Lockheed U-2 - Design

Design

The unique design that gives the U-2 its remarkable performance also makes it a difficult aircraft to fly. It was designed and manufactured for minimum airframe weight, which results in an aircraft with little margin for error. Most aircraft were single-seat versions, with only five two-seat trainer versions known to exist. Early U-2 variants were powered by Pratt & Whitney J57 turbojet engines. The U-2C and TR-1A variants used the more powerful Pratt & Whitney J75 turbojet. The U-2S and TU-2S variants incorporated the even more powerful General Electric F118 turbofan engine.

High-aspect-ratio wings give the U-2 some glider-like characteristics, with a lift-to-drag ratio estimated in the high 20s. To maintain their operational ceiling of 70,000 feet (21,000 m), the U-2A and U-2C models (no longer in service) must fly very near their maximum speed. The aircraft's stall speed at that altitude is only 10 knots (12 mph; 19 km/h) below its maximum speed. This narrow window was referred to by the pilots as the "coffin corner". For 90% of the time on a typical mission the U-2 was flying within only five knots above stall, which might cause a decrease in altitude likely to lead to detection, and additionally might overstress the lightly built airframe.

The U-2's flight controls are designed around the normal flight envelope and altitude at which the aircraft was intended to fly. The controls provide feather-light control response at operational altitude. However, at lower altitudes, the higher air density and lack of a power-assisted control system makes the aircraft very difficult to fly. Control inputs must be extreme to achieve the desired response in flight attitude, and a great deal of physical strength is needed to operate the controls in this manner.

The U-2 is very sensitive to crosswinds which, together with its tendency to float over the runway, makes the U-2 notoriously difficult to land. As the aircraft approaches the runway, the cushion of air provided by the high-lift wings in ground effect is so pronounced that the U-2 will not land unless the wing is fully stalled. To assist the pilot, the landing U-2 is paced by a chase car (usually a "souped-up" performance model including a Ford Mustang SSP, Chevrolet Camaro B4C, Pontiac GTO, and the Pontiac G8 GT) with an assistant (another U-2 pilot) who "talks" the pilot down by calling off the declining height of the aircraft in feet as it decreases in airspeed.

Instead of the typical tricycle landing gear, the U-2 uses a bicycle configuration with a forward set of main wheels located just behind the cockpit, and a rear set of main wheels located behind the engine. The rear wheels are coupled to the rudder to provide steering during taxiing. To maintain balance while taxiing, two auxiliary wheels, called "pogos" are added for takeoff. These fit into sockets underneath each wing at about mid-span, and fall off during takeoff. To protect the wings during landing, each wingtip has a titanium skid. After the U-2 comes to a halt, the ground crew re-installs the pogos one wing at a time, then the aircraft taxis to parking.

Because of the high operating altitude, the pilot must wear the equivalent of a space suit. The suit delivers the pilot's oxygen supply and emergency protection in case cabin pressure is lost at altitude (the cabin provides pressure equivalent to about 29,000 feet / 8,800 meters). To prevent hypoxia and decrease the chance of decompression sickness, pilots don a full pressure suit and begin breathing 100% oxygen one hour prior to launch to remove nitrogen from the body; while moving from the building to the aircraft they breathe from a portable oxygen supply.

The aircraft carries a variety of sensors in the nose, Q-bay (behind the cockpit, also known as the camera bay), and wing pods. The U-2 is capable of simultaneously collecting signals, imagery intelligence and air samples. Imagery intelligence sensors include either wet film photo, electro-optic or radar imagery – the latter from the Raytheon ASARS-2 system. It can use both line-of-sight and beyond-line-of-sight data links. One of the most unusual instruments in the newest version of the U-2 is the off-the-shelf Sony video camera that functions as a digital replacement for the purely optical viewsight (an upside down periscope-like viewing device) that was used in older variants to get a precise view of the terrain directly below the aircraft, especially during landing.