Ground effect
A physical phenomenon that is experienced by a body in a flow near the ground is referred to as a ground effect . Depending on the shape of the flow around the body, additional dynamic lift or downforce can arise.
principle
The air gap between the trailing edge of the wing and the ground becomes smaller, the smaller the ground clearance. The air is dammed up and the flow slowed down under the whole wing. This increases the pressure there, which leads to greater dynamic lift . At the same time, the center of lift moves backwards. The ground effect is based on the fact that an air cushion forms under the wing near the ground, which moves forward with the vehicle (aircraft). The buoyancy-to-drag ratio increases 2.5 to 3 times as you approach the ground. So there is an improvement in the efficiency of a wing and thus more lift.
From a fluidic point of view, the bottom effect can be described as follows: The pressure distribution on the profile changes and leads to a reduction in the induced drag . The drag of a wing is made up of the frictional drag , the pressure drag and the induced drag. This is created by the vortices at the wing tips, which are formed by the pressure equalization between the top and bottom.
This creates a vortex band and tip vortices behind the wing , which accelerate the air behind the wing downwards. The force created by this pressure acts perpendicular to the velocity vector . In addition to buoyancy, this creates a force that is opposite to the direction of movement, the induced drag. Near the ground there is hardly any space left for the air to flow downwards and it is forced to flow away almost horizontally. This reduces the induced drag. In this way, energy that is required to overcome the induced resistance can be saved.
The effect was observed by Gustav Lilienthal in the albatross in 1880 , but not understood, but recognized as early as the beginning of aviation at the beginning of the 20th century.
Examples of the floor effect
Fixed wing aircraft
The aerodynamic conditions of wings change near the ground . The lift increases, the center of lift moves backwards and the air resistance decreases. When flying a low- wing aircraft close to the ground, the ground effect means that the aircraft hovers much longer than is the case, for example, with a high- wing aircraft . A pilot must take this into account when landing with low-wing aircraft, especially on short runways .
In ground effect vehicles (Ekranoplan) is mostly seaplanes , which are designed for low-level flight taking advantage of the ground effect and are therefore suitable only for certain applications. Ground effect vehicles can on the one hand be fixed-wing aircraft that are also suitable for flight at greater heights. On the other hand, ground-effect vehicles are administratively managed as ships if the low flight altitude forces ground-effect vehicles to interact with ships and boats in terms of traffic , i.e. to submit to shipping law . For this reason, these devices are also called ground effect “vehicles” and not ground effect “aircraft”, although from a physical point of view they are aircraft that fly (dynamic lift, airplanes) and do not drive (static lift, airships). In the ground effect, gliders can cover a much longer distance than the one corresponding to the glide angle with less loss of altitude .
The Hughes H-4 , which was the largest "airplane" built from 1947 to 2019 , only performed a test flight at a height of 20 m. The flying ability of the type outside of the ground effect has not been proven.
Rule of thumb : The ground effect occurs when the flight altitude is equal to or less than half the wing span of the wing .
helicopter
The terms HIGE (for hovering in ground effect ) and HOGE (for hovering out of ground effect ) can be found in the service descriptions of helicopters . These terms are typically used when reporting the working limits of helicopters in relation to the necessary engine power. As with aircraft wings, they have a ground effect close to the ground which is responsible for increased lift as a result of an interaction of the main rotor's downward airflow with the ground. Air sinks into the rotor disk from above, is accelerated downwards and hits the ground. Since the ground prevents the air from flowing rapidly, the helicopter is also raised. At the same time, the rotor tip vortices are reduced.
The ground effect in a helicopter occurs when it is within half to a full rotor span above the ground (HIGE). It is less effective over water and tall grass, as these surfaces have an energy-absorbing effect, and it does not exist at all when the helicopter is moving at a great height (HOGE). The latter necessarily means a higher power requirement and greater fuel consumption.
The ground effect is definitely important for the working limits of helicopters. If you have to start in the event of limiting factors such as a high weight load, taking off from a high place or at high temperatures, the ground effect can expand these working limits through the additional buoyancy provided and thus in some cases make take-off possible in the first place.
Ground-effect vehicles based on the crowbar principle
A special type of ground effect vehicle based on the crowbar principle is a tandem Airfoil Flairboat. The tandem wing principle means that an inherent stability is achieved during the ground effect flight, which guarantees a safe flair condition within the ground effect. Leaving the ground effect as it is achieved with free-flight ground effect vehicles is not intended here.
The fact that a tandem Airfoil Flairboat cannot leave the ground effect already led to the traffic and approval-related classification as a type A ground effect vehicle in 1974.
As a result of the classification as a ground effect vehicle type A, a tandem Airfoil Flairboat is classified like a watercraft , so that a motorboat license is completely sufficient for operation.
Land vehicles
A reversal of the ground effect can be found in motorsport . Vehicles designed for road races, such as Formula 1 , have little ground clearance , not only because of their more favorable center of gravity . In 1977, the Lotus team no longer made the floor as flat as possible, but designed it according to aerodynamic criteria in order to exploit a negative floor effect and thus increase the contact pressure . The entire side bodies of the Lotus 78 were shaped as inverted wings and largely sealed on the outside by movably mounted aprons that rubbed the roadway. The cornering speed increased enormously, while the flow resistance increased less than with the corresponding rear and front wings. Driving over the curbs was extremely dangerous because the additional traction was lost as soon as more air destroyed this effect. Since the 1983 season, the Formula 1 rules have required a continuous ground clearance of several centimeters to limit the ground effect. Today this is done with a diffuser .
literature
- Götsch, Ernst: Luftfahrzeugtechnik , Motorbuchverlag, Stuttgart 2003, ISBN 3-613-02006-8
- K. Knowles, DT Donoghue and MV Finnis: A Study of Wings in Ground Effect , RAeS Vehicle Aerodynamics Conference, Loughborough University, May 18–19. July 1994
- K. Knowles and D. Bray: Ground Vortex Formed by Impinging Jets in Cross-flow , AIAA Journal of Aircraft, 30, 6, pp 872-878, November-December 1993
- K. Knowles: Impinging of Jet Flowfields for STOVL Ground Effect Research , RAeS Industry-University Aerodynamics Research Forum, London January 9, 1992
- K. Knowles and D. Bray: Recent Research into the Aerodynamics of ASTOVL Vehicles in Ground Environment , Proceedings ImechE Part G: Journal of Aerospace Engineering, 205, G2, pp. 123-131, 1991
- Lawson NJ, Knowles K., Hart RJE, Wray JN, Eyles JM: An Experimental Investigation Using PIV of the Underflow of a GA (W) -1 Aerofoil Section in Ground Effect , 4th MIRA International Vehicle Aerodynamics Conference, Session 6B, Warwick 16 . – 17. October 2002
- GW Jörg: Tandem Airfoil Flairboats as efficient WIG crafts; 2nd International EuroConference on High Performance Marine Vehicles Hyper'01, Hamburg 2. – 5. May 2001
