Flying oblique!

In Aeronautics, we see this obsession with symmetry and precision and it is quite understandableJ. When you cut an aircraft into two halves; left and right, everyone expects that the two halves should appear exactly symmetric. The general intuition is that an cleaner/smoother aircraft is a better flier. This might be one of the reasons why our civil aircraft haven’t seen any radical aircraft design which were commercially successful and this is why an oblique wing seems weird to many.

History

3-view drawing of AD1

Going by the history, an oblique wing design was originally proposed by Edmond de Marcay and Emile Moonen in 1912. The idea was to vary sweep of oblique wings for landing in sideslip. It was further studied by Richard Vogt in Germany for increasing wing sweep as the speed of the aircraft increases. R. T. Jones (then at the NACA Langley Memorial Aeronautical Laboratory) was introduced to oblique wings soon after and remained the most notable advocate of the concept. He initiated wind tunnel studies beginning in the late 1940’s on the merits of such a wing and how it could be integrated into a high-speed civil transport. However, this design got significant limelight only after the advent of NASA’s very famous AD1 experimental aircraft which was developed by Burt Rutan. This prototype was a result of NASA’s wind tunnel project conducted in 1979 which showed that a pivoting wing could increase fuel efficiency at supersonic speeds by as much as 100 percent. The AD-1 allowed the wing to pivot gradually as speed increased, always positioning it for maximum efficiency at the plane's current speed. However, testing also revealed that the aircraft became extremely unstable as the wing moved into an oblique position. A human pilot could not cope with the constant, minute adjustments necessary to maintain flight under these conditions. At the time, flight control computers were not sophisticated enough to manage it, either.

Concept of Drag Divergence Mach number and Sweep

As an aircraft approaches the speed of sound, shock waves form. These waves create a form of drag known as wave drag. This drag component drastically increases the total drag on the aircraft which requires substantial thrust from the engines to overcome. The mach number at which the drag starts drastically increasing is called Drag divergence mach number. Unswept wings are very bad at dealing with wave drag.

Swept wings reduce the wave drag by redistributing the shock waves along the plane's aerodynamic profile and shift this drag divergence mach number to a higher value. They are ideal for these high-speed conditions. Unfortunately, they are inefficient and burn too much fuel to stay aloft in low speeds.

Variable sweep and Oblique wings

F-14 Tomcats flying with three different
sweep positions

A design which is effective in both low and high speed regime could be something which would have unswept wings in low speeds and sweep the wing back as the aircraft move towards sonic speeds. In this regard, variable sweep and oblique wings are viable options.

In variable sweep aircraft like F14 Tomcat, B1B lancer and Su-24, the wings are attached to the fuselage using a pivot unlike conventional designs where they are completely fixed. These pivoted wings can be swiveled back and forth using actuators. The wings remain unswept during low speed flights, take off and landing and start sweeping back as they pick up speed and go supersonic. Again, these birds have their own set of disadvantages. The pivoting and actuators make the system complex and the structure further heavier. Also, since they are two separate wings which will not run through the fuselage,  the structure becomes heavier at the roots. This in turn increases the lift required to keep it aloft and the thrust required to push it forward. Also, since the wings sweep back heavily at transonic speeds, the aerodynamic centre (The imaginary point at which the total lift of the aircraft is acting) also moves further back. This creates a nosedown moment which requires a large horizontal tail to counter and trim the aircraft which in turn increases weight and also causes large trim drag burning up the fuel.

The significance of an oblique wing can only be appreciated when one knows the advantages and disadvantages of the variable sweep aircraft. The beauty of this type of wing configuration is that it keeps all the positives of the variable sweep and negates almost all the negatives.

The oblique wing works on the same principle of varying sweep with speed. However, unlike in variable sweep it is achieved by a single wing which is pivoted at a single point on the fuselage. So the wing basically rotates about the pivot point which at high speed configuration would have one side of the wing swept forward and other side backward.

  Studies show that the oblique wing produce much lesser drag than generally swept wing. As the wing is pivoted at only one point, there are fewer moving parts than the swing wing type which makes the aircraft much lighter and much less complex. Further, actuator loads are also reduced. As one portion of the wing is forward and other backward, the bending moments caused on one side of the wing is reacted by the bending moments on the other side avoiding the bending loads on the pivot. Also, the aerodynamic centre remains  almost constant as one side of the wing is forward and  the other side backward. This will help in designing a smaller horizontal tail. All these factors contribute in making a lighter and much efficient aircraft.

No Design is perfect

Sadly, oblique wing design also has its own drawbacks. The problem of controlling an oblique flying wing is more complex than other aircraft because this type of vehicle is considered to have multiple configurations based on the operation speed. The yawed wing causes a dynamic coupling between the degrees of freedom. The oblique wing causes some yawing and rolling. Pitch up and pitch down commands create rotation in other directions which complicates the situation further. Controllability of the aircraft was a major issue with no computers to assist in stability and control

SwitchBlade- Revisiting Oblique wing

The Switchblade was a proposed unmanned aerial vehicle with oblique wings developed by Northrop Grumman under the contract of United States Defence Advanced Research Projects Agency (DARPA). The program aimed at producing a technology demonstrator aircraft to explore the various challenges which the radical design entails. The proposed aircraft was a purely flying wing (an aircraft with no other auxiliary surfaces such as tails, canards or a fuselage).

Northrop Grumman Switchblade in Oblique configuration

The program entailed two phases. Phase I explored the theory and result in a conceptual design, while Phase II would have resulted in the design, manufacture and flight test of an aircraft. The outcome of the program would have resulted in a dataset that could then be used when considering future military aircraft designs. The flight of the Switchblade, which has a 61-meter long oblique wing perpendicular to its engines like a typical aircraft, was scheduled for 2020. As the aircraft speed increased, the wing would begin to pivot, so that when it broke the sound barrier, the wing would have swiveled 60 degrees, with one wingtip pointing forward and the other backward. The plane was to be totally controlled by an onboard computer, which would handle all the parameters needed for maintaining a stable flight during the mission, appropriately.

Though the program was cancelled in 2008, it opened up the possibilities for high speed UAV designs and utilization of long forgotten technology.