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Issue 19 - 2002 : The Art of Aerodynamics
After years of rigorous engineering, using the most sophisticated modeling techniques in the industry, Dassault Falcon unveils a true advance in wing design. A new-generation, high-transonic wing technology for the 7X ultra-long-range business jet. Consider the dimensions of the 7X wing as compared to the 900EX: sweep is 34 degrees for the inboard portion of the wing and 30 degrees for the outboard section versus 29 and 25 degrees, respectively, for the wing of the 900EX. The wing sweep of the 7X is still moderate compared to some recent business jet designs, preserving the excellent handling characteristics of Falcon jets, especially at low speeds. Aspect ratio is 9.0 for the Falcon 7X versus 7.6 for the 900EX. The 7X wing is slightly thicker at the wing/fuselage intersection, but tapers across its span and is thinner at the tip than the 900EX wing. Lift-generating capacity, or lift-to-drag ratio (L/D), is improved 35 percent over the 900EX, allowing, among other things, for it to make a direct climb to Flight Level 410 after a maximum gross weight takeoff-a 4000-foot altitude gain over the 900EX at the same Mach number. The extra altitude margin will allow pilots to quickly climb above the most crowded airspace, allowing more direct routings and fewer delays. At .80 Mach cruise, the Falcon 7X's buffet onset boundary is 20 percent higher than the 900EX, thus allowing the 7X to fly efficiently at higher altitudes. A typical flight profile for a 5700 nautical-mile journey would include an initial climb to Flight Level 410, followed by step climbs to Flight Levels 450 and 490 before descending for landing. Optimizing the Transonic Realm When the Falcon 20 was introduced, the computer design tools simply did not exist to run the hundreds of iterations needed to identify "hotspots" and adapt airfoil design to smooth out airflow. "We are working with 30 years of CFD design experience combined with wind tunnel observations," notes Rostand. "We can look at what the latest CFD codes predict on, say, a Falcon 2000, which we then confirm in flight test. It's a crosscheck by which we assure ourselves of the accuracy of our calculations concerning the Falcon 7X. "CFD is a very useful tool. It can also give very wrong results if you don't use it correctly. We are checking our results constantly against previous CFD results and real-world flight-test data, continually improving the accuracy of this process. That is where all our experience on both Falcon jet aircraft and fighters provides us an advantage."  Indeed, Dassault began using rudimentary CFD technology in the design of the Falcon 10, which first flew in 1970. It then used the evolving technology and more powerful com-puters to create a "supercritical" wing design and completely CFD-analyzed airframe on the Falcon 50, which had its maiden flight in 1976. A supercritical wing is distinguished by very little camber (curvature) in the forward portion of the wing and a highly cambered (cusped) shape to the rear portion. Such an "aft-loaded" wing delays the onset of air compressibility, wave drag and attendant Mach buffeting, allowing efficient cruise speeds at higher Mach numbers. The supercritical wing of the Falcon 50 pioneered the same basic planform and airfoil shape as used on larger Falcon jet aircraft. "Our first-generation supercritical wing had moderate camber, which already brought a significant decrease in drag at high cruise speeds. We did not possess the design tools to fully optimize camber, particularly flow-wise, across the wing's chord, as well as span-wise," says Rostand. "Significant margins had to be maintained with regards to flow separation." New CFD codes allowed Rostand and his team not only to adjust camber fore and aft and along the wing's span, but to globally optimize the wing shape with regards to transonic characteristics. The result was an ability to tailor airflow and prevent transonic spikes in a uniform fashion over the whole wing area. "With our new analytical tools we also have a better picture of viscous, or boundary layer, drag-the slowing of airflow due to friction with the wing's surface. Total airfoil drag is equal to wave drag plus viscous drag. One is not more important than the other, but you have to predict all terms and find the right globally optimized aeroshape that gives you the lowest overall drag level." For the first time, Rostand and his team were able to fully assess the effects of aeroelasticity, which is the effect of wing deformation on aerodynamics. All wings are flexible to some extent. To optimize wing per- formance under varying aerodynamic loads, wing bending has to be taken into account. "Aeroelasticity is something we can now predict by simulation and also validate in the wind tunnel using pressure sensitive paints. These have been just recently developed and allow you to visualize the full pressure distribution on a wind tunnel model that has flexible wings just like a real aircraft." The Low and Slow End of the Flight Envelope Optimization, however, consists also of using modern design tools to permit the widest possible performance envelope. The Falcon 7X, accordingly, is an ultra-long-range aircraft (see intro) that can zip along at more than 500 knots and approach for landing at less than 105 knots at maximum landing weight.  Dassault long ago opted for a wing with mobile lifting elements (flaps and slats) over conventional fixed leading edges, which are always a compromise between addressing low-speed approaches and high-speed cruise. As with other Falcons, the 7X will incorporate internal and external leading edge slats. External slats will deploy automatically at high angles of attack and low air speeds to guarantee good handling characteristics up to the stall. The aircraft will also have double-slotted trailing edge flaps. When deployed, the 7X's high-lift devices will transform a high-transonic wing into a deeply cambered airfoil yielding three times more lift, and allowing for slow approach speeds that translate into more versatility and greater safety. Ramp Appeal While the Falcon 7X pioneers a number of new technologies for business aviation, its heritage is unmistakable. Dassault has always subscribed to the belief that efficient design is inevitably eye appealing. This is the hallmark of all Falcon jet aircraft and it will be no different in the Falcon 7X. Transonic Primer A transonic aircraft is usually designed to fly slightly below Mach 1, with localized airflow over the wings, and perhaps other parts of the airframe, exceeding Mach 1 during high-speed cruise. Modern swept-wing jets operate in the transonic realm. Straight-wing aircraft tend not to, though even at relatively low cruise speeds of .50 to .70 Mach, air flowing over the upper portion of a straight wing can accelerate to Mach 1. Recall that air accelerates more quickly over the upper surface of a wing, causing an area of low pressure above and high pressure below, creating lift. Even though the aircraft's cruise speed or freestream Mach number is well below the speed of sound, when airflow over some part of the airframe accelerates to Mach 1, the aircraft is said to have reached its critical Mach number. Above critical Mach, airflow disruptions known as shockwaves may form. Exactly when and where they form has much to do with airfoil shape. Shockwaves are caused by rapid pressure changes due to the compressibility of air. Air acts like a fluid, after all. The faster you push an object through it, the more it tends to well up, like the bow wave on a ship. On typical commercial jet aircraft, shock formation leads to two forms of drag: wave drag and viscous drag, the latter due to boundary layer separation as airflow no longer hugs the wing behind the point of shockwave formation. As angle of attack increases, in a turn for example, the airflow across wing and tail surfaces becomes unsteady. The interactions between the shock and boundary layers on those surfaces unleash cascades of turbulence, or buffeting. The Mach number at which airflow separation begins is known as the Mach divergence limit. To achieve substantial improvement in high subsonic cruise speeds, the Mach divergence limit must be pushed closer to the speed of sound. Transonic airflows present some of the most complex challenges in aerodynamics because of the behavior of air as it transitions from subsonic to supersonic speeds. Formulae have long existed to calculate the effects of airfoil shape on airflow velocity. In fact, they were first postulated by two 19th-century mathematicians named Navier and Stokes. But it has only been with the advent of high-speed computing that it has been possible to run hundreds of iterations using the Navier- Stokes equations to model the transonic airflow over an entire airframe. As computing power has increased, so has the ability to accurately and quickly model transonic airflows. Hence we come to supercritical wings. If critical Mach is the speed at which localized airflows reach supersonic speeds, then a supercritical wing is one designed to delay the onset of supersonic airflows, and to be capable of flight above the critical Mach number without significant drag penalty. While a number of newer business jets employ "supercritical" wings, it is useful to consider that they exhibit supercritical benefits to varying degrees. Put more bluntly, some are "draggier" than others. What Dassault engineers have achieved with the Falcon 7X is a supercritical wing of extreme efficiency. While we can all admire it as an intellectual accomplishment, it is also a fundamental determinant in the operating cost for day-to-day business travel. Consider the dimensions of the 7X wing as compared to the 900EX: sweep is 34 degrees for the inboard portion of the wing and 30 degrees for the outboard section versus 29 and 25 degrees, respectively, for the wing of the 900EX. The wing sweep of the 7X is still moderate compared to some recent business jet designs, preserving the excellent handling characteristics of Falcon jets, especially at low speeds. Aspect ratio is 9.0 for the Falcon 7X versus 7.6 for the 900EX. The 7X wing is slightly thicker at the wing/fuselage intersection, but tapers across its span and is thinner at the tip than the 900EX wing. Lift-generating capacity, or lift-to-drag ratio (L/D), is improved 35 percent over the 900EX, allowing, among other things, for it to make a direct climb to Flight Level 410 after a maximum gross weight takeoff-a 4000-foot altitude gain over the 900EX at the same Mach number. The extra altitude margin will allow pilots to quickly climb above the most crowded airspace, allowing more direct routings and fewer delays. At .80 Mach cruise, the Falcon 7X's buffet onset boundary is 20 percent higher than the 900EX, thus allowing the 7X to fly efficiently at higher altitudes. A typical flight profile for a 5700 nautical-mile journey would include an initial climb to Flight Level 410, followed by step climbs to Flight Levels 450 and 490 before descending for landing. Optimizing the Transonic Realm When the Falcon 20 was introduced, the computer design tools simply did not exist to run the hundreds of iterations needed to identify "hotspots" and adapt airfoil design to smooth out airflow. "We are working with 30 years of CFD design experience combined with wind tunnel observations," notes Rostand. "We can look at what the latest CFD codes predict on, say, a Falcon 2000, which we then confirm in flight test. It's a crosscheck by which we assure ourselves of the accuracy of our calculations concerning the Falcon 7X. "CFD is a very useful tool. It can also give very wrong results if you don't use it correctly. We are checking our results constantly against previous CFD results and real-world flight-test data, continually improving the accuracy of this process. That is where all our experience on both Falcon jet aircraft and fighters provides us an advantage." Indeed, Dassault began using rudimentary CFD technology in the design of the Falcon 10, which first flew in 1970. It then used the evolving technology and more powerful com-puters to create a "supercritical" wing design and completely CFD-analyzed airframe on the Falcon 50, which had its maiden flight in 1976. A supercritical wing is distinguished by very little camber (curvature) in the forward portion of the wing and a highly cambered (cusped) shape to the rear portion. Such an "aft-loaded" wing delays the onset of air compressibility, wave drag and attendant Mach buffeting, allowing efficient cruise speeds at higher Mach numbers. The supercritical wing of the Falcon 50 pioneered the same basic planform and airfoil shape as used on larger Falcon jet aircraft. "Our first-generation supercritical wing had moderate camber, which already brought a significant decrease in drag at high cruise speeds. We did not possess the design tools to fully optimize camber, particularly flow-wise, across the wing's chord, as well as span-wise," says Rostand. "Significant margins had to be maintained with regards to flow separation." New CFD codes allowed Rostand and his team not only to adjust camber fore and aft and along the wing's span, but to globally optimize the wing shape with regards to transonic characteristics. The result was an ability to tailor airflow and prevent transonic spikes in a uniform fashion over the whole wing area. "With our new analytical tools we also have a better picture of viscous, or boundary layer, drag-the slowing of airflow due to friction with the wing's surface. Total airfoil drag is equal to wave drag plus viscous drag. One is not more important than the other, but you have to predict all terms and find the right globally optimized aeroshape that gives you the lowest overall drag level." For the first time, Rostand and his team were able to fully assess the effects of aeroelasticity, which is the effect of wing deformation on aerodynamics. All wings are flexible to some extent. To optimize wing per- formance under varying aerodynamic loads, wing bending has to be taken into account.
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