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Aviation History
1964
1964 - 1468.PDF
FLIGHT International, 14 May 1964 815 IN writing about the Dart I must be even more unofficial than indealing with the Foka, having been mixed-up with variousaspects of the Dart project since it was just a faint gleam in Fred Slingsby's eye. Whilst this article is based on flight tests of the prototype, I have tried to restrict it to those aspects which interest the pilot flying it for fun, making due allowance for some features of the prototype which are to be altered in production. Slingsby's previous 15-metre glider was the Skylark 2, which first flew in 1954. A machine of much merit, it became distinctly out- classed by later standard-class designs, so the T.51 Dart represents the firm's concept of the new look in 15-metre ships. The overall specification was not easy; it had to be a really first-class glider by international standards, fulfilling both the letter and the spirit of the standard-class rules, whilst being suitable for light-hearted operation in the United Kingdom with a high degree of toughness and reliability. It was firmly decided not to compromise the competition poten- tial of the Dart by trying to make it suitable for floating round the hill at the Lower Muddlecombe Gliding Club at 30 knots. To this end, the fashionable process of optimization by computer (and Bill FLYING THE Slingsby T-51 Dart By FRANK IRVING Slater's slide-rule) was applied, together with wind-tunnelry at a well-known Seat of Learning, where I discovered just how messy a mixture of paraffin and fluorescent pigment can be. So far as the wing is concerned, the outcome may look a little odd by convention- al standards because of the low taper-ratio. Also, the aircraft is very long and has an all-moving tail with geared tabs to confer some stick-free stability. External excrescences are notably rare. Rigging is simple and rapid, with excellent access to all the gubbins in the centre-section. Indeed, there is a vast hole in the top of the fuselage in this region, giving plenty of room to get at much of the machinery. In the top of the fuselage just aft of the centre- section is a commodious stowage space—which looks likely to deposit one's pyjamas and toothbrush into the aileron mechanism until you realize that it is closed-off by the rear of the centre- section fairing. In contrast to the Foka, the Dart has a large wheel well forward, and normally sits in the tail-down attitude. A small nose skid is provided, primarily to protect the front fuselage when the wheel meets a mole-hill. This arrangement gives a fairly hefty tail weight, particularly when devoid of pilot, but the risk of slipped discs when lifting the tail is diminished by a con- veniently placed detachable handle. The seating position is basically like that of the Skylark 4 but, as on the Foka, the control runs are along the cockpit sides and the pilot sits virtually on the bottom of the fuselage, making it several inches lower than the "4." The pilot's attitude might be described as semi-reclining, being appreciably less extreme than the Foka: it seems to be a good compromise in terms of comfort, view and reasonably small frontal area. The bulges along the inner cockpit sides which accommodate the controls rather diminish the sandwich- stowage capability, but there is plenty of room for equipment behind the seat. Controls mostly come comfortably to hand with 'he trimmer on the left. The slight awkwardness of the airbrake and trimmer controls of the prototype is to be altered in production.1 ^e rudder pedals and seat-back slope are adjustable over a wideran ge, and most sizes of pilot can find a happy compromise. Tail-down on the ground, the forward view seems slightly restricted but this impression disappears in the air. In fact the view 's extraordinarily good, and at aero-towing speeds one can see downwards over the nose at about 20° to the horizontal. Anyonew ho has flown a Skylark 4 will feel instantly at home. The elevator ^d aileron controls are remarkably friction-free, because of the use of push-pull rods throughout. The rudder uses conventionales for simplicity, and the low aerodynamic forces tend to accentuate such friction as there is. The cockpit cover is locked by a wiirivance which can only be described as substantial, and which tends to give me a slight guilt-complex whenever I see it, despite the Slingsby refinements. But the drizzle of detached cockpit covers has ceased since its introduction. Minor controls all are coloured according to the OSTIV recommendations. Take-off on aero-tow presents no difficulty. On one's first trip, the lightness of the elevator comes as a surprise, and one may be tempted to indulge in slight vertical oscillations. But you soon learn that the machine itself is remarkably non-twitchy and, once your personal servo has become attuned to the situation, a very com- fortable ride results. In free flight at, say, 50kl, the outstanding impression is of light effective controls. Despite the lightness of the elevator, the feel does not alter greatly over the wide permitted e.g. range and, surprisingly, any feeling of balancing on a pin-point is quite absent. The stick-force per g is low (about 4-51b), which makes for very pleasant thermalling characteristics, with the implication that care must be taken when performing aerobatics by those accustomed to more brutal gliders. The rate of roll is excellent (3.8sec to reverse a 45° banked turn at 50kt) and feels better than that of many other gliders with the same nominal figure, since the light ailerons allow you to use it all. Although the ailerons have no mass-balancing they are free from inertia tweaks in rough air and, although the rudder looks small, it is quite adequate to deal with the low aileron- drag and the ten-knot cross-wind take-off case. On decreasing speed in straight flight, slight buffet starts at 38kt and control is lost at about 35kt i.a.s. If everything were kept pre- cisely symmetrical in calm air and the pilot had his hair parted in the middle, the Dart might be persuaded to indulge in a straight nose- drop. In practice it usually drops one wing or another, but there is no difficulty in regaining control. The discerning reader will guess that certificating authorities are becoming more resigned to this sort of behaviour than heretofore. Spinning is entirely conventional by glider standards: e.g. for- ward, it indulges in a spiral dive, which one does well to arrest before the speed and g become excessive; e.g. aft, a genuine spin occurs with the a.s.i. showing some nonsensical reading because of the draught blowing sideways at the static vents. Recovery is rapid and straightforward, but it may pay to use the airbrakes, since the speed builds up very rapidly in a nose-down attitude. The airbrakes are of the standard DFS variety, with foam sealing to stop the leakage through the slots rattling-up the boundary-layer on the top wing surface. They are entirely effective, but with the markedly non-linear characteristics which seem almost inevitable with this arrangement. How does the Dart perform ? At the time of writing, no cross- countries and relatively little soaring have been done in it. Nor has the performance been measured, although some rather rough comparisons with certain other celebrated types give grounds for considerable hope. It is faster than most previous British types, so one expects it to rush about in larger circles in a thermal and to leave the rest of the field panting in the rear when it dashes off to the Continued on page 818, after double-page drawing of the Dart "Sling" and Dan: a recent picture of F. N. Slingsby, managing dir- ector of Stingsby Sail- planes Ltd, seen with a Dart fuselage under construction at his Kirbymoorside factory
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