An aircraft fitted with fixed wings 14 and a conventional empennage 12, which would be ineffective with the machine hovering or moving forward at very low speed, is rendered stable under such conditions by fitting slotted flaps 15,16 to the wings 14 to deflect downwardly in the direction of the arrows 18, 18 the slipstreams from symmetrically disposed propellers 17 capable of tilting in the pitching plane about axes 21 and located sufficiently forward of the centre of gravity 20 to ensure stability, i.e. by more than 80 per cent of the mean wing chord. In the extreme adjusted positions the propeller thrust lines 23,24 lie above and below the centre of gravity respectively. FIG. 2 shows the disposition of the controls for an aircraft incorporating two pairs of propellers 50, 50b and 62, 62b which are operable conjointly with the normal control surfaces to produce moments augmenting those of the surfaces. Movement of the control column 42 to operate the elevators 45 also effects similar tilting of the inboard propellers 50, 50b in the pitching plane through links 46, 48, 67, 67b, while operation of the wheel 43 controlling the ailerons 56,56b causes a cable loop 58 to rotate a nut 59 to effect differential adjustment of the blade pitches of the outboard propellers 62, 62b. Movement of the rudder pedals 44, 44b results in rotation of a nut 51 by a cable 53 to cause tilting of the inboard propellers 50, 50b in opposite directions. Collective blade pitch adjustment of all four propellers is effected by a hydraulic jack 63. The inboard propellers may be capable of bodily tilting movement, or the blades may be cyclically adjusted in pitch by a swash plate to produce virtual tilting.
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