Единственный экземпляр двухместного спортивного биплана Imp с двигателем Armstrong Siddeley Genet II мощностью 80 л. с. был сертифицирован в мае 1928 года. Позже самолет Imp использовался для летных испытаний звездообразного двигателя Pobjoy P мощностью 65 л. с., в декабре 1933 года он был списан.
Flight, April 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 conscience. And the view is probably very nearly as good as it is possible to obtain in an aeroplane of the tractor type. At the same time, the performance is approximately the same 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 cantilevers, wood-planked, and by giving the top plane a pronounced sweep-back. The latter feature is clearly visible in the photographs, and particularly in the scale drawings.
As the biplane wings are the foundation of most of the unusual features of the "Imp," they will be dealt with first. 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 thick section necessary, since the lower wing is a cantilever and has, in addition to its own load, to carry the load of the 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. The upper wing is built in two halves, hinged to a central cabane of streamline steel-tube struts, and the load is transmitted via one interplane strut on each side. As the lower wing is a cantilever, while the upper is hinged, it will be seen that as far as pure bending is concerned, the inter-plane struts are tension members. With fore and aft movement of the centre of pressure, however, the struts are also called upon to act as compression members, and thus are designed to fulfil both functions.
Constructionally the wings are unusual, in that they have been 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 is obtained a wing of exceptional torsional rigidity, and a wing, 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 as usually understood is entirely absent. A fabric covering is used, but it is not doped on, nor is it finished off with the usual 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 construction 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 proportioned 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 doubtless be criticised by some: there is no provision made for folding the wings. Mr. Bolas has a rather convincing, and certainly very 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. A hangar, 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 the private owner of an aeroplane can keep his machine in a small shed in his garden, and in any case, a fairly large field is still necessary for taking off and alighting. That being so, ground space for a hangar is not a serious consideration, and there remains only the question of the extra cost of the slightly larger hangar, which is met by the dictum quoted above. 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, where it is held in position by a pin through a small fitting. The man on the wing tip now lowers this slightly, when the second man releases the hook attachment to the cabane and the one half of the top plane is carried away. The process is repeated for the other half.
The lower plane has, as we have said, quick-release attachments to the fuselage at what would normally be the location of the rear spar. These are released first. When this has been done, the lower wing is prevented by a spring from falling down. The two persons catch, hold of a wing tip each, and press the wing backwards against the spring. After a backward movement of a little more than an inch the two pins in the front of the bottom plane come out of their sockets, and the plane is then free of the fuselage and can be carried away. The erecting of the wing is carried out in the reverse sequence.
Ailerons are fitted to the bottom plane only, and run the whole span, chiefly in order to simplify the quick-release control attachments.
The fuselage is of orthodox construction, with a light skeleton of spruce, covered with ply-wood. It is, of course, flat-sided, and has a flat bottom, but a cambered deck fairing in the usual way. The top longerons are placed rather higher than in some machines, i.e., the deck fairing forms a smaller percentage of the overall fuselage depth, and one result of this is that it is just a little awkward to get into the cockpits, as one has to step over the top longerons. There is, however, a step on the starboard side, so placed that it is used for both cockpits.
The cockpits are roomy and comfortable, and owing to the sweepback and large cut-out of the top plane, the view from both is exceptionally good. Dual controls are fitted, and the two seats rest on a box running through both cockpits.
The Armstrong-Siddeley "Genet" engine is mounted on tubes, and separated from the cockpit by a fireproof bulkhead. The petrol tank is placed in the deck fairing ahead of the front cockpit, where it gives sufficient head for gravity feed to the engine. Thus the necessity for a centre-section tank is avoided, which would be difficult to fit on the wings designed, and would moreover obstruct the view somewhat.
The undercarriage is the usual vee-type, with oleo and spiral spring "legs." The travel is about 5 ins., and the track is wide.
The main dimensions of the Parnall "Imp" are shown on the general arrangement drawings. The weight of the machine empty is 850 lbs. (386 kgs.). The loaded weight depends, of course, on the weight of the occupants, but 1,320 lbs. (600 kgs.) is given as an average figure. With the factors of safety as high as they are, the machine should be quite strong enough for any load with which it could get off. The wing area being 176 sq. ft., the wing loading is 7-5 lb./sq. ft. The top speed is approximately 102 m.p.h. (164 km./h.), and the landing speed about 40 m.p.h. (64 km./h.).
At present we do not know at what price the Parnall "Imp" will be marketed, but doubtless an announcement concerning this will be made shortly.
Flight, July 1928
THE PARNALL "IMP”
Armstrong-Siddeley "Genet" Engine
THE "Imp" was produced this year by George Parnall and Co. of Bristol, and is the latest design in the light aeroplane class. The engine is the Armstrong-Siddeley "Genet" radial, air-cooled, and the machine has an entire absence of wire bracing which gives a very clean appearance. Simplicity of design and a good view have been obtained by making the wings as cantilevers, wood-planked, and giving the top plane a pronounced sweep-back.
Fuselage. - This is of orthodox construction with a light skeleton of spruce, covered with ply-wood. It is, of course, flat-sided, and has a flat bottom, but a cambered deck fairing in the usual way. The top longerons are placed rather higher than in some machines, i.e., the deck fairing forms a smaller percentage of the overall fuselage depth with the result that one has to step over the top longerons to get into the cockpits, which are roomy and comfortable. Dual control is fitted.
Wings. - The wing section employed - a modified R.A.F.31 - is a fairly thick one with its centre line curved to a medium camber. The lower wing is in one length but the top wing is built in halves, hinged to a central cabane of streamline steel tube struts. A spruce veneer covers the wings and a fabric covering is used but not doped on. Both wings are easily detachable. Ailerons run the full length of the bottom plane and are not fitted to the top plane.
Engine Installation. - The "Genet" engine is mounted on tubes and separated from the cockpit by a fireproof bulkhead, whilst the petrol tank is placed in the deck fairing ahead of the front cockpit, where it gives sufficient head for gravity feed to the engine. Thus the necessity for a centre section tank is avoided, which, in any case, would be difficult to install with the existing wing design.
Undercarriage. - This is the usual V-type with oleo and spiral spring "legs." The travel is about 5 in. and the track is wide.
THE PARNALL "IMP": Three-Quarter rear view. Note that both cockpits are clear of the top trailing edge, and that thus the view is very good.
THE PARNALL "IMP": Rear view. The raked inter-plane struts transmit the lift from the top plane to the lower, cantilever, wing. Ailerons run the whole span of the bottom plane.
THE PARNALL "IMP": Front view. The engine is an Armstrong-Siddeley "Genet." By removing the top plane, cabane and struts, the machine can be converted into a low-wing, cantilever monoplane.
A NEW LIGHT AEROPLANE: The Parnall "Imp" with Armstrong-Siddeley "Genet" engine is of somewhat unorthodox design. The top plane has a pronounced sweep-back, and the wings are cantilevers. The wing arrangement was evidently chosen so as to give a good view from both cockpits.
Two-seat civil sporting biplane initially powered by an 80hp (59kW) Annstrong Siddeley Genet II five-cylinder radial, and later a 65hp (48kW) Pobjoy P seven-cylinder radial. Illustrated is Bolas beside the only example, G-EBTE, in 1927. Here powered by an 80hp (59kW) Armstrong Siddeley Genet II air-cooled radial. The upper wings featured a pronounced sweepback but the lower wings were straight.
TWO "IMPS": On the right is Mr. Bolas, designer of the Parnall "Imp" which won the Balloon Bursting Competition. With him is Flt.-Lieut. Bonham Carter, who flew it.
SOME MORE COMPETING MACHINES: The Parnall "Imp" (Siddeley "Genet II") with Flying-Officer W. Bonham-Carter up. The Parnall "Imp" is unusual in having its top plane arranged with a pronounced sweep-back.
THE PARNALL "IMP" WITH ARMSTRONG-SIDDELEY "GENET" ENGINE: This aerial view, taken from a "Moth" kindly lent by the Wessex Club 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.
Balloon "Strafing": Flight-Lieut. Bonham Carter well on his target in the Parnall "Genet-Imp." The swept-back top plane of this machine caused an onlooker to describe it as "An 'Autogiro' with one vane seized up."
The "Pobjoy Imp": A Parnall 2-seater light biplane, equipped with the 60 h.p. Pobjoy engine, with which an air speed of 100 m.p.h. is obtained. Parts of the Pobjoy engine (which has previously been described in "Flight") are being made by A.C. (Acedes) Cars, Ltd.!
This is a general view of the machines grouped round the enclosures with the Parnall "Imp" in the foreground.
THE PARNALL "IMP" LIGHT AEROPLANE: Some Constructional Details. The engine plate is supported on a system of square tubes and angle sections, as shown in 1. The quick-release attachment of the aileron controls is shown in 2, while 3 illustrates the attachment of the interplane struts to lower plane. The same sketch also shows the general construction of the strut. The wing construction, consisting of false spars or stringers, and ribs or formers, with spruce planking, is indicated in 4, and the aileron and elevator construction in 5.
Parnall "Imp" Armstrong Siddeley "Genet" Engine