This post is part of a series that introduces and makes the case for Electric Short Take-Off and Landing (eSTOL) aircraft.
We are at the beginning of a new and exciting chapter in aviation and transportation enabled by electric propulsion. The change from using internal combustion engines (ICE) to electric motors is not just an incremental improvement in aircraft performance but in fact allows for radical improvements in multiple key performance parameters.
More than just the promises of carbon-neutral emissions and lower operational and maintenance costs, the integration of electric propulsion system boasts several significant improvements in aircraft performance:
Electric Short Take-Off and Landing (eSTOL) aircraft greatly benefit from electric propulsion technology. This post gives an overview of the five key engineering technologies that enable eSTOL aircraft to operate safely and consistently:
Electric propulsion components are significantly improving in efficiency and reliability year by year. This includes components such as motors, inverters, generators, transmission systems, and energy storage systems. Though development of lightweight, powerful motors was initially driven by the automotive industry, the aviation industry is quick to catch on — the Pipistrel Velis Electro recently became the first electric-powered aircraft to receive a European Type Certificate. This aircraft is intended for pilot training and has an endurance of 50 minutes (plus reserves).
Compared to an internal combustion engine, an electric propulsion system has fewer moving parts, significantly lower maintenance costs, and is more flexible in its design to meet requirements on power, torque, and rotational speed. Although the specific energy density of aviation gasoline is significantly higher than that of today’s lithium-ion batteries, internal combustion engines are inherently inefficient due to the nature of their thermodynamic cycles. To better understand the energy losses from the energy source to thrust production, a “well-to-wheel” energy chain is often analyzed that steps through each component’s efficiency. A comparison done by Martin Hepperle approximates that turboprop and turbofan have a total propulsive efficiency of 39% and 33% respectively, whereas battery powered systems have a total propulsion power efficiency of 73%.
For applications in urban and suburban areas where current lithium-ion batteries can meet the energy requirements of the short-range missions envisioned, eSTOL aircraft can outperform traditional ICE-powered aircraft on energy consumption.
Unlike internal combustion engines which suffer drastically in performance as they are scaled down in size and power output, electric propulsors are essentially scale-free in performance, meaning that consistent efficiency and power-to-weight ratio is achievable regardless of the sizing of the propulsor — “motors of 1 hp, 10 hp, or 100 hp can all achieve essentially the same efficiency and power-to-weight ratio” — Mark Moore.
The characteristic of having a scale-free propulsion system is significant to aircraft designers because this opens up a degree of freedom in aircraft configuration which has not been available before. The result of this is the ability to design and distribute multiple compact propulsors across the aircraft — a configuration known as Distributed Electric Propulsion.
Distributed electric propulsion (DEP) is a lot more than just placing multiple propulsors on the aircraft. DEP is an integration technology that deals with the synergies of multiple engineering disciplines.
DEP expands vehicle control strategies in multiple ways. Having multiple propulsors on the aircraft provides safety through redundancy. DEP aircraft can be designed to handle not just one, but even multiple instances of motor failures, by redistributing the amount of thrust required to balance out and compensate for the failed motor(s).
For eSTOL aircraft, DEP improves the performance in one of the most important legs of any mission: the landing. More precise landing approaches can be performed by having the ability to increase drag by windmilling a subset of motors, and all or a subset of motors can be configured for reverse thrust on touchdown. Having a safe and robust method of windmilling and reversing the thrust on the propellers allows eSTOL aircraft to achieve precise approaches and exceptionally short landing distances.
Aero-propulsive interactions have long and extensively been researched. Research has shown that propulsors can be strategically placed around or along the wing to improve both aerodynamic and propulsive performance. A “boundary layer ingestion” DEP configuration has been computationally shown to improve propulsion efficiency, and placing propulsors on the wingtips can suppress induced vortices which results in overall decreased drag, hence improving cruise performance.
For eSTOL aircraft, the most significant aero-propulsive synergy is the “blown wing” concept. By having an array of propulsors along or in front of the leading edge of the wing, propulsors will continuously blow air over the wing behind them — effectively increasing the dynamic pressure on the wing thus increasing the lift generated. More on the blown wing concept will be elaborated in the next section.
Probably the most well-known aircraft that can be studied to understand the benefits of DEP is the NASA X-57 Maxwell. Detailing all the engineering aspects of the X-57 is beyond the scope of this post, but the designs for aero-propulsive synergies are visible in the intended DEP system.
Two sets of motors are utilized to improve the takeoff and cruise performance of the aircraft. The high-lift motors are designed to produce additional lift during takeoff and landing, and the cruise motors are designed to perform optimally during the cruise phase. Computational studies have shown a 1.7x increase in lift and an increase in efficiency through drag reduction.
A derived characteristic of high-lift aircraft is possessing a low stall speed (the minimum airspeed required to prevent the aircraft from stalling). A low stall speed enables an eSTOL aircraft to:
In addition to allowing the aircraft to operate at lower speeds, strategically placed propulsors can enhance the effectiveness of control surfaces to provide added maneuverability and safety, particularly during the landing phase.
There is another approach to take advantage of the lift generated from a blown wing. An aircraft with a blown wing system can be designed to achieve the same amount of lift with a smaller wing as its non-blown counterpart. Simply put, an aircraft with a smaller wing produces less drag and thus improves cruise performance. It is important to note that when optimizing aircraft performance, there is a trade off between takeoff and landing performance and cruise performance. Thus a DEP high-lift system allows the aircraft designer to meet a broader spectrum of performance requirements.
It is also worth mentioning that the blown wing concept is not new, and in fact has been a research topic for military transport aircraft in the past. Namely, the YC-14 aircraft developed in the 1970s utilized an “Upper Surface Blowing” concept, and the current C-17 aircraft (developed in the 1980s) utilizes an “externally blown flap” concept.
While a blown wing high-lift system enhances the takeoff and landing performance for an eSTOL aircraft, thrust reversal capabilities take the landing performance one step further.
As previously mentioned, reversing the direction of the thrust allows for the aircraft to land and decelerate in a very short distance. For ICE-powered aircraft with propellers, this is done by reversing the pitch angle of the propellers by using a variable pitch mechanism on the propeller hub. An electric propulsion system can utilize a variable pitch mechanism, but also has the option to reverse the motor rotational direction directly.
Airliners today use thrust reversal, and you have probably experienced this after touching down on the runway of your destination airport. A video of a turboprop aircraft using reverse thrust upon touchdown can be seen from this video of a Bombardier Q400 below (starts at 1:17):
Pilot assist systems envisioned in eSTOL aircraft such as an autopilot and a precision landing system will help capture the full integration benefits of DEP to fly safely and reliably. There are three immediate use cases for pilot assistance systems:
In this post we introduce the key engineering components of eSTOL aircraft. The advancement and implementation of these technologies allow eSTOL aircraft to operate more safely, consistently, and efficiently than conventional STOL aircraft in applications such as cargo and passenger transport.
For those who have not already read the previous post on eSTOL, A New Aircraft for Urban Air Mobility, please click here.