Flight, October 1922
THE PEYRET TANDEM MONOPLANE
Details of the Winning Machine
UNUSUAL as is the aerodynamic design of the Peyret tandem monoplane, its construction appealed to all experts at the first glance, and even before the machine took the air it
was conceded that, whatever peculiar views M. Peyret might have on the subject of disposition of wing surfaces, side areas, etc., he knew how to build a glider. The three-ply mahogany fuselage was evidently the work of an expert, and the wing bracing with one set of "N" struts on each side gave great rigidity in spite of evidently slender spars. Also, the differential control arrangement certainly looked extremely unlike what one would expect from an amateur enthusiast. Yet the fact remains that no one, ourselves included, really took the machine seriously until it had left the ground. It is useless to pretend that it was otherwise. Everybody was "taken in” and before the start most of us expected to see the machine crash on the hill-side. No sooner was it in the air, however, than it became evident that here was a machine which had something that most of the others lacked, i.e., controllability to an amazing extent. That it was as efficient, regarded purely as a glider in still air, as some of the others might be open to doubt, but that the pilot did have abundant control could not be doubted for one moment. The short, sharp turns bore testimony to that, as did the fact that even the most vigilant observation of the machine failed to reveal more than the slightest movement of the ailerons, indicating that at no time were the controls used to anything approaching their fullest extent. The rudder could be seen moving slightly from side to side, but otherwise only a minimum of control appeared to be required. Having thus frankly admitted our failure to appreciate at first sight the merits of the Peyret monoplane, we may proceed to give a detailed description of the machine.
The Peyret monoplane glider is of a type which has for many years been considered rather inefficient. Wind tunnel experiments on tandem surfaces have shown the rear plane to be far less efficient than the front one, and consequently, when large area was required combined with small span, designers got into the habit of choosing the biplane or triplane arrangement. Nor should we like to assert that they were wrong where power-driven machines are concerned. To employ the large rear plane means carrying about a considerable amount of rather superfluous structure weight which can ill be afforded. In a glider, however, this is but a small price to pay for the amazing controllability which the tandem arrangement gives, and to M. Peyret is due the credit for having realised this. Not only so, but having failed to find a French constructor willing to build the machine, M. Peyret set to work and built it himself. The whole of the work, therefore, is his own, from the first conception to the finished machine.
The tandem arrangement of the lifting surfaces necessitates a very strong fuselage; especially is this so when one considers the torsional stresses to which the fuselage may be subjected owing to a gust striking one wing before the other. Now the ply-wood covered type of fuselage is exceptionally good in torsion, hence this form of construction was chosen. The internal framework is very light, consisting of spruce longerons and struts, with here and there a multi-ply former (reduced to a mere skeleton by sawing away most of the material except diagonals) where local stresses call for extra rigidity. Over this light framework is placed a covering of three-ply mahogany, approximately 3/32 in. thick.
In section the fuselage varies considerably from point to point. At the cockpit the section is approximately rectangular, while forward and aft of this point the sides are of the "tumble-home" variety. The front two-thirds of the body is surmounted by a triangular section, which towards the stern merges into the inward-sloping sides of the main structure. Immediately in front of, and also behind, the cockpit the triangular section has its sides curved inwards, so as to improve the pilot's view. Along the ridge of the triangular "roof" runs a nearly straight longitudinal member, which is extended over the pilot's cockpit. Thus a weakening of the structure at this point is avoided.
The wings are characterised by a pronounced dihedral and back-sweep. The former has doubtless been necessitated by the large side area of the fuselage, and the latter by a desire to place the pilot behind the trailing edge and yet get the centre of gravity sufficiently far forward to make the front plane more heavily loaded than the rear, thus obtaining longitudinal stability by placing the front wing at a larger angle of incidence than the rear.
Constructionally the wings, front and rear of which are of equal span and area, consist of light ribs, placed somewhat far apart, carried on tubular spars of Duralumin. Short nose ribs extending from leading edge to front spar are placed half-way between main ribs. The wing section used is fairly thin, although of considerable camber, and the wings are braced by a single pair of "N" struts (Duralumin tubes enclosed in wood fairings) on each side. These struts work in compression as well as in tension, there being no top bracing. Attachment of spars to the fuselage is by short “T"-shaped bolts passing through the ends of the tubular spars and through short roots, made of wood, built into the fuselage just below the "ridge pole."
The undercarriage is of somewhat unusual design, although being of the simple two-wheel type. The axle passes across underneath the floor of the fuselage, but another member, also a tube, shaped somewhat like a very flat inverted "U," passes through slots in the sides and carries the rubber-cord shoe absorbers. Radius rods in the form of tubes run from the axle forward to points underneath the floor of the body, a short distance behind the nose. The undercarriage is divided in the centre so as to allow of removing the inverted "U," the two halves fitting into a central sleeve, where they are held by bolts.
As the greatest merit of the Peyret monoplane lies in its very effective controls, these will be dealt with at some length, and in order further to facilitate understanding of their working we would refer readers to the accompanying diagram. The essential feature of the controls themselves, apart from the system of working the four wing flaps simultaneously, is the use of a differential, formed by three bevel gears. One of these is mounted on the control column, and is placed in a transverse plane. This wheel engages with two others of the same size, but placed in a longitudinal plane. The action is similar to that of the differential of a motor-car. When the control column is moved in a fore-and-aft direction, the two side wheels move with it, without any movement relatively to one another. When, however, the control column is moved from side to side, the two side wheels move in opposite directions. To the two side wheels are attached short shafts, carrying on their outer ends cranks from which cables run to the wing flaps. The manner of connecting up the control cranks and flap king-posts will best be understood from a reference to the diagram. From this it will be seen that when the "stick" is pushed forward the flaps of the front wing are raised, those of the rear wing lowered. When the control column is pulled back, the reverse is the case. When the column is pushed to the left the starboard flaps of both front and rear wings are depressed, those on the port side being elevated at the same time. Pushing the "stick" to the right has, of course, the reverse action, while any combination of lateral and longitudinal movement is possible.
The same movement could, of course, be obtained by the use of crank levers, but arrangement would have to be made for the ends of one lever sliding in slots in the other, and the use of bevel gears results in a much more positive, and mechanically better, action.
Directional control is by rudder worked from a foot bar in the ordinary way. We are not quite certain as to how great effect the rudder has, placed as it is so close behind the rear plane, but it appears probable that, even assuming the rudder to be not particularly effective, the machine can be easily turned by the use of the wing flaps. In other words, the machine can probably be turned easily with the flaps, even with the rudder central, whereas we are somewhat doubtful regarding directional control with the wing flaps locked in the neutral position. Nor does it appear to matter very much, as the machine can undoubtedly be turned in a very short radius by the combined use of rudder and flaps.
Apart from its extraordinary controllability, the Peyret monoplane would appear to possess certain other advantages. Thus the view obtained from the pilot's cockpit is exceptionally good, owing to the fact that the front wing is placed relatively high so that the pilot's eyes are approximately in line with the wing chord, while he is situated behind the trailing edge and therefore has an unrestricted view downwards. Also, in a crash, the machine should offer a maximum of protection to the pilot, as there is a good deal of fuselage both in front and behind him, while laterally the two pairs of wings project a considerable distance, reinforced by the "N" struts.
Altogether, the Peyret monoplane is a machine of more than ordinary interest, and there can be little doubt that it will, as the French say, faire ecole. The tandem arrangement offers so many advantages that it is not to be doubted that machines of similar design will appear, probably incorporating detail improvements, but following the Peyret (and incidentally S. P. Langley, whose "aerodromes" were of the tandem type, and whose ideas have been vindicated to no small extent by the performance of this machine) in the disposition of their lifting surfaces.
The weight of the Peyret glider is given by M. Peyret himself as 67 kgs. (147.4 lbs.). The machine looks as if it might be heavier than that, but the wings are certainly very light, and the fuselage is probably lighter than it appears to be. The undercarriage, however, is probably fairly heavy. Assuming the weight to be 150 lbs., and the same figure for the pilot, the total weight is 300 lbs., which, with an area of 153 sq. ft., gives an average loading of 2 lbs./sq. ft. Actually the loading of the front wing is greater and that of the rear wing smaller than this figure.