Comparing Use Cases for eSTOL and eVTOL Aircraft

Marc Ausman
December 18, 2021
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5 minute read

Electric aviation promises to move people quickly, quietly, and cost-effectively, freeing them from ground-based commutes and traffic congestion. To do this, many types of aircraft will be needed and multiple categories are already emerging in the market.

Two primary types of aircraft are promising to fulfill this mission: One is fixed-wing and the other is rotary-wing. 

Fixed-wing aircraft include electric Short Take-Off and Landing (eSTOL) aircraft and electric Conventional Take-Off and Landing (eCTOL) aircraft. eCTOL aircraft are similar to eSTOL aircraft in many ways yet have different performance characteristics. One difference, for example, is that eSTOL aircraft can operate fully-loaded at even the smallest regional airports. Both types of aircraft can take off and land on existing runways using the wings to generate lift. For this discussion, we will focus on eSTOL aircraft. 

The other is rotary-wing aircraft and includes electric Vertical Takeoff and Landing (eVTOL) aircraft. These aircraft use rotors to lift off vertically like a helicopter and then fly horizontally using a wing for lift, finally transitioning back to vertical flight using direct lift from rotors for landing.

Visual differences between eSTOL and eVTOL

eSTOL and eVTOL aircraft are complementary to each other and will be used for different markets and missions. In our many talks with executives of charter operators, they shared with us how they view these new aircraft and how they plan to use them. This post describes an operator’s considerations when deciding how to use each type of aircraft.

Bear in mind that we’re talking about the usage of these aircraft at a high level and it is not our intention to discuss the specifics of each manufacturer. Each aircraft model is unique and will vary from what’s described herein. We want to share ideas that apply widely to the majority of designs in the industry.

One assumption that broadly covers the industry is to say that eSTOL aircraft use a hybrid-electric powertrain whereas eVTOL aircraft commonly use a battery-electric powertrain. While this is not true 100% of the time, it does represent the majority of today’s configurations. Also, we are considering eVTOLs with wings in this analysis although some eVTOLs called multi-copters do not have wings and generate 100% of their lift from their rotors in all phases of flight.  


Range Considerations

The power required by an eSTOL for takeoff and landing is substantially less than that required by an eVTOL. An eSTOL uses fixed wings for lift and the motors are only needed to propel the aircraft forward. The forward motion of the air across the wings creates lift for flight. On the other hand, an eVTOL uses the propellers as the sole source of lift for takeoff and landing. The propellers point vertically to lift the aircraft up, then (typically) tilt forward in order to transition to forward flight. At the end of the flight, the propellers again point vertically and provide the lift necessary for landing. 

Ultimately, this means eSTOL aircraft require less power and energy for takeoff and landing than eVTOL aircraft. It also means that for similar weight aircraft, an eSTOL can carry more than twice the payload of an eVTOL. For more in-depth technical information, please read the MIT study here.

The different ways to achieve lift combined with different powertrains used by each type of aircraft mean they are each suited for different and complementary missions. The inherent efficiencies of an eSTOL aircraft allow it to fly 50 to 500 mile missions from airport to airport or new, very short runways. 

Comparing range of eSTOL and eVTOL

eVTOLs, on the other hand, are best suited for short-range missions between 15 to 50 miles due to the limitations of today’s batteries and the large amount of energy required for vertical takeoff and landing. Any route under about 15 miles is generally faster to drive when transfers at each end of the flight are considered.


Understanding Operating Costs

The cost to operate an aircraft has a direct bearing on the cost of tickets for the passengers. Therefore, the lower the operating cost, the lower operators can drive the ticket price. One way the industry looks at this is to use cost per seat mile (CSM). This is a common unit of measurement used to compare the efficiency of most aircraft. CSM is calculated by taking the cost to fly a mile (hourly operating cost divided by the block speed) and dividing it by the number of passenger seats. If we use all-in operating cost numbers and compare a typical 4-seat eVTOL with a 9-seat regional eSTOL aircraft, we get the following figures.

Cost per seat mile:
eSTOL, 9 seats: $0.50
eVTOL, 4 seats: $2.19

Comparing CSM by aircraft type

The figures assume 600 hours/year utilization, all-in costs, and are calculated in statute miles.

We can see an eVTOL is more than 4 times the cost per seat mile of an eSTOL!

Based on these operating costs, a savvy commercial operator will use eVTOL aircraft ONLY where there is a true need for vertical lift, such as a helipad on top of a skyscraper. A popular use case for this could be the airport-to-city-center routes. There’s practically no commercially-sound reason to fly an eVTOL from airport to airport because vertical lift capabilities are simply not needed.

In use cases where the aircraft flies from an airport to another airport, the logical aircraft to use is an eSTOL with its significantly lower operating costs and ability to use existing infrastructure and air traffic control procedures. 


Weather is Important

One of the most important considerations for commercial operations, aside from safety, is the ability to fly in almost all weather conditions. Below we explain why an eSTOL aircraft is a reliable transportation means whereas an eVTOL is useful in only the best of weather conditions. Let’s look at the spectrum of operating rules (simplified) that cover flying in various weather conditions:

* Day VFR (visual flight rules) - flights in good weather in the daytime.

* Night VFR - flights in good weather at night.

* Day/Night IFR (instrument flight rules) - flights under operating rules designed for when the pilot has no visible horizon and cannot see other aircraft for some or all of the flight.

* FIKI (flight into known icing) - Flights that are flown under IFR with equipment on the aircraft that prevents the buildup of ice on the wings, propellers, and other surfaces in conditions where ice formation is likely. 

eSTOL aircraft, especially the ones developed by Airflow, can fly in all the weather conditions stated above. eVTOL aircraft, because of the newness of their design and proximity to obstacles, will start flying in day VFR conditions and progress to night VFR flights over time. 

An IFR approach is a formal procedure that defines how an aircraft flies from the en route portion of the flight to the runway or helipad. It’s unclear how current real-world IFR operations will support the short flights expected by eVTOL aircraft, particularly approaches to rooftop helipads. There are several issues that will limit eVTOL IFR operations into urban landing sites for the foreseeable future. 

First, there are currently no IFR approaches to the vast majority of existing helipads in urban areas. An operator would have to create their own approach and go through a lengthy approval process. A solution for this would be a completely new all-digital air traffic control (ATC) system such as Unmanned Traffic Management (UTM). I estimate it will take one to two decades to develop a system that integrates passenger-carrying eVTOL aircraft with regular airspace and operating procedures. 

Second, since today’s ATC radars are not designed to provide high traffic resolution, if any at all, for aircraft flying low over cities, it seems reasonable for ATC to block off large swaths of space over cities for EACH IFR approach and departure in order to safely separate aircraft. This means only a limited number of takeoff and landings will be approved at a certain point in time. Such a restriction significantly limits the scalability of operations in marginal and bad weather conditions until a UTM system is in place.

Third, because eVTOL are very sensitive to weight and have very complex configurations, it is unlikely that anti-ice equipment can be easily added at any time in the foreseeable future. Such restrictions limit the flights of these aircraft to fair weather operations. It is extremely rare for even today’s most sophisticated helicopters to have FIKI capability.

Given these limitations, customers cannot depend on reliable eVTOL transportation schedules as the weather can sometimes cooperate and sometimes not. This means customers have to build in extra travel time to account for a Plan B such as driving. 

While no aircraft can fly in all weather conditions, eSTOL aircraft can fly in most weather conditions and make practical use of today’s existing airport and ATC infrastructure that allows for high-volume operations.  


Conclusion

As with today’s aircraft, tomorrow’s electric aircraft will come in many forms for many different missions. Two of the most promising types are eSTOL and eVTOL aircraft. These aircraft will be complementary to each other and serve different mission profiles to bring new benefits to society.

eVTOL aircraft will be used for short flights within urban areas in fair weather conditions.

eSTOL aircraft will be used for short to mid-range regional flights between airports in almost all weather conditions. Lower operating costs will enable the growth of the regional air mobility market and the expanded use of existing infrastructure.