Abstract: A tilt-rotor aircraft (1) comprising a pair of contra-rotating co-axial tiltable rotors (11) on the longitudinal center line of the aircraft. The rotors (11) may be tiltable sequentially and independently. They may be moveable between a lift position and a flight position in front of or behind the fuselage (19).

Abstract: An apparatus for generating a digital count proportional to the frequency of a pulsed input signal has two binary rate multipliers both of which provide output frequency signals dependent on the instantaneous count in a counter. A first one of the multipliers is responsive to a fixed input frequency and the second multiplier is dependent on the output frequency of the first multiplier. The value in the counter is decremented from an initially-set maximum by the output of the second multiplier. The apparatus is operated for a time dependent on the cycle time of the unknown pulsed input frequency. Decrementation of the count is inversely proportional to time. The count is thereby directly proportional to the unknown frequency.