FARNBOROUGH 2008: Full circle for Rotary

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The dominant engine technology from the dawn of aviation is making a comeback at the light end of the market. Mistral Engines of Geneva is undergoing certification testing of is 300 hp G300 rotary engine, and has penned a deal late last year to power the Russian Kazan Aktai three-seat helicopter.

Rotary engines have no standard cylinder block or rotating crankshaft. Rather, the engines that powered Bleriot’s monoplane across the English Channel in 1909 and saw heavy use throughout World War I relied on a cylinder block that rotates around a fixed crankshaft, giving the “rotary” engine its name.

Increases in horsepower forced early aviation engine makers away from the rotary – but now Mistral and its US subsidiary in DeLand, Florida, believes its new rotary engines offer comfortable 3,000-hour TBO and “turbine-like smoothness resulting in higher comfort and lower cabin noise”.

Mistral believes the engines will be attractive to the retrofit market, aided by dual ignition and dual injection that “works on most grades of gasoline whether leaded or unleaded, Avgas or Mogas, mixtures thereof, and can readily accept ethanol-blended fuels”.

Mistral plans to use its Digital Engine Management (DEM) system on a twin-rotor, 200hp version called the G200, and the company is developing turbocharged versions of both. Within five years they mean to produce 2,000 engines a year. First flight was in 2004, when Mistral began taking orders at AirVenture in Oshkosh.

The Geneva assembly facility opened in 2005, and Mistral raised $8 million in 2007 from private investors to bring these designs to market. Collaborators include Embry-Riddle Aeronautical University in Florida, the Federal Institute of Technology in Lausanne, Switzerland and Hartzell Propeller Inc.

Though rotary engines have hade a long-term role in ground transportation, the last aircraft to regularly use the technology were both with the Royal Air Force. The Sopwith Snipe fighter used the Bentley BR2 rotary and the Avro 504K trainer used several models before the mid 1920s.

The first rotary engine was invented in 1889 using compressed air and was designed specifically for the upcoming aircraft industry. A major advantage of rotary designs is the rotation of the majority of the engine’s mass, which creates a gyroscopic effect to smooth out power and reduce vibration. Heavy flywheels were added to conventional motors just to reduce vibration.

Rotary engines became obsolete when companies found they could not design one to operate efficiently at speeds greater than 1,600rpm. But some applications do not need high revs, and more than ten years ago PATS Inc. introduced its small rotary auxiliary power unit for business jets to generate electricity while the main engines are not running and to help the mains start. PATS claims it burns half the jet fuel of a conventional turbine auxiliary power unit, and the Maryland company was acquired by DeCrane in 1999.

The unit is based on the Wankel engine design originally used in automobiles and motorcycles, which Mistral has emulated and which Diamond Aircraft also looked to when they started offering a “super efficient” rotary engine for their DA20 Katana motor glider in 2007.

Microlights never need high power, and what began as an engine for cart racing is also headed for the skies, as well as into boats and ATVs. Paul Mather of M-Squared was searching for a lightweight engine to provide 30hp for his new three-axis ultralight and found it from Revolution Rotary Engines Inc. He will soon be distributing those engines alongside two other US agents.

Mather is used to using Rotax engines, but the weights are too high, he says. The wing of his aircraft now can spare the strength to use struts instead of cables. Of his new 35hp engine and the 50mph cruise it provides, he says: “The same horsepower engine a year ago weighed twice as much.”

A German version is powering UAVs and ultralights and the Ontario-based importer and developer is exploring modifications and applications for its Revolution R301A. George Payne, CEO and chair of RRE, says: “Because of new materials, not only in the castings, new materials in the seals, and just the design improvements, make it a viable long-life engine.”

Rotomax is powering the latest incarnation of the Vampire, a Light Sport Aircraft that has the same rotary engine installed as one Sport Hornet LRS. Higher Class Aviation President Robert Gaither was happy to strike a long-term deal with Rotomax to use its 65hp single-rotor. Rotomax also offers a twin-rotor currently rated at 130hp.