Variable Sweep Wings and Aerodynamic Characteristics

  
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2) INTRODUCTION 
 
 
Presumably, after decades of designing and building airplanes, a person would suspect that all 
airplanes with a similar purpose would look like the same. All fighters would look alike, all 
passenger jets would look alike, and so on. Nevertheless, this has not happened. Even more than five 
decades after the dawn of the jet age (1940s), jet fighters designed around the same time can look 
radically different. The level of technological capability can explain most of these differences; 
cultural influences are also a factor, although generally of less importance. Aeronautical engineers, 
like any other group of people sharing similar interests, can become enthusiastic about certain ideas 
at the same time and overlook their drawbacks. That is what happened with variable-sweep wings. 
The swept-back wing used for the B-47 and common to many aircraft that followed demonstrated 
that airplanes could use wing designs other than the straight wing to achieve certain performance 
goals. Straight wings were clearly advantageous for short takeoffs and landings, low speed, and fuel-
efficient flight, but swept wings were ideal for high-speed, particularly supersonic flight. 
All aircraft represent numerous compromises made by their designers. Some aircraft need to fly at 
very high speeds whereas others need to fly very slowly. Some have to be highly fuel-efficient 
whereas others have to accomplish their missions without regard to cost. Designers, therefore, try to 
optimize each aircraft so that it accomplishes its mission as best as it can. But sometimes aircraft are 
required to do things that demand design features that oppose each other. The best example is an 
aircraft that can fly at high supersonic speeds but still needs to land at relatively low speeds, such as 
on the deck of an aircraft carrier. 
Experiments with variable-sweep wings began in France around 1911. Dr. Adolf Busemann, a 
German designer, presented a theoretical concept for a practical moveable wing at a convention in 
Rome just before World War II. His theory, supported with research by Dr. Albert Betz of the 
Göttingen Aerodynamics Research Institute, led Messerschmitt to begin developing in 1942 a 
variable-sweep wing design, the P-1101. The war ended before the aircraft could be produced. It is 
also doubtful that existing engines provided high speed enough for the design to make an 
appreciable difference in performance. 
Beginning in the late 1940s, as technological capabilities improved, designers in the United States 
began to examine the possibility of moving the wing of an aircraft while it was in flight, so that it 
extended straight out from the fuselage for takeoff and landing and swept back for high-speed 
operations. Such an airplane might be able to accomplish its demanding missions without paying a 
performance penalty. 
In the early 1950s, Bell Aircraft built the experimental X-5 aircraft for the U.S. Air Force and the 
National Advisory Committee for Aeronautics (NACA). It had a wing that could be moved 
backward and forward in flight - a variable-sweep wing. Grumman Aircraft built the F-10-F for the 
U.S. Navy, also with variable-sweep wings. Both designs proved that the concept of movable wings 
worked. 
A few years later, in 1959, engineers at NASA's Langley Research Centre discovered the two-pivot 
variable-sweep concept-as opposed to the single-pivot used in earlier experiments. This 
development led to the success of the variable-wing idea. 
  
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In the late 1950s, the U.S. Air Force submitted specifications for a plane with variable-sweep wings 
that could fly at supersonic speeds and cruise for long distances at high altitudes to reach its distant 
targets. Secretary of Defence Robert McNamara wanted to combine this requirement with Army 
needs for close air support and a Navy requirement for air defence of the naval fleet. This led in 
December 1960 to the Tactical Fighter Experimental (TFX) program. This consisted in an aircraft 
suitable for use by the Air Force, which would operate in the close air support role for Army ground 
forces and in its own roles, and by the Navy in aircraft carrier roles. 
The Department of Defence selected General Dynamics to develop two versions of the TFX, the F-
111A for the Air Force and the F-111B for the Navy. Roll out took place on October 15, 1964, and 
Secretary McNamara stated that the Air Force and Navy now had an aircraft with the range of a 
transport, capacity and endurance of a bomber, and agility of a fighter. However, the Navy cancelled 
its portion of the program, in August 1968, finding that the plane was too heavy for use aboard 
aircraft carriers. Commonality between the aircraft used by the two services had also dropped 
significantly by that time with the adoption of different engines. The Navy subsequently selected 
Grumman Aircraft to develop the F-14 Tomcat interceptor, which also had a variable-sweep, or 
"swing-wing". 
The F-111 had numerous problems during its early service, including problems with cracks in the 
large gearbox used to move the wings. The plane saw service in Vietnam, and the F-111F saw 
considerable action during the Gulf War. Perhaps its greatest success occurred in a combined U.S. 
Air Force and U.S. Navy attack on Libya and its terrorist government in mid-April 1986, at El 
Dorado Canyon. 
The F-14 adopted by the Navy incorporated a swing-wing that could be manually controlled by the 
pilot or shifted automatically according to the plane's speed. It moved forward to allow the plane to 
land on tiny aircraft carrier decks at relatively low speeds and backward as the plane dashed out to 
intercept Soviet bombers. More than 700 F-14s were produced, in several variants, and more than 70 
of them were exported to Iran in the 1970s. It first entered service in the mid-1970s, and was used 
„till 2001. Despite its long service, the F-14 has been the most expensive interceptor aircraft to 
operate in the U.S. Army. 
During the 1970s, an Israeli Air Force (IAF) pilot evaluated the Navy F-14 and the Air Force F-15 
Eagle for service in the IAF. He walked around both airplanes and counted their control surfaces 
such as ailerons, flaps, slats, and speed brakes. The F-14 had more control surfaces and the pilot 
determined that this would make it more difficult and expensive to maintain; for this and many other 
reasons, the IAF subsequently purchased the F-15 Eagle. In many ways this was an omen, for the 
variable-sweep wing, the largest moving part ever developed for an aircraft, proved to be more 
uncertain than it was worth for many aircraft. 
But for a period, variable-sweep wings were in vogue and more than a half dozen major military 
aircraft were designed with variable sweep wings during the 1960s and 1970s, with the number of 
swing-wing aircraft numbering in the thousands. The Soviets developed the Su-24 and MiG-27 
attack planes and the MiG-23 fighter, all with swing-wings. The European consortium Panavia 
developed the Tornado, produced in both ground attack and interceptor versions. It too had a 
variable-sweep wing. Aircraft designers also applied variable-sweep wings to large bomber-size 
aircraft. North American Rockwell began the B-1A bomber in the early 1970s as the U.S. Air 
Force's new strategic bomber (it was cancelled in the late 1970s and revived a few years later). The 
Soviet designed firm Tupolev to develop the Tu-22M Backfire naval attack bomber and the Tu-160 
Blackjack strategic bomber. 
  
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By the 1980s, however, no one was designing variable-sweep aircraft and no improvements on this 
technology have been incorporated in new production military aircraft in the last 15 years, although 
work is still being carried out on wings that move in other ways. The technology of variable-sweep 
wings lasted little more than 20 years before being phased out, although hundreds of aircraft 
continued to fly for years. 
There were several reasons for the move away from this technology, but the primary one was that 
the large metal gearbox needed to move the wings was complicated and heavy. This increased 
maintenance requirements and decreased fuel performance. An aircraft capable of moving its wing 
forward for fuel-efficient flight could never be as efficient as an airplane equipped with a straight 
wing. The same was true for aircraft with swept-back wings; they would always be more efficient 
than swing-wings aircraft. The B-1B Lancer, for example, has never been able to achieve its original 
range requirements and has to refuel in the air more often than planned. It also rarely flew at the 
high speed, allowed by sweeping back the wings. 
 
Ultimately, however, a tendency inversion in the research activities seems to be happened. In the last 
fifteen years, DARPA (Defence Advanced Research Projects Agency) and NASA (National 
Aeronautic & Space Agency) have invested millions of dollars on an experimental secret program to 
develop a high supersonic manoeuvrability aircraft. Their aim was reached by the “X29” project, 
with the creation of an aircraft with forward swept wing. The aircraft had the expected project 
requirements but was very unstable in the subsonic regime and could not reach speeds higher than 
Mach 2. Thus, the two agencies continued their research activities „till arriving to the “Bird of Pray” 
project realization. A new era opened: the first forward variable sweep wings aircraft was created. 
Being the X29 successor, it maintained  the high manoeuvrability capability and , moreover , it was 
able to fly „till Mach 3, thanks to the dart configuration (with the wings completely closed). On 
September 2002, it was unveiled for the first time as, it was no more necessary to conceal its 
existence.     
  
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3) THE AIRCRAFT 
 
 
 
3.1) Aerodynamic Drag 
Very narrow gains (1 % or less) can translate into a change of technology. It is widely assumed that 
the fuel crisis of the 1970s created the need to invest in drag reduction technology for aircraft 
transport. However, the problem is wider than that, since all the aerodynamic systems use external 
power that is partially dissipated due to drag forces.  
 
Effects of Drag Reduction 
 
For example, a reduction in the drag coefficient of an ordinary passenger car from CD = 0.4 to 
CD=0.3 would improve the fuel consumption by 7.5 %. This saving multiplied by the number of 
road vehicles in Europe and North America yields a figure (at least 10 billion gallons/year) that 
could affect the price of the crude oil in the world markets.  
The reduction of 10 % drag on a large military transport aircraft would save over 10 million gallons 
of fuel over the life-time of the aircraft. A 15 % drag reduction on the Airbus A340-300B would 
yield a 12 % fuel saving, other parameters being constant ( Mertens , 1998).  
The effects resulting from using VSWs, instead of fixed one, will be described later in terms of 
reduction of fuel consumption and flexibility of mission profile. 
 
Flow Physics 
 
The fundamental mechanisms by which drag is produced in steady state conditions can be reduced 
to the following: 
‘Viscous drag’ (or skin friction drag) is due to the stresses on the aerodynamic surfaces and in the 
boundary layer. The decreased momentum in the flowfield results in a corresponding loss of 
momentum of the aerodynamic system. Some of the physical aspects involved in the viscous drag 
loss are: presence of shear layers, turbulent transition, and boundary layer separation.  
If we are in a laminar flow conditions it is computed by:                
 
  
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While if the flow becomes turbulent it‟s evaluated by the formula:            
 
The amount of energy losses depends largely on the aero-hydrodynamic system. On a sailing boat it 
can range between 1/3 and (nearly) the total drag, depending on the speed of the craft (at low speeds 
the viscous drag is large, in percent, whereas the wave drag is low). Some typical viscous losses are 
listed below:  
Table 1: Summary of viscous drag 
Supersonic fighter  25-30 %  
Large transport aircraft 40 %  
Executive aircraft  50 %  
VTOL aircraft  70-80 %  
Underwater bodies  70 %  
Ships at low/high speed 90-30 %  
 
 
‘Drag due to lift’ : this type of drag is due to the vorticity produced by a lifting wing (induced drag 
or vortex drag) and it is expressed as momentum deficiency in the wake. This type of drag can be a 
drawback of high-lift systems.  Vortices are released during flow separation and trail downstream to 
form structured or unstructured wake patterns. 
The dissipation of these vortices farther downstream is one of the sources of loss, but this is of 
viscous nature. The inviscid nature of the drag is also due to the downwash created in the slipstream, 
which on turns is related to an induced angle of attack.  
To calculate the induced drag, we can use the relation:  
2' LDi CkC    
, where k‟ is  defined as  
Re1' Akk  Σ    . 
It is function of: the Oswald Coefficient e , the wing geometry ( throughout the aspect ratio AR ) and 
the k factor, that shows the dependency from another drag typology, called Form Drag, due to  
  
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pressure variations of the flow along the wing surface. Usually it is considered as a percentage of the 
induced drag for the reason  that  it cannot be estimated numerically. 
For a straight wing it is:  
 
While for a swept wing, it varies with the sweep angle Ο and the Aspect Ratio:  
 
 
‘Interference’ is the effect of the presence of one body on the aerodynamics of a second body. The 
interference drag is a system drag that is present even in absence of viscous effects (ideal fluid) and 
non-lifting conditions. Since interference occurs in many practical situations, interference drag is a 
separate topic.  
 
‘Wave drag’ in aerodynamics is drag associated with the shock wave and shock-induced separation. 
More exactly, it is associated with the energy radiated away from the vehicle in the form of pressure 
waves in much the same way as a fast moving ship causes waves in the water surface. It appears at 
transonic and supersonic speeds.  The problem is more general in hydrodynamics, since wave 
propagation occurs at all speeds for all types of sailing vessels and for most cases of submerged 
bodies.  
There are several ways of dealing with wave drag: use of transonic/supersonic area ruling for wing-
body combinations; use of supercritical airfoils, thin wing sections, wing sweep, low-aspect ratio 
wings, boundary layer control, blunt leading edge (at hypersonic speeds). Less orthodox methods 
include oblique and anti-symmetric wings (wings never built, in fact).  
At transonic speeds, some of the main concerns are driving the drag divergence upward, removing 
the buffeting and the possible shock stall.  
At supersonic and hypersonic speeds a few peculiar problems appear: namely, aero-thermodynamic 
heating, and structural stiffness compatible with volume distribution and wing thickness.  
 
 
 
 
 
  
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Summarizing all the previous considerations about drag, one can describe the drag acting on a finite 
span wing or on a complete aircraft as:  
DwDiDD CCCC       0  
The first term 
0DC  is the drag coefficient, also known as Parasite drag coefficient, because of its 
independence of the lift coefficient. In fact, it therefore exists when the configuration generates zero 
lift and brings inside the skin friction and form drag components. Usually its value for a jet fighter is 
about 0.015  0.03, where the wings may account for the 50% of it. 
 The second term DiC  is the drag due to lift coefficient that has a quadratic correlation with the lift 
coefficient and accounts for deviations from straight wing planforms like low aspect ratio and 
sweptback wings. 
The third term DwC  is the wave drag coefficient and represents the compressibility effects together 
with the skin friction drag and form drag, included in the drag due to lift coefficient. It is generated 
when the free stream Mach number is sufficiently large so that regions of supersonic flow exist in 
the flowfield. The designer can delay and/or reduce the compressibility drag rise by using low aspect 
ratio wings, by sweeping wings, and by using area rules. 
 
Speed-induced Drag 
 
Another classification sometimes used is that according to speed. The speed (e.g. Reynolds and 
Mach numbers) have, in fact, one of the most important effects on both the drag build-up and the 
drag level.  
„Speed‟ is related to the flow regime: laminar, transitional, and turbulent. This is a major problem in 
all aerodynamic systems. Laminar boundary layers are characterized by minimum skin friction drag. 
Laminar boundary layers are generally assumed to keep laminar at Reynolds numbers , to be 
transitional at about , and turbulent above this value.  
The actual transitional Reynolds numbers may depend on the specific case and several side 
constraints.  
Flat plate, circular cylinder, sphere and cones have been widely studied over the years, and the 
amount of data collected is staggering: in particular, drag data are available from the smallest 
Reynolds numbers (unity and below), to the largest Mach numbers (hypersonic speeds). The data 
witness the importance of this set of bodies as a limiting case of real life problems.