From the combustion chamber, the hot gas goes first to the high-pressure turbine wheel, which is on the same shaft as the compressor system, and which drives it. This completes the gas generator circuit. In fact, if the engine were a pure jet, the gas would then be discharged as a hot jet, direct to the atmosphere. In the case of the PT6 design, however, the gas is fed, not into the free atmosphere, but into the low-pressure turbine wheel; this second, or L.P., wheel is co-axial with, and close to the H.P. wheel, but there is no mechanical connection between the two. It is the L.P. wheel that provides the power, through a two-stage planetary gear train, giving a reduction ratio of 15:1, to the propeller. Maximum propeller speed is 2,200 r.p.m., and at normal cruise considerably less. When taxi-ing, very little power is put through the L.P. end of the engine, and as a result the powerplant of the Twin Otter is unusually quiet both on the ground and in the air. With every justification D.H. Canada refer to their Twin Otter as the "quiet commuter aircraft". After passing through the L.P. stage, the gas is then ejected to the atmosphere, through pipes which allow use to be made of the small residual thrust. The exhaust pipes on CF-YFT permit the gas to flow over the cowling and wing, making it dirty. Future aircraft will incorporate a modified jet efflux pipe to avoid fouling the clean surfaces.

        Fuel is supplied from two completely independent tanks, each of which comprises four interconnected flexible fuel cells. Both systems are located underneath the fuselage floor, the forward one supplying the starboard engine, and the after one the port. A crossfeed system permits, one or both engines to feed from either tank; but except in special circumstances, each tank and each engine are operated as two independent systems, and the fore and aft disposition of the tanks results in no great alterations of aircraft balance as fuel is used up. Fuel used is the normally available grades of aviation kerosene; or in an emergency aviation gasolene or diesel fuel to specification CPW 46. The design provides for additional fuel tanks in the wing as an optional extra.

        Particular attention has been paid in the powerplant installation to easy access, in addition to which the forward half of each engine, complete with airscrew and gear, can be quickly removed as a complete unit, thus permitting visual inspection of both turbine wheels. Operators whose intensive use of the Twin Otter require the speediest possible servicing have found that an inspection of this sort can be completed overnight, and the aircraft returned to use, fully serviceable, in the morning.

        One object of the Twin Otter is versatility. The cabin is therefore designed so that the loading can be varied in as many ways as possible between freight and passengers. It can accomodate twenty passengers and allow each of them to take 40 lb. of baggage; or it can load an equivalent weight of 5,300 lb. of pure freight, or any combination of these. There is provision for fitting floats, and operation as a seaplane; and the aircraft has been demonstrated with an under-belly tank as a water-bomber. The payload/range capabilities of the aircraft are shown in Fig. 2.

Pilot's cabin

        It is tempting to refer to the "Flight Deck" because the equipment of CF-YFT, is very complete, and fully up to anything; but the most specialised airline standards. To do so, however, would give a completely erroneous impression of complexity; for the pilot’s cabin of the Twin Otter is an exceedingly well laid out workspace, and the result of good design is that flying the aircraft is not at all complicated.

        There is provision for two pilots and most controls are either accessible to both pilots or duplicated. The exceptions are the nosewheel steering and the parking brake which are fitted on the port side only. The main array of flight instruments is on the port side of the panel, engine instruments are arranged centrally, and radio and other instruments for the second pilot either centrally or to starboard. It should be made clear, however, that instrumentation and equipment are entirely to customers’ requirements.

        The engine controls are located overhead in a position accessible to both pilots; at first sight the arrangement looks awkward, and this impression was confirmed by banging one’s head on the power lever quadrant in the process of getting into the pilot’s seat. This first impression proved entirely wrong, however, and was completly reversed by the end of an hour and a half at the controls. It is very comfortable in practice, easy to use, and frees the floor of the cockpit from a lot clutter which would otherwise have to be located on the more customary central console. Another excellent and unusual feature are the two doors, one on either side, so that either pilot can enter or leave the aircraft without going back through the cabin, and having to climb over freight. The same doors also provide easy access to the deck of the floats when in use as seaplane, and must be a great boon when coming up to a lonely mooring, or docking the seaplane.

        Both pilots’ seats are adjustable over a fore and aft range of just over 4 in.; the seats also move vertically up and down, and the rudder pedals are adjustable for reach. The seats have armrests, and are very comfortable. Having adjusted every comfort and ease of control, this pilot found that the distance of the eye to the nearest instrument on 27 1/2 in., which is almost exactly the ideal. The pilot’s cabin is very roomy, being 5 ft. wide. There is thus easy access from either seat into the main cabin by way of the central aisle, and the width of the instrument panel is sufficient for almost any instrumentation that a prospective operator might like to specify.

Handling

        The engines start from internal batteries, which spin up the gas-generator element of the engine; this soon settles down to a constant speed and temperature, and can normally be disregarded for the rest of the flight. The two "Power levers" can then be used in exactly the same way as the throttle levers on a conventional piston engine, and so can the twin airscrew pitch controls. So far as the pilot is concerned, this makes the PT6 turboprop extremely simple to handle, and there is no problem at all in acclimatising from piston engines.

        The view from either pilot’s seat is excellent. The aircraft can be taxied very accurately by means of the nosewheel steering -- it is normal to steer by the nosewheel, and not the brakes -- and apart from a little extra power to get moving, the aircraft proceeds comfortably at the normal ground idle speed of the turbines.

        There are two alternative take-off techniques. The normal one is to use 10 deg. of flap, rotate at 70 knots I.A.S., and initial climb at 80 knots. When clear of obstacles the flaps can be raised and the climb continued at 100 knots for best rate of climb at full power, or at whatever greater speed is desirable to suit the circumstances at the time; comfortable cruise climb speeds would be about 130 knots I.A.S. Using this method keeps well away from the placard single-engine minimum control speed with 10 deg. of flap, which is 66 knots, and is in practice very comfortable and easy. There is no tendency to swing-the Twin Otter is in fact cleared for a maximum 90-deg. cross-wind component of 27 knots. On the day of our test there was no wind at all, and bonfire smoke was going up vertically; but despite this, the take-off roll was very short. For shortest ground rail, flaps are set at 20 deg. And power increased against the wheel brakes until the aircraft is on the point of sliding. Using this method the take-off is startling, and despite a far from smooth grass surface and nil wind, we were airborne at 50 knots I.A.S. in something not far in excess of 120 yards. We were not, of course, at maximum weight; the maker’s figures for STOL take-off and landing capability in nil wind at full load are shown in Fig. 3 (on next page).

Steepest climb

        For maximum angle of climb, the aircraft can then be held at 75 knots I.A.S. – again well clear of the minimum control speed – and flaps raised when required, after which normal climbing techniques are resumed. There is no great change of trim felt by the pilot as flaps are raised (or lowered) except for the initial 10 deg. from the zero position. In this range, lowering the flaps results in a nose-up tendency; throughout the remainder of the movement, the elevator tab on the starboard side, which is coupled to the flap actuating mechanism, compensates for trim changes as the flaps move.

        A cruise setting for the engines of 45 lb. torque, and 75 per cent r.p.m. on the pitch control, at 8,000 ft., gave 145 knots I.A.S. (173 true). At this height we did some general handling. The controls are in general nicely harmonised. Ailerons are by no means light, but they are very positive, and the aircraft is not unduly affected by bumps and turbulence. Once in the groove, it stays there nicely, and the weight and feel of the lateral control is just right for the kind of aircraft this is. The aircraft has neutral lateral stability. Directional stability, as one would expect from the appearance of the fin and rudder, is very positive, and in fact there is no need to fly with one’s feet on the rudder in normal smooth air. Elevators feel just right, with the right amount of weight in them, except momentarily when the flaps are moving, and in consequence the trim tab linked to them is also moving, so there is a change of loading on the stick, lighter or heavier as the case may be; one very quickly becomes accustomed to this feel, and it causes no problems.

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