Stalling

        The stall straight and level, with flaps up, occurred at 62-63 knots I.A.S. and proved gentle and uneventful. There is not much pre-stall warning, but the red stall-warning light comes on at about 4 knots before the stall; to meet U.K. requirements the aircraft would also be equipped with a stall-warning horn, which Canadian and U.S. regulations do not call for. Loss of height from the level stall is of the order of a few hundred feet only, and recovery normal and quick.

        With 20 deg. of flap - i.e., in the approach configuration - we recorded a stalling speed of 50 knots I.A.S. with the stick fully back. Possibly because all controls were not centralised, there was a slight right wing drop; a second similar stall demonstrated by Captain Appleton produced no wing drop. Recovery in each case was speedy and normal, though with 20 deg. of flap the nose dropped more steeply than when stalled clean.

        The Twin Otter performs very well on one engine. From normal cruising power at 8,000 ft. the sudden loss of one engine produces remarkably little swing, and can easily be held straight by moderate rudder pressure; using trim eliminates the rudder pressure needed entirely, and the aircraft can then be flown in cruise condition on one engine feet off. Using full power on one engine, and 10 deg. of flap, speed was reduced to 60 knots I.A.S.; climbing turns at this speed either with or against the live engine presented no difficulty. The recommended procedure for best single engine climb in straight flight is to bank 5 deg. towards the live engine; the best rate of climb will be obtained at 79 knots I.A.S. at full load. At the reduced load at which this test was done, the Twin Otter was climbing under these conditions with a steady 1,000 ft./min. showing on its V.S.I.

        It is worth noting at this point that the powerplant incorporates an automatic feathering device which comes into operation if an engine fails. Using this system during take-off relieves the pilot of a great deal of work at a critical moment should he be unfortunate enough to experience an engine failure.

        Having lost height from 8,000 ft. down to 2,000 at 2,000 ft./min., using reduced power, and flying at the limiting speed of 170 knots I.A.S., we returned to the circuit, where the Twin Otter showed that the outlook from the pilot’s seat is specially good for the kind of work it is intended to do. This results in large part from the fact that on the approach the aircraft points straight do its glidepath at the intended point of landing. As speed is reduced, the flaps obviate the unpleasant nose-high hanging on the engines feel which some aircraft give; from the Twin Otter the pilot has an excellent and easy view of where he is going. Surprisingly for an aircraft with a fixed undercarriage, it does not lose speed easily, and to reduce to the maximum speed at which flaps may be lowered (102 knots for the first 10 deg., and down to 95 knots I.A.S. for anything greater) it was necessary to reduce power as registered on the torque meters to 10 lb.., Thereafter, with 15 lb. torque and 10 deg. of flap, the aircraft flew very nicely in the groove at 90-95 knots. Increasing the flap setting and after turning finals to 20 deg. produced quite a steep glidepath, despite the lack of wind, so that the aircraft must be rotated through a fairly large angle during the flare-out, which. was initiated over the threshold at 80 knots. There is very little float, and the aircraft sits down in a satisfactorily squashy manner without rebound; without using brakes to any great extent, and without reverse thrust, the landing run approximated to the distance required for a well-handled Cherokee.

        Using full flap on the approach results in an unusually steep glidepath, and consequently greater angle through which is necessary to rotate on flare-out. It was while doing this kind of landing that we also discovered the powerful braking effect of the airscrews when power is fully reduced. Passing the threshold at 75 knots, and on touching down using both wheel brakes and reverse thrust resulted in a landing roll of a hundred yards or thereabouts, and one which this pilot would find difficulty in bettering even in the Nipper which is his more usual mount. As we turned off the strip, the smoke from the bonfire on the perimeter was still ascending quite vertically.

Summary

        In many ways the Twin Otter is one of the most interesting aircraft we have tested. It possesses a unique combination of STOL qualities, rugged construction reliability and performance, and at the same time is pleasant and easy to fly, and is the sort of aircraft which pilots enjoy flying. It has no peculiarities of construction, or handling; it obtains its STOL performance without any trick features and can maintain its capabilities in ordinary every-day use with only the normal maintenance and servicing now available almost anywhere in the world. To date 281 Twin Otters have been sold, and of this number 274 have already been delivered and are in use. Users include the Argentine Air Force, Army andNavy, the Peruvian Air Force, the Ontario Department of Lands and Forests, New York Airlines, Sudan Airways of Alaska and Aero Commuter of Los Angeles.

        The price of a Twin Otter equipped similarly to the one used for this air-test would be $460,000 at Downsview, Ontario.

        Air Pictorial would like to express its gratitude to Captain Tom Appleton of D.H. Canada, who went out of his way gramme; and to Air Associates Ltd., 40 St. Peters Road, Hammersmith, London, W.6 (Tel: 01-748 0222), who are the U.K. Agents for the Twin Otter, for their help in arranging this test.