NASA Greased Lightening (GL-10) hovering, Virginia, USA.

Greased Lightning (GL-10)
NASA Langley
Hampton, Virginia, USA
www.nasa.gov/langley

In 2015, NASA Langley researchers completed a carbon fiber hybrid-electric Vertical Take-Off and Landing (VTOL) drone prototype named Greased Lightning (or GL-10) and was successfully remotely piloted from a hover to wing-borne flight back to landing, in tests at Fort A.P. Hill, Virginia, USA.

The purpose of this research project was to take advantage of new technologies, electric engine propulsion and affordable drone flight controllers (closed loop controllers) and make and aircraft which would take on the best aspects of an airplane and helicopter and fly with superb reliably, affordably and with excellent stability. We believe the project was started in 2014.

The Greased Lightning prototype has a 10-foot wingspan, with 10 electric engines, weighing about 55 lbs (25 kg), is a hybrid-electric Vertical Take-Off and Landing (VTOL) prototype aircraft, has been reported to travel up to 61 kts (mph or kmh) – possibly faster speeds, testing included unpowered glides and can take off like a helicopter and fly efficiently like an airplane.

Greased Lightning (GL-10) is an aircraft configuration that combines the characteristics of a cruise efficient airplane with the ability to perform vertical takeoff and landing (VTOL). This aircraft has been designed, fabricated and flight tested at the small unmanned aerial system (UAS) scale.

The GL-10 design utilized two key technologies to enable this unique aircraft design; namely, distributed electric propulsion (DEP) and inexpensive closed loop controllers. These technologies enabled the flight of this inherently unstable aircraft. Overall it has been determined thru flight test that a design that leverages these new technologies can yield a useful VTOL cruise efficient aircraft.

There are three primary advantages to Greased Lightning technology relative to conventional rotorcraft. The first is no speed limit due to retreating blade stall.

The second significant advantage over rotorcraft is improved aerodynamic efficiency. Typical fixed wing aircraft achieve a best lift to drag ratio of 14 to 20. Rotorcraft typically have an effective lift to drag ratio of 4 to 5. Note this equation can also be applied to a fixed wing aircraft and the result is equal to the aerodynamic lift to drag ratio. Aircraft with low effective lift to drag ratio, specifically rotorcraft, are limited in range and consume more energy to fly the mission. This leads to higher operating costs of the aircraft.

The third shortcoming of rotorcraft is they have multiple single point of failure modes, for example the pitch links. Granted that with proper inspection and maintenance the likelihood of a pitch link failure is very low, but these inspection and maintenance requirements increase operating costs.

There are many reasons for the limited success of previous VTOL aircraft, but they fall into four main categories. First, the resulting useful load fraction of the aircraft was small because the cross shafting and other VTOL systems significantly increased the empty weight of the aircraft. Second, the Effective L/D (Lift to drag ratio) of the aircraft is noticeable less than their fixed wing counter parts, and therefore overall performance suffers. Third, while these aircraft were flyable, their handling qualities were usually poor and required highly trained and skilled test pilots to safely operate.

Additionally, in general many of these aircraft would be disturbed by wind gusts more than rotorcraft and thus had smaller allowable wind environment envelopes. Fourth, the inspection and maintenance requirements were large due to the numerous single points of failure. Considering these previous shortcomings, when new technology can be infused into products, it is worthwhile reinvestigating concepts that were previously considered infeasible, or iterating from previous lessons into new concepts.

NASA July 2017 Greased Lightning Flight Testing PDF report (page 3-4) .


A very unique aspect to NASA’s project was the willingness for their engineers to accept risk in the planning of the project. NASA’s approach to flight testing for the Greased lightning had several phases to make this happen. First was using a “Foamie” aircraft frame to test the functionality of the avionics hardware and flight software. There was no intention to fly this model. Then a Greased Lighting Almost Ready to Fly (GLARF) realistic model was made and used as a flying simulator for multiple tests. Then the actual Greased Lighting flying made its first flight and more testing continued. NASA built 12 prototypes during this project.

Some highlights of NASA’s Greased Lightning VTOL prototype: A 10-foot wingspan (3.05 meters), eight (8) electric motors on the wings, two (2) electric motors on the tail, batteries in each nacelle, two (2) diesel engines turning alternators, one (1) fuel tank, a satellite communications system, maximum weight of 62 pounds (28.1 kilograms) at take off, has retractable landing skids and has a payload area in the front of the fuselage (where a cockpit would typically be in an airplane).

NASA stated that the use of the technologies in a VTOL aircraft enabled the flight of this inherently unstable aircraft, achieves excellent lift to drag ratio, can safely and repeatably transition into cruise efficient wing born flight, can robustly handle disturbances throughout the transition corridor, have ultra safe operations, have the cruise efficiency of a fixed-wing aircraft and still have true VTOL capability, is resilient to any motor failing, can be scaled to have a maximum take-off weight of approximately 3,000 lbs (from 1 to 4 people), is easy to fly and has low noise.

While not formally verified, NASA believed in 2015, the Greased Lightning set the record for the highest lift to drag ratio VTOL aircraft that has flown and transitioned.

References:

Tags:

NASA, NASA Greased Lightning, Greased Lightning, GL-10,
Vectored Thrust, Scaled Prototype, 0 Passengers, Hybrid-Electric VTOL,
Remote Controlled

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