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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
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| 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
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| 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.
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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).
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Northrop Grumman Switchblade in Oblique configuration
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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.
