Mario Asselin, author of Operational Aircraft Performance and Flight Test Practices and An Introduction to Aircraft Performance, discusses the emerging market of electric aircraft, why the current airport model is set up for “old technology”, and how this affects electric aircraft missions and airport procedures.
MISSION PROFILES FOR ELECTRICALLY POWERED AIRPLANES
People always try to fit electric airplanes into the ‘known’ profile for turboprop or jet airplanes ie. fly at high altitude as much as possible to minimize fuel burn… One must remember that this flight profile was developed ‘because’ of the economics of the gas turbine and that low altitude flights would make the economics even worse.
The question this author asked to a team of Fellows once, “why are we trying to design these new planes using the ‘old’ mission profiles?” The questions become 1) what are we designing? 2) why are we designing?
If the answer for the first question is, for example, a “regional commuter airplane”, then the answer for the second question is typically “to make money”. With this in mind, you need to define a market the airplane will need to address so that the design can be optimized for that market.
The Basic Mission
A typical mission could be:
- Engine starts and systems checkout
- Taxi out
- Takeoff and initial climb
- Climb to cruise altitude
- Approach and land
- Taxi in
- Engine shut down
Of course, people like to do ‘marketing missions’ when comparing airplanes and this typically entails 10 minutes taxi phases at the start and at the end of the flight. This may be fine if one considers that the airplane starts taxiing at the exact time the engines start, we can even find statistics on line to support these numbers as exemplified below, Figure 1.
Figure 1: Taxi times (https://www.planestats.com/aptot_2016mar)
The reality is that airplanes start engines more than 10 minutes before taxi. Some newer planes with more complex systems or more ‘modern engines’ may need as much as 30 minutes prior to the flight (system setup, engine warm up, etc) and 10 to 15 minutes minimum at the end of the flight with engines (and/or APU) idling. Gas turbines are very inefficient then with a specific fuel consumption easily 3 to 5 times higher at idle than at maximum power (see Figure 8.36 of reference 1). During this initial system checks, electric airplanes do not have to power up the electric motors and thus use very little to no energy.
Now to define the proposed mission, one may look at existing fleets as a starting point and see what could exist. This does not represent the full potential market as an airplane with new capabilities will typically develop new markets. This is what happened with the Boeing 787 and Bombardier CSeries – now Airbus A220, where the introduction of these very efficient airplanes opened new, profitable, direct routes for airlines that were not possible before.
When trying to define the basic mission model, one can use available tools and databases that can be found online. Some of those tools include FlightAware and FlightRadar24; they track flights of airplanes and are a useful source of information. A FlightAware extract from 11 July 2021 in the morning for the Beech 1900 fleet flying over North America showed the following flights (see Figure 2 below). We note how ‘short’ these missions are, with an average of about 40 – 45 minutes of flight time (taxi and engine start/runups not included).
Figure 2: Extract from FlightAware, Beech 1900 fleet over North America, 11 July 2021
Observations from some of those flights, Figure 3 below for an example, clearly show the non-optimal flight profile used (airplane descends early, burns more fuel than minimum for the mission), often driven by ATC requirements. In addition, one does not necessarily see how much time is spent on ground with engines at idle.
Figure 3: Extract from FlightAware for a flight of a Beech 1900
It is interesting to note that the Beech 1900D fleet was extensively evaluated by the US DOT and the FAA; several reports on commuter airliner ops were done. DOT/FAA/AR-09/15 offers a glimpse in the use of the Beech 1900D fleet. Although this commuter airplane is advertised has having up to 1300 nm range capability (ferry range), the reality is that typical missions only cover 100 to 150 nm (flight times between 30 and 45 minutes, Figure 4).
Figure 4: An extract of DOT/FAA/AR-09/15 (Figure 7 of that report)
From the above chart, one can extract that an airplane designed to perform a mission of about one hour in duration could cover about 89% of the flights shown above (for the same mission speeds of course). That mission is about the equivalent of a 175nm range (about 325 km). A UK study (Distributed Aviation, A new economic model for electric aviation) indicated that currently (2019 study), there were about 116 city pairs that were served by turboprops for an airplane with a design range of 400 km, but there were at least 793 possible city pairs possible with the right airplane economics, Figure 5. So, a properly optimized airplane with good economics could actually serve a significant market.
Figure 5: 2019 UK study, city pairs
The Flight Profile
If you have flown before, you expect the airplane to takeoff and climb for a while, typically 20 to 25 minutes, to achieve a high altitude for cruise, staying a little lower for shorter distances. Then, near the end of the flight, there will be another 30 minutes or so to descend to approach height. This profile is not just because pilots like to fly at higher altitude, there is an economic impact that drives this as discussed in Chapter 10 of reference 1.
Turboprops, like jets, follow the same general profile and will typically climb between 10000 and 20000 ft for cruise. The theory is that the extra full burn used in the climb at high power will be more than compensated by the lower fuel burn at cruise at the higher altitude. The descent is typically done with the engines either at idle or with just enough power to maintain a certain rate of descent (say 2000 fpm). There is still fuel used during the descent, but it is ‘relatively’ small.
For conventional takeoff and landing (CTOL) electric airplanes, we note the following differences on the approach to resolve a mission.
- The taxi phase of a mission is typically modelled as 10 minutes with engines at idle; the engines providing the required power for the systems. When we turn to the electric airplane, and we ask: What is idle? Should that not be modeled as zero energy use? We take one step further and compute the power required to taxi at 20 knots ground speed as an estimate of the power consumed, but then this same approach should also be used by the turboprop airplane.
- For takeoff, the power available may not be impacted as much by temperature but for our Sea level standard condition we will assume similar performance.
- For the climb to altitude, the electric airplane may assume constant power available vs altitude but the turboprop would most likely have reducing power with altitude, extending the time to climb.
- For an electric airplane, altitude does not play as much of a role in their overall propulsive efficiency. Electric motors have a wide range of torque and RPM where they remain very efficient and this offers the operator more flexibility in selecting a cruise altitude than for turboprop where the higher cruise altitudes are required to extract better fuel economy. So, for electric airplanes, flying above or below the weather may not impact economics as much, in fact, flying higher for a pressurized airplane may actually result in a much larger energy use (due to the pressurization system). We note, as we did in reference 1, that the much more efficient electric airplanes highlight the energy ‘leakage’ of the other systems onboard more than it did for the turboprops.
- The descent is usually modeled either as an idle descent if the rate of descent (ROD) is reasonable or it is modeled with the power required for no more than about 2000 fpm. An electric airplane with a design small ROD (high aspect ratio wing for example) can execute the descent with zero energy use and with good lift-to-drag (L/D), it can cover a good part of the descent distance with zero energy use.
- For the approach to land and landing, both airplanes can be analyzed in a similar way, they both need enough power to maintain a -3 degree flight path on a 5-mile approach.
- For the taxi in, the same approach used for point number 1 above is used.
Side by Side
To better visualize the difference, we create a model electric airplane that we compare with a turboprop capable of carrying the same payload. The airplanes will have the following characteristics, and all equations are extracted from Ref. 1&2. To ‘simplify’ the discussion, we assigned them similar aerodynamic efficiency (we note that each OEM tailor the airplane for a given mission and that two airplanes are rarely alike) and we focus on the differences in energy use due to the powerplant used. We assigned them the same wing loading (W/S) and same power loading (P/W). We expect the empty weight of the electric airplane to be higher (due to the weight of the batteries and structural weight to carry them) than the turboprop airplane, but we also note that the turboprop has to carry fuel (weight, diminishing during the flight). We pick an airplane capable of carrying 19 passengers.
Table 1: Model airplanes
As both airplanes have the same wing loading and same aerodynamic coefficient, they both have a similar minimum drag airspeed of about 130 KTAS at sea level standard day. We note that the electric airplane is heavier than the turboprop (operating empty weight of 15000 lb instead of the 9500 lb), but the electric airplane does not add fuel weight for the mission (essentially flying at equivalent full fuel all the time. Some may note that the electric airplane has a MTOW greater than the 19000 lb of Part 23 requirements, it is something that new electric programs will have to handle. We keep the weight for the purpose of this analysis.
We break down the mission into simplified segments to see the general differences expected from each airplane. We offer the table 2 below to support the discussion for a 200 km (108 nm) mission with 45 minutes reserves. We have the turboprop fly at 10,000 ft while the electric airplane flies at 5,000 ft.
Table 2: Sample mission
We note that we converted the fuel used into equivalent energy (we should not confuse energy used for the flight with energy required for the flight, the first one is driven by engine efficiency and airplane aerodynamic efficiency while the second is only due to aerodynamic efficiency). The turboprop airplane carried 1000 lb of fuel for the trip (371.2 lb were required for the flight and we estimated another 380 lb required for reserves, we rounded up to 1000 lb). The weight of the electric airplane is unaffected by the reserve energy (once the size of the batteries is set). We observe that even with the heavier airplane weight, the electric airplane still consumed much less energy than the turboprop airplane to perform the mission.
We note that flying at the lower cruise altitude increased the overall mission time some (about 6 minutes). This is also in part due to the choice of climb and descent speeds between the two planes. We note that the electric airplane used no energy for the descent but that the turboprop airplane used over 195 kWh.
Also note the large difference in taxi energy used. The turboprop airplane used about 2.5 US gal of Jet A fuel for the engine start and taxi (modelled as 10 minutes at idle). Had the ground operation been much longer (here set to zero beyond the 10 minutes of taxi), this quantity of energy would have been much greater.
So, with all the small difference in mission profiles between the two types of airplanes, it can be very ‘hard’ to do a quick side by side comparison using a traditional turboprop mission only, one could come up with the wrong conclusions. We have seen some authors simply use the energy used by a turboprop (fuel used converted to energy) to then compute a battery weight for the electric airplane to guess what the airplane weight could be.
We need to address each plane as they are with their respective best operating mode.
The Ups and Downs of Flying High
We want to add a note on the flying ‘high’ scenario (for a turboprop, 10000 to 20000 feet). This usually places the airplane into the worst location for icing risk (combination of true airspeed, temperatures, and liquid water content). We ran a few cases using a typical Part 23 airfoil (NACA 23015) with the airplane crossing a maximum continuous icing conditions defined by Part 25 Appendix C (17.4 nm horizontal extent, Figure 6) at a cruise speed of 150 KCAS (best cruise condition for a given turboprop) and -10°C OAT, one airplane flying at 5000 ft (KTAS = 157) and one at 20000 ft (KTAS = 210). Per appendix C, one should expect a Liquid Water Content (LWC) of about 0.42 gr/m3 for max continuous icing for a mean effective drop diameter of 20 microns at -10°C (some may argue that the temperature is colder at higher altitude, this author says it just depends on the time of the year. In the summer, lower altitudes are above freezing while flying at 20000 ft can result in icing conditions, as shown on Figure 7).
Figure 6: Maximum Continuous Icing Environment, FAA Part 25 Appendix C
Figure 7: Forecast icing condition on 12 July 2021, Source: NOAA
A quick run of our internal icing code with a typical commuter airline airfoil (NACA 23015) under cruise conditions (same weight, same KCAS, thus same angle of attack) shows that one airplane covers the 17.4 nm icing conditions in 399 seconds (low altitude, lower KTAS) while the other covers the same distance in 298 seconds (high altitude, higher KTAS). The code also shows that although one airplane covers the distance in a shorter time, for the conditions shown, it collected more ice (faster speed, greater ice collection efficiency) and further back on the top surface (more adverse for follow-on stall condition) then the other, Figure 8.
Figure 8: Icing run for 150 KCAS cruise condition at two different altitudes
The Flight Energy Reserves
Flight reserves (energy reserves) for electric airplanes to support the design mission is also an interesting subject which we will discuss in a future blog. It has enough material to be a standalone discussion.
When comparing two airplanes, care should be taken not to try to fit one airplane into the design mission of another one. Each airplane must be analyzed individually to extract the maximum performance for each. There is currently clear evidence that electric airplanes show great promises on short range missions. And these airplanes do have a capability that is clearly of benefit to the airline industry as seen in the recent announcement from United Airlines on the purchase of Heart Aerospace ES-19. This indicates the industry is ready for such an airplane.
- M. Asselin, Operational Aircraft Performance and Flight Test Practices, AIAA Education Series, 2021.
- M. Asselin, An Introduction to Aircraft Performance, AIAA Education Series, 1997.