EAGES Proceedings - S. AUBIN & J. MONCHAUX

Easy Ways to Study Ground EffectsPrepared for the EAGES 2001 International Ground Effect Symposium

Toulouse, France

June 2001

Jan Monchaux & Stephan Aubin

SUPAERO

10 avenue Edouard Belin

31055 Toulouse Cedex 4

France

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Easy Ways to Study Ground Effects

Jan Monchaux & Stephan Aubin

ABSTRACT

This paper discusses all the simple methods developed in SUPAERO to study the differentaerodynamic ground effects and understand their main characteristics. It gives a student pointof view of this complex phenomena, and illustrates some possibilities of easy studies of both theVenturi and the ”lifting” ground effects. It is clearly focusing on experimental approaches, andthe results must be considered for their tutorial aspect. After a short presentation of the differentconstraints due to the presence of the ground and the ways used in the case of the paper to solveit, the case of the Mercedes CLK-GTR and its study will be presented, followed by a more classicalstudy of the WIG phenomenon in wind tunnel.

ABOUT THE AUTHORS

Jan Monchaux and Stephan 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 Stephan is studying infinite swept wings separation for the ONERAToulouse.

ACKNOWLEDGEMENTS

The authors would like to thank Allan Bonnet for his valuable help, his feedback and hisknowledge, Francis Marty for his experience and his kindness, and all the SUPAERO aerodynamicslaboratory.

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Jan Monchaux & Stephan 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 aerodynamicproperties 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 andcomputational testing can provide good approximations and are usually validated in the real worldof true flying machines. In the case of Wing in Ground Effect Crafts (WIGs), the wind tunnelresearch as well as the computational work has to correctly take into account the presence of theground, 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 ableto provide reasonable results at a reasonable cost, the latter being of course the Gordian Knot ofany school . . .

BOUNDARY CONDITIONS IN GROUND EFFECT

For a person standing on a sea shore watching a WIG flaring , what immediately occurs isthe fact that the craft is flying over an almost steady ground. For the pilot of the same WIG, theground is definitely moving under his craft. The relativity of these two configurations is the key tounderstand 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. TheIF model is solution of what is commonly called the Euler Equations. This simple model givessatisfactory results, and can be coupled with the boundary layer model to compute flows insteadof solving the true Navier Stokes Equations. In the case of simple GE, we shall consider the flowto be subsonic with no supersonic points. In that case, the most know result for a wing in free IFis that its only drag is induced by the lift it creates. For 2D flows, this becomes what is called thed’Alembert’s paradox :

CD = 0

In IF, the only (and sufficient) boundary conditions that can be applied to a flow is setting atangential condition on any surface :

−→Vr.−→n = 0

with Vr : relative local speedOn an airfoil, this condition simply means that no flow passes through its surface.

Figure 1 : tangential condition on an airfoil

218 EAGES Proceedings

On the ground, this only means that the ground has to be a stream surface. One cannot setthe 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 mirrorbeing the ground itself. The geometry symmetry forces the pseudo mirror to be a streamline. Theupper side of the configuration is necessarily ruled by the same equations with the same boundaryconditions than the simple problem in GE. Its solution can then be assumed to be identical to theone searched.

This method is commonly used for computation or analytical research and leads to reasonableresults. Of course, it does not take into account the pressure and viscous drags, as it is easy toprove 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 relativespeed. 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 windtunnel section can be assumed to represent the ground. The boundary layer that develops on thatground, due to the relative speed between the ground and the wind changes all the properties ofthe 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 grounddesigned. In fact, one must not forget that the wind tunnel develops itself its own boundary layerwhich can also affect the flow studied.

Some solutions . . .

The most basic solution is to throw a model with a catapult and study its movement. Thissolution has been developed in Lille for instance, but is clearly too expensive for teaching.


Figure 3 : ONERA Lille Catapult

Another approach is to move the model over the ground, and weight the efforts. This solutionis 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 idealconditions 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 thebelt. Two wind tunnels are equipped with this system that has another advantage that is thatstudies 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 systemlocated 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 toimprove 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 runfaster lap times. If an aerodynamicist is asked to recommend a configuration for such a circuit withhigh downforce and relatively low drag, then very likely his choice will be based on an invertedwing in ground effect, as shown in Fig.5. The first design that used aerodynamics in Racing Carsother that body streamlining did not appear until the 1960s. This idea was technically developed


by Chevrolet-Chaparral in 1966 with their Can-Am racing car. The engineers mounted an invertedwing on two vertical struts above the rear axle. The wing could be pitched during racing to providethe optimal value of downforce. These ideas were developed quickly, and a few years later severalFormula 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 effectcar was introduced by Team Lotus in their 1978 Formula 1 car. By shaping the underbody withappropriate channels, and using side pods to increase the effective area, the car provided muchlarger values of downforce. The idea, again, was simple. Ground effect was a known concept inaerodynamics. 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 focusedon chassis characteristics, and give strict limits to size, position, and type of devices allowed andprohibited. 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 notslide. While cornering the car undergoes a centripetal acceleration, that is turn into a centrifugalforce on the wheels. This force can be cancelled by static friction (that limits sliding). Until a givenlimit 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 writtenin function of the speed and the corner radius. More precisely,


m.V 2

r= kf .(mg + Fz)

with : Fz =12ρV 2CL

Generally the next formula is preferred and give the minimal radius of cornering of the car :

rmin =V 2

kfg(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 atcutting the aerodynamic downforce, to secure safe races besides great shows. This may seem acontradiction, because it seems to undermine the primary motivation of car racing : speed. Duringthe ”24 Heures du Mans 1999” the Mercedes CLR-GTR had a spectacular accident. The car tookoff like a plane and had a furious crash, nobody was seriously injured but Mercedes decided to stoptheir race after this accident. We decided to try to understand what happened and what couldexplain this phenomenon by building a 1 to 10 scale model of the Mercedes CLK-GTR, whichis similar to its sister the CLR-GTR. The car was equipped with pressure captors all over theupper and lower centreline. The aerodynamic laboratory of SUPAERO had already a model of aFormula1, but we focused our study on the working of the Mercedes underbody. Through windtunnel testing in SUPAERO, we were able to place some important phenomenon, that characterizethe 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”


Influence of the ground clearance

The effect of ground proximity have an important influence on the aerodynamic coefficients ofthe 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 decreasingground clearance (Fig.7) Of course, the underbody channels are not Venturi tubes. But there isclearly 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 pressuredecreases at the same time. Note that the Bernoulli equation is true out of the boundary layer andit is acceptable to neglect the viscosity once out of the boundary layer. Up to a point it doesn’t workanymore, the lift increases when decreasing ground clearance. The converging part is obstructedby 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


The fact that the drag decreases with ground clearance could be a result of the faster airflowemerging 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 toseal the gap between the vehicle body and the ground. They were either rigid or flexible (slidingup and down) and were ”skirts”. They prevented the airflow from penetrating the low pressurearea under the car. In addition to that, it is clear that such a configuration can generate very largelift/drag ratios, as long as the flow is kept close being two-dimensional.

Our results show this trend :

Without skirts With skirtsCD = 2, 0 CD = 2, 0CL = −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 therecompression in the underbody channels (Venturis) is weaker. The peak reached at the narrowestflow passage is probably generated by the airflow traditionally penetrating under the car whenthere 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.


Influence of the rear wing

The experience showed that rear wings can be used to increase the flow under the car toaugment the body’s contribution to the downforce. The rear wing, which is an inverted airfoilgenerate a negative Cp zone between the ground and the airfoil. It helps to increase the velocityof the underbody flow. This zone is clearly more negative with the wing-on configuration thanwith the wing off one as shown in Fig. 10. The aerodynamic coefficient measured confirmed thisexplanation :

Wing-on Wing-offCD -2.8 -1.9CL 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 negativeincidence 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 airflowentering under the car was quite different of a normal configuration. In order to recreate thoseconditions we blocked the flow entering under the car with a little girder placed just in front of the


car as shown on this figure. The results were spectacular. The lift coefficient increased considerablyand was not far away of becoming positive. The CL reached the value of -0,9◦ (-2,8◦ without thegrid) ! In addition to that, the center of pressure moved backward not far away from the rear wheelaxe and the Cmt (its reference is the rear wheel axe) reached the value of -0,16 (-2,11 withoutthe grid). Those values shows that the longitudinal stability of the car was weak and that the carwas not far away to become a plane. Any road irregularity could become the divergent factor thatbrought 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. Thepressure under the car (until the channels) stays approximately constant is quite greater than inthe normal case. The recompression is in this case weaker than normally, the ground effect can notappear 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 Seriescar. It is then culturally hard to understand how an enormous machine like the KM could takeadvantage of the GE to fly. The first task is then to truly experiment the flight in GE. It can bevery 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, butpreferably look at the state of the art. In our case, the blank sheet will be enough ! The Tottori

1Tough nowadays it is limited on such cars . . .


University in Japan has developed a simple paper made RAM that can be easily built. After somecuttings, 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 merecharacteristics 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 RAMalso led to even more violent stability problems. Eventually, a tandem was developed but stabilitywas 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 GEaerodynamics and helped finding an experimental method to characterize what Irodov called thecentre of height : for a given angle of incidence, the dependence of the momentum on the liftcoefficient, with height moving, put in evidence a point where the momentum does not depend ofthe height. This point, as the aerodynamic centre, will appear to be determinant for the stabilityof the WIG, giving to its aerodynamics an pre-eminent role.

Figure 18 : position of the centre of height (slope), only one incidence tested


The small Aspect Ratio (AR) of the Tottori RAM also permitted to make very interesting flowvisualisations called tomoscopies. The tomoscopy is a technique that visualises a flow with smokein a certain chosen plane enlightened by a wide laser beam. As the RAM had a low AR, its wingtip vortices were quite big compared to its size, and the tomoscopy put in evidence the fact thatthese wing tip vortices were very much reduced when getting closer and closer to the ground. Itvisually explained the reason why drag was reduced in GE. These vortices are also visible on mostof 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 theextractor on racing cars, it was clear that the shape of the lower surface of the airfoil was ratherimportant. A classic NACA 0012 would probably not fit, as its symmetrical shape would behavelike a F1 in GE, i.e. create a Venturi that would lead to anti lift2. Hence, the airfoil chosen had aconcave shape, avoiding any possibility to create a Venturi. It had no name, having probably beencreated through an inverted method years ago. Tests were conducted with AR=3.

2for low angles of attack, of course.


Figure 22 : Concave airfoil

On this airfoil, the influence of incidence was measured - it was avoided on the Tottori RAMdue to its sharp leading edge. Height was always measured from the trailing edge, and incidenceas it is classically done. Hence, when the incidence was changed, as the tests were run for a givenincidence 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 whereno lift is created .Written in another way :

CL(α, h) ' k(α− α0 +4(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. Writingthe 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 theground is not accompanied by a Drag increase, but by a Drag reduction.


The aerodynamic and height centres were both put in evidence, but it appeared that the airfoilwas not naturally stable. It has been shown by Irodov that the aerodynamic centre has to be placedbehind the height centre. On this airfoil, the aerodynamic centre is at 25% of the aerodynamicchord, 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 backwardthe aerodynamic centre, the only natural solution is to place another lifting surface behind theprincipal one. But it must not move the height centre, or not much, so that the stability criterionremains satisfied. The best solution is then to place the second lifting device (let us call it astabilizer) high enough to be out of the ground influence. Being given the characteristics of thetwo lifting surfaces, one can write this simple equation :

xα1 − xα =xα1 − xα2

1 +CLα1

S1

CLα2S2

considering that xα1 ,xα2 and xα are the nondimentionnal positions of the aerodynamic centresof 1, 2, 1 and 2, etc. The aim of the designer is then to find a proper configuration that gives agood 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 CLα2will 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 placedas 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.


Optimal Airfoil

The easiest way to really understand the necessity of a good stability is to try and make somescale models. The concave airfoil was then put on a quite simple model with no horizontal tail -asort 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 acommon corridor WIG , it was decided to put a flexible plate at the trailing edge, that modifiedthe 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 enhancethe stability or critically degrade it. The noticeable pitch down tendency of the WIG without theplate, probably due to a bad centering, was accompanied by a quick pitch up if the craft was notproperly 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 alsopointed out that the approach used on the DHMTU airfoils was close to our approach. The ideaof these airfoils, that was experimentally and by try and see method developed in our case, wasto design a little extractor at the end of the lower side to be as close as possible of the stabilitycriteria 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 toa 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 wasadapted for tests. She has a removable tail (that can be changed with a not swept one), changeableflaps and a blowing system that will probably be implemented this very year.


Figure 29 : Orlyonok model

Figure 30 : changeable flaps

The numerical approach uses a perfect fluid singularity model of the flow, that represents theGE via the mirror image technique. The first results are quite satisfying, and this approach willhelp determining a good airfoil with reasonable aerodynamic centres.

Figure 31 : 2D configuration of an ekranoplan to put in evidence to role of the stabilizer.


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 toshow that with only simple material, the very basis of GE can be put in evidence. The wind tunnelwith a moving belt is certainly much more expensive and complicated to run, but it is the key tosimply start studying all the aerodynamic phenomena relative to the ground presence.

REFERENCES

1. J.Katz Race car aerodynamics, Bentley

2. A. Bonnet - J. Luneau, Theorie de la dynamique des fluides, Cepadues 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 dynamictesting of the airfoil modelizing the inertia of the WIG craft, using a system of springs to hold theairfoil in the wind tunnel.

Engineering

EAGES Proceedings - S. AUBIN & J. MONCHAUX