Flight, November 1924
THE CASPAR "C.17" LIGHT MONOPLANE TWO-SEATER
A.B.C. "Scorpion" Engine
SOME months ago we mentioned in FLIGHT that the first A.B.C. "Scorpion" for light 'plane use had been sold, and that, curiously enough, it had been sold to a German
firm, the Caspar-Werke of Travemunde. The machine for which the "Scorpion" was intended was finished some time ago, and has, we understand, proved to fly quite well with the small British power plant. The "C.17" - as the two-seater light monoplane is called - was designed by the chief designer of the Caspar works, Dipl.-Ing. Ernst Ritter von Loessl, and incorporates several unorthodox features. For the explanation of the objects which the designer had in mind, and the reasons for some of the arrangements chosen, we are indebted to Herr C. W. Erich Meyer, Editor of Deutsche Motor-Zeitschrift, who gives an authoritative analysis of the machine in his journal. Incidentally, it may be mentioned that Herr Meyer, who was, during the War, a member of the famous Richthofen squadron, carried out the test flights on the Caspar "C.17," so that he writes with first-hand knowledge.
The requirements which the designer set out to meet were that the machine should have as nearly as possible the minimum amount of engine power so as to follow in this respect the experience gained with light 'plane single-seaters. The fuel consumption was to be kept low so that the cost of petrol should not work out at any more, proportionately, for the two-seater than for the single-seater. In spite of this the machine was to have a performance and manoeuvrability suitable for school work, sporting flying, and touring. In spite of these somewhat difficult requirements the construction to be adopted was to be such as to make for cheapness, as it was considered that otherwise the whole object of the machine would be defeated.
The requirements briefly set out above influenced the designer in planning his machine, and the decision to make it a low-wing monoplane of high aspect ratio was, of course, the result of a desire for high aerodynamic efficiency. This was essential in order to give the machine, without any undue increase in engine power, a power reserve comparable with that found desirable in commercial aircraft. Constructional details were planned so as to give low production cost, not by building the machine of inferior and cheaper materials, but by simplifying as far as possible the constructional work, reducing the number of parts to a minimum and standardising certain sizes of members for use throughout the machine (Thus, to take an example, the stringers used in fuselage, tail and wings are all of the same dimensions, and are of triangular section.)
As the accompanying illustrations will show, the Caspar "C.17" low-wing monoplane is of rather typical "German" lines, i.e. the high aspect ratio, large span wing, and very short fuselage are features which one has come to associate with aeroplanes of German origin. In this country the tendency has been rather to lengthen the body so as to get the tail out of the downwash and acting on a longer lever arm. In the Caspar, however, the designer had clearly in mind what he wanted to attain, and the short body is not merely a conventional proportion, but has been deliberately chosen. The high aspect ratio was, of course, adopted from considerations of efficiency. It was also desired to make the machine as easy as possible to fly, so that even a pilot of indifferent skill might handle it with comparative safety. In order to attain this end the designer adopted a novel principle of wing attachment by which, under the action of gusts, the wings can adapt themselves to some extent to the new conditions without upsetting the balance of the machine. A glider designed by Herr von Loessl incorporating this feature was flown in the Rhon by Herr Meyer in 1922, the flexibly-mounted wings being intended to take advantage of the so-called "Knoller-Betz Effect." In the Caspar "C.17," however, the designer has gone a step farther, and has so interconnected the elevator and the wing that the two move in unison, maintaining the trim of the machine under all conditions, and retaining the controllability. These considerations and a desire to make the machine as little prone to spinning as possible are the main reasons for the short fuselage. The wing section also was chosen with a view to helping in this direction, that employed being the "Gottingen No.348."
Constructionally the Caspar "C.17" is of interest mainly on account of the principle employed of building up the machine as a series of complete units which, when finished, are attached to a central framework of steel tubes, which forms the backbone of the machine and serves as a central point of attachment for fuselage, wings, engine unit, undercarriage and the weight of pilot and passenger.
The fuselage is of rectangular section, and consists of a framework of stringers with planking of three-ply wood. All stringers are of the same triangular section as used in the wing, and the fuselage struts, which are arranged to form a triangulated structure, are of the same section. At the stern the fuselage proper terminates in a rectangular frame, to which the elevator and rudder are hinged, and which carries the tail skid. The streamline is maintained by a Duralumin pyramid bolted to the stern frame, the elevator being divided to clear it. There is no fixed tail plane, nor any vertical fin in front of the rudder, which latter is mounted above the fuselage.
The wing has two box-section spars, placed relatively close together, and the total number of ribs, as well as the number of different ribs, has been kept as small as possible for the sake of cheapness. Thus the rib spacing is 0-8 to 1 m. (2 ft. 7 ins. to 3 ft. 3 ins.), and there are but three different sizes of ribs. The space between the two main spars is braced by tubes running from, the lower edge of the front spar to the top edge of the rear spar, as shown in the illustration, while another series of tubes run from about the neutral axis of the rear spar to the trailing edge. The wing covering is in the form of three-ply attached to the widely-spaced ribs. Stringers running parallel with the spars stiffen the covering against local buckling.
Reference has already been made to the springing of the wings. One of our illustrations shows the springs on the inner ends of the wing spars. The wings pivot around a point just aft of the front spar, the fitting being shown in the photograph. The long bolt, with wedge-shaped nuts at top and bottom for engaging with the slotted steel plates, is a feature of all Caspar machines, and is employed for making a quickly-detachable joint where a wing is built in several sections. The addition of the springs is, however, new. Herr Meyer states that during the test flights which he carried out the sprung wings were found to render the machine particularly insensible to gusts, steady flight being maintained even on a very "bumpy" day. Whether the other object of this wing arrangement, i.e. to take advantage of pulsation and to gain extra lift therefrom, was attained is not stated, and, in any case, it would appear that the amount of extra lift that can be obtained in this manner must be too small to be easily noticeable.
The A.B.C. "Scorpion" engine is mounted on a metal frame work and drives a Bork airscrew of 1-2 m. (3 ft. 11 ins.) diameter. The amount of fuel carried is sufficient for 2 to 2 1/2 hours. It is stated that the engine has a power output of 30 h.p. at 3,200 r.p.m., and during test flights it was found that the machine would fly without losing height with the engine throttled down to 2,200 r.p.m., corresponding to approximately 16 h.p. The amount of reserve power should, therefore, be satisfactory, and allow of normally flying with the engine throttled.
The empty weight of the machine is 145 kgs. (320 lbs.), and the total loaded weight (with a useful load of 180 kgs.) is 715 lbs., which is extraordinarily good and compares favourably with the lightest (the Hawker) machines in the Lympne competitions. No speed tests over a measured course have been carried out, but, according to the air-speed indicator, the speed in horizontal flight was between 110 and 120 kms./hour (69 to 75 m.p.h.), with the engine running at 3,200 to 3,300 r.p.m. During climb at 3,000 to 3,100 r.p.m. the airspeed indicator showed 100 to 105 kms./hour (62 to 65 m.p.h.). An altitude of 300 m. (985 ft.) was reached in about 2 mins., and the machine climbed to 1,000 m. (3,280 ft.) in 12 mins.
On starting for the first test flight, Herr Meyer states that the machine got off in 60 m (67 yards) against a wind of 4 to 5 m. per second, and on a second attempt this distance was reduced to 40 m. (45 yards). The first time the machine was flown with a passenger (Herr von Loessl) on board it had to take off in a cross wind, and the run required was then less than 80 m. On another occasion, and piloted by another pilot, the C.17, again carrying a passenger, reached an altitude of 1,450 m. (4,750 ft.), but could be taken no higher on account of clouds. The estimated ceiling is 3,500 m. (11,500 ft.). The landing speed is 50 to 55 kms./hour (31 to 34 1/2 m.p.h.). Assuming a maximum power of 30 b.h.p., the power loading of the Caspar C.17 is 23-85 lbs./b.h.p., while the wing loading is 4 1/4 lbs./sq. ft.