Plane sailing: the seaplane of the future?

Researchers have developed a design concept for a transatlantic flying boat featuring a blended wing body.

Classic flying boats lost popularity  in the 1950s because they were inefficient compared with more aerodynamic airliners able to fly large numbers of passengers directly to land-based airports.

However, as rules on pollution and noise get ever tougher, limiting expansion at many major airports, the flying boat could be on its way back.

Researchers at Imperial College London have developed a design concept for a transatlantic flying boat that would move  the low-level flight paths of large aircraft offshore, away from heavily populated areas.

The design uses a blended wing body and looks far removed from classic flying boats with V-shaped hulls such as the Short Sunderland, the Saunders-Roe Princess or the Hughes H-4 Hercules ‘Spruce Goose’, but it does meet all airworthiness requirements.

“What we really wanted to do with this project is take a look whether the application  of new technology, and the new ideas coming into industry such as blended wing bodies, would actually result in an aircraft that is designed both conceptually, so in the overall configuration, and in the preliminary design process, that can actually alleviate the historical downsides of a flying boat,” said Dr Errikos Levis, a teaching fellow at Imperial’s Department of Aeronautics.

“Seaplanes of the past had a weight penalty and an aerodynamic penalty, and fuel consumption is inversely proportional to both, basically. So the bigger the weight penalty, the more fuel inefficient you are, and the bigger the aerodynamic penalty, the more fuel inefficient you are.”

The team designed a range of aircraft  from a 200-passenger model capable of flying 5,600km to a 2,000-passenger behemoth able  to fly 15,000km.

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Landing at sea requires no runway, but the technical dififculties are considerable

The 2,000-passenger model is about 80m long and 20m high from bottom to tip, and  has a 160m span. The Airbus A380 is 72.72m long and 24.09m high and has a wingspan of 79.75m. The A380 has a range of 15,200km and typically seats 544 passengers, although it can carry a maximum of 853.

As far as operating on choppy water  and aircraft efficiency was concerned, the researchers found that biggest was best. “The fact you are operating from seas means that you will have to either make a choice to  put in wave barriers, maybe somewhere offshore to cut down the intensity of waves coming in, or you are going  to have to accept that you are not going to be able to take off some of the time,” Levis said.

“Overall, size actually  solves the problem, in addition to making the aircraft more efficient overall. The bigger  you go, the more likely you are going to be able to use it 24/7, 365 days a year.”

Size also helped to solve the problem of emergency egress from a blended wing aircraft caused by the large number of passengers and the placing  of emergency exits dictated by the shape of  the craft.

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The 2000 seat seaplane would be considerably larger even than an Airbus A380

“With traditional blended wing bodies, it  is actually a pretty big deal, but in our design, because we have to raise the wings high enough above the waterline so that they  don’t get hit by spray, there is a very nice, almost vertical or slightly sloped side just underneath the wing that extends the entire cabin length,” Levis said.

“Now obviously similar issues to a standard blended wing body will appear here because you have a very high passenger density in the middle and a smaller perimeter area from which they can egress, but this design tries  to maximise the surface area available for emergency exits to be placed. It doesn’t completely solve the problem but it does  go some way towards a solution.”

Classic flying boats suffered increased drag and structural weight because their fuselage had to be shaped and reinforced to allow them to operate on water. While the blended wing body design allows the aircraft to float, it offers reduced drag when it is in the air.

“What we found is that by using this particular configuration, we could get rid  of a lot of the structural penalties that were associated with things such as tip floats,”  Levis said.

Tip floats ensure flying boats are laterally stable on the water surface. They provide  drag and weight, not only by their use but because their weight has to be counteracted  by strengthening the wing to take on the extra weight at the tip.

“What we did instead was say is there any way we could use part of the fuselage or part of the wing to provide lateral stability,” Levis said. “What you see is there is a hull that rides up. Outboard of that there is a proportion of the fuselage that stays almost parallel or even has  a little anhedral [goes downwards]. That means that the aircraft can actually right itself and maintain itself on the water using a very thick piece of structure that doesn’t need to be strengthened substantially because it is already supposed to take substantial loads when flying.”

The design needs to have excellent fuel efficiency if it  is to compete with traditional aircraft. Current state-of-the-art aircraft require around one to 1.1 megajoules per available seat kilometre, Levis said. “For  a 550-passenger aircraft, we  are getting 1.149 megajoules per available seat kilometre, and going up to 2,000 we can get that down to 0.94, so it  is substantial improvements, although that assumes that  we can fill the aircraft.”

The engines are on top of the fuselage, which limits the effects of spray. The design  was found to provide a massive amount of empty volume in  the lower part of the fuselage, as passengers cannot be underneath the waterline for safety reasons. This led to the idea of using alternative fuels such as hydrogen, which is more environmentally friendly but takes up about four times the volume per energy given compared with Jet A-1, according to Levis

Using advanced materials would also help provide the improved efficiency required. “As far as materials available we were thinking of using advanced composites throughout,” Levis said. “The problem is that in early design you can’t really quantify the exact effect that a particular composite would have, so we actually stuck with assuming a conservative assumption of five to 10 per cent weight reduction due to  that, which seems to me the value that aircraft designers are hoping for to be achievable when using composites and advanced methods.”

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The Saunder-Roe Princess was developed for transatlantic routes, but never went into production

The aircraft would be fuelled in the  same way as a conventional airliner, while maintenance could be undertaken in a dry dock. Another option might be a beaching cradle allowing the aircraft to get in and out  of the water using its own power, as seen with the Martin SeaMaster in the 1950s, although the sheer size of the design might prevent this.

However, that size allows more passengers, and could lead to less metal in the sky, and aircraft flying to coastal airports, according  to Levis. Although more infrastructure would be needed than for an old-style flying boat,  it should be smaller and cheaper than that needed to build or expand inland airports to allow them to welcome ultra-large aircraft.

“Out of a number of the biggest hubs out there or the many cities that are expected to become massive, a big percentage of those  are either on the coast already, and the airport is already coastal, or the city itself is within  50 miles from a coast, which is reasonable enough for transport with high-speed railway or something.”

Levis stressed that the aircraft was a non-optimised design and more detailed modelling was required. One issue the team faced was a lack of data on the constraints  of using the ocean as a runway. To overcome this, very conservative estimates were used  in terms of weight and power, so more detailed modelling could lead to an aircraft offering even more efficiency.

“Things we are looking at are ground  effect aerodynamics; how the aerodynamics  of the aircraft would work in this particular configuration in ground effect,” Levis added. “One of our hopes is that it would possibly allow us to land slower, thus relieving some  of the stresses on the fuselage, thus allowing the fuselage to be lighter as well.

“We are trying to develop better methods  for the weight estimation of the fuselage  itself, something that is rapid but accurate,  as within an optimisation framework speed  of computation is actually very important.

“We are also looking at the aerodynamics  in a bit more detail and the hydrodynamics of how it is going to take off; how the unique intricacies of this particular design affect it;  and how can we get more information about the shape of the hull into the design process earlier and see how they interact with other design considerations that we have.”

In the end, the use of such aircraft will only take off if they are more efficient than conventional airliners.

“Big business would have  to be the one to take this on and convert it because there is no way a start-up could look at something this big,” Levis said. “Truthfully, 2035 onwards is where we are looking at right  to have a step-change. That is where the planning is for the step-change in aviation to occur.

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Boeing and NASA have already experimented with blended-wing aircraft such as the X-48

“Planning this far ahead  is required for projects where there are so many unknowns, and that is not only for flying wing seaplanes; that is also  for flying wing landplanes. You look at the NASA N+1,  N+2 and N+3 [a silent aircraft that sends no carbon into the atmosphere], and the kind  of timeframe they have set;  it is really around the 2040s they are expecting to have  such bold ideas materialise.”

Meanwhile, major aerospace organisations are looking into the possibilities of the aircraft configurations the Imperial team has been studying, including NASA and Boeing, which collaborated on an experimental blended wing body called the X-48. “Boeing has ongoing blended wing body research activity to advance the state of the art and be prepared to shape the market with appropriate products,” said Katie Zemseff of the Seattle aerospace company’s engineering, operations and technology department. “Our research on the topic did not end with the completion of the X-48 work. Most of the current research, done together with NASA, is in wind-tunnel testing to investigate design concepts. Boeing studies many concepts for future commercial aircraft, taking into account market demand, customer requirements and production capacity, and makes decisions based on this research. Boeing believes the concept could be developed in the next 15–20 years for military applications such as aerial refuelling and cargo missions.”