The XBT-1 handled well in level flight, but progressively steeper dives with the brakes open produced severe tail flutter. Heinemann rode behind the test pilot and photographed the tail during repeated dives and nine-g pullouts. His measurements indicated that the horizontal tail tips moved through about two feet. Airflow vortices behind the open flaps disrupted the aircraft's stability, while the tail's strength kept the vibration from tearing it away. Rounding parts of the flap edges made little difference. The bomber could not perform its intended role unless the brakes could open without threatening structural failure or loss of control. Heinemann also recalls that designers commonly accompanied test flights before wartime practice treated the risk as unnecessary.
Notable Point · 2:20:31-2:23:02
Photographing the XBT-1's dangerous tail flutter
Repeated dives and onboard photographs revealed two-foot tail-tip movement, leaving the XBT-1 unable to use its brakes safely.