August 16, 2023 - No. 034 In This Issue : US airlines scramble to avoid cancellations after massive recall of 1,200 jet engines : Benefits and Downsides of Geared Turbofan or GTF Engines : Safety Concern For Boeing 737 MAX : Maintenance issue leads to RV-3’s engine to quit in-flight : IT WASN’T THE ENGINE THAT DID IN THE WESTLAND WHIRLWIND — WHAT WAS IT? : Modern jets are essentially enclosed propellers powered by a turbine. Could a piston powered enclosed propeller reach similar levels of power/propulsion? Profile photo for Huang ZheYu Huang ZheYu : The Canadian jet that could’ve changed aviation history : The Strut Rides Again : Can A Blended Wing Body Airlifter Make The Military Cut? US airlines scramble to avoid cancellations after massive recall of 1,200 jet engines By Ariel Zilber August 14, 2023 11:13am Updated Note: See graphics and photos in the original article. Several US-based airlines are reportedly working hard to avoid flight cancellations after Pratt & Whitney announced a massive recall of some 1,200 jet engines. JetBlue, Spirit, and Hawaiian are among the companies that are shifting ground crew and changing flight schedules after last month’s announcement by P&W that it was removing the Geared Turbofan (GTF) engines that were found to be tainted with microscopic contaminants in a metal piece of its core. The contamination is said to pose a danger that could cause cracks in certain parts of the engine, according to the company. The GTF engine, which is said to be used by dozens of foreign and domestic airlines, is one of two that can be fitted to the Airbus A320neo, which is the top-selling aircraft in the world. European carrier Wizz Air and India’s Go First are among the carriers that have had to ground aircraft as a result of the engine recall. Spirit Airlines is among several US domestic carriers that are scrambling to avoid flight cancellations following a jet engine recall by Pratt and Whitney.CAROLINE Spirit, the seventh-largest domestic airline in the US, told investors earlier this month that the recall will leave the company with fewer aircraft to fly as well as an over-staffing problem which will impact operations in the fourth quarter and early next year, according to the Financial Times. JetBlue COO Joanna Geraghty, who helps run the country’s sixth largest domestic carrier, told investors earlier this month that the firm would look to lease engines in hopes of minimizing the damage. “We are trying to take whatever self-help measures are available,” she said. “But as you know, the supply is pretty constrained.” Hawaiian Airlines warned it might have to adjust its capacity, but said recently that it was too early to assess the impact. Pratt & Whitney announced last month it was removing the Geared Turbofan (GTF) engines that were found to be tainted with microscopic contaminants in a metal piece of its core. The GTF engine, which is said to be used by dozens of foreign and domestic airlines, is one of two that can be fitted to the Airbus A320neo, which is the top-selling aircraft in the world. The carrier said last month that the impact would depend on availability of the parts that require replacement. The limited availability of replacements for these engines had already limited Hawaiian’s ability to make full use of its Airbus fleet. It had grounded some planes due to lack of engines. P&W is a subsidiary of RTX, the aerospace and defense contractor formerly known as Raytheon Technologies Corporation. The contaminant found in the engine poses a risk that certain parts of the machine could crack. RTX CEO Greg Hayes told investors on an earnings call last month that the company plans to compensation airlines, though he downplayed the recall as “not an existential threat” to his company or its subsidiary. Nonetheless, Hayes acknowledged that the snafu involving the engines “will be expensive.” Shares of RTX were trading down by more than 0.5% on Wall Street on Monday. The Post has sought comment from P&W, JetBlue, Hawaiian, and Spirit. Pratt & Whitney’s parent company RTX acknowledged that the recall “will be expensive.” The air travel industry has already been beset by turmoil in recent months. Bad weather, staffing shortages, and technical glitches led to massive disruptions, flight delays, and cancellations over the Fourth of July as well as the Christmas holiday periods. Benefits and Downsides of Geared Turbofan or GTF Engines June 29, 2023 Mond Ortiz Note: See graphics and photos in the original article. We’ve all heard the recent news about the grounding of several Airbus A320neo family jets in the Philippines and elsewhere around the globe. Reports attribute these disruptions to a shortage of components for their Geared Turbofan (GTF) engines. Considering these are newer models leveraging the latest technology, you’d naturally expect them to be more reliable than their predecessors using conventional turbofan engines. So what makes these GTF engines, adopted in modern jet airliners like the Airbus A320neos, so distinct and yet more prone to maintenance? Let’s peel back the layers of GTF technology and understand its strengths, its pitfalls, and the reasons behind the maintenance challenges. Understanding GTF Engines and Their Role in Modern Aviation Airbus A320neo and Boeing 737 MAX, among the most advanced commercial jetliners, both employ GTF engines. However, the A320neos are unique in offering two GTF options, the PW1100G and the CFMI CFM LEAP-1A. The Boeing 737 MAX uses a single type of GTF, the CFM LEAP-1B. Each engine has its unique selling points, with the PW1100G excelling in fuel efficiency and the CFM LEAP demonstrating superior reliability. In the Philippines, both Philippine Airlines and Cebu Pacific opt for PW1100G GTF engines for their A321neos and A320neos. The AirAsia group’s A320neos and A321neos, on the other hand, are powered by CFM LEAP-1A GTF engines. Photo: Gary Sato Breaking Down the Mechanism of GTF Engines A geared turbofan is a specialized type of turbofan aircraft engine, equipped with a planetary gearbox situated between the low-pressure compressor/turbine and the fan. This strategic placement allows each component to spin at its optimal speed. The primary advantages of this design include substantial reductions in fuel consumption and operating noise. However, these benefits come with a trade-off – increased weight and complexity. A traditional turbofan engine consists of a single “low-pressure” or LP shaft connecting the fan, the low-pressure compressor, and the low-pressure turbine. This design necessitates a cap on the maximum speed for the larger radius fan, which limits the rotation speed of the LP shaft, and consequently, the LP compressor and turbine. In high bypass ratios, this calls for additional compressor and turbine stages to maintain optimal efficiency levels. In contrast, a geared turbofan employs a planetary reduction gearbox between the fan and the LP shaft. This design allows the LP shaft to operate at higher rotation speeds, cutting down the need for additional stages in the LP turbine and the LP compressor. The resulting improvements in efficiency and weight reduction, however, are slightly offset by the weight of the gearbox and the heat produced within it. Furthermore, there are considerations regarding manufacturing cost and reliability. GTF engines’ ability to run at lower fan speeds enables higher bypass ratios, leading to reduced fuel consumption and significantly less noise. The BAe 146, equipped with geared turbofans, remains one of the quietest commercial aircraft. Bin im Garten | Wikimedia Commons The Challenges Faced by GTF Engines Despite their superior technology and benefits, GTF engines, such as the PW1100G, have been plagued by issues ranging from oil leaks and inflight shutdowns to higher maintenance cycles. Compared to the robust CFM56 conventional turbofan engines, which require maintenance only after 20,000 cycles, the PW1100G necessitates maintenance every 6,000 cycles. However, manufacturers like Pratt & Whitney are consistently working towards enhancing the durability and reliability of their GTF engines. Notable progress includes the introduction of the latest- configuration Block D hardware, currently deployed in sixty percent of the fleet and projected to cover over ninety percent within the next two to three years. Block D enhancements comprise improved hot section durability, new erosion coatings, and the extension of rotating part lives. Significant strides have also been made in reducing engine removal rates on the PW1100G-JM for the A320neo family, thanks to a new oil seal design. Similar updates, categorized as Block D.1, are expected to be available in 2024 for the PW1500G and PW1900G engines serving the Airbus A220 and Embraer E2-series. Moving Forward with GTF Technology GTF engines, despite their occasional challenges, represent a leap in aviation technology that is here to stay and continue to evolve. Key players such as Pratt & Whitney and CFMI remain committed to enhancing the overall performance of GTF engines, spanning fuel efficiency, operational efficiency, and maintenance costs. As technology continues to evolve, we can expect GTF engines to become even more integral to modern aviation. Safety Concern For Boeing 737 MAX 11/08/2023 DANIEL FOWKES Stephen Brashear The Federal Aviation Administration has been forced to issue an airworthiness directive for the Boeing 737 MAX. The AD issued relates to a safety risk around the engine powering the 737 MAX, specifically through its anti-ice or EAI system only under certain conditions. CFM engines, specifically the Leap-1B engines, are the centre of the focus for this AD. The FAA says that if the system was to be used for five or so minutes or even more than during certain conditions, there is the risk that overheating can occur of the engine inlet inner barrel. Boeing has said that it supports the Federal Aviation Administration in its findings. While there isn’t a safety incident yet is aware that there is a potential for a portion of the inlet to exceed its designed temperature range. The American plane maker further adds that it is working diligently with customers impacted to deploy the appropriate measures to ensure no safety risk occurs. This is because Boeing has sourced the measures to mitigate the problem, but a more permanent fix is still required. An inlet loss can cause fuselage or window damage, which has potentially severe implications for those inside the aircraft and the stability of the 737 MAX being flown. From here, the process will be to find ways to mitigate the problem under a permanent fix can be determined, but the vision would be for this not to impact deliveries or broader operations. Maintenance issue leads to RV-3’s engine to quit in-flight By General Aviation News Staff · August 8, 2023 · This is an excerpt from a report made to the Aviation Safety Reporting System. The narrative is written by the pilot, rather than FAA or NTSB officials. To maintain anonymity, many details, such as aircraft model or airport, are often scrubbed from the reports. While descending into ZZZ from ZZZ1, I experienced light to moderate turbulence (nothing uncommon). While descending into ZZZ, I was a little high, so I called ZZZ2 to request a descent through their airspace into ZZZ. Shortly after I checked in, the engine on my Van’s RV-3 quit. I immediately ran a checklist ABC, airspeed pitched to 90 KIAS, then had to quickly decide where to land. I decided to continue towards ZZZ. I then ran some checks. First I checked mixture (full in), secondary fuel pump on, no change, then I checked throttle, no change, then I switched full tanks (left side had approximately six gallons) to right side (approximately eight gallons,) no change. I then checked ignition, with no change. I then immediately requested priority handling. I told them I had a total engine failure and would attempt to glide to ZZZ. I was approximately five to six miles out at approximately 6,500 feet MSL. I was able to safely glide to ZZZ and was assisted by ground personnel. No damage to myself, aircraft, or personal property. The home base for the aircraft is ZZZ, so aircraft was towed back to hangar and the culprit to the engine out was quickly identified. A ground wire came loose and landed itself onto the ignition switch (where both left and right ignitions attach), which caused both ignition systems to ground and, in turn, shut off ignition to engine during flight. Approximately two weeks prior to this incident the radio (which sits above the ignition switch on the panel) was worked on and the shielding to the wire harness was rewired. I believe it was this work that inadvertently caused the ground wire to become loose and ultimately land itself on the ignition switch and cause it to ground and shut off both ignitions simultaneously. Primary Problem: Human Factors IT WASN’T THE ENGINE THAT DID IN THE WESTLAND WHIRLWIND — WHAT WAS IT? The twin-engine, single-seat Westland Whirlwind could have been a formidable fighter, but one overlooked modification changed everything. By MATT BEARMAN 8/2/2023 A Westland Whirlwind shows what it can do on April 20, 1944. By then the airplane had been classified as a failure. At low altitudes the Whirlwind was formidable, but its performance declined as it climbed. (Charles Brown/RAF Museum, Hendon) It could have been a game-changer. The twin-engine, single-seat Westland Whirlwind, produced by a small company in southwest England, looked like a formidably potent weapon. The four 20mm cannon packed close together in the nose could take out a tank when nothing else flying could. It was also innovative. It had a bubble canopy, intakes in the wing’s leading edges, slats and Fowler flaps. It had a slab-sided fuselage over the wing, which was the ultimate solution to high-speed interference drag. When it first flew on October 11, 1938, the Whirlwind was arguably the fastest, most heavily armed fighter in the world. Today, very few have heard of it. Westland built only 114 and the Royal Air Force sent the Whirlwind to Scotland to keep it out of the fighting during the Battle of Britain. It has often been labeled a failure. The reason given has always been the inability of its two 885-hp Rolls-Royce Peregrine engines to deliver speed at altitude. The RAF’s testing program had given the aircraft a clean bill of health and a ceiling of 31,000 feet. However, as the first trickle of aircraft began to arrive with No. 25 and 263 Squadrons in 1940, service pilots began to question why the altitude performance wasn’t what it was during test flights. “It must be emphasised…that the performance of the Whirlwind above 20,000 feet falls off rapidly, and it is considered that above 25,000 feet its fighting qualities are very poor,” read one report. “The maximum height so far attained is 27,000 feet but on every occasion that a height test has been carried out there has been a minor defect, either in airscrew revolutions or in lack of boost pressure.” The reply from the technical director of the test facility was straightforward—the aircraft in service were identical in all respects to the one tested, so the difference couldn’t be explained. The Whirlwind’s own designer, the eccentric W.E.W. “Teddy” Petter, blamed a fall-off in boost pressures delivered by the superchargers with height at “twice the rate anticipated.” By doing so, he placed the blame with the Rolls-Royce engines division. The Whirlwind prototype undergoes testing in 1938 at the Royal Aircraft Establishment’s giant wind tunnel at Farnborough. (RAE, Farnborough) The key difference between the tested prototype and the Whirlwinds in service was considered too minor to be worth commenting on officially at the time. It was the propeller. The prototype sent by Westland to the RAF had a one-off Rotol propeller design, not the de Havilland/Hamilton propellers that the production Whirlwinds received. The metal blades of the de Havilland props were very thick for a high-performance fighter—they had a 9.6% thickness-to-chord ratio (at the standard measuring point 70% of the way out along the blade). For comparison, the Spitfire’s blades were similar, but at 7.6% the ratio was smaller. This wouldn’t matter at low speed, but in a climb at 15,000 feet the tip of the Whirlwind’s propeller moved at Mach 0.72. Here, the difference between the 6% ratio at the tip of the Spitfire’s prop and the 8% of the Whirlwind’s was literally critical, meaning the tips approached the speed of sound. But an even bigger problem came with the combination of a thick profile blade with a constant speed mechanism. That mechanism was—and remains—a widely used solution for keeping an engine turning at the optimum speed to produce maximum horsepower, whether the aircraft is moving slowly or at its maximum speed. This is done by changing the “bite”—the angle of attack of the propeller blades. Increasing the blades’ angle of attack increases the drag and thus the braking effect on the engine. To maintain a constant RPM at varying speeds, the pilot controls the propeller’s pitch. A constant speed unit automates the process, with the pilot setting the desired RPM. The unit senses if shaft speed drops, and “fines” the blades appropriately by changing pitch. The Whirlwind had two de Havilland constant speed units under its sleek cowls. Dynamic tests in 1938 showed that the massive onset of drag above critical Mach would cause the blades to pivot—reduce pitch—as the constant speed mechanism hunted for a lower-drag condition to maintain RPM. Mach number lowers with altitude, so as the Whirlwind climbed at a constant RPM, relative Mach over the blades increased. Moving steadily inwards, more of the blade “went critical” as the phenomenon of compressibility created shock waves, drag rose exponentially and the blade turned farther to compensate. The prototype had different propellers from those used in the production airplanes, the reason for the drastic difference in performance at altitude. (RAF Museum, Hendon) This could reduce the blade angle of attack beyond zero. Add in any amount of aircraft pitch (and in a climb at altitude the aircraft would be pitched several degrees higher than line-of-flight) and shock waves would run up and down the blades as they spun. Wildly varying dynamic pressures would pass into the ram-air intakes, which sit immediately behind the blades. The intermittently windmilling prop would produce fluctuating boost pressures on top of reduced RPM. It was very shortly after receiving the report about “very poor” fighting qualities above 25,000 feet that Sir Hugh Dowding, the head of Fighter Command, made his decision to keep the Whirlwind away from any fighting in the south, sealing its reputation as the fighter that missed the Battle of Britain. “The limiting factor in the present fighting against ME 109s in the South of England is the performance, manoeuvrability and climb at high altitudes, and a difference in service ceiling of 2,000 feet is a very important advantage,” Dowding said. “It therefore seems to me quite wrong to introduce at the present time a fighter whose effective ceiling is 25,000 feet.” Ultimately the cancellation of the Whirlwind in November 1940 was an economic decision. Rolls needed to concentrate on developing and producing Merlin and Griffon engines, and it was never too sensible (“extravagant,” as Dowding called it) to produce a fighter that required two engines to do what another might with one. Contrary to popular belief, the Whirlwind went on to serve successfully for another three years, unaltered, in the role of a low-level strike aircraft over the English Channel and occupied France. More than one veteran has commented that they felt comfortable taking on Fw-190s in 1943 in the unmodified, undeveloped 1938 Whirlwind. Down low nothing could catch a Whirlwind. It was maneuverable, practically viceless and its pilots learned to love it. There is little doubt that the thick blades with the wrong airfoil section held the Whirlwind back. By the time fighters were doing 420 mph and higher at altitude with two-stage superchargers and blade tip speeds of over Mach 1, the blades were very thin and had profiles that had been developed to negate compressibility completely. But by then the time had passed for the Whirlwind and the much-maligned Peregrines that powered it. Modern jets are essentially enclosed propellers powered by a turbine. Could a piston powered enclosed propeller reach similar levels of power/propulsion? Huang ZheYu It would be a terrible idea to do so. Here is a comparison. This is Lycoming XR7755, which was the biggest radial engine ever built. It had a diameter of 1.5 meters, weighted 2.7 tons and could deliver 5000 hp. For comparison, this is the core section of GE90, which is slightly bigger in diameter and weights 6 tons. But do you know how many horsepower it can deliver? The answer is 115,000 hp, which is over 20 times as powerful. Even putting 3 XR7755 together is in no match. So you can see that piston engines are nowhere near the power required to drive ducted fans. It’s interesting to note that turbofan was invented precisely because jet engines are way too powerful. You can see that late WWII fighters had more propeller blades than earlier models. For example, P-47, P-51, and F8F Bearcat had 4 blades while Spitfire Mk.24 and Hawker Sea Fury had 5 blades. The reason is that as engines became more and more powerful, propellers needed to spin faster to match the power output of the engine. However, if propellers are spinning too fast, they will break the sound barrier at the tips and cause a huge waste of energy. So instead of spinning faster, later models used more propeller blades to increase the thrust. However, jet engines are even more powerful by an order of magnitude, which means to make full use of their output, you need to add as many blades as possible to the hub and enclose them in a shroud, which enables them to reach supersonic speeds at the tips while not losing too much efficiency. That’s why modern jet engines use ducted fans. The Canadian jet that could’ve changed aviation history By Logan Nye Updated on Jul 28, 2023 Full-sized replica of an Avro Arrow on display at the Canadian Air and Space Museum, Downsview (Toronto). A Canadian jet captured the imaginations of aviation enthusiasts for decades, especially those within Canada, since the 1950s. The Avro Arrow looked to be the fastest and highest-flying jet in the world for much of its development. But complicated politics, time and budget constraints, and global competition eventually brought an amazing jet down before its production release. The road to the Avro Arrow We take it for granted today that the Cold War grew out of World War II. But the Soviet Union was one of the strongest Allied Powers. Not everyone prepared for war with the Soviet Union right out of the gate. When Canada reduced its aviation workforce in 1945 at the close of World War II, the British aviation company A.V. Roe went to Canada. It established a new subsidiary, A.V. Roe Canada, which quickly designed and produced the first Canadian jet fighter. The company built 692 Avro CF-100 Canucks. Soon, Canada asked for a much more ambitious aircraft: An all-weather nuclear interceptor capable of Mach 2 at 60,000 feet. And Avro came up with a futuristic design to make this possible. It used a delta-wing configuration, internal weapons bays, and fly-by-wire controls. Avro hired thousands of workers and subcontractors to make the plane possible. It built pre-production Avro Mark I planes for testing as well as a series of rocket-powered models. At the project's peak, over 20,000 workers toiled on the plane. When the Canadian public saw the Arrow for the first time, they fell in love. Over 14,000 people watched the public unveiling in October 1957. The first Avro Arrow, RL-201, is officially rolled out on 4 October 1957. Libraries and Archives Canada MIKAN 3596416, PA-210520. The unfortunate end of the Arrow Historians point to a number of potential causes for what happened next. America and Britain announced fighters that could fly higher than the Arrow at a likely lower price point. Ballooning costs for the project threatened other Canadian budget priorities. The Cold War nuclear threat transitioned from manned bombers to ballistic missiles. And the new prime minister of Canada, elected in 1957, reportedly had a bad relationship with the head of A.V. Roe Canada. Canadians sometimes blame America for the cancellation, though historians have found evidence that America actually planned to buy Arrows for the Royal Canadian Air Force to improve Canada's muscle against Soviet attacks. According to Canadian historian Jack Granatstein, "There’s no doubt that the American aircraft industry would have been exceedingly unhappy if the [United States] had bought aircraft from Canada, but to say that the Americans killed [the Arrow] is, I think, simply not true.” Whatever the constellation of causes, A.V. Roe attempted to find a new partner or buyer for the jet, but could not. And in February 1959, just 16 months after the jet's public debut to massive fanfare, Canadian Prime Minister John Diefenbaker canceled the project. The company president announced the project cancellation on February 20, 1959, with a profane rant over the loudspeaker where he called the prime minister, "That f------ prick in Ottawa." The Canadian aviation industry calls it "Black Friday." At least 14,500 people were directly employed on the project and lost their jobs immediately, with subcontractors and others quickly laid off right after. Modern estimates point to job losses of about 25,000, many of them highly skilled. The legacy of the Avro Arrow Many of those workers moved to the United States to work for NASA on the Apollo program. Everyone involved in the cancellation claims they didn't order the destruction of the project materials, but someone did. The pre-production planes got cut apart with blowtorches, blueprints were shredded or burned, and models were sunk into a nearby lake. This was reportedly to prevent the Soviet Union from stealing any research. A few items survived. At least one draftsman smuggled out his blueprints and some sections of the jet are now on display. But the plane is lost to history, and Canada's aviation industry never recovered. The Strut Rides Again In its next concept aircraft, Boeing floats a classic way to reduce weight. By Peter Garrison August 11, 2023 In January, NASA contracted with Boeing to build a proof-of-concept airplane called TTBW, for Transonic Truss-Braced Wing. [Courtesy: Boeing] The Cessna 120, introduced in 1946, bequeathed its strut-braced wings to nearly all of its successors, making struts and single-engine Cessnas almost synonymous. It wasn’t always so. In the 1930s, Cessna built airplanes like the C-34, a clean radial-engine four-seater with a cantilever wing. The demise in 1954 of the C-34’s all-metal descendant, the 190/195, left the Cessna universe to strut-braced singles. So things remained until 1967, when cantilever wings appeared more or less simultaneously on the 177 Cardinal and the 210. Struts and other external bracing were going out of use in fast monoplanes during the 1930s. Round wires generated as much drag as airfoils 10 times thicker. So-called “streamline tubing,” like that used for under-wing struts, produced more than 10 pounds of drag per square foot of frontal area at 100 knots. Since its pre-war designs had cantilever wings, why did Cessna revert to external bracing when it resumed business after the war? The likely answer lies in the interplay of power, weight, and drag. The high drag of external bracing was never a secret. Fokker and Junkers had already recognized it a decade after Kitty Hawk and were building some cantilever monoplanes during World War I. But if reduced drag was one path to higher performance, reduced weight was another, and external bracing reduced the weight of wings. So long as airplanes didn’t go too fast, wings could be skinned with fabric or very light gauge aluminum; the only path to further reducing their weight was to make the spars lighter by adding a strut, which greatly alleviated loads in the otherwise highly-stressed inner portion of the spar. Weight principally affects takeoff distance and rate of climb, whereas drag affects speed. For any airplane, there is going to be a crossover point at which the overall performance gained from a lighter spar is erased by the losses resulting from a strut. That may not matter, however, in general aviation at least, because there is an even more important crossover point at which sex appeal outweighs rational design decisions. Cessna evidently reached that point with the 210 in 1967. Its struts looked stodgy compared with the cantilevers of its principal competitors, the Beechcraft Bonanza/ Debonair and the Piper Comanche; ergo, the 210 got a new wing, cantilever—and with laminar-flow airfoils to boot. The Cardinal, which was introduced at about the same time, also got a cantilever wing, which it hardly needed because with 150 horsepower it was, although a sweet airplane in many respects, grievously underpowered—a poor climber and quite slow. Logic would have given the Cardinal a strut, but it was intended to be a modern successor to the dowdy 172, and the cantilever wing fairly screamed: “Modern!” That was more than half a century ago. Today the strut-braced wing is popping up again, this time in a most unlikely place: a proposed Mach 0.8 jet transport. Theoretical study has been going on for more than a decade to establish the feasibility of greatly reducing the fuel consumption of airliners by increasing the aspect ratio of their wings. The idea isn’t new. In the 1950s, a French firm, Hurel-Dubois, produced strut-braced transports with sailplane-like aspect-ratio-20 wings. Respected aeronautical engineer Werner Pfenninger proposed a transonic airliner on the principle in 1975. For a given wing loading and speed, induced drag—the drag due to lift—is inversely proportional to the aspect ratio. At the altitudes and speeds at which airliners fly, induced drag can be as much as half of the total drag of the airplane. Thus, doubling the aspect ratio of the wing could, in principle, reduce the total drag and fuel consumption by a quarter. These elementary facts are not lost on Boeing, Airbus et al. But there’s a hitch. A higher aspect ratio means greater wing root stresses and hence greater structural weight. Current designs, products of decades of computer-aided refinement, already represent optimal compromises between aspect ratio and weight. Re-enter, with fanfare, the lowly strut. If you put the fuselage under the wing rather than on top of it and add a Cessna-style strut around mid-span, you can not only greatly reduce the structural weight of the inner part of the wing but also reduce the chord and thickness of the entire wing—both changes reduce drag—and increase its span. In other words, the strut allows you to increase the aspect ratio without a weight penalty, and the induced drag reduction more than pays for the drag of the strut and the various junctions it adds. You can even afford to add a third member, a short “jury strut” running from the main strut to the wing, to further reduce structural weight. In January, NASA contracted with Boeing to build a proof-of-concept airplane called TTBW, for Transonic Truss-Braced Wing. The project, budgeted at over a billion dollars, will mate the somewhat shortened fuselage and the empennage of a McDonnell-Douglas MD-90 to a thin, short-chord wing with half again the span of an equivalent cantilever-wing transport. There will be challenges. Elasticity and possibly flutter are liabilities of a very long, thin swept wing; active controls that react instantly to changing flow conditions might be a solution. Diminished fuel volume is another disadvantage; some fuel may have to be carried in the fuselage. Yet another is a lack of space at airports; an airplane of abnormally long span would have to fold its wingtips. (The Boeing 777X has folding tips that reduce its span by 22 feet; the feature is thought to entail a weight penalty of around 3,000 pounds.) Illustrations accompanying the announcement of the Boeing contract show an MD-90 airframe with a very long, narrow high wing. The high wing provides plenty of ground clearance for big fat engines—good news if it’s a 737 replacement—but no place to mount landing gear. The artist has graciously omitted the bulging pods that will likely be required for that mundane purpose. Both NASA and airframe manufacturers like to publicize futuristic studies like this from time to time. The last Next Big Thing was the BWB or Blended Wing-Body, which was preceded by Boeing’s improbable Sonic Cruiser, a Mach 0.95 aircraft with a canard strongly resembling the hapless Beech Starship. The cycle reminds me of the “dream cars” that General Motors used to cook up in the 1950s for its annual motoramas. The interval between a novel conception and actual production is nicknamed the “valley of death.” The phrase comes from a Tennyson poem, “The Charge of the Light Brigade,” about a gallant but suicidal British cavalry charge against overwhelming Russian artillery in the Crimean War in 1854. The Sonic Cruiser quietly expired without flying in any form. The BWB got as far as a large RC model. The TTBW will become a full-scale test article. Let’s see if it fares any better than the Light Brigade. Can A Blended Wing Body Airlifter Make The Military Cut? Steve Trimble August 14, 2023 Partnering with Northrop Grumman, JetZero has proposed the Z-5 for the U.S. Air Force’s program to build a large-scale advanced tanker-transport demonstrator. Credit: JetZero A decision will be announced by the U.S. Air Force on Aug. 16 that could decide the future shape of the strategic mobility fleet. The Air Force will unveil the winner of a Defense Innovation Unit (DIU)-managed competition to build and fly a Boeing 767-sized, blended wing body demonstrator within four years. The demonstrator could inform decisions in the late 2020s on the next air refueling aircraft to succeed the KC-46, as well as replacements for the Lockheed Martin C-5M and Boeing C-17 in the decades ahead. The terms of the DIU solicitation are clear that the demonstrator will be a blended wing body (BWB), a three-decade-old concept that optimizes what would be the inboard section of a traditional wing to carry a payload and serve as an aerodynamic control surface. The demonstrator’s scheduled first flight in 2027 will make it a contender to fill the Air Force’s emerging requirements for a Next Generation Air Refueling System (NGAS) and a Next Generation Airlifter (NGAL). To be sure, the BWB still faces competition. The Air Force Research Laboratory’s (AFRL’s) Rocket Cargo program proposed to transport C-17-sized cargo or personnel anywhere around the world in 1 hr. with a suborbital space vehicle. DARPA’s ongoing LibertyLifter program aims to demonstrate an experimental, C-17-sized seaplane that can haul cargo or personnel across the Pacific mostly in wing-in-ground effect, but with the capability to fly up to 10,000 ft. if necessary. New ideas for standard tube-and-wing aircraft designs, including Boeing’s NASA-funded Transonic Truss-Braced Wing (TTBW) demonstrator, also could be in the mix, as well as incrementally better versions of standard military airlift designs today. But the BWB program has a few advantages for a new demonstrator program. Since McDonnell Douglas engineers conceived the concept more than 30 years ago, the design has been thoroughly researched, including data from 122 flights of the NASA-funded, subscale Boeing X-48 between 2007 and 2013. A follow-on plan to test a NASA-funded, full-scale demonstrator led nowhere. Congress cut NASA’s X-plane funding in 2018, forcing the agency to choose only one project to follow the Lockheed X-59 low-boom supersonic demonstrator. In 2020, the agency selected Boeing’s TTBW over the BWB for a demonstrator to prove out technology for the next generation of single-aisle airliners. Meanwhile, the Air Force favored more ambitious technologies, such as AFRL’s Rocket Cargo program. Starting in 2022, however, support for the BWB emerged from a perhaps unlikely source: the Air Force’s Office of Energy, Installations and Environment. During Aviation Week MRO Conference in April 2022, the office unveiled plans to launch a BWB demonstrator. “The BWB is one of the single most impactful technology opportunities for future U.S. Air Force aircraft, both in terms of capability improvement and greenhouse gas emissions reduction,” the Air Force said in a follow-up fact sheet released in June 2022. Congress offered full support. The office received a $41.9 million budget in fiscal 2023 to kick off the DIU competition. The Air Force requested another $88.2 million for fiscal 2024. The goal is now to verify that the promises of the BWB configuration—including 30% less fuel consumed than a C-17, and 60% more off-load capacity at range compared to a KC-46—are achievable. The demonstrator stops short of a mission-focused prototype aircraft. The selected BWB will not be equipped with a refueling system or a cargo ramp. The Air Force mobility fleet may need a new aircraft in two or three decades and perhaps a new tanker even sooner. The time has finally come to find out if a BWB aircraft will be competitive. Curt Lewis