1) The document discusses easy experimental methods for studying ground effects, including using a moving belt wind tunnel system to simulate the movement of the ground. 2) It presents a case study on the Mercedes CLK-GTR race car where a 1:10 scale model was tested in wind tunnel to understand flow under the car and ground clearances. 3) Key findings included that downforce increases with lower ground clearance up to a point where the boundary layer blocks the underbody flow, and that side skirts and rear wings help increase downforce by preventing crossflows and increasing underbody velocity.

1. Easy Ways to Study Ground Effects
Prepared for the EAGES 2001 International Ground Effect Symposium
Toulouse, France
June 2001
Jan Monchaux & St´ephan Aubin
SUPAERO
10 avenue Edouard Belin
31055 Toulouse Cedex 4
France
213

2. 214

3. Easy Ways to Study Ground Effects
Jan Monchaux & St´ephan Aubin
ABSTRACT
This paper discusses all the simple methods developed in SUPAERO to study the different
aerodynamic ground effects and understand their main characteristics. It gives a student point
of view of this complex phenomena, and illustrates some possibilities of easy studies of both the
Venturi and the ”lifting” ground effects. It is clearly focusing on experimental approaches, and
the results must be considered for their tutorial aspect. After a short presentation of the different
constraints due to the presence of the ground and the ways used in the case of the paper to solve
it, the case of the Mercedes CLK-GTR and its study will be presented, followed by a more classical
study of the WIG phenomenon in wind tunnel.
ABOUT THE AUTHORS
Jan Monchaux and St´ephan Aubin were both second year student in SUPAERO at the mo-
ment EAGES happened. Their common interest for ground effects led them to very different tastes,
though they have in common a passion for racing cars. Jan is now doing his third year at Impe-
rial College, London, while St´ephan is studying infinite swept wings separation for the ONERA
Toulouse.
ACKNOWLEDGEMENTS
The authors would like to thank Allan Bonnet for his valuable help, his feedback and his
knowledge, Francis Marty for his experience and his kindness, and all the SUPAERO aerodynamics
laboratory.
215

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5. Jan Monchaux & St´ephan Aubin Easy ways to study ground effects 217
INTRODUCTION
Flying in Ground Effect (GE) implies to be very close to the ground. This particular characteris-
tic has very interesting properties in terms of induced drag, and can modify the main aerodynamic
properties of any body in the vicinity of that ground. Well used, it can provide more or less Lift.
But to know how the body will behave in GE, one must first test it. Usually, wind tunnel and
computational testing can provide good approximations and are usually validated in the real world
of true flying machines. In the case of Wing in Ground Effect Crafts (WIGs), the wind tunnel
research as well as the computational work has to correctly take into account the presence of the
ground, which make these tests a bit more complicated. We will present the different simple me-
thods we used to characterize GE, which are not exhaustive of course. This will give solutions able
to provide reasonable results at a reasonable cost, the latter being of course the Gordian Knot of
any school . . .
BOUNDARY CONDITIONS IN GROUND EFFECT
For a person standing on a sea shore watching a WIG flaring , what immediately occurs is
the fact that the craft is flying over an almost steady ground. For the pilot of the same WIG, the
ground is definitely moving under his craft. The relativity of these two configurations is the key to
understand the boundary conditions of the GE aerodynamic problem.
Perfect Fluid Boundary Conditions
The Inviscid Flow (IF) model of a flow considers that the viscous effects do not exist. The
IF model is solution of what is commonly called the Euler Equations. This simple model gives
satisfactory results, and can be coupled with the boundary layer model to compute flows instead
of solving the true Navier Stokes Equations. In the case of simple GE, we shall consider the flow
to be subsonic with no supersonic points. In that case, the most know result for a wing in free IF
is that its only drag is induced by the lift it creates. For 2D flows, this becomes what is called the
d’Alembert’s paradox :
CD = 0
In IF, the only (and sufficient) boundary conditions that can be applied to a flow is setting a
tangential condition on any surface :
−→
Vr.−→n = 0
with Vr : relative local speed
On an airfoil, this condition simply means that no flow passes through its surface.
Figure 1 : tangential condition on an airfoil

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On the ground, this only means that the ground has to be a stream surface. One cannot set
the speed of the flow on the ground.
Hence, one solution to set this boundary condition, if one considers the ground to be flat ,
is to study a new configuration made of the body itself and its mirror image, the pseudo mirror
being the ground itself. The geometry symmetry forces the pseudo mirror to be a streamline. The
upper side of the configuration is necessarily ruled by the same equations with the same boundary
conditions than the simple problem in GE. Its solution can then be assumed to be identical to the
one searched.
This method is commonly used for computation or analytical research and leads to reasonable
results. Of course, it does not take into account the pressure and viscous drags, as it is easy to
prove that the d’Alembert paradox is still verified in ground effect.
Real Fluid Boundary Conditions
For the person on the sea shore, the air does not move, as well as the sea. They have no relative
speed. For the pilot, the ground is moving at the same speed as the air his craft is flaring through.
The boundary condition becomes that for any point on the air-sea interface :
−→
Vr =
−→
0
It is then insufficient to consider that a simple flat ground, for instance the bottom of a wind
tunnel section can be assumed to represent the ground. The boundary layer that develops on that
ground, due to the relative speed between the ground and the wind changes all the properties of
the flow.
Figure 2 : Side view of a RAM with endplates in GE without moving ground
The last figure assumes the boundary layer to start at the beginning of the part of the ground
designed. In fact, one must not forget that the wind tunnel develops itself its own boundary layer
which can also affect the flow studied.
Some solutions . . .
The most basic solution is to throw a model with a catapult and study its movement. This
solution has been developed in Lille for instance, but is clearly too expensive for teaching.

7. Jan Monchaux & St´ephan Aubin Easy ways to study ground effects 219
Figure 3 : ONERA Lille Catapult
Another approach is to move the model over the ground, and weight the efforts. This solution
is once again really complex to develop and very expensive to run.
The solution has to be found with easy methods, and must not take too much place. The ideal
conditions would be to have a system the size of a common wind tunnel. To satisfy this condition,
the easiest way is to consider that the bottom of the wind tunnel must move. This is called the
belt. Two wind tunnels are equipped with this system that has another advantage that is that
studies can be run on flying crafts or land crafts like cars.
Figure 4 : moving belt system
The figure 4 shows our latest moving belt system that has a boundary layer suction system
located upstream the belt, avoiding the problem of the wind tunnel inner boundary layer.
This system was first developed to study racing cars ground effects that are determinant to
improve performances.
AN APPROACH OF THE GROUND EFFECT IN CAR RACING
On most racing circuits with medium to high speed turns, vehicles with high downforce can run
faster lap times. If an aerodynamicist is asked to recommend a configuration for such a circuit with
high downforce and relatively low drag, then very likely his choice will be based on an inverted
wing in ground effect, as shown in Fig.5. The first design that used aerodynamics in Racing Cars
other that body streamlining did not appear until the 1960s. This idea was technically developed

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by Chevrolet-Chaparral in 1966 with their Can-Am racing car. The engineers mounted an inverted
wing on two vertical struts above the rear axle. The wing could be pitched during racing to provide
the optimal value of downforce. These ideas were developed quickly, and a few years later several
Formula 1 teams mounted inverted wings on the rear axle, combined with a smaller front wing.
Introduction of Sealing Skirts and Ground Effect Cars
The addition of side fins to seal the airflow from the sides considerably increases the downforce
(since the lift of a two-dimensional airfoil is larger than that of a low-span wing). The ground effect
car was introduced by Team Lotus in their 1978 Formula 1 car. By shaping the underbody with
appropriate channels, and using side pods to increase the effective area, the car provided much
larger values of downforce. The idea, again, was simple. Ground effect was a known concept in
aerodynamics. It just needed a technical solution to be fully exploited.
Figure 5 : Description of the inverted airfoil shape of the side pods on the Lotus Type 79. In practice
just the sliding skirts were visible.
Importance of downforce
Since the introduction of aerodynamics, it has been clear that the proper distribution of down-
force has the most evident impact on car performances. Racing regulations are strongly focused
on chassis characteristics, and give strict limits to size, position, and type of devices allowed and
prohibited. Nevertheless, cornering speeds have reached 4 G (four times the acceleration of gra-
vity). The car needs road grip, it means friction. In order to be efficient the wheels should not
slide. While cornering the car undergoes a centripetal acceleration, that is turn into a centrifugal
force on the wheels. This force can be cancelled by static friction (that limits sliding). Until a given
limit the maximum lateral force can be written :
Fmax = kf .R
where :
– R is the ground reaction
– kf is the maximal static friction coefficient (mostly lightly greater than the dynamic friction
coefficient.
Suppose that the car is in an horizontal corner, the ground reaction equals the car load, it
means the weight and the downforce m−→g +
−→
Fz. In addition, the inertial force can easily be written
in function of the speed and the corner radius. More precisely,

9. Jan Monchaux & St´ephan Aubin Easy ways to study ground effects 221
m.
V 2
r
= kf .(mg + Fz)
with : Fz =
1
2
ρV 2
CL
Generally the next formula is preferred and give the minimal radius of cornering of the car :
rmin =
V 2
kf g 1 + Fz
mg
It is also important to see that the maximal lateral acceleration is given by :
Γmax =
kf
m
(mg + Fz) = kf 1 +
Fz
mg
g
Downforce is also of very importance during acceleration and braking.
Effects of Legislations
The step from one development phase to another was interrupted by legislations aimed at
cutting the aerodynamic downforce, to secure safe races besides great shows. This may seem a
contradiction, because it seems to undermine the primary motivation of car racing : speed. During
the ”24 Heures du Mans 1999” the Mercedes CLR-GTR had a spectacular accident. The car took
off like a plane and had a furious crash, nobody was seriously injured but Mercedes decided to stop
their race after this accident. We decided to try to understand what happened and what could
explain this phenomenon by building a 1 to 10 scale model of the Mercedes CLK-GTR, which
is similar to its sister the CLR-GTR. The car was equipped with pressure captors all over the
upper and lower centreline. The aerodynamic laboratory of SUPAERO had already a model of a
Formula1, but we focused our study on the working of the Mercedes underbody. Through wind
tunnel testing in SUPAERO, we were able to place some important phenomenon, that characterize
the GE in race car, in a prominent position.
Figure 6 : A flying Mercedes CLR-GTR during the 1999 edition of the ”24 Heures du Mans”

10. 222 EAGES Proceedings
Influence of the ground clearance
The effect of ground proximity have an important influence on the aerodynamic coefficients of
the car. As shown in Fig. 6 for an enclosed-wheel car, drag and lift decreases with ground clearance.
The increase in the downforce can be explained by the lower pressure under the car, with decreasing
ground clearance (Fig.7) Of course, the underbody channels are not Venturi tubes. But there is
clearly similitary between their pressure distribution. So as the boundary layer increases with x,
the flow accelerate until the channels (or Venturis). The Bernoulli equation assure that the pressure
decreases at the same time. Note that the Bernoulli equation is true out of the boundary layer and
it is acceptable to neglect the viscosity once out of the boundary layer. Up to a point it doesn’t work
anymore, the lift increases when decreasing ground clearance. The converging part is obstructed
by the thickness of the boundary layer.
Figure 7 : What happens under the car
Figure 8 : Influence of the ground clearance on Drag and Lift
Figure 9 : Influence of the ground clearance on the pressure under the car

11. Jan Monchaux & St´ephan Aubin Easy ways to study ground effects 223
The fact that the drag decreases with ground clearance could be a result of the faster airflow
emerging from under the vehicle, reducing the size of the rear flow separation.
Influence of the side skirts
They are now forbidden, but they were at their time of prime importance. The concept was to
seal the gap between the vehicle body and the ground. They were either rigid or flexible (sliding
up and down) and were ”skirts”. They prevented the airflow from penetrating the low pressure
area under the car. In addition to that, it is clear that such a configuration can generate very large
lift/drag ratios, as long as the flow is kept close being two-dimensional.
Our results show this trend :
Without skirts With skirts
CD = 2, 0 CD = 2, 0
CL = −3, 0 CL = −3, 4
Figure 10 : Influence of skirts
This graphic shows that the pressure under the car is lower when the car has skirts. But the
recompression in the underbody channels (Venturis) is weaker. The peak reached at the narrowest
flow passage is probably generated by the airflow traditionally penetrating under the car when
there are no skirts. This could explain why it is weaker with skirts.
Figure 11 : Typical underbody channel on an enclosed wheel race car and lateral penetrating airflow.

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Influence of the rear wing
The experience showed that rear wings can be used to increase the flow under the car to
augment the body’s contribution to the downforce. The rear wing, which is an inverted airfoil
generate a negative Cp zone between the ground and the airfoil. It helps to increase the velocity
of the underbody flow. This zone is clearly more negative with the wing-on configuration than
with the wing off one as shown in Fig. 10. The aerodynamic coefficient measured confirmed this
explanation :
Wing-on Wing-off
CD -2.8 -1.9
CL 2.0 1.57
Figure 12 : Effect of the rear wing on the ground effect existing under the car
Influence of incidence
Here is presented the effect of pitch on the car. Fact is that downforce increases with negative
incidence until a precise angle, while drag seems to stay constant. Here for incidence less than
-0,75◦
the viscous effects block the underbody flow and stop the increase in downforce.
Figure 13 : Influence of incidence for the Mercedes
A reason that could explain why the car took off
The videos showed that the cars that took off were often placed behind other cars. The airflow
entering under the car was quite different of a normal configuration. In order to recreate those
conditions we blocked the flow entering under the car with a little girder placed just in front of the

13. Jan Monchaux & St´ephan Aubin Easy ways to study ground effects 225
car as shown on this figure. The results were spectacular. The lift coefficient increased considerably
and was not far away of becoming positive. The CL reached the value of -0,9◦
(-2,8◦
without the
grid) ! In addition to that, the center of pressure moved backward not far away from the rear wheel
axe and the Cmt (its reference is the rear wheel axe) reached the value of -0,16 (-2,11 without
the grid). Those values shows that the longitudinal stability of the car was weak and that the car
was not far away to become a plane. Any road irregularity could become the divergent factor that
brought the car to behave like a plane.
Figure 14 : Experimental assembly
Figure 15 : the ground effect does not work anymore . . .
The lower centerline pressure distribution shows clearly that the undertray does not work. The
pressure under the car (until the channels) stays approximately constant is quite greater than in
the normal case. The recompression is in this case weaker than normally, the ground effect can not
appear clearly. The car becomes quite instable and dangerous.
A WING IN GROUND EFFECT SIMPLE STUDY
GE is usually associated with racing cars in Western Europe. We understand it as a enhan-
cement of the anti Lift created by the aerodynamic devices of a Formula 11
or a Le Mans Series
car. It is then culturally hard to understand how an enormous machine like the KM could take
advantage of the GE to fly. The first task is then to truly experiment the flight in GE. It can be
very simple and cheap, and the results give the very basic characteristics of the flight in GE.
The Tottori RAM
When someone wants to study a new concept, he or she will not start from a blank sheet, but
preferably look at the state of the art. In our case, the blank sheet will be enough ! The Tottori
1
Tough nowadays it is limited on such cars . . .

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University in Japan has developed a simple paper made RAM that can be easily built. After some
cuttings, a first WIG is ready to take off from any corridor, propelled by a simple rubber elastic.
Figure 16 : The Tottori RAM
Trying to change the basic configuration led to different problems, putting in evidence the mere
characteristics of WIGs. The position of the centre of gravity (CG) appeared to be determinant,
too big (and heavy) vertical tails leading to pitch instability, while changing the shape of the RAM
also led to even more violent stability problems. Eventually, a tandem was developed but stability
was too hard to find. It was time for wind tunnel testing.
The first object put in the wind tunnel was a brass version of the Tottori RAM.
Figure 17 : influence of the moving ground
It gave very interesting results, proving that the ground had an important role in the GE
aerodynamics and helped finding an experimental method to characterize what Irodov called the
centre of height : for a given angle of incidence, the dependence of the momentum on the lift
coefficient, with height moving, put in evidence a point where the momentum does not depend of
the height. This point, as the aerodynamic centre, will appear to be determinant for the stability
of the WIG, giving to its aerodynamics an pre-eminent role.
Figure 18 : position of the centre of height (slope), only one incidence tested

15. Jan Monchaux & St´ephan Aubin Easy ways to study ground effects 227
The small Aspect Ratio (AR) of the Tottori RAM also permitted to make very interesting flow
visualisations called tomoscopies. The tomoscopy is a technique that visualises a flow with smoke
in a certain chosen plane enlightened by a wide laser beam. As the RAM had a low AR, its wing
tip vortices were quite big compared to its size, and the tomoscopy put in evidence the fact that
these wing tip vortices were very much reduced when getting closer and closer to the ground. It
visually explained the reason why drag was reduced in GE. These vortices are also visible on most
of the common ground effect machines.
Figure 19 : Out of ground effect wing tip vortex
Figure 20 : In ground effect wing tip vortex
Figure 21 : KM wing tip vortices
Airfoil testing
The next step was to study a true airfoil in ground effect. After studying the role played by the
extractor on racing cars, it was clear that the shape of the lower surface of the airfoil was rather
important. A classic NACA 0012 would probably not fit, as its symmetrical shape would behave
like a F1 in GE, i.e. create a Venturi that would lead to anti lift2
. Hence, the airfoil chosen had a
concave shape, avoiding any possibility to create a Venturi. It had no name, having probably been
created through an inverted method years ago. Tests were conducted with AR=3.
2
for low angles of attack, of course.

16. 228 EAGES Proceedings
Figure 22 : Concave airfoil
On this airfoil, the influence of incidence was measured - it was avoided on the Tottori RAM
due to its sharp leading edge. Height was always measured from the trailing edge, and incidence
as it is classically done. Hence, when the incidence was changed, as the tests were run for a given
incidence and a variable height, the trailing edge height had to be measured again.
Figure 23 : Influence of the ground on Lift coefficient
It put in evidence that one could assume the ground to almost only modify the incidence where
no lift is created .
Written in another way :
CL(α, h) k(α − α0 + (h))
Figure 24 : influence of the ground on induced Drag
It also proved that the induced drag was reduced in GE. As the airfoil went closer to the ground,
the parabolic behaviour of CD changed, the AR being apparently higher than out of GE. Writing
the induced drag as follows, as in Prandtl’s approach :
CD =
CL
2
Πλe
with λe as effective AR, it leads to an effective AR of 5 to 6 instead of 3 for the geometric AR.
GE, in the case of WIGs, can thus be considered as free Lift , as the augmentation of Lift with the
ground is not accompanied by a Drag increase, but by a Drag reduction.

17. Jan Monchaux & St´ephan Aubin Easy ways to study ground effects 229
The aerodynamic and height centres were both put in evidence, but it appeared that the airfoil
was not naturally stable. It has been shown by Irodov that the aerodynamic centre has to be placed
behind the height centre. On this airfoil, the aerodynamic centre is at 25% of the aerodynamic
chord, while the height centre is at almost 35% of the chord.
Figure 25 : Aerodynamic centre position (slope)
Figure 26 : Height centre position (slope)
It is interesting to look a bit closer at the importance of these two points. To move backward
the aerodynamic centre, the only natural solution is to place another lifting surface behind the
principal one. But it must not move the height centre, or not much, so that the stability criterion
remains satisfied. The best solution is then to place the second lifting device (let us call it a
stabilizer) high enough to be out of the ground influence. Being given the characteristics of the
two lifting surfaces, one can write this simple equation :
xα1 − xα =
xα1 − xα2
1 +
CLα1
S1
CLα2
S2
considering that xα1
,xα2
and xα are the nondimentionnal positions of the aerodynamic centres
of 1, 2, 1 and 2, etc. The aim of the designer is then to find a proper configuration that gives a
good compromise between weight, lift and performance. Thus :
– Minimizing weight implies a small S2 and a small xα1 − xα2
– S1 is necessarily high, compared to S2 (it is at least the aim of the designer)
– CLα1
and CLα2
will probably remain of the same magnitude.
– All these assumptions lead to contradictory effects . . .
It is then clear that the aerodynamic centre of the principal wing of a WIG has to be placed
as close as possible to its height centre, in order not have to place a too big or too far stabilizer,
naturally behind it. The choice of the principal airfoil is then determining.

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Optimal Airfoil
The easiest way to really understand the necessity of a good stability is to try and make some
scale models. The concave airfoil was then put on a quite simple model with no horizontal tail -a
sort of flying wing. The CG was placed before the 25 % of the chord.
After some trials that put in evidence that the roof was not the ideal place to reach for a
common corridor WIG , it was decided to put a flexible plate at the trailing edge, that modified
the shape of both the upper and lower side of the concave airfoil. This plate, when correctly set,
completely annihilated the pitch instability. For different settings, this plate allowed to enhance
the stability or critically degrade it. The noticeable pitch down tendency of the WIG without the
plate, probably due to a bad centering, was accompanied by a quick pitch up if the craft was not
properly thrown. With the plate properly set, everything was going smoothly.
Figure 27 : Concave airfoil with flexible plate
This illustrated the fact that the design of the lower surface was really determining. It also
pointed out that the approach used on the DHMTU airfoils was close to our approach. The idea
of these airfoils, that was experimentally and by try and see method developed in our case, was
to design a little extractor at the end of the lower side to be as close as possible of the stability
criteria without using a stabilizer that can then be as little as possible.
Figure 28 : DHMTU airfoil
Next developments
The next researches are conducted on the influence of the stabilizer and flaps in wind tunnel,
on the influence of the lower side of the airfoil through a numerical approach, that should lead to
a reasonable WIG dynamic model . The results will be tested on a scale rubber propelled model,
as always, but this time, the configuration will be set before testing, to validate calculations.
The influence of the stabilizer and the flaps will be tested on an Orlyonok scale model which was
adapted for tests. She has a removable tail (that can be changed with a not swept one), changeable
flaps and a blowing system that will probably be implemented this very year.

19. Jan Monchaux & St´ephan Aubin Easy ways to study ground effects 231
Figure 29 : Orlyonok model
Figure 30 : changeable flaps
The numerical approach uses a perfect fluid singularity model of the flow, that represents the
GE via the mirror image technique. The first results are quite satisfying, and this approach will
help determining a good airfoil with reasonable aerodynamic centres.
Figure 31 : 2D configuration of an ekranoplan to put in evidence to role of the stabilizer.

20. 232 EAGES Proceedings
Figure 32 : Pressure distribution around the airfoil and the stabiliser.
The acceleration at the end of the lower side is due to what we called the extractor.
CONCLUSION
A lot a new developments have been tested since this paper was written, but the point was to
show that with only simple material, the very basis of GE can be put in evidence. The wind tunnel
with a moving belt is certainly much more expensive and complicated to run, but it is the key to
simply start studying all the aerodynamic phenomena relative to the ground presence.
REFERENCES
1. J.Katz Race car aerodynamics, Bentley
2. A. Bonnet - J. Luneau, Th´eorie de la dynamique des fluides, C´epadues editions
3. Edwin van Opstal, The WIG Page - http ://www.se-technology.com/wig
DISCUSSION
The discussion was not recorded, due to a VRC malfunction. Hanno Fischer proposed a dynamic
testing of the airfoil modelizing the inertia of the WIG craft, using a system of springs to hold the
airfoil in the wind tunnel.