The changes to the A350-900ULR

Discussion
Why no forward cargo bay for the A350-900ULR?

The new Aircraft Characteristics for Airport and Maintenance Planning for A350-900 and -900ULR lists the forward cargo bay for the ULR as deactivated. This is because the -900ULR will rarely carry cargo in its operation.

Cargo revenue is about 20% of the revenue for a normal Economy passenger per kilo hauled. For a ULR aircraft, which carries Business and Premium economy and no Economy passengers, the revenue difference per kilo payload is increased to more than 10 times.

It, therefore, makes little sense to have the equivalent of 36 cargo positions on the ULR version of the A350-900. These won’t be used. A cargo compartment like the ones for the A350 is equipped with walls/floors (called linings) and a loading system for Containers/Pallets. Rather than flying around with empty fully equipped cargo holds, Airbus decided to reduce the number to one and leave the other hold unfurnished. This reduces the empty weight of the aircraft equivalent to a payload of 10 passengers or more.

Which cargo hold shall be left unfurnished? It’s all about how the cabins look and the resulting center of gravity for the aircraft. To fly far, a ULR aircraft must be configured for minimum drag. Part of the drag of an aircraft is the induced drag from the horizontal tailplane. The induced drag from the tailplane is dependent on how large downforce the tailplane needs to exert to keep the aircraft stable.

An airliner needs to have positive stability in pitch (military aircraft are allowed to be unstable in pitch to reduce drag). This means the tailplane needs to generate a force pointing down. This creates induced drag from the tailplane, larger the more forward center of gravity the aircraft has. An increased downforce of the tail will increase the load on the wing, by it increasing its induced drag as well.

With the premium heavy cabins used for ULR aircraft, the empty airplane turns nose heavy. Figure 1 shows the difference in the extent of heavy seats for a normal A350-900 and the A350-900ULR.

Figure 1. The typical A350-900ULR cabin as given by the Airbus planning guide. Source: Airbus.

A business class seat is more than five times heavier than an economy seat. With the difference in how many fits abreast per length unit of the fuselage, we talk about a cabin which is over three times heavier than the cabin at the tail of the aircraft.

The passenger load works the other way. Forward of the Center of Gravity we now have about four passengers per length unit and in the rear seven or eight passengers per length unit, dependent on the seats abreast in the Premium economy section.

At low load factors, the aircraft will, therefore, have a forward center of gravity (the heavier forward seating dominate), at high load factors the passenger load even out the effects of the cabin. This is when we count the passenger as one unit, passenger and bags, weighing the IATA recommended 100kg.

A typical passenger payload is composed of two components, the passenger and his baggage. Let the passenger count as 80kg and his bags as 20kg. The 80% of the load goes into the seats, the 20% we can move around within limits. Now let’s see which cargo hold shall be deactivated and what effect it will have?

Cargo hold to deactivate

As explained before we can gain additional efficiency if we can fly the aircraft with an aft center of gravity. Airbus has for the A330, A340 and A380 a tailplane fuel tank which can be filled to balance the aircraft tail heavy during the cruise. This lowers drag, improves the aircraft’s efficiency and extends the aircraft’s range.

This complication was spared for the A350, as the morphing wing (spoilers and flaps can micro move during a flight to configure the aircraft for lowest drag) to some extent could replace the tailplane tank.

But the morphing wing can’t move the center of gravity around. In the choice of a cargo bay to block, Airbus choose the forward cargo bay. The empty weight reduction of the bare aircraft is ahead of the center of gravity and the baggage for the passengers is placed in the rear cargo compartment. This moves the center of gravity back, Figure 2. This is advantageous for the aircraft’s efficiency and range, especially when flying with low load factors.

Figure 2. A350 cargo compartments with the Center of Gravity (CG) in the middle. Source: Airbus.

The performance impact of an aft center of gravity

Our performance model takes the stability margin and by it the induced drag of the horizontal tail into account. When moving the CG further back, by it reducing the static stability margin to half, the range of the A350-900ULR is increased with 100nm.

Airbus wants to optimize the range and efficiency of the A350-900ULR. The optimal solution was to block the forward cargo bay which would hold no cargo on ULR flights anyway. The rear compartment is large enough for all baggage. For a normal ULR mission, the baggage will need around five LD3 containers and the rear cargo compartment takes 16 containers.

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