The field of configuration aerodynamics encompasses the role of aerodynamics in aircraft configuration design. Even though all aerospace engineering students take the basic aerodynamics theory classes, several topics related to applying the theory to aircraft design are usually omitted due to lack of time, and some other topics need additional reinforcement. This book serves as a bridge between theory and practical applications. The target audience is engineering students who wish to engage in the aerodynamic design of aircraft configurations. 

1.1 Purpose of Configuration Aerodynamics

Experience has shown that the basic, analysis-oriented aerodynamics class doesn’t provide an understanding of how to approach the configuration aerodynamic design process. Although many of the classical approaches to aircraft design are now highly refined, we frequently have to investigate innovative designs, some examples of which are presented in section 1.2. We need to be able to understand these innovative designs and predict their performance potential. To develop an appreciation of these concepts, we must assess them using first principles. Many times, concepts are proposed that don’t obey the laws of physics. A successful design must!

To get some insight into aerodynamic thinking, it is worthwhile to read about the experience of one of the more successful aerodynamic designers, Irving Waaland, whose 1991 Wright Brothers Lecture shows how an aerodynamicist approaches design.^[1]

1.2 Examples of Innovative Configuration Concepts

This section includes three innovative aircraft concepts to illustrate the type of configurations that aerodynamicists are called upon to investigate. Two of the concepts, covered in sections 1.2.1 and 1.2.2, have the potential to reduce the cost of subsonic commercial transportation. The third, in section 1.2.3, has the potential to increase the efficiency of supersonic flight. In section 1.2.4, we present additional examples of a range of proposed concepts, many developed by student design teams. Some of these are impractical for aerodynamic or other reasons. Yet all have required an aerodynamic assessment, and several could benefit from a better understanding of the aerodynamic design principles based on the physics of flow fields.

1.2.1 The Blended-Wing Body

The blended-wing body (BWB) concept, shown in figure 1-1, combines the fuselage and wing into a concept that offers the potential aerodynamic advantages of the flying wing while providing the volume required for commercial transportation. This configuration offers the potential for a large increase in aerodynamic efficiency characterized by the lift-to-drag ratio, L/D, and an associated large reduction in fuel use and in maximum takeoff gross weight (MTOGW). The major overview was given by Robert Liebeck,^[2] who predicted that the BWB has an 18% reduction in MTOGW and 32% in fuel burn per seat compared to the then-proposed A380-700. This Robert Liebeck paper is an archival publication of Liebeck’s Wright Brothers Lecture at the 40th Aerospace Sciences Meeting and Exhibit, Reno, Nevada, in January 2002.

Note that the BWB does not have large moment arms for generating control moments and also requires a nontraditional passenger compartment. That is why the design is more difficult than traditional designs and requires the use of multidisciplinary design optimization methods to obtain the predicted benefits.^[3] Roman et al. have demonstrated a significant speed advantage over other commercial transports.^[4] Because of the advantages of this concept, it has been extensively studied by many design groups. More references on the BWB concept are available on one of my websites.^[5]

1.2.2 The Strut-Braced Wing

In the early 1950s, Dr. Werner Pfenninger, Swiss-born aerodynamicist, suggested the truss-braced wing as a promising concept for long-range transport missions. His extensive research in laminar flow control (LFC) using suction demonstrated significant reductions in skin friction drag on wings, even at high subsonic Mach numbers and high Reynolds numbers. LFC prevented laminar separation, and it extended laminar flow up to 100% on some wings. To take full advantage of the reduced skin friction (or parasite) drag, the induced drag needs to be significantly reduced as well; this is because the maximum L/D occurs when the induced and parasite drag are equal.^[6] In the mid 1970s, Pfenninger shared his research on design considerations for LFC-enabled large global-range high subsonic speed transport aircraft at a special course organized by AGARD.^[7] This research formed one part of a comprehensive lecture on Laminar Flow Control Laminarization that he gave at this special course. In 1992, Pfenninger and Vemuru shared their insights into the design philosophy of long-range LFC transports with advanced LFC airfoils.^[8] The key observations and issues may be summarized as:

• The tight coupling between structures and aerodynamics requires the use of modern multidisciplinary design optimization (MDO) methodology to make it work.
• The strut allows a thinner wing without a weight penalty, with a higher aspect ratio, and with less induced drag.
• Reduced thickness-to-chord ratio (t/c) allows less sweep without a drag penalty.
• Reduced sweep leads to even lower wing weight.
• Reduced sweep allows for some natural laminar flow and thus reduced skin friction drag.

The benefits of this concept are similar to the benefits cited for the BWB configuration in section 1.2.1. This concept has the advantage that it doesn’t have to be used on a large airplane. The key issue is the need to provide a mechanism to relieve the compression load on the strut under negative g loads. Work on this concept was done at Virginia Tech.^[9],[10] Figure 1-2 shows the design resulting from a joint Virginia Tech–Lockheed Martin study. More references on strut-based concepts are available on one of my websites.^[11]

1.2.3 The Oblique Wing

Supersonic airplanes are especially difficult to architect for efficient flight. In addition to the issue of spanloading for low induced drag, the designer has to contend with supersonic wave drag associated with the volumetric distribution and the wave drag due to lift. One unique concept proposed by R. T. Jones^[12] helps resolve the aerodynamic dilemma. Jones points out that using an obliquely swept wing allows the lift to be distributed longitudinally in an elliptic distribution as well as laterally. At the same time, the maximum cross-sectional area can be reduced and the area distribution can be very smooth. Aerodynamically this is the right configuration for supersonic flight. Of course, the unusual asymmetric configuration introduces other challenges that must be overcome. Numerous models were built and flown to demonstrate that these configurations could, in fact, fly. One small manned aircraft, the AD-1, was built and flown successfully, as shown in figure 1-3.^{[13],[14],[15]}

Because of the possible adverse wing-body interference associated with this configuration, the natural choice for the oblique wing is a flying wing configuration. This has been the focus of further configuration studies.^[16],[17] More references on oblique-wing concepts are available on one of my websites.^[18]

1.2.4 Additional Examples

Figure 1-4 shows an example of the strut-braced wing (SBW) concept implemented on an A-7 for possible demonstration in 2000 by a Virginia Tech student team, Team REVCON, consisting of Chad Leigh, Josh Frommer, Conor Haines, Eric Kromer, Chris Lampman, Adam Lessey, Todd Norell, Chris Rourke, Joe Vasquez, Kurt Werner, and Han Woo. The team derived their motivation from ongoing SBW studies, as mentioned in section 1.2.2. This model was included as one of the examples in a Virginia Tech report^[19] prepared for NASA LaRC under contract.

The arch-shaped strut concept, presented in figure 1-5, illustrates the result of some imagineering. (Imagineering is an American English word that is a combination of imagine plus engineering. Collins dictionary defines this word to mean the implementing of creative ideas into practice.) It is a concept by Prof. Joe Schetz of Virginia Tech. The model was made using rapid prototyping.

Figure 1-6 shows a concept that is the result of a collaborative senior design project between Loughborough University (in the UK) and Virginia Tech. This concept draws on the idea of increased efficiency made possible by the use of the box-plane concept.

Figure 1-7 is also from a senior design project.^[20] It is a roadable aircraft, sometimes called a flying car. There has been additional study into some of the details of this concept and its further refinement.^[21]

The next example is Askin Isikveren’s innovative X-Wing concept^[22] known as TOLS (Twin Oblique Lifting Surfaces). Shown in figure 1-8, this concept is intended to take advantage of the strut-braced wing concept by using the engine pylons to reduce wing weight and to employ the advantages of an oblique/forward-swept wing to reduce drag (described in more detail in Isikveren’s AIAA paper).

Figure 1-9 is an example of the so-called inboard wing configuration^[23] based on the concept initially proposed by Leroy Spearman.^[24] This concept is intended to address the needs of very large aircraft. Spearman expected to reduce, or even eliminate, induced drag by having the fuselages act as endplates to the wing. An assessment of the viability of this concept requires the aerodynamicist to test the validity of Spearman’s flow hypothesis. Note that although we don’t show it, there is another variation of the twin-fuselage concept due to Houbolt with wings extending outboard of each fuselage.^[25] That concept may have a stronger basis in physics.

Figure 1-10 shows the Boeing Sonic Cruiser studied by Boeing as a potential new aircraft concept in 2001 and 2002. Hepperle^[26] performed independent analysis of this configuration. Boeing dropped the Sonic Cruiser from further development in 2003.^[27]

Figure 1-11 shows the Bird of Prey, a concept built and flown by the Boeing Phantom Works in the 1990s, presumably as a stealth concept.^[28]

There are myriad other examples, including UAVs, from micro UAVs to Global Hawk. Figure 1-12 shows one of them: the AeroVironment Black Widow micro UAV.^[29] This concept is remarkable, as it has flown for nearly an hour while sending a video feed back to a base station.

Figure 1-13 is an artist’s rendering of the X-65 technology demonstrator being developed by Defense Advanced Research Projects Agency (DARPA) under its Control of Revolutionary Aircraft with Novel Effectors (CRANE) program.^[30] Virtually every aircraft since the Wright Flyer of 1903 has used a system of movable, external control surfaces for flight control. The X-65 breaks this design paradigm by using jets of air from a pressurized source to shape the flow of air over the aircraft surface, with active flow control (AFC) effectors embedded in several flying surfaces to control the plane’s roll, pitch, and yaw moments. The elimination of external moving parts is expected to reduce weight and complexity and to improve performance of future operational aircraft.

The X-65 is being developed by Aurora Flight Sciences under a DARPA contract. The aircraft has a unique design with a distinctive joined tandem wing in a diamond configuration. The wingspan is around 9 meters (30 feet). It has twin vertical tails, and is powered by a single turbojet engine. Its weight is targeted to be around 3,200 kilograms (7,000 pounds), and its cruise speed is targeted to reach up to Mach 0.7. It will have modular construction so that the AFC effectors and wing shapes can be changed relatively easily for test purposes. The aircraft will initially be equipped with both traditional flaps and rudders, as well as the AFC effectors, to provide a baseline for performance comparison. Flight testing is expected to start in late 2025. Anyone interested in learning about the fundamentals of modern flow control technologies and their applications may check out the book by Joslin and Miller.^[31]

This brief survey of current unconventional concepts being investigated illustrates the wide range of shapes that have garnered interest. All require an aerodynamic assessment of their potential performance.

1.2.5 Morphing Airplanes

We’ve seen that specific airplane configurations are related to specific requirements. What if an airplane could change its shape in flight to respond to different flight requirements? The oblique wing configuration shown in figure 1-3 is one example with a variable-sweep wing that changes sweep to best meet the flight performance requirements. There is considerable interest in possibly using advances in structures, actuators, sensors, and controls to design an airplane that can achieve significantly increased performance by changing its shape in flight; this process is called morphing.^[32] This is a true multidisciplinary design activity, and a number of different advanced concept investigations fall within this general category, including recent work at Virginia Tech.^[33]

Although a wide range of configuration options are discussed in this section, there is still room for dreamers. We don’t yet know what the ultimate airplane concept is! Concerning the configurations described here, all of them are appropriate in the right circumstance. After all, they always say, There is a time and place for everything.

1.3 Decision Issues

With the airplane’s mission requirements and configuration options discussed above in mind, designers create a configuration that best satisfies the requirements. The following list summarizes the possibilities that have to be considered.

• Should it be a canard? aft tail? tailless? three-surface? sensitivity to CG travel?
• Should the wings be swept? forward or aft?
• Should it be a high aspect ratio, low aspect ratio, or slender wing?
• What is the control concept? Should it be stable or unstable?
• Should thrust vectoring be used as a control? How is the propulsion system integrated?

Designers make decisions about the specific options to incorporate into their configuration based on an understanding of the connection between the mission requirements and the strengths and weaknesses of the various configuration concepts.

Knowledge of previous designs is important in making these decisions. As an example, consider the Joint Strike Fighter (JSF) competition of the 1990s. Back in the 1960s, the US had tried to design a plane to be used by both the Air Force and Navy. The result was the F-111, which proved to be too heavy for use on a carrier. The JSF project stipulated a requirement that the new plane should be designed to meet the needs of the Navy, Air Force, and Marines. As it turned out, the resulting F-35 is not a single airframe, but comes in different versions for each service, with considerable amount of commonality among them. Anyone interested in learning more about the F-35 project should consider reading a book published by AIAA in 2019 titled “The F-35 Lightning II: From Concept to Cockpit” authored by Jeffrey W. Hamstra.

Another example of a key design constraint is the eighty-meter gate box-based span limit that affects the A380’s design. If you simply scale up previous Airbus, designs you discover that the wingspan would be much larger than 80 meters. So it is clear that aerodynamic efficiency has been compromised to meet the span limit constraint.

1.4 Design Approaches

1.4.1 Traditional approach

A matrix of concepts is usually defined by the designers in trying to create the best concept to satisfy a design requirement. The concepts are then evaluated against each other. There is not usually enough time to make a thorough evaluation of each design concept, and often not much detail is used. Relatively crude approximate analyses are used to choose “the configuration.” Once the concept is selected, more detailed design studies begin.

At this point, the disciplinary specialists (including the aerodynamicists) on the design team are given the basic design parameters such as the wing planform (sweep, aspect ratio, etc.) and maximum t/c, among others. At this stage, there is usually limited opportunity to change the basic design parameters. The aerodynamicist defines the detailed airfoil shapes, twist and camber, high lift system, and for supersonic aircraft, the area ruling.

1.4.2 Multidisciplinary Design Optimization (MDO) Approach

This is a new approach to computational design technology that seeks to use the more detailed analyses and design methods typical of preliminary and detail design much earlier in the design process. This allows the configuration to be defined using much more accurate information, with less uncertainty and risk. This approach has become known as MDO. A classic example that can be addressed using MDO is the aerostructures trade-off:

• A thinner wing is better for aerodynamics, worse for structures (fuel weight versus structural weight).
• More sweep reduces transonic wave drag, increases wing weight.

Numerous airplane design research studies using MDO have been done recently. We refer interested readers to two representative examples.^[34],[35]

1.5 The Role of Aerodynamicists Within the Overall Design Process

Aerodynamicists play a pivotal role throughout the entire aircraft design process because they “own” the outer mold line (OML), as asserted by Nicolai and Carty.^[36] If you are an aerodynamicist on the design team, you have to be the advocate for your discipline by considering the following:

• Which aerodynamic concepts are appropriate?
• Consider the appropriate uses and risks of advanced technology.

To be effective, you must:

• Show the aerodynamic benefits of the configuration very clearly.
• Accommodate other team members.
• Be ready to change.

Finally, the best airplane is not the one with the highest (L/D)_max, but most likely the one with the lowest cost. There are many costs that should be estimated. Ultimately there is the life cycle cost (LCC), but other costs are frequently more important. Examples include the direct operating cost (DOC) and the acquisition cost.

Once the concept is selected, many more details of the aerodynamic configuration will still have to be determined. The material in the remainder of the book addresses many of these details.

1.6 What’s Left Out?

In this book, we have omitted discussion of numerous aspects of configuration aerodynamics in favor of the absolute essentials. Even the concepts discussed aren’t necessarily covered in depth, and the interested reader can use the book’s references and appendices as a start toward more detailed studies of the concepts. Omitted topics include:

• Aeropropulsion integration
• Unsteady flows (which may in fact become important in design and aeroelastic effects)
• Helicopter aerodynamics
• Missile aerodynamics

Figure References