Aircraft that can alter their wing configurations in mid-flight have been in development since World War II. With different wing positions allowing for greater efficiency and performance in various flight modes, these aircraft are more versatile than aircraft with fixed wings. Although a few models have made it into production, the limitations of engines, mechanics and computers have kept these aircraft from coming into widespread use. Now technology has finally caught up with the concept, and Northrop Grumman is in the process of building an unmanned shape-shifting plane: the Switchblade.
If you look at aircraft from World War I or World War II, you'll notice that the wings are almost always perpendicular to the fuselage, with only a few degrees of backwards sweep, if any. At the time, airplane engines couldn't propel the planes any faster than about 375 mph. At these low speeds, a perpendicular wing configuration allowed for maximum lift and maneuverability.
After World War II, the development of jet engines led to an enormous boost in aircraft speed. Traditional wing shapes weren't as efficient at high speeds (particularly supersonic speeds), so jet fighters began sporting tapered wings. The F-4 Phantom II is a good example of this type of wing profile. However, this increase in high-speed performance came with a trade-off -- the planes were not very effective or efficient at lower speeds.
An aircraft that can alter its wing configuration in mid-flight has variable-wing geometry. This gives the plane the best possible performance characteristics at a given speed. The German Messerschmitt company first tested planes with variable wing geometry during World War II. The Messerschmitt P-1101's wings could be moved to different sweep angles, only while the plane was on the ground. Based on the Messerschmitt design, the U.S. developed a working test craft, the Bell X-5, which was slightly larger than the P-1101 and could change its wing-sweep angle while in flight.
This type of technology, also known as "swing wing," first appeared in a production aircraft in the late 1960s with the General Dynamics F-111. This plane had three different wing positions to give it maximum efficiency at all speeds. In fact, a variation of this plane, the FB-111A strategic bomber, carried the unofficial nickname "Switchblade." [ref].
Several other fighter jets used variable-geometry wings in the next few decades, including the Tornado and the F-14 Tomcat. The unswept wing position made these planes exceptionally maneuverable and easier to land on the short flight decks of aircraft carriers. However, the mechanisms required to make the wing movements function were complicated and heavy. They also took up a lot of space, cutting down on efficiency and payload. Swing wings were largely abandoned for more simplistic designs.
In the next section we'll look at the Switchblade plane design.
Switchblade Plane Design
Designers of planes with variable-geometry wings also had to consider agility. While sweeping the wings back made for more stable aircraft at high speeds, forward-swept wings allowed the aircraft to be exceptionally agile, an especially desirable characteristic for fighter aircraft. The German Ju-287 was the first aircraft with forward-swept wings, but the X-29 is the most well-known example [ref]. While this drastic configuration rendered the X-29 unstable, computers could control the aircraft with fly-by-wire flight control systems. The next swept-wing aircraft incorporated aspects of the previous designs.
In the 1990s, Northrop Grumman tested variable-geometry wings on another plane with the "Switchblade" nickname. The Northrop Bird of Prey had three wing configurations:
- full-back position - The wings were perpendicular to the fuselage for low-speed flight.
- intermediate position - The wings were swept forward for exceptional maneuverability.
- full-forward position - The leading edge of the wings folded in against the fuselage, allowing the trailing edge to become the front of the wing for high speeds. This resulted in a triangular, or delta wing shape.
Patents for the variable-wing geometry are public knowledge, and there were reports of a test squadron flying these aircraft. Documents showing the existence of a Bird of Prey test craft were declassified, but this declassified craft did not incorporate "swing wing" technology. Details of the three-position "Switchblade" Bird of Prey remain classified.
The Defense Advanced Research Projects Agency (DARPA) is the Pentagon's high-tech military technology research branch. It has allocated $10.3 million to Northrop Grumman to develop a Switchblade preliminary design by late 2007. The company beat two other proposals for the design project, and work will proceed at Northrop Grumman's El Segundo, California headquarters. Northrop Grumman plans a scaled-down test model with a 40-foot wingspan for 2010, with a full-size, fully-functional Switchblade ready for flight in 2020. As the project moves into the scale and full-sized phases, costs will likely escalate into the billions of dollars.
Next, we'll look at how wing position affects a plane's performance and learn more about the Switchblade.
Before we look at the specific technology involved in the design of this new Switchblade, we'll discuss exactly how the position of a plane's wings affect its performance.
Unswept wings are efficient at low speeds, providing a great amount of lift compared to the amount of induced drag exerted on the plane. Induced drag is essentially another component of the force that allows the plane to fly. As air flows around the wings, the resultant turning of the airflow deflects the plane up, counteracting gravity pulling the plane down toward the ground. However, some of that force also resists the plane's forward movement, resulting in drag. As speed increases, this drag becomes even more problematic.
As an aircraft approaches and passes the speed of sound, shock waves form (pressure waves heard as a "sonic boom" by observers if the plane is supersonic). These waves create a second form of drag known as wave drag. When a shockwave forms, it changes the aerodynamic profile of the plane. Instead of a streamlined aircraft shape cutting smoothly through the air, this large pressure wave adds a bulky impediment that must be pushed through the air. It's sort of like running into the wind carrying a mattress. Unswept wings are very bad at dealing with wave drag.
Swept wings cut down on drag caused by turbulence at the wingtips. But the real advantage of swept wings comes in supersonic flight -- the configuration cuts down on wave drag by redistributing the shock waves along the plane's aerodynamic profile. They are ideal for these high-speed conditions. Unfortunately, they do not allow for heavy payloads at lower speeds. Swept wings are also inefficient and burn too much fuel to stay aloft, which reduces the range of the aircraft.
So why is the U.S. military bringing back variable-geometry wing technology? Technological advances mean better wing transition mechanisms, advanced wing shapes and computer systems that can control unstable aircraft. The Pentagon has identified a need for a plane that can remain aloft for long periods close to enemy territory, and then switch to a high-speed mode to rush in and deliver a blow before rushing back out at supersonic speeds. These two modes of flight require drastically different wing profiles for maximum efficiency. Northrop Grumman's new Switchblade is unlike any "swing wing" aircraft previously imagined.
In the next section we'll look at the Switchblade technology.
This Switchblade is a flying wing with a pod attached underneath to carry the engines, surveillance equipment and weapons. It doesn't have fuselage, a tail, tail fins or other extraneous parts: it's literally one giant wing. For most of its mission, the Switchblade will cruise at high altitude for as long as 15 hours, waiting for the signal to strike. For that part of the mission, the wing will be perpendicular to the direction of flight, like a traditional aircraft. This will minimize fuel burn, and maximize the time aloft, much like a glider.
When the time comes for a strike, the entire wing will pivot 60 degrees relative to the direction of flight. This will leave the right wing tip pointing forward, while the rest of the wing slants back. The resulting aerodynamic profile will be ideal for a high-speed assault -- in this case, up to Mach 2 for a distance of 2,500 miles. This type of pivoting wing is an oblique wing design. With the wing in the oblique position, supersonic shock waves disperse, rather than "piling up" in front of the craft and creating drag.
The Switchblade will have a 200-foot wingspan. The pod suspended beneath the wings will hold two advanced jet engines, cameras, flight computers and any missiles or bombs required for the mission. It won't have a cockpit, because it won't have a pilot. Flight control computers will handle all the flying because of the unstable wing configurations. This also prevents problems with pilot fatigue during extremely long missions.
For lots more information about the Switchblade project, flying wings and variable-wing geometry, check out the links on the next page.
The Switchblade is not the first attempt to develop an oblique wing aircraft. A NASA wind tunnel project conducted in 1979 showed that a pivoting wing could increase fuel efficiency at supersonic speeds by as much as 100 percent [ref]. Aircraft designer Burt Rutan developed a AD-1 oblique wing prototype that helped prove the viability of an oblique wing system. (Rutan is most famous for designing SpaceShipOne, the first privately owned and funded craft to carry a human into space.)
The AD-1 allowed the wing to pivot gradually as speed increased, always positioning it for maximum efficiency at the plane's present speed. NASA hoped that the technology could lead to a more efficient supersonic commercial transport. However, testing revealed that an aircraft became extremely unstable as the wing moved into an oblique position. A human pilot could not cope with the constant, minute adjustments necessary to maintain flight under these conditions. At the time, flight control computers were not sophisticated enough to manage it, either.
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More Great Links
- AD-1 Oblique Wing http://www.dfrc.nasa.gov/gallery/Photo/AD-1/HTML/index.html
- DARPA Oblique Flying Wing http://www.darpa.mil/tto/programs/ofw.htm
- "F-22 Raptor Cost." GlobalSecurity.org, March 15, 2006. http://www.globalsecurity.org/military/systems/aircraft/f-22-cost.htm
- "The FB-111A 'Switchblade' History." FBI-111a.net, June 16, 2006. http://www.fb-111a.net/History.html
- Shachtman, Noah. "U.S. Miltary Plans Shape-Shifting Supersonic Bomber." Popular Science, July 2006. http://www.popsci.com/popsci/aviationspace/0f2505a52aceb010vgnvcm1000004eecbccdrcrd.html
- Cramer, M.S. "Sonic Boom, Sound Barrier, and Condensation Clouds." Fluid Mechanics, Cambridge University Press, 2004. http://www.fluidmech.net/tutorials/sonic/soundbarrier.htm
- "Switchblade." AirAttack.com, February 1, 2006. http://www.air-attack.com/page/9/Switchblade.html
- "USA: Northrop Awarded Contract to Develop a Conceptual Design." TMC.net, March 19, 2006. http://www.tmcnet.com/usubmit/2006/03/19/1470791.htm