"THE DE HAVILLAND Twin Otter Series 300 is a twin turboprop aircraft seating up to twenty passengers in either airline type or foldaway utility seats." This is the initial statement in one of the D.H.C. brochures. What makes the aircraft truly remarkable is that to do this it requires a landing strip no bigger than is necessary for a Tipsy Nipper; and its steep descent to land, and climb after take-off permit it to land and take off over obstacles which would render such a small strip useless to almost any other kind of aircraft except helicopters or balloons. Couple this with an approved overhaul life for its PT6 turbines of 3,000 hours for commuter operations (in some special cases the F.A.A. have approved 4,500 hours), a rugged and maintenance free airframe, and it is apparent why the Twin Otter is established as one of the foremost STOL light twins in the world.

        The subject of this air-test was the D.H.C. demonstrator CF-YFT. The aircraft was on a tour of Europe following the Hanover Air Show, and was made available for Air Pictorial through the kindness of Air Associates Ltd. of London, the U.K. agents, and Captain Tom Appleton, the demonstration pilot, who flew the aircraft over from Downsview, Ontario. The prototype DHC-6 Twin Otter, registered CF-DHC-X, first flew in May 1965, and was followed by the initial production version known as the Series 100. These are fitted with 579-e.s.h.p. PT6A-20s, have a maximum permitted all-up weight of 11,600 lb., and can be distinguished from the later landplane variants by a shorter nose. The Series 200 which followed has a lengthened nose (except when fitted with floats) and increased baggage space in the after compartment, but is otherwise similar in all main respects to the Series 100. The Series 300 is distinguishable externally from previous versions by having escape hatches on both sides of the fuselage, and by its vortex generators which are fitted on the leading edge of the tip of the fin in addition to those at the T-junction of the tail unit on all versions. Main feature of the Series 300, however, is that it has the more powerful PT6A-27 turbines delivering 652 e.s.h.p. each. These engines are flat-rated to 91 deg. F. at sea-level, with consequent beneficial effects on performance under hot and/or high conditions. Finally, the maximum permitted all-up weight of the Series 300 is increased to 12,500 lb., with consequent increased load-carrying ability.

        The principal material used throughout the aircraft is aluminium alloy. The fuselage, having frames, stringers and a stressed skin, comprises three main elements; the front section, containing the baggage and avionics compartment, and the pilot’s cabin; the centre portion, which is the main cabin; and the rear portion, which contains another baggage compartment, and to which the tailcone, fin and tailplane are attached. The floor of the main cabin is an aluminium-faced sandwich structure designed so that the aircraft can be used with equal facility for passengers or freight, or mixtures of both. The usable volume of the main cabin is 384 cu. ft. There is a passenger door to starboard, having a height of 451 in. and width of 301 in.; and opposite it on the port side the design permits double doors with an opening of 50 in. height and 56 in. width. One of these doors may be an air-stair door if so desired by the customer, and CF-YFT was so fitted. Front and rear baggage compartments have capacities of 38 and 88 cu. ft. respectively.

 Wing structure   

        The wing has a rectangular plan form without taper and is of constant thickness. Each half of the strut-braced mainplane is attached to the fuselage by two bolts. The main structure of the wing is a box, comprising front and rear spars, and top and bottom surfaces. The top skin panels are stiffened by corrugations attached internally with adhesive bonding.

        An important feature of the design is the double-slotted flaps and ailerons. When the flaps are lowered, the ailerons are also deflected, but only by half the movement of the flap. There are no leading-edge slots, and this not only makes it possible to fit full de-icing equipment without difficulty, but also results in a considerable nose-down change of attitude of the aircraft when the flaps are lowered ---something which does not occur to the same extent when flaps are used in conjunction with leading-edge slots. This is an important factor in the handling of the Twin Otter which, on the approach at low speeds, does not hang on its engines with its nose in the air, but instead points down its glide path straight at the point of intended touch-down.

        There are pilot-operated trim tabs giving trim adjustment about all three axes. In addition, the trim tab fitted to the starboard elevator is mechanically linked to the flap-operating mechanism. This tab gives automatic adjustment of fore and aft trim to compensate for the raising and lowering of the flaps; it is completely effective over the range of flap movement between 10 deg. and the maximum deflection of 40. Once the aircraft is set up, therefore, with 10 deg. of flap at circuit speeds, there is no further need for the pilot to make fore and aft trim adjustments. The tab is much less effective, however, over the first 10 deg. of flap movement, and this will be noticed by the pilot, as mentioned in the part of this description relating to handling.

        The landing gear is a non-retracting tricycle. The mainwheels are carried on steel cantilever beams, and the springing is by means of urethane synthetic rubber blocks, resistant to turbine oils and fuels, which provide both for compression loads and rebound shock-absorbing. There is no maintenance involved in the system, other than replacement of the blocks after long use. The suspension is most effective, and provides a very smooth landing and ride. The nosewheel is hydraulically steered on an oleo-pneumatic leg; it is self-centring in flight. Disc brakes, hydraulically actuated, are fitted to the mainwheels.

        A feature of the Twin Otter is its thorough anti-corrosion protection throughout the structure. The external surfaces are epoxy primed; internal detail parts are "alodined" and zinc-chromate primed before assembly; extrusions, plates and similar are chromic acid anodised and then zinc-chromate primed; details within the engine nacelles are primed with an epoxy primer resistant to turbine engine oils and fuels; both faying surfaces of lapped skin joints are treated before assembly, and in fact every item is given careful and special treatment appropriate to the job it has to do. The result of this was evident in the demonstration aircraft which, despite having been used on floats and as a waterbomber against forest fires, when tested for this report, at the conclusion of a tightly scheduled European tour, looked as though it had come straight from the factory. Its appearance was in fact the best possible tribute, not only to the excellence of the design, but also to the Canadian craftsmen at Downsview who built it, and to the crew looking after it on its tour.

Powerplant

        The powerplant comprises two United Aircraft of Canada PT6A-27 turbopropeller engines driving Hartzell threebladed reversible and fully feathering propellers. Designed and built in Canada, the PT6 is a particularly interesting engine; it comprises a gas generator, which incorporates the free turbine element of the engine, and turbine driving the screw. The essential feature is that there is no mechanical connection between the two turbines. In consequence, the gas generator can run automatically at whatever speed is most efficient in order to produce the gas needed to drive the propeller turbine wheel at the power required. At the pilot’s control is a power lever, which in practice is used exactly as the throttle lever in any ordinary piston engine; he observes the power being produced, not in boost, but as torque on a gauge reading in lb. per sq. in. In case this should appear confusing, the p.s.i. calibration of the instrument results from the pressure instrument method of reading the torque, and is not a direct measure of actual torque, which would of course be measured in in. or ft./lb.

        The gas flow through the turbine is shown in Fig. 1. From this it will be seen that the annular air intake is at the rear of the engine, and that a three-stage axial-flow compressor, followed by a single-stage single-sided centrifugal compressor, feed the air to an annular reverse-flow combustion chamber.

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