Contrary to past thinking, there is no reason to believe that the effects of icing are cumulative in any linear or proportional sense. Larger ice shapes can have substantial effects on lift, drag and pitching moment. However, considerable work over the past sixty years has repeatedly determined that very little surface roughness is required to generate significant aerodynamic effects.
In all cases, these effects are dependent on angle-of-attack which, consequently, makes control of the angle-of-attack absolutely critical. Perhaps the most insidious aspect of icing effects is that there is often no aerodynamic warning of a departure from normal performance. A thin ice surface roughness, often characterized as hoar frost , can cause a very early and abrupt peak on the lift curve, followed by a precipitous drop in lift.
The insidious aspect of this is that, up until the peak is reached and flow separation occurs, the lift curve may be quite normal. Consequently, there is no reason to expect aerodynamic warning. Further, the flow separation can occur well in advance of artificial stall warning. Testing has indicated that the drag curve for hoar frost may yield little degradation until the angle of attack is very near that at which the early stall will develop.
Even then, the increase in drag may be easily manageable. Hoar Frost has been identified as causal in a large number of accidents. Typically, this results from the flight crew either not detecting the thin ice accretion before increasing the angle of attack, or the flight crew delaying operation of the ice protection system until a minimal ice thickness has developed.
Larger ice shapes may create similar effects.
In some cases, the lift curve may peak at an angle of attack substantially lower than that for Hoar Frost. This reinvigoration will require an increased level of investment as well as careful assessment of investment strategy. A strong need exists to strengthen the weakest link in the technology development chain: namely, technology validation for risk minimization. In recent years, the National Aeronautics and Space Administration NASA has recognized the importance of technology development needs for high-speed civil transports HSCTs , advanced subsonic transports, and hypersonic vehicles of all types, as well as the need to maintain its facility capabilities through the Wind Tunnel Revitalization Program.
In each of these areas, however, overall constraints have limited NASA's ability to establish the kind of aggressive research efforts needed to maintain U. This chapter charts a course for NASA, industry, and academia to pursue toward the specific aerodynamics goals needed to achieve competitiveness. The boxed material summarizes the primary recommendations that appear throughout the chapter, with specific recommendations given in order of priority, and the benefits that can be gained through aerodynamics research and development.
Although discussion of individual technical disciplines is facilitated by categorization, progress in technical fields is enhanced by an interdisciplinary approach rather than by the more traditional sequential application of various technical skills. In the past, the disciplines of. General NASA must continue to provide the necessary resources for aerodynamics research and validation, including resources focused on specific key technologies, resources to maintain and enhance ground and flight test facilities, and resources for enhanced analytical and design capabilities.
The following research topics should serve as the focus of NASA's research effort in aerodynamics:. The following should be the focus of a program to enhance NASA's ground-based experimental facilities:. The following should be the focus of a program to enhance NASA's experimental flight facilities:. Propulsion system analysis proceeded independently and was integrated later into the design. Such a process requires several cycles of iteration to converge on a suitable design, and. Also, not only does this procedure not guarantee an optimum configuration satisfying multiple design constraints, it almost precludes such a configuration because the efforts devoted to the initial steps in the design process become progressively harder to change.
In some cases, cost and time constraints preclude more than one or two iterations. The increasing maturity of computational approaches in the various design disciplines provides new opportunities to couple the disciplines more tightly earlier in the design process. Routinely treating aerodynamics, structures, propulsion, and controls virtually simultaneously and continuously throughout each step of design has very large payoffs that fall into several categories.
First of all, the resulting design is truly optimized and is, therefore, superior to those derived through the older, sequential approach.
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A second major payoff is a significant reduction in the time required to evolve a final design. Attainment of this goal is critical to the success of the transport aircraft industry.
NASA has recognized the potential advantages of multidisciplinary analyses and plans to focus considerable attention on this in the Computational Aerosciences portion of the national High Performance Computing and Communications program, which was initiated in fiscal year The goal for the s is to develop the capability to computationally couple aerodynamics, structural response, propulsion system effects, and active controls into a single computation and to structure the resulting codes to take advantage of massively parallel computational system architectures that are expected to be in widespread use after the mids.
These NASA efforts are highly endorsed by the Committee and complement similar industry activities that will include manufacturing considerations in the trade-off decision-making process. The impact of rapid design processes that allow time for examining the trade-offs between various disciplines was evident in the design of the folding wing tip of the Boeing Three configuration options were examined: an external hinge, an internal hinge retaining the original wing surface contours, and an internal hinge with a local thickening of the wing.
An early wind tunnel test provided some baseline aerodynamic information, but the subsequent design iterations and final design decisions were made rapidly by using computational fluid dynamics CFD. A final verification wind tunnel test could be performed only after the design was frozen.
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This use of computational aids is being extended into multidisciplinary analyses coupling aeroelastic effects with structural analyses. The use of CFD for preliminary aerodynamic load predictions early in an airplane development program will significantly shorten the development cycle.
Use of CFD to permit flutter prediction early in the design process will reduce reliance on traditional after-the-fact remedies such as mass balancing. Early in this study the Committee decided not to address the hypersonic flight regime and to set the upper limit of vehicle speeds to encompass the HSCT. This section of the report, therefore, does not address the aerodynamics of hypersonic flight. Before moving to the areas considered, however, some comments on the state of hypersonic research in the United States are appropriate.
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Current U. Today, the nation's hypersonic capability is largely applied to the National Aerospace Plane NASP program, which is focused on the specific objective of the technologies necessary to design and build the X aerospace plane capable of single stage to orbit. As a consequence, few resources are available for generic hypersonic research that does not support NASP. The key aerodynamics technologies have been divided into the following categories, each of which is discussed in corresponding sections of this chapter:.
Low speed and high lift for subsonic configurations : This portion of the chapter examines the approach and takeoff flight phases for subsonic aircraft, encompassing high lift system performance in detail and overall takeoff and landing performance in general. Noise is included because this is the flight phase in which it is most troublesome. In addition, the integration must also recognize the need for low noise emission during takeoff and landing.
Lower drag relates directly to lower fuel costs and higher profits for the air carrier. Low speed and high lift for supersonic configurations : Supersonic aircraft shapes are strongly influenced by the need for economical supersonic cruise performance. Virtually all the design decisions to improve performance in this.
The requirements for improving the technology base for the design of supersonic aircraft are discussed in this section. For supersonic designs in particular, aerodynamic interaction between the airframe and the propulsion system is a critical task, made more difficult by the need to minimize takeoff engine noise.
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Aerodynamics of rotorcraft : Aerodynamics of rotorcraft and tiltwing aircraft are especially complex, because they require investigations of exceptionally wide speed ranges and varying angles of airflow. Test facilities : The numerous new or updated flight and ground test facilities that will be required to accomplish various technical goals set forth in the report are discussed in this section. The use of computational tools and associated computers that can predict complex flow fields around aircraft in all speed ranges is also of growing importance.
Specific applications of CFD are discussed in detail in the preceding categories, but this section of the chapter contains a more general summary. A generally recognized and acceptable measure of aerodynamic efficiency is the lift-to-drag ratio.enter site
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The efficiency and effectiveness of the low-speed, high-lift systems employed by subsonic jet transports for takeoff and landing have a major impact on overall economic performance. The payoff from continued improvements in high-lift systems is large. For an airplane the size of the forthcoming Boeing , a small increase in lift coefficient, 0. Simplifying the geometry of the flap system while maintaining aerodynamic performance yields large benefits in terms of cost and maintainability.
The Basic Principles of Aerodynamics
One of the stated goals for subsonic transport design is the reduction of acquisition and maintenance costs by 25 percent relative to current production airplanes. Future advances in high-lift system technology can contribute significantly to the achievement of this goal. Because of the complex physics and complex geometries associated with low-speed flight, CFD has not had a major impact on the technology of high-lift system design.
The primary design tool has remained the wind tunnel, with some design guidance from CFD. Work over the past decade has verified the strong influence of Reynolds number on aerodynamic performance of high-lift systems—aerodynamic performance of high-lift systems in many cases does not scale predictably with Reynolds number. Figure shows the experimentally measured maximum lift coefficient of a simple swept wing over a range of Reynolds numbers, which displays large and unpredictable variations in maximum lift with Reynolds number. Such experiments demonstrate that the best aerodynamic performance and lowest risk can be achieved only by carrying out the aerodynamic design and validation at flight Reynolds numbers.
Because of this, airframe companies do their development work in high Reynolds number wind tunnels—which, for the design of the new Boeing , meant extensive development work in European wind tunnels. The following developments in the technology for advanced high-lift system designs are needed:. Improved understanding and measurement of the detailed flow physics at wind tunnel and flight Reynolds numbers : A better understanding is needed of turbulence and its modeling; of boundary layer transition, laminar bubbles, turbulent reattachment and relaminarization phenomena; and of merging boundary layers and wakes, including the detailed behavior of the viscous layers and wakes in the high adverse pressure gradient region above the training edge flaps.
Some progress is being made in the United States in this area of research. In contrast, the Europeans have greatly exceeded U. For the United States to compete in this area, greater investment is needed in the experimental capabilities and flow physics studies that lead to breakthroughs in high-lift capabilities. Improved CFD : The challenge is formidable in view of the complex physics, complex geometries, and numerous length scales that must be resolved numerically.
Pacing items are the flow models modeling of turbulence, transition, bubbles and reattachment, merging sheer layers, and relaminarization ; the sheer size of the computational problem, including proper numerical resolution of all the important length scales of the physics; and code validation, particularly at flight Reynolds numbers where few data exists. What is needed is a CFD development program that is closely coordinated with other key elements of flow physics and high Reynolds numbers testing, and which is constrained to be economically viable and run able in a timely manner when hosted on today's computers.
The need for such validation has been recognized for some time as critical to the maturation of such computational capabilities for ultimate use in design method applications. Experimental research and developmental testing at the highest achievable Reynolds numbers : There is a major need for a low-speed, low-disturbance testing facility in the United States, one that produces the highest reasonable Reynolds numbers at the right Mach numbers and closely simulates the freestream environment of flight with levels of productivity that are needed to support developmental testing.
It is no longer acceptable to develop a candidate design at low Reynolds numbers and validate it at high Reynolds numbers. There is also a strong need for obtaining aerodynamics measurements at flight Reynolds numbers on full-scale flight vehicles.
Nonintrusive, highly accurate, and responsive instrumentation is urgently needed to make detailed measurements of various flow conditions on and away from the surfaces of a test article. The current level of U. In , NASA developed a broad plan for research in advanced aeronautics measurement technology; unfortunately, the plan was never implemented.
Other phenomena associated with high-lift systems are airframe noise and wake vortex prediction and alleviation. Continuing advances in engine noise reduction will mean that the airframe is responsible for a growing portion of the overall noise profile of an airplane.