Graham Warwick/ATLANTA
A US patent HAS been granted to 3X Jet for an unconventional propulsion-system concept for twin-engine aircraft. The configuration uses two different-sized jet engines mounted on the aircraft centreline and is claimed to offer reduced operating cost, improved climb performance and range, and increased cruise-altitude capability and safety.
According to 3X Jet, the larger of the two engines has around twice the thrust of one of the engines in a comparable twinjet aircraft and the smaller engine about the same thrust,. In normal operations, the smaller engine is shut down in the cruise.
3X Jet says that the larger engine is sized to power the aircraft at its normal design cruise speed and altitude without assistance from the secondary engine, which is sized to meet engine-out requirements.
In normal operations, the smaller engine is used for start-up and taxi. Both engines, are used for take off and climb, the larger engine is used for cruise and both engines, are used for descent and landing. In "high-performance" operations both engines are used in the cruise.
Richard Bacon, president of Golden, Colorado-based 3X Jet, says that two "major aircraft companies" are studying the concept, with one at the preliminary-design stage and the other "further along". Bacon says that independent studies of the concept have confirmed his "conservative" projections of the expected benefits of the configuration.
Bacon is projecting a 30% reduction in direct operating costs, citing the lower fuel consumption and maintenance costs (per unit thrust) of a larger engine.
A University of Kansas study comparing the Cessna CitationJet (two Williams-Rolls FJ44s) with the equivalent 3X Jet configuration (one AlliedSignal TFE731 and one FJ44) projected a 34% reduction in operating costs.
Bacon says that the effect of carrying the "dead weight" of the smaller engine during cruise is more than offset by the reduction in fuel consumption which the configuration offers.
Source: Flight International
