Aircraft may be affected by a number of
ground effects, aerodynamic effects due to a flying body's proximity to the ground.
One of the most important of these effects is the
Wing In Ground effect, which refers to the reduction in drag experienced by an
aircraftAn aircraft is a vehicle which is able to fly by being supported by 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 An aircraft is a vehicle which is able to fly by being supported...
as it approaches a height approximately equal to the aircraft's
wingspanThe wingspan of an airplane or a bird, is the distance from the left wingtip to the right 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,...
above ground or other level surface, such as the sea. The effect increases as the wing descends closer to the ground, with the most significant effects occurring at an altitude of one half the wingspan. It can present a hazard for inexperienced
pilotAn aviator is a person who flies aircraft for pleasure or as a profession. The first recorded use of the term was in 1887 as a variation of the French 'aviation', from the latin 'avis', coined 1863 by G. de la Landelle in "Aviation ou Navigation Aérienne"...
s who are not accustomed to correcting for it on their approach to landing, but it has also been used to effectively enhance the performance of certain kinds of aircraft whose
planformA planform or plan view is a vertical orthographic projection of an object on a horizontal plane, like a map.In aviation, a planform is the shape and layout of an airplane's wing and fuselage...
has been adapted to take advantage of it, such as the
RussiaRussia , officially known as both Russia and the Russian Federation , is a country in northern Eurasia . It is a semi-presidential republic, comprising 83 federal subjects...
n ekranoplans. The first to give scientific description of the ground effect and to provide theoretical methods of calculation of air cushion vehicles was
Konstantin TsiolkovskyKonstantin Eduardovich Tsiolkovsky was an Imperial Russian and Soviet rocket scientist and pioneer of the astronautic theory. He is considered by many to be the father of theoretical astronautics...
in his 1927 paper "Air Resistance and the Express Train".
Principle of Ground Effect
Wingtip vorticesWingtip vortices are tubes of circulating air which 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. To first approximation, these low-pressure regions form with...
are a major cause of induced drag, which refers to any drag created as a side-effect of generating
liftA fluid flowing past the surface of a body exerts a force on it. Lift is defined to be the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is defined to be the component of the fluid-dynamic force parallel to the flow...
. Reducing this form of drag leads to a number of widely-used design considerations found on many aircraft.
GliderUnpowered aircraft are a group of aerial vehicles that can fly without propulsion. They can be classified as gliders, balloons and kites. In this instance, 'flight' means a trajectory that is not merely a vertical descent such as a parachute. In the case of kites, the flight is not free, but...
s, for instance, use very long wings with a high
aspect ratiothumb|The low aspect ratio wing of a [[Piper PA-28 Cherokee]].In aerodynamics, the aspect ratio of a wing is defined as the square of the wing span divided by the wing area....
in order to reduce the size of the wingtip in relation to the size of the wing as a whole, thereby reducing the contribution of induced drag. Other aircraft sometimes include winglets or end-plates to actively disrupt the airflow over the tip.
The phenomenon of wing in ground effect is caused 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 experiencing the ground effect.
Factors affecting Wing in Ground effects
The wing in ground effect is affected by numerous factors, including the wing's area, its chord length, and its angle of attack as it nears the surface, as well as the weight, speed, and configuration of the aircraft, and
wing loadingIn 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 comes into close proximity with 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 a wingspan of the ground, the effect can increase lift by as much as 40%. Due to the effect of
spoilerIn 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 will increase drag to the point where ultimately a crash may occur).
GliderUnpowered aircraft are a group of aerial vehicles that can fly without propulsion. They can be classified as gliders, balloons and kites. In this instance, 'flight' means a trajectory that is not merely a vertical descent such as a parachute. In the case of kites, the flight is not free, but...
s may be less affected by 'wing in ground effect' due to the short chord and very long wingspan (in other words, high
aspect ratioThe 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 dragParasitic 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 relatively 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.
Some critics of
Howard HughesHoward Robard Hughes, Jr. was an American aviator, engineer, industrialist, film producer, film director, philanthropist, and one of the wealthiest people in the world. He gained fame in the late 1920s as a maverick film producer, making big budget and often controversial films like Hell's Angels,...
' massive Spruce Goose claim that the famous
flying boatA flying boat, properly known as an aquatic plane, is a specialised form of aircraft that is designed to take off from and land on water, using its fuselage as a floating hull. Such aircraft are sometimes stabilised on water by underwing 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.
Wing in ground effect craft
Many aircraft have been designed that make 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, ekranoplan wing or tandem wing.
Wing in ground effect with helicopters
Rotary wing aircraft also experience performance changes associated with wing in ground effect. A
helicopterA helicopter is an aircraft that is lifted and propelled by one or more horizontal rotors, each rotor consisting of two or more rotor blades. Helicopters are classified as rotorcraft or rotary-wing aircraft to distinguish them from fixed-wing aircraft because the helicopter achieves lift with the...
hovering close to the ground will require less power than when hovering out of ground effect. The effect occurs when the helicopter is within one blade length of a hard surface.
Following an engine failure (depending on weight and environmental conditions), a multi engine helicopter may be able to hover in 'wing in ground effect' but not outside it.
This effect can be described at three different levels of understanding: basic, intermediate and advanced.
Basic
As with a fixed wing surface a helicopter's blades produce a cushion of air when close to the ground which helps to support the aircraft. Wind, surface slope and surface texture all have an effect on how effectively the high pressure cushion is maintained under the rotor.
Intermediate
When hovering in free air a rotor disc produces a duct effect with an induced downwards movement of air above the rotor. Rather than being stationary, when the air hits the rotor it is already moving downwards as it fills the region of low pressure left by air below that has already been forced down by the rotor. To hover, the rotor disc must produce a vertical force in the manner of
force = mass × acceleration where mass is dependent on number of rotors, RPM and rotor diameter and acceleration by the difference in the initial air vector and its resulting vector. Assuming mass moved is constant then a helicopter hovering in free air will be produce a force equivalent to V2 (speed after passing through the rotor) - V1 (starting speed of air) where V1 is some value above 0 given that the air is already moving downwards before reaching the rotor.
When close to the surface (generally considered 1/3-2/3 of the rotor diameter), air forced downwards through the rotor disc is restricted in its flow by the ground. This produces an area of high pressure below the disc and in turn, reduces the duct effect and hence the downwards velocity of the air above the disc. This reduces V1 while V2 remains relatively static, so the value of V2 - V1 increases. From the equation
F = M x A we can see that the 'lift' of the rotor disc is greater when in ground effect.
Advanced
The relative air flow meeting the advancing face of a rotor blade in a hovering helicopter is determined by the speed of rotation (the blade moving forward into the air) and the induced flow (the induced downwards movement of air above the rotor). Rotational air flow is taken as horizontal, induced flow as vertically down and the relative airflow as the resulting vector.
Through geometry the
angle of attackAngle of attack is a term used in fluid dynamics to describe the angle between a reference line on a body and the vector representing the relative motion between the body and the fluid through which it is moving. In general, the reference line could be any line on any arbitrarily shaped body in...
of the rotor blade will decrease as the inflow increases and the relative airflow direction moves closer to the chordline.
When the helicopter is in wing in ground effect the induced flow is decreased as described above. This moves the relative airflow vector closer to the horizontal and increases the AoA for a given blade pitch. This increases the lift produced by the rotor disc and the helicopter will start to accelerate vertically. The vertical movement will induce its own inflow reducing the AoA again until a point of equilibrium is reached.
In reality a pilot will lower the collective slightly -> reducing rotor pitch -> reducing AoA -> reducing lift and the helicopter will hover in ground effect with a lower power setting than that required out of ground effect.
Wing in ground effect with airships
Airships too can take benefit of the wing in ground effect. Feasibility studies have already been made of especially designed airships making advantage of the effect.
See also
- Coandă effect
The 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.-Discovery:An early description of this...
- Ground effect vehicle
- Ground effect in cars
Ground 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...
- Hovercraft
A hovercraft is a craft capable of traveling over relatively smooth surfaces supported by a cushion of slow moving, high-pressure air, ejected against the surface below, and contained within a "skirt." Although supported by the air, they are not usually considered to be aircraft.Hovercraft are...
- Vortex ring
A 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...
- List of ground effect vehicles
External links