Wave drag
Encyclopedia
In aeronautics
, wave drag is a component of the drag
on aircraft, blade tips and projectiles
moving at transonic
and supersonic
speeds, due to the presence of shock wave
s. Wave drag is independent of viscous effects.
s around the body. Shock waves radiate a considerable amount of energy, resulting in drag on the body. Although shock waves are typically associated with supersonic flow, they form at a lower speed at areas on the body where local airflow accelerates to sonic speed. The effect is typically seen on aircraft at transonic
speeds (typically about Mach 0.8
), but it is possible to notice the problem at any speed over that of the critical Mach
of that aircraft. The magnitude of the rise in drag is impressive, typically peaking at about four times the normal subsonic drag. It is so powerful that, prior to the 1940s, it was thought that aircraft engines would not be powerful enough to overcome the drag, which led to the concept of a sound barrier
.
were able to dramatically reduce the magnitude of wave drag, and by the early 1950s the latest fighter aircraft
could reach supersonic
speeds.
These techniques were quickly put to use by aircraft designers. One common solution to the problem of wave drag was to use a swept wing
, which had actually been developed before WWII and used on some German wartime designs. Sweeping the wing makes it appear thinner and longer in the direction of the airflow, making a "normal" wing shape closer to that of the von Kármán ogive, while still remaining useful at lower speeds where curvature and thickness are important.
The wing need not be swept as it is possible to build a wing that is extremely thin. This solution was used on a number of designs beginning with the Bell X-1
, the first manned aircraft to fly at the speed of sound. The downside to this approach is that the wing is so thin it is no longer possible to use it for storage of fuel or landing gear.
Fuselage shaping was similarly changed with the introduction of the Whitcomb area rule
. Whitcomb had been working on testing various airframe shapes for transonic drag when, after watching a presentation by Adolf Busemann
in 1952, he realized that the Sears-Haack body had to apply to the entire aircraft. This meant that the fuselage needed to be made narrower where it joined the wings, so that the cross-section of the entire aircraft matched the Sears-Haack body, not just the fuselage itself.
Application of the area rule can also be seen in the use of anti-shock bodies
on transonic
aircraft, including some jet airliner
s. Anti-shock bodies, which are pods along the trailing edges of the wings, serve the same role as the narrow waist fuselage design of other transonic aircraft.
is a new wing design that results in reasonable low speed lift like a normal planform
, but has a profile considerably closer to that of the von Kármán ogive. All modern civil airliners use forms of supercritical aerofoil and have substantial supersonic flow over the wing upper surface.
Busemann's biplane
avoids wave drag entirely, but is incapable of generating lift, and has never flown.
Aeronautics
Aeronautics is the science involved with the study, design, and manufacturing of airflight-capable machines, or the techniques of operating aircraft and rocketry within the atmosphere...
, wave drag is a component of the drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
on aircraft, blade tips and projectiles
Shell (projectile)
A shell is a payload-carrying projectile, which, as opposed to shot, contains an explosive or other filling, though modern usage sometimes includes large solid projectiles properly termed shot . Solid shot may contain a pyrotechnic compound if a tracer or spotting charge is used...
moving at transonic
Transonic
Transonic speed is an aeronautics term referring to the condition of flight in which a range of velocities of airflow exist surrounding and flowing past an air vehicle or an airfoil that are concurrently below, at, and above the speed of sound in the range of Mach 0.8 to 1.2, i.e. 600–900 mph...
and supersonic
Supersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...
speeds, due to the presence of shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...
s. Wave drag is independent of viscous effects.
Overview
Wave drag is caused by the formation of shock waveShock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...
s around the body. Shock waves radiate a considerable amount of energy, resulting in drag on the body. Although shock waves are typically associated with supersonic flow, they form at a lower speed at areas on the body where local airflow accelerates to sonic speed. The effect is typically seen on aircraft at transonic
Transonic
Transonic speed is an aeronautics term referring to the condition of flight in which a range of velocities of airflow exist surrounding and flowing past an air vehicle or an airfoil that are concurrently below, at, and above the speed of sound in the range of Mach 0.8 to 1.2, i.e. 600–900 mph...
speeds (typically about Mach 0.8
Mach number
Mach number is the speed of an object moving through air, or any other fluid substance, divided by the speed of sound as it is in that substance for its particular physical conditions, including those of temperature and pressure...
), but it is possible to notice the problem at any speed over that of the critical Mach
Critical Mach number
In aerodynamics, the critical Mach number of an aircraft is the lowest Mach number at which the airflow over any part of the aircraft reaches the speed of sound....
of that aircraft. The magnitude of the rise in drag is impressive, typically peaking at about four times the normal subsonic drag. It is so powerful that, prior to the 1940s, it was thought that aircraft engines would not be powerful enough to overcome the drag, which led to the concept of a sound barrier
Sound barrier
The sound barrier, in aerodynamics, is the point at which an aircraft moves from transonic to supersonic speed. The term, which occasionally has other meanings, came into use during World War II, when a number of aircraft started to encounter the effects of compressibility, a collection of several...
.
Research
When the problem was being studied, wave drag came to be split into two categories – wave drag caused by the wing as a part of generating lift, and that caused by other portions of the plane. In 1947, studies into both problems led to the development of perfect shapes to reduce wave drag as much as theoretically possible. For a fuselage the resulting shape was the Sears–Haack body, which suggested a perfect cross-sectional shape for any given internal volume. The von Kármán ogive was a similar shape for bodies with a blunt end, like a missile. Both were based on long narrow shapes with pointed ends, the main difference being that the ogive was pointed on only one end.Reduction of drag
A number of new techniques developed during and just after World War IIWorld War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
were able to dramatically reduce the magnitude of wave drag, and by the early 1950s the latest fighter aircraft
Fighter aircraft
A fighter aircraft is a military aircraft designed primarily for air-to-air combat with other aircraft, as opposed to a bomber, which is designed primarily to attack ground targets...
could reach supersonic
Supersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...
speeds.
These techniques were quickly put to use by aircraft designers. One common solution to the problem of wave drag was to use a swept wing
Swept wing
A swept wing is a wing planform favored for high subsonic jet speeds first investigated by Germany during the Second World War. Since the introduction of the MiG-15 and North American F-86 which demonstrated a decisive superiority over the slower first generation of straight-wing jet fighters...
, which had actually been developed before WWII and used on some German wartime designs. Sweeping the wing makes it appear thinner and longer in the direction of the airflow, making a "normal" wing shape closer to that of the von Kármán ogive, while still remaining useful at lower speeds where curvature and thickness are important.
The wing need not be swept as it is possible to build a wing that is extremely thin. This solution was used on a number of designs beginning with the Bell X-1
Bell X-1
The Bell X-1, originally designated XS-1, was a joint NACA-U.S. Army/US Air Force supersonic research project built by Bell Aircraft. Conceived in 1944 and designed and built over 1945, it eventually reached nearly 1,000 mph in 1948...
, the first manned aircraft to fly at the speed of sound. The downside to this approach is that the wing is so thin it is no longer possible to use it for storage of fuel or landing gear.
Fuselage shaping was similarly changed with the introduction of the Whitcomb area rule
Area rule
The Whitcomb area rule, also called the transonic area rule, is a design technique used to reduce an aircraft's drag at transonic and supersonic speeds, particularly between Mach 0.75 and 1.2....
. Whitcomb had been working on testing various airframe shapes for transonic drag when, after watching a presentation by Adolf Busemann
Adolf Busemann
Adolph Busemann was a German aerospace engineer and influential early pioneer in aerodynamics, specialising in supersonic airflows...
in 1952, he realized that the Sears-Haack body had to apply to the entire aircraft. This meant that the fuselage needed to be made narrower where it joined the wings, so that the cross-section of the entire aircraft matched the Sears-Haack body, not just the fuselage itself.
Anti-shock body
An Anti-shock body is a pod positioned on the leading or trailing edge of an aircraft's aerodynamic surface to reduce wave drag at transonic speeds .-Wing trailing edge:...
on transonic
Transonic
Transonic speed is an aeronautics term referring to the condition of flight in which a range of velocities of airflow exist surrounding and flowing past an air vehicle or an airfoil that are concurrently below, at, and above the speed of sound in the range of Mach 0.8 to 1.2, i.e. 600–900 mph...
aircraft, including some jet airliner
Jet airliner
A jet airliner is an airliner that is powered by jet engines. This term is sometimes contracted to jetliner or jet.In contrast to today's relatively fuel-efficient, turbofan-powered air travel, first generation jet airliner travel was noisy and fuel inefficient...
s. Anti-shock bodies, which are pods along the trailing edges of the wings, serve the same role as the narrow waist fuselage design of other transonic aircraft.
Other drag reduction methods
Several other attempts to reduce wave drag have been introduced over the years, but have not become common. The supercritical airfoilSupercritical airfoil
A supercritical airfoil is an airfoil designed, primarily, to delay the onset of wave drag in the transonic speed range. Supercritical airfoils are characterized by their flattened upper surface, highly cambered aft section, and greater leading edge radius compared with traditional airfoil shapes...
is a new wing design that results in reasonable low speed lift like a normal planform
Planform
In aviation, a planform is the shape and layout of a fixed-wing aircraft's fuselage and wing. Of all the myriad planforms used, they can typically be grouped into those used for low-speed flight, found on general aviation aircraft, and those used for high-speed flight, found on many military...
, but has a profile considerably closer to that of the von Kármán ogive. All modern civil airliners use forms of supercritical aerofoil and have substantial supersonic flow over the wing upper surface.
Busemann's biplane
Busemann's Biplane
Busemann's Biplane is a conceptual airframe design invented by Adolf Busemann which avoids the formation of N-type shock waves and thus does not create a sonic boom....
avoids wave drag entirely, but is incapable of generating lift, and has never flown.