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previous issue review: popular science: how many types of wing shapes are there on airplanes? (1)

variable sweep wing: the take-off and landing speeds and maneuverability of large swept wings, delta wings, and trapezoidal wings are not as good as those of straight wings, but the resistance of straight wings at high speeds is too large. so, through mechanical means, the sweep angle of the wing can be changed as needed during flight. isn't it the best of both worlds? this is the origin of variable sweep wing.

the concept of variable-sweep wing seems simple, but there are many problems in its realization. first, there is the problem of flight stability. as the wing sweep angle increases, the lift center gradually moves backward, and soon there will be a problem that the lift center is far away from the center of gravity. even if the super-large horizontal tail can suppress it, it will bring huge resistance, which is not worth the loss. in order to reduce the movement of the lift center, the variable-sweep wing can only be divided into two sections, with the hinge set on the outside of the fixed inner section, and the movable outer section is reduced, sacrificing the effect of the variable-sweep wing to simplify the engineering design.

in order to minimize the problem of flight stability, the su-17's movable section only accounts for half of the wingspan; the f-14's movable section is a little larger, but it still has a large fixed section. there are many specific problems with variable-sweep wings: it is not easy to find a place for the underwing landing gear to take root, the movable section cannot be designed with an in-wing fuel tank, which greatly reduces the total in-wing fuel tank space, and the underwing weapon rack needs to rotate synchronously with the movable section to keep the mounted weapons pointing forward. in addition to the inherent mechanical problems of the variable-sweep wing, the variable-sweep wing will eventually become very heavy, greatly offsetting the aerodynamic advantages of the variable-sweep wing.

after a brief period of popularity in the 1960s and 1970s, variable-sweep wings are rarely used today, with the tu-160, which first flew in 1981, being the last new variable-sweep-wing aircraft to go into production.

for the forward-swept wing, the large-swept wing and the delta wing reduce drag by the sweep angle, but the air is actually only interested in the angle of the sweep, and does not care whether the wing is swept back or forward. so what are the benefits of the forward-swept wing? the spanwise flow of the airflow on the forward-swept wing is inward, and the body will eventually naturally block the spanwise flow, improving the efficiency of the wing in generating lift.

more importantly, the forward-swept wing greatly delays the problem of wingtip stall. air is viscous, and this viscosity forms a boundary layer (also called boundary layer) on the wing surface. the airflow is stagnant in the boundary layer, and the effect of generating lift is lost. when flying at a large angle of attack, the airflow flows along the span of the swept wing, causing the boundary layer to accumulate toward the wingtip, causing the wingtip to stall first, causing the lift center to move toward the wing root, causing the nose to rise further, and eventually causing the entire wing to stall.

the forward-swept wing is different. the wing tip is in a "clean" airflow, the boundary layer accumulation occurs at the wing root, the lift loss is small, and the aileron maintains effective roll control. the forward-swept wing will not have the problem of wing tip stall until almost the entire wing is stalled, which is much later than the swept wing entering stall, which is conducive to enhancing maneuverability.

the air is only interested in the "sweep" of the wing. it doesn't matter whether it is swept forward or backward, so the wing can also be swept forward. the picture below should be the american x-29, which is another forward-swept wing research aircraft.

however, the forward-swept wing also has an essential flaw: the aeroelastic divergence problem. the wing is not rigid, but has a certain degree of elasticity.

the airflow over the wing surface generates lift, which acts on the wing, so the wing tip has a tendency to twist upward with the wing root as the fulcrum. since the fulcrum of the forward-swept wing is behind the wing tip, the wing tip of the forward-swept wing has a natural tendency to twist backward and upward. the upward movement increases the local wing angle of attack, generates greater lift, and further intensifies the twist backward and upward.

if not controlled, the structure will soon be damaged due to excessive twisting. the fulcrum of the swept wing is in front of the wing tip. the wing tip has a natural tendency to twist forward and upward under the action of lift. if the local angle of attack is reduced, this problem will not occur. in the early days, due to material limitations, the forward-swept wing could not solve the aeroelastic divergence problem, and the swept wing became the only option. after the emergence of composite materials, the so-called "aeroelastic tailoring", that is, through the clever arrangement of the fiber direction, the structural rigidity in the normal direction can be made higher than the span direction, cleverly overcoming the problems caused by aeroelastic divergence.

about slant wing

both swept wings and forward-swept wings are symmetrical, either both sides are swept back or both sides are swept forward. but from the perspective of drag reduction, there is no reason why one side cannot be swept forward and the other side swept back, forming an asymmetrical oblique wing.

since it doesn't matter whether it's swept forward or swept back, it's also possible to have one side swept forward and the other side swept back. this is the oblique wing, which is the us ad-1 research aircraft.

(during takeoff and landing, it is flat, just like an ordinary straight-wing aircraft. after taking off and flying at high speed, it becomes an oblique wing, with one end swept forward and the other end swept backward)

compared with swept wings and forward swept wings, the total cross-sectional area of ​​the oblique wing along the fuselage axis is more evenly distributed, which is conducive to satisfying the transonic area law and reducing transonic drag.

fixed oblique wings have their advantages, but variable-sweep oblique wings are their shining point.

traditional variable-sweep wings are troubled by the hinge position, but the variable-sweep oblique wing has only one ideal hinge position: in the middle, and other positions are superfluous. since the weight on both sides is balanced, the variable-sweep oblique wing is a little simpler in mechanical design, which is like the difference between holding a bucket with both hands stretched out and carrying a load on the shoulder. in terms of aerodynamics, the change in the sweep angle also keeps the movement of the lift center roughly unchanged, simplifying the design of flight stability.

the oblique wing unexpectedly solved the problem of variable sweep wing, but the place where the oblique wing can better demonstrate its superiority is the flying wing.

the wings and fuselage of traditional aircraft are separate. the wings generate lift, and the fuselage carries people and cargo. however, the fuselage does not generate lift and is "dead weight". this problem causes high forces on the wing root, which is structurally inefficient. the best way is to put all the load inside the wing, so that the structural strength requirements are minimal. theoretically, if the lift and gravity at every point on the wing are exactly offset, you can make an airplane out of paper, and minimize the structural weight. of course, this is impossible in reality. before it takes off, the weight has already crushed the paper skin. however, this shows that the general direction of the flying wing with no fuselage and only wings is correct.

the flying wing uses the wing structure to carry the load, which maximizes the aerodynamic efficiency of the structure and eliminates the problem of wing root stress. this is the case with the us b-2

X47B

elliptical wing

if there is no thrust, all the energy transferred to the air by the aircraft will form resistance, and the wingtip vortex is a very important part of the flight resistance. reasonable design of lift distribution will make the area near the wing tip less lift, and the resistance generated by the wing tip flow will naturally decrease. this is the idea of ​​​​elliptical wings. the famous elliptical wings of the british spitfire fighter in world war ii came from this.

the famous elliptical wing of the british spitfire fighter was designed to reduce wingtip flow and optimize lift distribution, as shown in the figure above.

a natural extension of the elliptical wing is the circular wing. the circular wing not only concentrates the lift generation area toward the wing root, but also conforms to the area law more, especially in the case of a circular flying wing without a fuselage. this kind of flying disc is not only theoretically suitable for all speed ranges from hovering to supersonic speed, but is also a favorite of science fiction people and an ideal design that is hard to let go in aircraft design. however, the problem of flight control is more difficult to solve. not only is the control arm very short, but the design of the engine, nozzle, and control surface must be reconsidered.

the more extreme one is of course the flying saucer. this is the canadian avrocar, which was designed for the us air force, so it is painted in the us air force color scheme, as shown above.

winglets

another way to solve the wingtip flow is to use winglets, which are vertical wings erected on the wingtips to directly prevent the wingtip flow. aerodynamically, winglets are equivalent to extending the effective wingspan and increasing lift. if designed properly, winglets can achieve an effective wingspan that exceeds the actual "wingspan", but winglets also increase drag and weight, and also bring aerodynamic interference drag at the wing surface transition.

if you don't use an elliptical wing or a flying saucer, the winglets can effectively reduce the impact of wingtip flow.

winglets can be extended both up and down, or just up. the choice between the two is a trade-off between increasing lift and reducing weight and drag. winglets are very effective when the old design is being used to tap its potential, or when the wingspan is limited by airport conditions. but when designing a new wing, increasing the wingspan is often more concise and effective.

a brief history of winglet development

the winglets can not only be turned up, but also drooped down. this is the winglet of the a320.