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Aviation History
1928
1928 - 0268.PDF
APRIL 12, 1928 [ ' FLIGHT " Photograph THE PARNALL "IMP" WITH ARMSTRONG-SIDDELEY " GENET " ENGINE : This aerial view, taken from a " Moth " kindly lent by the WessexClub and piloted by Mr. Bartlett, the Club Instructor, gives a good idea of the unusual arrangement of the•wings. On this occasion the " Imp " was piloted by Mr. Harold Bolas, its designer. moreover, which is to all intents and purposes weatherproof,as it does not depend upon doped fabric but has a wooden skin protected by paint. In point of fact, doped fabric asusually understood is entirely absent. A fabric covering is used, but it is not doped on, nor is it finished off with theusual pigmented dope, but with ordinary paint. In order to make full use of the skin as the main load- bearing structure of the wing, a somewhat unusual construc- tion has been developed. In place of the usual two-spar internal structure, there are in the " Imp " a number of light stringers, whose function is to resist shear and not bending. In place of the usual ribs there are formers of the external shape of the aerofoil section. The skin itself is a spruce veneer, and is tapered in thickness according to the location in the wing. Furthermore, the spacing of the ribs is propor- tioned to the local stresses in such a way as to enable the skin to stand up to its compression loads without secondary buckling. It will be realised that a wing of this design is not amenable to ordinary stress calculation, and a series of tests have been carried out to determine the most economical spacing of ribs and the best thickness distribution in the skin. As a matter of fact, the resulting wings have proved a good deal stronger than required. For instance, the top wing has a factor of about 12, and the lower wing of about 8, which is higher than deemed necessary even in a single-seater fighter of more than six times the power ! One feature of the " Imp " wing arrangement will doubtlessbe criticised by some : there is no provision made for folding the wings. Mr. Bolas has a rathef convincing, and certainlyvery neat, reply to any such objection. " One gets," he says, " much more shed than aeroplane for a pound sterling."In other words, it is better economy to spend the extra cost of any folding arrangement on increased shed space. Ahangar, Mr. Bolas argues, if it is to be large enough to enable the owner to work around the machine in the folded condition,will be very little smaller than one large enough to house the machine erected. The time has not yet come when theprivate owner of an aeroplane can keep his machine in a small shed in his garden, and in any case, a fairly large field is stillnecessary for taking off and alighting. That being so, ground space for a hangar is not a sesious consideration,and there remains only the question of the extra cost of the slightly larger hangar, which is met by the dictum quotedabove. That, in brief, is Mr. Bolas's argument, and certainly there is a good deal of common sense in it. The time when wing folding is really an advantage is after a forced landing in a field. But Mr. Bolas contends that when, after such an event, the machine has to be pushed through a gateway, there will usually have gathered quite a number of people, and as the wings of the " Imp " can be dismantled in about two minutes by two people, the loss in time is not likely to be of any consequence. At present no provision has been made for carrying the dismantled wings on the fuselage, but this will be done in future machines. The one-piece lower wing rests in a cut-out in the bottom of the fuselage, and quick-release fittings are used for securing it in place and for disconnecting the aileron controls. The procedure of dismantling the wings is as follows : One man supports the upper wing tip, while a second releases the strut attachments to the lower plane. The strut can then be laid flat along the under side of the top plane. ' ' ["FLIGHT" Photograph THE PARNALL " IMP •*• : Rear view. The raked inter-plane struts transmit the lift from the top planeto the lower, cantilever, wing. Ailerons run the whole span of the bottom plane. •* 240
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