wing folding design whish rotated the outer panel about 135 degrees from the spread position. The navy representative said, “You don’t understand- the overall height of the airplane with wings folded must be less than the ceiling height of the hangar deck. There can be no waiver or compromise of that requirement.”
This called for some ingenious design, and one of the engineers proposed a mechanism using a “four-bar linkage” which would allow folding the wings about 160 degrees. He showed us a sketch of how this would work as the outer wing panel rotated. Even with the sketch, we had trouble visualizing how it worked, so he redrew his sketch, holding the outer wing panel still and rotating the airplane through a 160 degree arc. Now it became clear to us how it would work.
When the first few airplanes rolled out the door, something was obviously wrong. The outer wing panels would be folded at different angles. It was hard to believe that the production crew would roll those airplanes out without asking for help or advice on this obvious problem.
When it was reported to us, engineer Gordon Buchan and I went to the station on the production line where the wing rigging was performed. (Engineering had written a detailed procedure for rigging.) We asked to be allowed to witness a rigging procedure. It soon became clear to us that they had misinterpreted the procedure. One pivot bolt of that four-bar linkage wasn’t being installed until after the rigging procedure was performed. We very politely explained the correct procedure to them, and from then on, the folded wings matched each other.
Those first few airplanes were soon undergoing carrier trials. We got an alarming message from Larry McBee, our engineering representative on the aircraft carrier, observing these trials. He said that when the planes were being readied for catapulting, (take-off), the airplane in the number two position, with wings still folded but subject to the jet-blast of the airplane on the catapult, the tips of those outer wing panels were bouncing up and down by four feet. Not good.
It was decided that a hydraulically actuated latch mechanism was needed to secure the wing panel in the folded position,, and engineer Harry Pingel was to be the designer. Harry’s design was a beefed-up version of the latches we used to hold landing-gear doors closed in flight. The latch would engage a strut which protruded from the outer wing panel when in the folded position. I was tasked to create a test fixture which would simulate the loading this mechanism would experience in the airplane.
My test fixture duplicated about a square yard of wing, including the latch mechanism, with a hinged stub of “outer wing” using hydraulic cylinders to produce the weight load. This test installation uncovered an immediate problem.
The wing-fold actuator had a poor mechanical advantage when the wing was fully folded. The latch mechanism had to give the wing panel a boost for the first few degrees of motion, until the main actuation cylinder could take over. But the latch produced a side-load which broke that protruding strut. I proposed a fix, and was told ,“Do it!” This consisted of designing the latch hook with a new contour which would keep the “boost” load parallel to the outer-wing strut, minimizing the sideload. It worked. The life-cycle test was also successful.
We had a “static-test” (non-flying) A3D, and we had the new latch mechanism installed on one wing. Ed Heinemann, our chief engineer, was famous for his hands-on tests, and when the new installation was ready, he was informed. So Ed and his assistant chief engineer Leo Devlin came out and climbed the ladder we had set up for them to get on top of the wing. They put their effort into trying to deflect the outer wing panel, but there was no perceptible motion. In less than 30 seconds, they were climbing down, satisfied that the flapping wing problem was solved.
There were 282 A3D Skywarriors, in several versions (bomber, tanker, electronic countermeasures) delivered to the Navy starting in 1956. The last one was retired from service thirty-five years later, in 1991.