Picking the right tools for the job


Aircraft design is ruled by the propulsion system on the airframe — you can tell a helicopter, piston or turbine aircraft without seeing it. 

So there is little gain to be made from combining technologies that do not match with each other, even if they are great technologies. (You wouldn’t combine a Cessna 172 with a CFM 56 turbofan, though they are both excellent at what they do.) That said, combining technologies that do make sense like electronic power distribution and hybrid-propulsion system running on e-fuels can bring operating costs down by 40% or more depending on routes flown. 

Batteries, electric motors and electrification are all tools and when used for what they’re good at, the results can be transformational. “When we use these tools incorrectly though, we wind up with billions of dollars of investment that are the equivalent of someone using a hammer on a bolt,” Eric Bartsch, CEO at VerdeGo Aero tells Revolution.Aero. Electrification is the third big evolution in aircraft technology after pistons and jets and it is going to change the way aircraft are designed, according to Bartsch.

‘Common misconceptions’

Electric aircraft should look like electric aircraft, says Bartsch. It is a common misconception that electric aircraft will look the same as today’s but be driven by motors and an electric power source. Heart Aerospace, the Swedish firm building a 30-seat regional aircraft, realised this and has switched focus from pure battery to hybrid-electric propulsion. Upon unveiling Heart Aerospace’s ES-30, founder and CEO, Anders Forslund said the firm is building an airplane that the industry can actually use. It will have an electric range of 200km and a hybrid range of 400km with 30 passengers. With the ES-30 we can start cutting emissions from air travel well before the end of this decade,” said Forslund. Heart is targeting commercial entry in 2028.

Another misconception tied into that is the assumption electric aircraft are all about the batteries. Whilst the energy density question dominates discussions around electrification, Bartsch argues it is the arrays of motors which make an electric aircraft different, the batteries are invisible, housed within the airframe. 

“Piston and jet engines have huge economies of scale. You want the powerplants to be as big as possible, whilst still having whatever level of redundancy you need. With electric motors, they scale differently. If you need 1,000 horsepower on your aircraft, you could have one 1,000  horsepower electric motor or you could have 20 motors with 50 horsepower and there is not as much of a weight difference, They’re also simple, responsive and reliable,” says Bartsch. “If you tried to put 20 piston engines on an aircraft that would be crazy.”

“Electrification is interesting, not for the batteries, but because electric motors do things that piston and jet engines don’t do,” he adds. That’s why small electric consumer drones work, if electric motors were not high torque, responsive and simple, hobbyists would still fly R/C helicopters. Now they largely fly drones and the design has changed to take advantage of the new technology. 

So whilst batteries lag behind required outputs, by as much as 20X according to Bartsch, VerdeGo Aero has been building, and now delivering, hybrid-electric propulsion systems. These systems rely on liquid fuels and can operate using sustainable aviation fuel (SAF). When comparing batteries and liquid fuel as primary sources of power on an aircraft there is little competition, says Bartsch. “Liquid fuels get lighter as they are used, can trade energy for payload,  infrastructure exists, up to 20X lighter, certified, cheaper at present and, lastly, the last drop is a good as the first.”

That last point is key. Performance manuals and procedures for flying aircraft are all written around the assumption that as long as there is energy in the aircraft, it is all equally good until it runs out. “But the energy later in a flight is not as good as the energy at the beginning and that is a huge problem if the battery is a primary source of energy. Imagine having to go around right before you land at the end of a flight, and not having full power available,” says Bartsch.

Middle and last mile logistics

“In a nutshell, hybrid-electric enables what our customers want,” Dave Merrill, founder, Elroy Air tells us. The San Francisco-based large drone manufacturer is building the Chaparral, an autonomous VTOL cargo aircraft with hybrid-electric powertrain that can travel up to 300 miles (482km) carrying cargo weighing up to 300lbs (136kg). “Electric propulsion adds safety through redundancy, and it’s the right choice for autonomy. Like many in this space, we began our journey looking at battery-electric given the simplicity and elegance a purely battery-electric vehicle provides. But the energy density just isn’t there yet on batteries that are available and have high enough TRL [technology readiness level].”

There is a long list of use cases for advanced aircraft, so different propulsion systems can make sense for different missions. For Elroy’s middle and final mile logistics, hybrid was the only viable option, according to Merrill. “Customers in humanitarian, commercial and defence all told us: if you build a VTOL delivery aircraft that can go few hundred miles and can carry a few hundred pounds, that fits a big unmet need in our networks and in today’s logistics industry.”

Questions over range and payload aside, Merrill says the assumption that charging infrastructure will be available in all the locations where it would be needed is not valid. “Especially in the early growth chapter of this market, the diversity of route endpoints will far outstrip the availability of the battery charging infrastructure. Military, humanitarian, and early commercial logistics will operate in austere locations and we can’t assume that charging will be available”

“We’re designing aircraft for how the world looks today and for the next decade-plus,” adds Merrill. “..and a series-hybrid powertrain architecture like we’ve developed for Chaparral is future-proofed, ready for the upgrade to full battery-electric as cells improve” Designing for current infrastructure reality is key, using the “best of both worlds” by combining distributed electric-propulsion (enabling safety, redundancy) with the mission range and operational flexibility accessible through fuel-based combustion.

Regional use cases

The founders of Ampaire, a hybrid-electric powertrain systems developer based in California, realised hybrid was the only viable pathway for their vision when flying short-hop flights in regions from Norway to South America. “Regional airlines in these locations do things like turn around in — I timed it once — three minutes from when I got off the Caravan to when new passengers were loaded and it was already taking off again,” Cory Combs, co-founder tells Revolution.Aero. “There is not any charger in the world that can recharge that aircraft in three minutes.” Also, in some instances there are no ground support staff on hand. “I’ve been on a nine-seater seaplane where the pilot had to cut the engine, jump up on the dock, grab the plane with a hook, load all bags off and on again and takeoff, That was the entire infrastructure.”

Hybrid-electric propulsion systems, like those Ampaire is developing, are also the most easily scalable to larger aircraft where carbon emissions and fuel burn problems have the most impact, says Combs. (Commercial air travel is also the biggest market too.) The firm is building its initial propulsion systems for small fixed-wing aircraft up to 19 seats like the Cessna Caravan. But the same technology can scale to traditional rotorcraft, eVTOL systems, and even includes, in the near term, the ability to scale up to larger aircraft for onboard power and as power distribution systems. “So even if the aircraft are not being propelled electrically you are going to start seeing some of the ground infrastructure go electric and onboard systems be used as auxiliary power on larger planes,” explains Combs.

Boosting profit margins

More cost-effective operations are one of the biggest selling points hybrid-electric proponents have in their arsenal. But quantifying how much operators can save when the majority of technologies are in the test phase and there is very little operational data to go off can be a challenge. One metric that can offer insight is fuel burn, as it is one of the largest economic impacts on any mission. On average, using Ampaire’s AMP Drive propulsion system, fuel burn is reduced by as much as 50%, according to co-founder Kevin Noertker. “Before I came out with these numbers I had our team triple check them,” he says. “Cruise performance is great and takeoff and climb performance is even better.” With the electric system providing extra boost during takeoff and climb, Ampaire is seeing burn reductions upwards of 70% during that phase.

“It is game changing from an environmental point of view,” says Noertker. “But also the economics of that too. Even when you’re doing inflight recharge which does take additional fuel during cruise relative to on the ground, you’re still looking at an average fuel savings of over 40%. It is those economics, when an airline is normally spending 30-40% of their operating costs on fuel, that is transformative as far as the economic value proposition.” And it is that proposition which is key to getting customers onboard. Whilst the pressure of climate change is enough for some, cash remains king for many.

There are certain tools for certain jobs

VerdeGo’s Bartsch says he is not religious about hybrid-electric propulsion and there are use cases where it does not make sense. One example is on long haul airliners where the design is already very efficient, and using SAF in the existing engines may make the most sense for reducing net emissions. “If the aircraft is configured to the same existing airframes, then it is probably optimal for its current propulsion, and hybridisation wouldn’t enable significant new capabilities or efficiencies,” said Bartsch. The mission also impacts the best fit for the propulsion system. Many next generation electric VTOL aircraft use electrification to enable flight in both vertical and horizontal modes in a manner such that hovering flight is far less efficient than a helicopter, but cruising flight is far more efficient. Bartsch says that on these missions the efficiencies gained whilst in cruise mode would outweigh the efficiency lost during lift and hover. “Cruising on a wing rather than a rotor is a really good use case for eVTOLs going point-to-point and this plays to the strengths of hybrids where the engine can be optimised for the cruising phase of flight with batteries augmenting power for the short hover at the beginning and end. However a VTOL search and rescue mission is very different, requiring extended durations in hovering flight, and that plays to the strengths of a conventional helicopter. For search and rescue, using SAF in the existing powertrain of a helicopter is likely to be the best path forward,” he explains. 

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