The science behind NASA’S insane tilt-wing concept
When it is not occupied with extravaganzas like sending people to Mars, NASA does some interesting work on terrestrial aerodynamics. Three years ago, I wrote about a NASA project called LEAPTECH (Leading Edge Asynchronous Propeller Technology) that involved stringing 18 electric motors and propellers along the leading edge of a very skinny wing.
The idea was to blow high-speed air over the wing during takeoff and landing, augmenting lift and allowing the wing area (and weight, and drag) to be reduced by two-thirds. In cruise, 16 of the motors would shut down and their propellers would fold flush with the sides of their nacelles; thrust would then be provided by two motors at the wingtips. These would rotate top-blade outward, weakening the tip vortices and shaving off some induced drag. A full-scale piloted test item is now being built, using the fuselage and empennage of a Tecnam P2006T, an Italian-built light twin. It is expected to fly in 2018 or 2019, and will, as NASA modestly states, “allow engineers to compare the performance of the flight demonstrator with that of the original P2006T.”
In the meantime, another NASA group, this one at the Langley Research Center in Virginia, has been developing a concept that is similar in some respects to LEAPTECH, but in others more radical. It is called GL-10. The GL stands for Greased Lightning, not because it is expected to be exceptionally fast, but because in its final form it will use a hybrid power system consisting of an internal combustion engine burning fossil fuel or discarded french-fry oil — hence the “greased” — and driving a generator to supply power to electric motors — hence the “lightning.”
The 10 stands for the number of motors and propellers. Each wing has four motors, spread out so that the entire wing is bathed in prop wash; in that respect, its technology is similar to LEAPTECH’S. Two more motors are on the tail. One project engineer described it as a tandem-wing design, as opposed to wing-and-stabilizer, because the CG is somewhat farther aft than it normally would be and so the horizontal stabilizer/aft wing contributes about a sixth of the lift in cruising flight. The reason for this design choice will become apparent shortly.
What is unusual about Greased Lightning is that it’s a VTOL — vertical takeoff and landing — machine that converts into an airplane for cruising flight. Unlike the tiltrotor V22 Osprey, whose wing is fixed while its engines and rotors tilt, Greased Lightning is a tilt-wing. Its motors and propellers are fixed with respect to the wing, which tilts up to a vertical incidence for hover.
This configuration has been around a long time. Canadair studied experimental tilt-wing models in the late 1950s, and the 1964 Ling-temcoVought XC-142 was quite a big thing, with four interconnected 2,850 hp turboprops and a max VTOL weight
of 45,000 pounds. Although neither airplane went into production, both, along with other flying test-beds, executed many transitions to and from hover under direct pilot control. A notable characteristic of both was an unusually low power loading. The XC-142’S was about 4 pounds per horsepower; the Canadair’s 2. For comparison, power loadings of conventional utility transports like the C-130 Hercules or CASA CN-235 are in the range of 8½ to 9½ pounds per horsepower. The excess power is needed for the vertical phases of flight; as any bird knows, it takes far more power to levitate in place than to fly forward.
The aim of a “tilt” design, be it tiltrotor or tilt-wing, is to combine the range, speed and efficiency of a conventional airplane with VTOL capability. In its cruising configuration, Greased Lightning — a 10-foot-span, 62-pound radio-controlled half-scale model — consequently looks pretty much like a conventional airplane. Unlike the Osprey, whose huge propellers only permit it to take off and land vertically, or nearly so, Greased Lightning could, if its pilot wished, operate from a runway like an ordinary aircraft, particularly because the variable-incidence wing would allow its fuselage to remain horizontal during flare or rotation.
Pitch and roll are controlled in hover by separately varying the speeds, and therefore the thrust, of the motors. Ailerons, blown by the wing motors, control yaw. The reason for the aft location of the CG should now be apparent: For the two aft motors to bear their share of the load during takeoff and hover, the CG must be located at about 20 percent of the distance between them. Aft loading is detrimental to longitudinal stability, however, and so the wing is mildly swept. When it rotates to a horizontal position its center of lift moves aft, reducing the distance to the CG.
During the transition from hover to cruise, the wing gradually tilts forward so that some of the thrust begins to accelerate the airplane horizontally. Wing-tuft video of Greased Lightning, which can be found on Youtube, shows that the wing is entirely stalled while the airplane gains speed with the wing partly tilted. The airflow on the wing is a mix of propeller-driven and ambient flows moving in different directions, and the propellers themselves operate at an angle to the local airflow. “There’s a lot going on,” says project manager Robert Mcswain. Some sense of the sheer power required for hover and transition can be gleaned from one statistic: The power that GL-10 requires to hover is 16 times the power it requires to fly in airplane mode.
The controls used in hover are different from those used in cruise; the segue from one to the other requires management of many variables. This is the sort of situation that cries out for a fly-by-wire control system, which Greased Lightning has. An inexpensive off-the-shelf hobbyist’s controller translates the pitch, yaw, roll and power outputs from a conventional RC transmitter into the combinations of commands to motor, wing and control surfaces that make transition possible.
The target of the program is an autonomous 20-foot-span surveillance airplane that would cruise 200 miles at 100 knots, loiter at 21,000 feet for 20 hours or so and return to base. The current test article, which was preceded by several smaller ones, is half-scale; it is battery-powered, and its endurance is limited. The full-size airplane would use liquid fuel to power its engine-generator unit, or genset. The energy content per pound of liquid fuel is much greater than that of batteries, and allows long flights — which are the reason for a fast cruiser in the first place — at a weight that still permits vertical takeoff and landing. Surge power for takeoff, hover and transition would be stored in batteries that the genset would recharge while cruising.
A NASA technical memorandum summarizing results of the GL-10 experiments thus far concludes by suggesting three potential roles for such aircraft: search and surveillance, package delivery and, in a 3,000-pound version, on-demand personal transportation for four people over greater distances, and at higher speeds, than a pure rotorcraft could achieve.