U.S. patent number 3,583,660 [Application Number 04/851,009] was granted by the patent office on 1971-06-08 for lift and control augmenter for airfoils.
This patent grant is currently assigned to Lockheed Aircraft Corporation. Invention is credited to Charles H. Hurkamp, Willi F. Jacobs.
United States Patent |
3,583,660 |
Hurkamp , et al. |
June 8, 1971 |
LIFT AND CONTROL AUGMENTER FOR AIRFOILS
Abstract
Means is herein provided for augmenting the lift and
controllability of an aircraft by blowing compressed air through
spanwise slots in a multiple trailing edge flap system on the
aircraft's airfoils. The mechanism consists of a main flap, with
rotary and translation motion relative to the associated airfoil; a
lower surface flap which is hinged to both the airfoil and the main
flap; and an airflow directional control device rotatably mounted
to the aft end of the main flap. Downward deflection of the flap
mechanism creates a spanwise duct between the main and lower
surface flaps, through which compressed air, extracted from a
suitable source, is allowed to flow. The air is ejected at high
velocity from multiple spanwise slots provided in the flap
mechanism. The resultant jet sheets impart a downward momentum to
the air passing over the associated airfoil increasing the
circulating flow around its chordwise sections and imparting an
upward reaction. The airflow directional control device may be
utilized both to vary lift symmetrically on the aircraft and to
provide roll control by differential action.
Inventors: |
Hurkamp; Charles H. (Atlanta,
GA), Jacobs; Willi F. (Atlanta, GA) |
Assignee: |
Lockheed Aircraft Corporation
(Burbank, CA)
|
Family
ID: |
25309717 |
Appl.
No.: |
04/851,009 |
Filed: |
August 18, 1969 |
Current U.S.
Class: |
244/207;
244/212 |
Current CPC
Class: |
B64C
9/146 (20130101); B64C 21/04 (20130101); B64C
9/38 (20130101); B64C 2230/06 (20130101); B64C
2230/04 (20130101); B64C 2230/16 (20130101); Y02T
50/10 (20130101); Y02T 50/166 (20130101) |
Current International
Class: |
B64C
21/04 (20060101); B64C 21/00 (20060101); B64C
9/14 (20060101); B64C 9/38 (20060101); B64C
9/00 (20060101); B64c 021/04 () |
Field of
Search: |
;244/42,42.4,42.41,42.6,42.61,42.62 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Buchler; Milton
Assistant Examiner: Rutledge; Carl A.
Claims
What we claim is:
1. A lift and control augmenter for an airfoil of an aircraft
comprising:
a fluid-conveying duct mounted internally of said airfoil;
an articulated flap mechanism pivotally connected to the aft end of
said airfoil and formed by hinged flap elements defining an
internal fluid chamber communicating with said airfoil duct, said
flap mechanism including a main flap, a pivotal and translational
connection between said main flap and said airfoil, an upper and
lower flap element hinged at one end at and along the aft end of
said airfoil and adapted to overlie and define at least a portion
of the exterior upper and lower surfaces respectively of said main
flap, and an actuator between said airfoil and each said upper and
lower flap element operative to deflect the latter relative to the
former and concurrently move said main flap rearwardly along a path
determined and described by said pivotal and translational
connection;
a first fluid outlet from said chamber at and along the upper
surface of said flap mechanism medially of the aft end thereof and
the aft end of said airfoil;
a second fluid outlet adjacent the aft end of said flap mechanism,
said first and second outlets being in a relatively closed position
when said flap mechanism is disposed in its undeflected position
relative to said airfoil; and
means to locate said outlets in a relatively open position and to
regulate the effective transverse dimension of said second outlet
concurrently with the deflection of said flap mechanism.
2. The augmenter of claim 1 including a control associated with
said second outlet to vary the direction thereof angularly relative
to said flap mechanism.
3. The augmenter of claim 1 wherein said pivotal and translational
connection includes an arcuate slot in said main flap, and a roller
connected to and carried by said airfoil in constant engagement
with said slot.
4. The augmenter of claim 1 wherein said pivotal and translational
connection includes a lever pivotally connected at one end to the
forward end of said main flap and at the other end to a fixed point
relative to said airfoil aft of said forward main flap end when
said flap mechanism is undeflected as aforesaid.
5. The augmenter of claim 4 wherein said lever is one arm of a
bellcrank, the other arm of which is pivotally connected to a link
pivotally connected in turn at its other end to said lower flap
element aft of said fixed point.
6. The augmenter of claim 1 including an auxiliary flap at the aft
end of said main flap mounted for rotation thereon to and from
various deflected positions.
7. The augmenter of claim 6 wherein said auxiliary flap is
rotatably mounted at the aft end of said main flap with its
opposite surfaces disposed in the plane of the corresponding
surfaces of the upper and lower flaps when in the neutral position,
and said second outlet is disposed in a predetermined position
about said auxiliary flap so as to discharge fluid when operative
in preselected quantities adjacent the upper and lower surfaces of
said auxiliary flap.
8. The augmenter of claim 6 wherein said auxiliary flap is
rotatably mounted at the aft end of said main flap and is pierced
by a passage in communication with said fluid chamber.
9. The augmenter of claim 7 wherein said quantity of fluid
discharged adjacent the upper surface of said auxiliary flap is
appreciably less than that discharged adjacent the lower surface of
said auxiliary flap.
10. The invention of claim 1 wherein said airfoil is a fixed wing
on each side of the fuselage of the aircraft and one said lift and
control augmenter is associated with each said wing.
Description
This invention relates to lift an control surfaces for aircraft,
and more particularly to such surfaces incorporating augmenters to
enhance the operation and performance of the aircraft over a wide
range of flight conditions and regimes.
Heretofore various schemes have been proposed, including
arrangements based on the so-called "boundary layer control" and
the "jet flap" principles in order to augment the lift and
controllability of an airfoil or airplane wing when moving through
the air. Both the "boundary layer control" and "jet flap"
principles are predicated on an expulsion of a jet sheet of
compressed air or gas from the rearward portion of the control
surface or wing. The action of this jet sheet imparts a downward
momentum to the air passing over the associated wing surface
increasing the circulatory flow around the chordwise sections of
the wing as well as imparting an upward reaction to the wing due to
the downward force of the jet.
Boundary layer control by blowing is usually defined as blowing the
jet sheet from a slot located on the upper surface of the wing at
or near the junction between the relatively fixed structure of the
wing and the downwardly moving or hinged trailing edge flap.
Sufficient momentum is provided to entrain and increase the speed
of the otherwise slow-moving low-energy boundary layer air adjacent
the wing surface. This effect in turn causes the flow of free air
to turn or bend downward following the contour of the flap in its
deflected position. Without the jet, the free air tends to become
detached or separated from the flap creating turbulence and
limiting the amount of lift provided by the wing.
It has been found that a relatively small amount of blowing air
momentum is required to maintain airflow attachment to the wing
surface. Additional air momentum, i.e., increasing the mass and
velocity of the airflow over the wing surface causes the lift to
increase, although at a lower rate reaching a practical maximum as
determined by the available supply of blowing air and the maximum
limits of the duct cross section available.
The jet flap principle is usually defined as directing the jet
sheet of gas or air from a slot or nozzle located in the trailing
edge of the wing in a generally downward and rearward direction.
This, in effect, creates the equivalent of a rearward extension of
the wing chord thus increasing the effective angle of attack and
the circulation around the wing. The jet flap sometimes employs a
high-energy jet blast of air or gas blown from a slot or nozzle at
the hinge line or knee of the deflected trailing edge flap. In this
way, both boundary layer control and high-lift supercirculation are
accomplished while the flap acts as a deflector.
Although the jet flap can and does provide high-lift
characteristics, the high local velocities associated with blowing
over the upper surface of the flap increase friction drag which
deenergizes the jet stream. Also high-velocity blowing over the
angularly disposed or deflected flap is generally accompanied with
some airflow separation or detachment along the rear of the flap
with the result of further diminishing the effectiveness of this
type of jet flap arrangement.
The present invention endeavors to combine the principles of
boundary layer control and jet flap in such a manner that they
complement one another in optimum fashion and the objectionable
features of the jet flap system referred to above are avoided. At
the same time the arrangement herein proposed is so designed that a
relatively small amount of air is directed out through a slot in
the knee of a jet deflector flap to provide flow attachment while
the bulk of the blowing air is discharged through a slot in or near
the trailing edge of the flap where it will produce maximum
effect.
In substance, therefore, the concept underlying the instant
invention is the combination with the aerodynamic characteristics
of a mechanical flap of a blowing boundary layer control and a jet
flap. This allows for controllability of the aircraft by the highly
effective jet deflection control of the mainstream at the trailing
edge of the flap. Moreover, the arrangement allows for a spanwise
duct for the passage of the compressed air or gas by an enlargement
of the flap cross section in the deflected position. This
constitutes a real advantage in present day aircraft by removing
this structure from the primary wing interior where space is at a
premium for fuel storage compartments and various accessories
necessary for the operation of the aircraft.
More specifically, the instant lift and control augmenter comprises
an articulated trailing edge flap mechanism pivotally connected to
the aft end of the relatively fixed wing structure. This mechanism
is formed by multiple flap elements pivotally associated one with
another and with the wing structure to provide an automatic
expansion in profile upon its deflection producing an internal duct
communicating with an appropriate fluid pressure conduit in the
wing structure. During such expansion fluid discharge slots are
produced at and along the hinge line formed by the flap and the
adjacent upper wing structure, and also adjacent the trailing edge
of the flap. The effective size of these slots may be made to vary
by configuring the adjacent flap and wing structure surfaces as a
function of their deflection. Preferably, the slots associated with
the trailing edge of the flap include actuators for their
additional, independent regulation.
With the above and other objects in view as will be apparent, this
invention consists in the construction, combination and arrangement
of parts all as hereinafter more fully described, claimed and
illustrated in the accompanying drawings wherein:
FIG. 1 is a transverse section taken through the rear portion of an
airfoil such as, for example, an airplane wing showing an
articulated flap mechanism designed and constructed in accordance
with the teachings hereof with the several components thereof
disposed in their inoperative position, i.e., in the
high-performance flight condition of the aircraft, the operative
position of said components, i.e., their other extreme position,
being shown in broken lines.
FIG. 2 is a section taken along line 2-2 of FIG. 1 to show
primarily the mounting and connection of the main flap element to
the relatively fixed wing structure by which it is capable of
concurrently rotating and linearly translating;
FIG. 3 is a section taken along the line 3-3 of FIG. 1 to show
primarily the interconnection of the several flap elements in the
area trailing edge slots;
FIG. 4 is a section similar to FIG. 1 of an alternate form or
embodiment of the invention showing a somewhat different manner of
interconnecting the several flap elements to permit substantially
the same articulation thereof;
FIG. 5 is a section taken along line 5-5 of FIG. 4 and corresponds
to the section shown in FIG. 2;
FIG. 6 is a section similar to FIGS. 1 and 4 of still another
alternate form or embodiment of the invention showing a different
linking arrangement interconnecting the several flap elements to
accomplish substantially the same articulated movement thereof;
FIG. 7 is a section similar to FIGS. 1, 4, and 6 of the aft
extremity only of the mechanism showing an alternate form of
coacting flap elements in the area of and regulating the effective
discharge direction of the trailing edge slot or slots; and
FIG. 8 is a section similar to that shown in FIG. 7 of another
alternate arrangement of coacting flap elements in the area of the
trailing edge slots.
Referring more specifically to the drawings, FIGS. 1, 4, and 6
depict alternate mechanical means of achieving the general
objectives of the invention. Each of these embodiments shows a
typical cross section taken through the aft portion of an aircraft
wing 10 which can be considered as representative of sections taken
along the entire span of the exposed portion of the wing. In all
instances, fixed structure of the wing is provided with one or more
internal ducts through which compressed air or gas is supplied. The
source per se of this air or gas is unimportant to the invention,
it being merely provided by either bleed air from the primary
propulsion system of the aircraft or by some appropriate, auxiliary
power unit or units.
Considering first the embodiment illustrated in FIGS. 1, 2, and 3,
10 designates an aircraft wing and 11 designates fixed structure
thereof including an internal duct 12 adapted to convey air or gas
from a suitable source. The structure 11 carries a series of
supporting brackets 13 immovably secured thereto and projecting in
an aft direction therefrom. At their aftmost extremity these
brackets 13 each retain a set of rollers 14 which are rotatably
mounted thereon, and which are adapted to mount within a curved
slot 15 in a primary flap 16. The flap 16 is thereby supported and
capable of rearward movement thereon relative to the fixed wing
structure 11. To this end, the end of each bracket 13 is pierced by
a hole adapted to receive and mount a bolt 17 which serves as an
axle on which a roller 14 is mounted on each side of the bracket
13. The rollers 14 are retained in position by the head and the nut
of bolt 17 working in conventional manner in opposition one to
another and appropriate spacers 19. Thus located, each roller 14 is
adapted to engage a surface or track 18 that defines the slot 15 in
the flap 16 which thereby permits the flap 16 to roll freely
thereon.
At its rear extremity the flap 16 carries a series of supporting
brackets 20 rigidly attached thereto as at 21 and extending
aftwardly therefrom. At its aft end each bracket 20 mounts a pivot
bearing 22 to which an auxiliary flap 23 is hinged. A series of
power actuators 24 pivotally connected at opposite ends to the
primary and auxiliary flaps 16 and 23 serves to control the angular
movement of the auxiliary flap 23 with respect to the main flap 16.
These actuators 24 may be hydraulically, electrically,
mechanically, or otherwise operated, the means for the adjustment
thereof being unimportant so far as the present invention is
concerned.
A lower surface flap 25 is continuously hinged as at 26 along its
forward edge to the fixed wing structure 11 and similarly along its
rear edge to each of the auxiliary flap support brackets 20, e.g.,
through the pivot bearing 22. Thus mounted, the lower surface flap
25 is adapted to move in a generally angular motion to and from
extended and retracted positions being actuated by appropriate
means such as a power cylinder 27 pivotally secured at opposite
ends to a convenient part of the fixed wing structure 11 and the
lower surface flap 25. Thus, actuation of the cylinder 27, i.e.,
the extension and contraction thereof, serves to move the main flap
16 and the lower surface flap 25 away from each other producing a
fluid chamber 28 in communication with the duct 12. This chamber 28
terminates in an outlet 29 between the aft end of the lower surface
flap 25 and the main flap 16. Due to the sliding pivotal connection
through the rollers 14 operating between the main flap 16 and the
fixed wing structure 11, the main flap 16 is concurrently
translated in a generally aft direction as it is rotated
downwardly.
The mounting of the brackets 20 and their respective pivot bearings
22 is such that the auxiliary flap 23 is disposed in spaced
relation to the aft end of the main flap 16 creating a fluid
passage 30. The effective size of the passage 30 is small relative
to the outlet 29, being intended to pass only sufficient fluid when
operative to maintain the airflow over the upper surface of the
auxiliary flap 23 attached at all times. This leaves the maximum
amount of fluid available through duct 12 and chamber 28 to pass
through the outlet 29.
Preferably an upper surface flap 31 is also provided between the
fixed wing structure 11 an the main flap 16. This upper flap 31
overlies and encloses the several brackets 13, being continuously
hinged as at 32 at its forward end to the fixed wing structure 11.
Its angular motion relative to the structure 11 is regulated by a
series of power actuators 33 connected at one end to the under
surface of the upper surface flap 31 and at the other end to the
associated bracket 13. The several actuators 33 are interconnected
through suitable means for operation in unison following any
conventional practice. Extension of the actuators 33 thus serves to
rotate the upper surface flap 31 with respect to the brackets 13
and associated surface of the main flap 16 producing a rear slot 34
of desired cross-sectional dimension. Where a constant cross
section of this slot 34 is desired, the upper surface flap 31 may
be merely fixed and contoured with respect to the associated
surface of the main flap 16 whereby deflection and rearward
translation of the main flap 16 as above described automatically
produces the slot 34.
In the embodiment of the invention illustrated in FIG. 4, the same
end result is obtained by the operation of the several flap
elements through a slightly modified structural arrangement. In
this case, a series of supporting brackets 13a carried by and
extending rearwardly from fixed structure 11a of the wing 10a
retain pivot connections 35 on which a main flap 16a is mounted for
free and unrestricted rotation by appropriate actuating means such
as one or more cylinders 27a.
An auxiliary flap 23a is hinged to the main flap 16a through a
series of pivot bearings 22a each supported by a bracket 20a fixed
to and projecting rearwardly from the main flap 16a. Power
actuators 24a are used to control the angular movement of the
auxiliary flap 23a relative to the main flap 16a. The mounting of
the brackets 20a and their respective pivot bearings 22a is such
that the auxiliary flap 23a is disposed in spaced relation to the
aft end of the main flap 16 creating a passage 30a.
A lower surface flap 25a is continuously hinged as at 26a along its
forward edge to the fixed wing structure 11a and at its rear edge
is provided with a series of rigidly connected brackets 36 each of
which supports a pair of rollers 14a. Each pair of rollers 14a is
mounted on a bolt 17a acting as an axle on which one roller 14a is
disposed on each side of its bracket 36. Thus located each pair of
rollers 14a operates within complemental curved slots 15a in the
main flap 16a adjacent the aft end thereof. The curvature of the
slots 15a and the lower contour of the main flap 16a is such that a
flush lower surface is established when the flap mechanism is in
the inoperative position, i.e., undeflected. This is accomplished
by forming the main flap 16a with a depression or cavity 37 having
a base wall disposed in the plane of the brackets 13a so that when
the lower surface flap 25a is retracted its inner surface abuts the
brackets 13a and main flap 16a and its outer face forms an
aerodynamically clean and smooth continuation of the adjacent
surfaces of the wing structure 11a and main flap 16a.
Also when the flap mechanism is operative, i.e., extended or
deflected, a fluid chamber 28a with outlet 29a is produced between
the main flap 16a and the lower surface flap 25a, while a fluid
passage 30a is opened between the main flap 16a and the auxiliary
flap 23a to discharge air or gas from the duct 12a over the upper
surface of the auxiliary flap 23a The dimension of the chamber 28a
is adjusted or regulated over a desired range of angular movement
of the flap mechanism during its deflection.
As in the case of the embodiment of FIG. 1, the size of the outlet
29a is substantially greater than that of the exit from the passage
30a so that maximum fluid from the duct 12a and chamber 28a is
employed in the jet flap function and only enough fluid employed in
the boundary layer function to lubricate the upper surface of the
auxiliary flap 23a.
Also, an optional upper surface flap 31a is hinged as at 32a to the
wing structure 11a along the length thereof adjacent its aft end.
The angular motion of this upper surface flap 31a is regulated by
one or more power actuators 33a, the opposite ends of each of which
are secured to the upper surface flap 31a medially of its length
and to the bracket 13a. The operation of the several actuators 33a
in unison by appropriate interconnecting means thus serves to
regulate the effective size of an opening or slot 34a between the
aft end of the upper surface flap 31a and the main flap 16a where
it is desired that this be variable.
FIG. 6 shows still another alternate construction of an articulated
flap system which is preferred in certain applications. The fixed
structure 11b of the wing 10b includes rigidly attached brackets
13b intermittently spaced and each provided with a pivot 38 to
which is mounted a bellcrank lever 39 consisting of two radial
angularly spaced mounting pivots 40 and 41. One of these pivots,
e.g., pivot 40, is attached to the main flap 16b while the other
pivot 41 is attached to a link 42 which in turn is attached to the
lower surface flap 25b as at 43. This bellcrank 39 is rotated by a
linear powered actuator 27b or the like which causes the leading
edge of the main flap 16b to move in a circular path or arc in a
rearward and upward direction whereby the aft end thereof is
deflected downward.
The lower surface flap 25b is concurrently forced downwardly in an
arc or a circular path about its hinge by the combined action of
the main flap 16b and the link 42 acting on it. Any slight binding
created by these combined actions will be relieved by deflection of
the main and lower flap structures 16b and 25b. Moreover, the use
of the link 42 to assist in rotating the lower surface flap 25b is
optional, since it is not necessary to achieve the desired result
as described. It is preferred, however, in providing the lower
surface flap 25b with an intermediate point of support against the
load created by pressure against its interior surface exerted by
the fluid or compressed air from the duct 12b and in the chamber
28b.
FIG. 6 also shows an optional configuration for the auxiliary flap
23b which is interchangeable with those shown in FIGS. 1 and 4. In
this instance a single slot 44 is provided adjacent the upper
surface between the main flap 16b and the auxiliary flap 23b while
the joint between the lower surface flap 25b and the auxiliary flap
23b is substantially sealed as for example by rubbing contact. A
powered linear actuator 24b which may be attached to either the
main flap 16b or the lower surface flap 25b as desired in the
manner shown for example where it is pivotally connected to the
lower surface flap 25b. The length of the auxiliary flap 23b may be
selected to any predetermined extent, the minimum being
substantially a cylinder rotating about its pivot 22b.
In FIGS. 7 and 8, there is shown alternate structures and
arrangements for the auxiliary flap 23, 23a and 23b as previously
described in connection with the embodiments of the invention
illustrated in FIGS. 1, 4, and 6. Referring to FIG. 7, this
alternate auxiliary flap is formed by a continuous rotatable,
generally cylindrical body 23c hinged as at 22c to the trailing
edge of the main flap 16c along a hinge line which may be supported
by intermittent brackets or ribs 20c. The ribs 20c connect the main
flap 16c with a continuous lower surface slat 45 which is hinged to
the lower surface flap 25c by means of a continuous hinge 46. Fluid
escape outlets 47 are formed between the end of the slat 45 and the
rotatable cylinder 23c and also between the main flap 16c and the
rotatable cylinder 23c. Thus fluid is discharged from the chamber
28c at the aft end of the articulated flap mechanism to perform the
jet flap function. In this case the actuation means for adjustment
and deflection of the auxiliary flap 23c may be in the form of an
appropriate series of cables 24c which are wound over and around
grooves i the flap or cylinder 23c.
Referring to FIG. 8, a continuous rotatable, generally cylindrical
body 23d is employed that incorporates a series of intermittent
slots 48 in communication with the chamber 28d defined by the main
flap 16d and lower surface flap 25d/ slot 45d for the discharge of
fluid therefrom. The cylinder 23d is hinged to the trailing edge of
the main flap 16d through ribs 20d which interconnect the main flap
16d with the slat 45d hinged as at 46d to the lower surface flap
25d. The main flap 16d and the lower continuous slat 45d are
intended to substantially bear against the periphery of the
cylinder 23d so that the sliding joint thereby created may be
considered as tight. This is not critical, however, inasmuch as
fluid leakage around the cylinder 23d does not prevent the
operation thereof as a jet flap. The rotation of the cylinder 23d
is accomplished by means of and through a cable arrangement 24d as
previously described in connection with the mechanism shown in FIG.
7.
In any of the systems shown in FIGS. 1, 4, or 6, the extension of
the main actuators 27, 27a, or 27b and the admission of compressed
air or gas into the chamber 28, 28a or 28b causes the flap
mechanism to assume a predetermined angular position as shown in
phantom lines in each of the FIGS. 1, 4, and 6. Several such
positions may be available through the use of stops or locks which
per se are not a part of the invention and therefore have not been
shown. The fluid entering the flap chamber 28, 28a, or 28b is
constrained by the main/upper surface flap 16, 16a and 16b/31, 31a
and 31b and lower surface flaps 25, 25a and 25b respectively and
will travel in both spanwise directions thereof. Simultaneously the
fluid will be ejected at relatively high velocity through selected
spanwise slots or outlets at 29 and 30, 29a and 30a, 44, 47, or 48,
as the case may be, followed by optional fluid ejection through
another spanwise slot or outlet at 34, 34a, or 34b. These fluid
sheets in combination with the external flow of free air produce
the boundary layer control and jet flap effects previously
described.
In any fixed position of angular deflection, the auxiliary flaps
23, 23a, or 23b of either FIG. 1, FIG. 4, or FIG. 6 or the rotary
cylinders 23c or 23d of FIGS. 7 and 8 may be moved at the will of
the operator or pilot. Symmetrical movement of these auxiliary flap
elements 23, 23a, 23b, 23c, or 23d on the opposite wings 10 of the
aircraft can be employed to provide limited orientation of the
resultant lift and drag vectors for control of the climb or glide
slope of the aircraft. On the other hand differential movement of
these elements 23, 23a, 23b, 23c, or 23d can be employed to provide
correction of asymmetrical lift or lateral trim as well as primary
roll control. Another means of providing lateral control is that of
differentially regulating the height of the upper surface flap 31,
31a, or 31b and the adjacent opening 34, 34a, or 34b. The two means
can also be used in various combinations.
In each case, i.e., the embodiments of FIGS. 1, 4, and 6, opposite
ends of the flap elements 16, 16a, and 16b, 31, 31a, and 31b, and
25, 25a, and 25b are provided with suitable means to close the
respective chambers 28, 28a, and 28b at all times. Such closure
means may comprise any conventional structure and arrangement such
as for example bellows or other expandible and collapsible sheets
of suitable material. Elastic fabrics have also been employed in
similar type applications. Since such closure means per se forms no
part of the present invention, it is not illustrated lest it
detract from the invention claimed.
While shown and described in what is believed the most practical
and preferred forms or embodiments, it is apparent that departures
from the illustrated specific structures will suggest themselves to
those skilled in the art. Such variations and innovations may be
made without departing from the spirit and scope of the invention
as covered by the appended claims.
* * * * *