1. 1.1 2. 2.1 2.2 2.3 2.4 3. 3.1 3.2 3.3. 3.4. 4.

INTRODUCTION................................................................................................. 2 OBJECTIVES ................................................................................................... 3 GROUND EFFECT AERODYNAMICS.............................................................. 4 CHORD DOMINATED GROUND EFFECT ................................................... 4 SPAN DOMINATED GROUND EFFECT....................................................... 5 AERODYNAMIC CENTERS IN GROUND EFFECT..................................... 6 AERODYNAMIC EFFICIENCY IN GROUND EFFECT ............................... 7 EKRANOPLANS.................................................................................................. 9 CONFIGURATION LAYOUT ......................................................................... 9 POWER AUGMENTATION RAM (PAR)..................................................... 12 LONGITUDINAL STABILITY...................................................................... 14 LATERAL STABILITY ................................................................................. 15

1.

INTRODUCTION

Wing-in-ground effect applies to vehicles design to fly at very low altitudes to take the advantage of increased in aerodynamic lift and reduced drag which occurs when a wing is in ground effect. The phenomenon of ground effect was observed as early as the Wright Brothers’ Wright Flyer I which flew in the presence of ground effect. During World War II, war planes which are low on fuel flew in ground effect in order to fly back to base in order to make use of the increase in efficiency when operating in ground effect. Despite this phenomenon of ground effect was discovered very much earlier before the cold war, the main advances in ground effect technology took place during the 1960s in the Soviet Union by a Russian engineer, Rostislav E. Alexeyver, and his Hydrofoil Design and Construction Bureau. Alexeyver and his company design and build a number of very successful WIG vehicles known to the Soviet Union as Ekranoplans. The end of the cold war sees the end of the development of WIG vehicle in the Soviet Union. Several European countries too are involved in developing ground effect vehicles. In particular, Dr. Alexander Lippisch, the famous German aircraft designer and widely known for his invention of delta wing aircrafts have made significant contribution in the development of WIG vehicles. WIG vehicles, based on the reverse delta wing which is pioneered by Lippisch, still existed today and can is said to be a much better design to the Soviet Union’s Ekranoplan. The world’s first commercialized WIG vehicle today is base on the Lippisch concept (See Fig. 1).

LIPPISCH’S REVERSE DELTA WING................................................................ 17 4.1. 4.2. STABILITY .................................................................................................... 18 MANEUVERABILITY AND CONTROL...................................................... 18

5.

OTHER WIG CONCEPTS ..................................................................................... 21 5.1. 5.2. BOEING’S PELICAN..................................................................................... 21 HYBRID AIRSHIP IN GROUND EFFECT ................................................... 23

6.

CONCLUSION ....................................................................................................... 25

REFERENCES................................................................................................................ 26

2.

GROUND EFFECT AERODYNAMICS

When a wing approaches the ground, an increase in lift as well as a reduction in drag is observed which results in an overall increase in the lift-to-drag ratio. The cause of the increase in lift is normally referred to chord dominated ground effect (CDGE) or sometime also known as the ram effect. Meanwhile, the span dominated ground effect Fig 1. WG-8 in flight (With courtesy of Wigetworks Pte Ltd.) (SDGE) is responsible for the reduction in drag.

1.1

OBJECTIVES

2.1

CHORD DOMINATED GROUND EFFECT

The aim of this paper is to look into the historical developments as well as the different designs of WIG vehicles. Currently, all the WIG vehicles that have been developed up to date can be broadly classified under two main schools of thoughts: ekranoplan and Lippisch Reverse Delta. The conceptual design philosophy WIG vehicles discussed here in this paper with the emphasis on the ekranoplans which are the first WIG vehicles ever build. As most WIG vehicles generally shared the same design principles, only the important features and differences of the will be discussed.

In the study of CDGE, one of the main parameters which one considers is the height-tochord (h/c) ratio. The term height here refers to the ground clearance between the ground surface and the airfoil or the wing. The main causes for the increased in lift is due to the increased of the static pressure creates an air cushion when the height decreases. This result in a ramming effect whereby the static pressure on the bottom surface of the wing is increased which leads to higher lift. Fig. 2 shows the difference between an airfoil without ground effect (a) and with ground effect (b). Theoretically, as the height approaches 0, the air will become stagnant hence resulting in the highest possible static pressure with a unity value of coefficient of pressure.

a

b Fig.2.2. Vortex strength of an aircraft in flight; Left: Out of ground effect. Right. In ground effect 2.3 AERODYNAMIC CENTERS IN GROUND EFFECT

Fig.2.1. Contour plot of static pressure on an airfoil; a. Out of ground effect. b. In ground effect

2.2

SPAN DOMINATED GROUND EFFECT

Similar to classical aerodynamics, a convenient point known as the aerodynamic center is define where the moment acting on the body is independent of the angle of attack. However, in the presence of ground effect, the force and moment acting on the body changes with height. Hence for a WIG vehicle, there are two aerodynamic centers. The former, which is the same as the aircraft aerodynamic centers, is known as the center of pitch. The latter where the moment is independent of height is known as the center of height. These two centers can be obtained by considering the Lift and Moment curve with respect to angle of attack and with respect to height. They can be calculated by the following given relationship [1]: xα =

On the other hand, the study of SDGE consists of another parameter known as the heightto-span (h/b) ratio. The total drag force is the sum of two contributions, profile drag, and induced drag. The profile drag is due to the skin friction and flow separation. Secondly, the induced drag occurs in finite wings where there is a ‘leakage’ at the wing tip which creates the vortices that decreases the efficiency of the wing. In SDGE, the induced drag actually decreased as the strength of the vortex is now bounded by the ground. As the strength of the vortex decreases, the wing now seems to have a higher effective aspect ratio as compared to its geometric aspect ratio therefore a reduction in the amount of induced drag.

Cmα CLα Cmh CLh

(2.1)

xh =

(2.2)

where xα and xh are the center of pitch and the center of height, respectively. In this paper, the sub-subscript means the differential.

By applying the Thin Airfoil theory, it is found that for a flat plate with infinite aspect ratio under the influence of ground effect, the Lift and Moment taken from the leading edge are given as: CL = α h α 3h (2.3)

found to have a parabolic spanwise distribution. Therefore to minimize induce drag and achieve the optimum Lift to Drag ratio for a ground effect wing, the angle of attack and the chord along the span can be made to vary in a parabolic distribution as well as increasing the aspect ratio of the wing.

CM = −

(2.4)

In equation 2.3 and 2.4, the angle of attack is express in radians and height h is express as a fraction with reference to the chord length. Hence through the relationship given by 2.1 and 2.2, the both xα and xh located at 1/3 of the chord which also coincides with the center of pressure. This however is only applicable for the case of a flat plate with aspect ratio larger than 2. For the case of a cambered airfoil, the two aerodynamic centers and the center of pressure most likely do not coincide on the same point. Recall that for a symmetrical airfoil out of ground effect, the center of pitch and pressure are located at ¼ of the chord.

2.4

AERODYNAMIC EFFICIENCY IN GROUND EFFECT

For a finite wing, the Prandtl’s lifting line theory has shown that the induce drag is given by: Cdi =
CL πeAR
2

(2.5)

For a wing out of ground effect, the circulation along the spanwise direction of the wing follows an elliptic distribution. However in extreme ground effect, the circulation is

3.

EKRANOPLANS

generally with an aspect ratio much smaller than an aircraft which is typically less than 3.5. The purpose of the main wing is to generate lift and sometimes end plates are mounted at the wing tip in order to reduce induce drag and decrease the loss of lift due to leakages at the tip. Flaps are also attached on the aft of the wings which will be deployed during take off in order to increase the lift force required to lift the hull off the water. Another distinct feature of an ekranoplan is the relatively large horizontal stabilizer as compared to an aircraft. The size of the horizontal stabilizer of the first generation ekranoplan is about 50% of the main wing area and has the same span as the wing. The horizontal stabilizer is also mounted very high up the fuselage in order to keep it out of the influence of ground effect. The purpose of the horizontal stabilizer is to provide longitudinal trim, stability and control with the aid of the elevator. Directional control and lateral stability are provided by the rudder and vertical stabilizer respectively. The propulsion systems of ekranoplan are generally classified into two types: Separate power plant and integrated power plant. For a separate power plant approach, two groups of engines are use. One group is for cruising mode, another group is use during the starting of the vehicle for aiding take off which is known as Power Augmentation Ram (PAR) effect and will be discussed in depth under section 3.2. The other approach is to use an integrated power plant which can operate in both PAR mode and cruise mode. The advantage of using the separated power plant system as seen in the KM, Fig.3.1 and
Fig.3.1. KM in cruise.

In Russian, the word ekran means a screen whereas plan means a lifting surface of an airplane. Hence Ekranoplan is a vehicle that is supported by aerostatic lift created by its propulsion system and aerodynamic lift created by ground effect. Perhaps the most famous and the largest of all the ekranoplans, known to the west as the Caspian Sea Monster, is the KM designed by Alexeyver and his Hydrofoil Design and Construction Bureau. Its dimension was documented to have reach a wing span of 40m, a length of 100m, with a maximum take off weight to reach 540 tons and had a cruising speed of over 400km/h. Unfortunately, the KM was destroyed in 1980 when its pilot let it rise out of ground effect where it lost its lift and crashes into the sea. Attempts were made to recover it but failed.

Fig.3.2, is that the rudder will be more effective at lower speed as they are just in the jet stream of the engine. The main drawback will be the excessive drag from the PAR which is not use during cruise mode. The opposite goes for the integrated power plant system which is employed in the Lun as shown in Fig.3.3.

3.1

CONFIGURATION LAYOUT

The general configuration layout view of an ekranoplan is shown at Fig. 3.2 and Fig. 3.3. The main features of the ekranoplan consist of a lowly mounted simple rectangular wing,

3.2

POWER AUGMENTATION RAM (PAR)

One of the major obstacles a WIG vehicle face, like all marine vessels, is the large hydrodynamic drag the vehicle will face during take off. In order minimize the power for the vehicle to take off; one such solution is to blow air under the main wing from engines mounted just in front it. This mode of operation is therefore known as Power 2 1 3 4 5 Augmentation Ram (PAR) effect. The principle behind this is simple, by blowing or injecting the air under the wing, the wing will see jet of high stream gas heading towards it even when the vehicle is traveling at lower speed. As the jet of air headed towards the bottom surface of the wing, the deceleration of the air causes an increase in static pressure under the wing hence creating a static air cushion which leads to an increment in
Fig.3.2. Plan view of KM. 1 – Main Wing. 2 – Horizontal Stabilizer. 3 – Vertical

lift. This jet of air however does not only propagate in the longitudinal direction of the wing but in other direction as well. Therefore in order to maximize the efficiency of the PAR by preventing air from escaping or leaking out, the bottom surface of the wing is sealed except for the leading edge during take off. This enclosed or sealed region is achieved by attaching end plates on wing tip, the fuselage which is attached to the root of the wing and the flaps that are deploy during take off mode. Other than increasing lift by creating an air cushion underneath the wing, the PAR can also be use to increase lift by allowing part of the jet stream to flow above the wing. This enables two advantages. One of which is to prevent flow separation from occurring at the upper surface of the wing. Due to the higher adverse gradient of the airfoil on the upper surface in the presence of ground effect (See Fig.3.4), separation of flow occurs at a lower angle of attack than an airfoil in the absence of ground effect. With the PAR

Stabilizer. 4 – Power Augmentation Ram System. 5 – Power Plant

Fig.3.3. Plan view of Lun

however, the air will have a larger momentum to overcome the adverse pressure gradient

to prevent separation from occurring. In addition, the higher momentum of the air above the wing will also leads to an increased in the suction force and hence resulting an increment in the total lift force.

3.3.

LONGITUDINAL STABILITY

Stability is a very important criterion in the design of aircraft. For aircraft, two conditions must be met for longitudinal pitch stability [ ]:
Cmα < 0 and Cm 0 > 0 .

(3.1)

Physically, equation () states that when the vehicle is cruising at it’s trimmed angle of attack, the vehicle will return to it’s trimmed condition if the change of it’s angle of attack cause by disturbance is small. For WIG vehicle, however, there is also a need for height stability in addition to pitch stability. The height stability criteria is necessary for the vehicle to be bounded to the ground, meaning when there is a small disturbances acted on the vehicle while cruising,
Fig. 3.4. Static pressure plot on a NACA2412 airfoil. Left: absence of ground effect.

the vehicle will have the ability to return to it’s own cruising height. Therefore for both height and pitch stability to be satisfy, it is proven mathematically by Routh – Hurwitz criteria that the height center of the vehicle must be in front of the center of pitch.

Right: In ground effect.

xh − xα > 0

(3.2)

xh

xθ
Fig. 3.6. Criteria for height and pitch stability of WIG.

Generally for a positive camber wing, the height stability can never be satisfied as the
Fig.3.5.

center of height will be located behind the center of pitch hence for a wing alone design, the vehicle is unstable. Therefore normally to achieve the Routh – Hurwitz criteria of stability, a large horizontal stabilizer is needed and is typically mounted high up such that it is out of the influence of ground effect. The idea behind keeping the horizontal

stabilizer out of ground effect is to keep the position of the center of height unchanged but shift the center of pitch to the aft of center of height. Theoretically this can be proven by observing equation 2.1 and 2.2 which relates the two aerodynamic centers to the lift and moment curves slope with respect to angle of attack and height. From equation 2.1, by mounting the horizontal stabilizer will causes changes to the gradient of the lift and moment curves hence shifting the center of pitch backwards, similar to the aircraft. But if the horizontal stabilizer is mounted out of ground effect, the lift and moment curves with respect to ground clearance h will remain unchanged theoretically since the tail is not in the influence of ground effect, henceforth the center of height remain unchanged. h + ∆h CL ↓ h
Fig. 3.7. WIG craft subjected to a roll motion

h – ∆h CL ↑

3.4.

LATERAL STABILITY

Ailerons are typically absent in an ekranoplan, unlike the aircraft which is needed to achieve lateral stability. Ekranoplans have the advantage over the aircraft whereby lateral stability can be achieve naturally in the presence of ground effect. Consider Fig. 3.7 when the Ekranoplan is subjected to a roll angle. The wing on the right now operates in a lower ground clearance than the wing on the left, base on ground effect aerodynamics, the right wing now experiences an increase in lift whereas the left wing experiences a loss in lift. This creates a natural stabilization moment on the vehicle due to the different in the lift forces acting on the two wings. However, do note that this is only possible if and only if the vehicle is operating in ground effect.

4.

LIPPISCH’S REVERSE DELTA WING

4.1.

STABILITY

While Alexeyver was working on a boat that flies, Dr Alexander Lippisch on the other hand took the approach of designing an aircraft that flies in ground effect. The main feature in the Lippisch’s design is the use of a reverse delta wing. Currently, the Lippisch’s design seems to be more favorable as it has better stability and control over the ekranoplans. Other features of the Lippisch’s design includes having a smaller horizontal stabilizer as compared to the ekranoplans and the wings are anhedral instead of dihedral as seen in a conventional aircrafts.

As discussed in section 2.3, the aerodynamic center of pitch of a wing in the presence of ground effect will shift aft as the ground clearance decreases. For a non-symmetrical positive cambered airfoil, the center of pitch may shift to as back as to 50% of the chord length. The variation of the position of the pitch center causes the moment acting on the vehicle to change as the altitude of the vehicle varies. This is especially so if the wing platform is rectangular hence ekranoplans uses a very large tail to counter the effect of the varying moment acting on the vehicle so as to obtain a higher static margin of stability. The Lippisch’s reverse delta wing platform however is very insensitive to the changes in the moment acting on the wing with respect to ground clearance. Therefore the reverse delta wing is said to have inherent stability since the position of the pitch center will not varies as big it would on the rectangular wing platform. This made the vehicle more stable which is why the Lippisch design need not require a big tail to achieve stability and the structural weight can be reduce since only a smaller tail is require for balancing of the aerodynamic forces.

4.2.

MANEUVERABILITY AND CONTROL

Due to the insensitive response of the center of pitch with respect to ground clearance, in addition to stability, the Lippisch design possesses better maneuverability and control and
Fig. 4.1. Plan view of X-114

it has the ability to jump out of ground effect for obstacle avoidance. Some of the WIG design by Lippisch, like the X-113 has the ability to operate out of ground effect as an aircraft although it is not as efficient when it is out of ground effect.

To achieve better roll control, Lippisch place the ailerons on the winglet instead on the main wing. The advantage of this is to reduce the disruption of the lift on the main wing since the main wing profile remains unchanged which will ensure there is sufficient lift to keep the vehicle on air while executing a turn. The winglets are also place at a very big dihedral angle to prevent it to touch the water while banking hence preventing the skipping stone effect.

Fig. 4.2. X-113 flying out of ground effect

As the wing platform is of a reverse delta form, it means that the chord length varies in the span wise direction and is much longer at the root and shorter at the tip. This variation allows the wing to be anhedral to the horizontal since the root can be elevated higher than the tip while keeping the same chord to height ratio hence the same Coefficient of Lift. By placing the wing anhedral allows the Lippisch design to be more maneuverable than the ekranoplan since it can now bank at a larger angle hence requires a smaller turn radius when it is executing a turn.
Fig. 4.4. Winglets with ailerons as seen on the Airfisch 3

The only drawback with the Lippisch design however will be that very large power is required during take off. Unlike the ekranoplans which has power plants to assist take off, the Lippisch WIG will have to over come the large hydrodynamic drag before it can achieve enough speed to become airborne.

Fig. 4.3. X-114 executing a bank turn

5.

OTHER WIG CONCEPTS

Besides the ekranoplans and the Lippisch reverse delta wing, other WIG concepts have also been proposed and some are in the process of being developed. In this section, some of the WIG projects….
5.1. BOEING’S PELICAN

In year 2002, Boeing unveils its own WIG project name Pelican. With a wing span of 152m and a fuselage length of 109m, the Pelican will be the largest aircraft ever build in the world and also the first non-Russian large WIG. Being built as a military transport vehicle, the Pelican is design to carry a payload of more than 1400 tonnes which can transport 17 M-1 tanks at a time, and will fulfill the US military's goal of the ability to deploy a whole division in five days, or five divisions in 30 days. Cruising at 6m above water at 480km/h and powered by four turboprop engines, the Pelican if necessary can also fly at 20 000feet in the air. Boeing's claim that the Pelican is capable of transporting 750 tons over 18,530 km when cruising in ground effect, but can carry the same load only 12,045 km when out of ground effect.
Fig. 5.2. Comparison between the Pelican and the B747

Skeptics have pointed out however that Boeing’s Pelican is not really a WIG vehicle unlike its Russian counterpart. Given the wing span and the capabilities of the Pelican to fly out of ground effect at 20 000feet, the Pelican is just merely a huge airplane that take the advantage of ground effect when conditions allows. However, the current status of this project is unknown and is most probably shelved like other Boeing’s projects, the
Fig.5.1. Artist’s impression of Boeing’s Pelican WIG

sonic cruiser and the blended wing transport aircraft.

5.2.

HYBRID AIRSHIP IN GROUND EFFECT

Despite Calkins concluded that the hybrid airship will yield a 43% higher annual profit over the conventional airship, to date no further developments on this concept is being undertaken by any companies, government bodies or institutes.

Calkins proposed the concept of a hybrid airship which operates in ground effect for the purpose of transoceanic cargo transportation. A normal airship requires a very huge blimp to obtain the sufficient buoyancy that is needed to be effective enough to carry any useful load. However by extending the concept into ground effect, the performance of the hybrid airship will be enhanced further as given the gigantic size of the airship and the advantage of ground effect, a new mode of cargo transport vehicle can be developed.

The hybrid airship concept consists of three aerospace technologies: aerostatic lift due to buoyancy, lifting body aerodynamics and ground effect augmented lift. The basic configuration of the hybrid airship consists of a forward canard control surface to provide pitch trim control, high winglets mounted the wing tips to act as a vertical stabilizers as well as to reduce induced drag and powered by to Pratt and Whitney JT-9D turbo fan engines. To reduce the projected side are and decrease the problem of gust loading, the hull takes the shape of a rectangle instead of a conventional circular fuselage.

Fig. 5.3. Artist’s impression of the hybrid airship concept

6.

CONCLUSION

REFERENCES
1. H.H. Chun, C.H Chang, Longitudinal stability and dynamic motions of a small passenger WIG craft, Ocean Engineering 29, 2002, pp 1145-1162

The potential benefits of ground effect are indeed attractive however the future of WIG remains uncertain. To date, only the Soviet Union is the only country that ever put serious money into developing them and after the cold war, the project was being abandoned. Current developments in WIG by many private companies are of a smaller scale which many are still in the development stages. Even the commercialize WG-8 as seen in Fig. 1 is a dwarf when compared to the Caspian Sea monster. The drawback of smaller WIG is that it lacks the efficiency and usefulness that the large vehicles can deliver. However, larger WIGs are harder to build and design. Although the Russians are very successful WIG builders, some skeptics pointed out that the Russian WIG are not that well design after all as it consists of a large heavy hulls, low aspect ratio wings which violates the law of aerodynamic efficiency and the banks of turbojets as seen on the Caspian Sea Monster. With Boeing pulling out of the Pelican project, any future developments of large WIG comes to a halt for the time being hence for aircrafts is currently and will still be preferred as the mode of transoceanic transport. On the other hand, with the developments on smaller WIG vehicles by private organizations, these smaller WIGs will be most likely be used in archipelago islands like Indonesia and the Maldives. Perhaps one day, WIG will be the next generation of land transport where cars will be riding on air cushion instead of wheels.

2. Robert C. Nelson, Flight Stability and Automatic Control, 2nd ed., McGraw-Hill, 1998

3. V. Bebyakin Ed., EKRANOPLANS: Peculiarity of the theory and design, Saint Peterburg, "Sudostroeniye", 2000

4. K.V. Rozhdestvensky, Aerodynamics of a Lifting System in Extreme Ground Effect, 1st ed., Springer-Verlag, 2000

5. J.D. Anderson Jr., Fundamentals of Aerodynamics, 3rd ed., McGraw-Hill, 2001

6. Chin-Min Hsiun, Cha’o-Kuang Chen, Aerodynamic characteristics of a twodimensional airfoil with ground effect, J. Aircraft v33 (2), 1996, pp 386-392

7. M.R. Ahmed. S.D. Sharma, An investigation on the aerodynamics of a symmetrical airfoil in ground effect, Experimental Thermal and Fluid Science, In Press, 2004

8. Ron Laurenzo, A long wait for big WIGs, Aerospace America AIAA, June 2003, pp 36-40

9. D.E. Calkins, Feasibility Study of a Hybrid Airship Operating in Ground Effect, J. Aircraft Vol.14, No.8, August 1977, pp 809 – 815.