Ground effect
Encyclopedia
As it pertains to fixed wing aircraft
, "ground effect" refers to the increased lift and decreased drag that an aircraft airfoil
or wing
generates when an aircraft is about one wingspan's length or less over the ground (or surface). Ground effect often gives pilots and/or passengers of light aircraft the feeling that the aircraft is "floating", especially when landing.
that is approximately at or below the same distance as the aircraft's wingspan
there is, depending on airfoil and aircraft design, an often noticeable ground effect. This is caused primarily by the ground interrupting the wingtip vortices
and downwash
behind the wing. When a wing is flown very close to the ground, wingtip vortices are unable to form effectively due to the obstruction of the ground. The result is lower induced drag, which increases the speed
and lift
of the aircraft while it is in the ground effect.
A wing generates lift, in part, due to the difference in air pressure gradients on the wing surfaces: both upper and lower. During normal flight, the upper wing surface experiences reduced static air pressure and the lower surface comparatively higher static pressure, these air pressure differences also accelerate the mass of air downwards. Flying close to a surface increases air pressure on the lower wing surface, (the ram or cushion effect) improving the aircraft lift to drag ratio. As the wing gets lower the ground effect becomes more pronounced. While in the ground effect, the wing will require a lower angle of attack
to produce the same amount of lift. If the angle of attack and velocity
remain constant, an increase in the lift coefficient will result, accounting for the "floating" effect. Ground effect will also alter thrust
versus velocity in that reducing induced drag will require less thrust to maintain velocity.
Low winged aircraft are more affected by ground effect than high wing aircraft. Due to the change in up-wash, down-wash and wingtip vortices there may be errors in the airspeed system while in ground effect due to changes in the local pressure at the static source
.
as it nears the surface, as well as the weight, speed, and configuration of the aircraft, and wing loading
(aircraft weight per unit-area of wing).
The wing in ground effect, often described as a 'cushion', is thought to be an increase in air pressure which occurs below a wing when it gets close to the ground. The effect begins to be noticeable when the aircraft's altitude is within 1–1.5 times the length of its own wingspan and, when the altitude is within about half the wing chord, the effect can increase lift by as much as 10%. Due to the effect of spoiler
s and high wing loading, this effect is only dramatically noticed in smaller, less complex aircraft, usually weighing less than 12,500 lbs (5,670 kg). Ground effect is a major factor in aircraft "floating" down the runway, and is the reason that low-wing aircraft have a tendency to float more than the high-wing varieties.
Wing in ground effect during take-off is thought to be a cause of many aircraft accidents. A small plane loaded beyond gross weight capabilities may be able to take off under ground effect, due to the 'artificially' low stall speed due to the decreased induced drag. However, once the aircraft climbs to a height at which wingtip vortices can form, the wings will stall, and the aircraft will suddenly descend — usually resulting in a crash. (Note that the ground effect cushion does not of itself reduce wing vortices; rather, on leaving ground effect the pilot of an overloaded aircraft must increase the angle of attack to keep flying. This action may result in the wing exceeding the critical angle of attack, thus inducing a stall at low altitude.).
Gliders
may be less affected by wing in ground effect due to the short chord and very long wingspan (in other words, high aspect ratio
) for weight, which minimizes the effect of induced drag caused by wingtip vortices. On the other hand, since gliders generally are built to minimize all form drag and parasitic drag
as well, the reduction in the induced drag caused by ground effect can in fact effectively increase flight performance, resulting in an enhanced glide ratio. Pilots of gliders who seek to exploit this phenomenon on landing are said to be performing a "penetration approach." A successful penetration approach would involve diving at a speed higher than the usual optimal glide speed for a given glider (which would result in a sub-optimal glide ratio on the descent), and then flaring and holding the lowest possible altitude above the ground, at this high speed. Theoretically the positive effect of wing in ground effect — decreased drag — could result in a final stopping place farther than would have been achieved, had the pilot simply flown the speed resulting in the best glide ratio. This approach is risky, and it is not a sure bet that performance will be increased; thus it is not a generally recommended means of improving glide distance. This should also not be confused with a glider performing a penetration approach into wind; the fact that the wind generally is less strong near the ground improves the achieved groundspeed enough to offset the higher airspeed drag penalties.
Wingtip vortices are a major cause of induced drag, which refers to any drag created as a side effect of generating lift. Reducing this form of drag leads to a number of widely-used design considerations found on many aircraft. Gliders
, for instance, use very long wings with a high aspect ratio
to reduce the development of spanwise flow. As the wing has a smaller chord length over wing length, spanwise flow has less time to develop and therefore the angle at which the upper and lower airflows converge is reduced. This smaller angle creates vortices of less magnitude and therefore produce less induced drag. Other aircraft sometimes include winglets or end-plates to decrease the pressure differential between the upper and lower wing. This barrier method increases the distance air has to flow from HIGH to LOW thereby reducing the speed at which this air flows. To grasp how it does this think of weather maps and isobars. When isobars (regions of similar pressure) are closer together, the pressure differential is greater and the wind speed will be higher and vice versa. This reduced pressure differential results in a reduction in spanwise flow, the angles at which the airflows meet and ultimately induced drag.
Some critics of Howard Hughes
' massive Spruce Goose claim that the famous flying boat
's first (and only) flight was due entirely to wing in ground effect and that the craft was incapable of sustaining flight above a very low altitude. It is probably true that the Spruce Goose was underpowered in its current configuration; development of the engines the plane was designed to carry was canceled before completion, and as a result, the Goose was running at probably 60% of its designed power. Nevertheless the power necessary to bring a seaplane to flight speeds is greater than land-planes, and the height the Goose reached was probably well beyond the ground effect for such a heavy-lifter.
Although all airplanes fly through ground effect at some point, craft that do so in a dedicated manner must be designed in such a way that their wings are either unsuitable or unable to take them into flight out of ground effect (free flight). Those that can fly out of ground effect are so out of their element that they are only capable of short distance hops into free flight. Because of this, these craft do not meet the required legal criteria to be referred to as airplanes and the operators of such craft need not possess a pilot's license to fly them.
These specially designed craft include airplanes with inverse delta wing
, ekranoplan wing or tandem wing
.
hovering close to the ground will notice that less power is required than when hovering further away. The apparent effect (that can easily be explained by Newtons Laws) occurs because the ground is pushing back on the air being forced down by the aircraft. As the aircraft moves higher the force pushing back become less to the point at which gravity and force created by the momentum of the particles being pushed out by the blades is only acting.
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...
, "ground effect" refers to the increased lift and decreased drag that an aircraft airfoil
Airfoil
An airfoil or aerofoil is the shape of a wing or blade or sail as seen in cross-section....
or wing
Wing
A wing is an appendage with a surface that produces lift for flight or propulsion through the atmosphere, or through another gaseous or liquid fluid...
generates when an aircraft is about one wingspan's length or less over the ground (or surface). Ground effect often gives pilots and/or passengers of light aircraft the feeling that the aircraft is "floating", especially when landing.
Principle of ground effect
When an aircraft is flying at an altitudeAltitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
that is approximately at or below the same distance as the aircraft's wingspan
Wingspan
The wingspan of an airplane or a bird, is the distance from one wingtip to the other wingtip. For example, the Boeing 777 has a wingspan of about ; and a Wandering Albatross caught in 1965 had a wingspan of , the official record for a living bird.The term wingspan, more technically extent, is...
there is, depending on airfoil and aircraft design, an often noticeable ground effect. This is caused primarily by the ground interrupting the wingtip vortices
Wingtip vortices
Wingtip vortices are tubes of circulating air that are left behind a wing as it generates lift. One wingtip vortex trails from the tip of each wing. The cores of vortices spin at very high speed and are regions of very low pressure...
and downwash
Downwash
In aeronautics downwash is the air forced down by the aerodynamic action of a wing or helicopter rotor blade in motion, as part of the process of producing lift....
behind the wing. When a wing is flown very close to the ground, wingtip vortices are unable to form effectively due to the obstruction of the ground. The result is lower induced drag, which increases the speed
Speed
In kinematics, the speed of an object is the magnitude of its velocity ; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance traveled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as...
and lift
Lift (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...
of the aircraft while it is in the ground effect.
A wing generates lift, in part, due to the difference in air pressure gradients on the wing surfaces: both upper and lower. During normal flight, the upper wing surface experiences reduced static air pressure and the lower surface comparatively higher static pressure, these air pressure differences also accelerate the mass of air downwards. Flying close to a surface increases air pressure on the lower wing surface, (the ram or cushion effect) improving the aircraft lift to drag ratio. As the wing gets lower the ground effect becomes more pronounced. While in the ground effect, the wing will require a lower 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...
to produce the same amount of lift. If the angle of attack and velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
remain constant, an increase in the lift coefficient will result, accounting for the "floating" effect. Ground effect will also alter thrust
Thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....
versus velocity in that reducing induced drag will require less thrust to maintain velocity.
Low winged aircraft are more affected by ground effect than high wing aircraft. Due to the change in up-wash, down-wash and wingtip vortices there may be errors in the airspeed system while in ground effect due to changes in the local pressure at the static source
Pitot tube
A pitot tube is a pressure measurement instrument used to measure fluid flow velocity. The pitot tube was invented by the French engineer Henri Pitot Ulo in the early 18th century and was modified to its modern form in the mid-19th century by French scientist Henry Darcy...
.
Factors affecting wing in ground effect
The wing in ground effect is affected by numerous factors, including the wing's area, its chord length, and its angle of attackAngle 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...
as it nears the surface, as well as the weight, speed, and configuration of the aircraft, and wing loading
Wing loading
In aerodynamics, wing loading is the loaded weight of the aircraft divided by the area of the wing. The faster an aircraft flies, the more lift is produced by each unit area of wing, so a smaller wing can carry the same weight in level flight, operating at a higher wing loading. Correspondingly,...
(aircraft weight per unit-area of wing).
The wing in ground effect, often described as a 'cushion', is thought to be an increase in air pressure which occurs below a wing when it gets close to the ground. The effect begins to be noticeable when the aircraft's altitude is within 1–1.5 times the length of its own wingspan and, when the altitude is within about half the wing chord, the effect can increase lift by as much as 10%. Due to the effect of spoiler
Spoiler (aeronautics)
In aeronautics, a spoiler is a device intended to reduce lift in an aircraft. Spoilers are plates on the top surface of a wing which can be extended upward into the airflow and spoil it. By doing so, the spoiler creates a carefully controlled stall over the portion of the wing behind it, greatly...
s and high wing loading, this effect is only dramatically noticed in smaller, less complex aircraft, usually weighing less than 12,500 lbs (5,670 kg). Ground effect is a major factor in aircraft "floating" down the runway, and is the reason that low-wing aircraft have a tendency to float more than the high-wing varieties.
Wing in ground effect during take-off is thought to be a cause of many aircraft accidents. A small plane loaded beyond gross weight capabilities may be able to take off under ground effect, due to the 'artificially' low stall speed due to the decreased induced drag. However, once the aircraft climbs to a height at which wingtip vortices can form, the wings will stall, and the aircraft will suddenly descend — usually resulting in a crash. (Note that the ground effect cushion does not of itself reduce wing vortices; rather, on leaving ground effect the pilot of an overloaded aircraft must increase the angle of attack to keep flying. This action may result in the wing exceeding the critical angle of attack, thus inducing a stall at low altitude.).
Gliders
Glider (sailplane)
A glider or sailplane is a type of glider aircraft used in the sport of gliding. Some gliders, known as motor gliders are used for gliding and soaring as well, but have engines which can, in some cases, be used for take-off or for extending a flight...
may be less affected by wing in ground effect due to the short chord and very long wingspan (in other words, high aspect ratio
Aspect ratio
The aspect ratio of a shape is the ratio of its longer dimension to its shorter dimension. It may be applied to two characteristic dimensions of a three-dimensional shape, such as the ratio of the longest and shortest axis, or for symmetrical objects that are described by just two measurements,...
) for weight, which minimizes the effect of induced drag caused by wingtip vortices. On the other hand, since gliders generally are built to minimize all form drag and parasitic drag
Parasitic drag
Parasitic drag is drag caused by moving a solid object through a fluid medium . Parasitic drag is made up of many components, the most prominent being form drag...
as well, the reduction in the induced drag caused by ground effect can in fact effectively increase flight performance, resulting in an enhanced glide ratio. Pilots of gliders who seek to exploit this phenomenon on landing are said to be performing a "penetration approach." A successful penetration approach would involve diving at a speed higher than the usual optimal glide speed for a given glider (which would result in a sub-optimal glide ratio on the descent), and then flaring and holding the lowest possible altitude above the ground, at this high speed. Theoretically the positive effect of wing in ground effect — decreased drag — could result in a final stopping place farther than would have been achieved, had the pilot simply flown the speed resulting in the best glide ratio. This approach is risky, and it is not a sure bet that performance will be increased; thus it is not a generally recommended means of improving glide distance. This should also not be confused with a glider performing a penetration approach into wind; the fact that the wind generally is less strong near the ground improves the achieved groundspeed enough to offset the higher airspeed drag penalties.
Wingtip vortices are a major cause of induced drag, which refers to any drag created as a side effect of generating lift. Reducing this form of drag leads to a number of widely-used design considerations found on many aircraft. Gliders
Glider (sailplane)
A glider or sailplane is a type of glider aircraft used in the sport of gliding. Some gliders, known as motor gliders are used for gliding and soaring as well, but have engines which can, in some cases, be used for take-off or for extending a flight...
, for instance, use very long wings with a high aspect ratio
Aspect ratio (wing)
In aerodynamics, the aspect ratio of a wing is essentially the ratio of its length to its breadth . A high aspect ratio indicates long, narrow wings, whereas a low aspect ratio indicates short, stubby wings....
to reduce the development of spanwise flow. As the wing has a smaller chord length over wing length, spanwise flow has less time to develop and therefore the angle at which the upper and lower airflows converge is reduced. This smaller angle creates vortices of less magnitude and therefore produce less induced drag. Other aircraft sometimes include winglets or end-plates to decrease the pressure differential between the upper and lower wing. This barrier method increases the distance air has to flow from HIGH to LOW thereby reducing the speed at which this air flows. To grasp how it does this think of weather maps and isobars. When isobars (regions of similar pressure) are closer together, the pressure differential is greater and the wind speed will be higher and vice versa. This reduced pressure differential results in a reduction in spanwise flow, the angles at which the airflows meet and ultimately induced drag.
Some critics of Howard Hughes
Howard Hughes
Howard Robard Hughes, Jr. was an American business magnate, investor, aviator, engineer, film producer, director, and philanthropist. He was one of the wealthiest people in the world...
' massive Spruce Goose claim that the famous flying boat
Flying boat
A flying boat is a fixed-winged seaplane with a hull, allowing it to land on water. It differs from a float plane as it uses a purpose-designed fuselage which can float, granting the aircraft buoyancy. Flying boats may be stabilized by under-wing floats or by wing-like projections from the fuselage...
's first (and only) flight was due entirely to wing in ground effect and that the craft was incapable of sustaining flight above a very low altitude. It is probably true that the Spruce Goose was underpowered in its current configuration; development of the engines the plane was designed to carry was canceled before completion, and as a result, the Goose was running at probably 60% of its designed power. Nevertheless the power necessary to bring a seaplane to flight speeds is greater than land-planes, and the height the Goose reached was probably well beyond the ground effect for such a heavy-lifter.
Ground effect craft
Many aircraft have a design that makes use of the wing in ground effect.Although all airplanes fly through ground effect at some point, craft that do so in a dedicated manner must be designed in such a way that their wings are either unsuitable or unable to take them into flight out of ground effect (free flight). Those that can fly out of ground effect are so out of their element that they are only capable of short distance hops into free flight. Because of this, these craft do not meet the required legal criteria to be referred to as airplanes and the operators of such craft need not possess a pilot's license to fly them.
These specially designed craft include airplanes with inverse delta wing
Delta wing
The delta wing is a wing planform in the form of a triangle. It is named for its similarity in shape to the Greek uppercase letter delta .-Delta-shaped stabilizers:...
, ekranoplan wing or tandem wing
Tandem wing
thumb|right|QAC Quickie Q2A tandem wing aircraft usually involves two full-sized wings, both of which are full airfoils. Sometimes an aircraft of this configuration can look like a variation on the biplane, but is in fact very different. The forward wing is often technically a canard, fitted with...
.
Ground effect with helicopters
Pilots of Rotary wing aircraft also feel changes associated with being close to a flat surface such as the ground. A pilot of a helicopterHelicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally...
hovering close to the ground will notice that less power is required than when hovering further away. The apparent effect (that can easily be explained by Newtons Laws) occurs because the ground is pushing back on the air being forced down by the aircraft. As the aircraft moves higher the force pushing back become less to the point at which gravity and force created by the momentum of the particles being pushed out by the blades is only acting.
Ground effect with airships
Airships can benefit from the wing in ground effect, too. Feasibility studies have been made of specially designed airships taking advantage of the effect.See also
- Coandă effectCoanda effectThe Coandă effect is the tendency of a fluid jet to be attracted to a nearby surface. The principle was named after Romanian aerodynamics pioneer Henri Coandă, who was the first to recognize the practical application of the phenomenon in aircraft development....
- Ground effect vehicle
- Ground effect in carsGround effect in carsGround effect is term applied to a series of aerodynamic effects used in car design, which has been exploited to create downforce, particularly in racing cars. This has been the successor to the earlier dominant aerodynamic theory of streamlining...
- HovercraftHovercraftA hovercraft is a craft capable of traveling over surfaces while supported by a cushion of slow moving, high-pressure air which is ejected against the surface below and contained within a "skirt." Although supported by air, a hovercraft is not considered an aircraft.Hovercraft are used throughout...
- Vortex ringVortex ringA vortex ring, also called a toroidal vortex, is a region of rotating fluid moving through the same or different fluid where the flow pattern takes on a toroidal shape. The movement of the fluid is about the poloidal or circular axis of the doughnut, in a twisting vortex motion...
External links
- Universal Hovercraft Hoverwing(tm) ACV/WIG
- SE-Technology, engineering explanation
- Aerospaceweb.org | Ask Us - Ground Effect and WIG Vehicles
- Wing in Ground Effect Craft Review DSTO, Australian Government.
- Wing in Ground Effect and helicopters
- Film showing several experimental wing in ground effect aircraft in operation (SkyShip)
- Scientists from Tongji University in Shanghai announce that they have designed a new model WIG 14 July 2007
- Ground-effect gliding
- Numerical Analysis of Airfoil in Ground Proximity