The Hybrid Airplane Concept: A Unique Power Pairing
The automobile hybrid employs two different power sources: an internal-combustion piston engine and an electric drive. The automobile hybrid achieves above-average fuel efficiency by drawing from the powerplant optimal for conditions. For example, the gasoline motor may augment the electric drive during periods of high power demand – accelerating from a stop, climbing hills – but use only the electric drive for most cruise conditions.
Applying this approach to aircraft, the hybrid airplane employs two types of turbine engines which use the same type fuel: a jet engine and a turboprop. Both are used for take-off and rapid climb, but only the turboprop is used in cruise and descent phases. This unique design stores the jet engine in the nose section when it is not needed, using the more fuel efficient turboprop in cruise. The jet is still available for contingencies and the aircraft is able to operate safely on either. Thus the hybrid airplane retains performance and redundancy benefits of twin-turbine aircraft while providing the improved economy of the more efficient single-engine turboprop.
The benefits are:
Economy: The turboprop engine mounts on the rear of the aircraft while the jet is installed on a retractable mount below and ahead of the cockpit. In normal cruise flight, the rear engine running alone delivers design cruise speed using 40% less fuel and producing 40% less exhaust emissions compared to conventional turboprop twins. Fuel savings are even better compared to twin jets.
Fuel is both expensive and heavy. Since the hybrid lacks the drag of wing-mounted engines and therefore needs far less fuel, it can be built 25% lighter than a comparable conventional twin, compounding both operational and production savings.
Safety: Since both engines mount on or near the aircraft centerline, the hybrid aircraft can fly on either engine without the dangerous asymmetrical thrust inherent in conventional twins when forced to fly with one engine. The additional takeoff power provided by the jet improves safety margins when flying from smaller airports.
Cabin Comfort: Part of the popularity of jet airplanes stems from passengers’ disdain for the cabin noise levels associated with wing-mounted propjet twins. The hybrid design avoids this problem because the turboprop is located behind the passenger compartment and most noise radiated from the propeller is behind the aircraft.
Performance: The engines on a conventional twin, or the primary engine in the case of the hybrid, are typically sized for the cruise power required. When the hybrid airplane jet is used during takeoff and climb the aircraft has effectively more than 100% power than would be available to a conventional twin. Therefore, the climb-out from the airport will be steeper (smaller airport noise footprint) and the aircraft will reach cruise altitude faster than the twin.
Scalability: Aircraft from 6 seats up to regional airline size can be designed under this concept. The basic concept is covered by US patent #7,549,604, which is owned by Joseph A. Hutterer. The patent does not limit aircraft size or restrict engine types.