Recently, a team composed of Boeing, GE, and Georgia Tech completed an 18-month study on future commercial airplane concepts under the NASA N+3 Program. The goal of the program is to explore revolutionary aircraft concepts aimed at entry into service date of 2030 and beyond, with aggressive noise, emissions, and fuel burn targets. The program looked at both subsonic and supersonic aircraft concepts. The Boeing-led team looked at five different subsonic concepts as part of the Subsonic, Ultra Green Aircraft Research (SUGAR) project. Concepts ranged from a conventional tube and wing design (SUGAR Free and Refined SUGAR) to a high span, strut-braced wing aircraft (SUGAR High and SUGAR Volt), and a hybrid wing body configuration (SUGAR Ray). To read more about Boeing’s technology research and the future of aircraft design, click here.

While not all concepts met the ambitious goal set by NASA, the SUGAR Volt concept, which adds an electric battery gas turbine hybrid propulsion system, can reduce fuel burn by greater than 70%. It also reduces overall energy use by 55% when battery energy is included. With the fuel burn improvement, the aircraft has an added benefit of large reductions of CO2 and nitrous oxide emissions.

What’s preventing engineers from designing such aircraft today? One major challenge is battery technology. While there are rapid advancements in battery technology, a level of energy density suitable for aerospace application is still years away.  Will a hybrid electric open rotor propulsion be the game changing technology the industry is seeking? We hope so. There have been many innovative aircraft concepts that were not adopted due to either the infrastructural or operational constraints. As the price of fuel continues to increase, the industry will hopefully be more acceptable to these innovations.

Image credit: NASA/The Boeing Company

In my previous blog from Oshkosh, I talked about Sikorsky’s ambitious Firefly project, an all-electric helicopter powered by batteries. Sikorsky had planned for first flight sometime in 2012, but apparently someone has beaten them to the punch! Pascal Chretien, a French electrical/aerospace engineer and helicopter pilot, has designed and built a fully electric coaxial rotor helicopter by himself (almost) and took his machine into the air for a two minute flight.

The challenges for electric fixed-wing flight are well documented, and they are even greater for electric rotary-wing flight. For a fixed-wing aircraft, it will only require max power during takeoff, but once in flight, the power requirement is reduced. On the other hand, a helicopter requires high power throughout its flight profile, so it will require a tremendous amount of energy to stay in the air.

For Pascal, he had to design a helicopter with minimum weight, so he had to adopt a different configuration than the standard single main rotor. He picked the coaxial design so all the power is going toward lifting the vehicle off the ground, versus a 90:10 split with the conventional helicopter. Instead of cyclic for directional control, he uses a weight shifting system. This design is particularly dangerous for two reasons. The first reason is that you are shifting the C.G. of the vehicle, which could be catastrophic if the weight is shifted beyond design envelope. The second reason is that the control is now backwards compared to the regular cyclic.

As a throwback, perhaps homage, to the early aviation pioneers, Pascal did not recruit a test pilot to fly his contraption. Instead, he took it to the air himself. So far the flight testing has been limited to within ground effect while he makes final tweaks. Eventually, Pascal and his sponsor, Solution F, is targeting 10 to 12 minute flight time, which is similar to the Firefly.

Back on June 30, 2011, DLR of Germany demonstrated electric taxiing using a novel fuel cell-powered electric landing gear (press release here and video link here). The premise of electric taxiing is to postpone engine start and using it as the source for propulsion and electricity while the airplane is still on the ground, turning the engine on once the aircraft is ready to take off. By doing so, this would help reduce fuel consumption and wear on the engine while the aircraft is sitting in queue waiting to takeoff, and now days the queue seems to get longer and longer.

This concept sounds great at first glance, but let’s dig a bit deeper. Fuel burn savings during taxi are really dependent on the proportion of time spent on ground taxi relative to the entire mission. For example, a 737 flight involves around 20% of its mission time on ground taxiing, while a 777 flight involves only 6%. Therefore, electric taxiing creates more value for a 737 versus 777 because the fuel consumption reduction due to electric taxi is greater on a 737 (~16%) than a 777 (~5%). The savings is not 20% for 737 because you still have to burn some fuel to power the electric motors, either through an APU or in the future, a fuel cell.

While ~16% savings is pretty significant, there are tradeoffs to consider. To start, one will have to offset the gain with the added complexity and weight of adding motors and related controls robust enough to handle the rigorous landing environment. Also, aircraft engines need to warm up prior to takeoff, depending on ambient temperature and whether it is the first flight of the day, so the real savings might be less than the 16%.

Are there other ways to achieve the same results? What about using ground tugs to tow aircraft to the runway? What do you think?

In my earlier blog on the 787, I talked about the dramatic increase (5X) in electrical power on board the aircraft versus its predecessor due to increased demand. With all that power, the weight of the electrical power system is sure to go up and at some point, the electrical power system weight will drive decisions and compromises in aircraft design. A step change in technology is needed to overcome the weight challenge.

Current power electronic components are silicon (Si) based and as power devices are forced to achieve higher efficiency, the silicon core is operating closer to its temperature limit. Extra cooling packaging is required to maintain reliability, which becomes a key driver of the weight increase. A breakthrough in reduced cooling requirements is anticipated with the introduction of silicon carbide (SiC) based devices. Key advantages of SiC are that they can operate at higher temperatures (more than 50% higher than Si-based devices) and at faster switching speeds, especially at high voltage (> 600V). However, fundamental challenges associated with reliability and yield of the SiC devices are preventing mass adoption in many markets.

Many leading power electronics manufacturers are focusing on the SiC device area. Cree has launched the first commercial SiC-based MOSFET.  The product’s performance is slightly better than Si-based devices, and more companies will join suit in the next two to three years with more capable devices. Why the focus? The industry is anticipating the next generation hybrid-electric and pure-electric vehicles to adopt SiC technology, making the vehicles more efficient than current generations. Companies are jockeying for a piece of this huge market.

GE is certainly not standing still on the sideline in SiC development. GE Aviation Systems, in collaboration with GE’s Global Research Center (GRC), has a an industry leading  cooperative project with AFRL at Wright Patterson AFB to develop an advanced solid-state primary power distribution technology using Silicon Carbide (SiC) high power switches. GE also announced introduction of a new line of SiC-based power conversion devices at the 2011 Paris Air Show. GE anticipates that its proprietary SiC technology will be world class, addressing fundamental challenges of gate oxidation and switch reliability. Once the technology matures, we can imagine that the next generation of aircraft will have electrical power equipment that enables aircraft manufacturers to continue up the power curve without lumbering heavy electrical power systems.

Click here to see GE’s SiC MOSFET.

 Click here to see lower switching loss of GE’s all SiC module vs. hybrid Si-SiC module.

After three years of extensive construction and renovation, GE’s Electrical Power Integration Centre (EPIC) located in Cheltenham, England, is set for its grand opening in September!

EPIC represents a major investment by GE Aviation Systems, along with local government, to the development and application of end-to-end electrical power systems for aviation and defense vehicles. EPIC, along with its planned sister EPISCENTER facility in Dayton, Ohio, will become the centroid of GE’s electrical power application research & development and help define and deliver the future vision of electrical power.

The 30,000 square foot EPIC facility went through an extensive makeover to update itself to the modern GE standard. The only original structure left is the brick work, and lab equipment is moving in now.

Over the next six months, EPIC will began testing on a number of key electrical power programs. The first involves advanced solid state primary distribution equipment utilizing silicon carbide (SiC) technology developed at GE’s Global Research Center (GRC), and soon after that the Advanced Power Management System (APMS), which enables end-to-end electrical power management, from generation to distribution and control, will begin testing and validation.

The following are some snapshots of the new facility. Pretty impressive.