A propeller system comprising: a pair of rotors and a pair of blade sets (24, 28) and at least one drive input to drive a first of said rotors and blade sets and a second of said rotors and blade sets; said blade sets (24, 28) being positioned such that when both are driven, air will be driven across said first blade set, and then across said second blade set; there being a pitch change mechanism (40, 41) to change an angle of incidence in at least one of said first and second blade sets (24, 28), and there being a device for selectively stopping rotation of said at least one of said first and second blade sets (24, 28) while still allowing rotation of the other of said first and second blade sets (24, 28); and said pitch change mechanism (40, 41) of said at least one of said first and second blade sets (24, 28) still allowing change of said angle of incidence when said device has stopped rotation; wherein a control selectively changes the angle of incidence of said one of said first and second blade sets (24, 28) and also selectively controls said device, while said at least one of said first and second blade sets has its rotation stopped; said selective change includes changing the angle of incidence as flight conditions of an associated aircraft change, and one of the two following changes occur; a) when said second rotor and second blade set are stopped from rotating, said second blade set acts as guide vanes positioned to turn a swirling flow of air from said first blade set generally in an axial direction, to convert energy from the swirling flow into thrust; and b) when said first rotor and first blade set is stopped from rotating, said second blade set being positioned to swirl incoming flow such that flow exiting the second blade set is more pre-dominantly in an axial direction, thus generating more thrust; said control causing said at least one of said first and second blade sets (24, 28) to be driven when at higher power operation of the associated aircraft, and stopping rotation of said one of said first and second blade sets (24, 28) at lower power operation of said aircraft; wherein said higher power operation includes a take-off condition for the associated aircraft; and wherein said lower power operation includes a cruise condition of the associated aircraft. a pair of rotors and a pair of blade sets (24, 28) and at least one drive input to drive a first of said rotors and blade sets and a second of said rotors and blade sets; said blade sets (24, 28) being positioned such that when both are driven, air will be driven across said first blade set, and then across said second blade set; there being a pitch change mechanism (40, 41) to change an angle of incidence in at least one of said first and second blade sets (24, 28), and there being a device for selectively stopping rotation of said at least one of said first and second blade sets (24, 28) while still allowing rotation of the other of said first and second blade sets (24, 28); and said pitch change mechanism (40, 41) of said at least one of said first and second blade sets (24, 28) still allowing change of said angle of incidence when said device has stopped rotation; wherein a control selectively changes the angle of incidence of said one of said first and second blade sets (24, 28) and also selectively controls said device, while said at least one of said first and second blade sets has its rotation stopped; said selective change includes changing the angle of incidence as flight conditions of an associated aircraft change, and one of the two following changes occur; a) when said second rotor and second blade set are stopped from rotating, said second blade set acts as guide vanes positioned to turn a swirling flow of air from said first blade set generally in an axial direction, to convert energy from the swirling flow into thrust; and b) when said first rotor and first blade set is stopped from rotating, said second blade set being positioned to swirl incoming flow such that flow exiting the second blade set is more pre-dominantly in an axial direction, thus generating more thrust; a) when said second rotor and second blade set are stopped from rotating, said second blade set acts as guide vanes positioned to turn a swirling flow of air from said first blade set generally in an axial direction, to convert energy from the swirling flow into thrust; and b) when said first rotor and first blade set is stopped from rotating, said second blade set being positioned to swirl incoming flow such that flow exiting the second blade set is more pre-dominantly in an axial direction, thus generating more thrust; said control causing said at least one of said first and second blade sets (24, 28) to be driven when at higher power operation of the associated aircraft, and stopping rotation of said one of said first and second blade sets (24, 28) at lower power operation of said aircraft; wherein said higher power operation includes a take-off condition for the associated aircraft; and wherein said lower power operation includes a cruise condition of the associated aircraft.