The completion of the Mü 28 in the early 1980s set new standards for glider aerobatics. However, aerobatics have their origins in powered flight. If you look at a typical powered aerobatic aircraft, it has a very good power-to-weight ratio thanks to its pronounced lightweight construction (simple design with minimal systems), extreme agility with good controllability and low control forces as well as good flight performance, including good climb performance. These characteristics are not only advantageous for aerobatics, but also in terms of efficiency and safety when towing.
And so the basic idea for the Mü 30 “Schlacro” was born, a motorized aircraft for SCHLepp und ACRObatik. Die notwendigen Kompromisse, um beiden Betriebsarten gerecht zu werden, sollten die Eignung für jede der beiden Einsatzszenarien nur geringfügig beeinträchtigen. Die hohen Kosten beim Betrieb als Kunstflugzeug sollten außerdem durch den Einsatz als Schleppflugzeug kompensiert werden.
The Engine
At the beginning of the 1980s, Porsche began developing a modern aircraft engine based on the air-cooled 6-cylinder boxer engine of the 911 sports car. The PFM 3200 was type-approved in 1984 and the approval of a further development suitable for aerobatics was also intended. This engine was made available free of charge by Porsche for the Mü 30 and, due to its suitability for car gasoline (“Mogas”), would also have been very favorable in terms of operating costs compared to engines requiring aviation gasoline (“Avgas”). The reduction gear of the PFM 3200 also enables a lower propeller speed and thus lower noise emissions compared to conventional “direct drives”.
With the availability of a state-of-the-art engine, the group decided in 1984 to build the Mü 30 rather than the Mü 29, a racing class glider for cross-country flying.
The First Project Phase
The Mü 30 was built with the Porsche engine in mind. The engine had its origins in the automotive industry, which is why the arrangement of the mounting points was somewhat unusual for aircraft. This type of arrangement was referred to as “bed-mounted”. Typically, engine mounts for bed-mounted engines are more complex than rear-mounted designs. In addition to being more complex, they are usually also heavier. To avoid committing unnecessary weight sins with one of the first components of the Mü 30, the strength calculation was carried out using an FEM model on a high performance computer and optimized in terms of weight. The engine mount was subjected to a static load test up to the safe load. Strain gauges were used to measure the strains and thus validate the FEM calculation. Next, the fracture strength was to be verified purely analytically. However, the test results did not fully confirm this approach. The deviations between the calculation and the test were too large and points were identified where the strength was insufficient.
In the mid-1980s, construction with aramid fiber composites (commonly known by the brand name “Kevlar”) became increasingly popular. The material was quite promising: lighter than carbon fibers, with fantastic (tensile) strength, but rather unwieldy to work with. Kevlar promised considerable weight advantages when used as a covering for sandwich shells in the wing shell and tail fins. Binding dimensioning and material parameters were not yet available at the time, but “conservative” values were assumed, which were expected to be confirmed in future material tests.
In 1990, a Kevlar-reinforced spur bar was tested under load and broke. A second, improved version suffered the same fate, which cast doubt on the assumed values of the aramid composites. As a result, a test program to determine the previously uncertain values was set up and carried out in 1991. This confirmed that the material parameters that could be used for dimensioning were worse than the assumptions.
With regard to the engine mount, a solution was finally found, albeit an unsatisfactory one: Porsche announced that the aerobatic version of the engine would not be approved for the load multiples of ±10 g that were targeted by the Akaflieg for the Mü 30. The strength of the engine mount in its current design was sufficient for reduced load multiples.
A Forced Change of Course
In 1992, the PFM 3200 engine program was announced to be discontinued by Porsche. There were no plans to complete the development of the engine's aerobatic capability before the program was discontinued. With the loss of the engine, an essential basis for the Mü 30 project had been lost. The molds for the wing, tailplane/fin and canopy as well as the welded tubular steel fuselage frame had already been completed. The control system on the fuselage side was well advanced and the horizontal and vertical stabilizers were almost ready for rough construction. The design, including power calculation, center of gravity determination and load assumptions were no longer up to date with the elimination of the engine. The dimensioning values used for the Kevlar material also lost their validity.
Ultimately, the decision was to cancel the project or to redesign it by selecting a new engine. The choice ultimately fell on a Lycoming AEIO-540-L1B5. At the time, it was the most powerful engine available on the market in the western world and approved for aerobatics. The six-cylinder boxer engine produced 300 hp at 2700 rpm from a displacement of almost 9 liters. Alternative approaches such as Russian radial engines (even more powerful), propeller turbines (expensive and limited aerobatic suitability) or engines with less power (4-cylinder, with 200 hp) were rejected.
The Second Project Phase
With the selection of the Lycoming AEIO-540-L1B5 engine began the second phase of Mü 30 construction. In 1993, the load assumptions were revised (higher empty weight, higher take-off masses), a new engine mount was designed and work was carried out on the electrically powered flap drive. The ailerons were replaced by a variant with the highest possible roll rate possible with the Mü 30 at a wingspan of 8.96 meters. The roll rate thus increased from 150 °/s to 200 °/s. The tail unit was recalculated with the new loads and the Kevlar dimensioning values now available.
Es wurden diverse Beschläge verstärkt und die Schalen wurden durch weitere „Angstlagen“ von außen versteift. Der Rumpf wurde durch ausgetauschte Rohre lokal verstärkt und an die neuen Lasten angepasst. Alte Flügelzeichnungen waren nichts mehr wert. Statt einer Kevlarschale mit Kohleholmgurten und einem Glasfasersteg, wurde ein Vollkarbonflügel beschlossen. Ein überholter AEIO-540-L1B5D wurde in den USA bestellt. Der als Einbaudummy verwendete Motor der Firma Mühlbauer wurde der Akaflieg dankenswerterweise endgültig überlassen und als sogenannter „core“ für den zu überholenden Motor in die USA geschickt.
Die gesamte Gewichts- und Schwerpunktrechnung wurde aktualisiert und das v-n-Diagramm auf den aktuellen Stand gebracht, da aufgrund des neuen Motors ganz andere Massen zu berücksichtigen waren. Es wurde darauf geachtet, hier Reserven einzubauen, um im Zuge des weiteren Baus auf Eventualitäten vorbereitet zu sein. Da der (rechnerisch vorherbestimmte) Schwerpunkt nicht mehr mit dem flugmechanisch gewünschten Schwerpunkt in Einklang zu bringen war, wurden die Tragflächen um knapp 150 mm nach hinten verschoben und die Flugmechanik aktualisiert. Nebenher wurde an den Zulassungsunterlagen gearbeitet.
Maiden Flight and Flight Testing
Der Antrag auf die vorläufige Verkehrszulassung (VVZ) wurde beim Luftfahrt Bundesamt (LBA) abgegeben und mit Gültigkeit vom 29. Dezember 1999 lag die erste VVZ für die Mü 30 vor. Nach der Aktualisierung des Flughandbuchs war der Flieger am Ostermontag 2000 technisch und von der Papierlage her klar zum Erstflug.
The final approvals (locally at Königsdorf airfield and at the South Bavarian Air Office) were obtained and taxi tests (including a “high-speed taxi”) were scheduled. On Thursday, the “intention for maiden flight” was announced by the Akaflieg to their members. In the evening, the first “high speed taxi test” followed in a “two-point position” with the tail in the air. Controlled and without any negative findings.
Am Freitag, dem 28. April 2000, erfolgte dann ein weiterer „high speed taxi test“ und am Nachmittag bei leichtem Seitenwind der erste Start auf der „10“ in Königsdorf von der Grasbahn. Nach diesem „inoffiziellen“ Erstflug wurde der Flieger zusammengepackt und zum Institut für Aeroelastik der DLR in Göttingen zum Standschwingversuch verbracht. Zurück in Königsdorf erfolgte dann der offizielle Erstflug am 16. Juni 2000 mit vielen Gästen und Presse.
The following three years of flight testing then revealed a number of problems, which led to a longer interruption in flying activities from 2003 onwards. In particular, this concerned the initially inadequate cooling of the engine, the conversion to a new exhaust system and a complete redesign of the air intake. The latter was associated with the new construction of the cowling, which also had cracks due to strength issues. In 2007, the conversion work was successfully completed and certification-relevant flight characteristics and performance were tested, including final spin flights. In 2009, the completion of testing was confirmed and certification as a single unit was applied for. On February 17, 2011, the Mü 30 finally received the hoped-for certification as a single aircraft in the (unrestricted) special class from the LBA.
Operation Since Certification
The engine was completely overhauled in the winter of 2013/2014.
In 2014, the Mü 30 flew in motorized aerobatics championships for the first time: Sascha Odermann won both the Bavarian and German championships with the Schlacro.
2016 wurde eine Rauchgasanlage (Smokesystem) eingerüstet und mit der Erprobung begonnen, bis sie 2022 zugelassen wurde. Mit diesem System wirken die Flüge der Mü 30 auf Flugschows noch beeindruckender. 2023 erlangte der Schlacro sogar nationale Bekanntheit, also er mit Sascha am Steuer ein riesiges Herz und eine Brezn in den Himmel über dem Oktoberfest zeichnete. Diese Aktion wurde 2024 unter großem Jubel wiederholt.
In 2019, the tiresome issue of a susceptible and short-lived tail wheel was improved by installing a modified wheel.
In 2020, the Mü 30 reached the 20-year service life originally specified in the certification (with over 1,000 flight hours). To overcome this limit, the maintenance manual was expanded to include an inspection program to be carried out every 20 years or 3,000 hours. The corresponding inspections and maintenance work were carried out successfully, providing up to two decades of operation again.
Conclusion
The Schlacro has a very eventful history from its first idea to its certification, which required a lot of perseverance, especially in the phase after the end of the Porsche engine until the first flight. In the course of construction, many doubts arose whether this “mighty machine” could ever be put into meaningful flight operations within the Akaflieg given the costs, effort and aeronautical demands. In the end, the actual development was much more positive than what was feared. Operation as a tow plane is possible and affordable, not at least thanks to the commitment of INTEC as a sponsor. The Schlacro is the figurehead of our Akaflieg and has earned more media attention than any other prototype thanks to campaigns such as the heart over the Oktoberfest or the spectacular double tow of the Red Bull Blanix.
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