Airplane Origami: How Folding Wings Work

By Terry Dunn

Terry's recent visit to the Cavanaugh Flight Museum in Dallas sparked an exploration of folding wing design for airplanes.

I recently found myself with a few hours to kill while in the Dallas, Texas area. On the advice of a Tested reader, I made my way to the Cavanaugh Flight Museum, and I'm glad I did. Cavanaugh is my favorite kind of aviation museum to visit. It has a very eclectic mix of static and airworthy aircraft that spans from WWI to the modern era. Several of the airplanes in the museum's collection are combat veterans as well.

Of all the various aircraft vying for my attention, the one that I spent the most time with was a humble-looking former navy machine, the Grumman S-2F Tracker. While I had seen various versions of the cold-war-era S-2 at airshows, this was the first opportunity I'd had to get a good up-close look at its wing folding mechanism.

The many components that are visible in the wing fold of the Grumman S-2F Tracker piqued my interest in the intricacies of folding wing design and operation.

The S-2F was parked outdoors with its wings folded as if it were on an aircraft carrier. I spent several minutes analyzing the various parts that were visible at the wing folds while trying to figure out the purpose of each. The functions of some components seemed obvious, but most remained a mystery. I walked away utterly fascinated by the intricacies of folding wings and determined to learn more.

It's All About Elbow Room

The concept of folding wings is nearly as old as aviation itself. Irish airplane company, Short Brothers, developed a series of biplanes with folding wings prior to the start of WWI. The idea has persevered with most modern naval aircraft, and even the Boeing 777X passenger jet. The goal of folding wings in every instance is to give the airplane a smaller storage footprint when not in use.

The F4F Wildcat was the first airplane to use Grumman's Sto-Wing hinge design, which mimics how birds rest their wings.

Wide-spread implementation of folding wings came about during WWII with the emergence of the aircraft carrier as the prime offensive naval weapon. Folding wings allowed up to 50% more aircraft to be stored aboard these ships. By the end of the war, folding wings were standard equipment on nearly every carrier-based aircraft. There have been very few exceptions in the decades since.

Two examples of a milestone in folding-wing design were located in a hanger right next to the Tracker. The F4F Wildcat and TBM Avenger were WWII aircraft from Grumman that utilized a folding wing design called "Sto-Wing". Rather than simply folding the outer wing panels upward, Sto-Wing mimics the way that resting birds tuck their wings against their bodies.

The wing joint on the Wildcat is very simple. Note the stowed hand crank that is used to fold and extend the wing.

The simultaneous twisting and back-folding of the wing that is accomplished by the Sto-Wing mechanism suggests that there are two distinct motions involved. In actuality, the outer wing panel swings on a single pivot joint through one axis of motion. Legend has it that then-president of the company, Leroy Grumman himself, worked out the geometry of the hinge using paper clips stuck in a soapstone.

The Sto-Wing was first incorporated into the Wildcat. A hand crank on the underside of the wing allowed flight deck personnel to fold or unfurl the fighter's wings. Folding the wings reduced the Wildcat's wingspan from 38 feet to 14.3 feet. Subsequent Sto-Wing-equipped aircraft used hydraulics to actuate the folding mechanism.

Grummans's TBM Avenger was the first Sto-Wing aircraft with a hydraulically operated wing joint.

The fundamental aspects of Grumman's original Sto-Wing hinge design are still being used today on the E-2 Hawkeye and the C-2 Greyhound. The E-2 is the US Navy's airborne early-warning radar platform. Although the Greyhound has a different fuselage to accommodate its role as a cargo hauler, it has essentially the same wing as the Hawkeye. These airplanes replaced the E-1 Tracer and C-1 Trader (both were variants of the S-2) during the 1960s. The Hawkeye and Greyhound are still in production 50 years later.

Insider Viewpoints

To further my knowledge of folding wings, I contacted experts who know the E-2/C-2 wing inside and out. Will Dossel is a retired navy captain who served as a naval flight officer. He logged more than 3500 hours in the E-2 with 525 carrier landings. Tom Hunt is a senior vehicle engineer at Northrop-Grumman. He has worked on several aspects of the E-2 and C-2 during his 35 years with the company.

The Grumman C-2 Greyhound is an active carrier-based aircraft that utilizes the Sto-Wing design. (photo courtesy of Brian Lockett, Air-and-Space.com)

Airplane wings are subjected to enormous structural loads both in flight and on the ground. This is especially true for carrier-based airplanes that undergo catapult launches and arrested landings. Constructing a wing to adequately withstand these loads and also fold for storage requires beefier-than-normal construction. I asked Hunt if there are any rules of thumb that approximate how much heavier a folding wing would be in comparison to a static wing of the same size:

"I couldn't tell you what we're paying for in extra weight by having to fold the wing, but it's not insignificant. But if it's a carrier airplane, it's got to [have a folding wing]. You have to make the assumption that if you want to save weight, you've got to do it in other places. You can't have a folding wing and not pay that penalty. There's no magic there."

Both Dossel and Hunt used superlatives when describing the substantial structural components that make up the wing joints. Hunt elaborated:

"There are very large fittings on each side of that joint for accepting the huge, hydraulically-actuated pins that lock the wings in place. There are two pins on the front spar, two pins on the center spar, and the rear spar is the pivot. While it is not highly loaded, the pivot is also a loaded structure. It is not just a simple hinge that supports the weight of the wing when it is swinging. It has to carry some aerodynamic load too."

The E-2/C-2 wing does not have fuel tanks in the folding panels. The additional weight of fuel would necessitate overly large and heavy wing joints. Hunt pointed out that "wet" outer wings do not typically jibe with the stresses seen by carrier-based aircraft.

"Logistically, if you made the wing joint strong enough to carry any fuel in the outer wing panels, you'd have to make sure that it [the fuel] was gone when you came home. You don't want all of that mass hitting the deck. The other issue is that the fuel lines would have to cross the break, and that's not an easy thing to do."

The flight manual for the E-2 indicates numerous areas of the wing joint that require attention during pre-flight inspection of the airplane.

Despite the absence of fuel tanks, there are other, non-negotiable systems in the outer wing such as control surfaces and lights. The electrical wires, control cables, and hydraulic lines that link these components to the cockpit must somehow bridge the wing fold. The methods for routing systems through the wing joint that Grumman has employed over the years range from ingeniously simple to seemingly Golbergian. In the case of the E-2/C-2, the myriad systems with junctions at the wing break make it a complex area of the aircraft. The navy's flight manual for the E-2 indicates more than 20 individual areas on each wing joint that must be checked out during every pre-flight inspection.

The E-2/C-2 wing is built using the same robust construction philosophy that earned the factory its nickname, "The Grumman Iron Works". Yet, there are practical limits to what the wing's pivot joint can withstand. Hunt relayed the story of an E-2 crew that began folding the wings as they sped over a rough patch of taxiway at a naval air station. The subsequent jostling of the unsecured wings caused one of pivots to fail and the attached wing fell to the ground.

Fitting In

With a wingspan of more than 80 feet, the amount of flight deck real estate that an E-2 or C-2 requires makes the folding of its wings a critical function. According to Dossel, the wings are kept stowed until just before the aircraft is attached to a catapult for launch. Likewise, the wings are folded as soon as the tailhook is released from the ship's arresting cable after landing.

Dossel pointed out that an E-2/C-2 with mechanical problems can create havoc on a carrier's flight deck. "That has always been the aircraft handler's nightmare…that you have an E-2, right after it traps [lands] that is unable to fold the wings. There isn't enough room on a flight deck, with an air wing on board, to try and taxi an E-2 with the wings spread."

This E-2 is seen folding its wings while taxiing. In this position, the entire weight of the each outer wing is supported by a single hinge joint. (photo courtesy of Brian Lockett, Air-and-Space.com)

Even when there are no apparent problems with the hydraulic system, the folding mechanism sometimes needs a little boost the get the 3000-pound outer wing panels into position. Hunt indicated that in videos showing launch operations of early model Hawkeyes, you can often find a broom leaning against the ship's island. The broom was used to push upward on the Hawkeye's wingtips and help the folding joints lock into place before launch. Dossel noted that even recent versions of the Hawkeye sometimes require a little human intervention to get the wings locked into their stowed orientation.

Blowin' In the Wind

Quite obviously, folding the wings on the E-2/C-2 renders the aircraft incapable of flight. Less obvious is the dramatic impact on the airplane's ground handling caused by stowing the wings. The upturned and rearward-swept wing panels add a tremendous amount of vertical surface area to the aft end of the airplane. This gives the Hawkeye a profound tendency to behave like a weathervane and point itself into the prevailing wind while parked or taxiing.

The difficulty of taxiing the Hawkeye is exaggerated by the rearward weight shift that accompanies folding of the wings. As the heavy outer panels swing aft, less weight is placed on the nose wheel, which reduces its steering authority. This was among the reasons Dossel listed that his crew rarely folded the airplane's wings when they landed at shore bases, where the plentiful parking space did not often necessitate it.

The E-2 gets a little light on the nose when the wings are folded. The flight manual warns pilots of a potential consequence.

The E-2 flight manual contains other dire warnings regarding ground handling with the wings stowed. One section points out that it is possible for the 25-ton airplane to be blown sideways across the flight deck when it is broadside to the wind. Dossel explained:

"If you're on a carrier that's been on an extended deployment, the non-skid that's on the flight deck has been pretty well worn down and saturated over the course of months with oil, grease, and everything else…especially if you've been operating in a hot climate. You tend to have greater issues from about mid-cruise on in that case. Literally, you could be…with a strong enough wind…sliding across the deck."

Folded wings even have an effect when parking the Hawkeye and Greyhound. The adjustable-pitch propellers can be set to provide reverse thrust for backing the airplane into a parking spot. The flight manual warns that if the pilot hits the brakes too hard while reversing, the aircraft may tilt back onto its tail. If this happens while the tail is overhanging the edge of the flight deck, well, it's a long way down.

Closing Thoughts

I'd like to express my sincere thanks to Will Dossel and Tom Hunt for sharing their E-2/C-2 experiences with me. They revealed numerous aspects of folding wings that I had never considered. I have a feeling that my education on this topic is only just beginning.

Terry spent 15 years as an engineer at the Johnson Space Center. He is now a freelance writer living in Lubbock, Texas. Visit his website at TerryDunn.org and follow Terry on Twitter: @weirdflight