The Klemm L.25 Monoplane
A 40-H.P. really light aeroplane of outstanding design and performance.
During the past half-dozen years the trend of light aeroplane design has progressed along two distinct lines. In England the tendency has been to get away from the simpler type of machine and the low-powered plane has been almost wholly superseded by machines of 80 to 100 h.p. and there are indications that the open cock-pit type will give place to the coupé and cabin type before very long. Thus, British manufacturers appear to be concentrating on producing machines of the purely utility-cum-luxury class with high performance and high power.
On the Continent, however, particularly in France and Germany, the small-powered aeroplane is still being made and developed and vies with English type of so-called light plane in general popularity.
Foremost amongst these little machines is the Klemm low-wing monoplane, which to many readers of MOTOR SPORT is no stranger, since examples of the latest type and many of its predecessors have been amassing an enviable reputation by their performances ever since the first edition appeared in 1919. It may be remembered that this early Klemm was powered, like other light planes, with a motorcycle engine—a Harley-Davidson— but in spite of its limited power it flew well as a two-seater, and a number of remarkable flights were made with it both as a land and seaplane.
In the more advanced type of Klemm—the L.20—the engine used was a 20 h.p. Mercedes horizontally opposed twin. This type is still available and is being used all over the world, notably in Germany, France, South America and Australia, and one of the most recent achievements carried out with this little craft is a round-the-world flight by a young German pilot.
In 1926, it may be recalled, an L.20 was flown (with passenger) from Stuttgart to Vienna and Budapest and back and the route taken entailed the crossing of the Gross-Glockner Alps at a height of over 12,000 feet in very bad weather. Considering the size of the engine, which had bore and stroke dimensions of only 75 mm. x 100 mm. this flight was no mean achievement. Recently we had the opportunity of inspecting and trying one of the latest Klemm-L.25's, which has been used during the past two years for demonstration purposes by the English concessionaires, S. T. Lea, Ltd., of 141, New Bond Street, London, W.1.
The salient constructional features of the Klemm are the all-wood construction, the cantilever high-lift wing, and the entire absence of wire bracing. All the covering of the machine, with the exception of the controlling surfaces and the section of the mainplanes between the rear spar and trailing edge (which are fabric-covered), are of plywood. Both the wing thickness and the chord of the wings diminish towards the tips and the dihedral angle is 2°. The wings are quickly detachable for storage purposes. The cock-pits are in tandem, the instruments being in the rear, but dual control is fitted as standard. The elevator, rudder and ailerons are worked through cables which pass to their respective kingposts over pulleys, and it is noticeable that care has been taken in the design to avoid running the cables through acute angles.
The undercarriage is of the split axle type and is of steel tube construction. Landing shocks are conveyed to the rubber-chord absorbers via two vertical tubes, the former being neatly disposed in the wing roots close to the fuselage.
The power unit is the well-known AD9 Salmson nine-cylinder radial of 40 h.p. and the propeller is of wood, with metal covered tips. A fireproof bulkhead is placed between the engine and the gravity tank which is situated in front of the front cock-pit. The total capacity of the petrol tanks is 22 gallons, the major quantity of which is carried in the main tank housed in the port wing root. The fuel is pumped from this tank by means of a hand pump in the cock-pit, to the gravity tank and a very simple float-type indicator gives the pilot warning when a further supply is necessary. In our test flight, which was carried out at Heston, we found the Klemm remarkable in a number of ways. In the first place although the machine is a really light aeroplane and the weather was by no means ideal, she showed herself to be exceptionally steady ; that is to say, we were not "chucked about the sky" in the way that is usual under such conditions. The bumps and gusts which struck us did not " flick " us suddenly over on one wing tip, or lift the machine and drop it in the way which to the average passenger is so distasteful. All uneven movements were easy and cushioned and indeed, we felt that we were in a full-sized machine rather than in one which weighs but little over a thousand pounds when fully laden.
No Blind Spots.
Another feature which appealed to us very much was the excellent view, which of course, is made possible by the low-wing construction. The question of the pilot's view is one of growing importance since aerial traffic around all public 'dromes has now become quite formidable and the danger of collision by no means remote. With the Klemm one is freed from a lot of anxiety when approaching a crowded aerodrome, for there are no blind spots when flying level or when on the glide.
The cock-pits are deep and snug and singularly free from draughts and the stick and rudder bar are so placed as to give an easy and comfortable position for the pilot. The only criticism which one might make is that glass windscreens might be an improvement on the existing celluloid type. As to the general performance of the machine we were truly astonished with its capabilities. In the take-off, for instance, the Klemm got off after a very brief run and in the able hands of Mr. J. Rogers, Messrs. Lea's demonstration pilot, nosed skywards in a most impressive manner. In the air the control, both fore and aft and lateral, was light and easy but not over-sensitive and the rudder, similarly so. The Salmson, too, was absolutely dead smooth at all throttle openings and although fitted with only short exhaust stubs was not unduly noisy.
On the glide, after flying other machines, we at first found the Klemm almost disconcerting ! We anticipated a stall, so shallow was the gliding angle and so low the speed. But, of course, the L.25 is renowned for its landing speed of 25-30 m.p.h. This quality, coupled with its other attributes, to our mind, makes the Klemm an exceedingly pleasant machine for long distance touring because one is enabled to cruise at an altitude sufficiently low to make the panorama interesting without undue risk of mishap in the untoward event of a forced landing. Normally, cross-country flying, it will be agreed, can be somewhat tedious, especially for the passenger, for if one is to be ordinarily cautious one must travel at an altitude where the ground below appears flat and details are lost.
The top speed, flying level with full load is 80-85 m.p.h. and the cruising speed, with the engine throttled down, about 70 m.p.h., so that it will be seen that while the machine is docile and comparatively low-powered it is at the same time a really useful craft. Moreover it has a cruising range of 700 miles and the moderate fuel consumption of but 4 gallons per hour.
Altogether we were most favourably impressed with the Klemm and because of its qualities of safety (Mr. Rogers informs us that it is impossible to make it spin), its controllability at low m.p.h., speed and cruising range, together with its simplicity and low running costs it undoubtedly approaches an ideal type of craft for which there is a big potential market.
The following is the main data respecting the machine : Span, 42 feet 8 inches ; length, 23 feet 8 inches ; height, 7 feet 3 inches ; area of supporting surfaces, 215 square feet ; weight empty, 600 lbs. ; useful load, 700 lbs. ; total weight, 1,300 lbs. ; wing loading per square foot, 6½lbs. ; power loading 29 lbs. per h.p. ; ceiling with full load, 15,000 feet ; ceiling with pilot only, 22,000 feet.