Project MX-1016 Tip-Tow
During World War II, fighters, such as the legendary P-47 Thunderbolt and P-51 Mustang, escorted bombers over their targets. However, the postwar jet-propelled fighters, such as the F-80 Shooting Star and F-84 Thunderjet, used fuel at a much higher rate and were, therefore, range-limited. They could no longer escort the bombers.
To be sure, the Air Force regularly transferred fighter units overseas, particularly after the outbreak of the Korean War on June 25, 1950. The standard method involved using U.S. Navy or Navy contact vessels -- primarily World War II-era escort aircraft carriers -- to physically ship the aircraft. This took weeks, and -- more often than not and despite protective efforts -- upon arrival the fighters required extensive maintenance because of salt air exposure and corrosion.
The answer was simple: find a way to extend the range of fighter aircraft. Initial efforts included projects with the names of Tip-Tow, Tom-Tom and FICON (for "Fighter Conveyor"). These did not involve actual in-flight refueling, but instead involved literally "towing" fighter aircraft, albeit under rather unusual circumstances.
The principle of wing-tip coupling, whereby a small aircraft is towed from the wing tips of a larger aircraft, is not new. First recorded experiments were in Germany in 1944-45. In 1949 and 1950, the Wright Air Development Center at Wright-Patterson Air Force Base developed and tested a system in which a Q-14 aircraft was towed on one wing tip of a C-47 aircraft.
Starting in August 1949, Major Clarence E. “Bud” Anderson did remarkable experiments with a system of wing tip coupling of two aircraft in flight. The smaller single-seat Culver Q-14 Cadet had a pointed lance mounted on its wing tip and flew that into a ring, mounted on the C-47 wing tip, as such making a coupled pair. This increased the aspect ratio of the wing, with more wing surface from another plane, offsetting the additional drag of the smaller plane. The the smaller plane could fly ‘for free’ with its engine shut off, as a parasite coupled with the larger plane.
The advantages of such a system are obvious. The range of the towed aircraft, for strike purposes or reconnaissance purposes, is increased enormously. Moreover, this range increase is obtained at but slight expense to the carrier range because the increase in wing aspect ratio of the composite configuration reduces induced drag below that for the carrier alone. Project Tip-Tow employed a modified B-29 and two F-84Ds, which attached to the bomber at either wingtip through a clamping device. Project Tom-Tom, tested in 1953, was similar and involved the coupling of RF-84F reconnaissance aircraft to the wing tips of a modified B-36. The FICON proposal involved the actual carriage of a modified RF-84F in the bomb bay of a B-36, slung beneath a trapeze.
Despite 15 hours of coupled flying time with both EF-84Ds attached, on 24 April 1953, disaster struck. During testing, flying the fighters in close proximity to large bombers and hooking up proved supremely challenging, even in perfect weather. Right after the autopilot was switched on in a left wing tip coupling between an EB-29A and a F-84D, the nose pitched up and the F-84D Thunderjet rolled suddenly to the right over its fixed coupling or “hinge” with the mother ship. In that flip, the F-84 turned upside down and hit the outer wing panel of the B-29. Explosive bolts for jettisoning the F-84 went off but too late to release the Fighter in time and prevent her from slamming into the Bomber’s outer wing. The Super Fortress’ left wing folded due to the impact and the combo crashed. Fighter pilot John Davis and 5 crew members in the B-29 were killed [some sources report one crewmember survided] in this fatal accident.
This fatal crash involving the Tip-Tow B-29 and one of the F-84s in April 1953 reinforced this and led to the cancellation of Tip-Tow. Doing such hook-ups operationally, possibly in combat and most likely at night and in bad weather, made the efforts even more risky. Fortunately, advances in air refueling of fighters made all three of these difficult and complex "towing" methods superfluous.
It was a dangerous feat to hook the fighter to the wing tip of the Peacemaker by flying the fighters' wing tips through the vortices streaming from the bomber's wing tips. When an airplane wing is moving through the air, there is an area of low pressure over the top of the wing and an area of high pressure under the wing. This lift enables the aircraft to remain airborne. At each tip of the wing, therefore, the air under the wing flows up into the low pressure area on top, forming a whirl or vortex at the wing tip.
The flight of aircraft generates vortices at the wing tips. Such vortices are the result of the flow of air around the wing tip, that is, usually from bottom to top thereof, and with a lateral component relative to the direction of travel or flight of the craft. By the 1930s it was recognized that this vortex added considerable disturbance and drag to the airplane. Hence, if this vortex was reduced or eliminated the drag would be decreased and the speed of the airplane increased.
Such vortices persist in the wake of the flight of aircraft for an appreciable period of time before they are dissipated. The wake or vortices or turbulence of a craft are characterized by counter-rotating spirals of air currents which appear to behave much like horizontal tornadoes. Aircraft flying in the vortex wake of a large commercial or military transport can be subjected to unexpected rolling effects or downdrafts due to these air currents with resulting dangerous effects.
As a wing moves through the air, lift is generated when a low pressure field is hydrodynamically produced on the top surface or suction side of the wing and a relatively higher pressure field is produced on the bottom surface or pressure side of the wing. This large pressure difference between the two sides of the wing causes the fluid to accelerate around the tip of the wing from pressure side to suction side, thus forming a vortex. For a lifting wing, the air pressure on the top of the wing is lower than the pressure below the wing. Near the tips of the wing, the air is free to move from the region of high pressure into the region of low pressure. The wing tip vortices produce a downwash of air behind the wing which is very strong near the wing tips and decreases toward the wing root. This vortex trails from the wingtip and remains relatively strong for many chord lengths downstream. The dual wakes from big aircraft are each usually eight to ten feet wide, tubular-shaped and counter-rotating at a velocity of 100 to 200 feet or more per second.
Winglets oppose the drag wingtip vortices create by harnessing the vortices' airflow. NASA engineer Richard Whitcomb ["the" Richard Whitcomb who previously had helped pave the way for supersonic flight by narrowing the fuselage in what's known as the area rule] pioneered the technology in the 1970s, and they've become a fixture on almost every modern jet.
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