Several years ago I was touring the facilities of a major aerospace company and was disappointed to hear the visit to the wind tunnel was cancelled as it was temporarily ‘off-limits’. Why’s that? I asked – are you testing a classified combat aircraft, a stealthy UAV or perhaps an innovative design that will revolutionize commercial aviation?

“No, nothing quite like that,” they said. It was something “way, way more interesting, and just as classified.” What could that possibly be? Reverse engineering of alien technology perhaps? No, nothing like that either. Turns out it was a band new racing car design for Formula 1 - the most popular form of world motor racing.

Wind tunnel testing is emblematic of the close relationship between F1 and aerospace which every year seems to grow stronger. Beyond high-speed aerodynamics and drag reduction, the two industries share secrets in advanced, lightweight materials, systems and sensors.

A new link between aerospace and F1 was established today when Rockwell Collins announced an agreement with Caterham F1. The deal will see the two “collaborate to define aviation technology that will be adapted in the team’s quest to win the Fédération Internationale de l’Automobile (FIA) Formula 1 World Championship.”

Caterham driver Heikki Kovalainen practises for this weekend's Spanish Grand Prix

Typically for such an agreement that’s about as detailed as it gets. It is also hard to guess more given the breadth of Rockwell Collins’ expertise in avionics and systems, though it could range from advanced displays to improved sensing. There’s no telling yet when we might see the first evidence of the collaboration with Caterham which was only granted entry in 2009 for the F1 World Championship in 2010. Based in the UK, Caterham raced as Team Lotus in 2011, winning 10^th place in the Championship and already showing signs of moving quickly up this highly contested field. Let’s see how they get on now.

Boeing is readying its hydrogen-powered Phantom Eye unmanned air vehicle demonstrator for first flight following the completion of taxi tests at Edwards AFB, Calif. The testing, which included a high speed taxi run down the centerline of one of the runways at the base’s dry lakebed, concluded on April 21. The company adds that the 150-ft span vehicle achieved “all required test points in its preparation for first flight.”

Click to view a new Boeing video of the high-speed taxi test.

Photo: Boeing

As my colleague Graham Warwick noted recently, Phantom Eye is powered by a pair of Ford truck engines modified to run on hydrogen and triple-turbocharged to operate at high altitude. Phantom Eye is designed to stay aloft at 65,000 ft. for four days, as a precursor to a 10-day-endurance UAV.

A strange, 16-propeller vertical take-off and landing craft has been awarded the Lindberg Prize for Innovation at the Aero-Friedrichshafen 2012 airshow in Germany. It is an example of the unusual configurations made possible by distributed electric propulsion.


Photos: e-volo

The VC1 was first flown by Karlsruhe-based e-volo in October 2011 as a proof of concept for its Volocopter, an aircraft that uses multiple small, electric-powered, fixed-pitch propellers for lift and flight control. The array of battery-driven props provide redundancy for safety and are individually controlled via a fly-by-wire system to provide flight control via differential power by varying motor rotational speed in response to joystick inputs.



The single-seat, four-armed VC1 measures 17ft x 17ft and weighs 80kg empty, and could fly for about 20 minutes on current battery technology, says e-volo. The company is now designing a two-seat VTOL aircraft, the Volocopter VC Evolution 2P, that mounts its 18 props and motors on an umbrella-like structure above the helicopter-style fuselage. The aircraft is being designed to comply with European ultra-light (ULM) regulations.


Concepts: e-volo

Fitted with a pusher propeller for forward flight, this aircraft will have a take-off weight of 450kg, speed exceeding 54kt (100km/h), altitude capability of at least 6,500ft and a flight time of more than an hour thanks to a serial hybrid power system - with a "range-extender" internal-combustion engine that will recharge the batteries in flight.

In addition to being able to land safely with several props failed, the VC Evolution 2P will have additional batteries in case the range extender fails, and an airframe parachute to lower the aircraft to the ground is all else fails - an additional layer of safety not possible with a helicopter or autogyro because of the rotor, e-volo notes.



Awarding the prize to e-volvo, Erik Lindbergh (Charles Lindbergh's grandson) said: "We believe that the development of the Volocopter holds significant promise to radically change short-distance transportation." The Lindbergh Foundation's prizes are intended to promote advances in green aviation.

This article was originally published as the Leading Edge column in the Apr. 16 issue of Aviation Week & Space Technology.

Photo: US Navy

Quicker Thinking

Budget cuts not only put pressure on the scope of defense research, but also emphasize the speed with which results can be fielded to improve today’s weapon systems.

The Office of Naval Research (ONR)—responsible for science and technology (S&T) across sea, air, land and space realms for the US Navy and Marine Corps—is looking to field technologies faster to meet the objectives of the Defense Department’s new strategic guidance.

“ONR’s strategic plan, released in the fall of 2011, refocused us in the right areas,” says Rear Adm. Matthew Klunder, chief of naval research. “We knew anti-access/area-denial would be important, so we refocused our core S&T.” Focus areas that support assured access, he says, include autonomy and unmanned systems, expeditionary and irregular warfare, information and cybersecurity, power projection and integrated defense.

“We knew times are going to be tough, so we reduced from 13 focus areas to nine,” Klunder says. ONR also diverted some funds from foundational “discovery and invention” research to programs that could deliver results more rapidly. Some of these ONR has been working on for several years, and could be in the fleet within 1-3 years, he says.

Others are “leap-ahead” technologies that ONR wants to accelerate. “We are absolutely committed to prototypes to get capabilities out to the fleet more quickly,” Klunder says. “We feel confident, in a tougher budget, that we have [up to] three leap-ahead technologies that will get into the fleet.”

Examples include the electromagnetic rail gun. The Navy was aiming for a weapon with a muzzle energy of 64 megajoules (MJ), an exit velocity of Mach 7.5 and a range greater than 200 nm, for fielding by 2025. Now the program is focused on a smaller, 32-MJ, 100-nm weapon that can be delivered sooner and integrated more easily onto existing ships such as the DDG-51.

BAE Systems and General Atomics have delivered 32-MJ tactical prototypes for test firings. Now they are developing pulsed-power and thermal-management systems. They plan to deliver prototype multi-shot, actively cooled rail guns for firing tests in 2017, leading to a multi-mission weapon.

The free electron laser (FEL) program has also been restructured. Plans to develop a 100-kw prototype as a step toward the envisioned megawatt-class weapon have been put on the back burner. Instead, ONR will mature solid-state laser technology, already being pursued by the Army and Air Force. A solid-state laser would be less powerful, but smaller and could be adapted more quickly for shipboard use.

The Navy continues to believe FEL is the best solution for a naval directed-energy weapon, as its beam is tunable to minimize atmospheric absorption and distortion in a maritime environment, but “[we] also realize that some of the technologies still have a ways to go,” Klunder told Congress last month.

ONR’s two newest prototype programs, meanwhile, have an aviation focus. The Variable-Cycle Advanced Technology (VCAT) effort will take the Air Force Research Laboratory’s Adaptive Versatile Engine Technology (Advent) demonstration and apply it to propulsion systems for next-generation manned and unmanned carrier-based strike and surveillance aircraft. Under Advent, Rolls-Royce and General Electric will test variable-cycle engines combining high thrust for supersonic speed with low fuel burn for subsonic endurance.

The Autonomous Aerial Cargo/Utility System (Aacus) program will enable unmanned vertical-takeoff-and-landing aircraft to drop off and pick up loads in adverse weather and harsh terrain. The “platform-agnostic” system will allow the aircraft to autonomously avoid obstacles, select an unprepared landing site and touch down precisely, with the ability to react to unplanned events.

Aacus and VCAT support the newest of ONR’s five “national naval responsibilities” (NNR)—sea-based aviation. NNRs are “areas where the other services, the federal research establishment and the private sector may not have the incentive to investigate, [so] the sole responsibility rests with the Navy,” says Klunder. Aviation NNRs focus on challenges associated with launch, recovery and deck operations of manned/unmanned aircraft on carriers and other air-capable ships (see photo).

A key goal of an NNR is to provide funding stability to maintain an industrial capability unique to the Navy. “Supporting the industrial base is very important,” says Klunder “If we tell them where our priorities are, and don’t go off on tangents, industry is then willing to invest its independent R&D dollars. The trend line is getting better; we are providing clearer direction and reducing wasteful dollars.”

Klunder is also keen to bring the acquisition community into the S&T process earlier, to ease the transition from R&D to procurement. “It is always a challenge integrating new technologies on to platforms,” he says. “As we develop prototypes, we need to bring on engineers from the acquisition side to tell us what interface standards they would prefer. We can compress the acquisition process when we work side-by-side.”

This article was originally published as the Leading Edge column in the Mar. 12 issue of Aviation Week & Space Technology.

Cold Comfort

Icing has been a threat to aviation since the dawn of the aeronautic age and, as manufacturers pursue efficiency improvements measured in fractions of a percent, the power demanded to protect aircraft—whether by bleed air or electrical heating—is coming under attack.

At the same time, unmanned-aircraft developers see the need to operate in all weather conditions, but cannot afford the power and weight penalties associated with traditional anti-icing systems. Nanotechnology may hold the answer, at both ends of the spectrum.

U.S. research and development organization Battelle is mixing conductive carbon nanotubes (CNT) into aircraft paint to produce a lightweight, low-power, anti-icing coating that is easy to apply and repair. General Electric Global Research is developing nanostructured surfaces that delay the formation of ice and reduce its adhesion.

Columbus, Ohio-based Battelle has dispersed nanotubes into a coating solution to make it conductive, so it heats up like a resistor when power is applied to a wing’s surface, says John Ontiveros, operations manager for the program. The coating has been tested in an icing tunnel using a scrap wing from a UAV.


Photos: Battelle (left image shows ice-free heated area in center)

The trick is to suspend the right amount of CNTs in the solution to achieve the resistivity needed in a coating that can be applied by a spray gun like regular paint, he says. The coating “stack-up” consists of primer over the bare metal, the nanotube heater coating, a soy-based protective layer and then the normal top coat of paint.

The coating is only 0.020-0.030-in. thick and four times more power-efficient (in watts/area) than other electrothermal anti-icing systems, Ontiveros says. The lower the resistance, the more efficient the heat generation. “We are down to low single digits, which is pretty awesome.”

Because the heating is on the wing’s surface, not under its skin, there is no thermal lag, according to Ontiveros. The coating heats up rapidly, and power can be cycled quickly between different sections of the leading edge to minimize consumption. Power leads are laid flush to the surface then the coating is sprayed on, minimizing the need to penetrate the wing skin. The coating can also be repaired or replaced easily, he says.

Battelle is talking with aircraft and paint manufacturers, looking at both large and small platforms, while continuing risk-reduction work in areas such as coating application and power management. “We need to look at the full regime of icing,” Ontiveros says, and operating strategies need to be developed based on an aircraft’s flight envelope and available power. He notes, “We are ready to look a full-scale integration and test on an aircraft.”

GE’s Niskayuna, N.Y.-based Global Research Center, meanwhile, is continuing research into superhydrophobic materials with nanotextured surfaces that repel water and prevent ice from forming and adhering. GE’s interest is in anti-icing surfaces and coatings for both aircraft and engines as well as wind turbines, where increased drag caused by ice buildup can significantly reduce the power-generating capacity and efficiency.


Photos: GE Global Research (see bouncing water video here)

Having demonstrated via icing-tunnel tests that microscopically textured aerodynamic surfaces can dramatically reduce ice adhesion, GE says it has now shown that they can significantly delay ice formation. “Compared to standard surfaces . . . nano-enabled anti-icing surfaces would delay ice formation for more than a minute on their own,” says a company representative.

Nanotextured coatings on aerodynamic surfaces and engine blades “could one day reduce and possibly even eliminate the need for existing anti-icing measures,” the company states. But it cautions that further development is required before such surfaces are durable enough to be ready for commercial application.

EADS Innovation Works, the research and technology arm of Airbus’s parent company, is investigating water-repellent coatings to prevent ice adhesion and accretion at a new laboratory-sized cryogenic tunnel in Ottobrunn, Germany. Mississippi State University is providing computational fluid dynamics modeling of supercooled water droplets impacting aerodynamic surfaces. The work is in support of Europe’s Aeromuco research program, which is to include flight tests of ice-resistant coatings.


Image: NASA

NASA, meanwhile, has begun modifying a Gulfstream G-II to investigate how ice crystals—formed in high-altitude, warm-weather storms—can enter the core of an engine (see photo), melt and then refreeze, causing loss of power or shutdown. Trial flights are planned for Florida in August, ahead of a test campaign set for January-March 2013 over Darwin, Australia, an area known for storms with high levels of ice crystals

A team of MIT, Aurora Flight Sciences and Pratt & Whitney are beginning key windtunnel tests to prove whether an innovative aircraft configuration can deliver big fuel-burn reductions without resorting to exotic airframe and engine technology.


Graphics: MIT

The MIT-lead tream developed the twin-aisle "double-bubble" D8 configuration during NASA-funded N+3 studies completed in 2010. In NASA paralance, N+3 means an aircraft that could enter service around 2035. In those studies, the team concluded that the D8 could reduce fuel burn 70% relative to today's 737-800.

Of that saving, 49% comes from the configuration, with its wide lifting-body fuselage, almost-unswept wing and reduced cruise Mach number, and rear-mounted engines ingesting the fuselage boundary layer. The rest of the 70% reduction comes from 2035-timeframe airframe and engine technologies.



Under a Phase 2 contract from NASA, the team will conduct a series of windtunnel tests to validate that 49% fuel saving from the configuration alone. These have begun with tests of a 1/20th-scale unpowered model of the D8 in MIT's Wright Brothers Wind Tunnel (above) to measure the aerodynamic benefit of the airframe without its integrated engines.

Mark Drela, the MIT professor who developed the TASOPT optimization tool used to design the D8, says the aerodynamic advantages come from the lifting fuselage, which shrinks the wing; the nose-up pitching moment from the upturned nose, which shrinks the horizontal tail; and reducing Mach number to 0.72 from 0.80, which allows a lighter, more-efficient low-sweep wing.

Next will come tunnel tests of 1/11th-scale powered models to measure the benefit of boundary-layer ingestion (BLI). This will involve testing the model first with conventional podded engines...


...then with the engines integrated into the aft fuselage.

Nestled into the rear fuselage, the engines ingest the slower-moving fuselage boundary-later and re-energize the fuselage wake, reducing drag. Slowing the flow into the engines also increases their propulsive efficiency, reducing fuel burn. The D8's lower cruise speed helps, Drela says, as the air entering the fans has a local Mach number of around 0.60, which minimizes flow distortion.

Other advantages of the flush-mounted engines include the ability to minimize the size, weight and drag of the nacelles by using the rear fuselage to align the flow with the engines; shielding engine noise from the ground using the fuselage and "pi-tail"; and reducing engine-out yaw moments, which shrinks the vertical tails.

A disadvantage, says MIT professor Edward Greitzer, is the possibility of engine fratricide - an uncontained failure in engine also taking out the adjacent one. This could be overcome by extra containment or by staggering the engines longitudinally. The team is also studying small-core ultra-high-bypass engines in which the core is aerodynamically coupled via a free power-turbine to an aft fan  - an arrangement that could allow more flexibility in lining up the disk burst zones.

The final round of tests, with a 1/4th-scale powered model in NASA Langley's 14 x 22ft subsonic tunnel, will focus on the aero-acoustic performance of the full configuration, with the goal of "traceably comparing" the D8 with the 737-800 to assess the benefits of BLI and identify any unanticipated losses from propulsion/airframe integration, such as the effects of flow distortion into the fans.

NASA is claiming a breakthrough in quiet supersonic aircraft, with successful windtunnel tests of designs that combine low sonic boom and low cruise drag - characteristics once thought to be mutually exclusive.


Photo: Boeing

Commenting on the completion of boom and performance tests by Boeing and Lockheed Martin, NASA Supersonic Fixed Wing project manager Peter Coen says: “This is a breakthrough. It’s the first time we have taken a design representative of a small supersonic airliner and shown we can change the configuration in a way that is compatible with high efficiency and have a sonic signature than is not a boom.”

The trick is in the shaping of the airframe to tailor the shockwaves so that they produce a sinusoidal pressure signature at the ground, rather than the abrupt N-wave shape of a traditional "double-bang" sonic boom. Instead of powerful bow and aft shockwaves, the aim is to generate lots of smaller shocks along the airframe that attenuate more quickly as they travel through the atmosphere. Boeing's model (above) shows the result of all that subtle shaping.

Boeing's design for a 202ft-long, 30-passenger supersonic airliner acheived a boom level of 81PLdB at Mach 1.8 in NASA Ames' 9 x 7ft tunnel. Lockheed's design for a 230ft-long, 81-passenger trijet (below) achieved 79PLdB at Mach 1.6. NASA's goal is 85PLdB. "That's 25dB less than Concorde and 20dB less than the best we achieved under HSR [NASA's High Speed Research program]," Coen says.


Concept: Lockheed Martin

Both concepts for a 2025-timeframe small supersonic airliner achieved the cruise lift-to-drag ratio required to meet their design range of 4,000nm. "We've broken the low-boom/low-drag paradox, where you could get one, not both," he says. "They achieved low boom with a good level of supersonic lift-to-drag."

But we can do better, it seems. Longer term, NASA is targeting a boom level of 65PLdB, and 70PLdB is widely regarded as the threshold for public acceptance of unrestricted supersonic flight over land. Both the Boeing and Lockheed designs are at the high end of what would be acceptable. “We’ve learned that shaping technology will improve, and we'll probably be able to reduce the boom further,” Coen says.

So Boeing and Lockheed will further refine the shaping of their designs in Phase 2, for which NASA has two goals: to reduce boom levels across the full 60-mile-wide ground "carpet", and not just under the aircraft's track, and to measure the effects of engine inlet and nozzle shock systems on the boom signature. Refined models will go back in the tunnel in October/November.


CFD graphic: Lockheed Martin

Getting the aft-fuselage shockwave shaping right is tricky. The 3D geometry optimizer used "makes some pretty fine adjustments to the aft end of the aircraft," says Coen. "Shock position is pretty important, and even small shocks from the nozzle flow could have an effect." The models tested this year had only representative flow-through nacelles. but the new ones will have accurately modelled inlets and nozzles, with their shock systems.

The Skunk Works' John Morgenstern says Lockheed's design achieved a full-carpet low boom averaging 79PLdB "with a cruise L/D impact of less than 10%". He is aiming for a boom of less than 78PLdB, and ideally less than 73PLdB, with the refined design, which will also introduce natural laminar flow to further reduce drag. Boeing Research & Technology's Todd Magee says his team will evaluate ramp and conical inlets and take another look at how the upper-mounted engines on their design are integrated with the body.

All this success is increasing pressure on NASA to find the funds - from somewhere - for a low-boom supersonic flight demonstrator. The agency has been working with Gulfstream on design of the X-54A Low Boom Experimental Vehicle (LBEV - below), but does not have the budget unless it cuts other programs or persuades Congress or its government and industry partners to put in money.


Concept: NASA

For now, NASA uses an F-18 to fly a dive maneuver that creates a shaped boom at a certain point on the ground. But to assess the public acceptance of shaped booms, it needs to get away from the boom-inured populace around Edwards AFB. “Ultimately we would like to do a flight demonstration of low boom in steady level flight, as a way to look at community acceptance,” says Coen.

Aeronautics doesn't have to be about aeroplanes. Altaeros Energies, an MIT spin-off, has demonstrated an airborne wind turbine (AWT) that uses lighter-than-air technology adapted from tethered aerostats.

It's 35ft-diameter subscale prototype has just demonstrated it can lift an off-the-shelf wind turbine to 350ft altitude, where it produced more than twice the power generated at conventional tower height, Altaeros says.


Photo: Altaeros Energies

Transported and deployed from a docking trailer, the prototype climbed to 350ft, produced power at altitude and landed in an automated cycle. Altaeros' goal is to develop an AWT that reduces energy costs up to 65% by harnessing the stronger winds above 1,000ft. It could be deployed in just days to replace diesel generators at remote locations.

Altaeros is not alone in this field. Makani Power is developing a tethered unmanned aircraft that autonomously climbs to altitude to produce power at half the cost of a conventional wind turbine, and uses 90% less material. The automous flying wing can handle sudden shifts in wind speed and direction, the company says.


Photo: Makani Power

Once aloft between 800ft and 2,000ft, and controlled by an onboard computer, the tethered wing travels in a circle mimicking the tip path of conventional wind-turbine blade. Wing-mounted rotors generate the electricity, and can keep the wing aloft if the wind dies.

Makani is developing its AWT with funding from the Advanced Research Projects Agency - Energy under its Wing 7 program, and has flown an 8m-span, 30kW prototype. It's goal is to develop a 28m-span, 600kW commercial version capable of delivering energy at an unsubsidized cost competitive with coal, says ARPA-E.

In December, Altaeros says, the FAA released draft guidelines for siting airborne wind systems. Longer term, both companies have their sights set on the deep offshore wind-energy market.

When NASA talks about turboelectric distributed propulsion, it talks in terms of superconducting generators and motors in a hybrid wing-body concept aircraft it calls the N3-X. But there are other ways of approaching all-electric propulsion.

Empirical Systems Aerospace (ESAero), a small advanced-design house based in Pismo Beach, California, assumed superconducting technology when it produced a concept (below) for a 2030-35 timeframe turboelectric-powered 150-seat airliner, the ECO-150.


Graphics: ESAero, via NASA

Designed to meet NASA's N+3-generation goals, which include a 60% reduction in energy consumption relative to the CFM56-powered 737-800, the ECO-150 features electrically driven fans embedded in the inboard wing sections, powered by mid-span turbogenerators supported by bracing struts attached to the tails. There are eight fan propulsors per side.

High-efficiency superconducting generators and motors require cryogenic cooling to work, and ESAero president Andy Gibson says the company did not have enough information on cryocoolers so it used liquid hydrogen to cool the electrical systems. Hydrogen is contained in tanks along the top of the fuselage and, after cooling the superconducting systems, is burned in the turbogenerators.

The required superconducting technology does not exist today but ESAero, continuing internal studies into hybrid propulsion systems, became convinced that conventional, non-superconducting electrical systems could be made to work in a large aircraft. It was funded by NASA Ames to take the ECO-150 concept and rework it around ambient-temperature generator and motor technology available to meet NASA's 2020-25 timeframe N+2 goals.

To the evident surprise of both ESAero and NASA, the N+2 ECO-150 (above) design closed - met its requirements - despite having a significantly heavier turboelectric distributed-propulsion system using technology available today in industries outside aerospace. "Our main interest was could we even get the aircraft to close, and the answer is yes," says Gibson.

"This is our first shot at getting the aircraft to close, and performance is about equal to a CFM56-powered 737-700," he says. Without the benefit of high-efficiency superconducting motors and generators, the propulsors are significantly larger (below, superconducting on the right and non-superconducting on the left). Gibson says ESAero might redo the N+2 ECO-150 design and increase fan diameter, which would allow the motors to be shorter.

Ben Schiltgen, Gibson's business partner and architect of the propulsion system, says past studies of hybrid turboelectric power concluded it would take electric-motor power densities of 10hp/lb to make a design work. Current technology is around 4-5hp/lb. "Technology is not even close to 10hp/lb, but it appears we do not need that kind of power to close an aircraft," he says. The N+2 ECO-150 has generators and motors in the 2.4-4.5lb/hp range.

The result of all this work is growing military interest in turboelectric propulsion - superconducting and non-superconducting - and a Large Electric Aircraft Propulsion Technology (LEAPTECH) workshop was held in Dayton, Ohio, in January, where NASA and the US Air Force, Navy and Army shared their ideas in an effort to identify opportunities for collaboration - including the potential for a dual-use demonstrator.

Under its N+2 study contract from NASA, ESAero has also produced a dual-use commercial-airliner/military-transport concept. The two variants of this design have essentially the same fuselage - housing 130 seats as an airliner and 52,000lb of payload as an airlifter. The wing is scaled up 20%-plus for the military version (below).

Although ESAero believes the technology for such an aircraft is available, it is outside aerospace and needs to be scaled up. "The motors are about an order of magnitude larger than exist today," says Schiltgen. Even using today's non-superconducting technology, the time needed to scale up the motors, make sure they work at altitude and find ways to dissipate the heat they generate, would put a turboelectric-powered aircraft out into the 2025 timeframe, he believes.

While NASA believes ambient-temperature turboelectric propulsion could be used in a demonstrator aircraft, it continues to pursue cyrogenic superconducting technology to get the power density and energy efficiency it is seeking. To that end, it has awarded contracts to Rolls-Royce Liberty Works to design a 50MW-class propulsive electric grid; Advanced Magnet Lab for a fully superconducting motor/generator; Creare for a flight-weight cryocooler; and MTECH Laboratories for a cryogenic inverter/rectifier.

NOTE: Post corrected to identify NASA Ames as the sponsor of ESAero's N+2 and dual-use concept work.

Biplanes are slow, right? Well researchers in Japan and the US are investigating a biplane configuration to reduce the shockwave drag and sonic boom of supersonic transports.


Concept: MIT/Tohoku University

The basic idea, first proposed by aerodynamicist Adolf Busemann at the 1935 Volta Congress, is that the shockwaves from the two airfoils cancel each other out, virtually eliminating wave drag.

It's more complicated than that, of course, and researchers at Tohoku University in Japan and Massachusetts Institute of Technology and Stanford University in the US are wielding the latest computational fluid dynamics tools in a bid to turn the Busemann Biplane into a practical design.


Graphics: Tohoku University (from this review of their work)

A wing generates shockwaves through two different mechanisms: lift and thickness. The Busemann Biplane (left, above) splits the supersonic airfoil into two and divides the lift between them. As wave drag is proportional to the square of lift, this reduces drag. Between the airfoils, compression and expansion waves from the upper and lower surfaces also cancel each other out, almost eliminating wave drag from thickness. Together, these two mechanisms reduce drag dramatically from a conventional supersonic aerofoil.

That works for the design point, but the Busemann Biplane has poor performance "off design", at Mach numbers other than the aircraft's intended supersonic cruise speed. As the aircraft has to accelerate through lower Mach numbers to get to its cruise speed, that's a pretty big disadvantage for the biplane configuration.

The problem is that the biplane acts like a nozzle and flow between the airfoils chokes at transonic speeds, dramatically increasing drag. As the aircraft accelerates, the choking persists to higher Mach numbers - a phenomenon called hysteresis. Only when the choking has cleared does the biplane generate less wave drag than a conventional supersonic airfoil.

Follow the sequence of images below, from M0.6 at the top left to M2.18 at the bottom left, and you can see a strong bow shock form and the biplane remain choked until the shock attaches to the leading edges and is swallowed - the desired low wave-drag condition.



Tohoku researchers are tackling the choking problem with variable geometry. They looked at three options (left to right, below): morphing to change the throat area between the airfoils; slats and flaps to move the leading and trailing edges; and a Fowler action to extend the wing chord. They selected the slats and flaps, which can also be used as high-lift devices for take-off and landing.



At MIT and Stanford, profs Qiqu Wang and Antony Jameson and postdoc Rui Hu took a different approach - modeling 700 wing configurations to come up with the optimum fixed shapes for the airfoils. Keeping the distance between the airfoils constant, the leading and trailing edges were moved until choking was minimized. In addition, the airfoils were modified from triangles to diamonds to further improve performance.

The result, says MIT, is an aircraft concept with half the drag of Concorde. According to Hu, compared with a classic Busemann Biplane, the optimized design has a smaller off-design wave-drag penalty from Mach 1.2 to Mach 1.5. Choking has disappeared before the wing reaches its design point at Mach 1.7, where wave drag is dramatically lower than for a conventional supersonic airfoil. The next step is to turn the two-dimension airfoil work into a three-dimensional wing model - something Tohoku has already done with its design.