Redefining the 757 replacement: Requirement for the 225/5000 Sector, Part 6.

Discussion

As a continuation from Part 5 in our series around the 225 passenger and 5000nm aircraft segment we now introduce what Airbus alternative could be to a new development from Boeing. To make the comparison as valid as possible we have also tuned the principal models we developed in our previous articles. We thereby use the latest information from Boeing around their range and passenger capacity.

Boeing’s focus is to avoid making “too much airplane” in this segment. This would mean larger dimensions and more weight which would open up a gap to single aisle alternatives. Part of that process is to find the right payload capability for passengers and cargo and transport these over the necessary range but not longer. Also to use the most efficient build technique for the resulting aircraft to limit weight and drag increase over a similar capability single aisle aircraft.

Optimized MOM models

We have therefore adjusted studied aircraft’s design range to 4,750nm (Boeing has said customers indicate they want around 20% more than today’s 757) and introduced Boeing’s patented elliptical fuselage type as one alternative in the comparison, Figure 1. Boeing presented this fuselage type in three patent applications between 2001 and 2010 aimed specifically at this market segment. It minimizes the wetted area and weight for a dual aisle aircraft using the smaller container type LD3-45 by shortening its dimensions in height. Fuselage circumferential area is saved and the resulting smaller skin area creates less drag and less weight.

The reason why this fuselage shape has not been used to date is that a non-round fuselage creates cyclical bending in the fuselage structure when the cabin gets pressurized (a round fuselage creates no bending, just tension forces). With fatigue-insensitive Carbon Fiber Reinforced Plastic, CFRP, as fuselage material this can be mastered, especially as its directional material characteristics can be used to put the carbon fibers in the optimal direction to handle the cyclical bending and tension loads.

Higher local loads from an elliptical fuselage can also be covered for free in the thicker material dimensions that is needed for fuselages due to dimensioning for damage tolerance from hail, bird strike or ramp rash. These forces together with minimal thickness criteria (due to manufacturing needs) forms a thicker CFRP fuselage skin that can accommodate the extra loads in many areas of the fuselage “for free.”

Airbus response

Airbus took Boeing by surprise with its A321LR. This means Airbus is already present in the segment with the 97 tonne variant of A321neo. It will be hard to stretch this variant further as the wing is loaded to its limits by the 97 tonnes takeoff weight. In our comparison table, Figure 2, we have exchanged the Airbus wing reference area of 123m2 with the actual A321neo wing area of 129m2. This lowers the wingloading number from 790 to 750kg/m2 but in reality nothing has changed; we are just showing the actual wingloading, not the reference area-based one as we did before. Wingloading is still high which results in acceptable but not good take-off performance and low initial cruise Flight levels.

To increase the passenger capacity to 200 normalized passengers, which is the equivalent of 225 passengers if using short to midrange configuration rules, one need to add 10 frames to the fuselage stretching it to almost 50 meter. An aircraft with such a fuselage and a range of 4,750nm would be pushing toward 110 tonnes max takeoff weight, 13 tonnes higher than today’s A321LR. To carry this extra mass a larger wing is needed. Takeoff speed is decided by wing area and to keep things competitive around 40 m2 additional wing area would be needed.

It is not possible to extend the present wing to such dimensions. In addition, the wingbox is too small to hold the necessary 40,000L /10,600 US Gallons of fuel needed for the range with reserves. The fuselage has competitive dimensions and it can be extended to this length. The fineness ratio is then past 12, which costs weight but it is still in an acceptable region. To the longer fuselage, a CFRP-based wing of 165-170m2 area and an aspect ratio of 10 or more should be added to keep drag and MTOW within limits. Main landing gear would have to be extended to a tandem bogie.

MOM versus A321/A322

Figure 2 shows the results when we have feed our proprietary model with the described prerequisites. We have restricted the models to the competitive types, therefore no clean sheet single aisle alternative shown in Figure 1, NSA6-200 is there for reference. The optimized MOM7 is compared to the circular NLT7, both now have 18 inch aisles to make them more comparable to the competing A321/A322 and to show how an optimized dual aisle stack up against today’s single aisles.

Passengers are counted with 210lb per pax+bag or 95.3kg. The cargo area can just hold the bags stored in containers for A321LR and leaves little space for revenue cargo for the other types, thus a MOM aircraft is a passenger aircraft leaving belly cargo to be handled by the full LD3 capable dual aisles.

Figure 2. A321LR and stretched A322 versus most likely Boeing MOM alternatives. Source: Leeham Co.

As can be seen, the empty and takeoff weight of MOM7 is in the area of the equal capacity A322. This is due to the elliptical shape lowering the surface area which decreases both empty weight and wetted area. CFRP makes this possible without a significant weight penalty due to elliptical shape. A cylindrical variant, NLT7, gains around 2.5 tonnes empty mass and 3.5 tonnes MTOW over a more optimal elliptical variant. The result is better fuel consumption for the MOM7, in total 3% better than the equally configured circular MOM model on our 3,400nm sector and 2.7% better than a single aisle stretch of Airbus A321.

The efficiency of the clean sheet MOM alternatives and the re-winged/re-engined A322 fulfill the efficiency improvement criteria we set in Part 1, more than 20%. This is necessary to motive new designs as these costs in order of $10-15 billion dollars to develop. This investment would have to be amortized with around $10-15M per sold aircraft, so a substantial improvement over existing aircraft is necessary.

We have limited ourselves to the efficiency domain in this comparison. Should we have added Cash and Direct operating costs and revenue, the higher utilization of the dual aisle alternatives would have increased the competitive advantage versus a stretched Airbus alternative.

Summary

Our article series has shown that the MOM market is best served by a twin aisle aircraft if a clean sheet design shall be made. With classical fuselage design techniques, a twin aisle aircraft has a higher wetted area and higher empty weight. Boeing has patented an elliptical shape that can bring that close to the same values as a single aisle aircraft if CFRP design techniques are used.

The higher production costs of a CFRP dual aisle aircraft means it has to use its advantages in turn around time to increase revenue and spread fixed cost over a larger transported passenger base to amortize a higher price than a competing aluminum aircraft. For the same reason Airbus’ most likely move to a Boeing MOM development would be a further stretch of A321 to a larger A322 carrying 30-40 more passengers. Equipped with a CFRP wing and the same engine generation as a Boeing MOM design, it would only trail with around 3% in fuel consumption. Due to lower development and production costs it should be able to be competitive by using purchase price as the lever.

Airbus, which has a competitive existing cross section for the market, would therefore most likely refrain from a clean sheet design and stay with a lower risk stretch alternative. Boeing has no choice, to participate in this market a new aircraft is needed.

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