Group of five airmail covers flown on experimental helicopters, all with stamped cachets filled out with information on the flights, signed by the respective pilots. Helicopters and dates are: Kellett KH-15 Rocket-Powered Helicopter (January 4, 1956); de Lackner Aerocycle DH-4 (January 20, 1956); XRON-1 Rotocycle (April 3, 1956); XROE-1 Rotocycle (May 4, 1957); and Sikorsky's HSS-T-58 Turbo-Shaft Powered Helicopter (September 27, 1957). In overall fine condition.
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