Golden Era of Aeronautics, Part One

Image of Patrick Stoliker, Deputy Director of NASA Dryden Flight Research CenterPatrick Stoliker

Deputy Director 
NASA Dryden Flight Research Center

On May 2, 1952—virtually 60 years ago—36 people boarded BOAC’s De Havilland Comet DH 106, known as “Yoke Peter” to its crew for last letters of its registration G-ALYP, took their seats and readied for a long journey. These were the first paying passengers of the modern jet age, departing London for a 7,000-mile trip to Johannesburg, South Africa. Powered by four Ghost jet engines (also made by De Havilland) and with a cruise speed listed at about 500 mph, the passengers rode in pressurized comfort. About the only significant difference between that day and now was the legroom and the ratio of passengers to fight attendants. But if we pay too much attention to the similarities we’ll miss what is remarkable about the span of time involved. It’s been 60 years since the first passengers rode on a jet airliner and we’re still flying at the same speeds and the same altitudes.

There was a brief interval during which the wealthy could travel at Mach 2 on the Concorde (slightly faster if they wanted to fly the Soviet Tu-144), but neither airplane was ever a commercial success: both were just items of national prestige.

If it seems as though little has changed in the last 60 years—I said seems—consider how much changed between 1903 and 1952: call it the first 50 years of flight. It was on a wintry day in 1903 that two brothers from Dayton, Ohio, succeeded where no one else had, on the sand dunes of Kill Devil, North Carolina. Wilbur and Orville Wright were hardly the only ones trying to figure out flight at that time; there were quite a few in the US and in Europe, and most were further along than the two brothers. Even worse for the brothers, their competitors seemed to be better educated or better funded, as in the case of Samuel P. Langley of the Smithsonian Institution, or at least further along in the quest. 

The brothers made up for this with an intellect that the made their lack of a high school diplomas irrelevant. They learned the value of a wind tunnel by building their own, and they were smart enough to figure out that everyone before them had been wrong about the relationship of lift, wing area, and velocity, to say nothing of airfoils shapes. They had that rarest of talents, what Eugene Ferguson called “engineering in the mind’s eye,” the ability to move back and forth between the abstract and the concrete when trying to solve an engineering problem. They recognized propellers as rotating wings in the process. They understood wing warping and, more importantly, figured out the vertical stabilizer as the solution to adverse yaw, which they encountered on their glider, and they managed to do so largely because of how they mingled the abstract and the concrete. With Charlie Taylor, they put together a 180 pound, 12 horsepower engine with a good enough thrust to weight ratio to fly. It was an ungainly airplane, unstable in all axes, and I’m certain that only the hours of practice with the gliders gave them the skills needed to control the airplane. Then again, the same goes for riding a bicycle: it too, is unstable and takes time to learn how to ride, but you are then rewarded with a contraption that is nimble as can be, as opposed to a four-wheeled wagon. Progress came quickly. Although canard configurations have returned, the Wright’s pursued their original aircraft configuration until 1909, when they added an elevator and larger vertical tails. The 1910 Wright Model B had no canard and engines with 28-42 hp (with a production run of about 100 airplanes).

Their competitors settled into the more conventional wing/rudder/elevator configurations and everywhere advancements in capabilities and performance quickly followed (ditching wing warping in favor of ailerons was a way to control the airplane and try and avoid the Wright’s patent). Aircraft manufacturers proliferated across Europe and the United States. Although we think of this as the era of the biplanes (and triplanes), monoplanes like the Nieuport 2 were being built by 1910! The Loughead Brothers (later Lockheed) formed their first aircraft company in 1912 in Santa Barbara. The first aviation meet in America was held in Los Angeles (Dominguez Hills) January 1910 and airplanes were still so relatively new enough that dirigibles were a huge draw even then.

The speed with which aeronautical technology developed is astonishing and I consider this first decade one of the golden eras of aeronautics. We made big steps from the first ‘practicable’ airplane to modern aircraft. War had much to do with this, as it often does–in this case it was WWI. That war brought the first monococque fuselage, the interrupter gear, vastly improved flight instruments, the first UAV (the Kettering Aerial Torpedo, whose flight was made possible by Lawrence Sperry’s autopilot of 1914), the first all metal aircraft (Junkers J1, J2, and finally, the truly functional J4), just to name a few. The rapidity of the developments and their dramatic appearances likely makes us think war is the prime driver in such changes. But if we look at the bigger picture, this seems to be less the case.

After all, the Wrights achieved flight in a period of peace and were themselves not driven by any war, even if their first customer was the US Army Signal Corps. The first four-engine aircraft, Igor Sikorsky’s Ilya Mourometz appeared in 1914 in Russia, before The War. It was during the interwar years, the period between WWI and WWII, that NASA’s predecessor agency, the NACA (National Advisory Committee for Aeronautics) developed or assisted in developing some of the most fundamental changes to aeronautics, including retractable landing gear, the variable pitch propeller, deicing boots, engine cowlings, and the world famous NACA airfoils. It was in this same era that the first superchargers were developed to allow piston engines to gain higher altitudes (and speeds), and we saw the first genuine pressurized fuselages. It was in 1928—long before WWII began—that Englishman Frank Whittle conceived of the turbojet, and still before WWII that German Hans von Ohain succeeded in flying the world’s first turbojet, developed independently of Whittle. And while people first paid to travel by air before the War, it was in the interwar years that commercial aviation actually took off, so to speak.

Image of X-1WWII drove more aeronautical developments, of course, one of the subtler ones being the abandonment of seaplanes as the preferred way to carry passengers on long routes. The war resulted in plenty of new runways all over the world as well as an abundance of multi-engine aircraft with long range, making the big seaplanes a dying breed. It also helped that long-range navigation and radar approaches were improved and created, respectively, because of WWII. Piston engine/propeller aircraft had been having trouble with the transonic realm before WWII; figuring out supersonic flight was the next big challenge, and while folks in the Army Air Corps and the NACA expected it to be a big challenge, I don’t think they realized just how big the leap into the unknown would be. Getting to Mach 1 could be done with a turbojet engine, but if you were in a hurry to do it—and the AAF was—you were going to need a rocket plane. Jet engines of the era simply weren’t powerful or reliable enough to do the job-yet. This led to the Bell X-1, two X-1s actually, and the quest for supersonic flight. This was dawn of the golden age of flight research and X-planes, of almost infinite questions and purpose-designed aircraft like the X-3, the X-4, and the X-5.

Change came fast.

In 1961 TWA introduced in-flight movies on its Boeing 707s. By 1964 we had gone Mach 3 in a jet powered aircraft (A-12/SR-71), faster in rocket planes; we’d been to space and back in capsules and a space plane (the X-15), and supersonic flight was routine, at least for the military. In 1970 Boeing introduced first the “jumbo” airliner, the 747, and we’ve been packing in more and more passengers ever since. Yet we’re still flying at virtually the same speed and altitude as those 36 passengers on “Yoke Peter” in 1952. Have things really not changed? Are we in the doldrums, and if so, why?

Aerodynamic Trucks Make Me Smile

Image of David McBride, Director, Dryden Flight Research Center By David McBride

Director, NASA DrydenFlight Research Center

Over the last severalmonths, I have read many news stories and web accounts about rising and falling fuel pricesand how some companies are rediscovering efficiencies by making trucks more aerodynamically efficient. These makeme smile as it reminds me of the early aerodynamic truck studies conductedalmost 40 years ago at NASA’s Dryden Flight Research Center on Edwards AirForce Base. Fuel efficiency in long haul trucks was never much of an issueuntil the first peacetime gas crisis, in the early 1970s. In 1973 anaeronautical engineer at NASA Dryden began musing over ways to cut theaerodynamic drag of over-the-road trucks. He led a small team of researcherswhose results had an extraordinary, if little recognized, impact.

The center’s firstexperiment involved a passenger van modified into a driving laboratory. Weattached an aluminum rectangular box to the vehicle—hence the nicknameShoebox—and over successive experiments, changed elements of the box.We rounded the vertical and horizontal corners, sealed the entireunderbody including the wheel wells, and even added a “boat tail” tothe rear of the vehicle, finding out what benefits each had on the overallaerodynamic drag. Road tests of the Shoebox, with rounded vertical andhorizontal corners front and back, lowered the vehicle’s aerodynamic drag by 54percent. Sealing the van’s underbody and wheel wells reduced drag another 15percent. Road test showed a mileage increase of between 15 and 25%. Mileage mayvary, of course, depending on conditions and styles, which is why Dryden’sengineers were fond of testing outside of wind tunnels.

Image at right: 1970s van fitted with square corners, top image, and round corners bottom. Tufts of yarn attached to the sides of the van indicate air flow around the vehicle.

Our second experiment wasconducted on a cab-over-engine tractor-trailer, again modified by rounding allof its front corners and edges. In addition, technicians attached sheet metalfairings over the cab’s roof and sides, reaching as far back as the trailer;this completely closed the open space between the cab and trailer. While stilllooking like a tractor-trailer, it was a radical departure from anything on theroad in the 1970s. Researchers found that in highway driving at 55 miles perhour, these changes resulted in 20 to 24 percent lower fuel consumption over anidentical but unmodified tractor-trailer they tested against it.

At the time of NASADryden’s research, which extended off-and-on until 1982, the majority oflong-haul tractors were cab-overs. This was because the Federal Aid Highway Actof 1956 – formally known as the National System of Interstate and DefenseHighways Act – placed a limit on the length on the total vehicle.

Image to the left: As depicted by this 1975 image, sheet metal is attached to a heavy haul truck rounding the front corners of the truck and adding a fairing between the cab and the trailer of the truck.

The Surface TransportationAssistance Act (1982), which came primarily in response to the impact of thegas crisis on the trucking industry, required states to permit trucks withtrailers as long as 48 feet on both interstate and intrastate highways, andeffectively ignored the tractor altogether. This small detail of the bill wasresponsible for the shift from cab-overs to conventional engine-in-fronttractors, a much more fuel-efficient design because of its shape.

In 1985 Kenworthintroduced the T600, the first tractor manufactured with factory-built fairingsthat reflected the empirical research done at NASA Dryden. It is encouraging tosee the continuing improvements to the shapes of both tractors and trailerstoday, all of which reflect research conducted at Dryden at the time orperformed at three universities under Dryden’s guidance.

Hence, when you see fairingsthat narrow the gap between tractor and trailer, side skirts on the trailer, orboat tails on the back of trailers, you’re looking at the results, in part, ofNASA and NASA-sponsored empirical research whose benefits have a tangibleimpact on our daily lives. It is especially gratifying when I think of the veryreal increases in fuel efficiency these trucks have realized, and the benefitswe all derive as a result

Laminar Flow and the Holy Grail

By Al Bowers
Associate Director for Research
NASA Dryden Flight Research Center

For aerospace engineers, the holy grail of low drag means conquering laminar flow. NASA (and the NACA before us) has spent a LOT of effort and money to make laminar flow work in real-world applications, which would mean dramatic improvements in fuel efficiency.

Image: The black test section of the upper wing skin on this NASA Gulfstream III research aircraft has a line of miniscule bumps at the leading edge that allows the boundary layer airflow to remain stable and smooth over most of the wing’s upper surface. The tiny vertical airfoils mounted outboard of the black test section are vortex generators that keep the airflow attached over the wing surface at cruising speed.

Laminar flow is essentially the way airflow travels above and below wing surfaces. A certain amount of air turbulence occurs on the surface of most aircraft wings, regardless of their shape and size. As air moves across a wing, it’s altered by the friction between it and the wing’s surface, changing from a laminar, or smooth, flow at the forward area to more turbulent flow toward the trailing edge. The ideal would be laminar airflow across the entire surface of the wing with no sign of turbulence, which hinders flying performance by increasing aerodynamic drag and fuel consumption.

In various efforts dating back decades, NASA has attempted to achieve that ideal. Research by the NACA began in the 1930s with smoke trails photographed in a Langley wind tunnel and continued through the 1990s using such test beds as a Lockheed JetStar and an F-16XL. Today, a new program is getting under way at NASA Dryden that will use the center’s Gulfstream III aircraft and build on the work of the world’s most knowledgeable researchers in this area, Bill Saric and Helen Reed of Texas A&M University.

The idea Saric and Reed had is so good it’s simply sheer genius. It’s a known fact that if airflow is excited to a HIGHER frequency than the unstable frequency, waves are stable. Let me say that again: if waves are excited to a higher frequency, airflow is stable; that is, it remains laminar and does not immediately break down and transition to turbulent flow.

Saric and Reed’s simple but brilliant idea was to put bumps on the laminar-flow part of a test wing. By carefully adapting the size of the bumps to the depth of the boundary layer (that part of the air flowing next to the skin of the wing), a stable wave can be established in the boundary layer and this allows the flow to remain laminar for long runs (30 to 50 percent of the upper surface) over the wing. The Air Force Research Laboratory issued a grant to Saric and Reed for an experiment that flew to Mach 0.3, a lift coefficient of 0, and a Reynolds number of about 7 million, and showed laminar flow back to about 70 percent over a 30-degree swept wing.

Fay Collier, NASA’s expert in laminar flow, was so interested in their idea that he wanted to pursue it further. He was instrumental in getting the Gulfstream project funded to see whether laminar flow could be sustained at the full cruise flight conditions of a modern airliner. The goal will be to achieve significant runs of laminar flow at Mach 0.75, a lift coefficient of 0.3, Reynolds numbers of 25-30 million with laminar flow back to 60 percent over a 30-degree swept wing. These numbers correspond to those of medium-size airliners – somewhere between a 737 and 757. Dryden’s team will be focused on achieving that goal for NASA.

To do the job, NASA needed an airplane that had properties similar to aircraft in this size range and could be flown cost-effectively. The Gulfstream III fit a lot of the criteria. The G-III’s wing is big, and the aircraft cruises easily at the necessary flight conditions. Most important, should NASA achieve the proposed laminar flow runs, the promise of a 20-30 percent reduction in fuel burn might save a lot of fuel and energy.

Okay. Those of you who are truly interested in the technical aspects of all this and want to dive into the real nuts and bolts, keep reading.

So what was the big hold-up in the research all these years? Making laminar flow work in the real world isn’t easy. Minor imperfections in manufacture – things like ripples, wrinkles, rivet heads, bugs, small imperfections in shape, waves in the wing – all prevent laminar flow. Worse, many of these imperfections can be invisible in casual inspection by observers, and prevent laminar flow. And even if all these problems could be solved, it’s still possible to fail in achieving significant runs of laminar flow. It turns out that to cruise at Mach 0.7 to 0.8, the sweep of the wing is an enemy to laminar flow. And cruising at Mach 0.7 to Mach 0.8 is where we want to cruise with modern airliners.

In a straight wing, airflow is “pulled” along from the leading edge of the wing to near the wing’s point of maximum thickness, and this helps promote laminar flow. At the maximum thickness, airflow is at its lowest pressure (the low pressure on the upper surface is lower than that of the lower surface, and this pressure difference is the lift; discovery of this phenomena is attributed to eighteenth-century Dutch-Swiss mathematician Daniel Bernoulli). From the max thickness point back to the trailing edge, the air is increasing in pressure. This can be thought of as the air “coasting” uphill against the pressure. As the air does this, the subtle variations in the smoothness of the air are amplified. These small perturbations cause waves in the boundary layer and the flow abruptly breaks down and becomes turbulent. This turbulent flow “scrubs” against the surface of the wing and causes the skin-friction drag of the wing to rise dramatically. Turbulent flow isn’t all bad, as the additional energy in the boundary layer helps prevent flow separation from the surface of the wing (which would cause even more drag than the increased skin friction of turbulent flow). To maximize the amount of laminar flow on a straight wing, designers use very carefully tailored shapes to move the maximum thickness very far aft on the wing. Laminar flow runs of 70 percent on the upper surface and nearly 100 percent on the lower surface are possible if caution is used. The resulting drag is very low compared to conventional turbulent airfoils producing the same lift, as much as 70 percent less. So all this is on the straight wing.

A swept wing, which is necessary for flight at high Mach numbers (like Mach 0.7 to 0.8), has a different problem. In this case, the swept leading edge causes an immediate transition from laminar to turbulent flow. The culprit is called crossflow transition. As the flow meets the leading edge, it’s easier for the air to move along the leading edge with the sweep than for it to move over the wing, as it would have on an unswept (or straight) wing. So the flow starts out moving towards the wing tip, and then it curves over the upper or lower surface and finally moves aft toward the trailing edge. But once the flow starts out toward the tip in crossflow the boundary layer transitions from laminar to turbulent and, once transitioned, it is nearly impossible to make the airflow laminar or smooth again.

Remember those unstable “waves” in the airflow on the straight wing? The unstable waves in crossflow can be calculated, and are dependent on flight condition. One oddity is that these waves are inherent in the air, and not related specifically to the size of the aircraft; waves don’t scale up or down with the size of the aircraft – wavelength is an inherent property of air. So a T-38 and a 747 (if they had the same wing sweep and wing shape) would experience the same wavelength and pattern.

Saric and Reed’s idea resolved the question of what to do about these unstable crossflow waves. With the latest Gulfstream research effort, Dryden hopes to build on their accomplishments as well as on NASA/NACA laminar-flow research spanning nearly 80 years. Here’s hoping that we’re getting closer and closer to that holy grail of ideal conditions and greatly improved fuel efficiency, which will pay off in the form of reduced cost for all kinds of air travel.