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Researchers Unveil Quieter, More Fuel-Efficient Aircraft Concept

Four views of the SAX-40. Click to enlarge.

Researchers from Cambridge University and the Massachusetts Institute of Technology (MIT) have unveiled their revolutionary concept for a “silent aircraft”: the SAX-40 (Silent Aircraft eXperimental).

Originally conceived in 2003 to make a huge reduction in the noise experienced by people in the vicinity of airports, the design of the SAX-40 also offers improvements of around 25% in the fuel consumed in a typical flight compared to current aircraft.

The designers currently predict the SAX-40 will deliver:

  • 149 passenger-miles per UK gallon of fuel (124 passenger miles per US gallon) compared with about 201 for the best current aircraft in this range and size). This is equivalent to the Toyota Prius Hybrid car carrying two passengers.

  • A noise of 63 dBA outside airport perimeter. This is some 25dB quieter than current aircraft.

The design is intended for the generation after next of aircraft for entry into service in 2030. The design looked at improving the airframe as well as the engines as half of the noise from a landing plane comes from the airframe. Some of the key design features employed are:

  • The overall shape of the aircraft which is a single flying wing. This shape allows the body to provide lift as well as the wings allowing a slower approach, thereby reducing noise. The shape also improves fuel efficiency in cruise.

  • Flaps and slats have been eliminated. These are a major source of airframe noise when a plane is landing.

  • The undercarriage has been simplified and its aerodynamics improved.

  • The engines are mounted on the top of the aircraft which screens much of the noise from the ground.

  • Novel ultra-high bypass engines, which have variable-size jet nozzles to allow slower jet propulsion during takeoff and climb for low noise, and optimization for maximum efficiency during cruise which requires higher jet speeds.

The Silent Aircraft Initiative was funded by the Cambridge-MIT Institute (CMI) in 2003 as a collaboration led by Prof Ann Dowling at Cambridge University Engineering Department and Prof Ed Greitzer, Aeronautics and Astronautics at MIT. The project, with a grant from CMI of £2.3 million, brings together teams involved in different aspects of aircraft design for a multidisciplinary approach. The Initiative has involved 40 researchers from the University of Cambridge and the Massachusetts Institute of Technology (MIT).

The project has had significant collaborations from all parts of the civil aerospace/aviation industry including: British Airways, BAA, Boeing, Brüel & Kjær, the Civil Aviation Authority, Cranfield University, DHL, easyJet, Eurocontrol, HACAN Clearskies, Lochard, London Luton Airport, Marshall of Cambridge Aerospace, National Air Traffic Services (NATS), Nottingham East Midlands Airport, the Royal Aeronautical Society and Rolls-Royce.

My first reaction on hearing of the Silent Aircraft Initiative was profound scepticism. Three years on, I have to concede that the SAI has surpassed my expectations by quite a margin. The team has produced a high-risk but credible design that is predicted to meet the original target. In retrospect, I ought to have expected a team from Cambridge and MIT, supported by Rolls-Royce and Boeing, to achieve something special. A radical approach to the challenges of the future comes more naturally from Academia than Industry, but the outcome will carry credibility only if the team is sufficiently strong and if it has the support of Industry and access to modern design methods. The SAI team has shown how this can be done.

—Dr John Green, Chairman of the Science and Technology Sub-group of Greener by Design



Sid Hoffman

Flying wings have been around since the 1950's and lifting body concepts for at least 30 years. The biggest problem is the same as the Concord - the lack of flaps means landing must be in an extreme upward pitched condition in order to achieve the same landing speed as an aircraft with flaps.

The Concord used a rotating nosepiece so the pilot can still see the runway but without that you either need a very high bubble cockpit or high landing speeds, which of course decreases safety and increases runway length requirements.

One of the other issues is for passenger comfort. Typically aircraft with conventional wings and a tail have higher wing loading and are less prone to getting tossed around in turbulance. A low wing loading design (as a lifting body, flying wing structure with a low landing speed would be) would tend to get tossed around by turbulance a lot more. People would very likely be willing to pay more just to keep from having to fly on something that will make them sick.

Lastly, take a look at the most efficient aircraft of all - gliders. None of those are a lifting body or flying wing design. Gliders are so aerodynamically efficient that they need no engine and can gain altitude merely through heat pockets over the ground. Gliders demonstrate there is great inherent efficiency to the "wings on a tube" style of aircraft.




Sid that was interesting to read. I thought that the whole point of flying wing or lifting body was that everything that produces drags is also part of the lifting system. Gliders are impressive, they often fly over the local ski area. But when an aircraft is really large and heavy exotic designs like this pay off. Turbulance on an aircraft weighing 6 digits is different than on small aircraft. That and the fact that aircraft cruise pressurized to 'get over the weather' makes turbulance less of an issue.

Here's how I read this one. The Airbus A380 is awsome. The A380 can carry more people/cargo farther than a Boeing 747-400 and be more efficient doing it. The Airbus A380 was designed with an aluminum wing to save money. Not having the more expensive composite wing made the aircraft heavier but still beat the current champ the Boeing 747-400. That left the door open for boeing to design a new composite 747-800 that is more efficient than the A380 and didn't break the bank designing an whole new aircraft. Boeing had already bought Northrup and hence the B2 Spirit bomber all composite technology. The B2 is a heavy bomber that cost more than it's weight in gold to build. Actually 90% was designing the technology. Now Boeing could adapt a composite wing with the new 787 Dreamliner and upgrade the 747 to the new 747-800 composite standard.

Airbus spent something like 10 Billion dollars on the A380 and that money saving heavy metal wing is biting them in the ass now. To come up with a new generation of A380 when this one isn't paid for yet... Now Boeing are spending there money and time getting the next generation of engineers to see their good work. As for ultra high bypass engines that's great. The more air pumped by fans the more efficient the emgine.

Rafael Seidl

Sid -

it's not necessarily true that modern BWB designs, derived from concepts developed by Junkers and Horten in WW2, don't feature flaps. A rival concept for 900-1000 passengers by Hamburg University of Applied Sciences does, for example:,5538,12641,00.html

The main obstacles appear to be the lack of window seats, potentially high vertical acceleration for the outermost rows and, the need for automated stability control (presently strictly military technology). Video screens providing a view of the outside will be mounted to the interior partitions, whose primary purpose is to stiffen the pressure cabin. Approach routes to airports may need to be redesigned to allow for sufficiently gentle turns.

Gliders owe their efficiency to very long, slender wings without any sweep angle. This admittedly ideal configuration does not scale to large passenger counts nor to high speeds. Also, airport parking spots have a limited width, originally designed to accommodate the 747. The A380's wingspan is no greater.


The article mentions that the SAX-40 will be about 25% more fuel efficient than comparable current aircrafts.
So maybe the figure of 201 passenger-miles per US gallon
(for current aircraft) is wrong ?
If the predicted fuel consumption for the SAX-40 is
124 passenger-miles/US gallon, then,
the consumption for comparable current aircraft is perhaps about 99 passenger-miles/US gallon ?


What could they do if they eliminated the "silent" part and just went for efficiency ?
You could have viewing cameras feeding LCDs in every seat back ( as we have now ). You could have a few general cameras and people could pay to own the right to slew and zoom a camera around for 10 minutes at a time.
Evacuation might be a problem in the event of a crash.
Otherwise it could be interesting.

Roger Pham

Sid made a good point that this new design does not necessarily reflect the most efficiently-possible design. Indeed, the article states that the estimated passerger-mileage for this new design is 149pmi/ImpGal, whereas the most efficient in this class (?Boeing 787?) can get 201pmi/ImpGal. Of course, in their research before building the Dreamliner B-787, the most efficient and best-selling new airliner todate, Boeing has looked at BWB concept, among others, and deemed that the conventional configuration still represents the most efficient possible.

The main advantage of this new BWB concept as stated in the article is noise reduction.

However, I see potential problem in the turbofan intake placed above the aft fuselage section. At high enough angle of attack, or with wind gust or atmospheric turbulences, there may be enough airflow separation in the aft fuselage area to stall the turbofan blades on the center engine, and that'll mean impaired takeoff performance. The two outer engines have intakes that could be behind the path of the vortices that would be formed at the junction of the wing to the fuselage at the angle of attack typically required for takeoff and climb. This is turbulent air and may not be good for the fan blades. Hopefully, the vortices would move outward enough as they move backward and will miss the engine intake entirely. Furthermore, if ice is formed in the front fuselage and breaks loose and travels on top of the fuselage to the back, some of it may be ingested by the turbofan blade, and that would be major bad news.

Roger Pham

On the other hand, Jorge may be right.

Looking up in

at the passenger capacity of the B-787-9 and multiply by the max range and divide by the total fuel load reveals only 63 pmi/USgal for the B-787. So, somebody has got to be wrong somewhere! The 129 pmi/USgal for this new BWB may be too good to be true.


the passenger capacity of the B-787-9 and multiply by the max range and divide by the total fuel load reveals only 63 pmi/USgal for the B-787

That's assuming that all the fuel gets consumed, which it doesn't. Even so, at maximum seating, range, and fuel capacity numbers, the 787-9 would rate at 72.6 passenger-miles/gallon, not 63.

Sid Hoffman

Roger makes a good point about the location of the engines I didn't even think about. Commercial aircraft are supposed to be SAFE above all. Nothing makes headlines like having a couple hundred screaming Americans go down into a smouldering hole in the ground.

Now the first thing that got me when I looked at this design is that they elected not to use canards. Very often you'll see canards used even when not required so as to provide some additional safety margin. Why? Because canards are designed to provide some lift at the nose and be shaped so they will allways stall well before the main wing stalls. When the canards stall, it drops the nose and prevents a main wing stall. To be honest, I actually have no idea if a BWB is even capable of recovering from a full stall, even with maintenance of power.

Now throw in the fact that a full stall would possibly (likely?) cause compressor stall such as plaged the early F-14's at low speed and high angles of attack and you can have a situation where your nice BWB when coming in for landing at high angle of attack hits a big wind sheer gust that kicks the nose way up, causes compressor stall, kills your engines, stalls the main wing and is followed by uncontrolled flight into terrain. That's not a pretty situation.


CNN quoted a Boeing spokesman giving a figure of 100 passenger miles per gallon for the 787.


Another factor to PMI/G is the price of fuel. With jet fuel 2-3X more expensive vs earler this decade, airliners are taking steps, like using weather forecasts to plot fuel efficient paths around storms, and to the use jetstream more. For example, the Asia-Northeastern NA trip used to pass near the North Pole. Now, many are taking advantage of polar jet forecasts, either to shorten the Eastbound route, or to avoid flying too long in it Westbound. This saves fuel, flight time for pilots (pilot fatigue safety and contract rules and regs) and crews, and helps the bottom line.


Does anyone know what the equivalent passenger-miles/gallon are for various high speed rail systems around the world? I've seen something like 250pmpg for local rail, but the system is too small to be an effective alternative.

Sid Hoffman

Keep in mind these figures are not always comparable anyway. A jet flying from Phoenix to Denver will only cover 500 miles but to DRIVE from Phoenix to Denver via interstates is closer to 900 miles. Nevermind the fact it's a 1.5 hour flight versus a 16 hour drive. There is a value to human life, and time costs human life, in effect.

Same applies for rail, you might have to cover 25-100% more miles to reach the same destination by rail as you would by air. Rail might be a little quicker than ground travel in some countries, but in the US I don't think that is true. Either way it will still never be as fast as aircraft for distances over 500 miles or so when factoring in check-in and security check time.

This is even more true for geographic areas separated by water because that will not affect the air travel by much at all, but ground/sea travel is going to be even more limited in route and vehicular selection. Commercial air travel is a wonderful thing. I don't want anyone to take my criticism of BWB as criticism of air travel in general. I welcome any viable improvements to air travel.


Of course a 1.5 hour flight is no longer a 1.5 hour flight. It's half an hour or more to the airport, half an hour to find parking (no rapid transit yet), 30 minutes to get checked in, another 30 minutes (or longer) to get through security, 15 minutes to board, 1.5 hours of flight time, 1 hour of luggage retrieval and then another hour by cab, rail of transit to where you're going. Trains are so civilized.


Trains are so civilized.

In the US? Ha.


Boeing does not own Northrop


Let's see - we lower drag and put in more efficent fan-jets and get 25% better fuel economy. We take-off and land at slower speeds and put the engine exhaust above the wing to act as a sound wall. I do not see anything which would require MIT engineering. Now directed thrust would allow very low take-off and approach speeds, and the flying wing provides a wide area for mounting thrust fans, but neither is mentioned. Is the future so hidden from view?


pose significant threat, mechanically, to the engine. I owned a slipstream genesis aircraft with a pusher propeller. The prop was in the most aerodynamically dirty position it could possibly be in, and the plane shook all over during taxi and ground roll (smoothed sid hoffman - The blended wing body is fantastic at stall recovery, I watched an engineer throw a small BWB mockup into a vertical wind tunnel, spinning it like it was a boomerang. The 3 foot wide mockup stopped spinning,aimed itself down at about a 60 degree angle and sat perfectly stable in the upward airstream.

BTW, there are alot of flying wing gliders out there.

roger pham- The engines being in an aerodynamically "dirty" location during rotation for takeoff doesn't out once you were in flight). It never had any effect on the reliability of the engine.

I agree with you on the ice though.


WOW! my last post really got scrambled somehow.

sid hoffman - The blended wing body is fantastic at stall recovery, I watched an engineer throw a small BWB mockup into a vertical wind tunnel, spinning it like it was a boomerang. The 3 foot wide mockup stopped spinning, aimed itself down at about a 60 degree angle and sat perfectly stable in the upward airstream.

roger pham- The engines being in an aerodynamically "dirty" location during rotation for takeoff doesn't pose significant threat, mechanically, to the engine. I owned a slipstream genesis aircraft with a pusher propeller. The prop was in the most aerodynamically dirty position it could possibly be in, and the plane shook all over during taxi and ground roll (smoothed out once you were in flight). It never had any effect on the reliability of the engine.

I agree with you on the ice though.


Northrup was aquired


OK Northrop merged with Grumman and is largely owned by a larger company.


I'm going to have to second Neil on the trains... while they aren't nearly fast enough for coast to coast travel, they would be really nice within the area east of the Mississippi, and they can take you right into city center instead of leaving you with a 1 hour journey after getting off the plane. Too bad Amtrak is so bad and there's generally no other options. Bear in mind that both airports and airlines are hugely subsidized, while Amtrak is weakly subsidized and has to make do largely with renting tracks owned by freight lines.

Roger Pham

coal burner,
It's fantastic that you own a BWB like the Genesis. That airplane looks very clean and efficient. For a small personal aircraft with cargo loading in only a narrow range with respect to the aircraft CG, the tailess BWB is Okay. However, for a commercial aircraft with cargo loading throughout the fuselage length, a good size of horizontal taiplane (stabilizer) is required for trimming and balancing against CG changes due to uneven loading longitudinally.

Properly trimmed with CG is the right position, any airplane shape can be made to recover spontaneously. However, for maximum efficiency, a load-carrying aircraft will be more efficient with a aft CG position, and an aft CG position will impair natural stall recovery, making pilot corrective action absolutely crucial in the stall recovery phase. Most business jets have servos connected to the control wheel to push the control wheel forward to push the nose down upon stall detection, as well as shaking the control wheel as warning for the pilot, since the pilot may not react fast enough. Once stall has progressed to a deep stall, recovery action may not be possible anymore.

Sid's point regarding adding a canard is a good one. Look at the Piaggo Avanti, the most efficient business aircraft, and it has a canard configuration.

The best engine mounting position in this BWB design for minimum noise signature would be in a pod above the wing in the root section, one engine on each side. The pod should be high enough to keep the engine intake above turbulence stall wake and icing run off bacward. Look at the HondaJet in Honda's website for to get a visual idea about my suggestion. Only two engines should be installed for maximum efficiency. More than two engines and the plane won't be as efficient nor would be competitive commercially with two engines version. All three-engine airliners are being phased out, replaced by more efficient twin-engine airliners.

Aft-mounted propeller receives much higher stress level and vibration level than front mounted propeller, and tends to be noisier. Just ask Burt Rutan!


I too do not understand the reasoning behind having three engines. If either of them stops the control of the plane must be very difficult indeed. The best approach would be to use only one engine and to direct the thrust such that better control is achieved at lower speeds which will decrease the needed runways and the noise at take of and landing.

Regarding the danger associated with ice i think this problems boils down to inspection control and proper preparation. If this is done correctly ice will not pose a safety problem.

Turbulence at take of is also a minor risk since we are talking about a plane with considerable enertia. Never the less i would prefer directional thrust to exert greater control faster and i think the added cost is very well spent as this will provide access to smaller airports with shorter runways.

I find it strange that they even bother to put pilots up front. Cameras are far cheaper than cockpit windows and can provide better information, so in fact the cockpit could be anywhere.

Regarding power efficiency I think it should be considered to fly at higher altitudes which may imply that a Stirling engine should be considered because they increase thrust with altitude because of the wide temperature span between combustion and ambient temperature at high altitudes. Also their external combustion is super silent and vibration free.

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