sentences:  Service use of the T40 revealed problems with integrity of the propellers and gearboxes with at least one instance each of a propeller breaking free and a gearbox separation.One unusual problem was discovered during ground running of the Republic XF-84H Thunderscreech turboprop powered fighter; the special transonic 3-bladed single propeller set up harmonics that proved to be harmful to humans within a certain distance from the aircraft.The major problems of the T40 were its fragile gearbox, and the propeller control system which used 25 vacuum tubes, and was far from reliable.