Germany’s Take Off GmbH
Boxer engine conversion experts
By Kent P. Misegades, EAA 520919, for Experimenter
R1100 in Spang BX-2 Cherry.
During an extended tour of European general aviation airports in the spring of 2010, one engine gaining popularity among homebuilders caught my eye. Whether used as a pusher in the Merlin weight-shift trike or mounted under the cowl of a Breezer or BX-2 Cherry, the two-cylinder, four-stroke, air-cooled boxer motors from the German company Take Off Ultraleichtflug GmbH represent a very interesting alternative powerplant in the 90- to 115-hp range.
First, though, a few thoughts on vehicle engine conversions: If we’re ever going to significantly lower the cost of experimental aviation, we must exploit every reasonable volume production of materials, fuel, electronics, and engines. Vehicle engine conversions provide us with the multiple benefits of lower acquisition and replacement-part costs, as well as the natural ability to operate on a variety of lower-cost, lead-free fuels.
Modern engines developed for small automobiles and motorcycles are potentially well aligned with the needs of aircraft designers, since they’re typically relatively light in weight, reliable, and robust; feature full authority digital engine controls (FADEC) with electronic fuel injection and electronic ignition; and exhibit low fuel consumption, ultraclean emissions, low vibration, and noise levels. Their widespread use in all continents of the world means that parts are generally available and inexpensive compared to the much smaller production numbers and support organizations found for purpose-built aircraft engines.
These vehicle engines achieve their high horsepower-to-weight ratios partially through higher operation speeds, usually requiring the use of reduction gearboxes as well as liquid cooling for the most critical components. The excellent reputation of the products from Rotax in recent years, with their 5,000-plus rpm operational speeds, water-cooled heads, reduction gears, and 2,000-hour TBOs, have proven that engines of this class are quite suitable for sport aircraft.
I’ve often thought that a modern, four-stroke motorcycle engine would serve as an ideal basis for such a conversion; after all, motorcycle engines too must be light, low in vibration, efficient, and quiet. Wilfried Bleidiesel, founder and president of Take Off Ultraleichtflug (whose name, ironically, translates to “Lead Diesel”) obviously sought the same engine characteristics for the Merlin 1100/1200 trikes his company makes and for an array of light-sport aircraft one finds in Europe.
R1200 top view.
An electrical engineer by trade, Wilfried found his way into a cockpit as many others have— through a fellow model airplane enthusiast who invited him one day to try the real thing. Wilfried, though, was quick to give his lovely wife, Doris, credit for how he became a pilot. “We were dating during my studies, and she suggested we build model airplanes together as a means to spend more time with each other,” he said. While the two never did build a model as planned, they built a family and a business that they’ve run since 1989.
Wilfried’s search for an engine led him to a highly respected Bavarian manufacturer of vehicles that produces 250 finely crafted two-cylinder boxer motors every day. In recent years, these engines have gone under the name R1100, R1150, or R1200, depending on the total displacement in cubic centimeters. As you’ve likely guessed, the company’s name is Bayerische Motoren Werke, better known as BMW.
Spang BX-2 Cherry powered by a BMW R1100.
Wilfried explained that BMW doesn’t formally support his efforts, and they don’t publicly endorse the use of their engines for anything other than motorcycles. (This ought to keep their lawyers happy.) Anyone with a basic knowledge of aviation history knows, however, that BMW started its illustrious history as the manufacturer of aircraft engines, not cars and bikes. Its logo represents the white blades of a propeller cutting through the blue sky and also demonstrates the company’s loyalty to its home in the “Freistaat Bayern” with its blue and white checkered flag.
During World War II, BMW produced a series of powerful radial and in-line piston engines. More important, however, was its advanced BMW 003 axial-flow turbojet which served as the basis for the Atar line of jet engines from Snecma when the BMW engineering team was lured to France after the war. In short, BMW is no stranger to aircraft engines; one can imagine a few retired engineers quietly enjoying the use of their technology in aircraft again.
Having seen several installations of Take Off’s motorcycle engine conversions in trikes and homebuilts throughout Europe the past few years, and having studied every word and diagram on the company’s detailed website, I arranged to stop by the company’s modern facilities located just south of Hamm during a visit to my German-born wife’s family in early November 2011. Take Off starts with stock R1100/R1150/R1200 engines which already include several key features aircraft designers should like:
- Four-stroke, twin-cylinder boxer engine
- four radial valves per cylinder
- double overhead camshaft
- central balance shaft to reduce vibrations
- air-cooled cylinders
- thermostat-controlled, engine oil-cooled heads (requires small oil cooler)
- Electronic engine management with
- electronic intake pipe fuel injection
- use of fuel injection eliminates risk of carburetor icing
- overpressure of fuel system lowers possibility of vapor lock
- electronic engine management
- twin solid-state spark ignition
- a computer that “learns” in the first few minutes of operation, optimizing performance while minimizing fuel consumption and emissions
- automatic, altitude-adjusting pressure sensor
- starter relay
- belt-driven, 600-720 watt alternator
- Regulated, three-way catalytic converter and efficient muffler
- Maximum power of 90 to 115 hp (depending on displacement) at 7,000-plus rpm
- Recommended 1,000-hour TBO (very reasonable if 100 hours flown annually).
Wolfgang Bleidiesel with two R1200s.
In addition to these impressive features, since 1989, Wilfried and his staff of experienced technicians and machinists have been producing an array of important components needed to convert the engine into a reliable powerplant for light aircraft. These include:
- A variety of lightweight reduction gearboxes with ratios from 2.8:1 to 3.5:1. These may be mounted in either “high” or “low” orientations on the crankshafts, providing designers more options.
- Custom wiring harnesses including a single, wide connector for all required engine sensors, simplifying panel installations.
- Custom-designed, one-piece dry centrifugal clutch that engages the propeller above 2,000 to 2,500 rpm.
The clutch has the ability to help protect the engine from damage in the event of a propeller strike, and it does this by transmitting torque from the engine’s crankshaft to the reduction gear via a flexible coupling. It also can significantly damp rotational fluctuations, resulting in low overall vibrations that some have compared to modern four- or even six-cylinder aircraft engines.
Intake and Exhaust
Filtered air enters each cylinder separately through independent throttle bodies—ideally located on the front face of the cylinders—allowing a slight overpressure for increased power. Exhaust gases exit the opposite face into a pair of manifolds that are typically joined just upstream of the combination catalytic converter-muffler unit. This unit is enclosed in a tubular heat shield of elliptic cross section, serving as an ideal muff for cabin heat.
As with any engine, adequate cooling of the Take Off powerplant is essential. Unlike other engines, however, these boxers make use of engine oil forced by a separate pump to cool the cylinder heads. One starts the engine from an idle throttle setting. The computer measures ambient and engine conditions to meter the correct fuel-air mixture automatically. The warm-up period is short because cooling oil is pumped to the heads only when needed by a thermostatic control.
Take Off produces the R1200-powered Merlin 1200 Trike.
Cooling with oil is so efficient that just a small heat exchanger is required, and the head remains warm to the touch even after running at high power settings. This quality also means that a tight-fitting cowl without baffles in the head region is possible, reducing the fuselage cross section and drag around the engine. The boxer at its widest point is only 0.747 meter (30 inches); Wilfried recommends at least a 1-centimeter (0.5-inch) gap from the head to the inside cowl surface.
The lower portion of the cylinders is cooled by air. Here Wilfried recommends an airflow of at least 45 kilometers/hour (41 feet/second) measured 1 centimeter above the top of the inner cylinder fins. Cooling air is also required on the belt-driven alternator mounted at the top of the rear section of the engine. All in all, it shouldn’t be too difficult to provide adequate cooling to the engine, despite its high power density.
R1200 side view.
According to industry reports, the next generation of these boxers will see a further increase of power and the need for water-cooled heads instead of oil. Given water’s higher capacity to remove heat, the additional complexity of a water mantel, pump, and radiator should be a small price to pay for the increased power. Typical fuel consumption during cruise flight is less than 10 liters/hour (less than 2.6 gallons/hour), but that’s at a very low power setting. The entire powerplant, including starter, generator, exhaust system, oil heat exchanger, and reduction gear unit, weighs a mere 80 kilograms (176 pounds), yielding a low 1.53 pounds/hp for the 115-hp R1200 engine.
The engine is mounted at four of the multiple, pre-tapped holes provided by the manufacturer, giving designers great flexibility to use the system for tractor or pusher aircraft. As previously mentioned, the reduction gear provided by Take Off can be mounted in “high” or “low” thrust-line positions, another benefit for experimenters.
Wilfried strongly recommends propellers from Drensteinfurt-based Neuform, a few kilometers from Take Off’s office in Hamm, Germany. Neuform’s three-bladed propellers are constructed from carbon fiber and fiberglass and offered in ground-adjustable, hydraulic variable-pitch or electronic constant-speed versions. The two companies get along so well that Neuform uses the Drensteinfurt airfield developed by Take Off for its trike business and flight school.
SH: Summing Up
I first saw one of these remarkable engines in a BX-2 Cherry, a popular two-seat design that originated in Switzerland and is now supported by Wolfgang Spang of Sauerlach, Germany. Spang, who spent some years with the German Luftwaffe and in the United States during the airplane’s construction, provides a great amount of detail and images on his website.
Two R1200s being prepared for shipping; the one in foreground has reduction gear in “high” position.
A complete powerplant based on the R1200 boxer is offered by Take Off for €6,910 ($9,269). For a number of reasons, Wilfried can’t ship the actual boxer motor to North America, but he will supply everything else that’s needed for the conversion. I’ve found one good supplier of stock boxer motors at Beemerboneyard of Morristown, New Jersey. A quick review of the R1150s and R1200s in the company’s inventory showed several low-mileage units for around $1,000.
The owner of Beemerboneyard will also find you a specific engine to meet your requirements and budget. Even with the freight costs from Germany, it should be possible to build a Take Off powerplant for under $10,000. Given the system’s low weight, compact dimensions, high specific power, low fuel burn, vibration and noise levels, it’s no wonder that European homebuilders increasingly favor the products from Take Off Ultraleichtflug.
Kent P. Misegades has 30-plus years of experience as an aerospace engineer and engineering business owner. He had started his career as an aerodynamicist for Dornier in Germany. Kent has been flying light aircraft since 1973, has written extensively for the EAA and General Aviation News where he co-authors a blog on aviation fuel, and is president of EAA Chapter 1114 in Apex, North Carolina, one of the largest chapters in the country. Together with fellow chapter members, Kent is restoring a rare Stits SA-7D Skycoupe that will be flown to EAA AirVenture Oshkosh 2012. For the past year, he has been designing a highly efficient, two-seat, cross-country aircraft that will make use of the BMW R1200 engine developed by Take Off GmbH.