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Aviation History
1949
1949 - 1607.PDF
FLIGHT, 22 September 1949 371 " Flight '" pltoto(jia[>h New engine dials and the revised four-lever pedestal will be noticed at once by pilots familiar with the standard Dakota. are now exactly level with the pilot. This is not in accord- ance with I.C.A.O. regulations and, though convenient from the e.g. point of view on this first machine, it will be altered. The Mambas were in this instance started up with the aid of external compressed-air bottles, but electric starters are now available. There was nothing unusual about the procedure or their behaviour although they happened to be the first pair of Series 2 engines for flight-test with the 1,475 e.s.h.p. rating. When idling they produced very little noise or vibration anywhere in the aircraft, even though they were separated from the listener by nothing more substantial than the 20-gauge fuselage skin. The cockpit of the Dakota shows little change, and the standard fuel-tanks and lines are employed. In place of the normal six levers on the central pedestal (throttles, mixtures and airscrews) there are only four: two power levers and two high-pressure cocks. Certain instrument changes are also apparent: new r.p.m. indicators reading up to 15,000 or more and of two-needle (sensitive-altimeter) pattern; jet-pipe temperature gauges; burner-pressure gauges ; rear-bearing temperature gauges—each pair neatly replaces or adds to the normal Dakota complement. On take-off at about 15,000 r.p.m. turbine speed the four-blade de Havilland airscrews gave forth an urgent roar very similar to that of the original Twin Wasp in fine pitch. Other than this there was very little noise and remarkably little vibration. Naturally, the more powerful and lightly loaded aircraft came off and climbed into the hazy Midland atmosphere with alacrity, and, as soon as the Mambas were eased back to cruising r.p.m., the airscrew noise was greatly reduced and the aircraft went scudding around the airfield steeply banked at about 180 kt. R&W. WASP NORMAL CRLHSE AT 8000ft It would be idle to expect silence in the main body or, for that matter, the crew compartment of the machine, but the noise was very appreciably less than in the standard aircraft. One could talk without shouting and clearly hear the wind-noises around the fuselage above the deeper note from the power units. The quality of the noise also lacked the oppressive, penetrating quality which makes one wish to plug one's ears, and which is a principal fatigue ingred- ient. In addition to the noise reduction there was a marked smoothness which is new to the Dakota. There is little doubt that a soundproofed and fully furnished aircraft would offer a remarkable advance in silence and lack of vibration and thus match the qualities of the turboprop prototypes of the new generation of airliners. That the Mamba Dak has a lively performance is appar- ent both from the ground and in the air. It can cruise comfortably at 180 kt (200 m.p.h.) and with the starboard airscrew feathered (and for once stationary) it maintained a comfortable 130/140 kt (150 m.p.h.) turning easily to- wards the operative unit. Its maximum speed is over 225 kt (260 m.p.h.), but at this early stage of testing about 180 kt is considered sufficient at low level, as on a demonstration fly-past, and maximum weight is limited to 24,000 lb. Perhaps the most important performance consideration of all, however, is the ability to meet I.C.A.O. requirements for single-engine failure on take-off, at an all-up weight of 26,700 lb—only 1,300 lb less than the maximum all-up weight at present authorized in this country for take-off from first-class airfields. The Mamba Installation The new Mamba nacelles are unusually shapely, even though the maximum overall diameter governed by the original piston-engine installation is 55.5m, whereas the maximum diameter of the gas turbine is only 28in. The total installed weight of each power-plant with airscrew is 2,000 lb, which is some 500 1b lighter than for the Twin Wasp. Although it might be possible to reduce the size of the Mamba Dak to gain any real advantage would entail a new undercarriage. Such a major modification is opposed to the whole conception of the conversion, a guiding prin- ciple of which has been to install the Mambas with the very minimum of alteration to the Dakota airframe, control system, fuel system or nacelle aft of the main bulkhead. On the Mamba 3s as much of the available power as pos- sible is diverted to the airscrews—this is in keeping with the latest ideas on turboprops—and it is noticeable that the area of the jet pipes is considerably greater than on early installations. The exact form of the twin jet-pipes for each Mamba has not yet been settled, but the pipes illustrated represent a second long stage in development, and it is believed that they can now be cut back some i8in or 2ft at an oblique angle to blend with the wing contour without fear of damage from heat to skin or fabric. For the first few hours of ground running short pipes were fitted flush with the end of the nacelle fairing on top of the wing, but it was found that on the (Continued on page 374) <5POO A.U.W. OF WASP VERSION : 23£OOIb. All.W. OF MAMBA VERSION : 26,7OOIb. spool MAMBA OPTIMUM CRUtSE AT IQOOOft. NORMAL CRUISE AT 8OOOft. 3OOOI IOO ZOO 3OO 4OO 5OO 6OO 7OO 8OO 9OO 1OOO STILL AIR RANGE. INCLUDING ALLOWANCES (miles) )OO 2OO JOO 4OO 5OO 6OO TOO 8OO 9OO JPOO STILL AIR RANGE. INCLUDING ALLOWANCES Mes) Comparative payload/range curves for the piston-engined and Mamba 3 turboprop-powered Dakotas. Left, for the present authorized all-up weight of 28,000 Ib; right, with a.u.w. adjusted to comply with I.CA.O. take-off requirements. Range limits with present maximum fuel capacity are indicated by a break in the curves. B 3 ,-..,?:;•-••• ..:>.:•', ....; . ....•.•:•• ••• :: -.^. • •• ••.-...:*. .-,--> .••.-- -\ •• -. -••• ••• .
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