M.Simons The World's Vintage Sailplanes 1908-45
Theoretical work showed that for minimum rate of sink, a glider needed its wing area distributed over as great a span as possible: long, narrow wings were required, rather than broad, short ones, even though the structural difficulties
multiplied rapidly with every extra metre of wingspan. Almost nothing was known about large-span monoplanes in an era dominated entirely by strut and wire-braced biplane powered aircraft. When they designed the Konsul, the Darmstadt students swept aside all doubts. The wing span was 18.7 metres with an aspect ratio of 17. The cantilever wing was built in three parts, with the tips attached by a method devised by Junkers. The 8 metre centre section mainspar was a box in cross section, the tip spars were 'I'-sectioned, and they slid into a metal sleeve on the end of the inner spars, locked in place by a horizontal steel pin passing through. There was a light false spar to carry the aileron hinges and to stiffen the ribs of the inner panels. The joint in this member was a ball and socket bolt. After rigging the wing, the gap at the junction was closed by a simple plywood strip.
The ailerons were very large and broadened toward the tip. They were geared differentially, so that the outer aileron in a turn drooped only slightly whilst the inner one was raised more. This reduced aileron drag which acts against the turn. Ailerons and rudder were also linked so that the rudder would always assist the ailerons. Like them, the rudder was enormous. At a later stage the ailerons were reduced in size.
The aerofoil section was Goettingen 535. Wind tunnel tests showed it had an excellent performance at low speeds and high angles of attack, and it became one of the most popular profiles from this time on for fifteen years or so.
The wing was mounted directly on top of the fuselage with three bolts to the main vertical frames. The fuselage itself was a plywood shell with the minimum of internal stiffening. It had a lozenge or almond-shaped cross section, the pilot sitting upright just ahead of the wing. The Konsul was probably one of the first sailplanes to be fitted with instruments of any kind, an altimeter and an airspeed indicator, but sensitive variometers did not exist and a pilot could judge his success in soaring only by looking at the hill slopes nearby and watching the slight movements of the altimeter. Many earlier gliders had possessed a fairly elaborate undercarriage. The Konsul, by contrast, relied on a simple central wooden skid and thus adopted the characteristic ‘wing down’ attitude of all sailplanes ever since, when grounded. The tailplane was of generous span and huge elevators matched the rudder for area.
At the Rhoen in 1923, Botsch, one of the young designers, achieved a new distance record of 18.7 kilometres. In 1925 the Rhoen Competition took place as usual in August. A team came from the USSR, bringing their own sailplanes, but they did not have much success in the competition. A new pilot flew the Konsul, Johannes Nehring. He won the Wasserkuppe distance prize and made the best gain of height. Later in the year there was a national gliding competition in the Crimea, in the far south of the USSR. At the Wasserkuppe the Russians had extended an invitation which the Germans were delighted to accept. They took along several of their best sailplanes. Bubi (Kid) Nehring, as he was nicknamed, achieved a new world record of 24.4 km.
Back in Germany the Konsul continued in use for two more years until it was completely smashed in a bad landing when flown by a new pilot. The lessons it had taught the designers had by then been well understood. For the next few years virtually every new advanced sailplane that appeared could be described accurately as an improved Konsul type. Some, however, were not as good. In one leap, three years after the first gliding contest, the Darmstadt student group had achieved a plateau in sailplane development.
Konsul: Span, 18.70 metres. Wing area 21.00 sq m. Aspect ratio 16.66. Empty weight, 185 kg. Flying weight, 270 kg. Wing loading, 9.10 kg/sq m. Aerofoil, Goettingen 535. Best glide ratio. 1: 21.4.
Flight, January 1924
LIGHT ‘PLANE AND GLIDER NOTES
ONE of the most interesting of the machines which took part in this year's Rhon competitions was the Darmstadt "Konsul," on which Botsch covered a distance of 12 miles. By the courtesy of our German contemporary Flugsport we are able this week to publish the general arrangement drawings of this machine, and also a curve of gliding angles. The machine was designed for long-distance flights, as the Darmstadt students considered that the problem of soaring in an up-current had already been solved, the quality required being low rate of descent. For long-distance work, however, it was necessary that the machine should be able to cover the greatest possible distance for a given loss of height. High L/D ratio, or, in other words, a good gliding angle in still air, was wanted, and in order to cover a good distance under varying wind conditions a good gliding angle over a wide range of speeds was required. The result of these considerations was that the designers decided to "go the limit" on aspect ratio, and from the drawings it will be seen that this appears to have been done, the wing span being no less than 18-1 m. (61 ft. 4 ins.), while the chord is 1-2 m. (3 ft. 11 ins.), giving an aspect ratio of 15-6. It is a matter for surprise that such a wing structure has been possible for a reasonable weight, and that sufficient controllability has been obtained.
FROM the accompanying curve the gliding angle of the Darmstadt "Konsul" can be ascertained under varying conditions. The curve indicates that normally, i.e., in still air, the "Konsul" has an optimum gliding angle of 1 in 21-4, at a speed of 14-8 m. per second (48-5 ft./sec. = 33 m.p.h.). This figure is, of course, extraordinarily good, and probably marks the maximum ever attained by any glider. For the "Konsul" it is found that when flying with a following wind of 10 m./sec. (32-8 ft./sec. = 22-3 m.p.h.), and in a rising current of -5 m. per sec, the best gliding angle is something like 1 in 160 and occurs at a speed of 13-2 m./sec. The worst condition is, of course, gliding against a wind and in a down current. If the head wind is blowing at 8 m./sec. and the down current is -4 m./sec., the best gliding angle is 1 in 10-6, at a speed of 20 m./sec. (44 -5 m.p.h.).
THE construction of the "Konsul" is of special interest, as it might have been thought well-nigh impossible to build such a wing with sufficient rigidity against torsional stresses. The manner in which it has been done reflects credit on the designers. The monoplane wing, which is in three sections, of which the centre section is of 8 m. (26 ft. 2 ins.) span, is built up on a single spar, reinforced against torsion by covering the entire leading edge with three-ply wood. In the centre section the spar is of box section, while in the ends an I-section spar is used. As will be seen, the wing does not taper greatly in chord, but it does taper considerably in thickness, and the angle of incidence diminishes towards the tips so as to get a better load distribution and increase the aileron effect.
AN interesting feature of the "Konsul" is the interconnection of rudder and ailerons. This has been employed in order to increase the controllability, which, as a matter of fact, is reported to be quite satisfactory. For small rudder angles the ailerons are not affected, but once the rudder exceeds a certain angle the ailerons are automatically brought into play to assist the turn. Only in this way could controllability be obtained with such large span and comparatively small rudder leverage.
THE aileron control is so arranged that, when right rudder is put on, the right aileron moves up through a considerable angle, whereas the left aileron moves down through a much smaller angle. In other words, the action of the ailerons of the Darmstadt "Konsul" is very similar in principle to the de Havilland patented differential aileron control, which, for one thing, forms a convenient form of aileron balance, and also has the advantage of reducing the tendency to swing the machine into a spin, owing to the great drag on the lower aileron set up when no differential movement is provided. In the glider, however, the ailerons, in addition to their differential action, are connected to the rudder, which is not, of course, the case in the de Havilland system.
THE fuselage is of oval section, with sharp edges top and bottom. It is covered with three-ply, and the undercarriage consists of a single central skid, enclosed in fabric and sprung by rubber cords. The "Konsul" is somewhat faster than the designers bad expected, partly because the weight was greater than had been anticipated, and partly, it is thought, because the wing fabric sagged considerably between the ribs, the section being heavily cambered, and therefore the value of KL somewhat smaller than that of the sections upon which the design estimates were based. Nevertheless, we think the Akademische Fliegergruppe Darmstadt deserve the very greatest credit for their machine, especially when it is remembered that financial difficulties were very great indeed. We wish to goodness there was more of the Rhon spirit in this country.