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Aviation History
1975
1975 - 1373.PDF
TllCHT Internoti'onol. J/ Jdy Wi" Flying the Motor Cirrus Dr BRENNIG JAMES describes the machine in which he plans to explore the higher Himalayan peaks later this year (see Flight for July 10, page 48) THE SF-27MCy (SF for Scheibe Flugzeugbau, 27 for its design number, M for motor and Cy for Cirrus wings) seems to be one of the best motor gliders to have come along so far. Largely due to the efforts of German engineering students there is now a family of glassfibre gliders with lift-to-drag ratios of 40:1 or more at 60kt, which make them easily the most efficient means of transport now available in terms of energy con sumption. It is only a matter of time before these efficient airframes are married to suitable engines, and the RF-6 is a fair example of this trend, despite the fact that it is made of wood rather than glassfibre. The engine of the SF-27M is mounted on a boom which folds down into the fuselage when not in use, through doors which obligingly open and close as the boom is elevated and retracted. The original SF-27M had wooden wings and a Hirth four-cylinder, two- stroke engine, which could be hand- started using a cable. This is a familiar arrangement to many motor- lawnmower owners, and they all know the kind of language provoked by that method of cranking. A keen German amateur, Willibald Collee, modified his aircraft to take a Hirth two- cylinder, two-stroke engine running in the inverted position with a belt drive giving a 2 to 1 reduction in propeller r.p.m. The most important addition was an electric starter, which worked every time (well, nearly every time). The next step was to fit a decent pair of wings (those of an open class Cirrus) and to lengthen the fuselage to obtain the correct e.g. and tail moment. This is essentially the machine which is now in my posses sion. There has been little difficulty in obtaining UK certification for the aircraft. In order to drive a standard RF-6 propeller, the two-stroke engine has been modified to run in the Above, the SF-27M under power. Relow left, the engine emerging from the fuselage. Right, ready to start, with the doors again closed G-BCBH G-BCBH opposite direction, and the CAA was no doubt thrilled to hear that it was certificating an aircraft whose engine ran backwards. Rigging the aircraft is straight forward. The fuselage is heavy and has a high e.g. due to the high mount ing of the engine and fuel tank, so it must be carefully held upright. The drill for starting up is normal except for two details. The engine is held down in its compartment by a catch which is also a brake on the flywheel. This must be released before the engine can be raised, and is effected by turning a handle on the right-hand side of the cockpit four-and-a-half times. The run-up should give 5,400 r.p.m., which produces a sound like a sewing machine. Releasing the brake sees the aircraft airborne in 200yd. At 55kt there are 6,000 r.p.m. on the clock and a 450ft/min rate of climb, equivalent to a good aerotow. At a height of 10,000ft the rate of climb is still 350ft/ min, after which I tend to become bored and fold the engine away. The drill here is to throttle back (tickover on the ground is 1,200 r.p.m., in the air 3,000 r.p.m.), cut the ignition and fuel pump, and apply the engine brake. Compression obliges the propeller to stop either horizontally or verti cally; if horizontal a touch on the starter button will bring it vertical, whereupon the whole unit is wound down into its tunnel. Flown as a glider the SF-27M will fly at speeds low enough to be comparable with a good standard class ship. At speeds above 80kt it is much better than could be expected. With all that machinery and paraphernalia, a rough and noisy ride could be expected at speed. Not a bit of it: it is smooth, quiet and the controls are nicely harmonised. As the SF-27M is a heavy aircraft the landing is fast and long but the penetration
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