By Bjorn Fehrm
July 22, 2015 © Leeham Co. We will now finish our series around Singapore Airlines (SQ) need for an Ultra Long Haul (ULH) airliner by looking at what would be the technology and performance of the A350R that Airbus talked about as a possible future model at the launch of A350XWB in 2007.
The A350R as presented was quite different to the A350-900LR that we presented in the first analysis articles. Whereas the A350-900LR is essentially a new Weight Variant (VW) the A350R was an aircraft combining the wing, engines and main landing gear from A350-1000 with the fuselage of A350-900 to create an Ultra Long Haul aircraft (and a freighter variant).
Such an A350 variant could be an interesting aircraft for Singapore or other airlines with a need for a ULH aircraft. We will use our proprietary aircraft model to create the A350R and check its performance against the A350-900LR and Boeing’s 777-8X. This will give an understanding if it could be worth the development effort for Airbus.
At the launch of the A350XWB Airbus presented three main A350 models and two possible future variants. The main models were A350-800, -900 and -1000 and the future variants A350F freighter and A350R, a ULH aircraft. Both models would combine the higher Max Take-Off Weight central sections and wing+engines+main landing gear of A350-1000 with the front and rear fuselages of the -900.
The resultant lower weight and drag from the shorter fuselage with the lift and engine capability of A350-1000 would generate an aircraft with higher payload capability (Freighter) or longer range than the base variants (ULH variant). For A350R the talk was of a range of around 9,500nm, the same range bracket as the 777-8X that we analyzed last week. We will now use the model to create the A350R and pit it against 777-8X and see if it would be an interesting aircraft for Airbus to develop.
To create an A350R a high commonality of parts would be necessary with A350 base variants. The ULH market is small. The sales of Airbus’ A340-500 and Boeing’s 777-200LR have been dismal compared with the main variants. This means no large investments in new development of an ULH could be justified. Essentially an A350R would have to combine the center parts of A350-1000 with the front and rear fuselage sections and empennage of A350-900 with minimal changes to control cost.
Using the A350-1000 center section would make all strength-critical parts be designed for a Max Take-Off Weight of 308 tonnes, the high weight margin enabling the uploading of the fuel necessary for ULH missions. Combining this center section with the front and rear fuselages of A350-900 would create a shorter aircraft with less drag, both due to skin friction drag and drag due to weight as the shorter fuselage would reduce wetted area and empty weight.
Using the whole A350-1000 center section, including the center fuselage section, would be important as the landing gear for A350-1000 has three wheel bogies and requires a main landing gear well which is one fuselage frame longer than the for the two wheel bogie of A350-900.
Combining the -1000 center section with the -900 front and rear fuselage should be pretty straight forward. The extra 11 frames length for A350-1000 was in the front fuselage with seven frames and in the rear fuselage with four frames.
One area that would need extra attention would be the aircraft empennage and the “tail volume” it creates. “Tail volume” stands for the force moments that the empennage surfaces can generate to control the pitch and yaw moments of the aircraft. The critical moments are generated at low speed by take-off and landing, especially when One Engine is In-operative (OEI).
The OEI case is especially taxing on the corrective moment that the vertical tail can generate to counter-act the one sided thrust of a going engine versus the in-operative engine’s drag on the other side. This one sided thrust+drag generates a yaw moment which has to be checked by the Vertical Tail-Planes (VTP) side force. The VTP creates an counter yaw moment which has the size of the tails force multiplied by the length of the moment arm of the aft body.
This moment is called the tail volume of the VTP. Its size is determined by the thrust of the going engine-, the drag of the inoperative one, how far from the centerline these engines sits and also the aircrafts wing span. The destabilizing influence of a long fore-body is also included in the calculations.
Our model says we would need a VTP area of 80m2 to manage the yaw moments; the A350-900 has an area of 60m2. The classical tail volume rules are for aircraft which does not have Fly-By-Wire (FBW) artificial stability augmentation and the 60m2 area could be sufficient for an A350R as all Airbus aircraft has FBW. The VTP of the A350 was seized by the short fuselage A350-800 so it might well be that the VTP would be sufficient for a longer A350R even though it has stronger engines. This would at least be one of the areas that would have to be studied for an A350R.
Another area of interest would be the weight of an aircraft combining parts from A350-900 and -1000. Our model gives an empty weight of 146 tonnes which shall be compared to 139t for A350-900 and 155t for A350-1000.
Performance of A350R
An A350R as sketched would consume 105 tonnes of fuel on our typical missions of 8,500nm. The fuel consumption per seat mile would be 345 kg per seat and statue mile. It shall be observed that the per seat values from Part 2 was also per seat statue mile (all per seat mile values in the industry are with statute mile versus nautical mile).
The A350-900LR consumed 335kg per seat and statute mile; the 777-8X is 350kg. The A350-1000 is at 355kg but this is because it is fuel limited at that range. If something be done to its tank volume this figure would improve. The A350R would have the same tank volume problems, its fuel limit would be at 8,700nm without changes. The tank volume would have to be opened up to at least 170,000l for it to be effective.
The results are not surprising. An A350R is in the size class of the A350-900LR and should have a fuel consumption which would be better than a 777-8X since it is considerably lighter. It could easily lift the same payloads as -8X as it has the strengths of the center section of A350-1000 to counter wing bending moments. It would not be of much use however; the volumes of the -900 fuselage would not allow it to lift more than its passengers+bags and around 15 tonnes of cargo in the remaining 26 LD3 positions, in all around 45 tonnes out of a capability of 65 tonnes.
On our 8,500nm trip it could upload the same 10 tonnes of additional cargo that the 777-8X could. The difference would be in the passenger capacity, 313 passengers using our normalized two class seating instead of 777-8X’s 350 passengers.
An A350R would have the performance of Boeing’s 777-8X in terms of range and payload capability and it would beat it on fuel efficiency per aircraft and seat mile. It is a smaller aircraft and would pose less risk on thinner routes but it would leave around 35 passengers behind should the route have sufficient market potential.
Airbus is now entering a phase where it will avoid large new aircraft projects, rather creating derivatives of aircraft and aircraft parts that it already have developed. An A350R would fit in this category. It would require very little new development for its parts but Airbus could not avoid a rather substantial certification effort for what is essentially an A350-1000 and -900 hybrid.
The big question for such a variant would not be the technology or performance; it would be if there is a market big enough for such a variant to pay back the investments costs, especially as a 777-8X would beat it to that market.