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The Six-Engined VTOL Design

by John Reed on January 10, 2012

I was taking a peek at Lockheed Martin big defense contractors’ recent patent filings the other day when I came accross this whopper. A patent for a vertical take-off and landing aircraft with three wings and six tilting engines, notice I didn’t say tiltrotors.

Richard Oliver recently told Aviation Week that a DoD customer was interested in his six-engined Hexplane design (an unmanned version of which is shown above).

Yup, the patent filing, which lists Oliver as the inventor, calls for a six-engined craft that would eliminate the safety concerns associated with world’s only tiltrotor, the twin-engined Bell Boeing V-22 Osprey.

The patent filing claims the Osprey isn’t ideal for high-risk combat scenarios due to the fact that it cannot operate if one of its engines fails (kinda like a conventional chopper):

The current technology twin tilt rotor V22 Osprey’s recent performance is used as the best example of twin tilt rotor problems.

The failure of either propulsion unit to produce thrust, results in the loss of the aircraft and occupants. The cross coupled shaft system provides a marginal performance backup limited to engine failure, not gearbox or rotor failures. The loss of 50% of this aircraft’s power results in its’ inability to continue its’ mission and in many circumstances requires immediate landing. The present invention solves this problem with its’ fundamental design, providing for continued flight following the complete loss of thrust by any propulsion unit, no matter what the cause.

Inability to use evasive maneuvering tactics to avoid hostile fire, similar to that found in high threat landing zones. Quick maneuvering has resulted in the cracking and breaking of rotor root components which may result in the loss of a blade, and therefore the aircraft. The 38 foot rotors are limited to about 10 degree flapping angles. Quick maneuvering can (and has) easily exceed this angle resulting in the failure of a rotor and the potential loss of the aircraft. This limited maneuverability might be acceptable for commercial operations, but not for the intended applications of the military.

The document goes on to point out other shortcomings with the Osprey’s design, including a limited ability to allow troops to fast rope off the back of the aircraft due to the location of the Osprey’s center of gravity and rotor wash as well as the way the tiltrotor reacts to a vortex ring sate (the potentially deadly scenario where choppers and tiltrotors basically run out of air to lift them). In all of these cases, the patent filing says six propeller or jet engines, not rotors, would dramatically improve performance by providing increased stability, backup engines and better maneuverability. Props would be ideal for a medium-size transport while jet engines would be necessary for a faster, larger aircraft.

“While in vertical flight, the six propulsion units are arranged around the aircraft producing thrust. Imagine a round table with six legs,” reads the filing. “Remove one leg and the table remains standing! The center of gravity of this aircraft is generally located about its’ center. The propulsion units are placed such that the remaining thrust following the loss of thrust from one propulsion unit will maintain longitudinal and lateral static stability, therefore supporting the aircraft.”

As Oliver told AvWeek:

it has the unusual ability to survive the complete failure of any propulsion unit and remain statically stable. If you remove any one unit and wing in flight it will still fly safely, even a maximum gross weight and HOGE [hover out of ground effect].” The six engines are completely independent, with none of the cross-shafting you would see in a two– or four-prop tiltrotor.

The Hexplane design is all about survivability, he says, and about being able to continue the mission and not having to return and land after a failure. “I don’t see it as complex. There are not six throttles, just a single go lever and six-channel fly-by-wire,” Oliver says.

Click through the jump to read the patent filing’s detailed description of the design.

The term “Propulsion Unit” as used within this document refers to any method of producing thrust. The example of an engine driven propeller is chosen solely for illustration purposes and is not intended to limit the scope of this invention. The invention is valid for alternate means of propulsion including jet engines. When the propulsion units consist of jet engines this VTOL configuration is known as a “Tilt Jet” since the jets are tilted forward for forward flight and tilted vertically for vertical flight. The invention is valid for alternate propulsion unit tilting implementations. The engine (piston, turbine, rotary, electric or other power source) may be mounted on the wing and transfer power through a gearbox into its tilt-able propeller or rotor. This design allows the engine or motor to remain relatively fixed in a single position without having to operate in multiple positions of the propeller.

[0060] The aircraft configuration consists of a conventional aircraft fuselage, with a nose, with or without pilot and/or co-pilot crew stations in the case of Unmanned Aerial Vehicle (UAV) applications, a central cabin or payload area, and a tapering empennage. The aircraft has three wings, the front wing, middle wing, and the rear wing. Two propulsion units are mounted above, below, or on each of the three wings, yielding six propulsion units. The wings are fixed to the fuselage and the propulsion units rotate in unison to either of two (not including intermediate) positions, vertical or horizontal.

[0061] For electric motor, rotary, piston or turbine driven propeller or rotor embodiments, the propulsion units on opposite sides of the aircraft turn in opposite directions to cancel rotational moments about the yaw axis due to propeller or rotor torque. Small flapped wing panels are fixed outboard of the forward and rearward propulsion units. These wing panels are located within the propulsion units’ propeller slipstream. They provide yaw control during vertical flight. Their flaps are disabled in the neutral position once the propulsion units advance toward the horizontal position.

[0062] The main landing wheels are located at the rear end of the forward propulsion units. A retractable and steerable tail wheel is located on the center line of the fuselage near the rear of the aircraft and retracts rearward and upwards into the normally unused space in the tail cone or alternatively for applications which require rear doors or a ramp the tail wheel may retract forward into the bottom of the fuselage. This eliminates the normally complex and heavy landing gear retraction and extension system increasing payload capacity. When the engines or motors rotate with the propellers or rotors the main wheels are mounted to the aft end of the forward propulsion unit engine support structures. This takes advantage of the existing structural load path which already exists for the engine support. When the engines or motors are mounted in a fixed position with the propellers or rotors tilting, the main wheels may be attached to the aft end of the tilting assembly. The propulsion units are spaced further apart than typical main gear designs increasing ground stability. Gear up landings are not possible with this invention as the landing gear is always down when the aircraft is in vertical flight mode. Separate landing gear controls and systems are not required. Proper placement of the main gear below the nacelle center line and clam shell gear doors can enable partial conventional takeoff and landings (CTOL) to enable additional payload capability when a runway is available. This is accomplished by placing the propulsion units in an intermediate position considering ground clearance is provided for the propeller or rotor tips. With jet propulsion units this would not be a problem.

Click here to read the entire filing.


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