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Aviation History
1958
1958 - 0595.PDF
FLIGHT, 2 May 1958 DART HERALD . . . 611 a large flap area. The low floor is ideal for quick loading and thelarge propellers and comfortable tip clearances are valuable for operations from short, rough airfields. Short-haul passengers tendto study their local terrain rather than settle down to reading, and the high wing and enlarged windows of later Heralds give asplendid view. It has also been suggested that it is more reassuring for passengers to look at the smooth tension skin of an aircraft'swing rather than a wrinkled upper surface. If there is any one criterion that stands out above all others, itis that for the Herald type of operation, the structure should be absolutely sound and guaranteed for the life of the aircraft—quitea tall order for a pressurized airframe. As Handley Page point out, new spars at 20,000 or 30,000 hr are not included in costequations, but can add very considerably to the maintenance expense over the aircraft's operating life. The airframe must alsocontinue to give good aerodynamic performance when it has been operating hard for many years, when it is patched and dirty andeven a little dented. And to suggest that old aeroplanes will main- tain their new sparkle is plainly to close one's eyes to the obvious. The Herald airframe has been extensively restressed for Dartpower. The tail and 20in longer fuselage (the section has been added to adjust the centre of gravity and bring the crew doorforward of the plane of the propeller discs) has been tested to 100 per cent load under torsional fin and rudder side loads at37,000 1b gross weight and to the severe case of a propeller failing by going into flight fine pitch, which applies a severe asymmetricside load. Two loading cases have also been simulated on the fuselage and centre section: maximum shear on the centre sectionand a maximum bending case aft of the rear spar. A pressure test in air, with strain gauges at 3,000 different positions on the fuselagehas been carried out by H.P. Stress levels at the full differential pressure of 3.35 lb/sq in are less than 8,000 lb/sq in, except atisolated concentrations where a maximum of 15,000 lb/sq in is allowed, and the Air Registration Board have allowed the fuselagean infinite fatigue life. Certification testing will be completed by pressurizing the fuselage in the water tank at Farnborough at 2Pand then cycling it at 4.2 lb/sq in to determine the rate of crack propagation and the form of final failure. It remains for a tip-to-tiptest to be done on the wing in the structural test department at Radlett. Premature failure is frankly not expected, and fatiguetests can consequently be simplified. Various panels and joints have been fatigue-tested and discussions are now being held withthe A.R.B. on clearing the wing for infinite fatigue life. The test wing will be deliberately cut to prove its fail-safe properties byfurther testing in the Radlett laboratory. The Herald's wing section is substantial. It varies from NACA23016.5 at the root to NACA 4412 at the tip; the separation point is reasonably well forward on the chord and the wing isthus less sensitive to dirt and minor deformations than a more laminar flow section. The Alvis-engined Herald was designed asa simple single-component aircraft with easy interchangeafrility. The centre section spar flanges on the three spar webs were10 s.w.g. rolled angle and acted merely as shear carriers; bending was taken by the sandwich skin and centre spar flange composed ofa corrugated sheet core spot welded to the outer skin and Pop riveted to the inner one. A major change in the Dart Herald has been the addition to the40ft centre section of machined upper and lower centre spar flanges to react the increased bending moment introduced by the20 per cent higher maximum speed of the turboprop version and the lack of bending moment relief with only two engines. The Design for serviceability: the cabin roof panels drop down to expose electric cable and flying control runs. Thick looms have been avoided. Instrument panel of the prototype Dart Herald has been much simpli- fied with the reduction in the number of engines fitted. The second prototype and production aircraft will be further improved. opportunity has also been taken of working to C.A.A. as well asA.R.B. requirements, and 10,000 lb/sq in has been set as the limit—on fatigue considerations—for a lg stress. The new spar flange is an interesting example of fail safe designtechnique. The flanges are planks of 24 ST, a material chosen for its low crack-propagation qualities. The upper compressionboom is used as-rolled, and the tension boom is machined all over and routed to the required section. Both booms are split spanwisealong their complete length to avoid cracks propagating from one half of the boom into the next, so that the tension and com-pression booms form four separate units. The remainder of the centre section is made up of light front and rear spars, much asbefore, although skin gauges and sections have been stiffened throughout the wing. There are very few ribs in the centre sectionapart from the end ribs and undercarriage ribs, as a continuous rib is formed by stress-carrying sandwich covers attached to thecentre spar flanges and to the angles on the fore and aft spars. The critical tension flange is machined so that the lower surfaceforms part of the wing skin and is thus always open to inspection for fatigue cracks. This is a fail-safe structure by design and not only by demonstra-tion. Sixty-six per cent of the ultimate load can still be carried if one half of a centre spar flange fails, or a front or rear spar anda corrugated cover, or various other combinations or isolated instances of fairly major—and unlikely—failures. Crack propaga-tion is likely to be quite slow and incipient failure of a flange or a panel should not be difficult to spot at an early stage. This alsoapplies to the corrugated sandwich covers; it has been H.P.'s experience that a crack is likely to be seen on the outside manyhours before a complete failure can occur, and in practice they do not anticipate failure at all. Fatigue tests have demonstrated thatcracks on corrugated cores will spread to the outer skin before a failure takes place, and a special and simple technique for detect-ing a failure of hidden components is being developed. A novel technique employed in the centre section is the methodof lining the tank bay with nylon sheet to prevent the ingress of swarf and din. This has a very beneficial effect upon tank lifeand the bay has been passed by the A.R.B. for inspection at four- year periods. Should a leak occur (the flexible Fireproof tanksare buttoned and laced into position), a drain is led to a tell-tale spill outlet on the inboard side of the engine nacelle, where it isvisible from the cabin. The outer plane has been similarly affected by the increasedloadings imposed by installation of die Darts. Under Handley Page's direction, redesign and construction of the outer panels isbeing done by Boulton Paul at Wolverhampton. Heavier stringers, machined inboard, drawn strip outboard, have been added in wayof the centre spar to diffuse the loading from the two-spar outer wing. For the Dart aeroplane, an integral tank has been incorpor-ated in the outer plane, immediately outboard of the multi-bolted wing joint. Access holes large enough for two-handed maintenanceoperations have been added and these are closed by stress-carry- ing doors with machined stringers. Stringer joints have beenredesigned throughout and most of the plating has been up-gauged. Extensive use has been made throughout the wing of corrugatedsandwich skins and H.P. have developed their spot welding tech- nique for use all over the primary structure. For such applications
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