Electric WheelTug System to Move Planes on the Ground

As much as the flying public dislikes all the time airplanes spend on the tarmac before they take off, the airlines hate all that waiting time even more. It costs them millions of dollars, since their planes burn fuel while creeping to the runway for take-off. A new all-electric integrated tug system promises to make those lines to the runway a little less painful — at least for the airlines.

Based on a patented ac-induction motor from Chorus Motors, this WheelTug system takes its power from the plane’s APU and directly drives the nose wheel. It’s designed to move regional and larger aircraft on the ground at speeds up to 20 mph without running their turbines or hooking them up to an airport tug.

The resulting savings could be huge, which has enticed Delta Airlines to invest in the new technology, initially for its Boeing 737s. “Even a moderately sized fleet could save tens of millions of dollars per year,” says Walt Klein, Delta’s director of engineering, quality and training. WheelTug’s projections put the savings at $60,000 per month on a typical 737 involved in regional runs, according to Isaiah Cox, CEO of WheelTug, which is a subsidiary of Chorus Motors.

All those savings come from a variety of sources. One big one is the direct savings of burning less fuel when the turbines no longer have to push the aircraft to the runway. And there’s an indirect fuel savings, too. To account for taxi time, airlines often have to load more fuel onto the plane then they need for the flight itself. The weight of that extra fuel, if it’s not all burned on the ground, potentially poses a secondary drag on fuel economy.

Then consider the cost of airport tugs that also move planes on the ground, particularly near tight gates for safety reasons. “The tugs impose an obvious capital cost,” says Cox. And the tugs also burden the airlines with additional maintenance and labor costs.

The installed cost and maintenance of the WheelTug system may offset the potential savings a bit, but probably not by much. It’s too early in the systems’ development to quantify the economic case completely, but “we expect the cost to own and operate the WheelTug will be a mere fraction of what we save in fuel costs,” says Robert Cooney, Delta’s engineering manager for 787s, the first plane the WheelTug system targets. He says the weight of the production WheelTug system, predicted to be about 200 lb for a 737, will have truly “negligible” affect on fuel burn in the air and save lots of fuel on the ground.

Cox also ticks off a list of other potential benefits from the system, including reductions in greenhouse gas emissions, brake wear, damage to the planes from tugs and turnaround time since ground personnel won’t have to wait for engines to cool before working near them.

So if direct drive is such a good idea, why hasn’t it happened yet? The idea has been around for years. Until recently, though, traction motors lacked the torque density to both move a heavy plane and fit in the tight spaces of the nose gear. “Advances in motor technology have now brought the concept close to reality,” Klein says. Those advances are good news not just for the airline industry but also for any engineer working on torque-hungry mobile applications that don’t have a lot of packaging space for electric drives.

Motor Makes It Happen

The motor advance that makes the WheelTug possible is the Chorus’ patented Meshcon high-phase order AC induction motor and drive. Though patented back in 2000, the motor has just recently become commercially available. Initial general purpose units have been in the 1 to 20 HP range, though other sizes are in the works.

Some details of the production WheelTug remain up in the air, but the likeliest configuration consist of two Chorus Motor assemblies that almost entirely nestle into existing space in the opposing nose wheel hubs. Each motor’s thin stator mounts on the aircraft’s standard nose wheel strut, while the rotor and wheel interface mount on the axle. The rotors feature an integrated planetary gear assembly to drive the wheels via the wheel interface.

Cox declined to provide the dimensions, horsepower or its torque-speed curve for the specific motors used in the WheelTug. “All that would change based on the size of the aircraft and the expected operating conditions such as taxi speeds,” he says.

In an early demo of the WheelTug, a belt-driven configuration with motors mounted outside the wheel, Cox notes, “two Meshcon motors the size of a watermelon moved a 300,000 lb aircraft.”

Cox claims the chief advantage of the Meshcon motor in this integrated tug application comes down to its torque density. “Depending on the sizing of its electronics, the Meshcon has five to 10 times the torque as a conventional three-phase induction motor of the same size,” Cox says.

Meshcon gets that torque, particularly at low speeds, from the company’s patented approach to motor and inverter design. These motors use a mesh connection to connect high-phase-order inverters to the induction motor windings. This mesh connection, in which each winding termination connects to both an inverter output and to the termination of a different winding, provides access to a range of V/Hz ratios within the same system. “We can change V/Hz ratios on the fly using the drive electronics,” Cox says. “That essentially gives us the ability to use the full inverter capability at both high and low speeds.”

In another key aspect of its technology, Chorus enlists the naturally occurring harmonic components of the drive waveform to switch the V/Hz ratios electronically. In traditional three-phase induction machines, unchecked harmonics can act against the rotating field produced by the fundamental waveform, dragging down performance. In a multi-phase system, such as the Meshcon, careful design can allow the harmonics magnetic fields to rotate synchronously with that of the fundamental. By feeding the appropriate harmonics into the motor along with the fundamental, Chorus’ drive essentially “rewires” the mesh connections to give different V/Hz ratios — and ultimately dictate the torque generated at both high and low speeds. “Think of the harmonics as our gear ratios,” says Cox. (Get more information on this harmonic mesh effect).

Engineering Work Remains

Chorus isn’t the only one claiming extra torque density from the Meshcon motor. Delta’s engineers have noticed it, too. “I know this motor will push the aircraft and I’m confident it will fit in the space we have available for it,” says Klein.

Even so, Delta and WheelTug have plenty of work do before the WheelTug launches on Delta’s 737NG fleet. According to Cooney, Delta engineers still have to do some of the integration work. Part of that work involves how the WheelTug will be physically interfaced to the wheel and nose gear strut. Delta engineers also have to integrate WheelTug on a systems level. For example, they’ll verify the integrated tug system’s electrical loads don’t overtax the auxiliary power unit. And they’ll make sure the system doesn’t interfere with the functioning of the landing gear. “You have a substantial weight at the end of a pendulum and you want to be sure you can pull it all up into the hold,” says Cooney.

The fact the system will launch on a 737NG also complicates Delta’s validation work somewhat. “That aircraft operates everywhere,” says Cooney. So, Delta engineers will have to evaluate its performance in a wide variety of operating conditions that include differences in surface conditions, gradients and wide swings in temperature.

All of this work will take place as the system goes through its Federal Aviation Administration Certification process over the next few years. Klein estimates the system will earn its certification and be ready to retrofit on Delta’s 737s by 2009. “It won’t be the most difficult installation we’ve ever done,” says Klein. But it may be one of the most cost-effective. “Financially it’s a no-brainer,” he says.