By Dr. Troy Mestler, CEO, Skyfront
Tuesday, September 17, 2018
Electric propulsion has changed aircraft manufacturing. The reason is simple: wires bend. Shafts and gears do not. To build a helicopter engineers no longer have to design around a long mechanical shaft and heavy, failure-prone gearbox and clutch that spin to transmit power from the engine to the rotors.
With electric propulsion, power is routed to the propulsors over wires. Wires bend easily to accommodate a design optimized for aerodynamics, safety and ease of manufacturing. Power is provided wherever power is needed. This frees up space and reduces both weight and complexity, reducing production costs and time to market by an order of magnitude. This “propulsion by wire” is the reason there are so many drone companies today.
As great as electric propulsion sounds, there is one problem with it: batteries have nowhere near the energy density required for safe flight. The designers of manned multirotors forget that one of the principal causes of UAV failures is that batteries unexpectedly run out of power. Today, the total flight time of a battery-powered VTOL aircraft is equal to the reserve flight time required of a helicopter by the FAA. Nobody who understood the risks learned by the aviation community in the last 110 years would fly regularly in a battery-powered vehicle.
So, when are batteries going to get better?
A History of Lithium Ion
To answer that question, it is helpful to understand the history of the lithium-ion battery. The lithium-ion battery was first developed by M Stanley Whittingham in the 1970s while at Exxon. Exxon manufactured his battery, but the battery’s constituents were too expensive and too volatile to be made into a commercial product.
The term “lithium-ion” represents a class of batteries in which lithium ions move between the cathode and anode. The makeup of the cathode, the anode, and the electrolyte separating the two are what define the different subtypes of lithium-ion batteries. Over the years, scientists made many attempts to commercialize Li-Ion batteries by experimenting with different cathodes, anodes, and electrolytes to make them cheaper, safer, and more reliable. These advancements culminated in Sony releasing the first commercial Li-ion battery in 1991.
Today, nearly fifty years after Whittingham’s discovery, lithium ion and its subtype, lithium polymer (LiPo), have become the predominant battery type for energy-intensive devices from laptops to drones.
The reason it took so long was due to the sheer number of constraints that batteries must satisfy before entering the market. Energy density is but one constraint that batteries must satisfy in the aviation industry. Batteries must also be cheap, powerful, reliable, safe, versatile, and long-lasting.
For example, sulfur cathode lithium has four times the energy stored in a traditional LiPo battery, enough to fly an aircraft for four or more hours. But the problem is that they are prone to failure and typically need to be replaced after 50 charge-discharge cycles, which is far too few for a mission-critical application like aviation.
Similarly, silicon anode batteries can achieve two to ten times the energy density of existing batteries, but also have a limited number of charge-discharge cycles and cost tens of thousands of dollars per cell.
Battery technology is science, not engineering, and science always moves at a much slower, unpredictable pace. For the aviation industry, this means that a manned, long-range, commercial, battery-powered aircraft is many, many years away.
A Hybrid Approach to Aviation
Hybrid-electric propulsion combines the energy density of gasoline with the simplicity of electric propulsion. Today, hybrid multirotors can fly for up to 5 hours and eliminate most of the reliability and maintenance issues associated with gas-powered helicopters (e.g., engine vibrations, swash plates, gearboxes, and drive shafts).
Hybrid technology is more than just a compromise until batteries improve. It has many advantages of its own. For example, the battery can act as backup power if an engine failure occurs, eliminating power loss as one of the most common contributors to crashes. Other important advantages include:
– Fast refuelling/recharging
– Sub-zero temperature operation
– Eliminating the maintenance and transportation of many batteries
– Reducing battery replacements
– The ability to isolate and dampen the engine from the airframe
When applied to aeroplanes, hybrid-electric technology also improves fuel economy by downsizing the engine (which saves weight) and enables it to operate in its most efficient state during the cruise. Hybrid technology can also improve noise pollution by providing the option of only running the engine at altitude, far away from people.
While companies like Opener, Joby, and Uber are sticking to pure electric, many others have taken note of the benefits of hybrid technology. Rolls Royce, Bell Helicopter, and SureFly have all released plans for building full-scale hybrid VTOL aircraft. And Zunum Aero (funded by Boeing), NASA (in collaboration with ESAero and Launchpoint Technologies), Siemens, and Airbus have developed or begun developing hybrid-electric aeroplanes.
All the above is not to say that batteries will not improve eventually or that we should not invest in battery technology. But for the foreseeable future, hybrid technology will become the predominant source of power for electric-based aviation.
Troy Mestler, Ph.D., is the CEO of Skyfront (www.skyfront.com), a company that builds hybrid-electric multirotor drones that fly for up to 5 hours and over 100 miles.
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