Flight, January 1924
THE MAGNAN MONOPLANE GLIDER
A French Machine Designed for Gust-Soaring
WHILE soaring in an up-current of air, over the slope of a hill or range of hills, presents no particular difficulty, provided the rising current is strong enough
and the machine has a sufficiently good gliding angle and ample controllability, gliding in a horizontal wind, by making use of such turbulence as exists in the wind, is at present outside the practical possibility, although theory indicates that a certain amount of energy should be available for soaring flight. This form of gliding, which has been termed "gust-soaring" has been examined theoretically by Knoller and Betz, and experimentally in the wind tunnel at Gottingen. In both cases the conclusions reached were that a not inconsiderable amount of energy was present which, with a suitable machine, should be available for flight. A few experiments with man-carrying machines have been made, but with rather inconclusive results. It should be remembered that it is very often difficult to distinguish between true "gust-soaring" and gliding in a wind having, perhaps quite unsuspectedly, an upward vertical component, and that in many instances during a glide a machine may be helped partly by one and partly by the other form of energy.
Apart from the studies made in Germany, several French experimenters have attacked the problem of "gust-soaring," among them being Dr. Magnan, who has made a close study of bird flight, and who has now reached a stage when he feels that the theories involved are sufficiently well, although not completely, understood to justify the building of a machine for the purpose of carrying out actual flying experiments. This machine, which forms the subject of the following notes, is of unorthodox design, both aerodynamically and structurally, and for the information on which the description is based as well as for the illustrations, we are indebted to our French contemporary L'Air of January 1, 1924, in which appears an article by M. Andre Lesage dealing with the Magnan glider.
Basing his design to some considerable extent upon the characteristics of web-footed birds, Dr. Magnan has produced a cantilever monoplane, the wing of which is of uniform chord over approximately one-half of its span, but tapering to a point at the tips. The leading edge, it will be seen, is straight and the taper is provided solely by the trailing edge. Near the root the wing is swept down suddenly and sharply to form a pronounced dihedral angle. This angle, however, extends over but a few feet of the span, and the rest of the wing is at a smaller, although still considerable, dihedral. The wing tapers in thickness as well as in chord, and the angle of incidence is progressively altered, being in the neighbourhood of 20 degrees at the root, where the wing joins the body. In addition to the change in section and angle, the wing is unusual in that the ribs are so constructed that they are capable of being flexed to a very considerable extent under varying loads. Thus one can scarcely speak of any particular wing section in the Magnan monoplane since the number of variables is unusually great. It will be noticed from the drawings that no ailerons are fitted. Lateral control is by wing warping.
The fuselage is short in proportion to the span, and an exceptionally large portion of it projects ahead of the wing. This results in the tail being very close to the wing, only about one chord-length separating the trailing edge from the forward end of the fixed tail plane. It appears rather doubtful whether directional control will be sufficient under these circumstances, although a fairly large rudder is fitted. The "fin area" of the forward portion of the fuselage is very considerable, although of rounded, section, and altogether the machine does not look reassuring from the point of view of directional stability It would seem that the designer has attempted to copy the outward shape of a bird without taking into consideration that the range and number of movements of which a bird's wing is capable enable it to carry out with perfect safety manoeuvres which a machine outwardly resembling the bird but lacking its adaptability could not hope to imitate.
Constructionally no less than aerodynamically the Magnan glider is somewhat unusual. The monoplane wing has but a single spar, of box section and built of wood. The construction of this spar must have presented considerable difficulties, as there are two fairly sharp bends in each spar, one a few feet out from the body, where the horizontal cabane meets the spar, and another (this in a horizontal plane) a few feet from the tip. where the spar tip is swept forward to meet the straight leading edge. The ribs have top and bottom flanges of ash, the lower flange, which runs from leading to trailing edge, being screwed and glued to the lower face of the spar. The top flange stops short of the trailing edge, about one-third of the chord from it, and is so attached to the lower flange and to the spar that it can slide a short distance in a fore-and-aft direction, thus allowing the trailing edge to flex. The details of the arrangement are not available, but we understand that they constitute a patent. Near the wing tips the ribs slope outwards, and also they are so mounted on the spar as to give a pronounced "wash-out" to the wing. Lateral control is by warping, but instead of the warp causing a change of angle without sensible change in camber, in the Magnan monoplane both angle of incidence and camber are altered. It would appear that the force on the control column necessary to produce the required amount of warp must be very considerable. The fabric covering is applied in a special way, which is claimed to prevent wrinkling when the wing is being warped.
The fuselage is of egg-shape section, and is built up of formers alternating sloping back and forward, thus forming a series of Vees as seen in side view. To these formers are attached four main longerons and a great number of stringers, and wire bracing is employed for stiffening the structure against torsion. (The Vee formation of the formers plus the four longerons already provide a structure stable under plain bending loads.) The fuselage is fabric covered except at the extreme nose and stern.
The tail is of more or less orthodox design, but is supported on a duralumin cone bolted to the rear bulkhead of the fuselage proper.
A simple undercarriage consisting of two wheels carried on a duralumin axle is fitted, the axle being sprung by rubber cords anchored inside the lower portion of the fuselage.
The pilot's seat is mounted on longitudinal rails, somewhat like the sliding seat in a boat, and for fore-and-aft control he can alter the position of the centre of gravity by sliding the seat along. The ordinary controls are of the usual type.
The manner in which it is hoped to carry out gust soaring with the Magnan glider is as follows. The machine will be launched from a cliff on the coast, and will glide into the wind until fairly low over the sea. It is assumed that gusts will be present, and that these occur at such intervals as to enable the pilot to manoeuvre the machine in the manner required to extract energy from the fluctuations in the wind. During a gust the pilot will pull back the stick, and if necessary shift his seat back so as to bring the tail down quickly. As the gust dies down he will push the stick forward and slide his seat forward at the same time so as to avoid stalling the machine. During a lull it will be the pilot's endeavor to glide forward with the minimum loss of height, i.e., at the best gliding angle for the particular conditions. As soon as he feels another gust rising he will again elevate, and so the cycle is continued with alternate elevations and depressions. Dr. Magnan considers that another method would be to glide down-wind during the lulls and up-wind during the gusts, but that it is doubtful if the machine could be manoeuvred quickly enough to make this form of gust-soaring feasible.
Some preliminary tests over land have been made with the machine, piloted by Canivet, and these are stated to have indicated that the machine should, under suitable conditions, be capable of taking advantage of a gusty wind.
The main characteristics of the Magnan Monoplane, known as the "Type Marin M.2," are as follows: Length o.a., 4-95 m. (16 ft. 3 ins.); span, 11-5 m. (37 ft. 9 ins.); chord (root), 1-3 m. (4 ft. 3 ins.); wing area, 10 -25 sq. m. (110 sq. ft.); weight of wing, 60 kgs. (132 lbs.); weight per sq. ft. of wing, 1-2 lbs.; weight of machine (empty) 130 kgs. (286 lbs.); weight in flying trim, 200 kgs. (440 lbs.); wing loading, 19 kgs./sq. m. (4 lb./sq. ft.). As alighting on the sea will be one of the normal functions of the machine, the fuselage and wings have been made watertight, the opening for the wheel axle being bulkheaded off from the rest of the fuselage. The wing loading, it will be seen, is fairly heavy, and the structural weight appears to be greater than usual in gliders. Probably this is due, mainly, to the wing construction. Actual tests over the sea will be looked forward to with interest.