UK clean aviation research body FlyZero has detailed the three concept aircraft it has developed over the course of its year-long work programme.
Addressing the regional, narrowbody and midsize segments the trio use hydrogen as a fuel, but employ different propulsion architectures depending on the mission.
And while the notional performance and shape of the concept aircraft is broadly similar those of their current in-service equivalents, in each case, the storage requirements of hydrogen have driven a notable increase to the fuselage diameter.
Aimed at the regional market, FlyZero’s FZR-1E 75-seat concept features six electric propulsors – three on each wing – powered by fuel cells, which are located under the rear cabin floor in an unpressurised zone.
Compared to the ATR 72-600, FlyZero’s reference aircraft for the segment, the FZR-1E is slightly longer and has a larger wingspan, boasting respective figures of 28.8m and 31m, against 27.2m and 27.1m for the regional turboprop.
However, the fuselage diameter grows markedly, to 3.5m from 2.8m, driven by hydrogen storage considerations.
FlyZero adds: “Even with the increased fuselage diameter, extension of the landing gear fairings was required to accommodate all the system elements.” Heat exchangers needed for the electrical and fuel cell thermal management systems are located in the nacelles.
In addition, a water storage system is required to prevent water in the exhaust from being ejected onto the runway during taxi or take-off, with a potential impact on runway friction. That stored water can either be exhausted later, says FlyZero, or used for other aircraft systems.
Cruise speed is pegged at 325kt (601km/h) and range at 800nm (1,480km), above the ATR 72-600’s respective figures of 266kt and 448nm, in order to bridge the regional jet and turboprop markets, it says, while noting that “the cruise speed is within the capability of existing turboprops such as the Q400.”
FlyZero’s FZN-1E narrowbody is an even more striking departure from the project’s reference aircraft, in this case the Airbus A320neo.
Instead of the classic underwing engine configuration, FlyZero has opted instead for rear-mounted hydrogen-burning turbofans. These are located near to the two cryogenic fuel tanks and the rest of the fuel system in the rear of the fuselage, minimising the length of the hydrogen fuel lines.
Those design choices in turn require a T-tail and nose-mounted canards to address centre-of-gravity challenges.
However, FlyZero notes that an alternative configuration with underwing engines was explored, which offered similar performance levels.
The fuselage is also over 10m longer and up to 1m wider than that of the A320neo and has a variable cross section, becoming wider towards the rear.
In addition, the proposed wingspan of 39.3m takes the aircraft beyond current ICAO narrowbody limits, requiring the addition of folding wing-tips.
“Folding wing-tips have been demonstrated on the Boeing 777X, but further work is required to develop a lightweight, low-cost, and certifiable solution for a narrowbody aircraft,” says the report. FlyZero proposes a dry wing, allowing optimisation of the structure.
Range is given at 2,400nm with a 450kt cruise speed – in line with the performance of the A320neo.
For the mid-size segment, FlyZero based its FZM-1G concept against the Boeing 767-200ER, resulting in an aircraft that can fly 279 passengers on a trips of up to 5,750nm which would “enable flights to all major global destinations with… one stop.”
FlyZero points out that when designing an aircraft for longer-range operations “the location of the hydrogen storage is a key driver of aircraft architecture.” But those considerations are complicated by the need to ensure the fuel tanks are away from areas that could be hit by debris from a variety of sources – including the engines and tyres – and “also account for impact from tail strikes, bird strikes, belly landings and general crashworthiness”, plus trim and stability concerns.
These requirements have resulted in the adoption of underwing engines and “‘delta’ tanks” have been added in an unpressurised zone in front of the wing to ensure weight and balance stay within reasonable limits.”
In order to maximise the weight and volume efficiency of the hydrogen storage a wide fuselage has been selected. However, with a diameter of 6m, FyZero notes that this is closer to that of the A350 or 777X than the reference aircraft.
“To ensure that all hydrogen systems are located in unpressurised zones, the trailing edge of the wing root was also extended to provide suitable space for routing the fuel pipes from the rear hydrogen tank to the forward delta tanks and engines outside of the pressure vessel,” it says.
“This requires long liquid hydrogen fuel lines which introduce design challenges and attendant potential risks. Further work is required in this area.”
A dry wing is again preferred, and although the 52m wingspan is around 5m wider than the 767’s, the study says there is no need to exceed current airport gate limits, removing the complication of folding wing-tips.
In common with the narrowbody, FlyZero proposes a relatively narrow fan diameter as the fuel consumption penalty matters less than on a kerosene-fuelled aircraft. “With hydrogen the reduction in fuel mass is relatively small, so a smaller, lighter engine is the better option overall.”
Similarly, the lower mass of the hydrogen means that both the narrowbody and midsize aircraft can carry enough fuel for a typical return flight, without incurring a significant increase in fuel consumption.
This would remove the need to refuel at the destination airport, says FlyZero, potentially allowing more routes to be opened up closer to service entry as it would ease infrastructure constraints.
FlyZero says its concepts have been designed to be as safe as existing aircraft and in line with current regulations, but notes that “there are areas where the fundamental behaviour of cryogenic hydrogen is not well understood and it is possible the FlyZero designs incorporate features which may not be acceptable to future safety standards when they are defined”.
“Substantial further work is needed in this area and global collaboration on safety standards will be required.”
Additionally, FlyZero calls for research into six priority areas in the short term: liquid hydrogen behaviour and materials compatibility data; cryogenic fluid pumps; liquid hydrogen storage; hydrogen combustion systems; fuel cells and associated thermal management; aircraft integration of cryogenic hydrogen fuel systems.
These should ensure that the “significant technology challenges” to certification and service entry are “characterised and solved”.
FlyZero is run by the UK government-funded Aerospace Technology Institute, with staff seconded from industry. No second-phase work is currently proposed, although project leader Chris Gear has previously expressed hope that its term might be extended.