At last week’s AIAA Aviation Forum (Aug 2021), there was a lot of debate among the eVTOL, eCTOL, and eSTOL players (you can read more here on eVTOL.com) on the merits — and potential limitations — of our various approaches. A healthy dialogue was had, but I left feeling like we weren’t comparing apples to apples, which is confusing for the industry, investors, and ultimately the public. I wanted to summarize one of my primary concerns, which is around range.
Range describes how far an aircraft can fly on its primary mission from its departure point to its intended destination.
But we must take other factors into account that affect the practical range of an aircraft: altitude and speed, alternate landing site, flight rules, and extra fuel are all considerations. Depending on its use even the same aircraft can have different range estimates.
While defining range is not always straightforward, I believe that the industry must agree on standardized flight profiles so that range estimates are accurately compared between aircraft.
eVTOL aircraft, in particular, create even more challenges when defining range because of the various flight modes. These aircraft can take off and land vertically, like helicopters (lift comes from the rotors). They can also fly horizontally like a fixed-wing aircraft (lift comes from the wing). And they can transition between these two modes of flight (this is the tricky part). The power required for each of these flight modes is very different, and assumptions about the amount of flight time spent in each mode significantly impacts range.
So how does one understand the practical range of an electric aircraft used in commercial service, and how does anyone in this space make a fair comparison between aircraft?
Over the years, the aviation industry has defined “range” in different ways, a few of which are outlined here.
The first is a term called Specific Range, which is the distance an aircraft travels per unit of fuel consumed, and depends mainly on its cruise speed, altitude, and mass. Aircraft performance tables typically list the specific range for both long-range and high-speed cruise. For instance, an aircraft with a high-speed cruise specific range of 1.890 means for every pound of fuel used, that aircraft can fly 1.890 nautical miles.
The next method is to assume a flight profile based on FAA standard fuel reserves, which can range from 20 minutes to 45 minutes and may or may not include an alternate landing site. The key takeaway here is that a pilot cannot plan to land with zero fuel. All flight planning must include fuel reserves to account for unknowns (such as stronger headwinds than anticipated), and this extra fuel requirement effectively reduces the practical range of the aircraft.
The third method was developed decades ago by the National Business Aircraft Association, known as NBAA IFR Range. By using this method, business jet buyers are able to cut through the marketing hype and compare aircraft flight profiles on an apple-to-apples basis. It describes a specific flight profile to the intended destination, then another flight profile to an alternate landing site, and includes planned fuel reserves.
The methods above can be readily adapted to eSTOL and eCTOL aircraft, as these aircraft’s flight profiles match those of traditional fixed-wing aircraft.
However, eVTOL aircraft don’t fit easily into any method to objectively compare range. VTOL aircraft simply don’t exist outside of military circles so there hasn’t been a need to standardize.
Until now. With billions of dollars pouring into this market, we need to figure it out.
Let’s look at a recent example by SMG Consulting as their analysis explains two very different eVTOL flights. Note that these assumptions about the flight profile have a meaningful impact on the range of the aircraft.
Both aircraft are eVTOL battery-electric aircraft, but the lack of a standard flight profile makes it challenging to use these flights as a reliable indicator of range or to make any comparisons. One aircraft took off and landed like a conventional fixed-wing aircraft and did zero transitions. The other aircraft took off like a helicopter, transitioned to wing-borne flight, then transitioned to land like a helicopter at the end of the flight. There are also other differences noted in the small text above that you probably can’t read.
When considering battery-electric eVTOL aircraft, there are many other factors that must be taken into consideration as well because they impact range.
Two items, in particular, deserve attention: battery charge states at both takeoff and landing. These are important because batteries are not like fuel tanks, which can be filled to capacity and drained to empty. Batteries have limits on both charging and discharging.
When considering the limits on batteries, it’s possible that a battery-electric aircraft can take off with only 60% of the theoretical battery capacity. These numbers will vary depending on battery chemistry, but the point here is to understand the general idea. So, in order to accurately compare range, we need to know what state of charge is used for flight planning.
Another consideration is the number of transitions assumed. This is important because transitions and flight in vertical flight mode (or hover) require a lot of power and use a lot of energy, thereby affecting range. It seems reasonable, for flight planning purposes, to assume at least two transitions, one at the departure point and one at the point of intended landing. But what about alternate landing sites in case the aircraft can’t land at the planned destination? Should we then assume two more transitions are needed for safe planning?
So my question is, how do we get at the true commercial range of a battery-electric eVTOL, and how does it compare to what we’re anticipating for eSTOL and eCTOL aircraft?
There is no standard definition of range for electric aircraft today. I believe an independent third party is needed to define standardized range profiles for each aircraft that reflect how they’ll be used in commercial service. Doing so will aid buyers, investors, and industry observers as they compare aircraft and it will add more transparency to this rapidly emerging industry.