Wing in Ground Effects “The Basics
Wing in ground effects (ground effects or surface effects) occurs when the wing of an aircraft flies in close proximity of the ground (approximately one quarter the height of the wing span depending on the aspect ratio of the wing). There are a number of changes in the flow characteristics around the wing due to this effect, firstly, the layers of air flow between the ground and the wing are compressed in the direction perpendicular to the width (cord) of the wing decreasing the layers between the laminar flow and increasing the pressure and the velocity gradient on the trailing edge of the wing, this results in an increase in lift (L). Secondly, the lift the aircraft produces drag (lift-induced drag), a vast amount of this propagates down the wing and is shed at the wing-tips in the form of vortices caused by the laminar flow of the air trying to complete a circle from the underside of the wing to the low pressure area above via the tip, this generates a helical wake behind the wing tip bleeding energy from the vortex into free stream flow at the tip (bellow figure) and inducing drag (D) into the system. Flying in the ground effect region, almost totally destroys the tip vortex as the flow at the tip, interfaces with the ground causing a pressure increase between the ground and the wing, reducing the drag (D) to the aircraft. As a result, the overall effect is an increase of the Lift/Drag (L/D) ratio, so as the aircraft craft flies closer and closer to the ground, the Lift generated by the wing increases with the Drag decreasing as the tip vortices are suppressed, the craft becomes more efficient in ground effect. Therefore, flying in ground effects, results in a major lift induced drag reduction and outstanding cruise efficiency. As described above, ground effect increases lift. The air cushion under the wing creates high pressure that increases when the ground is approached, due to the laminar flow being compressed under the wings, this is sometimes referred to as ram effect or ram air pressure. When the ground distance becomes very small the velocity gradient directly at the wing surface, can stagnate giving the highest possible pressure, or pressure coefficient unity. In the early sixties Lippisch was showing that higher values could be reached in ground effect, with his X-112 craft, and is stated achieved as high as L/D27 in ground effect flight. Although most of the modern CFD analysis on wing in ground effects is now achieving an L/D ratio between 29 –35, by far exceeding that of any aircraft.
Wing in ground effects (ground effects or surface effects) occurs when the wing of an aircraft flies in close proximity of the ground (approximately one quarter the height of the wing span depending on the aspect ratio of the wing). There are a number of changes in the flow characteristics around the wing due to this effect, firstly, the layers of air flow between the ground and the wing are compressed in the direction perpendicular to the width (cord) of the wing decreasing the layers between the laminar flow and increasing the pressure and the velocity gradient on the trailing edge of the wing, this results in an increase in lift (L). Secondly, the lift the aircraft produces drag (lift-induced drag), a vast amount of this propagates down the wing and is shed at the wing-tips in the form of vortices caused by the laminar flow of the air trying to complete a circle from the underside of the wing to the low pressure area above via the tip, this generates a helical wake behind the wing tip bleeding energy from the vortex into free stream flow at the tip (bellow figure) and inducing drag (D) into the system. Flying in the ground effect region, almost totally destroys the tip vortex as the flow at the tip, interfaces with the ground causing a pressure increase between the ground and the wing, reducing the drag (D) to the aircraft.
As a result, the overall effect is an increase of the Lift/Drag (L/D) ratio, so as the aircraft craft flies closer and closer to the ground, the Lift generated by the wing increases with the Drag decreasing as the tip vortices are suppressed, the craft becomes more efficient in ground effect. Therefore, flying in ground effects, results in a major lift induced drag reduction and outstanding cruise efficiency.
As described above, ground effect increases lift. The air cushion under the wing creates high pressure that increases when the ground is approached, due to the laminar flow being compressed under the wings, this is sometimes referred to as ram effect or ram air pressure. When the ground distance becomes very small the velocity gradient directly at the wing surface, can stagnate giving the highest possible pressure, or pressure coefficient unity.
In the early sixties Lippisch was showing that higher values could be reached in ground effect, with his X-112 craft, and is stated achieved as high as L/D27 in ground effect flight. Although most of the modern CFD analysis on wing in ground effects is now achieving an L/D ratio between 29 –35, by far exceeding that of any aircraft.
When all of these benefits are taken into account, we find that a vehicle operating in ground effect has the potential to be much more efficient than an aircraft operating at high altitude. The aerodynamic efficiency of an aircraft is expressed through a quantity called the lift-to-drag ratio, or L/D. In steady, level, non-accelerating flight, a plane's lift is equal to its weight, and the amount of thrust required is equal to the drag it produces. Therefore, the L/D ratio is a measure of the weight that can be carried for a given amount of thrust. The higher the L/D, the more efficient the vehicle. Typical L/D values for conventional, subsonic aircraft are on the order of 15 to 20. By comparison, a ground effect vehicle could, in theory, achieve L/D ratios closer to 25 or 30.
Though ground effect has been known since the early days of flight, most pilots regarded it as nothing more than a nuisance that changed the flying qualities of their aircraft during takeoff and landing. Nevertheless, many researchers soon realized that this phenomenon could be exploited to produce a new class of highly efficient craft known as WIG vehicles. Most of the pioneering research into these vehicles was performed in West Germany and the Soviet Union.
Longitudinal (pitch) stability
Longitudinal stability refers to stability around the lateral axis of an aircraft. It is also called pitch stability and depends on the location of the centre of gravity (C of G.) Additionally, longitudinal stability is compromised by the fact that the Aerodynamic Centre (AC) must be close to the (C of G) since the net lift must act opposite to weight.
In order to achieve a good longitudinal stability, the (C of G) should be ahead of the (AC) of the aircraft. For wings such as triangular, trapezoidal, compound, etc. we have to locate the Mean Aerodynamic Chord (MAC), which is the average for the whole wing. For conventional designs (with main wing and horizontal stabiliser) the C of G location range is usually between 28% and 33% from the leading edge of the main wing's MAC, which means between about 5% and 15% ahead of the aircraft's AC. This is called the Static Margin, which is expressed as a percentage of MAC. When the static margin is zero (C of G coincident with AC) the aircraft is considered "neutrally stable". However, for conventional designs the static margin should be between 5% and 15% of the MAC ahead of the AC.
However, ever since the very first experimental Wing in Ground Effect vehicles were built, longitudinal stability has been recognised as a critical design factor, although, mainly due to a lack of understanding of the dynamics, many people try and draw similarities between the pitch-up tendency of powerboats, when they meet a wave and/or air pressure builds up under the hull and suddenly flip backwards, as happened to my grandfathers good friend, Donald Campbell in Bluebird k7 January 4, 1967, although the initiator to this event is not known, the probability of this event occurring had the k7 had a suitably positioned horizontal stabiliser would have been dramatically reduced if not totally.
The reason for this behaviour is as the pressure wave builds up rapidly under the front portion of the wing or body rapidly changing the laminar flow velocity gradient and affecting the Angle of Incidence, the lift line vector travels forward, and further away from the C of G, the more acute the angle the more air flow under the wing or body and the further forward the lift line travels, with the lift pressure equalling or greater the mass of the craft, and without any equal applied force, will loose its longitudinal stability in severe pitch-up, and can even flip backwards. Therefore, correct longitudinal stability is essential to any craft travelling at high speed, on or just above a surface.
This tendency was taken into account in the design and build of Thrust SSC by Ron Ayers, Chief Aerodynamicist where correct longitudinal stability was included in the design for any adverse ground effect transients.
Therefore, the correct placing of the C of G in relation to the AC the craft must have suitable longitudinal stability in the form of a stabiliser which flies in free air (uniform flow) out of ground effect and any turbulent flow generated by the lift wing in order to return the aircraft to an equilibrium effected by any transients, being equal to, or greater than this force.
Ground Effect Craft utilize a well know phenomena, where the ratio between lift and drag is highly improved, when a lift creating wing is operated in the vicinity of the ground. Beginning at a height of appr. ½ of the wing span, the glide ratio steadily improves up to a factor of 2.3 at close Ground Effect i.e. 5% of wing span, compared to free flight.
An vehicle with a glide ration of 10 in free flight therefore can improve the glide ratio up to 23 in close Ground Effect, if properly designed, indicating, that in Ground Effect the same craft only produces 40% of the drag, which would occur in free flight or on the other hand is able to carry up to 130% more weight with the same propulsive power. Fig. 1 shows the so-called "Wieselberger Slope" in which the physical phenomena of Ground Effect con be seen.
Fig. 2 shows the position of Ground Effect Craft in comparison to other means of transportation. It is clearly indicated, that Ground Effect Craft have no competitors for over water transport, if low power requirement i.e. economical transport at high speed is required.
Over years, one aerodynamic problem prohibited the utilization of Ground Effect Craft. This reason was the aerodynamic shift of the centre of pressure occurring in changing height from Ground Effect to greater heights. This shift of centre of pressure resulted in so called pitch up tendency, which was not acceptable for commercial applications.
Also for this reason, first manned Ground Effect Craft were designed along military specifications, where the intention was to fly under the enemy radar sight, while being able to temporary rise height in order to enlarge the own radar range. Consequently, those first Ground Effect Craft were registered as aircraft.
The most outstanding design in regard to performance and safety was developed by the German aerodynamist Dr. Alexander Lippisch, referred today to as "Lippisch Design". He combined a high positioned tail surface outside the ground effect with an inverted delta wing, and proofed this in flight as being an aerodynamic inherent longitudinal and height stable configuration.
This stability, together with the origin coming from free flight, results in of the biggest advantages of the Lippisch design: To cruise waves without loss of safety. In the design of the 8-seated over water taxi, developed by AFD GmbH, the customer specification required the ability to cruise over 2m high waves. Due to the small size of the craft, cruise over such wave height is not best economic, but allows safely to maintain flight schedules for the passengers.
Basically, the wave height to be cruised over in economic ground effect mode is determined by the wing span of the craft. As mentioned, economic ground effect begins at about 10% of the wing span. For the ability to cruise 3 m waves economical, a wing span of 30m would be required.
Another advantage of the Lippisch design is the manoeuvrability during turns. Fig. 3 compares the possible turn radii of aircraft, Hovercraft and Ground Effect Craft ("Airfoils"). The improvement compared to Hovercraft is significant, and ensures safety.
This basic design is still used for the HW-Tech, being target of the investigation in the scope of this Feasibility Study.
The above described advantage of efficiency is not the only benefit of Ground Effect Craft.
It is known, that High Speed Wave Piercing Catamarans were expected to be revolutionary for over water transport with their high speed. Actually, the results were disappointing, as under real operational wave conditions, the high speed could not be maintained due to the lack of passenger comfort.
The first proof of the concept by Dr. Lippisch
It was in the sixties, when Dr. Alexander M. Lippisch experimented with Ground Effect configurations. The result was the aerodynamic layout, in which the reversed delta wing was combined with a high positioned T-tail.
Until now, it is the only Ground Effect configuration, which proofed it’s inherent length and height stability in and out of Ground Effect theoretical and in operation.
Dr. Alexander M. Lippisch first designs were desigend as aircraft, for which the ability to safely operate in and out of Ground Effect was truly essential. On the other side, this original ability for free flight was great disadvantage to develop a Ground Effect Craft as a commercial useful product: The basic advantage of a Ground Effect Craft is it’s possible cheap operation.
To burden this advantage with the additional expenses to produce, register and operate a Ground Effect Craft as an aircraft,is completely unfeasible. Beside that, for very feasible reasons, a good Ground Effect Craft is a very bad aircraft, and vice versa.
Continous works of RFB (Rhein-Flugzeugbau GmbH)
In the late sixties, RFB with Mr. Hanno Fischer as a technical director started to evaluate Ground Effect Craft by order of the German MoD. With Dr. Lippisch as a consultant, they developed the X-113 as a successor of the X-112 in order to fullfil the task of the design evaluation.
After a successful series of tests with the X-113, a new design was started to cover the Navy requirement to observe the baltic sea as per NATO requirement of that time.
ifferent to the X-113, the X-114 was a catamaran design, where the fuselage was above the water, and the sponsons had to give the displacement needed.
Also this craft was successfully operated. To improve sea worthiness during take off and landing, the X-114 was modified to the X-114H with hydrofoils. Due an error of the test pilot that time, the X-114 got lost during trials, while the Pilot survived uninjured.
At the end of the both programs, the conclusion was, that the technical and the military suitability was proven.
However, for civil applications, the registration of both craft as aircraft in those early years, was an extreme disadvantage, which prohibited the attempt to approach the market.
Ongoing R&D by Fischer - Flugmechanik
It was in 1979, when Hanno Fischer together with his partner Klaus Matjasic started to operate the R&D company Fischer – Flugmechanik. The initial target was the design, construction and operation of a small vessel, that should be unable for free-flight, and beeing able to receive a registration as a boat.
The first craft as a proof of the ship concept was Airfish-1 , which made it’s maiden flight in 1987. It cruised at 100 km/h on a power setting of 13hp. It raised the interest of an american company, and a cooperation to market this craft begun.
Based on this market interest, Fischer – Flugmechanik granted a license to RFB in order to continue the further devlopment on a basis of shared tasks, wherin Fischer – Flugmechanik proceeded with R&D and consulting, and RFB made the hardware.
Considering the recreational boating market, it was figured, that a production craft should have an significant smaller wing span, in order to fit into regular marina facilities.
As Lippisch designed craft with extremely small aspect ratio n had not been tested until then, it was decided to modify Airfish-1 by designing new wings for Airfish-1 to evaluate the characteristics of such small aspect ratio wing in flight tests. The modified craft was designated Airfish-2, and was successfully tested in 1988 and 1989.
After those tests, FF and RFB were ready to design and build the prototype of the Airfish-3 , which was a kind of pre-production model for the 2 seated recreational craft. It made it’s maiden flight in 1990 and went through intense tests until 1992, when it was demonstrated during the Miami Boat Show.
As the American licensee breeched the contracts, and tried to build unauthorized copies of Airfish-3, the program was discontinued.
Together with the downgoing of RFB, the Airfish design in complete came to a temporary stop.
The present situation of the Airfish design
In 1997 Fischer – Flugmechanik entered into agreement with Flightship Ground Effect Pty for the commercialisation of an 8-seated ground effect craft. Flightshio Ground Effect Pty was a joint ventrure between Singapore company Flightship Pte. and Australian company in Cairns Flightship Gound Effect Pty.
According a customer specification, the initial design was frozen, and Fischer – Flugmechanik decided to add a production facility to it’s original engineering office.
Therefore, in 1997, the AFD Airfoil Development GmbH was founded as a sister company, and should become the first facility to develop and manufacture Ground Effect Craft towards commercial application. In order to safeguard a development along the state of the art, AFD settled it’s development on three bases. First was the know–how use, which was gathered by Fischer–Flugmechanik, second was a recognition by Germanischer Lloyd to be certified for the production of GRP components, and third was a certification along DIN ISO 9001 for the Development, Production and Commercialisation of Ground Effect Craft – the first certification of that kind at all.
Since 2004 Fischer – Flugmechanik entered into agreement with new Singaporean company Wigetworks Pte. after the liquidation of Australian company Flightship Ground Effect Pty for the restore and commercialisation of Airfish-8 - 001 an 8-seated ground effect craft, and now the prototype is certified by Lloyd's Register and Singapore's MPA Authorities for commercial operation.
R&D on the Hoverwing Technology
Fischer – Flugmechanik began to work on the design basis for bigger vessels, when in the early 90’s, after the reunification of Germany, a new situation occurred.
The German Ministry of Economics arranged a feasibility study to figure the best suitable product for the large shipbuilding industry at the baltic sea, which would be competitive on the shuipbuilding market despite the high production expenses in Germany.
The result was e requirement for a 80 seat Ground Effect Craft with a specified performance, which at that time was beyond the state of the art.
Based on that situation, the German Ministry for R&D supported a milestone program, in which initially all available know-how was concentrated in order to investigate possible technical solutions to fullfil the specification with a Ground Effect Craft. The end of the technical feasibility study Indicated, that none of the available designs could feasibly be scaled up to the recommended size.
But there were two suggestions for alternate designs, which appeared promising enough to be continued in the next milestone. Those were the design of Techno-Trans e.V. and the new design of Fischer – Flugmechanik, the Hoverwing.
Until 1996 Fischer – Flugmechanik continued the development of the new Hoverwing Technology together with it’s project partners, VBD Versuchsanstalt für Binnenschiffbau at Duisburg and others.
Then, in 1996 the new design appeared advanced enough to be tested with a manned test craft, which was a scaled down version of the projected 80-seated vessel. Under consultancy and supervision of Fischer – Flugmechanik, the Hoverwing 2VT was constructed by the VBD and several suppliers of components.
The maiden flight took place in 1997 on the inland waters of lake Baldeney nearby the city of Essen. After minor modifications, Hoverwing 2VT was transported to the Baltic Sea in order to compete with the alternate design of Techno Trans e.V.
Hoverwing 2VT continued several sea trials on the baltic, and the Markermeer in the Netherlands until 1999, in which it covered a distance of more than 3.000 km in all, without any damage or operational problem.
WSH craft series are based in well known patent WIG craft technology that we are called "Hoverwing".
One disadvantage of earlier Ground Effect Craft was the fact, that the take off from water required by far more engine power, than the operation in Ground Effect during cruise. To make Ground Effect Craft commercial feasible, one of the most important targets was to reduce this take-off drag by having a take-off-aid.
The R&D work of Fischer Flugmechanik were heavily sponsored by the German Ministry for R&D (BMB+F)and led to an air-cushion supported design, in which a part of the propeller slip-stream is taken during take-off, and guided under the catamaran fuselage to build up a static air cushion, which creates already 80% of the crafts weight as lift, while speed is still 0.
Further advantage of the Hoverwing Technology is the fact, that after take-off, the air inlets behind the propellers are closed, and the full thrust is available for cruise.
Thus the Hoverwing Technology is the bridge between ACV and WIG craft.
The Airfoil Development GmbH (AFD) was founded 1997 by Klaus Matjasic (has died on 2005 by brain tumor) and Hanno Fischer to evolve and produce, commissioned by an asian investor, an 8-seat ground effect vehicle by using the Airfish technology.
In result of this project the AFD GmbH gets the official approval as the first worldwide producer in case of ground effect vehicles by the "Germanischer Lloyd". The AFD also gets the certification DIN ISO 9001 by the "Technischer Überwachungsverein" (TÜV) for deplopment and production of ground effect vehicles.
FF was founded 1979 by Hanno Fischer based of his 30-years job experience as technical director of Rhein-Flugzeugbau GmbH (RFB), to implement his acquired knowledge in research, evolution and manufacturing of civil and military aircrafts, as well as military ground effect vehicles to business application.Based on work by Dr. Lippisch and Hanno Fischer together with his business partner Klaus Matjasic developed the first civil ground effect vehicles called Airfish.
Based on decades of experience in research oft he ground effect by Fischer Flugmechanik and its realisation into commercial products done by AFD Airfoil Development GmbH, the German Government "Bundesministerium für Bildung und Forschung" (BMBF) in cooperation with the research institute for "Binnenschiffbau Duisburg" (VBD), pushed further continously the developing of the second generation of ground effect vehicles, the protected Hoverwing Technology. The technical demonstrator HW 2VT, which is working in this advanced and patent protected technology, is another milestone in respect of the scaling to major sizes up to 200 seats.