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Aviation History
1969
1969 - 0044.PDF
32 Private Flying ... FLIGHT International, 2 January 1969 FLYING THE DIAMANT BY Dr BRENNIG JAMES I N GENERAL THE DEVELOPMENT OF GLIDERS is cheaper than the development of powered aircraft so that it is not surprising to find that many of the features of modern powered aircraft- such as the use of the leading-edge "D" as a torsion member first appeared on a sailplane. Laminar-flow airfoil development came with the War, but the widespread adoption of low-drag wings is now more apparent in sailplanes than in light aircraft. Glass-fibre construction of the whole airframe is now a feature of numerous modern gliders and one of the first examples of this type flown in this country was the 18m Diamant, so it would be reasonable to give some account of its features based on over lOOhr of flying during the last soaring season. The general arrangement, performance curve and construc tion are shown for the 18m aircraft. The wing has a glass-fibre spar, but otherwise the construc tion of the aircraft is pure monocoque without ribs, bulkheads, stringers or longerons. The skin consists of an outer layer of two plies of glass-fibre reinforced with epoxy resin immediately below which is a layer of 8mm cross-linked PVC foam (Klegecel) having the appearance of stale bread and a final inner skin of one ply of Fibreglass in the wing and three in the fuselage. The wings are made in upper and lower halves in female moulds and then assembled together with the spar, after which a lapping strip is cemented to the leading edge. The fuselage is likewise built in right and left halves joined in the vertical plane. The controls are light-alloy pushrods with ball joints running in integral plain delrin bearings. The wheel retracts into the fuselage, but since the cable attachment hook is mounted on the wheel strut the cable must be released before the wheel can be retracted. The suspension is by means of compression cylinders of Cellasto in the shock-absorbing struts. The aerodynamics are what is now coming to be regarded as conventional for an aircraft of this type. The wing has a Wortmann section with flaps adjustable from +16° to —10° while the aileron droop is automatically adjusted in unison. The aspect ratio is 22.7; there is no washout, but slight sweep forward of the aerodynamic centre. The high T-tail is all-moving and rotates about its centre of pressure and has no anti-balance tab, so that elevator feel (such as exists) is supplied by an adjustable spring in the tailplane circuit. The general impression is one of excellent workmanship and superb detail design. There are few instances of profound originality but many of the best features of numerous other sailplanes have been brought together in one aircraft, and this in itself is no mean feat. The cockpit is a close but comfortable fit for a man of average size, and is covered by a canopy derived from the Polish Foka but having an opening mechanism of extraordinary complexity and con venience. The instrument panel rises on a central pedestal, again in Foka fashion, but it is easily removable together with its base for servicing, although it is quite accessible in situ with the canopy removed. Rudder pedals, seat back and head rest are readily adjustable, but only the latter is adjustable in flight. The pilot sits on the floor, the control rims passing along the fuselage sides in the arm rests so there is nothing under him but a centimetre of fuselage skin, something to think about in a wheel-up landing. The reclining position is comfortable with good visibility over a wild field of view, the canopy giving surprisingly little distortion. The cockpit is such a close fit that unfolding a map is a major undertaking; it is, however, comfortable for long periods of flight. Cockpit ventilation is contrived through a complex piece of plumbing in the nose which does its job effectively but rather noisily. A silent cockpit is the aesthetic ideal, of course, and a rushing noise seems a heavy price to pay for ventilation and a clear canopy. Rigging is a major effort; there are six fittings in the wing which must be perfectly aligned before they can be joined. One can sometimes struggle for half an hour before they slip easily home with a faint click. Maddening since one kt FORWARD SPEED Performance of the 18m Diamant is typical of today's top sailplane: a minimum gradient of 1.45 and good high-speed penetration doesn't know what was wrong before, but practice yields some proficiency in this subtle art. Once home the wings are secured by a massive king-pin, itself secured by a pip-pin in true belt and braces fashion. The only other connections to be made are two pip-pins through the aileron bell-cranks; the ball joints for flaps and drive brakes join up automatically. The tailplane hooks readily on to the elevator linkage and is secured on the fin by a single bolt. The cockpit checks are extensive and differ little from the usual practice. Take-off procedure is normal and neutral flap is recommended as it gives better aileron response. Once off the ground application of full flap brings the nose down and gives a better view of the tug. One danger on take-off is that a mild ground loop quickly turns into a spectacular "christie." This is partly because the tow rope attachment on the undercarriage generates no righting moment and partly because the centre of gravity (being behind the wheel) gives a strong turning moment once rotation is initiated.
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