Note: Descriptions are shown in the official language in which they were submitted.
WO 95135236 219 2 9 3 9 PCTfGB95101410
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This invention relates to a seaplane hull and particularly to the
form thereof.
By seaplane hull is meant an integral part of the fuselage of a
flying boat or the floats on a floatplane. The design criteria of
sponsona or floats mounted outboard on a flying boat which, arising
from their displacement, commonly provide flying boats with lateral
static stability, contrasts to the design criteria for seaplane hulls
and this invention does not necessarily relate to their design.
Since the inception of seaplanes, hull development has revolved
around the planing hull concept, so much so that the hydrodynamic form
to achieve efficiency when in displacement mode, that is to say below
planing speed, has been generally ignored. By combining aspects of
design established in racing catamarans and passenger catamaran ferries
and innovating to enable such hulls to achieve seaplane design
criteria, a new, highly original and advantageous seaplane hull has now
been developed.
In plan form, conventional hulls almost universally have a
resemblance to an elongated rain-drop with a rounded bow and an
afterbody trailing to a point. Though this form is aerodynamically
very efficient, it is wholly unsuited to motion on the surface of
water. It creates a large bow-wave which creates high drag, and the
Coanda effect draws it powerfully down into the water preventing it
from taking-off in addition to creating a powerful nose-up pitching
tendency. This is in contrast to the comparatively sharp bows and fine
bow entry angles of recent catamarans.
The obfect of conventional seaplane hull design has been to
minimise these hydrodynamic penalties while incurring as mild
structural and aerodynamic penalties as possible. To prevent the
Coanda effect sucking the hull down into the water, hulls are generally
fitted with a transverse abrupt "step" below the centre of gravity of
the seaplane to separate the water flow from the hull. To enable them
to climb over the bow wave, they feature hard edged forebody chines and
powerful planing surfaces from the bow back to the step. The said
forebody chines and the step are angled steeply across hydrodynamic and
aerodynamic streamlines and create substantial vortices which both
damage directional stability and create substantial drag. The result
W0 95135236 PCT/GB95I01410
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is undesirable qualities from the point of view of hydrodynamic loading
and hydrodynamic resistance during take-off. They demand an aircraft
with high power-to-weight ratio and result in limited payload capacity
for s given size of aircraft.
Variable geometry hulls, hydrofoils, sir blowing and other '
devices have been used to try and maintain a clean aerodynamic hull
form. However, they all have limitations and add considerably to the
weight and complexity of an aircraft. None of these attempted
solutions have proved Certifiable and/or commercially viable though
various ideas have been tested in prototype and experimental form.
Some improvements however have been demonstrated by the use of
high length-to-beam ratio hulls and this was researched in some depth
prior to 1950 and is well reported in "Development of High-speed Water-
based Aircraft", by Earnest G Stout in the Journal of the Aeronautical
Sciences Vol. 17 August 1950. This discusses tests on hulls with
length/beam ratios of up to 12 though the advantages indicated were
little exploited as there has since been little seaplane development
work. The US Navy's flying boat XP5Y - 1 (first flown in 1950) had a
length/beam ratio of 10. However, it had all the shove-mentioned
features of a conventional seaplane.
The top speeds of offshore racing multihulls and passenger
catamarans approximately doubled in the 25 years to 1994. These hulls
have also demonstrated major sea-keeping advantages, but despite the
demonstration of the advantages of this type of hull, no seaplane has
incorporated this type of design. The appropriate use of these types
of hull for seaplanes is not obvious as there are two potentially
problematic factors in their application to seaplanes. Firstly, they
have a large wetted surface area at high speed which results in high
frictional drag when approaching take-off speed. Secondly, there
appears to be a widespread assumption amongst aircraft designers that
the Coanda effect remains problematic on any hull without a central ,
step.
According to the invention there is provided a hull of a seaplane
which can be partially immersed in water and is able to take off from
water, the hull having an underside formed as a planing surface defined
at each aide by a chine;
wherein any waterline likely to be encountered in operation and
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positioned a distance equal to not less than one tenth the maximum beam
of the planing surface above the chines has a bow entry angle in plan
form, except at and immediately proximate the bow, of less than twenty
four degrees; and
' 5 ' a length dimension from a bow of the hull to the aftermost edge
of the underside is more than seven times the maximum width of the
narrowest waterline above the chines.
Preferably each chine, viewed in bodyplan section, has a radius
of curvature less than 25X of the maximum width of the underside, the
chine distinguishing the planing surface forming the underside from a
topside thereabove.
Advantageously the beam of the underside at any station more than
80X of hull length aft of the bow is less than 20X of its maximum beam.
Preferably, viewed in profile, a plane containing a waterline
which is parallel to the chine at a station common to the seaplane
centre of gravity and passing through a position not more than 20X of
the maximum beam of the underside higher than either the forwardmost
extremity of the chines or, if the chines extend forward of a station
20X of hull length from the bow, through the intersection of the chine
with said station 20X of hull length aft of the bow, has that waterline
WL1 fair from any position within 5X of hull length of its forwardmost
extremity to a station common to the centre of gravity of the seaplane.
Advantageously any section of said fair waterline of a length
equal to 3X of hull length and located between !OX and 50X of hull
length from the forwardmost extremity of that waterline, has less than
a 5° change in mean direction over that section.
Preferably any waterline aft of the centre of gravity converges
toward the central plane, is fair having no discontinuity or abrupt
angle or edge along its whole length formed by chine, edge or other
structure and wherein the aftermost extremity is forward of the
aftermost extremity of any waterline thereabove.
The aftermost extremity of the underside is preferably forward of
a station 80X of hull length aft of the bow.
Advantageously any waterline has a radius of curvature at its
forwardmost extremity viewed in planform of less than 2X of hull
length.
Preferably any waterline has a radius of curvature, at eny
WO 95/35236 219 2 9 3 9 PCTIGB95101410
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station between lOx and 50x of hull length measured from the bow of not
less than half the length of the hull.
Advantageously a concave hollow is provided shave each chine and '
is formed by a curved or angled face.
Preferably the forwardmost extremity of each chine is at a
station not more than 10X of hull length aft of the bow and wherein
forward of this station, the topside and hull underside are faired
together so that neither chine nor edge distinguishes topside from hull
underside.
Thus the hull has a high length-to-beam ratio and a fine bow. In
planform, the bow proximates to a fine wedge and the waterlines are
preferably fair with large radii of curvature over the forward 50x of
hull-length. When the hull is level but submerged to its mean static
waterline, the waterlines above the underside of the hull extend from
proximate the bow to aft of the centre of gravity and a main planing
surface, and these are fair throughout their length.
This fine entry angle enables the hull to slice the water without
the creation of a significant bow-wave and to reach a considerable
speed before residuary drag (wave-making) and skin friction prevent
further acceleration. By this speed, a seaplane can carry a
substantial proportion of the weight aerodynamically, such that any
hydrodynamic lift required for planing can be provided by the planing
surface being relatively small and elegant. In combination, these
forces lift the hull to cause it to start to plane at higher speed than
is conventional, then substantially reducing the wetted area for low
frictional drag for the latter part of an acceleration.
To reduce the wetted area further, the planing surface preferably
finishes Forward of the transom or stern. This is achieved by the
lateral chines defining the planing surface finishing on the central
plane substantially forward of the stern. The chines substantially
define a plane (i.e. two dimensions) and their effectiveness is not ,
necessarily dependent on them being fair or continuous. It is however
advantageous if the waterlines immediately above these chines are fair
for hydrodynamic performance in displacement mode (at low speed) and
For reducing aerodynamic drag in cruise.
The chines are designed to encourage ventilation under (i.e.
separation of water flow from) the afterbody aft of the main planing
WO 95/35236 219 2 9 3 9 pCTlGB95/01410
surface. This is achieved by encouraging hydrodynamic vortices in the
water above the chines when these chines are set at an incidence to the
hydrodynamic streamlines. These vortices draw channels of air along
the upper side of the chines and aft on to the underside of the hull
5 aft of the planing surface. This separates water from the afterbody
which reduces wetted surface area and drag at speed and prevents the
Caanda effect pulling the afterbody down.
The finishing of any waterlines at an aft transom or stern
atemline is not critical in design as the airflow so far aft tends to
be turbulent (the boundary layer having broken down) irrespective of
the fairness of streamlines. A mild abrupt transom at the stern may be
advantageous for structural or stowage purposes and will affect the
performance little.
This hull shape enables continuity of all aerodynamic and
hydrodynamic streamlines to be maintained but the configuration of
surfaces can still be optimised for handling and performance criteria.
This is not achieved by other previously proposed seaplane hulls which
have forebody chines, step or afterbody chines breaking waterlines and
aerodynamic streamlines. There is no requirement for mechanical or
other device to enable acceleration to take-off speed and release from
the water.
The invention is diagrammatically illustrated by way of example
in the accompanying drawings, in which:
Figure 1A and 1B show respectively a plan and a profile of an
example of hull shape conventionally used by seaplanes with their
predominant features;
Figure 2 shows in perspective from underneath an embodiment of a
seaplane hull according to the invention, incorporated into an aircraft
configuration;
Figure 3 shows in perspective a diagram of the outlines of
. equidistant bodyplan sections numbered sequentially from the baw to the
stern and the edges of the main planing surface of a seaplane hull
according to the invention;
Figure 4 shows the equidistant bodyplan sections of the hull of
Figure 3 superposed;
Figure 5 shows a second embodiment of the bodyplan sections of a
seaplane hull with hollows extended forward and also defines two
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WO 95135236 PCT/GB95/01410 .
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waterlines;
Figure 6 shows in plan view chines defining the planing surface
on the underside of the hull and the two waterlines shown in Figure 4;
Figures ~A, 7B and 7C show respectively a side elevation, a half
front elevation and a half plan view of a flying boat having a hull
according to the invention; and
Figure 8 shows a profile section of the flying boat of Figure 7.
In this specification the following definitions apply.
Fair Continuous and regular curve (though the radius
of curvature may change along the curve)
Streamline The contour that a particle of air or water
follows in relation to the hull (commonly
described by a longitudinal section)
Waterline The level on the hull of a vessel to which the
surface of the water comes when it is afloat or
lines parallel thereto. The waterlines are
assumed to be substantially parallel to the
plane defined by the chines on the edges of the
underside of the hull directly below the centre
of gravity of the aircraft.
Bodyplan sections Viewed parallel to the longitudinal axis the
cross-sections of the outer akin of the hull
Bow The station containing the forward most
extremity of any waterline
Stern The station containing the aftermost extremity
of any waterline
Hull length Distance from bow to stern at any waterline
Length/beam ratio The ratio of overall hull length at the static
waterline to the maximum beam of the planing
surface on the underside of the hull.
As shown in Figures 1A and 1B a conventional seaplane has a hull
with an elongated teardrop shape, a length to breadth ratio of about
seven and a broad planing surface on the underside finishes at its aft
end in a substantially full-hull width step. Hard chines cross the
streamlines WLC1, WLC2 and WLC3 of the hull.
Referring to Figures 2 to S and initially to Figures 1 to 4, a
seaplane hull comprises a hull underside 1 and a topside 2 at each
W0 95/35236 PCTIGB95101410
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side. It has a high length 1 to beam h ratio as shown in Figure 6 of
over ten, and viewed in planform a waterline WL1 along the topside
(above the hull underside) has a large average radius of curvature.
Between 14x and 50X of the hull length 1 from a bow 9 this radius of
curvature is not less than twice the hull length. The waterline WLi is
fair and continuous and extends from the bow 9 to a stern stemiine 12.
Assuming a length/beam (1/h). ratio of twelve and a constant curvature
from bow to stern, this results in a radius of curvature of over three
times the hull length 1 and an angle Q between the waterline WL1 and
the central plane CP at the bow of 9.52°.
Viewed in planform the stemline of the bow 9 is formed by a hard
edge or a radius of less than 0.5x of the hull length. Referring to
Figure 5 and Figure 6 the waterline WL1 is fair from the radius at the
bow stemline 9 to the stern 12. Waterline WL2 (vertically between
chines 4 forming the planing surface and WL1) is fair from proximate
the bow 9 to finish on the central plane substantially forward of the
stern stemline 12.
Over sections at the bow, as can be seen at stations ST1, ST2 and
ST3, the surfaces forming the hull underside 1 and topsides 2 are
faired together, resulting in a section with substantially rounded
lower edges 3.
Aft of the bow sections i.e. aft from station ST4, the lateral
extremities of the hull underside 1 become increasingly pronounced,
developing into the hard edged chines 4 through the midbody sections of
the hull (at stations S'I~ to ST10). Where these chines 4 are
pronounced they define the lateral extremities of the underside 1 and
the lower extremities of the topsides 2.
The edges of the port and starboard chines 4 extend aft and
toward the central plane, forming a tail 5. The planform of this tail
is not critical, and can take the form of a V U or W (as demonstrated
~ by many windsurfers). The tail 5 is located substantially forward of
the stern extremity of the hull ie. the stern stemline 12. The section
of the topsides 2 adjacent and immediately above the chines 4 is
cambered, that is to say recessed forming a hollow 6. As the hull
accelerates, low hydrodynamic pressure in the hollow 6 draws air along
it, the sir spreading across the underside of an afterbody 7.
A planing surface formed on the hull underside 1 over the length
WO 95135236 219 2 9 3 9 PCT/GB95101410 l
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of the chines 4 comprises one or more concave curves spanning between
the chines. The depth of these at any position along the hull does not
exceed 20x of the beam h. Referring to Figure 4, double concave curves '
form an edge on the central plane CP which is exaggerated toward the
trailing edge to form a skeg 8, Figure 2, which adds to the directional '
stability of the hull. In profile, the underside of the planing
surface is longitudinally substantially straight to prevent the Coanda
effect creating a low pressure area and to prevent hydrodynamic pitch
instability.
A spray dam 10 is located on the bow stemline 9 protruding
forward of the bow stemline. This spray dam continues aft of the bow
to form a lengthways knuckle 1l which continues over the length of the
hull. The knuckle consists of a hard edge and the topside 2 is
cambered and flared upward and outward to the knuckle edge.
Where the port and starboard knuckles 1I coincide immediately
above the stern stemline, this forms an inverted flat 13 acxross the
central plane.
An air rudder 14, Figure 2. is mounted on a hinge line above the
convergence of the knuckles 11. The rudder extends to below the flat
13 thereby forming a water rudder 15. The leading edge of the rudder
is forward of the rudder hinge line and aft of the stern stemline 12.
From the bodyplans of Figure 4 and Figure 5 it can be seen that
the topsides of the hull incorporate two knuckles on each side. These
and the outward flare of the topsides maximise displacement volume for
a given draught, and incorporated into a flying boat can provide a
broad and voluminous fuselage.
Figures 4 and 5 show three waterlines WLO, WL1 and WL2. WLO is
a static waterline the position of which will vary with loading of the
seaplane. WL1 is the likely operational waterline at half to two
thirds of take-off speed. WL2 is the likely operational waterline at
90x of take-off speed.
Referring to Figures 7A to 7C, protruding from the higher of the
two knuckles is a laterally extending stub wing 16 to which is attached ,
a sponson 1'7. The underside of the stub wing 16 and the underside of
the higher knuckle 11 are faired together to form a substantially
continuous surface. The buoyancy of the sponson provides lateral
stability when the aircraft is static and also has a shined underside
W0 95135236 PCTIGB95/01410
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which provides planing lift at speed. When the aircraft is loaded to
its maximum and the aircraft is level and submerged to its loaded
static waterline. each sponson 17 displaces approximately half its own
volume. The outer edge of the sponsons are reinforced and padded for
berthing against hard surfaces (i.e. betties). The stub wing 16 serves
as a walkway for passengers and crew to gain access to the fuselage.
Protruding from the top of the fuselage is a mast 18 to which a
nacelle 19 is attached. An engine 20 is mounted in the front of thin
nacelle 19 and the main wings 21 are attached laterally. To transmit
loads between the nacelle 19 and the fuselage, ring frames 22, 23,
Figure 8, are bonded into the fuselage and extend upward through the
mast 18 to the nacelle 19, forming a continuous frame or bulkhead in
both. The forward ring frame 22 forms a firewall 24 behind the engine
20. The use of the two ring frames 22, 23 displaced longitudinally
imparts strength and rigidity in all axes between the fuselage and the
nacelle 19.
A strut 25 is attached at one end to a pivoting attachment 26 on
the side of the fuselage. The pivoting axis of this attachment 26 is
common to a hinge attachment 2~ of the wing 21 to the nacelle 19 and is
typically at 70x of wing chord from the wing leading edge 28.' The
opposite end of the strut 25 is attached rigidly to a locator 29
outboard on the wing 21, thus bracing the wing to carry flying loads
and alleviating the hinged wing attachment 27 of bending (cantilever)
loads. The leading edge of the wing is detachably fixed in a socket 30
in wing fairings forming part of the structure of the nacelle 19. A
retractable flap 31 extends across the trailing edge of the nacelle 19.
This inboard flap assembly 31 can be hinged upward and this enables the
wing to rotate aft by a full 90 degrees. This brings the leading edges
28 of the wings 21 to lie within the beam defined by the outer edges of
the sponsons 17.
A frame 32 to which the hinged strut 25 attachment to the
fuselage connects accommodates the loads of a main undercarriage 33~
A nose wheel 34 is provided at the front of the fuselage.
Frames of side canopies 35 locate forward and aft in the
structure formed by the ring frames 22,23. A central structural member
36 extending down the centre of the front cockpit provides the hinge
attachment of port and starboard front canopies 37. When these are
WO 95/35236 ~ ~ g 2 9 3 9 PCTIGB95101410
open they form a substantial shield immediately behind the propeller
(not illustrated) protecting any occupants from the risk of making
contact with the propeller for example when mooring. "
Aerodynamic roll control is achieved conventionally using either
5 or both spoilers and ailerons 38. To achieve maximum lift coefficient,
the ailerons 38 droop simultaneously with the extension of the flaps 31
and flaps 39 while still retaining a differential control facility,
thus forming "drooperons".
A tail fin 40, the air rudder 14, a tailplane 41 and an elevator
10 42 are all conventional. The tailplane 4i is mounted in the wash (i.e.
slipstream) of the engine propeller to provide a positive and powerful
pitch control facility during take-off independent of the airspeed of
the aircraft.
Referring to Figure 8, the lowest edge of any fixed part of the
water rudder 15 is above a line which passes thxrough the aftermost edge
of the tail and makes an angle to the plane described by the chines 4
below the C of G of 5 degrees. This enables the aircraft to rotate for
take-off without the water rudder 15 penetrating the water at take-off
speed. The water rudder is thus intended only to be effective at
displacement speeds.
The Iift characteristics of the flaps 31.39 (designated by flap-
chord, type and angle of deflection) are adjusted so that the nose down
(negative) pitching moment created by flap deflection is compensated by
the resulting negative Lift on the tailplane caused by the downwash aft
of the wing and flap creating an equal and opposite (positive) pitching
moment. The geometry and rate of deflection of the inboard flaps 31
and outboard flaps 39 differ and the gearing of the flap controls is
adjusted so that any change in power setting or any change in mean flap
angle results in little or no trim change or requirement for
complementary pitch control input.
The flap 31,39 is such that it can be set with reflex, i.e. it ,
can be rotated upward, reducing the lift coefficient of the wing. This
enables the wing incidence in relation to the planing hull to be set .
for optimum take-off performance, and using reflex, the attitude of the
hull in cruising flight to be set so that the knuckles 11 and chines 4
are substantially parallel to aerodynamic streamlines thereby
minimising aerodynamic drag in cruise.
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The engine 20 exhausts upward to reduce the noise level and noise
print below.
Fuel is contained in the nacelle 19, the main fuselage hull and
the sponsons 17.
Although not illustrated, further features may include, for low
speed water handling, a thruster unit driven by an electric motor and
mounted transversely in the bow to steer the bow at low speed
independently of forward speed to help berthing. Either a second
thruster unit can be mounted in the stern or a water propeller can be
mounted on the submerged lower section 15 of the sir rudder 14. This
propeller can be driven by an electric motor mounted in an extension of
the rudder forward of the rudder hinge line, thus also serving as a
control weight balance.
For seasonal operations the undercarriage could be removed and a
simple cover plate attached over the cavity normally containing the
retracted undercarriage.
Wing and undercarriage retraction systems can use hydraulics with
a common drive unit.
