Flight, October 1923

THE PARNALL "PIXIE" LIGHT 'PLANE
Douglas Engine

   OWING to the fact that we were unable to obtain from the manufacturers details and illustrations of the machines entered by George Parnall and Co., of Coliseum Works, Bristol, for the Lympne light 'plane competitions, it was not possible to include with the comparative scale drawings published in our issue of October 11 the drawings of the "Pixie," We have now, however, obtained these drawings, and have therefore thought that a brief illustrated description of the machines may be of interest. In our issue of October 18 several sketches showing certain constructional details of the "Pixie" were published, and to these we would refer readers, as space does not allow of publishing them again this week. As entered for the Lympne competitions the Parnall "Pixie," designed by Mr. Bolas, chief designer to George Parnall and Co., was built in two types, one with large wings and a 500 c.c. Douglas engine, and the other with small wings and a 750 c.c. Douglas. Instead of building two complete machines, the one fuselage, chassis and tail were made to serve for both types, the machine being changed from one type to the other by merely changing the engine and wings. The constructional details are the same for both types, so that the following remarks may be taken, except when otherwise stated, to refer to both. Incidentally, the idea of interchangeable wings of various areas might be worth further development when we come to consider the marketing of light 'planes, and it would appear that the idea might be extended to include a change from single-seaters to two-seaters. This we do not claim as an original idea, as something of the sort has already been done (by Capt. Barnwell while he was associated with the Bristol Aeroplane Company), but it seems that in the case of a light 'plane the "side-car" idea has a good deal to recommend it. At any rate, we offer the suggestion for what it is worth. It should not be difficult to design a fuselage in which a central portion can be interchanged for one with two seats and carrying a larger wing or wings. Thus, if the owner-pilot desires to go alone, but to travel reasonably fast, he would use the machine with small wings and one seat. If he wished to take a passenger he would merely disconnect the central portion of the fuselage and replace it with the two-seater, large-wing portion. In other words, he would attach his "side-car."
   However, to return to the Parnall "Pixie." The machine is a low-wing monoplane, with the two halves of the wing hinged to the lower longerons of the fuselage and braced by two streamline steel tube struts. These struts are attached to the upper longeron of the fuselage by a very neat adjustable fitting, which allows of setting the angle of incidence and dihedral within very fine limits, A sketch of this fitting was published last week.
   The wings themselves are of normal construction, the spars being of built-up I-section, with the web resting in grooves in the top and bottom flanges, somewhat after the fashion of a wing rib. The material used is spruce. The wing design is, however, unusual in that, although over the inner portion of the wing the two spars are parallel, from just outside the points of attachment of the bracing struts the rear spar slopes forward to meet the front spar at the tip. Thus any tendency on the part of the rear spar to deflect under aileron loads is prevented by the front spar, which is placed at a deeper part of the section. In fact, according to how the spar positions are chosen, it would probably be possible so to design the wing that the tendency to warp under aileron loads was in a direction opposite to the usual, i.e., to produce auto-warping. The drawback to this construction appears to be that a joint in the rear spar is necessary, but probably it is no difficult matter to provide adequate strength at the joint without adding undue weight.
   The ailerons are of usual type, but are operated "differentially," in the manner invented by Mr. Hagg of the de Havilland Aircraft Company, i.e., the downward-moving flap moves through a smaller angle than that traversed by the upward-moving flap on the opposite side. A sketch showing the levers and setting used for this purpose was published last week.
   The fuselage of the Parnall "Pixie" is of rectangular section, fabric covered over the greater portion of its length. The longerons are of spruce, and diagonal struts, attached to the longerons by ply-wood plates, form the bracing of the structure. In the forward portion of the fuselage, where the stresses are more severe, these struts are attached to the longerons by metal plates. The pilot's cockpit is placed between, and above, the wing spar roots, and the view obtained is extremely good, except straight down, where it is, of course, obstructed by the wing. The controls are of usual type, i.e., a "joy-stick" for elevator and ailerons, and a foot bar for the rudder.
   The Douglas engine is mounted on a structure of six steel tubes from the front ends of the longerons proper, a fireproof bulkhead separating the engine from the pilot's cockpit. In both types a chain reduction gearing of 2 1/2 to 1 is employed, the propeller shaft, with journal and thrust ball bearings, being mounted on a sheet-steel bracket bolted to the top of the crank-case and surrounding the magneto. The petrol tank is carried in the deck fairing over the front portion of the fuselage, and as the engine is mounted low gravity feed has been possible. The induction pipe is jacketed and heated by exhaust gases carried to the muff by short branch pipes from the main exhaust pipes.
   The undercarriage is of unusual design, and consists of an inverted V of steel tubes, with its apex in the top of the fuselage and the "legs," which slope forward, carrying the tubular axle. The wheels are placed some distance out from the point of attachment of the struts, so that the axle itself not only gives a wide wheel track, but also provides a certain amount of springing. The elasticity of the tubes provides a certain amount of springing, aided by the deflection on the pneumatic tyres, and the amount of wheel travel actually obtained is approximately the same as that of other light 'planes using rubber shock absorbers, i.e., about 3 or 4 ins.
   The main characteristics of the Parnall "Pixies" are as follows: Length, o.a. (both types), 18 ft. 0 ins.; span (type I), 28 ft. 6 ins., (type II) 17 ft. 10 ins.; maximum chord (both types), 4 ft. 7 ins.; wing area (type I), 100 sq. ft., (type II) 60 sq. ft.; weight, empty (type I), 276 lbs., (type II), 279 lbs.; weight loaded (type I), 457 lbs., (type II) 460 lbs.; wing loading (type I), 4-57 lbs. per sq. ft., (type II) 7.66 lbs. per sq. ft.

Flight, September 1924

THE PARNALL "PIXIE III" LIGHT MONOPLANE (No. 17)
Bristol "Cherub" Engine

   THE two machines entered by George Parnall and Co., of Bristol, are identical, except that by adding a top plane one is converted into a biplane. The original machine remains unchanged by this addition, the necessary fittings being provided on fuselage and wings, and the original wing structure taking the stresses transmitted to it from the top plane via the inter-plane struts. In general lines the Parnall two-seater monoplane is similar to last year's single-seater, and constructionally also much the same principles have been adopted.
   Fundamentally, the Parnall monoplane two-seater is a low-wing monoplane with strut bracing and wings tapering in chord and thickness. The chord is uniform for approximately half of the span, and then the two spars converge to a point at the tip, the front spar running straight while the rear spar slopes forward to meet it. This it will be remembered, was a feature of last year's machine, and was chosen by Mr. Bolas as giving a torsionally stiff structure. The rear spar is pin-jointed where the change in direction occurs, giving zero bending moment at the joint. It would appear likely that aerodynamically the taper is also beneficial, so that there is very good reason for an arrangement which must necessarily be somewhat more costly to construct than one in which all ribs are alike. The wings taper in thickness from a maximum depth at the point of the strut attachments to a small depth at tip and root.
   The main wing spars of the Parnall "Pixie III" are of box section, as shown in sketch 5, and narrow strips are screwed to the flanges externally on which rest the three-ply webs of the ribs, thus relieving the flanges of shear loads. The rib flanges, incidentally, are of the divided type, over the nose portion, the webs rising to the full depth of the wing section. Tie-rod drag bracing is employed, and it is of interest to note that the lift struts are attached not direct to the spar flanges, as is usually done, but to the tubular drag struts. The arrangement is shown in sketches 5 and 6. This form of attachment avoids all twisting stresses on the spars, and as the strut fittings are placed close to the spar walls the tubular drag struts are not subjected to bending loads but merely to shear.
   The pin joints of the wing roots to the fuselage are not placed on the neutral axis, but on the lower flange, as shown in Figs. 1 and 2, and the front hinge is a universal joint about which the wing can swing when the rear spar pin and the strut attachments are cast adrift. The wing can then be tilted with its trailing edge upwards, and can be folded back along the sides of the fuselage. The machine is shown with wings thus folded in one of our photographs.
   Fig. 4 shows parts of the differential aileron control. The cables pass from the cockpit along the wing to a double-crank lever mounted on a vertical tubular pillar. At its upper end this pillar carries another small (single) crank, from which a short tube runs to the aileron crank on top of the wing. By suitable setting of this crank a differential movement is imparted to the ailerons - i.e. the rising one moves through a greater angle than the dropping one.
   The fuselage is of similar construction to last year's single-seater "Pixie," and consists of four longerons braced by triangulating struts attached by three-ply gussets. This form of construction gives a very light structure, and requires no trueing-up once it has left the stocks. A side elevation of the fuselage is given, from which, in conjunction with what has already been said, the construction of the fuselage should be fairly clear. At the stern the fuselage terminates in a vertical knife-edge formed by a channel-section metal strip. The short elevator control tube passes through an opening in this strip from the arms of the short layshaft inside the fuselage, so that the elevator has but one crank, placed underneath and just aft of the stern post.
   The undercarriage is of very simple type, and consists of two "legs" of steel tubing running through the floor and up to the top longerons where the upper ends are anchored to these tubes, at the lower end, the axle is secured, and there is neither lateral nor fore and aft bracing, the "legs" and axle being all cantilevers. The "legs" have an oleo-pneumatic type of shock absorber incorporated in them giving a fair amount of travel so that what with the movement of the telescopic tubes and the deflection of the axle the springing should be all that can be desired. The tail skid consists simply of three pieces of Malacca cane, each forming a 120-degree sector, glued and taped together. It is extremely light, but should be well up to its work.
   The Bristol "Cherub" engine is mounted on a tubular structure in the nose of the fuselage, and is, as the photograph shows, entirely cowled-in except for the cylinder heads. The petrol tank is mounted under the deck fairing, behind the engine bulkhead.
   Altogether, the Parnall "Pixie III" impresses one as being a most business-like proposition, the simplicity of the structure and the ease with which the wings can be folded and again erected without requiring any trueing-up being particularly valuable features, especially for the private owner-pilot of the future.

The Biplane (No. 18)

   Most of the foregoing remarks apply equally well to the Parnall biplane, with the exception that an Anzani engine is fitted, and with such extra remarks as the addition of the top plane requires. Reference has already been made to the fact that the biplane is in all essentials the monoplane with a top plane added, thus converting it into a biplane. FLIGHT suggested more than a year ago that some such arrangement be adopted so that the private owner could use his machine both for carrying an extra passenger and for flying somewhat faster when solo. In the case of the Parnall biplane the addition of the second wing was not so much, we believe, chosen from such considerations as with the idea of providing larger wing area and thus giving a lower landing speed for the purpose of the competition, where a premium is placed on low landing speed. That the top speed will be pulled down somewhat by the presence of the second plane seems likely, but even so it may well be that the biplane will collect more marks than the monoplane. At any rate, it will be interesting and instructive to compare the performance of the two.

Flight, September 1926

British Light ‘Plane Development & Lympne Meeting

THE 1926 MACHINES

No. 14. The Parnall "Pixie III" (Bristol "Cherub" Engine)

   It may be recollected that in the 1924 Lympne meeting one of the Parnall "Pixie III" machines was convertible into a biplane by adding to the normal low-wing monoplane a top plane of slightly smaller area, the sloping strut bracing of the monoplane serving also for the support of the top plane. This top plane was added purely by way of gaining extra marks, and Mr. Harold Bolas, chief designer to Geo. Parnall and Co., much preferred the machine in its original monoplane form. This may explain why, in this year's competition, the "Pixie III" is to fly as a monoplane, although possibly the addition of the top plane might have enabled a slightly greater useful load to be carried, with a resultant gain in points scored. However, the monoplane is undoubtedly a good deal faster, and if the week happens to be windy it will score by having a higher cruising speed, and may thus easily make up for a slightly smaller useful load.