Tip Tow to the Wingtips
Hitchhiker: Republic F-84 Thunderjet
Mothership: Boeing B-29 Superfortress United States, 1950
World War II “triple ace” fighter pilot Clarence E. “Bud” Anderson was no stranger to dangerous missions, and he wasn’t going to let the end of the war put a stop to his risk-taking. Anderson became a test pilot and took part in a highly unusual project to connect airplanes in flight.
Mothership-parasite attachments had taken many forms since the Bristol Scout was propped on crutches 35 years earlier, and now the U.S. military was testing a new, aerodynamic concept: a wingtip-to-wingtip configuration with the code name Tip Tow.
The idea came from German aircraft designer Richard Vogt, who came to the United States after World War II with a vision to gain something for nothing. He surmised that by attaching temporary fuel panels to the wingtips, an aircraft could achieve increased range for “free,” since the panels providing extra fuel would create their own lift, and the extended spar would reduce the induced drag and create a more efficient wing.
The U.S. Air Force was intrigued, but modified the idea to carry bomber-escort fighters for “free.” They designated the F-84 and the B-29 as the two airplanes for the job, with the ordinarily short-range fighter hitching a ride into faraway enemy territory alongside the bomber.
These craft were not the first to couple via wingtips: The Germans were experimenting in secret with the concept at the end of the war, but left little documentation. When the Americans considered the idea in 1947, they sent Anderson to test the concept with the Culver Q-14 target drone and a piloted Douglas C-47. Maneuvering the simple ring-and-hook coupling system “was like trying to thread a needle in the middle of a fire hose,” wrote Anderson in a 1979 article for the Society of Experimental Test Pilots. After some adjustments, the aircraft pair proved that wingtip coupling could be done, and beginning in 1950, tests proceeded with the F-84 and B-29.
Because the F-84 could take off and land on its own, the hookups were made in-air by a torpedo-shaped lance mounted to one of the fighter’s wings. Making the connection, says Anderson, required precision similar to inflight refueling using the probe-and-drogue system. The F-84 pilot had to insert the lance into an opening approximately one foot in diameter on the B-29’s coupler, which was on a retractable boom 19 inches off the end of its wingtip. Once inserted, the boom pulled the wingtips together into a rubber seal.
Throughout testing, the Tip Tow pilots completed 43 couplings and 15 hours of linked flight, with the longest dual-coupled flight lasting just over two hours. Tests showed that the induced drag reduction of the coupled airplanes, at higher gross weights and lower airspeeds, made them more efficient than a single B-29.
The connection wasn’t always graceful, and the pilot needed to control the F-84’s movement around the connection axis—the “rotation around the wing-tips like a hinge,” Anderson describes in an e-mail. “If it was not controlled properly, it could be dangerous for certain since [the connection] was locked and there was some structural bending going on in the bomber wing structure.”
The coupling mechanism was continually tweaked. At one point it included an automatic flight control system to find the proper damping frequencies that would hold the F-84 at the correct angle, but experiments proved deadly: One coupled F-84 and B-29 crashed when the system was activated and pitched the fighter onto the bomber’s wing. There were no survivors, and Tip Tow was cancelled.