The OBS Urubu

(Translated from "Flugsport" by The Sailplane magazine (1933) )

The two-seater Obs Urubu of the Research Institute olf the Rhön-Rossitten Gesellshaft was designed by A. Lippisch. The aircraft was intended in the first place for the investigation of therrmal up-current regions. In the summer of 1932 the Research Institute decided, on account of the therrmal currents at the Griesheim aerodrome near Darmstadt, that temperature records should be made over larger areas and at different height levels, and that regions of upward and downward currents should be traced out from the distribution of regions of warm and cold air in the free atmosphere. These records have shown that, at aerodromes which are specially " thermically " suitable, frequent and almost stationary regions of rising currents develop, which even in the absence of any tell-tale clouds can be easily sought out by a sailplane. The charting of these stationary up-current regions will make it possible in the future to make more use than hitherto of auto-towing for advanced soaring flight..

These temperature observations for the charting of stationary upwind areas will be carried out over an extended range by be sailplane OBS in the coming spring. The observer's cabin of the OBS has therefore been given such roomy proportions that, in addition to the meteorological observer, an extensive range of instruments can be included for carrying out the desired observatlons. Further, the OBS will be used in the further development of the aerological observation service, Now that aeroplane towed flight of gliders has been successfully developed, the entry of the sailplane into the field of aerological research is made possible over an extensive range.

It has been planned to have the sailplane OBS towed by an aeroplane of the meteorological service to a height of 5,000 to 6,000 m. (16,000 to 20,000 ft.). After having cast off at this height, it is possible for the sailplane, even without the help of up-currents, to remain in the air for about another three hours. By this means it will be possible for the OBS, undisturbed by engine vibrations or exhaust gases, to carry out aerological observations to a far more extended degree than has hitherto been possible with the meteorological service aeroplanes.

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Above all, this sailplane is specially suitable for researches Into atmospheric electricity, which are rendered very difficult in a power plane owing to the exhaust gases. The wings, which are strutted, have a span of 26 m. (85.3 ft.) and a surface of 38 sq. m , (409 sq. ft. ). The weight empty is 390 kgs. (860. Ibs.) and all-up weight 540 kg. (1,190 lbs.), glving a wing loading of 14.2 kg. per sq. m, (2.85 Ibs. per s q. ft. ). The a ircraft is in consequence comparatively slow in fllght. The wilngs are tapered, and as the trailing edge is straight, this results in a slightly V-shaped planforrn .

In order to obtain sufficient aileron efficiency wlfh such a large span, the wing has a slight twist. The aileron on either side is divided into three sections, eachl with differently formed slots. [ If our memory is correct, the outer aileron slots would only let the air through from below to above, i.e., with aileron raised, while the action of the inner ones was the exact contrary. It was also stated that the outer ailerons worked dlifferentially, and that the middle and inner ones could be used to Increase the camber, presumably by pulling all four down together. ~Ed.]. The trial flights so far undertaken have shown that the desired aileron efficiencv has been attained.

The fuselage is built up of welded steel tubing with wire cross-bracing in the rear part. The enclosed pllot's cockpit is entered through the detachable hood and the observer's door. The central section of the wing is covered with Cellon where it adjoins the fuselage in order to provide an upward view for radiation observations. The observer's cabin is of such Iarge dimensions that a table can be fitted for writing down notes and observations during flight.

The elevator is of pendulum type, balanced as regards its weight. [We were Inlormed that the C. P. of the wing was stationary and so the elevator was theoretically superfluous in normal flight. Ed.. ] The rudder adjoins a small fixed fin. For asslsting the action of the rudder, small end panel rudders are a ttached to the wingtips in such a way that one or other of them moves simultaneously with a movement of the rudder at the tail. These end panels have proved their worth in tailless aircraft, and they allow the fuselage to be kept short.

In place of the sprung skid which is usually to be found on sailplanes, two wheels are provided, capable of being braked. As the OBS, on account of its great span, has a very low sinking rate, an air brake cannot be dispensed with. Immediately behind the strut connection in the inner part of the wing are two braking flaps immediately in front of the spar, in a similar manner o those of the "Dresden No. 9" two-seater. These brakes, w'hen put into action, cause a powerful dlsturbance of the lift distribution and with its increrease in the induced drag. By this means the gliding angle Iis adversely affected while a high lift figure is retained. Air and wheel brakes are worked by the same hand lever; the air brakes by' 'Pushlng: it forward, and the wheels by pulling it back .

Since the OBS carried out a short towed flight during the Rhön competition [Short's the word as the towing aeroplane, a Flamingo, was unable to get off and crashed into some trees. Ed.] it has been brought out again in the last few weeks by F. Starner and its flying qualities tested . The OBS with either one or two up, can be launched with a double launching rope and a team of about 20 men.

In the coming spring it will be installed at the Griesheirn aerodrome near Darmstadt for its first introduction to the research programme already mentioned.