EAGES Proceedings - Hanno Fischer 1

The Hoverwing TechnologyBridge between WIG and ACVPrepared for the EAGES 2001 International Ground Effect Symposium

Toulouse, France

June 2001

Hanno Fischer

Fischer - Flugmechanik

Kickenstraße 88

47877 Willich / Germany

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The Hoverwing Technology

Bridge between WIG and ACV

Hanno Fischer

ABSTRACT

Wingships (WIG, Wing In Ground) utilise water as runways to reach their lift-off speed, whichis determined by the wing loading. High Wing Loadings are desirable for high cruising speeds withinherent height and Longitudinal stability.

To build up the necessary dynamic air pressure under the wings, they need roughly 3 times morepower to overcome the hydrodynamic hump-drag compared to the drag during ground effect flight.So it is necessary to develop suitable devices as Lift-off-aids in order to reduce the recommendedpower. With support of the German Ministry for R&D (BMB & F) Fischer-Flugmechanik (FF)has developed the ”Hoverwing - Technology” in order to further reduce the necessary lift-off power.The principle of this technology, for which FF has patent rights, is the building up of static airpressure between the catamaran float. After lift-off the dynamic pressure will replace the staticpressure and the craft operates as a WIG with high lift to drag ratios.

FF is developing the ”Hoverwing 80”, with the target to transport 80 passengers at 100 kts.Some test results with a scaled down two Seater will be demonstrated by video extracts.

ABOUT THE AUTHOR

Ing.Hanno Fischer was the Technical Director to Rhein- Flugzeugbau GmbH (RFB)in Germany.He has developed around 12 different aircraft like Fantrainer, Fanliner, RW 3 and the military usedWIGs X113, X114 and X114 H (X114 with hydrofoils) .They were designed as aircraft to fulfil themilitary requirement with free flight capability. The concepts were based on the works of Dr.Lippisch.

After retiring from RFB he founded the company Fischer - Flugmechanik together with hispartner Klaus Matjasic. Their target is to develop the ground effect technology towards commercialapplication.

Based on their patents, they successfully designed the first generation of WIGs for civil use-the Airfisch 1, to Airfish 3, for which they granted a production licence to RFB.

In order to achieve a higher economical efficiency, they have developed the Hoverwing techno-logy, which can be considered to be a basis for the second generation of WIGs. Their works aregovernment sponsored from the German Ministry of R&D.

Last design is the Airfisch 8 called now Flightship 8, a 8 seater which has made the maidenflight in February 2001 and is delivered to Australia after successful flight demonstration.Author of many articles and papers in the field of ground effects, for instance in Australia 1996.

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Hanno Fischer The Hoverwing technology - Bridge between WIG and ACV 49

INTRODUCTION

The transport velocity of passengers and goods is well above 100 km/h during surface transport.Changing from surface to water transport, the speed is reduced by as much as 20 % because of thelow speed of waterborne vessels, thus increasing the tendency to switch over to fast but expensive airtransport. Utilizing the so called ground effect, the gap between slow and inexpensive ships and fastbut expensive aircraft can be filled (Fig.1). As WIGs lift of from the water during cruise, they avoidthe high drag from the high density of the water. In order to achieve the necessary aerodynamiclifting forces for taking off, it is also necessary to overcome the hydrodynamic drag, which can beextremely high on a conventional WIG and determines the engine power to be installed. Contraryto aircraft, this excessive installed power cannot be used to increase the cruise speed. The economicefficiency of WIGs, therefore, relies on the drag being overcome during take-off.

The Hoverwing technology uses a small portion of the propeller slip stream to create a staticair cushion between the floats, which are designed, as catamarans. Thus the displacement of thevessel’s floats is reduced by 80 %. This results in a reduction of the wetted surfaces, that enablesa drastic reduction of installed engine power.

TAKE-OFF AIDS REDUCE THE ENGINE POWER TO BE INS-

TALLED

The attempts to reduce the hydrodynamic drag in order to achieve higher speeds led to nu-merous different designs. Figure 2-Morphologic Triangle shows the different systems known today,which are supported by static air cushions at increasing speed but which cannot avoid water contactcompletely. By using a static air cushion during take-off, which at take-off speed is replaced bya dynamic air cushion, the Hoverwing can lift of from the water completely, and reaches glideratios which are significantly higher than those of today’s vessels. Figure 3-Weight to Thrust Ratiogives an impression of the different thrust requirements of different vehicles. A seaplane or flyingboat has a very high thrust, as it is able to use this excessive power for cruise. WIGs, having thedisadvantage compared with water-planes in that they cannot change the angle of attack duringa take-off run in order to maintain the best lift to drag ratios.They must have an improved floatdesign, allowing them to take-off with a constant angle of attack. Tested WIGs show a thrust-to-weight ratio of about 1 : 4. The optimized float design of the Airfisch-3 increased that number to1 : 5.2 and with the manned Hoverwing Testbed we reached already 6.5 and we expect to come to1 : 8 with our current optimisation programm.

HOVERWING TECHNOLOGY

Figure 4-Technology shows the relationship of the different transport systems to each other.On the left hand side, those systems are shown which do not need any forward speed to carrytheir weight. On the right hand side, those systems are shown which require speed to create theiroperational lift, hydrofoils and aircraft. Correspondingly, the Hovercraft is the link between thedisplacement vessel and helicopter, while on the right hand side the WIG is the link betweenhydrofoils and aircraft. The Hoverwing Technology is the bridge between ACV and WIG. FF hasdeveloped the Hoverwing - Technology in which the catamaran float design is comparable withthe SES, but in which the supply of the air cushion is achieved by using a small part of the

50 EAGES Proceedings

propeller stip stream (Fig. 5). Hoverwing principle explains the working principle of the HoverwingTechnology. Between displacement and approximately 90 % of take-off speed, the air cushion is ableto lift up to 80 % of the vessel’s take-off weight. When reaching take-off speed, sealing of the aircushion by the catamarans and skirts cannot be maintained, and the dynamic pressure from the freeair replaces the static air cushion.The sealing finger skirts are moved by the dynamic air pressureautomatically until they lay flat on the underside of the hull, where the additional drag is minimisedin order to maintain the high glide ratio of WIGs. In the Hoverwing design, the cabin actually isa fall-out. The chord length of the of the airfoil section between the catamarans allows already acabin-height that 30 passengers can stay upright.The benefit of not having a separated fuselageto accommodate passengers or freight, results in a significant reduction of structure,weight, dragand construction expenses. To maintain the unconditional, inherent longitudinal stability of WIGsduring transition when taking-off, the aerodynamic centers of static and dynamic air cushions andaerodynamic outer wings may not shift. By adjusting the rear sealing of the static air cushion,which is a bag type skirt, and the forward sweep of the outer wings, the aerodynamic centers arekept at one longitudinal position. So even during take-off transition, the vessel maintains stability,and no manual control is necessary. As soon as the so-called ”flare” mode is taken after take-off,the inlet port behind the propeller is closed, and full thrust is available for cruise. Closing theinlet port also deflates the bag-type skirt sealing at the end of the air cushion, and is then alsofolded. Before landing, the inlet port is opened again, the rear sealing inflates immediately afterthrottling up when touching the water surface. The build up of the air cushion makes the frontfinger skirt sealing swing down automatically, so that the air cushion is fully working again duringtouch down. This makes the landing extremely soft and reduces structural loads. One problem ofthis basically new configuration was the catamaran float design. The best position for the step, thedesign of the sealing skirts and the best trim for take-off, were optimized in cooperation with theVersuchsanstalt fur Binnenschiflbau e.V (VBD) at Duisburg, Germany. Using scale models withextensive measurement equipment which transmitted the test data, the complete speed range fromdisplacement to flare mode was analysed in order to achieve reliable information about the dynamiccharacteristics and performance of the new configuration. The achievable reduction of drag, and theresulting possible reduction of power to be installed, is shown in Figure 6 Performance potential.

PROOF OF THE CONCEPT WITH HOVERWING 2 VT

In order to achieve operational experience under realistic conditions, the manned test craft HW2 VT was designed and built. This representative vessels is scaled down in the ratio of 1 :3.35 fromthe 80 Seater HW 80 and were tested 1997. Hoverwing 2-VT completely fulfilled the performanceand characteristics in accordance with the given requirements in regard to take-off, cruise, maneu-verability and landing, during tests on the Baltic Sea and on the IJsselmeer in the Netherlands in3.000 k m test flights. Fig.7 shows HW 2 Vt in Operation

CONCLUSION

Creating a static air cushion by using parts of the propeller slip stream during take-off, a reduc-tion of hydrodynamic drag comparable to that of an SES is achieved. During cruise the dynamicpressure of the airspeed replaces the static air cushion and automatically folds the sealing skirtsto the lower side of the catamaran body, which then creates aerodynamic lift and makes the vessel


become a ground effect craft. The catamaran body functioning as the main wing, offers, due to itsize and airfoil shape, a thickness which allows the comfortable accommodation of passengers insidewithout the need building a separate fuselage. FF holds patent rights for the Hover-wing Techno-logy and could confirm the required performance and characteristics as predicted by calculationand experiment with the manned test craft, the Hoverwing 2 VT.







Figure 7


HOW DOES THE HOVERWING STATIC AIR CUSHION

WORK?

TO UNDERSTAND THE PRINCIPLE THE FOLLOWING PARTS OF THE

CRAFT SHOULD BE NOTED


CRAFT IN DISPLACEMENT - ENGINE STOPPED

CRAFT IN DISPLACEMENT - ENGINE STARTED - PROPELLER BUILDS

UP AIRFLOW

CRAFT IN DISPLACEMENT - PROPELLER PRODUCES AIRFLOW


CRAFT STARTS TAKE OFF RUN


CRAFT SHORTLY BEFORE TAKE OFF

CRAFT IN TAKE OFF TRANSITION


CRAFT IN GROUND EFFECT

CRAFT IN LANDING CONFIGURATION


CRAFT LANDING

EMERGENCY LANDING


CRAFT LANDED

DISCUSSION

Bo Svensson B(BS), Euroavia Stockholm

I would like to know where you would place trucks on the slide you showed comparing price perkilo for aircrafts and boats ? Because in Sweden we have a long road on the coast and one couldimagine to fly along the coast. . .

Hanno Fischer (HF), Fischer Flugmechanik

The key point is the Lift to Drag ratio (L/D). You can roughly convert 75% of the horsepoweravailable into thrust. So with a craft that has a L/D of 20 -in comparison with an aircraft that hasa L/D of 14 like a B 747 1, you have directly the thrust you will need, hence the fuel you need,and so you have the efficiency that is the key factor. And believe me we can go to higher spansand there are chances to reach a L/D of 30 ! This is a question of dimensions ! If you have a L/D of30, your efficiency is twice the efficiency of a conventional aircraft. But for distances shorter than1000km. We cannot go faster than 200 km/h. The problem of ground effect crafts is to answerthe following question : how can we have a height stability for a given angle of attack ? When theaircraft goes faster, the necessary angle of attack decreases. Hence when we use the fuel power thatis determined by the take-off during the flare, we come to an angle of attack that is negative. Andbeing negative, we lose a great portion of the advantages of the ground effect and still we needthe height stability. In the future, we shall go step by step to higher wing loadings to increase thespeed or we could use another system that makes that the trailing edge can have a relative negativeangle, and then we will still have a positive angle of attack with the height stability, allowing usto go faster.

Ingrid Schellhaas (IS), BOTEC GmbH

You said that there is no need for a pilot license on your boat. But I learnt from Mr van Opstalthe your boat is a B-type. Is there any need of a licence for such a craft ?

HF

We have worked with the Germanischer Lloyd that checked our facilities and we are the first in theworld to be certified to build wing in ground effect crafts under their control with an ISO 9001 !

1The Airbus 340 has a L/D of roughly 17. The Editor


For jumping, we are using kinetic energy. Only kinetic energy ! We only have to pull the controldevice, the craft jumps up and returns automatically to ground effect. There is no need for anycontrol during the jump, the craft recovers automatically. And with another system I shall presenton my other paper, we are even able to forget all these things. This new system necessitates onlya small joystick for jumping, and the rest is automatic !

IS

So it is A+B, between A and B or B ?

HF

It is a B. We cannot remain in constant flight out of ground effect. We only use kinetic energy tojump. Someone told me once that jumping 20m required a pilot licence and I said ”Think aboutthe ski jumpers ! The fly at 100km/h and jump over 100m ! Do they need a pilot licence ? Theyonly use kinetic energy !”

Cornelius Dima (CD), EA Bucuresti

Regarding air leakages of the air cushion, could you estimate them and give us a way of compen-sating them ? More could you please say more about how you control the mast flow coming fromthe propeller ? Has it only to do with the propeller speed or is there any other intake ?

HF

We are only using the slip stream velocity of the propeller. Behind the propeller, there is a flowconcentration and one can estimate that behind the propeller, at a distance of roughly 50% of thepropeller diametre, the flow generated by this propeller has a diametre of 50% of the propellerdiametre with roughly twice the speed. At that point, we installed a little door that catch asmall proportion (7%) of the slip stream to fill the rear bag and the rest to goes between the twocatamarans and skirts. There is no other device to control the pressure.

But there is as you said a small speed range from 90% of the take-off speed to the take-offwhere there are some leakages, especially due to the sea state. The sealing of the catamarans is atits minimum. So we lose some air in this speed range. We would be able to compensate this witha bigger door catching 10 to 12% or the slip stream but we feel that we can overcome this humpdrag with acceleration.

Cornelius Dima (CD), EA Bucuresti

You said that such a vehicle could reach the airport ?

HF

We can fly at roughly 30% or the span of the craft. But we need the full power at such heights !You remember the graph showing L/D with respect to height divided by span 2. This would meanthat with a craft with a 18m span, we would fly at 6m . . .but it would not be economical.

To answer the question of the route we would take, I would say that we are fighting with ourGerman authorities and I must say that they are very bad ! It is a lot easier in Holland and this iswhy we are doing most of our flight testing there ! You probably know this old joke ”In Germany,what in not allowed is forbidden while in the USA what is not forbidden is allowed” and excuseme but it is almost as complicated in France ! We have now reached the point where our Ministryof Trafic will allow us next year3 to make an operation from Lubeck to Rostock and Strandstromto show that our ground effect machines are reliable and safe transport means that can operate in

2See P. 205. The Editor32002. The Editor


mixed trafic or also between boats. So we will start with short distances in areas where the wavesare not higher than 1.5m. So Hamburg is a long future . . .

Aji Purwanto (AP), ENSICA

I would like to ask you a question about the transition between the air cushion lift and the dynamiclift. You showed the state of the front skirt at this moment and its stability. Is there any stabilityproblem due to the pressure difference between the two sides of the skirts ? Thank you.

HF

The key point is the moment when we lose the sealing of the skirt. At this moment, the pressuredrops down in the air cushion and the static air cushion is replaced by the RAM air.

So when you see the videos you can observe, for a few seconds, the finger skirts move a littlebit because, as long as we have a sealed air cushion, the back pressure maintains them and whenthe nose of the craft goes up a little bit it creates this little movement. But we haven’t seen anyproblem of stability yet and the great advantage is, you’ll see it on the videos, that during thelanding, the opposite happens !

Hanno Fischer

Mats Larsson (MS), Euroavia Stockholm

I would like to ask you a question about your merit factor :

merit factor =payload× pay − volume× speed× range

purchase− price× operational cost(range used)

You are using the cabin a lifting-body. Are lifting-bodies a better concept for ground effectcrafts ?

HF

We tried the lifting-body on aircraft configurations in previous times. It’s a problem of dimensions.Men need at least a 1.80m cabin height. This gives thus the minimum height of the cabin.So weneed long chords to reach such height with an airfoil with roughly a 12% thickness. If our Hoverwingis smaller than an 8-seater, then it would not make sense. The chord would be too small and peoplewould not be able to seat in the cabin. The FS-8 is too small and this is why we didn’t select theHoverwing technology for that craft and preferred the Airfish technology. As you’ve seen on somepictures, our test pilot is sitting very uncomfortably in the HW 2 as the chord length is only 6m.

So we need at least 8 to 10 persons to have a configuration that can provide room for thepassengers and lift. 40% of the lift is generated by the cabin !


ML

But it is thanks to your low speeds that you can use such thick wings.

HF

We are between 10 and 13% of thickness for ground effect airfoils.

ML

Yes but there are no projects of an aircraft with a body-lift configuration with a speed range of400 to 500km/h and that would be comparable to your project !

HF

Of course, the fast aircraft chord thickness are roughly 9% but there is a real difference betweenan airfoil in free flight and an airfoil for ground effect. As you’ve seen this morning4, the pressureis different around the airfoils, the downwash is different, the angle of attack is different, on a sea-plane, you can rotate the craft to reach CLmax

, on a WIG you cannot, the flaps are not as efficienton a ground effect craft than on an airplane. So one cannot really compare the foils sections.

Jean Margail (JM), Airbus

Excuse me. As I already said, I am from Airbus and I remember seeing at the beginning of theA-3XX program some drawings, that was about 15 years ago, and they were studying a big flyingwing configuration, and the PAX number was excellent.

Bernard Masure (BM), Universite d’Orleans

For your demonstrator HW 20, the cruise height is 1.75m. Why is it so high, with a L/D of only18 ?

HF

This comes from the Baltic Sea ! The waves are lower than 1.5m high for 80% of the year !

Hoverwing 20

BM

So without waves, where would you fly ?

HF

Of course, we would fly closer to the ground. But from an economic point of view, we must be ableto fly for 80% of the year.

BM

But for the 747, the L/D is about 14.

4in Edwin van Opstal’s Lecture. The Editor


HF

In high altitude. The B 747 has such a L/D in high altitude. We are flying much lower !

BM

But the L/D are anyway quite comparable.

HF

If you really want to compare the two crafts, a B 747 should be compared with a craft with a spanof 60m if using the Hoverwing technology. This craft would have a L/D of more than 30 !

Jean Michel Duc (JMD), Association Aeronautique et Astronautique de France

I would like to come again on your merit factor because I have the feeling that you are countingthings twice, as the payload and the pay-volume are both on the formula. I would chose either oneor the other. Is it fair to count it twice ?

merit factor =payload× pay − volume× speed× range

purchase− price× operational cost(range used)

HF

OK there are two answers. When you make a competition glider, you have only a pilot with 90kg,but you sit very uncomfortably. Hence you have no pay-volume. The other point is when you flyin the first class cabin of an airliner, you have the same speed, you arrive at the same time butyou pay 50% more . . .only for the volume ! So the comfort and the payload are important. Maybethere should be some exposant to give from this factor a better estimation of the efficiency, butthe idea is here. You know, every craft has an optimum. For more that 1000km, I would chose the747.

Sasa Mavrovic (SM), Euroavia Zagreb

Your government said that the requirement for range was 800km but you make only 500km. Wouldthat be a problem getting financing ?

HF

No, the requirements are based on the 80-seater. But I said that the step from the 2-seater to the80-seater is too big. I prefer small steps rather than one too big. So we said that the 20-seater didnot need this range capacity.

Our government gave us studies from a research company based in Rostock on the Baltic Sea.They checked the market and I have the data here where they say how many passengers are goingfrom Copenhagen to St Petersburg, how much they pay, how long they travel to find out how bigthe gap is. And they found that roughly 30% of all the passengers moving on the Baltic Sea can betransported with ground effect crafts under these limits in distances . . .and this makes 320 crafts.So we have a big market !

Notice that with longer distances, the speed of the classic fast aircrafts reduces our potential,like for trains.


Hoverwing 80

Requirement based on Market Study

Hoverwing 80

Transport Baltic Sea condition

80 passengers

Speed 180 km/h

Range 800 km

Wave height Cruise 1.5 m

Wave Height take-off 1.5 m

Wave Height landing 2.5 m

Stop Distance emergency 400m

Turn Radius ≤ 1000m

Fuel Consumption 40g/Pax/km

Installed Power ≤ 150 kW/tonne

Span ≤ 20m

Jean Margail (JM), Airbus

Another question. An operating question. Maybe it is idiot . . .When you’ll be allowed to fly overrivers, what is your way to cross bridges ? Under ? Over ? And we know there is a ground effect. Isthere any ceiling effect ?

HF

We have three operating modes.When we are in high density rivers with boats or in a harbor, werun in displacement mode. We have on our wingtips two water propellers and a joystick in thecraft, and we control the two propellers to go backward, forward, only one forward so we are verymanoeuvrable. In high density, we have to go on displacement.

When we have a certain distance before us that is free, we go on step taxiing like sea-planes.We run with 80% of our take-off speed. We still have water contact, have a very short stop distanceand still a high manoeuvrability.

And only when we have a clear free distance before us then we go at 100% of the take-off speedand we’re in the air.

So we have to check like on an airplane. Jumping is only the last solution. You will see sometest results on jumping in my other presentation.

JM

But do you think you will be allowed to fly on rivers ?


HF

With the X113 that was able of free flight we have flown in the river Wiesel and we made a turnover the land as the river was small. But I would say it is not good for every pilot . . .

I personally flew our vehicles and this is a joke we have. When we were flying on the lake ofConstance, the first time, I remember that the water police was very concerned about our craftand we flew directly to the police boat at high speed and jumped over them ! It costed us everytimea bottle of whisky but is was very impressive, you’ll see it on the videos !

Engineering

EAGES Proceedings - Hanno Fischer 1