Something about the "hidden" wings of fighter planes
  ——On the popular styles and development trends of the world's mainstream fighter planes aerodynamic layout

  On July 15, 2020, the State Intellectual Property Office officially announced the review results of the 21st China Patent Awards, the design patent of the "lift body side strip wing-duck-type aircraft" of the Chengdu Aircraft Design Institute of China Aviation Industry Corporation Won the gold medal. As a result, the "hidden" wings of lift bodies, side strip wings, and canard layouts have once again become a hot spot for aviation enthusiasts. This issue of "Weapons Grand View" specially invites relevant experts to interpret relevant content for you.

The "Three Treasures" of Fighter Aerodynamics

  Except for a few vertical take-off and landing fighters that generate lift by changing the direction of air flow during the take-off and landing phase, most aircraft rely on the lift generated by the wings during the movement to overcome gravity.

  The causes of wing lift are more complicated, and there are many related theories. Few theories can completely solve all the secrets of lift generation. But one thing is certain, the lift of an airplane is mainly derived from the air pressure difference caused by the airflow velocity difference between the upper and lower surfaces of the wing. The magnitude of the lift is related to the speed of the airflow, the density of the air, the area of ​​the wing and the angle of the wing.

  Of course, aircraft will also encounter various resistances during flight, such as frictional resistance, differential pressure resistance, induced resistance, interference resistance, and wave resistance. Continuously optimize the aerodynamic layout, its role is to increase lift, thrust and safety, and reduce various resistance.

  The aerodynamic layout is one of the important factors that determine the maneuverability of a fighter. Compared with the wings, the lift body, side strip wings, and duck layout are not "conspicuous" and belong to the "recessive" wings of the fighter. In terms of function, they can be regarded as the "three treasures" of the aerodynamic layout of mainstream fighters in active service, and they play an important role in mainstream fighters in the world.

  Lift body layout. Compared with the wing design of traditional aircraft, the lift body is a completely different concept. It uses a three-dimensional design wing body fusion to generate lift. This design can obtain greater lift at lower speeds.

  Since two scientists, Igles and Allen, accidentally discovered the advantages of this design in 1957, the United States and the Soviet Union have begun related research. The half-scale prototype X-33 of the American space carrier "Adventure Star" and the X-38 prototype of the space rescue ship all adopt the lift body configuration.

  The three generations of aircraft that adopt the lift body design include Su-27, MiG-29, etc.; the fourth generation aircraft include F-22, F-35, Su-57, etc., of which the design features of the Su-57 lift body are more obvious, and the fuselage Larger, stronger lift and lower resistance.

  Side strip wing layout. As a design that appeared in the mid-1950s, the side strip wing is a pair of narrow, pointed leading edges, large sweeping slender, extended from the roots of the leading edge of the wing on the basis of the conventional sweep wing or delta wing layout. Long wings. Usually the side wing is designed to be completely integrated with the fuselage and main wing to form a composite wing, which can improve the maneuverability and post-stall characteristics of the fighter, especially the lift of the aircraft. It is usually divided into wing side strips and side strips.

  Canard wing layout. Also called front wing or front wing. As early as 1903, the Wright brothers' aircraft used a front wing configuration. The feature of this configuration is to place the horizontal stabilizing surface in front of the main wing.

  The aircraft layout with canard wings is called the canard layout, and the representative models include some Russian Su-35, Su-34, Su-30MKI, and European "Typhoon", "Rafale" and "Gryphon" fighters.

  Some fighters can't control the canard, while others can. The canards are steerable, such as the European "typhoons", "gusts" and "griffins". In addition to generating vortex lift, these canards are also used to improve the stability of the transonic process. When landing, the canard deflection angle can be manipulated to act as a speed brake.

  According to the installation position, the canard can be divided into three configurations: upper, middle, and lower. Taking into account factors such as lift and stall angle of attack, aircraft with canard layout often use upper and middle canards.

Combination is not a simple problem, it is about innovation and strength

  Different aerodynamic layouts have their own strengths in different flight conditions. The combat requirements are different, and the selection and combination of the "three treasures" of the aerodynamic layout of the fighters are also different.

  Combination of side wing and lifting body. For fighters, the use of side strips can improve flight performance and overcome the shortcomings of conventional swept-wing and variable-swept-wing aircraft in terms of low-speed characteristics, wave resistance, and flight stability. The use of lift bodies can increase the effective space inside the fighter, reduce flight resistance, and improve high-speed flight capabilities. Therefore, F-22, F-35, Su-57, Su-27 and MiG-29, etc. have adopted the layout of a combination of side wing and lift body.

  The first type to successfully adopt side wing and active control technology is the F-16 fighter, but the F-22 that combines side wing and lift body is better. Its side strip wing extends from the vertex of the nose to the root of the wing, and is integrated with the fuselage and air intake to form a lift body, which not only generates greater lift, but also increases flight stability.

  Combination of canard wings and side strip wings. In order to improve the lift coefficient of modern aircraft, many of them have chosen the coexistence layout of canard and side wing. For example, some of Russia's Su-35, Su-34, Su-30MKI, European "typhoon", "gust" and "Gryphon" are all the same.

  Fighters with this aerodynamic layout have better lift and agility. The canard is generally larger in size and separated from the wing. Under certain conditions, the additional lift generated by the wing is larger than that of the elongated wing. In some cases, side wing can produce more lift than canard wing. The combination of the two complements each other. In addition to increasing lift, canards also help maintain the flight stability and controllability of the aircraft.

  The fourth-generation fighter jets of the United States and Russia use a conventional aerodynamic layout instead of a duck aerodynamic layout. There are many reasons. One of them is that when the duck layout is adopted, the flight control processing is more difficult, and it will bring more difficulty to the stealth design of the fighter, so they avoided this layout design.

  The lifting body has a canard-like layout combination of side strips. The aerodynamic layout design of an aircraft must not only consider the maneuverability of the aircraft, but also a series of issues such as stability, maneuverability, strength, and rigidity. In particular, modern aircraft pursue high maneuverability and high speed, and have more stringent requirements for aerodynamic layout.

  There are very few fighters in the world that simultaneously adopt the aerodynamic layout of canards, side strips, and lift bodies. One of the main reasons is that such a combination design is very difficult. However, China Aviation Industry Corporation Chengdu Aircraft Design and Research Institute has obviously successfully overcome a series of related problems, which is why its design won the award.

  With the design of canard wings and side strip wing, it is necessary to face the work matching problem of canard, side strip wing and main wing. If these two are used together with the design of the lifting body, there will be more problems to be faced. Its size, shape, position and other parameters need to be obtained through accurate calculations and a large number of wind tunnel tests. In this way, the combination can reach the optimal. Moreover, how to reduce weight while increasing efficiency and how to achieve a balance between weight and flight efficiency are also issues that designers must face. All these will greatly increase the difficulty of related design.

  This combination is not a simple match, but the result of countless tests and wind tunnel tests. It is about innovation and strength.

"Three Treasures", can they fly with fighter jets in the future?

  In the future, the development of fighter jets will focus on improving the capability of full-height, all-weather, and all-round air combat. Some experts summarize its main features as the "six super", namely: ultra-flat shape, supersonic cruise, ultra-conventional maneuver, ultra-long-range strike, ultra-dimensional IoT, and ultra-domain control.

  Based on the expectations and understandings of various countries on the next-generation fighters, it is speculated that the next-generation fighters may be equipped with artificial intelligence and quantum radar, have the ability to perform combat missions without participation, can reach hypersonic speeds, enter and exit near space, and can be used New physical principles, weapons, etc. Therefore, manned/unmanned control systems, improved stealth, improved flight performance and communication systems are the key standards for the next generation of fighter jets. Among them, the manned/unmanned control system, stealth, hypersonic flight performance, etc., will all be based on the comprehensive innovation of the aircraft aerodynamic layout. This determines that aerodynamic layout design will still be an "old proposition" under the new situation.

  Lifting body layout is a popular model for future fighters. In the future, fighters are expected to have a large load capacity, which requires the use of a wing-body fusion lift body layout to increase the fuel capacity of the wings and increase the lift of the fuselage. In view of the rapid advancement of detection technology, in order to improve the survivability on the battlefield, future fighters will adopt a more powerful stealth design. Among them, the layout of the aircraft is very likely to adopt a supersonic tailless delta wing lift body design, and the flat tail and vertical tail are completely eliminated. The full wing body fusion and the lift body design with a large lift-to-drag ratio can enable the fighter to obtain higher mobility and stealth capabilities.

  The side strip wing layout may be the standard configuration of future fighters. The use of medium-swept wings to increase the side wing is a typical design of the third-generation high-maneuverability fighter. Fourth-generation fighters such as F-22 adopt a special side strip wing layout, and F-35 and Su-57 also adopt special side strip wing design, so they have excellent flight quality. Future fighters are generally an extension of existing fighter technology. Therefore, the side strip wing layout will still be an indispensable design for future fighters, but its design will be more advanced and reasonable.

  Future fighters may also adopt a duck layout. The duck-type fighter can basically eliminate the possibility of stalling, that is, the aircraft will not enter the "spiral." In order to avoid the shortcomings of the traditional duck layout, future fighters may be designed with a lift body duck layout, that is, the lift body fuselage can function as a canard, so that the fighter can fly and maneuver at high speed. This layout may become the basis of future aircraft.

  The waverider layout may be applied to future fighters. The concept of waveriders has now been embodied in some hypersonic missiles. The next-generation fighter aircraft has the characteristics of unmanned driving, fast speed, high altitude, long cruising distance, and strong penetration ability. It is likely to adopt an aerodynamic shape with high lift-to-drag ratio and strong maneuverability. The shapes suitable for hypersonic aircraft include lift body, wing body fusion body, axisymmetric body, wave rider, etc. Therefore, the waverider configuration may also be one of the future development trends of fighter jets.

  (Editor's note: For the sake of unification, the designation of fighters in this article is based on my country's military designation standards.)

  (Author's unit: Air Force Research Institute, Nanchang Hangkong University)

  Layout design: Liang Chen

  Photo courtesy of this edition: Shi Feng Fang Xiaopeng

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Yi Yanlei Jiang Tianhe Zhang Guangshan