The Control System
The craft is unique in not having rudders. However the float pylons do provide a large amount of fin area which ensures that the craft will, provided it is in a symmetrical configuration, always go (relative to the air ) in the direction in which it is pointing. For this reason alteration of course can be achieved without bank (a flat turn) by using differential engine thrust to yaw the craft. This yawing action will put incidence on the pylons and generate centripetal force to alter the course of the craft. Equally a conventional turn can be made by the application of bank. However both methods suffer the over-riding limitation that the outer tip airspeed is limited by the power available while the inner tip must maintain a safe margin over the stalling speed. Experiment will be necessary to choose which, or a combination of both, achieves the quickest time for 180°. At best guess the turn will take about 5.5 minutes.
For maximum turning rate turn the outer wing tip must be powered up to maximum air speed and the inner tip reduced to minimum safe air speed. These speeds are estimated to be 110 knots and 55 knots and require a bank angle of 1.5° for a perfect turn i.e. with the ball in the middle ².
Control round the other two axes is conventional using normal elevator type control surfaces situated in the slipstream of the engines. They are used symmetrically for pitch control of the whole aircraft but differentially for roll control. To facilitate the roll action the wing is designed to have low torsional stiffness so that relatively low control forces will twist the wing the required amount (perhaps 1 – 2°).
The above description of control deals with the simple problem of control about the three axes However there are further aspects to be dealt with. Artificial fore and aft stability may well be useful and there is definitely a need for engine thrust to be matched to the local requirement which will differ at all stations during turning flight and other manoeuvres.
Fortunately the modern auto-pilot has reached a high level of development and can easily meet all these requirements (and more) together with fully satisfying modern safety standards. The popular name for such a system is “fly-by-wire”: it describes a system in which the pilot has no direct connection with the control surfaces but commands the auto-pilot with the manoeuvre he requires. This could be simple e.g. ‘steer 210°’ or something much more complex such as ‘land’. It is even quite possible that some of the requirements will need reaction times shorter than those the pilot can deliver in which case fly-by-wire becomes essential.
² Theoretically there is another band of bank angles which will give higher turning rates but these are considered impractical for so large a craft