NASA Armstrong Flight Research Center is currently testing an incredible technology called LEAPTech. Mark Moore, NASA Langley Research Center’s Principal Investigator says, “This is the most exciting thing I’ve ever worked on. And that’s with 31 years of working on NASA projects.” NASA, in cooperation with Joby Aviation and ESAero, is creating a electric propulsion flight demonstrator that operates faster and five times more efficiently than a similarly sized general aviation aircraft. Most electric propulsion flight demonstrators today cruise at 40 - 80 mph. This new demonstrator is targeting a cruise speed of 200 mph. High efficiency is required because battery technology isn’t currently up to par to give an electric propulsion aircraft a practical range.
The key to this 500% efficiency increase is Leading Edge Asynchronous Propellers Technology (LEAPTech). LEAPTech consists of 18 small propellers along the leading edge of the wing. These small propulsors will create induced velocity to supercharge the airfoil in a technique called “blowing the wing.” This will make the wing smaller (31’ in this case), optimizing it for cruise flight. This smaller wing is less susceptible to turbulence, which increases ride quality.
These 18 smaller inboard propellers will be used only for takeoff, climb, approach and landing. During cruise flight, the inboard propellers will fold back against the nacelle and power will shift to one larger propellor at each wingtip, which are not currently installed on the test article. These propellers will gain thrust and efficiency by operating in the more favorable upwash fields inside the wingtip vortices.
LEAPTech will offer a 15 dB decrease in noise compared to a conventional single-engine piston-powered general aviation aircraft. To achieve this, the vehicle uses low tip-speed propellers which move at half the speed of a conventional propellor. By controlling the spacing between adjacent propellers, you can not only reduce noise, but change the tone of the aircraft to reduce true annoyance levels. The key lies in LEAPTech’s ability to offer asynchronous spreading by employing extremely precise digital motor controllers that keep exact azimuth spacing between adjacent propellers, even at a relatively high 6,500 rpm.
Most accidents in general aviation accidents happen in the low and slow flight regime. LEAPTech will make this flight condition much safer because it offers increased redundancy. If you suffer a motor failure, you have more than enough thrust from the remaining motors to fly safely. Because this is a powered lift wing, you have the option of varying thrust for aircraft control, as opposed to using control surfaces, which is much more effective at the slow flight condition.
During approach to landing, as per FAA regulation, aircraft are required to keep a velocity of 30% higher than stall speed. A LEAPTech aircraft may not need a stal margin as large as this. The vehicle can fly more slowly and at much higher angles of attack because the flow from the propellers effectively acts like a wing slat, forcing air over the top of the wing. If a gust of wing happens to decrease lift, you can instantaneously increase lift anywhere on the wing by increasing power.
If you wanted to test a vehicle in the 40 x 80 Wind Tunnel at NASA Ames Research Center, you would be added to a two year long waiting list after writing a check for two million dollars. In order to start wind tunnel testing more quickly and on a lower budget, Joby Aviation built the Hybrid-Electric Integrated Systems Testbed (HEIST) pulled behind a Peterbilt Semi-Truck. Results have shown only a 3% error compared to data collected in wind tunnel testing. This small error can easily be nulled by the fact that they can test as much as they want, whenever they want. Testing is a simple matter of firing up the big rig.
The HEIST vehicle was inspired by a previous test performed by Scaled Composites in which SpaceShipOne was evaluated in a similar way, placed on the back of a Ford F-250 pickup truck. This method of testing is perfect for building an aerodynamic database on a low budget, but it does pose some interesting challenges. One problem with this method of testing is vibration and ground noise. The HEIST vehicle solves this problem by isolating the test rig from vibration using four airbags, one of which is shown in photo seven.
On May 12, 2015, at NASA Armstrong Flight Research Center, Mark Moore saw the HEIST vehicle mated to the LEAPTech demonstrator aircraft for the first time as it rolled out onto Rogers Dry Lake Bed for a test. At this moment Moore exclaimed, “Even an ugly semi can look gorgeous right now. That’s the sexiest thing I’ve ever seen. My wife is going to kill me.”
Normally, HEIST operates in the early morning under calm wind conditions. On this day, the test was performed in windy conditions for the first time, which produced interesting data. This was the first test conducted with a newly installed air data probe and a secondary inclinometer which measures the tilt of the vehicle as it accelerates into the wind. The vehicle made two passes, both upwind and downwind. On the upwind leg, the vehicle’s ground speed was 40 mph with a 27 mph headwind. This produced a lifting force of 1,800 lb. On the downwind leg, the ground speed of the vehicle was 65 mph, with a tailwind which reduced lift.
Mark Moore says, "This is the most fantastic project team that I’ve ever been associated with, and I’m not just saying that. This is a really incredible team.” The team has already been approved to convert a twin piston-powered Tecnam P2006T into the first manned distributed electric propulsion (DEP) flight demonstrator. Their ultimate goal will be to use this technology in a vertical takeoff and landing vehicle, so we can operate an electrically propelled aircraft from our front yard. This exciting project could some day make the decreased emissions and operating costs of electric propulsion practical for all of us.
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