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Aviation History
1928
1928 - 0267.PDF
APRIL 12, 1928 THE PARNALL "IMP A New British Light Aeroplane with "Genet" Engine IN designing the " Imp," photographs of which were published in FLIGHT of March 29, 1928, Mr. Harold Bolas, George Parnall's chief designer, had two outstanding fundamental aims in view, simplicity and good view. An examination of the " Imp " shows that he has attained these objects. With its entire absence of wire bracing (and consequent avoidance of any need for ever trueing-up the machine once it has been assembled), its all-wood construction, and general weatherproof qualities, the " Imp " is simple enough in all The wing section employed is a modified R.A.F. 31, i.e,,a fairly thick section, with its centre line curved to a medium camber. The wing arrangement chosen made a fairly thicksection necessary, since the lower wing is a cantilever and has, in addition to its own load, to carry the load of th«unbraced hinged upper wing. The lower wing is made in one piece, with a span of 21 ft. 6 in. and a chord of 4 ft. Theupper wing is built in two halves, hinged to a central cabanc of streamline steel-tube struts, and the load is transmitted [" FLIGHT " Pkotegrapk THE PARNALL " IMP " : Front view. The engine is an Armstrong-Siddeley " Genet." By removing thetop plane, cabane and struts, the machine can be converted into a low-wing, cantilever monoplane. conscience. And the view is probably very nearly as goodas it is possible to obtain in an aeroplane of the tractor type. At the same time, the performance is approximately thesame as that of other and more orthodox designs of the same power and load-carrying capacity. Put as briefly as possible,the two main objects of the design, simplicity and good view, were obtained respectively by making the wings as canti-levers, wood-planked, and by giving the top plane a pronounced sweep-back. The latter feature is clearly visible in thephotographs, and particularly in the scale drawings. Wing Design As the biplane wings are the foundation of most of the unusual features of the " Imp," they will be dealt with first. via one interplane strut on each side. As the lower wing isa cantilever, while the upper is hinged, it will be seen that as far as pure bending is concerned, the inter-plane struts aretension members. With fore and aft movement of the centre of pressure, however, the struts are also called uponto act as compression members, and thus are designed to fulfil both functions. Constructionally the wings are unusual, in that they havebeen designed in such a manner as to utilise the skin or planking as the chief stress-resisting member. In doing this,it seems likely that a slightly greater wing structure weight per square foot is entailed, but for the price thus paid isobtained a wing of exceptional torsional rigidity, and a wing, THE PARNALL " IMP " [" FLIGHT " Pkotctraph Three-Quarter rear view. Note that both cockpits are clear of the top trailingedge, and that thus the view is very good. 239 c
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