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Un B-52 Stratofortress mostrando sus alas extendidas.

Un ala barrida es un ala que se inclina hacia atrás u ocasionalmente hacia adelante desde su raíz en lugar de en una dirección recta hacia los lados.

Las alas en flecha se han volado desde los días pioneros de la aviación. El barrido del ala a altas velocidades fue investigado por primera vez en Alemania ya en 1935 por Albert Betz y Adolph Busemann , y encontró aplicación justo antes del final de la Segunda Guerra Mundial . Tiene el efecto de retrasar las ondas de choque y el aumento de la resistencia aerodinámica que las acompaña debido a la compresibilidad del fluido cercana a la velocidad del sonido , mejorando el rendimiento. Por lo tanto, las alas barridas casi siempre se usan en aviones a reacción diseñados para volar a estas velocidades. Las alas barridas también se utilizan a veces por otras razones, como baja resistencia, baja observabilidad, conveniencia estructural o visibilidad del piloto.

El término "ala barrida" se usa normalmente para significar "barrida hacia atrás", pero las variantes incluyen barrido hacia adelante , alas de barrido variable y alas oblicuas en las que un lado se desplaza hacia adelante y el otro hacia atrás. El ala delta también es aerodinámicamente una forma de ala en flecha.

Características de diseño [ editar ]

Para un ala de envergadura determinada, barrerla aumenta la longitud de los largueros que la recorren desde la raíz hasta la punta. Esto tiende a aumentar el peso y reducir la rigidez. Si la cuerda de proa a popa del ala también permanece igual, la distancia entre los bordes de ataque y de fuga se reduce, lo que reduce su capacidad para resistir las fuerzas de torsión (torsión). Por lo tanto, un ala barrida de envergadura y cuerda dadas debe reforzarse y será más pesada que el ala no barrida equivalente.

Un ala barrida generalmente se inclina hacia atrás desde su raíz en lugar de hacia adelante. Debido a que las alas están hechas lo más livianas posible, tienden a flexionarse bajo carga. Esta aeroelasticidad bajo carga aerodinámica hace que las puntas se doblen hacia arriba en vuelo normal. El barrido hacia atrás hace que las puntas reduzcan su ángulo de ataque al doblarse, reduciendo su sustentación y limitando el efecto. El barrido hacia adelante hace que las puntas aumenten su ángulo de ataque a medida que se doblan. Esto aumenta su sustentación provocando una mayor flexión y, por lo tanto, aún más sustentación en un ciclo que puede provocar una falla estructural descontrolada. Por esta razón, el barrido hacia adelante es raro y el ala debe ser inusualmente rígida.

El "ángulo de barrido" característico se mide normalmente trazando una línea desde la raíz hasta la punta, típicamente el 25% del camino hacia atrás desde el borde de ataque, y comparándolo con la perpendicular al eje longitudinal de la aeronave. Los ángulos de barrido típicos varían de 0 para un avión de ala recta, a 45 grados o más para cazas y otros diseños de alta velocidad.

Aerodinámica [ editar ]

Vuelo subsónico y transónico [ editar ]

Yakovlev Yak-25 ala barrida
En la fase transónica, el ala barrida también barre el amortiguador que se encuentra en la parte superior trasera del ala. Solo se ve afectado el componente de velocidad perpendicular al impacto.

Cuando una aeronave entra en velocidades transónicas justo por debajo de la velocidad del sonido, las ondas de presión asociadas con el vuelo subsónico convergen y comienzan a incidir en la aeronave. A medida que las ondas de presión convergen, el aire frente a la aeronave comienza a comprimirse. Esto crea una fuerza conocida como arrastre de onda . Esta resistencia de onda aumenta abruptamente hasta que todo el avión es supersónico y luego se reduce.

Sin embargo, se pueden formar ondas de choque en algunas partes de un avión que se mueve a una velocidad menor que la del sonido. Las regiones de baja presión alrededor de una aeronave hacen que el flujo se acelere y, a velocidades transónicas, esta aceleración local puede exceder Mach 1. El flujo supersónico localizado debe regresar a las condiciones de corriente libre alrededor del resto de la aeronave, y cuando el flujo entra en un gradiente de presión adverso en la sección de popa del ala, surge una discontinuidad en forma de onda de choque cuando el aire se ve obligado a desacelerar rápidamente y volver a la presión ambiental.

Con objetos donde hay una reducción repentina en el perfil / grosor y el aire local se expande rápidamente para llenar el espacio ocupado por el objeto sólido o donde se imparte un cambio angular rápido al flujo de aire que causa un aumento momentáneo de volumen / disminución de densidad, un Se genera una onda de choque oblicua . Esta es la razón por la que las ondas de choque a menudo se asocian con la parte de la cabina de un avión de combate con la curvatura local más alta, que aparece inmediatamente detrás de este punto.

En el punto donde la densidad cae, la velocidad local del sonido cae correspondientemente y se puede formar una onda de choque. Esta es la razón por la que en las alas convencionales, las ondas de choque se forman primero después del Espesor / Acorde máximo y por qué todos los aviones diseñados para navegar en el rango transónico (por encima de M0.8) tienen alas supercríticas que son más planas en la parte superior, lo que reduce al mínimo el cambio angular de flujo a aire de la superficie superior. El cambio angular al aire que normalmente es parte de la generación de sustentación se reduce y esta reducción de sustentación se compensa con superficies inferiores curvas más profundas acompañadas de una curva refleja en el borde de fuga. Esto da como resultado una onda de choque estacionaria mucho más débil hacia la parte trasera de la superficie superior del ala y un aumento correspondiente en el número de mach crítico.

Las ondas de choque requieren energía para formarse. Esta energía se extrae de la aeronave, que debe proporcionar un empuje adicional para compensar esta pérdida de energía. Por tanto, los choques se ven como una forma de arrastre . Dado que los choques se forman cuando la velocidad del aire local alcanza velocidades supersónicas, existe una cierta velocidad de " mach crítico " donde el flujo sónico aparece por primera vez en el ala. Hay un punto siguiente llamado número de mach de divergencia de arrastre donde el efecto del arrastre de los choques se vuelve notable. Esto es normalmente cuando los choques comienzan a generarse sobre el ala, que en la mayoría de las aeronaves es la superficie continuamente curva más grande y, por lo tanto, la que más contribuye a este efecto.

El barrido del ala tiene el efecto de reducir la curvatura del cuerpo visto desde el flujo de aire, por el coseno del ángulo de barrido. Por ejemplo, un ala con un barrido de 45 grados verá una reducción en la curvatura efectiva a aproximadamente el 70% de su valor de ala recta. Esto tiene el efecto de aumentar el Mach crítico en un 30%. Cuando se aplica a grandes áreas de la aeronave, como las alas y el empenaje , esto permite que la aeronave alcance velocidades más cercanas a Mach 1.

Robert T. Jones ofreció una de las mejores y más simples explicaciones de cómo funciona el ala en flecha : "Supongamos que un ala cilíndrica (cuerda constante, incidencia, etc.) se coloca en una corriente de aire en un ángulo de guiñada, es decir, es Ahora, incluso si la velocidad local del aire en la superficie superior del ala se vuelve supersónica, no se puede formar una onda de choque allí porque tendría que ser un choque de retroceso, barrido en el mismo ángulo que el ala, es decir, sería un choque oblicuo. Este choque oblicuo no puede formarse hasta que el componente de velocidad normal se vuelva supersónico ". [1]

Un factor limitante en el diseño del ala en flecha es el llamado "efecto medio". Si un ala barrida es continua - un ala barrida oblicua , las isobarras de presión serán barridas en un ángulo continuo de punta a punta. Sin embargo, si las mitades izquierda y derecha se desplazan hacia atrás por igual, como es práctica común, las isobarras de presión en el ala izquierda en teoría se encontrarán con las isobarras de presión del ala derecha en la línea central en un ángulo grande. Como las isobarras no se pueden unir de esta manera, tenderán a curvarse en cada lado cuando se acerquen a la línea central, de modo que las isobarras crucen la línea central en ángulos rectos con la línea central. Esto provoca un "desbarbado" de las isobarras en la región de la raíz del ala. Para combatir esta imperturbabilidad, el aerodinámico alemán Dietrich Küchemannpropuso y había probado una muesca local del fuselaje por encima y por debajo de la raíz del ala. Esto resultó no ser muy eficaz. [2] Durante el desarrollo del avión de pasajeros Douglas DC-8 , se utilizaron superficies aerodinámicas sin cambiar en el área de la raíz del ala para combatir el desbarbado. [3] [4] De manera similar, se agregó un guante de raíz de ala decambered al ala del Boeing 707 para crear el Boeing 720 . [5]

Vuelo supersónico [ editar ]

A velocidades supersónicas, existe un impacto oblicuo frente al borde de ataque del ala. El componente de velocidad perpendicular al choque es diferente aguas arriba y aguas abajo del choque. El componente de velocidad paralelo al choque es el mismo en ambos lados del choque.
El ala delta del Convair F-106 Delta Dart es una forma de ala en flecha.

El flujo de aire a velocidades supersónicas genera elevación a través de la formación de ondas de choque, a diferencia de los patrones de flujo de aire por encima y por debajo del ala. Estas ondas de choque, como en el caso transónico, generan grandes cantidades de resistencia. Una de estas ondas de choque es creada por el borde de ataque del ala, pero contribuye poco a la sustentación. Para minimizar la fuerza de este impacto, debe permanecer "unido" a la parte delantera del ala, lo que exige un borde de ataque muy afilado. Para dar mejor forma a los amortiguadores que contribuirán a la elevación, el resto de una superficie aerodinámica supersónica ideal tiene una sección transversal en forma de diamante. Para la elevación a baja velocidad, estos mismos perfiles aerodinámicos son muy ineficientes, lo que conduce a un manejo deficiente y velocidades de aterrizaje muy altas. [6]

Una forma de evitar la necesidad de un ala supersónica dedicada es utilizar un diseño subsónico de gran barrido. El flujo de aire detrás de las ondas de choque de un cuerpo en movimiento se reduce a velocidades subsónicas. Este efecto se utiliza dentro de las tomas de los motores destinados a operar en el supersónico, ya que los motores a reacción son generalmente incapaces de ingerir aire supersónico directamente. Esto también se puede utilizar para reducir la velocidad del aire visto por el ala, utilizando los golpes generados por la nariz de la aeronave. Siempre que el ala se encuentre detrás de la onda de choque en forma de cono, "verá" el flujo de aire subsónico y funcionará con normalidad. El ángulo necesario para colocarse detrás del cono aumenta al aumentar la velocidad, a Mach 1.3 el ángulo es de aproximadamente 45 grados, a Mach 2.0 es de 60 grados. [7]Por ejemplo, a Mach 1.3, el ángulo del cono de Mach formado fuera del cuerpo de la aeronave será aproximadamente sinμ = 1 / M (μ es el ángulo de barrido del cono de Mach) [8]

Por lo general, no es posible colocar el ala de modo que quede completamente fuera del flujo de aire supersónico y aún tenga un buen rendimiento subsónico. Algunos aviones, como el English Electric Lightning, están sintonizados casi por completo para vuelos de alta velocidad y cuentan con alas muy extendidas que hacen poco o ningún compromiso con los problemas de baja velocidad que crea dicho perfil. [9] [10] En otros casos, el uso de alas de geometría variable , como en el Grumman F-14 Tomcat y Panavia Tornado , permite que un avión mueva el ala para mantenerla en el ángulo más eficiente independientemente de la velocidad, aunque los inconvenientes incurridos en una mayor complejidad y peso han llevado a que esta sea una característica poco común. [11][12]

La mayoría de los aviones de alta velocidad tienen un ala que pasa al menos parte de su tiempo en el flujo de aire supersónico. Pero dado que el cono de choque se mueve hacia el fuselaje con mayor velocidad (es decir, el cono se vuelve más estrecho), la porción del ala en el flujo supersónico también cambia con la velocidad. Dado que estas alas se mueven, a medida que el cono de choque se mueve hacia adentro, el vector de sustentación se mueve hacia adelante [ cita requerida ] ya que las porciones exteriores y traseras del ala generan menos sustentación. Esto da como resultado poderosos momentos de lanzamiento y los cambios de ajuste necesarios asociados.

Desventajas [ editar ]

Flujo spanwise de la capa límite

Cuando un ala barrida viaja a alta velocidad, el flujo de aire tiene poco tiempo para reaccionar y simplemente fluye sobre el ala casi en línea recta de adelante hacia atrás. A velocidades más bajas del aire hace tener tiempo para reaccionar, y es empujado en envergadura por el borde delantero en ángulo, hacia la punta del ala. En la raíz del ala, junto al fuselaje, esto tiene un efecto poco notable, pero a medida que uno se mueve hacia la punta del ala, el flujo de aire es empujado en sentido transversal no solo por el borde de ataque, sino por el aire que se mueve en sentido transversal a su lado. En la punta, el flujo de aire se mueve a lo largo del ala en lugar de sobre ella, un problema conocido como flujo en sentido transversal .

La sustentación de un ala es generada por el flujo de aire sobre ella de adelante hacia atrás. Con el aumento del flujo a lo largo del tramo, las capas límite en la superficie del ala tienen más tiempo para viajar, por lo que son más gruesas y más susceptibles a la transición a la turbulencia o la separación del flujo, también la relación de aspecto efectiva del ala es menor y, por lo tanto, fugas de aire "alrededor de las puntas de las alas reduciendo su efectividad. El flujo a lo largo de las alas en flecha produce un flujo de aire que mueve el punto de estancamiento en el borde de ataque de cualquier segmento de ala individual más debajo del borde de ataque, lo que aumenta el ángulo de ataque efectivo.de segmentos de ala en relación con su segmento delantero vecino. El resultado es que los segmentos de ala más hacia la parte trasera operan en ángulos de ataque cada vez más altos, lo que promueve la pérdida temprana de esos segmentos. Esto promueve la pérdida de la punta en las alas en flecha hacia atrás, ya que las puntas están más hacia atrás, mientras que retrasa la pérdida de la punta para las alas en flecha hacia adelante, donde las puntas están hacia adelante. Con las alas en flecha hacia adelante y hacia atrás, la parte trasera del ala se detendrá primero. Esto crea una presión de morro hacia arriba en la aeronave. Si el piloto no corrige esto, hace que el avión se incline hacia arriba, lo que lleva a que el ala se pierda, lo que lleva a un mayor cabeceo, y así sucesivamente. Este problema llegó a conocerse como el baile del sable en referencia a la cantidad de Súper Sables F-100 norteamericanos que se estrellaron al aterrizar como resultado.[13] [14]

La solución a este problema adoptó muchas formas. Uno fue la adición de una aleta conocida como cerca del ala en la superficie superior del ala para redirigir el flujo hacia la parte trasera; el MiG-15 fue un ejemplo de un avión equipado con vallas de ala. [15] Otro diseño estrechamente relacionado fue la adición de una muesca en forma de diente de perro al borde de ataque, como está presente en el interceptor Avro Arrow . [16] Otros diseños adoptaron un enfoque más radical, incluido el ala del Republic XF-91 Thunderceptor que se ensanchó hacia la punta para proporcionar más sustentación en la punta. El Handley Page Victor estaba equipado con un ala en forma de media luna, presentando un barrido sustancial cerca de la raíz del ala donde el ala era más gruesa, y reduciendo progresivamente el barrido a lo largo de la envergadura a medida que el grosor del ala se reduce hacia la punta. [17] [18]

Las soluciones modernas al problema ya no requieren diseños "personalizados" como estos. La adición de listones de vanguardia y grandes flaps compuestos a las alas ha resuelto en gran medida el problema. [19] [20] [21] En los diseños de caza, la adición de extensiones de borde de ataque , que normalmente se incluyen para lograr un alto nivel de maniobrabilidad, también sirven para aumentar la sustentación durante el aterrizaje y reducir el problema. [22] [23]

El ala barrida también tiene varios problemas más. Una es que para cualquier longitud dada de ala, la envergadura real de punta a punta es más corta que la misma ala que no se barre. La resistencia a baja velocidad está fuertemente correlacionada con la relación de aspecto , la amplitud en comparación con la cuerda, por lo que un ala barrida siempre tiene más resistencia a velocidades más bajas. Otra preocupación es el torque aplicado por el ala al fuselaje, ya que gran parte de la sustentación del ala se encuentra detrás del punto donde la raíz del ala se conecta al avión. Finalmente, si bien es bastante fácil pasar los largueros principales del ala a través del fuselaje en un diseño de ala recta para usar una sola pieza continua de metal, esto no es posible en el ala barrida porque los largueros se encontrarán en ángulo.

Teoría del barrido [ editar ]

La teoría del barrido es una descripción de ingeniería aeronáutica del comportamiento del flujo de aire sobre un ala cuando el borde de ataque del ala se encuentra con el flujo de aire en un ángulo oblicuo. El desarrollo de la teoría de barrido dio como resultado el diseño de ala barrida utilizado por la mayoría de los aviones a reacción modernos, ya que este diseño funciona de manera más eficaz a velocidades transónicas y supersónicas . En su forma avanzada, la teoría del barrido condujo al concepto experimental de ala oblicua .

Adolf Busemann introdujo el concepto de ala en flecha y lo presentó en 1935 en el 5. Volta-Congress en Roma. La teoría de barrido, en general, fue un tema de desarrollo e investigación a lo largo de los años 1930 y 1940, pero la definición matemática avance de la teoría de barrido se atribuye generalmente a la NACA 's Robert T. Jonesen 1945. La teoría del barrido se basa en otras teorías del levantamiento de alas. La teoría de la línea de elevación describe la sustentación generada por un ala recta (un ala en la que el borde de ataque es perpendicular al flujo de aire). La teoría de Weissinger describe la distribución de la sustentación para un ala en flecha, pero no tiene la capacidad de incluir la distribución de la presión a lo largo de la cuerda. Hay otros métodos que describen distribuciones por cuerda, pero tienen otras limitaciones. La teoría del barrido de Jones proporciona un análisis sencillo y completo del rendimiento del ala barrida.

Para visualizar el concepto básico de la teoría de barrido simple, considere un ala recta, no barrida, de longitud infinita, que se encuentra con el flujo de aire en un ángulo perpendicular. La distribución de la presión del aire resultante es equivalente a la longitud de la cuerda del ala (la distancia desde el borde de ataque hasta el borde de salida). Si comenzáramos a deslizar el ala hacia los lados (en sentido transversal ), el movimiento lateral del ala en relación con el aire se agregaría al flujo de aire previamente perpendicular, lo que resultaría en un flujo de aire sobre el ala en ángulo con el borde de ataque. Este ángulo hace que el flujo de aire viaje una distancia mayor desde el borde de ataque hasta el borde de salida y, por lo tanto, la presión del aire se distribuye en una distancia mayor (y, en consecuencia, se reduce en cualquier punto particular de la superficie).

This scenario is identical to the airflow experienced by a swept wing as it travels through the air. The airflow over a swept wing encounters the wing at an angle. That angle can be broken down into two vectors, one perpendicular to the wing, and one parallel to the wing. The flow parallel to the wing has no effect on it, and since the perpendicular vector is shorter (meaning slower) than the actual airflow, it consequently exerts less pressure on the wing. In other words, the wing experiences airflow that is slower - and at lower pressures - than the actual speed of the aircraft.

One of the factors that must be taken into account when designing a high-speed wing is compressibility, which is the effect that acts upon a wing as it approaches and passes through the speed of sound. The significant negative effects of compressibility made it a prime issue with aeronautical engineers. Sweep theory helps mitigate the effects of compressibility in transonic and supersonic aircraft because of the reduced pressures. This allows the mach number of an aircraft to be higher than that actually experienced by the wing.

There is also a negative aspect to sweep theory. The lift produced by a wing is directly related to the speed of the air over the wing. Since the airflow speed experienced by a swept wing is lower than what the actual aircraft speed is, this becomes a problem during slow-flight phases, such as takeoff and landing. There have been various ways of addressing the problem, including the variable-incidence wing design on the Vought F-8 Crusader,[24] and swing wings on aircraft such as the F-14, F-111, and the Panavia Tornado.[11][12]

Variant designs[edit]

The term "swept wing" is normally used to mean "swept back", but other swept variants include forward sweep, variable sweep wings and oblique wings in which one side sweeps forward and the other back. The delta wing also incorporates the same advantages as part of its layout.

Forward sweep[edit]

Main article: Forward-swept wing
LET L-13 two-seat glider showing forward swept wing
Grumman X-29 experimental aircraft, an extreme example of a forward swept wing

Sweeping a wing forward has approximately the same effect as rearward in terms of drag reduction, but has other advantages in terms of low-speed handling where tip stall problems simply go away. In this case the low-speed air flows towards the fuselage, which acts as a very large wing fence. Additionally, wings are generally larger at the root anyway, which allows them to have better low-speed lift.

However, this arrangement also has serious stability problems. The rearmost section of the wing will stall first causing a pitch-up moment pushing the aircraft further into stall similar to a swept back wing design. Thus swept-forward wings are unstable in a fashion similar to the low-speed problems of a conventional swept wing. However unlike swept back wings, the tips on a forward swept design will stall last, maintaining roll control.

Forward-swept wings can also experience dangerous flexing effects compared to aft-swept wings that can negate the tip stall advantage if the wing is not sufficiently stiff. In aft-swept designs, when the airplane maneuvers at high load factor the wing loading and geometry twists the wing in such a way as to create washout (tip twists leading edge down). This reduces the angle of attack at the tip, thus reducing the bending moment on the wing, as well as somewhat reducing the chance of tip stall.[25] However, the same effect on forward-swept wings produces a wash-in effect that increases the angle of attack promoting tip stall.

Small amounts of sweep do not cause serious problems, and had been used on a variety of aircraft to move the spar into a convenient location, as on the Junkers Ju 287 or HFB 320 Hansa Jet.[26][27] However, larger sweep suitable for high-speed aircraft, like fighters, was generally impossible until the introduction of fly by wire systems that could react quickly enough to damp out these instabilities. The Grumman X-29 was an experimental technology demonstration project designed to test the forward swept wing for enhanced maneuverability during the 1980s.[28][29] The Sukhoi Su-47 Berkut is another notable demonstrator aircraft implementing this technology to achieve high levels of agility.[30] To date, no highly swept-forward design has entered production.

History[edit]

Early history[edit]

The first successful aeroplanes adhered to the basic design of rectangular wings at right angles to the body of the machine, but there were experimentalists who explored other geometries to achieve better aerodynamic results. The swept wing geometry appeared before World War I, and was conceived as a means of permitting the design of safe and stable aeroplanes. The best of these designs imposed "self-damping" inherent stability upon a tailless swept wing. These inspired several flying wing gliders and some powered aircraft during the interwar years.[31]

A Burgess-Dunne tailless biplane: the angle of sweep is exaggerated by the sideways view, with washout also present at the wingtips.

The first to achieve stability was British designer J. W. Dunne who was obsessed with achieving inherent stability in flight. He successfully employed swept wings in his tailless aircraft (which, crucially, used washout) as a means of creating positive longitudinal static stability.[32] For a low-speed aircraft, swept wings may be used to resolve problems with the center of gravity, to move the wing spar into a more convenient location, or to improve the sideways view from the pilot's position.[31] By 1905, Dunne had already built a model glider with swept wings and by 1913 he had constructed successful powered variants that were able to cross the English Channel. The Dunne D.5 was exceptionally aerodynamically stable for the time,[33] and the D.8 was sold to the Royal Flying Corps; it was also manufactured under licence by Starling Burgess to the United States Navy amongst other customers.[34]

Dunne's work ceased with the onset of war in 1914, but afterwards the idea was taken up by G. T. R. Hill in England who designed a series of gliders and aircraft to Dunne's guidelines, notably the Westland-Hill Pterodactyl series.[35] However, Dunne's theories met with little acceptance amongst the leading aircraft designers and aviation companies at the time.[36]

German developments[edit]

Adolf Busemann proposed the use of swept-wings to reduce drag at high speed, at the Volta Conference in 1935.

The idea of using swept wings to reduce high-speed drag was developed in Germany in the 1930s. At a Volta Conference meeting in 1935 in Italy, Dr. Adolf Busemann suggested the use of swept wings for supersonic flight. He noted that the airspeed over the wing was dominated by the normal component of the airflow, not the freestream velocity, so by setting the wing at an angle the forward velocity at which the shock waves would form would be higher (the same had been noted by Max Munk in 1924, although not in the context of high-speed flight).[37] Albert Betz immediately suggested the same effect would be equally useful in the transonic.[38] After the presentation the host of the meeting, Arturo Crocco, jokingly sketched "Busemann's airplane of the future" on the back of a menu while they all dined. Crocco's sketch showed a classic 1950's fighter design, with swept wings and tail surfaces, although he also sketched a swept propeller powering it.[37]

At the time, however, there was no way to power an aircraft to these sorts of speeds, and even the fastest aircraft of the era were only approaching 400 km/h (249 mph).The presentation was largely of academic interest, and soon forgotten. Even notable attendees including Theodore von Kármán and Eastman Jacobs did not recall the presentation 10 years later when it was re-introduced to them.[39]

Hubert Ludwieg of the High-Speed Aerodynamics Branch at the AVA Göttingen in 1939 conducted the first wind tunnel tests to investigate Busemann's theory.[2] Two wings, one with no sweep, and one with 45 degrees of sweep were tested at Mach numbers of 0.7 and 0.9 in the 11 x 13 cm wind tunnel. The results of these tests confirmed the drag reduction offered by swept wings at transonic speeds.[2] The results of the tests were communicated to Albert Betz who then passed them on to Willy Messerschmitt in December 1939. The tests were expanded in 1940 to include wings with 15, 30 and -45 degrees of sweep and Mach numbers as high as 1.21.[2]

With the introduction of jets in the later half of the Second World War, the swept wing became increasingly applicable to optimally satisfying aerodynamic needs. The German jet-powered Messerschmitt Me 262 and rocket-powered Messerschmitt Me 163 suffered from compressibility effects that made both aircraft very difficult to control at high speeds. In addition, the speeds put them into the wave drag regime, and anything that could reduce this drag would increase the performance of their aircraft, notably the notoriously short flight times measured in minutes. This resulted in a crash program to introduce new swept wing designs, both for fighters as well as bombers. The Blohm & Voss P 215 was designed to take full advantage of the swept wing's aerodynamic properties; however, an order for three prototypes was received only weeks before the war ended and no examples were ever built.[40] The Focke-Wulf Ta 183 was another swept wing fighter design, but was also not produced before the war's end.[41] In the post-war era, Kurt Tank developed the Ta 183 into the IAe Pulqui II, but this proved unsuccessful.[42]

A prototype test aircraft, the Messerschmitt Me P.1101, was built to research the tradeoffs of the design and develop general rules about what angle of sweep to use.[43] When it was 80% complete, the P.1101 was captured by US forces and returned to the United States, where two additional copies with US-built engines carried on the research as the Bell X-5.[44] Germany's wartime experience with the swept wings and its high value for supersonic flight stood in strong contract to the prevailing views of Allied experts of the era, who commonly espoused their belief in the impossibility of manned vehicles travelling at such speeds.[45]

Postwar advancements[edit]

Artist's impression of the Miles M.52

During the immediate post-war era, several nations were conducting research into high speed aircraft. In the United Kingdom, work commenced during 1943 on the Miles M-52, a high-speed experimental aircraft equipped with a straight wing that was developed in conjunction with Frank Whittle's Power Jets company, the Royal Aircraft Establishment (RAE) in Farnborough, and the National Physical Laboratory.[46] Despite being envisioned to be capable of achieving 1,000 miles per hour (1,600 km/h) in level flight, thus enabling the aircraft to potentially be the first to exceed the speed of sound in the world,[46] in February 1946, the programme was abrupted discontinued for unclear reasons.[47] It has since been widely recognised that the cancellation of the M.52 was a major setback in British progress in the field of supersonic design.[31]

Another, more successful, programme was the US's Bell X-1, which also was equipped with a straight wing. According to Miles Chief Aerodynamicist Dennis Bancroft, the Bell Aircraft company was given access to the drawings and research on the M.52.[48] On 14 October 1947, the Bell X-1 performed the first manned supersonic flight, piloted by Captain Charles "Chuck" Yeager, having been drop launched from the bomb bay of a Boeing B-29 Superfortress and attained a record-breaking speed of Mach 1.06 (700 miles per hour (1,100 km/h; 610 kn)).[31] The news of a successful straight-wing supersonic aircraft surprised many aeronautical experts on both sides of the Atlantic, as it was increasingly believed that a swept-wing design not only highly beneficial but also necessary to break the sound barrier.[45]

The de Havilland DH 108, a prototype swept-wing aircraft

During the final years of the Second World War, aircraft designer Sir Geoffrey de Havilland commenced development on the de Havilland Comet, which would become the world's first jet airliner. An early design consideration was whether to apply the new swept-wing configuration.[49] Thus, an experimental aircraft to explore the technology, the de Havilland DH 108, was developed by the firm in 1944, headed by project engineer John Carver Meadows Frost with a team of 8–10 draughtsmen and engineers. The DH 108 primarily consisted of the pairing of the front fuselage of the de Havilland Vampire to a swept wing and compact stubby vertical tail; it was the first British swept wing jet, unofficially known as the "Swallow".[50] It first flew on 15 May 1946, a mere eight months after the project's go-ahead. Company test pilot and son of the builder, Geoffrey de Havilland Jr., flew the first of three aircraft and found it extremely fast – fast enough to try for a world speed record. On 12 April 1948, a D.H.108 did set a world's speed record at 973.65 km/h (605 mph), it subsequently became the first jet aircraft to exceed the speed of sound.[51]

Around this same timeframe, the Air Ministry introduced a program of experimental aircraft to examine the effects of swept wings, as well as the delta wing configuration.[52] Furthermore, the Royal Air Force (RAF) identified a pair of proposed fighter aircraft equipped with swept wings from Hawker Aircraft and Supermarine, the Hawker Hunter and Supermarine Swift respectively, and successfully pressed for orders to be placed 'off the drawing board' in 1950.[53] On 7 September 1953, the sole Hunter Mk 3 (the modified first prototype, WB 188) flown by Neville Duke broke the world air speed record for jet-powered aircraft, attaining a speed of 727.63 mph (1,171.01 km/h) over Littlehampton, West Sussex.[54] This world record stood for less than three weeks before being broken on 25 September 1953 by the Hunter's early rival, the Supermarine Swift, being flown by Michael Lithgow.[55]

In February 1945, NACA engineer Robert T. Jones started looking at highly swept delta wings and V shapes, and discovered the same effects as Busemann. He finished a detailed report on the concept in April, but found his work was heavily criticised by other members of NACA Langley, notably Theodore Theodorsen, who referred to it as "hocus-pocus" and demanded some "real mathematics".[37] However, Jones had already secured some time for free-flight models under the direction of Robert Gilruth, whose reports were presented at the end of May and showed a fourfold decrease in drag at high speeds. All of this was compiled into a report published on June 21, 1945, which was sent out to the industry three weeks later.[56] Ironically, by this point Busemann's work had already been passed around.

The first American swept-wing aircraft, the Boeing B-47 Stratojet

On May 1945, the American Operation Paperclip reached Braunschweig, where US personnel discovered a number of swept wing models and a mass of technical data from the wind tunnels. One member of the US team was George S. Schairer, who was at that time working at the Boeing company. He immediately forwarded a letter to Ben Cohn at Boeing, communicating the value of the swept wing concept.[57][58] He also told Cohn to distribute the letter to other companies as well, although only Boeing and North American made immediate use of it.[citation needed]

Boeing was in the midst of designing the B-47 Stratojet, and the initial Model 424 was a straight-wing design similar to the B-45, B-46 and B-48 it competed with. Analysis by Boeing engineer Vic Ganzer suggested an optimum sweepback angle of about 35 degrees.[59] By September 1945, the Braunschweig data had been worked into the design, which re-emerged as the Model 448, a larger six-engine design with more robust wings swept at 35 degrees.[37] Another re-work moved the engines into strut-mounted pods under the wings due to concerns of the uncontained failure of an internal engine could potentially destroy the aircraft via either fire or vibration.[60] The resulting B-47 was hailed as the fastest of its class in the world during the late 1940s,[61] and trounced the straight-winged competition. Boeing's jet-transport formula of swept wings and pylon-mounted engines has since been universally adopted.[citation needed]

In fighters, North American Aviation was in the midst of working on a straight-wing jet-powered naval fighter, then known as the FJ-1; it was later submitted to the United States Air Force as the XP-86.[62] Larry Green, who could read German, studied the Busemann reports and convinced management to allow a redesign starting in August 1945.[37][63][64] The performance of the F-86A allowed it set the first of several official world speed records, attaining 671 miles per hour (1,080 km/h) on 15 September 1948, flown by Major Richard L. Johnson.[65] With the appearance of the MiG-15, the F-86 was rushed into combat, while straight-wing jets like the Lockheed P-80 Shooting Star and Republic F-84 Thunderjet were quickly relegated to ground attack missions. Some, such as the F-84 and Grumman F-9 Cougar, were later redesigned with swept wings from straight-winged aircraft.[66][67] Later planes, such as the North American F-100 Super Sabre, would be designed with swept wings from the start, though additional innovations such as the afterburner, area-rule and new control surfaces would be necessary to master supersonic flight.[68][13]

A MiG-15 of the Polish Air Force

The Soviet Union was also intrigued about the idea of swept wings on aircraft, when their "captured aviation technology" counterparts to the western Allies spread out across the defeated Third Reich. Artem Mikoyan was asked by the Soviet government's TsAGI aviation research department to develop a test-bed aircraft to research the swept wing idea — the result was the late 1945-flown, unusual MiG-8 Utka pusher canard layout aircraft, with its rearwards-located wings being swept back for this type of research.[69] The swept wing was applied to the MiG-15, an early jet-powered fighter, its maximum speed of 1,075 km/h (668 mph) outclassed the straight-winged American jets and piston-engined fighters initially deployed during the Korean War.[70] The MiG-15 is believed to have been one of the most produced jet aircraft; in excess of 13,000 would ultimately be manufactured.[71]

The MiG-15, which could not safely exceed Mach 0.92, served as the basis for the MiG-17, which was designed to be controllable at higher Mach numbers.[72] Its wing featured a "sickle sweep" compound shape, somewhat similar to the F-100 Super Sabre, with a 45° angle near the fuselage and a 42° angle for the outboard part of the wings.[73] A further derivative of the design, designated MiG-19, featured a relatively thin wing suited to supersonic flight that was designed at TsAGI, the Soviet Central Aerohydrodynamic Institute; swept back at an angle of 55 degrees, this wing featured a single wing fence on each side.[74] A specialist high-altitude variant, the Mig-19SV, featured, amongst other changes, flap adjusted to generate greater lift at higher altitudes, helping to increase the aircraft's ceiling from 17,500 m (57,400 ft) to 18,500 m (60,700 ft).[75][76]

Germany's swept wing research also made its way to the Swedish aircraft manufacturer SAAB, allegedly via a group of ex-Messerschmitt engineers that had fled to Switzerland during late 1945.[77][78] At the time, SAAB was eager to make aeronautic advances, particularly in the new field of jet propulsion.[79] The company incorporated both the jet engine and the swept wing to produce the Saab 29 Tunnan fighter; on 1 September 1948, the first prototype conducted its maiden flight, flown by the English test pilot S/L Robert A. 'Bob' Moore, DFC and bar,[80] Although not well known outside Sweden, the Tunnan was the first Western European fighter to be introduced with such a wing configuration.[81][82] In parallel, SAAB also developed another swept wing aircraft, the Saab 32 Lansen, primarily to serve as Sweden's standard attack aircraft.[83] Its wing, which had a 10 per cent laminar profile and a 35° sweep, featured triangular fences near the wing roots in order to improve airflow when the aircraft was being flown at a high angle of attack.[83][84] On 25 October 1953, a SAAB 32 Lansen attained a Mach number of at least 1.12 while in a shallow dive, exceeding the sound barrier.[84]

The Avro Vulcan, flying at Farnborough, 1958.

The dramatic successes of aircraft such as Hawker Hunter, the B-47, and F-86 embodied the widespread acceptance of the swept wing research acquired from Germany. Eventually, almost all advanced design efforts would incorporate a swept wing configuration. The classic Boeing B-52, designed in the 1950s, continues in service as a high-subsonic long-range heavy bomber despite the development of the triple-sonic North American B-70 Valkyrie, supersonic swing-wing Rockwell B-1 Lancer, and flying wing designs.[85][86] While the Soviets never matched the performance of the Boeing B-52 Stratofortress with a jet aircraft, the intercontinental range Tupolev Tu-95 turboprop bomber with its near-jet class top speed of 920 km/h, combining swept wings with propeller propulsion, also remains in service today, being the fastest propeller-powered production aircraft.[87] In Britain, a range of swept-wing bombers were designed, these being the Vickers Valiant (1951),[88] the Avro Vulcan (1952),[89] and the Handley Page Victor (1952).[90]

By the early 1950s, nearly every new fighter was either rebuilt or designed from scratch with a swept wing. By the 1960s, most civilian jets also adopted swept wings. The Douglas A-4 Skyhawk and Douglas F4D Skyray were examples of delta wings that also have swept leading edges with or without a tail. Most early transonic and supersonic designs such as the MiG-19 and F-100 used long, highly swept wings. Swept wings would reach Mach 2 in the arrow-winged BAC Lightning, and stubby winged Republic F-105 Thunderchief, which was found to be wanting in turning ability in Vietnam combat. By the late 1960s, the F-4 Phantom and Mikoyan-Gurevich MiG-21 that both used variants on tailed delta wings came to dominate front line air forces. Variable geometry wings were employed on the American F-111, Grumman F-14 Tomcat and Soviet Mikoyan MiG-27, although the idea would be abandoned for the American SST design. After the 1970s, most newer generation fighters optimized for maneuvering air combat since the USAF F-15 and Soviet Mikoyan MiG-29 have employed relatively short-span fixed wings with relatively large wing area.[citation needed]

See also[edit]

  • Delta wing
  • Theodore von Kármán, first to recognize the importance of the swept wing[91]
  • Trapezoidal wing
  • Wing configuration

References[edit]

Citations[edit]

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Further reading[edit]

  • "The High-speed Shape: Pitch-up and palliatives adopted on swept-wing aircraft", Flight International, 2 January 1964

External links[edit]

  • Swept Wings and Effective Dihedral
  • The development of swept wings
  • Simple sweep theory math
  • Advanced math of swept and oblique wings
  • The L-39 and swept wing research
  • Sweep theory in a 3D environment
  • CFD results showing the three-dimensional supersonic bubble over the wing of an A 320. Another CFD result showing the MDXX and how the shock vanishes close to the fuselage where the aerofoil is more slender