It's all done with computers (and good old-fashioned hydraulics).
- By Joe Pappalardo
- Air & Space magazine, September 2006
In the 1960s, U.S. Navy strategists wanted an aircraft that could efficiently cruise at subsonic speed, maneuver well in high-subsonic dogfights, accelerate to above Mach 2, and yet remain stable during slow landings on an aircraft carrier. Considering these demands, the Grumman Aerospace Corporation gave them in 1972 the F-14 Tomcat, a fighter that could change the sweep of its wings depending on the widely varying speed regimes.
A variable-sweep wing imitates nature. To glide or slow down, birds extend their wings; to speed up, they tuck them close. But designing those capabilities into a metal airframe, using nuts, bolts, and gears that would mimic a bird’s muscle and bone, took decades of work by aircraft engineers.
The first aircraft capable of varying the sweep of its wings in flight was the Bell X-5, an experimental aircraft used by NASA in the 1950s to test wing angles. It was not the prototype of an operational aircraft, but a testbed to explore the aerodynamic effects of variable-sweep wings. Some of the design and even some parts were cannibalized from the Nazi-engineered Messerschmitt P 1101, a variable-sweep wing aircraft that never flew and was captured by U.S. troops in 1945.
Researchers found that as the X-5’s wings swept from a 20- to a 60-degree angle, the airplane’s center of gravity and center of pressure changed, requiring the entire wing assembly to move toward the nose in order to keep the aircraft stable. To achieve the 40-degree difference, rails inside the fuselage moved the wings about 27 inches forward. It took 20 seconds to complete the change—longer if the electronics malfunctioned and the pilot was forced to hand-crank the wings.
Higher-degree angles in those days presented their own challenges; they tended to make the aircraft more unstable, and the X-5 was notorious for its inability to recover from a spin. In 1953, Air Force Major Raymond Popson was killed when his X-5 spun into the ground, wings in a 60-degree position.
Three decades later, variable-sweep wings became the distinguishing feature of a new aircraft to be flown by both the U.S. Air Force and Navy, the General Dynamics F-111 Aardvark fighter-bomber. The engineering lessons from the F-111 would help create the F-14.
“The aircraft itself was very complex, and we built it without models or simulations,” recalls Chris Clark, who worked on the Tomcat as chief test engineer for Air Test and Evaluation Squadron 23 at Naval Air Systems Command (NAVAIR). “A lot of it boiled down to slide rules and hard paper calculations.”
The Tomcat’s wings could sweep from 20 to 68 degrees. That translates to the wingspan shrinking from 64 feet to 38 feet. The transformation occurred automatically, with the onboard Standard Central Air Data Computer (SCADC) using altitude and Mach number to determine the appropriate wing angle. The F-14 was the only aircraft in NATO that used a computer-controlled, fully automatic sweep. The SCADC activated the hydro-mechanical system that actually moved the wings and optimized wing positions for altitude and speed, but a Tomcat pilot could manually override the system in the event the SCADC did not work.
Each wing of the Tomcat was driven by a single actuator that could sweep at eight degrees a second. A hollow, crossover shaft of aluminum alloy kept the wings in synchronization. The shaft was riveted into an assembly that connected the left and right wing sweep actuator gearboxes. NAVAIR personnel say there have been only two failures of the crossover shaft in 30 years of F-14 operations in the Navy. In both cases, the aircrews landed safely.
The wings themselves were mounted to a titanium structure, called a wing box, that ran across almost the entire dorsal side of the fuselage and was connected to the wings at two pivot points upon which they rotated.
When the wing retracted, about 25 percent of its trailing edge tucked beneath an overwing fairing, which left a gap between the aft section of the wing and fuselage. Inflatable canvas bags attached to the fuselage closed the gap. The bags also provided a smooth contour to blend the wings’ trailing edges and the aft fuselage, allowing a smooth flow of air.
Each wing had a hydraulic motor that moved it either forward or aft. In flight, moving the wings forward required less hydraulic power than moving them back. The hydraulic flow needed to move the wings forward was about 15 gallons per minute and was handled by a fixed displacement pump. The flow needed to move the wings aft was about double that, and was accomplished with a variable displacement pump. The reason for the mismatch was that the positioning of the wing pivot in relation to the wing’s center of pressure made it easier to unsweep than to sweep.
On the F-111, the pivot locations were relatively inboard, resulting in excessive trim drag at transonic and supersonic conditions. Tomcat designers were not going to repeat that mistake.
“In those days, [the Navy] wanted high-altitude maneuverability,” says Tom Lawrence, a NAVAIR aerodynamics expert who evaluated this capability for the Tomcat. “If you had the wing pivots closer to the fuselage, you get a very large shift in the center of pressure” when the wing changes its angle of sweep. That could lead to the kind of instability that killed Raymond Popson in the X-5.
Designers attached the Tomcat’s wings so that the pivots were located at the most outboard position possible, at 8 feet, 11 inches from the fuselage centerline. The result: When the airplane changed shape, less of the wing was actually sweeping.
Though technology improved, the wing design remained basically the same, but Grumman replaced parts of the wing assembly with composite materials better able to handle heat and stress. The airplane’s role changed from chasing fast Soviet interceptors to supporting U.S. ground forces with bombing runs, and the Tomcat began showing its age.
“Back in the 1960s there was a need to vary the airplane’s geometry,” says Captain Don Gaddis of Naval Air Systems Command, a former Tomcat pilot and current program manager for its replacement, Northrop Grumman’s F/A-18 Hornet. On the F/A-18, “we’ve learned how to optimize the wing design so that the aircraft can carry out its functions” without changing geometry.