January 22, 2021, ©. Leeham News: In last week’s Corner, we looked at how the hydrogen consumed in the rear fuselage tanks changes the airliner’s Center of Gravity (CG).
Now we discuss how this change of Center of Gravity, limiting the aircraft’s load flexibility, can be mitigated with different concepts.
The Airbus ZEROe turbofan airliner concept in Figure 1 has two tanks placed in the rear, Figure 2. The rear placement is to not divide the cabin into two compartments with difficulty getting access to the front part of the aircraft, which would be the consequence of a full size front and rear tank setup.
But the drawback of the rear placement is CG shifts with up to 17% of the mean aerodynamic chord, Figure 3.
This is limiting the load flexibility of the aircraft. What can we do to limit this shift of CG?
Several Airbus aircraft types (A330, A340, A380) use tailplane fuel trim tanks to achieve an optimal CG change during a flight.
Can this be used for an airliner like the ZEROe? Yes, it can.
The Liquid Hydrogen (LH2) tanks of an airliner are less flexible in their form as they, for cryogenic isolation reasons, are cylindrical or tubular in design.
The ideal placement of LH2 trim tanks would be as far forward as possible. Liquid hydrogen then flows to the rear tanks as LH2 gets consumed in these.
In the last Corner, there were several ideas of how we could shift masses around in the aircraft to compensate for the CG shift. One idea was to put the freshwater tank upfront and the wastewater tank in the rear.
When passengers visited the lavatory, gradually the CG of the water system shifted back. It’s a good idea, but the mass of water on a domestic airliner is around 1kg per passenger, so we would have ~150kg to shift around.
The most efficient idea is if we can distribute the fuel of the aircraft without ending up blocking the passageways in the cabin. A design that is similar to the water one is shown in Figure 4.
Here the total fuel load of 4.1t is divided between two “cheek” tanks in the front, below the cockpit floor, and rear tanks. The distance to the front tanks is twice the average distance of the rear tanks, so if we could store 1/3 of our fuel upfront, we could get a balanced system.
Today, the space under the cockpit floor is used as an avionics compartment and for the nose landing gear bay. The nose landing gear has moved forward to under the weather radar for the A380 and A350 as this is structurally more efficient, and the avionics could move to the crown of the cockpit or into the compartments on the sides of the front cargo bay. This allows us to use the space below the cockpit floor for the tanks.
Modern airliners (Boeing 787, MC-21, C919) have more blunt nose profiles as this is more structurally efficient without compromising the aerodynamic efficiency. Modern CFD allows the designers to shape the nose so no supersonic pockets are created around a plump nose profile. The blunter nose gives more volume for the tanks and the repartitioned avionics compartments.
Such a split fuel system is a bit heavier than the original design, but it would fix the CG displacement problem and the efficiency loss due to a nose-heavy aircraft at the end of the flight.
The problem of how to store the LH2 in a hydrogen airliner is an area crying for innovation. We have discussed a couple of concepts; numerous others await discovery and presentation.