Mach tuck
Encyclopedia
Mach tuck is an aerodynamic effect, whereby the nose of an aircraft
tends to pitch downwards as the airflow around the wing reaches supersonic
speeds. The aircraft will be subsonic, and traveling significantly below Mach
1.0, when it first experiences this effect.
.
Mach tuck is the result of an aerodynamic stall
due to an over-speed condition, rather than the more common stalls resulting from boundary layer separation due to insufficient airspeed, increased angle of attack
, excessive load factors, or a combination of those causes. As the aircraft's wing approaches its critical Mach number, the aircraft is traveling below Mach 1.0. However, the accelerated airflow over the upper surface of the cambered
wing exceeds Mach 1.0 and a shock wave
is created at the point on the wing where the accelerated airflow has gone supersonic. While the air ahead of the shock wave is in laminar flow
, a boundary layer separation is created aft of the shock wave, and that section of the wing fails to produce lift. The image to the right illustrates this concept.
In most aircraft susceptible to Mach tuck, the camber at the wing root
, the section of the wing closest to the fuselage
, is more pronounced than that of the wing tip
. This design ensures that in a standard stall the root will stall before the tips. This allows the pilot to recognize the stall while still maintaining control of the ailerons to enhance stall recovery. However, this also means that when an airfoil exceeds its critical Mach number
, the shock wave, and resulting stall condition, will begin to form at the root.
A second design element that leads to Mach tuck is that many aircraft which will approach the speed of sound are designed with swept wing
s. The center of pressure of a wing is an imaginary point where the summation of all lifting forces across the wing's surface can be resolved into a single lift vector. When the wing root stalls, the center of pressure of the (reduced) lift being generated by the wing is shifted towards the wing tip. With a swept wing, this also means that the center of pressure travels aft (because it is traveling out from the wing root and therefore backwards as the wing sweeps). When the center of pressure moves aft, its movement rearwards compared to the unmoving center of mass
of the aircraft will generate a force which will act to depress the nose of the aircraft; this nose down pitching moment
is “Mach tuck."
As the wing becomes more affected by the shock wave the center of pressure will continue to travel aft, thereby causing a significantly higher nose-down force and requiring a nose-up input or trim to maintain level flight. Although Mach tuck develops gradually, if it is allowed to progress significantly, the center of pressure can move so far rearward that there is no longer enough elevator authority available to counteract it, and the airplane enters a steep, sometimes unrecoverable dive.
In addition, until the aircraft goes supersonic, as the shock wave goes towards the rear, because of the faster flow there the top shockwave will impinge upon the horizontal stabilizer and elevator control surfaces further back than the lower shockwave; this can greatly exacerbate the nose down tendencies. The horizontal stabilizer at the tail of the aircraft generates a downward force, so loss of effective horizontal stabilizer area will reduce this downward force, so the tail will pitch up and the nose will pitch down. If the shock wave affects the elevators, it may reduce their effectiveness, making it impossible for the pilot to alter the aircraft's pitch.
Finally, there is a related condition that can exacerbate Mach tuck. If enough of the wing surface becomes engulfed in the shock wave, the wing will not produce enough lift to support the aircraft, and a standard stall will occur. This often fatal combination of overspeed and aerodynamic stall can most easily be avoided by not allowing the effects of Mach tuck to develop beyond its incipient stage. This is best accomplished by retarding the throttle
, extending speed brakes
, and if possible, extending the landing gear
. Any actions, which would increase aerodynamic drag and thus reduce airspeed below critical Mach, will prevent further aggravation of the condition.
Historically, recovery from a mach tuck in subsonic aircraft has not always been possible. In some cases, as the aircraft descends, the air density increases and the extra drag will slow the aircraft and control will return.
For aircraft such as supersonic fighters/bombers or supersonic transports such as Concorde
that spend long periods in supersonic flight, Mach tuck is often compensated for by moving fuel between tanks in the fuselage to change the position of the centre of mass. This minimizes the amount of trim required and keeps the changing location of the center of pressure within acceptable limits.
Supersonic and subsonic aircraft often have an all-moving tailplane (a stabilator
) rather than separate elevator control surfaces. This avoids the shock wave making the control surfaces pitch downwards.
fighters with high power engines capable of producing extremely high airspeeds were the first aircraft to experience Mach tuck. Because research with supersonic airfoils was in its infancy, there were no wings with the design elements that aid in slowing the onset of the Mach tuck effects. Instead, the shock wave would engulf the entire wing, making recovery much more difficult.
The P-38 was the first 400 mph fighter and it suffered more than the usual teething troubles. It had a thick, high-lift wing for fast climb characteristics and for holding a large fuel supply. It also had three fuselages: the central weapon and pilot nacelle or gondola, and the twin booms containing engines and turbosuperchargers. Finally, it was a very densely weighted fighter for its day, and accelerated quickly to terminal velocity in a dive. Bernoulli's effect worked very strongly on the thick wing, and was even more pronounced where air was pushed out of the way by and compressed between the central nacelle and the engine booms. Mach tuck would occur when the aircraft attained Mach 0.68 at which point the air flow over the wing roots would go transonic. The wing would lose lift and the normal loading of the tail's horizontal control surfaces would move aft, leaving the elevator unloaded, bringing the nose further down in a Mach tuck. Lockheed engineers eventually found a solution whereby a small 'speed bump' flap on the underside of the wing would be engaged by a pilot initiating a dive. The flap changed the center of pressure distribution so that the wing would not lose its lift.
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
tends to pitch downwards as the airflow around the wing reaches 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. The aircraft will be subsonic, and traveling significantly below Mach
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...
1.0, when it first experiences this effect.
Causes of Mach tuck
Mach tuck is dependent upon the dynamics of liftLift (force)
A fluid flowing past the surface of a body exerts a surface force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction...
.
Mach tuck is the result of an aerodynamic stall
Stall (flight)
In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded...
due to an over-speed condition, rather than the more common stalls resulting from boundary layer separation due to insufficient airspeed, increased angle of attack
Angle of attack
Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...
, excessive load factors, or a combination of those causes. As the aircraft's wing approaches its critical Mach number, the aircraft is traveling below Mach 1.0. However, the accelerated airflow over the upper surface of the cambered
Camber (aerodynamics)
Camber, in aeronautics and aeronautical engineering, is the asymmetry between the top and the bottom surfaces of an aerofoil. An aerofoil that is not cambered is called a symmetric aerofoil...
wing exceeds Mach 1.0 and a 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...
is created at the point on the wing where the accelerated airflow has gone supersonic. While the air ahead of the shock wave is in laminar flow
Laminar flow
Laminar flow, sometimes known as streamline flow, occurs when a fluid flows in parallel layers, with no disruption between the layers. At low velocities the fluid tends to flow without lateral mixing, and adjacent layers slide past one another like playing cards. There are no cross currents...
, a boundary layer separation is created aft of the shock wave, and that section of the wing fails to produce lift. The image to the right illustrates this concept.
In most aircraft susceptible to Mach tuck, the camber at the wing root
Wing root
The wing root is the part of the wing on a fixed-wing aircraft that is closest to the fuselage. On a simple monoplane configuration, this is usually easy to identify...
, the section of the wing closest to the fuselage
Fuselage
The fuselage is an aircraft's main body section that holds crew and passengers or cargo. In single-engine aircraft it will usually contain an engine, although in some amphibious aircraft the single engine is mounted on a pylon attached to the fuselage which in turn is used as a floating hull...
, is more pronounced than that of the wing tip
Wing tip
A wing tip is the part of the wing that is most distant from the fuselage of a fixed-wing aircraft.Because the wing tip shape influences the size and drag of the wingtip vortices, tip design has produced a diversity of shapes, including:* Squared-off...
. This design ensures that in a standard stall the root will stall before the tips. This allows the pilot to recognize the stall while still maintaining control of the ailerons to enhance stall recovery. However, this also means that when an airfoil exceeds its critical Mach number
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....
, the shock wave, and resulting stall condition, will begin to form at the root.
A second design element that leads to Mach tuck is that many aircraft which will approach the speed of sound are designed with 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...
s. The center of pressure of a wing is an imaginary point where the summation of all lifting forces across the wing's surface can be resolved into a single lift vector. When the wing root stalls, the center of pressure of the (reduced) lift being generated by the wing is shifted towards the wing tip. With a swept wing, this also means that the center of pressure travels aft (because it is traveling out from the wing root and therefore backwards as the wing sweeps). When the center of pressure moves aft, its movement rearwards compared to the unmoving center of mass
Center of mass
In physics, the center of mass or barycenter of a system is the average location of all of its mass. In the case of a rigid body, the position of the center of mass is fixed in relation to the body...
of the aircraft will generate a force which will act to depress the nose of the aircraft; this nose down pitching moment
Moment (physics)
In physics, the term moment can refer to many different concepts:*Moment of force is the tendency of a force to twist or rotate an object; see the article torque for details. This is an important, basic concept in engineering and physics. A moment is valued mathematically as the product of the...
is “Mach tuck."
As the wing becomes more affected by the shock wave the center of pressure will continue to travel aft, thereby causing a significantly higher nose-down force and requiring a nose-up input or trim to maintain level flight. Although Mach tuck develops gradually, if it is allowed to progress significantly, the center of pressure can move so far rearward that there is no longer enough elevator authority available to counteract it, and the airplane enters a steep, sometimes unrecoverable dive.
In addition, until the aircraft goes supersonic, as the shock wave goes towards the rear, because of the faster flow there the top shockwave will impinge upon the horizontal stabilizer and elevator control surfaces further back than the lower shockwave; this can greatly exacerbate the nose down tendencies. The horizontal stabilizer at the tail of the aircraft generates a downward force, so loss of effective horizontal stabilizer area will reduce this downward force, so the tail will pitch up and the nose will pitch down. If the shock wave affects the elevators, it may reduce their effectiveness, making it impossible for the pilot to alter the aircraft's pitch.
Finally, there is a related condition that can exacerbate Mach tuck. If enough of the wing surface becomes engulfed in the shock wave, the wing will not produce enough lift to support the aircraft, and a standard stall will occur. This often fatal combination of overspeed and aerodynamic stall can most easily be avoided by not allowing the effects of Mach tuck to develop beyond its incipient stage. This is best accomplished by retarding the throttle
Throttle
A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...
, extending speed brakes
Air brake (aircraft)
In aeronautics, air brakes or speedbrakes are a type of flight control surface used on an aircraft to increase drag or increase the angle of approach during landing....
, and if possible, extending the landing gear
Undercarriage
The undercarriage or landing gear in aviation, is the structure that supports an aircraft on the ground and allows it to taxi, takeoff and land...
. Any actions, which would increase aerodynamic drag and thus reduce airspeed below critical Mach, will prevent further aggravation of the condition.
Dealing with Mach tuck
All supersonic aircraft experience some degree of mach tuck.Historically, recovery from a mach tuck in subsonic aircraft has not always been possible. In some cases, as the aircraft descends, the air density increases and the extra drag will slow the aircraft and control will return.
For aircraft such as supersonic fighters/bombers or supersonic transports such as Concorde
Concorde
Aérospatiale-BAC Concorde was a turbojet-powered supersonic passenger airliner, a supersonic transport . It was a product of an Anglo-French government treaty, combining the manufacturing efforts of Aérospatiale and the British Aircraft Corporation...
that spend long periods in supersonic flight, Mach tuck is often compensated for by moving fuel between tanks in the fuselage to change the position of the centre of mass. This minimizes the amount of trim required and keeps the changing location of the center of pressure within acceptable limits.
Supersonic and subsonic aircraft often have an all-moving tailplane (a stabilator
Stabilator
A stabilator is an aircraft control surface that combines the functions of an elevator and a horizontal stabilizer...
) rather than separate elevator control surfaces. This avoids the shock wave making the control surfaces pitch downwards.
History
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...
fighters with high power engines capable of producing extremely high airspeeds were the first aircraft to experience Mach tuck. Because research with supersonic airfoils was in its infancy, there were no wings with the design elements that aid in slowing the onset of the Mach tuck effects. Instead, the shock wave would engulf the entire wing, making recovery much more difficult.