The Scheibe SF-25 MotorFalke
The Scheibe SF-25 MotorFalke is a German touring motor glider developed from the earlier Bergfalke glider by Scheibe Flugzeugbau. Since May 2006 the business has been run by Scheibe Aircraft GmbH.
The company had produced the Motor Spatz but decided to produce a better light aircraft based on the Bergfalke glider. It had a new forward fuselage with an enclosed cockpit with two side-by-side seats and originally a Hirth F10A2a engine in the nose.
It first flew in May 1963 as the SF-25A Motor Falke. After an initial batch of aircraft the wing was lowered and it was renamed as just the SF-25B Falke. A number of variants were built with various engines and the type was licence built by Sportavia-Putzer, Aeronautica Umbra (Italy), Loravia (France) and Slingsby (United Kingdom).
The Scheibe SF25 Motorfalke was probably the first practical touring motorglider. It is certainly one of the most built types (around 1500 in total).
Attempts at motorising sailplanes of various descriptions have been made almost for as long as sailplanes have existed. Even in the 20’s and 30’s there were various attempts at creating some beast that would combine the best of both worlds: soaring like a true sailplane, but still being able to take off without assistance and return home when the thermals get weak.
As with many other things in aviation, it was with the development of suitable power plants that things really got underway. The traditional American aviation engines (Lycoming, Continental) were generally too big and heavy to be used in a light motorglider. But then came the ubiquitous VW Beetle, and with it a hundred adaptations of its air-cooled, 4 cylinder boxer motor. In the 60s and 70s Rectimo, Sauer, Stamo and Limbach were some of the European companies that led the way in this regard, producing practicable, reliable and light aircraft engines of 40-60hp – a perfect match for the early motorgliders.
Egon Scheibe was a young German aircraft engineer, who designed and built some of the most successful gliders before WW2. After the war, he founded Scheibe Flugzeugbau and built a large number of successful gliders, most notably the Bergfalke tandem 2-seater.
From the early 60's onward, latching on to what would be a 3-decade-long boom, the company worked on designs for a motorised glider. The design brief was a simple, easy to fly and cheap to maintain motorglider which gliding clubs could use for tuition and for flying during the winter months. Most German gliding clubs had plenty of experience with tube, wood and fabric aircraft, so expertise and tools for repairs and maintenance were readily available.
SF25-A: The SF25 Motorfalke first flew in 1963. The early A-models still had a somewhat finicky Hirth 2-stroke motor and high wings with plenty of dihedral, which led to a very stable aircraft that was rather difficult to thermal. Although it had certain flaws, 50-odd were produced over the next few years.
SF25-B: In 1967 the much improved B-version saw the light of day. Scheibe lowered to wing to the bottom of the fuselage, incorporated supporting wheels under the wings to allow for independent takeoffs and, most importantly, installed a VW-based 4-stroke Stamo motor of 1500cc and 45hp that vastly improved practicality. Continuous operation and touring flight was now feasible. The maximum all-up weight was also increased to 555kg. Nearly 400 B-Falkes were built before the C-version was released in the early 70s.
SF25-C: A slightly more powerful engine (initially 60hp, and later 80hp with the 2-litre Limbach installed in the SF25C-2000 model), an electric starter and some other minor changes augered in the SF25-C – the model which continues to be built to this day. In the mid-70s, the C-Falke received another design overhaul with a more bubble-shaped hatch, a suspension system for the main wheel, and a more streamlined tail. From the 80s, conventional landing gear with a steerable tail wheel, and even tricycle landing gear were offered. The aircooled VW-based Limbach and Sauer motors were eventually dropped in favour of the watercooled Rotax, with 80, 100 and 115hp variants being certified. Max all up weight has kept climbing steadily, from 580kg to 650 and finally up to 690kg in the latest versions. The SF25-C in all its variants is by far the most common Motorfalke model today. In its current Turbo-Rotax form it is used mainly as a glider tow rig. The powerful Rotax motors allow safe towing of even heavy modern 2-seater gliders with a max all up weight of 800-850kg, representing a true alternative to thirstier, traditional tow planes.
SF25-D: The D-model is essentially a factory-converted B-Falke with a more powerful 60hp Limbach engine, which necessitated strengthening the fuselage and slightly shortening the wings (to 14.7 instead of 15.3m). This came about when the then-new C-Falke was released and many owners of the older, underpowered B’s wanted to upgrade. In practice it is hard to detect much difference between a D and C-Falke in cruise or glide. All up weight was increased to 580kg, which means the useful load tends to be better on the D than many other Falkes.
SF25-E “Superfalke”: The fairly rare "Superfalke" got a set 18m wings and a featherable prop as standard equipment, which substantially increased the glide (around 28 as opposed to 22). Some of them also have wingtips that can be folded inwards, thereby reducing the wingspan to 10m to facilitate hangarage, similar to a Fournier RF5B.
SF25-K: A version with foldable wings - in this case with wings that fold back completely to dramatically reduce the required hangar space. The “K” stands for “Klappfluegel” – "folding wings". Very few of this variant were produced, as the folding mechanism was complicated and added a fair amount of weight.
Scheibe Flugzeugbau GMBH, the original manufacturer, eventually closed its doors in 2006, partly because of the lack of a successor for Egon Scheibe, but probably also due to the increasingly difficult trading conditions after the boom of Ultralight/Microlight aircraft that have taken over a large portion of the market for fun, relatively cheap, and easy to fly recreational aircraft - and arguably also the unwillingness of the company to embrace modern glass-fibre construction techniques, preferring to stick to the traditional mixed construction style (fabric-covered wooden wings and steel tube fuselage). The Motorfalke design was subsequently taken over by the newly formed Scheibe Aircraft GMBH based in Heubach (Germany), which has certified the Turbo-Rotax powered version with the glider towing market in mind.