VTOL Hybrids for General Aviation?

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Because of power required and cost, VTOL hybrids using wings for horizontal flight have been limited to the military. We look at two aircraft companies that want to bring the concept to general aviation.

Heavier-than-air Vertical Take Off and Landing (VTOL) flight has been around since not long after the Wrights began wowing the world with airplanes. The first (horribly unstable) quadcopters flew in 1907—however, the desire for a device to lift humans straight up into the sky and whisk them elsewhere is probably as old as conscious thought along the lines of "Get me out of here!"

Two VTOL hybrid (wings for horizontal flight) concept aircraft have recently made the news, one designed for cargo and one that is roadable and designed for personal transport. We sniffed around to learn more. Despite the huge engineering challenges, we think there is potential. Maybe we're overly optimistic, but we're hoping for VTOL hybrids to come to the general aviation world.

A Little Background

Despite the sheer power necessary to blast a vehicle straight up off the ground and back down again, VTOL flight developed remarkably quickly during the first half of the 20th Century. Helicopters were a success by the 1950s. Naturally, people weren't satisfied—primarily because of the limits on cruise speeds imposed by the rotor that allows VTOL in the first place. Could there be some sort of hybrid arrangement that used wings as well as the capabilities of the helicopter?

Doak VZ-4

The idea of what the FAA now calls powered lift but is popularly known as tiltrotor aircraft (a VTOL hybrid) came of age with the Doak VZ-4 in the late 1950s. With a tilting, ducted fan prop on each wingtip, shaft-driven by a single 1000-HP turboshaft engine, the VZ-4 proved the concept of powered lift and conversion to and from fast, horizontal flight. It also confirmed that a lot of power was needed—it had a takeoff weight of only 3200 pounds—and that the combination of VTOL and winged cruising flight meant weight for drive shafts and tilting mechanisms ate heavily into useful load.

Further, engine failure and what the FAA calls a "catastrophic single failure mode" (failure of the drive train to a rotor/ducted fan) proved to be a challenge. Further development of the Doak VZ-4 was not pursued.

Power

Power costs money, and those who fly helicopters recognize that vertical flight requires cubic money to be able to lift any kind of load. Thus far, power required, and the costs to apply that kind of power have been major reasons that successful VTOL hybrids have been not made it into general aviation.

Another reason is that the downwash involved with the brute force of VTOL has to be considered in every takeoff and landing—standing near the downwash of a helicopter ranges from uncomfortable to dangerous; the Harrier VTOL fighter will deafen bystanders and its jet blast has damaged concrete and asphalt. To get a feeling for what's involved as weight and desired performance go up consider that a new F-35 develops some 30,000 HP when launching into the sky vertically. While the comparison is imperfect, also consider the retired Space Shuttle, about the size of a DC-9—the power involved with launching it vertically meant that the distance one had to stand back was measured in miles.

With the challenges in mind, we took a look at the two VTOL hybrid concept aircraft that have made the news recently.

4 X 4 Aviation

Operating from London Ashford Airport, on the southeast coast of England, 4 x 4 Aviation, Ltd. is developing a VTOL hybrid machine it calls the VV (Versatile Vehicle)-Plane. It's intended to carry cargo in and out of areas of the world where ground transportation is difficult and runways scarce. On its website, the company says that the aircraft will be able to carry at least one shipping container—it doesn't specify the size and they range from 20 to 45 feet long—which it will be able to land on and have it become part of the fuselage. Whatever the length, the plan is to be able to lift as much as 30 metric tons, or some 66,000 pounds, more than twice the capability of the 12,300-HP VTOL Boeing V-22 Osprey.

The VV-Plane uses vectored thrust for the vertical takeoff and landing and wings for horizontal flight.

The aircraft is to be powered by a combustion engine, which drives a generator, which in turn powers 16 "electric turbines" that provide thrust. The turbines are grouped in four sets. Each set of four turbines is mounted on a gimbal that allows that set to direct its thrust appropriately for the mode of flight involved—and are operated by what the company describes as microcontrollers directed by a company-designed software system.

To keep weight down, each set of turbines on its gimbal is not controlled by a mechanical system that physically moves the set, it is directed by adjusting the power output of each turbine within the set—effectively via differential thrust. We are curious what happens when one electric turbine fails. Will that render the entire set useless because there is no mechanical means to aim the thrust of the remaining three in the proper direction—thus, does the failure one of 16 motors effectively reduces available power by 25 percent?

The power output of the individual "electric turbines" is not specified. While the artist's conceptions of the aircraft are admittedly early, the "electric turbines" appear to be ducted fans of relatively small diameter which raises questions regarding the intensity of the downwash generated in vertical operations and whether it would be feasible to land and takeoff from unpaved surfaces—a serious issue in areas of limited infrastructure, the target market for the aircraft.

VV-Plane in vertical flight.

4 x 4 Aviation describes the overall propulsion system us using a combustion engine that is a "multi-staged (gas and steam) thermodynamic process machine built from off-the-shelf components, that in the future will eventually lead to machines achieving over 75% fuel efficiency and zero emissions."

A part of the propulsion system includes batteries charged by the generator driven by the combustion engine, but also "e-Storage," which is "based around double-walled pressure vessels" that allows "fast charge and discharge of the stored energy." Thus, it appears the aircraft will store energy in the form of batteries and compressed steam.

We are eager to see breakthroughs in general aviation, yet we think 4 x 4 Aviation faces some big challenges. One of the sad axioms of aviation is that if the development and certification of a new airframe also includes the development and certification of a new engine, the chance of the aircraft reaching production is low. We saw it with the Bede-5, the Foxjet and the original version of the Eclipse. We also note that capital-rich Boeing was involved with a heavy lift VTOL some six years ago with an airship that proposed to use vectored thrust to be able to lift as much as 80,000 pounds. The project has been on hold since 2010, apparently due to lack of developmental funding.

We admire the audacity of 4 x 4 Aviation, and will be watching the VV-Plane project to see what develops—the company says they intend to fly the prototype in three years.

Personal VTOL—Krosswind Aerospace

On the other end of the size spectrum, Arizona-based Krosswind Aerospace has announced development of the SkyCruiser, a five-place, six-motor VTOL that will convert from a quadcopter configuration for takeoff and landing to a pusher monoplane that will cruise at over 260 knots. On top of that, it will be a street legal for the drive from home to the airport at up to 75 MPH.

SkyCruiser in roadable configuration.

The SkyCruiser is designed for what company CEO Dan Lubrich described to us as limited highway operations—with the wings, tail and pusher motors and props stowed.

Once at a suitable place for vertical takeoff, the stowed components are locked into position and the "switchblade" mechanism brings the four vertical propulsive motors and rotors from the aft fuselage into quadcopter configuration. A 400-HP internal combustion engine is fired up to drive an electrical generator. Once the four 80 HP electric motors get the SkyCruiser to something over 1000 feet into the air, the two 150-HP motors driving pusher props on either side of the tail begin to move the aircraft forward. At something over 100 knots the wing will support the weight of the aircraft (stall speed for the wing will be about 90 knots—it will have about half the area of current GA airplanes). Once the wing is doing the lifting, the quadcopter rotors are stopped and the switchblade mechanism stows the quadcopter components back in the fuselage.

Should a motor fail while in vertical operations, the switchblade mechanism moves the rotors from an "X" configuration to a "Y" shape, allowing continued flight. Lubrich says the SkyCruiser will at least hold altitude on three motors. Should the main engine fail, there is adequate battery power for a descent and normal landing. Lubrich said that because a quadcopter cannot autorotate, the system will provide redundancy to allow a powered landing from batteries rather than the internal combustion engine—redundancy instead of autorotation. He also pointed out that in a helicopter an autorotation is difficult, requiring constant practice, so a powered descent and landing is probably safer.

Proposed maximum takeoff weight is 3461 pounds and the target empty weight is 2216 pounds. We expressed concern about the weight of six electric motors, the necessary structure for a retractable quadcopter, the supports for the pusher motors, an electrical generator in addition to an internal combustion engine and a wing folding mechanism. Lubrich said that the additional weight will be 250 pounds because the small size of the wing saves some weight. He said the VTOL portion of the aircraft makes up about 15 percent of the total empty weight. Having watched the development of numerous general aviation airplanes, we know how optimistic initial weight forecasts are when compared to what finally evolves.

SkyCruiser in quadcopter configuration.

For comparison, a four-place Robinson R44 Raven II helicopter weighs about 1500 pounds empty and has a 2500-pound maximum gross weight. It has 245 HP available for takeoff and has an out-of-ground-effect hover ceiling of 7500 feet. The 3461-max gross SkyCruiser will have 240 HP available for vertical flight.

Krosswind has been flying a small UAS VTOL/quadcopter/winged aircraft for some time, so the underlying concept appears viable. Lubrich said that the company will probably begin selling the UAS soon.

Conclusion

The military has been using VTOL hybrids for specialized operations successfully for decades. While the concept has always required a massive amount of power and dollars, technology may allow viability in the world of general aviation. We're going to be paying attention. 

Rick Durden is the Features Editor of AVweb and the author of The Thinking Pilot's Flight Manual or, How to Survive Flying Little Airplanes and Have a Ball Doing It, Vol. I