EAGES Proceedings - Hanno Fischer 1

The Hoverwing Technology
Bridge between WIG and ACV
Prepared for the EAGES 2001 International Ground Eﬀect Symposium
Toulouse, France
June 2001
Hanno Fischer
Fischer - Flugmechanik
Kickenstraße 88
47877 Willich / Germany
45

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, which
is determined by the wing loading. High Wing Loadings are desirable for high cruising speeds with
inherent height and Longitudinal stability.
To build up the necessary dynamic air pressure under the wings, they need roughly 3 times more
power 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 recommended
power. 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 air
pressure between the catamaran float. After lift-off the dynamic pressure will replace the static
pressure 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 used
WIGs X113, X114 and X114 H (X114 with hydrofoils) .They were designed as aircraft to fulfil the
military 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 his
partner Klaus Matjasic. Their target is to develop the ground effect technology towards commercial
application.
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 are
government 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 maiden
flight 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.
47

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 the
low speed of waterborne vessels, thus increasing the tendency to switch over to fast but expensive air
transport. Utilizing the so called ground effect, the gap between slow and inexpensive ships and fast
but expensive aircraft can be filled (Fig.1). As WIGs lift of from the water during cruise, they avoid
the high drag from the high density of the water. In order to achieve the necessary aerodynamic
lifting forces for taking off, it is also necessary to overcome the hydrodynamic drag, which can be
extremely high on a conventional WIG and determines the engine power to be installed. Contrary
to aircraft, this excessive installed power cannot be used to increase the cruise speed. The economic
efficiency 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 static
air cushion between the floats, which are designed, as catamarans. Thus the displacement of the
vessel’s floats is reduced by 80 %. This results in a reduction of the wetted surfaces, that enables
a 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 contact
completely. By using a static air cushion during take-off, which at take-off speed is replaced by
a dynamic air cushion, the Hoverwing can lift of from the water completely, and reaches glide
ratios which are significantly higher than those of today’s vessels. Figure 3-Weight to Thrust Ratio
gives an impression of the different thrust requirements of different vehicles. A seaplane or flying
boat has a very high thrust, as it is able to use this excessive power for cruise. WIGs, having the
disadvantage compared with water-planes in that they cannot change the angle of attack during
a take-off run in order to maintain the best lift to drag ratios.They must have an improved float
design, 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 to
1 : 5.2 and with the manned Hoverwing Testbed we reached already 6.5 and we expect to come to
1 : 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 carry
their weight. On the right hand side, those systems are shown which require speed to create their
operational lift, hydrofoils and aircraft. Correspondingly, the Hovercraft is the link between the
displacement vessel and helicopter, while on the right hand side the WIG is the link between
hydrofoils and aircraft. The Hoverwing Technology is the bridge between ACV and WIG. FF has
developed the Hoverwing - Technology in which the catamaran float design is comparable with
the 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 Hoverwing
Technology. Between displacement and approximately 90 % of take-off speed, the air cushion is able
to lift up to 80 % of the vessel’s take-off weight. When reaching take-off speed, sealing of the air
cushion by the catamarans and skirts cannot be maintained, and the dynamic pressure from the free
air replaces the static air cushion.The sealing finger skirts are moved by the dynamic air pressure
automatically until they lay flat on the underside of the hull, where the additional drag is minimised
in order to maintain the high glide ratio of WIGs. In the Hoverwing design, the cabin actually is
a fall-out. The chord length of the of the airfoil section between the catamarans allows already a
cabin-height that 30 passengers can stay upright.The benefit of not having a separated fuselage
to accommodate passengers or freight, results in a significant reduction of structure,weight, drag
and construction expenses. To maintain the unconditional, inherent longitudinal stability of WIGs
during transition when taking-off, the aerodynamic centers of static and dynamic air cushions and
aerodynamic 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 are
kept 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 the
inlet port also deflates the bag-type skirt sealing at the end of the air cushion, and is then also
folded. Before landing, the inlet port is opened again, the rear sealing inflates immediately after
throttling up when touching the water surface. The build up of the air cushion makes the front
finger skirt sealing swing down automatically, so that the air cushion is fully working again during
touch down. This makes the landing extremely soft and reduces structural loads. One problem of
this basically new configuration was the catamaran float design. The best position for the step, the
design of the sealing skirts and the best trim for take-off, were optimized in cooperation with the
Versuchsanstalt für Binnenschiflbau e.V (VBD) at Duisburg, Germany. Using scale models with
extensive measurement equipment which transmitted the test data, the complete speed range from
displacement to flare mode was analysed in order to achieve reliable information about the dynamic
characteristics and performance of the new configuration. The achievable reduction of drag, and the
resulting 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 HW
2 VT was designed and built. This representative vessels is scaled down in the ratio of 1 :3.35 from
the 80 Seater HW 80 and were tested 1997. Hoverwing 2-VT completely fulfilled the performance
and 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 in
3.000 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 dynamic
pressure of the airspeed replaces the static air cushion and automatically folds the sealing skirts
to 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 it
size and airfoil shape, a thickness which allows the comfortable accommodation of passengers inside
without 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 calculation
and 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 per
kilo for aircrafts and boats ? Because in Sweden we have a long road on the coast and one could
imagine 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 horsepower
available into thrust. So with a craft that has a L/D of 20 -in comparison with an aircraft that has
a 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 spans
and there are chances to reach a L/D of 30 ! This is a question of dimensions ! If you have a L/D of
30, your efficiency is twice the efficiency of a conventional aircraft. But for distances shorter than
1000km. We cannot go faster than 200 km/h. The problem of ground effect crafts is to answer
the following question : how can we have a height stability for a given angle of attack ? When the
aircraft goes faster, the necessary angle of attack decreases. Hence when we use the fuel power that
is determined by the take-off during the flare, we come to an angle of attack that is negative. And
being negative, we lose a great portion of the advantages of the ground effect and still we need
the height stability. In the future, we shall go step by step to higher wing loadings to increase the
speed or we could use another system that makes that the trailing edge can have a relative negative
angle, and then we will still have a positive angle of attack with the height stability, allowing us
to go faster.
Ingrid Schellhaas (IS), BÖTEC GmbH
You said that there is no need for a pilot license on your boat. But I learnt from Mr van Opstal
the 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 the
world to be certified to build wing in ground effect crafts under their control with an ISO 9001 !
1
The 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 control
device, the craft jumps up and returns automatically to ground effect. There is no need for any
control during the jump, the craft recovers automatically. And with another system I shall present
on my other paper, we are even able to forget all these things. This new system necessitates only
a 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 to
jump. Someone told me once that jumping 20m required a pilot licence and I said ”Think about
the ski jumpers ! The fly at 100km/h and jump over 100m ! Do they need a pilot licence ? They
only use kinetic energy !”
Cornelius Dima (CD), EA Bu¸curesti
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 from
the 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 flow
concentration and one can estimate that behind the propeller, at a distance of roughly 50% of the
propeller diametre, the flow generated by this propeller has a diametre of 50% of the propeller
diametre with roughly twice the speed. At that point, we installed a little door that catch a
small proportion (7%) of the slip stream to fill the rear bag and the rest to goes between the two
catamarans 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-off
where there are some leakages, especially due to the sea state. The sealing of the catamarans is at
its minimum. So we lose some air in this speed range. We would be able to compensate this with
a bigger door catching 10 to 12% or the slip stream but we feel that we can overcome this hump
drag with acceleration.
Cornelius Dima (CD), EA Bu¸curesti
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 mean
that 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 our
German authorities and I must say that they are very bad ! It is a lot easier in Holland and this is
why 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 excuse
me but it is almost as complicated in France ! We have now reached the point where our Ministry
of Trafic will allow us next year3
to make an operation from Lübeck to Rostock and Strandström
to show that our ground effect machines are reliable and safe transport means that can operate in
2
See P. 205. The Editor
3
2002. The Editor

mixed trafic or also between boats. So we will start with short distances in areas where the waves
are 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 dynamic
lift. You showed the state of the front skirt at this moment and its stability. Is there any stability
problem 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 pressure
drops 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 little
bit because, as long as we have a sealed air cushion, the back pressure maintains them and when
the nose of the craft goes up a little bit it creates this little movement. But we haven’t seen any
problem of stability yet and the great advantage is, you’ll see it on the videos, that during the
landing, 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 effect
crafts ?
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 we
need long chords to reach such height with an airfoil with roughly a 12% thickness. If our Hoverwing
is smaller than an 8-seater, then it would not make sense. The chord would be too small and people
would not be able to seat in the cabin. The FS-8 is too small and this is why we didn’t select the
Hoverwing technology for that craft and preferred the Airfish technology. As you’ve seen on some
pictures, 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 the
passengers 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 of
400 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 between
an airfoil in free flight and an airfoil for ground effect. As you’ve seen this morning4
, the pressure
is 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 efficient
on 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 the
A-3XX program some drawings, that was about 15 years ago, and they were studying a big flying
wing configuration, and the PAX number was excellent.
Bernard Masure (BM), Université d’Orléans
For your demonstrator HW 20, the cruise height is 1.75m. Why is it so high, with a L/D of only
18 ?
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 able
to fly for 80% of the year.
BM
But for the 747, the L/D is about 14.
4
in 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 span
of 60m if using the Hoverwing technology. This craft would have a L/D of more than 30 !
Jean Michel Duc (JMD), Association Aéronautique et Astronautique de France
I would like to come again on your merit factor because I have the feeling that you are counting
things twice, as the payload and the pay-volume are both on the formula. I would chose either one
or 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 fly
in the first class cabin of an airliner, you have the same speed, you arrive at the same time but
you pay 50% more . . .only for the volume ! So the comfort and the payload are important. Maybe
there should be some exposant to give from this factor a better estimation of the efficiency, but
the idea is here. You know, every craft has an optimum. For more that 1000km, I would chose the
747.
Sasa Mavrovic (SM), Euroavia Zagreb
Your government said that the requirement for range was 800km but you make only 500km. Would
that 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 the
80-seater is too big. I prefer small steps rather than one too big. So we said that the 20-seater did
not 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 going
from Copenhagen to St Petersburg, how much they pay, how long they travel to find out how big
the gap is. And they found that roughly 30% of all the passengers moving on the Baltic Sea can be
transported 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 over
rivers, what is your way to cross bridges ? Under ? Over ? And we know there is a ground effect. Is
there any ceiling effect ?
HF
We have three operating modes.When we are in high density rivers with boats or in a harbor, we
run in displacement mode. We have on our wingtips two water propellers and a joystick in the
craft, and we control the two propellers to go backward, forward, only one forward so we are very
manoeuvrable. 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 distance
and still a high manoeuvrability.
And only when we have a clear free distance before us then we go at 100% of the take-off speed
and we’re in the air.
So we have to check like on an airplane. Jumping is only the last solution. You will see some
test 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 turn
over 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 of
Constance, the first time, I remember that the water police was very concerned about our craft
and we flew directly to the police boat at high speed and jumped over them ! It costed us everytime
a bottle of whisky but is was very impressive, you’ll see it on the videos !

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