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Патент USA US2408789

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Oct. 8, 1_946. A'
c. T. LUDINGTON ETASL
Filed June 16, 1939
2,408,788 .
5 sheets-,sheet 1
Oct 8, 1946-
- c.> T. LUDINGTÓN ETAL
'AIRFOIL
Filed June-16, _1939 '
2,408,788
v
3 Sheets-Sheet 2
Y.
26
Z7 I
30
2,408,788
Patented Oct. 8, 1946
UNITE-o STATES „PATENT >oF-Flcla‘.4 A,
AIRFOIL
Charles Townsend Ludington,- Ardmore, Pa., and
Roger W. Griswold, II, Qld Lyme, Conn.
Application June 16, 1939, Serial No. 279,416
1 Claim. (Cl. 17o-472)
conclusive,V` the following exposition is believed
to be accurate Within the limits of presently un
ticularly to >theprovision of rotatable airfoils by
which iiuid now control _relative to the airfoils
is established for` the normall high speed ranges
including the super-sonic.
derstood physical concepts and their attendant
terminology. While thepmethod of expression
'
used in the following discussion may be subject
Airfoils, whether of the ñxed or rotary wing
type, >are essentially dynamic energy converters,
the principal function of which is to produce the
to later revision, it is submitted that the under
lying principles and basic hypotheses will remain
optimum ratios of lift Vto drag in the normal op
erating range. Due to certain functional faults
inherent with airfoils of the prior art, the con
ventional type are seriously restricted in their
range of usefulness bydeiinite upper and lower
velocity limits, both critical, such upper limit of
most so-called modern sections occurring at ap 15
proximatelyV three quarters the speed of sound.
The critical lower limit is of course, the “stalL”
VThis invention is mainly concerned withthe high
speed characteristics of airfoils,> and herewith
discloses principles of super-sonic flow `Vcontrol
(self-energized in the case of rotatively operative
airfoils), which are believed to be new. These
principles, as will be apparent to those skilled in
the art as the description unfoldsfcombine in a
2
sequent knowledge of the phenomena is thus yin
This invention relates to airfoils and par
valid.
-
Though the several design parameters may
vary considerably, conventional airfoils have- in
common a rather> blunt or bulbous nose leading
edge, the curved surfaces ofA whichA diverge
through a wide angular range. to thepoint of
.maximum airfoil thickness. and then more gently
converge to join into a fairly sharp trailing edge.
In the highk speed range (relatively low angle of
attack) such airfoils divide the flow at some point
on the leading edge known as rthe stagnation
20 point-so-called, since the full impact of the free
stream flow impinges and is substantially stopped
in its path of travel at this theoretical point>
(changing with changesinthe angle of attack)
' with consequent conversion of its'dynamic (or
kinetic energy) pressure to a corresponding in
crease of static pressure at the leading edge. «E_X
dynamic laws, thus, for what is rbelieved tobe
tending either side of the stagnation point over
for the ñrst time, enabling operation of airfoils
constructed according to the principles offthe t a considerable lsection of the bulbous nose ¿and
moderately forward of the airfoil is a general
invention, well into the range of super-,sonic
velocities, with eiiiciency and economy( ‘While k30 region lof stagnation whereinY the flow is
>decelerated progressively less, the net result 'of
this disclosure reveals means to utilize `the ravail
able centrifugalv energyk of rotatingairfoils, Vit 1 the phenomena being the Vcreation of a sub
stantial ypressural drag throughout the stagna
must also bev clearly understood that the same
tion fregion. Pressural .Y drag isthe resultant
basic principles of super-sonic now Ycontrol areV
novel manner, proven and well established aero- k .
equally applicable to- fixed wing yaircraft whenr
other sources of energy >are introducedif
In the following description, the term “rotor”
is used to cover all applications of airfoils where
translation is combined with rotation, either nor
mal to, or in the plane of rotation, or any com
bination of the two, as with autogiros, heli
copters, gyroplanes etc., sometimes called direct
lift aircraft since the primary function of rotors»
is the generationof lift, and 'secondarily thrust
in some cases, while Vthe term “propeller” ap
plies to all airfoil propulsive systems where trans
lation is combined Vwith rotation substantially
normal to the plane of rotation to provide
thgrust primarily.A The invention applies pri
marily to both rotors and propellers.
Leading edge flow phenomena in the high
‘
speed range
'
f
As research onthefcompressibility burble is
' 4>rather incomplete, so far ,as known,_and con
35
down stream or rearward component of the nor'
mal pressures on the surfaces and varies with
different shapes and velocities. Being Van >in
escapable bye-product of the shape-Velocity factor
it represents -the irreducible minimum to which
drag might theoretically be reduced if frictional
or viscousdrag were eliminated. In the normal
cruising ran'geof present day fixed wing aircraft,
y pressural drag is an important, though notjby
any means a major part of Wing drag, but to
wards the sonic range of-velocity, as attained
over the tip sections of propellersand rotors it
becomes _a controlling factor-and the prepon
' Vderant part yof pressural drag under the latter
conditionsbuildsvup within the stagnation region
at the relatively blunt leading edge. Thus `the
shape of the latter is of paramount importance
for high speed. A
.
i
l
.
The stagnation area orl region is substantially
an effective function of the bulbousity or relative
thickness of the entering edge and can be sub-,I
2,408,788
3
4
stantially eliminated as the bulbousity disappears
about 550 M. P. H. When such critical velocity
and as the thickness of the entering edge be
pressure relationships are reached the normal
stability of the iiow breaks down resulting in a
comes reduced.
Under the combined inñuence of the excess
static pressure in the stagnation region, which
must of necessity be re-converted to kinetic
energy, and the crowding of the streamlines with
consequent flow convergence, caused by the wide
ly diverging surfaces of conventional leading edge
highly disorganized surging generation of turbu
lence in proximity to the stagnation region. This
phenomenon is known as the compressibility
burble. The resultant highly disorganized iiow is
chieily characterized by a phenomenal increase
of drag, and a very dynamically unstable pres
sections, a substantial acceleration is imparted 10 sure distribution over the airfoil.
to the ñow'over this part of the airfoil. The
Even though one might'overlook the deleterious
shape of the leading edge (generally referred to
effects of the severe loss in lift-drag eiiiciency,
as the camber) determines what might be called
which could not be justiñed for any practical ap~
the leading edge acceleration ratios. For» the
plication, a moment’s consideration of the de~
average conventional airfoil in the high speed 15 gree of dynamic instability induced by this con
range the acceleration ratio is such as to give an
increase in velocity of about one and one half
times that of the undisturbed free stream flo-w.
Quite obviously, then, sonic or super-sonic veloc
dition will make it apparent that it is very
dangerous to use conventional airfoils in the
super-sonic range. Since a large proportion of
the unbalanced load on the airfoil is concen
ities will be attained in this region when the speed 20 trated in the stagnation region at the leading
of such an airfoil itself is somewhat less than
edge due to the extreme pressural drag and since
that of sound, say of perhaps about 550 M. P. H.,
the stagnation region itself is teetering on a point,
approximately characterized as sub-sonic and
so to speak (it could hardly be otherwise with
which may be designated as the critical com
such turbulent flow to the rear), we have here
pressibility speed. The critical speed, of course, 25 the very combination of forces to set up resonant
varies with different airfoil proñles. Here again,
vibration in the structure, or aerodynamic flutter,
the leading edge camber is of vital importance
the disastrous consequences of which vare only
for sonic or super-sonic speed with economy.
too well known to the art. Quite clearly, such a
While excessive pressural drags and accelera
complete'break-down of functional characteris
tion ratios are important contributory causes of 30 tics (lift, drag and favorable (stable) pressure
presently encountered subsonic speed limitations
distribution), utterly destroys the airfoil’s use
inherent in the prior art, so far as understood,
fulness and the best that may be said is that the
by far the most serious and controlling consid
phenomenon is highly extravagant of energy,
eration is the iiow deiiection factor. According
most of which is dissipated as heat, some as
tothis theory a line drawn tangent to the leading 35 sound.
edge at the stagnation point is approximately
The entering wedge airfoél
normal to the direction of the undisturbed free
stream iiow inthe high speed range, which means
It should now be obvious, as our studies and
there is an equivalent extreme angular displace
experiments have led us to conclude, that an
ment of the ñow at vthis point, the deflection 40 approach to the elimination of the compressibility
being successively less with increasing distance
burble or its relegation to some higher super
from the stagnation point. The flow having
sonic velocity, must be initiated by a radical re
divided and been sharplydeflected into new paths
design of the conventional airfoil leading edge
of travel from which the leading edge surfaces
parameters with which this highly undesirable
increasingly diverse (curving away from deflected
phenomenon is inherently functional. Namely,
flow) ,it is known from Newton’s law that it will
we must‘do away with the bulbous nose section
continue in its state of uniform motion in a - and the highly divergent surfaces curved away
straight line unless it is compelled by external
from the' flow, We propose, then, to construct
forces to change that state. Fortunately for con~
the super-sonic airfoil in the general shape of
ventional airfoils in the sub-sonic range below the
a wedge, the relatively sharp edge of which is the
critical compressibility speed `(up to about 550
airfoil leading edge. This will, of necessity, re
M. P. H.) such an external force of suñicient
duce the stagnation region literally to a point or
magnitude is had in the available static pressure
line„ with corresponding reduction of pressural
of the atmosphere-_for again, according to New
drag-for all practical purposes one might con
ton’s law, change of momentum is proportional
sider the stagnation "point to have been elimi
to impressed force and takes place in the direc
nated. The surfaces diverge from the leading
tion in which the force acts. Thus the impressed
edge at a relatively small angle and are inclined
force of the atmosphere acting normally to the
towards the flow substantially over their whole
surface constrains and redirects the flow along
extent-_they may actually be fiat, slightly convex
the surface in the acceleration regions, but is
or even slightly concavely curved into the flow in
unbalanced in proportion to the kinetic energy of
the manner of a hollow ground razor or in any
the iiow, thus setting up an equivalent low pres
sure at the surface-in the stagnation region the
combination of the three basic surfaces just re
cited.` Such an airfoil will reduce flow deñection
impressed atmospheric force is augmented by the
from thatcaused by a plane normal to the flow,
excess static pressure, dynamically induced and
as functionally induced by the bulbous nose, to
one inclined >at a very small acute angle, and
a compressibility burble in this ì region would
accordingly be a physical impossibility. When
however, the flow'velocity over any part of the
airfoil attains the speed of sound, its dynamic
pressure reaches vaïcritical value relative to the
thus the most serious iiow disruption factor in
the sonic range will have been largely eliminated,
in fact, this combination of surfaces and factors
static pressure of the atmosphere (i. e. when the
ently appear.
Moderately diverging surfaces will cause cer
respondingly little flow convergence, with conse
quentlow acceleration ratios extending over the
cntireífairfoil. Accelerating-news induce a fall
dynamic pressure is approximately 53% of at
mospheric pressure) and we have seen that such
local iiow velocities will be attained with the
average airfoil of the prior Yart at a 'speed of
now serves to promote flow control, as will pres
2,408,783 l
5
6
duction in> viscous drag to about one-sixth that
attained by the prior art which efficiency increases
withhigher Reynolds numbers. The introduc
ing pressure gradient along the surface, having
characteristic laminar flow stability and extreme
ly small frictional or viscous drag. Since the
tion of fluid discharge energy jets into the flow
'may also- permit a slight curvature of the surfaces
away fromv .the flow, if that be desirable for any
surfaces are inclined into the ñow at every point,
there will be a small down stream component of
dynamic pressure (pressural drag) all along the
surface, additive to that of atmospheric impressed ’
reason.
»
'
In the case of rotors and propellers, such fluid
flow may be provided by the introduction of inlet
force, to maintain higher pressures outsideßthe
boundary layer, exerting a repressive effect on '
the flow within the boundary layer, which must 1.0 openingsl disposed along the blades, preferably
near or at the hub, connected by communicating
accordingly remain laminar so long aS this cir
passages within the blade’to rearwardly directed
cumstance prevails. The limiting factors of the
discharge jets disposed in the region of the blade
prior art have thus been so radically modified 'as
tip. The centrifugal force vdue .to blade rotation
to achieve completely Ynew functional vcharacter
>will `accordingly setup corresponding pressure
istics in the super-sonic range with an airfoil so
differentials between the inlet passages and the
constructed, and what formerly precipitated flow
discharge jets, thus inducing a flow into the
disruption is now made to actually facilitate flow
former, radially through the blade and rearward
control. It is further important to note from
ly ejected out through the latter (properly di
the above discussion .that in addition to the great
rected to the rear by deilector varies) in the man
reduction of pressural drag, it is more evenly dis
tributed over the whole surface of the entering
wedge airfoil, which factor, combined with the `
ner of any conventional type centrifugal blower
unrestricted discharge gives a complete con
version of centrifugal force .to kinetic energy, ex
cept for frictional losses in the system. ' It will be
laminarA flow stability, provides good dynamic
stability.
Since no conceivable shape Aof ccn
vergent ordinary airfoil surfaces’terminating in
observed that the proposed centrifuge flow sys
25
vtem is automatic in its operation since it utilizes
the usual .type of trailing edge would induce the
flow to follow such surfaces> in the super-sonic
the available rotational energy which thus elimi
nates the need for external sources of energy and
their necessary power converters, such as blowers
etc., with their attend-ant complications. With
range, due .to insufficient atmospheric impressed
force, it is generally quite useless toso terminate ,
the entering wedge airfoil and for super-sonic
Speeds it may as well thereforev remain a simple
3,0 correct application of the principles herewith
wedge shape having a blunt trailing edge, the face
of which> is substantially normal-.tothe undis
disclosed, our calculations indicate sufficient cen
y trifugal energy is available in present operating
'ranges of propellers and> rotors to achieve the de
turbed free stream flow. Such an airfoil will, of
results. Since both centrifugal force and
' course, have a large momentum loss wake 'but 35 'sired
dynamic pressure are a function of velocity
nowhere near lthat of the conventional airfoil at.
squared-the >system should be designed to sub
super-sonic velocities and it will therefore ‘effect
stantially balance these forces over the vtip sec
a great saving in drag; This is useable of itself
tions requiring super-sonic flow control-proper
but in order to reduce' or Veven eliminate this
momentum loss drag we propose to combine with 40 functioning of the systemin the case of propellers
or rotors will accordingly be independent of ve
the entering wedge airfoil and in some ycases even
locity, into some, as yet undiscovered, upper limit
with bulbous nosed airfoils and the like a'system
of:
.
l
e
'
’
of thesuper-sonic range. Obviously, the'centrif
`
ugally energized fluid flow energy balance prin
ciple is 4susceptible of -almost infinite detail modi
fication in the number, size and location of the
inlet 5and discharge passages and the arrange
Fluid disposal energy balance
' By dispensing with the conVentionalafter-body
, convergent surfaces and substituting therefore la
ment ofthe communicating ducts-for instance,
fluid vdisch-arge having substantially the same
kinetic energy and direction of flow as the adja
if some sacrifice of available centrifugal energy
is permissible', .the inlet passages may be radially
cent local stream when it leaves the trailing edges "
distributed over a considerable span of the blades
of the wedge airfoil, 'a harmonious lamination of
ldisposed on either the upper orlower airfoil sur
faces, or both, to act as flow control suction slots
such airfoil-ejected-flow with the free streamlines
may be obtained.
It may be considered that with .the enteringr
in >removing the turbulent boundary layers over
wedge 'airfoil described or with any other enter- F
such surfaces.` Many otherlvariations could be
j'enumerated but it should'be .apparent that the
ing edge as willvbe disclosed, with the trailing
proper vthickness and with its speed asl close as
,scope> of .the invention encompasses the basic
principles as set forth in the foregoing discus
sion and the appended claims ratherv than the
possibleV to that -of’th'e relative“ air stream at’such
ffeatures of any particular application.
_ edge thereof comprising a relatively blunt;end
from which a fluid propulsion is effected of the v
discharge points as to havenpractically no rela
tive motion With regard to such air stream, in
effect -creates a synthetic chordwise elongation of
the airfoil having no effectivefrictional or other
turbulence-creating character.
The slightly
It hardly seems necessary to mention that the
energy balance'super-sonic airfoilshould open
Vup substantial vopportunities in several fields of
applied aerodynamics, especially-with propellers
65
higher momentum energy'of the wake of such
an airfoil system relative to the undisturbed free
centrifuge airfoils) since thrust and lift go up as
the'square of the rotative speed andthe limiting
stream kinetic energy will readily re-establish at
increase in the latter would not be likely to arise
from the aerodynamic considerations. Since the
fluid ejectorV tip sections >would be operating
mospheric equilibrium without undue disturbance
' very shortly after passing of the airfoil.
The
significance of providing means to fully maintain
through laminar flow conditions, the super-sonic
region'. of the blade. would actually have farA
laminar flow’over theentire airfoil vand to elimi
nate the turbulent wake .in operating Reynolds
A.higher ïefüciency than the inboard „sub-,sonic
numbers (velocity-airfoil size factorslvvill'be ap
preciated when it is realized that it effects a re
«and rotors (suchrapplications might be called
75
2,408,788
7
8
It is among the objects of this invention: -to
provide improvements in airfoilsyto provide an
airîoil with which super-sonic speeds can Vbe at
tained without danger; to -provide airfoils >for use
at speeds approximating that` of sound without
attainment of a compressibilty burble; to provide
ble, showing diagrammatically the great increase
of drag.
Fig. 3 represents diagrammatically a profile of
an airfoil constructed in accordance with one
improvements in airfoils for use in the super
phase of the invention herein, in which the gen
erally wedge shape is formed‘of slightly concave
upper and lower surfaces with lines indicating the
sonic ranges; to obviate in airfoils certain of the
undesirable attributes of the conventional bulb-v
blunt trailing edge.
airñow over the airfoil and having an abrupt
i
ous nosed airioil at sub-sonic, sonic and super 10
Fig. 3A represents diagrammatically a proñle
sonic speed ranges; to provide an airfoil for
of a modification ofthe airfoil of Fig.«3 having
substantially sonic speeds in which the pressural
a wedge shaped leading edgeformed of con
drag incident to the relative impingement of air
vergentflattened arcs, with an elongatedV trail
upon a stagnation area is eii‘ectively eliminated;
imT edge formedon thesame arcs in place of the
to provide an airfoil for substantially sonic speeds
blunt trailing edge of Fig. 3.
which the acceleration of the relative air
Fig. 3B represents diagrammatically a profile
stream over the vleading edge of the airfoil is so
of a mcdincation of the airfoil of Fig- 3, having
as to raise the speed at which a com
a truly wedge leading» edgeformation with planar
pressibility bur'ble occurs appreciably into the
upper and lower surfaces, with a modiñed less
super-sonic; to provide an improved airfoil in 20 blunt trailing edge, and in which the airflow
which the new deflection of air iiowing relatively
would be substantially similar to that of Fig. 3.
past the airfoil leading edge during movement at
Fig, 3C represents diagrammatically a profile
super-so ic speeds is so reduced or minimized as
a still further modification of the wedge shaped
to be relatively. unimportant; to provide an air
airfoil oi Fig. 3, in whichthe surfacesl of the'lead
i'cil having a relatively abrupt physical trailing 25 ing edge are each convexy and the trailing edge
edge with a chordwise synthetic effective trailing
is a rounded iin'therfmodincation of the blunt
>fluid extension having a minimum coeiTi.
trailing edge of Fig. 3.
,
' -1to1? friction relative to the airíiow over the
Fig. 4 represents a diagrammatic proñle of a
ai fcil te. minimize the drag otherwise attaching
still further modiíied form of entering wedge air
tc the abrupt trailing edge; to provide an airfoil
>foil. in which the upper and lower surfaces are
for super-sonic speeds in which the-.energy other
slightly convex, and in which the blunt trailing
wise eiîective against a stagnation area is effec
edge
a rearward emission of projected fluid
tively transformed to reduce pressural drag of the
emerging substantially parallel to and of sub
airfcil; to provide a novel shape of airfoil;.to
stantially the same velocity relative tc the airfoil
provide an airfoil applicable 'to rotor or propeller 35 as the relative airstream passing about the air
use with the tip moving in the super-sonic range
Íoil and of the thickness of the airiîoil.
,
without creating a shock or compressibility burble
Fig. 5 represents diagrammatioally a‘plany par
wave; to utilize the rotational energy of a rotoror
tial'y in transverse section, of a movable airfoil,
propeller, to provide a gaseous emission creating
such as a, propeller or rotor blade ci varying cross
a synthetic chordwise trailing edge extension of
sectionalcontours, in which the fluid medium in
the rotor or propeller airfoil section to reduce the
which the airfoil moves may enter the blade ad
friction between the relatively iiowing air stream
jacent toits rotary axis, and by centrifugal force
and the airfoil to minimize the drag of the air
be thrown outwardly against rearwardly inclined
-foil; to provide an airfoil with atrailing edge or
blades behind a wedge shaped entering edge to
rearward gaseous emission to reduce drag; to pro 45 exit rearwardly from the peripheral tip to form
vide' a propeller with means such that the tips can
travel at super-sonic speeds with safety andeíiì
ciency; to provide a variable controlled iluid
omission from the trailing edge- of a rotor blade
>the control being consonant vwith changes in rela
tive velocities resulting from combination of ro
tational and translational velocities and being
either manual or automatically responsive kto
the synthetic trailing edge of the wedge shaped
’airfo-il‘tip having a minimized frictional coefficient
relative to the airstream flowing relatively about
the airfoil, to preclude the attainment of the
50 compressibility burble or other adverse drag ef
fects from the super-sonic velocities 'of the blade
tip.
'
‘
.
Figs. 6, '7, 8, 9 and 10 represent respectively
`changes in relative velocities;v to provide control
transverse diagrammatic proñles through the
means automatically or manually Operative to in 55 rotor blade of Fig. 5, showing the various pre
terruct, or modulate a trailing edge fluid emis
ferred contours, each of-Which 'is designed for
sion
a rotor airfoil or .tochange the shape >of
optimum efficiency at itsrespective relative op
an airfcil comprising a rotor blade to harmonize
erating speed.
'
,
with cr to accommodate the varyingk relative tip
Fig. 11 represents a diagrammatic fragmentary
speeds incident to the. combination of rotation 60 elevation of the tip end of the rotor blade of
with translation; tov provide an airfoil with mov
able elements such as to be eiiîcient at low as
Fig. 5.
_
-
'
Fig. l2 represents a diagrammatic plan view
similar to that 4of Fig. 5, of a Inodiiied form of
well as super-'sonic night speeds.
in the accompanying drawings forming part
of this description:
65 vair-foil, having intake suction slot boundary layer
Fig. i represents a diagrammatic proñle of a
icihand, illustratively having at the tip .rear
wardlyd'irected ica-illesy or deflectorsby vwhich the
indicating the stagnation region or area, the
its or substantial points of greatest accelera
fluid inwhich the blade moves is flung out by
70 centrifugal force, in the case of use of the airfoil
on cf the Vairstream, and the lift-drag and re
te» nt for a vector,
Fig. 13 represents a >diagrammatic chordwise
,.
'
flow control apertures in the surface of the air
typical conventional airfoil showing the relative
airflow under favorable iiight speed conditions,
Fig. ‘2 represents a similar diagrammatic pro
a rotor or propeller blade.
section through the einen of Fig.. 12.
Fig. la represents a diagrammatic `fragmentary
ñle of the same airfoil .after the “shock wave” has
formed as an incident of the compressibility bur 75 .plan ofthe tipsection of 4a vrotor .or propeller Vot
214085788
and having a rearward gas emission.
Figs. 15, 16 and 17 represent sections throughv
various airfoils in each of which is provided the
introduction of a moving stream of fluid into -the
airfoil and its emission at suitable points to facili
divergence of the surfacesis gradual so that the
. flow deflection isat avery small angle and the4
> acceleration of theair flowing relatively past the
airfoilis held `to small limits by the relative ab
sence of stagnation pressure energy 4and minimal
flow` deflection. The `only undesirable factor
of such an airfoilat the sonic velocities is the
blunt trailing “edge” I0, which _causes a high
momentum loss wake following the airfoil and
thus a high degree of drag, as will be understood.
tate the smooth laminar flow over the. outer sur
faces of the airfoils in accordance with the in
vention.
,
51’0
»line at I3 andi] respectively, and therefore has
very. little or agreatly reduced stagnation area
by which excessive pressural drag is created. The
conventional plan form partially broken" away
y
- Fig. 18 represents a fragmentary diagrammatic
section through a modified form of airfoil in which
the blunt trailing edge is replaced by controllable
flaps or closure doors for the emission of fluid
However this drag in many cases is insufficient to
ages or the like, so that the trailing edge may be 15 detract from the other highly favorable attributes
of the airfoil, especially as regards its freedom
closed when desired, either manually or auto
under pressure and is operative by suitable link
matically,
from compressibility burble which latter condi
~
tion, _of course, results in a_ far higher` degree of
Fig. 19 represents a similar section with the
drag. It is to be recognized that the entering
trailing edge closed and forming the conventional
20 wedge airfoil of itself is of great importance,
streamlined trailing edge.
either as a fixed wing, or `as the tip section of a
Fig. 20 represents a plan of the airfoil of Figs.
rotor or propeller or as the leading edge portion
18 and 19, showing an illustrative form of linkage
of an airfoil having various types of trailing
for the controlling purpose, in which the axis of
edges. Thus in Fig. 3 as noted the trailing edge
flapping of the blade is eccentric to the axis about
which a control link swings so that upward tilt 25 I Il is blunt and practically of the full width of
the airfoil. ¿As shown in Fig. 3A the airfoil sur
of the blade is accompanied by extension of Vthe
faces I and 2 may be flattened arcs meeting at
link or its retraction as desired.
the leading edg'evin a line I3 and converging upon
The disclosure of Fig. 1 is purely diagrammatic
a. thin or line trailing edge 3. In Fig. 3B, the
to illustrate the present day airfoil sections, typi
cal approximately of al1 types of airfoils having 30 trailing edge comprises the partiallyl convergent
surfaces 4 and 5> andthe vertical planar portion
more or less blunt leading edges C, a thick mid
6 whereas in Fig. 3C the trailing edge is a rounded
surface 'l analogous’to the present day conven
section, »A-B yand an after section D tapering
gradually down to a sharp trailing edge, and
having generally similar contours whether used
tional bulbous entering er leading edge con
Whether 0f propellers or rotary wing systems.
Thev entering wedge airfoil having no appre
ciable stagnation area, as disclosed in Fig. 3, and
for fixed Wing purposes, or for the blades of rotors, 35 struction.
Such present day airfoil sections usually have
a greater Icamber in the upper surface A than is
present inthe lower surface B in order to pro
duce, by means of the greater acceleration of the 40
air flow over the upper surface of the blunt lead- .
ing edge C the aero-dynamic phenomenon kno-wn
asA “top surface lift.” Thisfis brought about
through the creation above the airfoil of an area
of diminished pressure resulting from the rela
tively higher velocity ofthe flow in this region.
In the operating ranges of conventional airfoils
the air adjacent to the surfaces, known as the
“boundary layer” experiences a-transition from
the related figures, can be so treated as to form
the >theoretically vsubstantiallyideal airfoil for
`sonic andsuper-sonic speeds if the trailing edge
drag can beeliminated or sufficiently minimized.
The-diagrammatic illustration ~comprising Fig. 4
indicates the solution to the trailing edge drag
accordingto one phase of this invention.
Thus
D' an entering wedge airfoil I4 having the upper
l and lower surfaces respectively I5 and I6, meet
ing in 'the entering edge line I'I, has the rear
blunt trailing edge section I8, which is, however,
either open preferably, although not necessarily
the laminar form to that of turbulence in the ' 50 for the entirethickness of the airfoil, or for such
portions of the trailing edge as to enable a jet
region indicated in general at T in Fig. l.
Referring to Fig. 3 there is disclosed a Wedge
or plurality or multiplicity of jets of more or less
shaped _airfoil 9 exemplifying 4certain phases of
compressed fluid discharge rearwardly, of sub
the invention according to one portion of the
stantially the same sectional thickness vertically
problem. The airfoil shown in diagrammatic. 55 as the rear-for kphysical trailing edge or‘trailing
edges of the airfoil, and having the same high
elevation isv of the sort which we have -designated
velocity, preferably, relative to the airfoil, as the
as “entering Wedge” and comprises the blunt
latter has relative to the airstream adjacent to
trailing edge surface I0 which'may be considered
the rear part of the airfoil.v Thus the various '
as of two spaced trailing edges and the upper and
degrees of bluntness typified by the several figures
lower relatively divergent surfaces II and I2 di
verging from leading edge to trailing edge, as 60 may have various sizes of fluid stream. Thus the
trailing >edges lll and I8_ of Figs. 3 and 4 respec
viewed meeting at the apex as a point or line at
tively may-have fa rearward fluid stream of the
I3 and the angular divergence of which is prefer
full width of and emergent «between such trailing
ably not appreciably greater than 30°. Surfaces
I I and` I 2 are each shown as slightly concave, but 65 edges. A The trailing edge portions 4, 5, and 6 of
either or both ofthe surfaces may be planar to
form a true Wedge in profile asindicated in'Fi'g.
3B, or, as indicated in Figs. 3A and 3C, the sur
faces II and I2 may be more or less convexed
either as independently curved surfaces as in Fig. 'f
3C, or as flattened arcs of circles as shown in Fig.
3A. It is tobe understood that for many sub
sonic, and even higherspeed ranges the airfoils
-of Figs. 3, `3A, 3B, and 3Cl ande have great im- k
Fig..3B_-ìmay have >a fluid stream ofthe Width of
wall B, While the rounded trailing edge 1 of Fig.
>3C andthe tapering edge 3 of Fig. 3A may have
comparatively narrow streams of rearwardly pro
ljected high velocity fluid emission.
, The jet or gaseous discharge preferably is led
suitably. into the airfoil, vas by ducts, passages,
channels, and the like as ,I9 from any desired
source of power (centrifugalenergy, in the case
pertence, as the relative airstream meets a mere ‘ 75 Yof rotors or -propellers) or any other gaseous or
1'1
àióavss
other fluid pressure and is then guided outwardly
in a rearward stream as at 20, which is projected
rearwardly with such velocity (due to its pres
sure) as to merge without appreciable relative
movement with the upper and lower airstreams
respectively 2l and 22, as at 23. It will be clear
that such rearwardly propelled thick stream of
nuid under pressure will in effect create a syn
thetic chordwise extension of the airfoil to form
an effective trailing edge having a small frictional
coefficient relative to the airstream flowing over
the airfoil, so that an ideal situation of no vis
cous or frictional drag, or of inappreciable drag
will be presented. It is further to be understood
that in the preferred embodiment the jet or other
gaseous emission will not be such as to have a
thrust effect, as it is desired that there be no
rounded entering edge at 26, and the conventional
acute trailing edge 2l, through thinner and thin
ner sections, of decreasing bulbosity, until at the
tip the entering edge has changed from the bulb
ous at 2S, to the line 28 of the wedge shaped tip
end 3E?s and the trailing edge has changed from
the conventional convergent to the blunt en
larged trailing edge 3| having the elongated oval
shaped aperture 32 of Fig. 11 containing a plu
rality of angular baffles 33 so shaped and pro
portioned as to divert the course of air moving
spanwise through the blade, to direct a rearward
stream of air under low pressure and high veloc
ity at the blunt trailing edge of the tip as indi
cated by the arrows. While the fluid pressure
may be derived from any desired extraneous
source it is an important part of this invention
appreciable relative movement between the air
to utilize the centrifugal force available from the
stream and the emitted air and therefore the
rotational energy of the rotor or propeller itself
“jet” emission is normally to be distinguished 20 by providing longitudinal passages through the
from “jet propulsion.”
lblade leading to the baflies 33, from suitable open
In the broadest aspects of the invention it is
to be understood that the combination of enter
ing wedge airfoil and synthetic frictionless air
foil trailing edge, while the theoretical ideal, does
ings ínto the blade closer to or in the hub thereof.
Illustratively in Fig. 5 a series of elongated lon
gitudinal slots 34 close to the hub of the rotor
are provided through which air can enter to
not need to be used as such a combination under
satisfy the pressure differential created by the
certain conditions. It has been mentioned that
the entering Wedge airfoil may be used with any
centrifugal force acting upon the air in the rotor
blade ,as it is flung outwardly through the open
type of trailing edge, varying from the blunt large
ings between the baffles 33. It will be evident
end trailing edge to the convergent line trailing 30 that with knowledge of the tip speed to be at
edge, as shown respectively in the Figs. 3, 4, 3A,
tained by the rotor orpropeller blade, the area
3B and 3C. It is to be understood that the jet
of tip to be treated for super-sonic conditions will
or other gaseous low pressure high velocity flow
be evident, and the pressure to be developed and
at the trailing edge as a means of minimizing
volume of the airflow to be flung out will be Ysub
drag may be used with a conventional or other ‘
ject to variation by the size and location of the
bulbous-ncsed airfoil section. It is further to be
inlet apertures 34 and their spaced relation from
contemplated that the rearward gaseous projec
the discharge jets 32 as well as the rotational
tion, althotgh as noted being preferably of the
speed of the blade. It is preferred and the rotor
same relative veiocity as the passing airstream to
is so designed as to have the air volume and veloc
minimize drag, if desired under certain condi 40 ity such as to permit super-sonic motion of the
tions, may be used at a higher velocity such as
Íblade tip without any of the adverse factors at
to combine with the creation of the synthetic
taching to the use of the conventional airfoils
trailing edge or the like, of small drag, the jet
at such speeds. The advantages from the rotor
propulsion of the airfoil in accordance with the
and propeller standpoints, with the ability to uti
aims of certain experimenters seeking to obviate 45 lize small diameters operating at high rotational
the torque reaction otherwise attaching to the
speeds and to obviate the weight of 'the larger
power driving of single rotors or the like. The
diameters with their attendant reduction gears
entering wedge associated with such' jet propul
to drive the propellers or rotors at engine speed
sion makes for a highly eñicient type of rotor,
or higher, will be obvious.
especially when the projected stream is of the 30
While there are many modiñcations of the
same thickness as the airfoil which is a further
rotor or propeller blade assembly that may be
departure from early experimenters, so far as
resorted to, as will be obvious, mention may be
known.
made of a few 'that incorporate additional prin
Although the entering wedge type of airfoil
ciples. In the modification of Fig. 12, the blade
thus described is of great interest for the several 55 35 having the tip end provided With the rear
types of airfoil uses, including fixed wings, it finds
wardly disposed jets fro-m the baliles or other
an immediate use of high utility in connection
directing means 3B, has the inlets 31 for the air
with rotors and propellers, owing to the ease
to be eentrifugally inducted arranged in a series
with which the tip ends thereof attain sub-sonic,
of apertures at any desired points in the sur
sonic and super-sonic speeds, necessitating heavy 60 faces of the blade, and illustratively in points of
and expensive reduction gears in combination
the surfaces such as to act as flow control suc
with larger diameter heavier propellers and ro
tion slots for removal of the turbulent boundary
tors and the like to obviate such excessive tip
layers extending over the inboard airfoil sur
speeds while maintaining propeller or rotor ef
faces. rlî‘hus, as shown in Figs. 12 and 13 slots
65 3'1 are provided in the upper surface 38, while
iiciency.
For this purpose reference may be made to the
additional inlets a little further forward com
disclosure of Fig. 5 and its related figures. This
municating with a duct separate from that lead
ñgure illustrates a blade of a rotatable type, either
ing to the upper surface due to the relative
of a rotor or propeller 24 having a spar or shank
pressure differentialsV may be provided in the
25 arranged for mounting in a hub (not shown), 70 lower surface 40 and 4|. These dispositions are
with which it rotates about an axis normal to
of course purely illustrative and the location of
the shank 25. As shown in the progressive Fig
the slots at other points .to secure desired results
ures 6, 7, 8, 9, and 10, the blade contour of the
may equally well be resortedto.
rotor changes progressively from the root to the
In Fig. 14 asimilar rearward surface for a
tip from a _conventional bulbous airfoil having Va 75 normal plan form airfoilblade is shown at 45
2,408,788
14
"
13
in which the section is not acute. It >has been
shown that the modification of the nose with
its stagnation areas by providing the wedge
shaped entering edge marks a great advance in
boundary layer control. Diagrammatic power
eñiciency for high speed flight in the super-sonic
rotor, owing to the fact that the blade moving
against the air stream has much greater relative
range.
There are other ways of accomplishing
analogous results.
source |22 is shown in the entering edge portion.
It will be clear to those skilled in the art that
with certain rotors, the effect of the compres
sibility burble may vary between blades of theA
k
velocity than that which is moving with the air
stream. It is desirable that the relative changes
posite airfoil shown in Figs. 15, 16 and i7, which
in speed should not upset the smooth operation of
include the provision of the exit gap of substan 10 the invention as it affects the tip constructions.
tial or effective wing slots, may be summarized
As the relative direction of the blade, whether
as follows: As the air acquires high velocity in
into or with the airstream in the ordinary ilap
ñowing through the slot exit gaps due to the
ping rotor is accompanied by changes in blade
nozzle or slot conñnement effect, an unbroken
angle about the flapping aXis or relative change
15
flow over the surfaces of the main wing is thus
of blade incidence a simple solution to the prob
maintained which materially assists in improv
lem of differential rotor speeds is secured by the
ing the high speed characteristics of the com
device shown in Figs. 18, 19 and 20. A blade
posite airfoil.
~
|25 vhaving slots or the like in accordance with
In Fig. l5 a composite airfoil is disclosed in
the earlier discussion admitting air to the inte
which a forward main airfoil entering edge ele
rior of the blade has a tip end comprised of mov
ment »91 has an open wide trailing edge airfoil
able or bendable surfaces respectively |26 and
The advantages of the several forms of com
portion |0|, of smaller proñle thickness than
the forward part 91, having an entering edge
portion |112 within the spaced trailing edges 98
and, mi?.
|21. A link |28 pivoted to surface |26, is pivoted
, also to the link |30 which is pivoted to the sur
face |21. A bell crank lever has one arm |3|
25
A suitable source of air pressure |03
pivoted to the common center of the two links,
is provided by means of which air at high velocity,
equaling that ñowing over the trailing edges 98
and |60, flows out through the slots |04 and |05
and the other _arm engages a link |32 extended
beyond the end of the blade toward the hub to
a transverse pivot |33 eccentric to the pivot |34
to establish and maintain laminar flow over the
upon which the blade is ilappingly pivoted. Ob
rear airfoil section and to preclude any appre 30 viously as the blade rises and falls the link |32
ciable viscous drag building up over the after
is moved outwardly and inwardly,vactuating the
part of the composite airfoil.
bell crank to exert force upon the links |28 and
In Fig. 16 a further modified form of airfoil
|49 like a toggle, to open the trailing edge to
is disclosed formed by taking the entire airfoil
of Fig. l5, and adding to it a substantially wedge 35 permit the rearward emission of the air flung
centrifugally or otherwise from the blade, or to
shaped entering edge portion |06 forming slots
close the trailing edge to preclude the >emission
the
entering
edge
of
the
, |01 and |08 relative to
of such `air as shown in Fig, 19. Application of
secondary airfoil element H0. The latter 'has
similar provisions for other types of rotors will
the rear spaced trailing edge lips Hl and H2
defining slots H3 and H4 relative to the rear 40 be obvious.
wardly convergent trailing edge portion H5. A
source of air pressure or the like H6 furnishes
power to cause smooth laminar flow over portion
H0, and an analogous source of pressure and
power H1 functions relative to the slots H3 and
H4, as has been described of the slots |04 and
We claim as our invention:
An airfoil for rotor use comprising a blade
the tip end region only of which has a wedge
shaped entering edge and an open trailing edge,
intake means on the blade spaced hubwardly
from the tip, and the whole so arranged that
air is drawn into the intake and centrifugally
S65 in Fig. l5.
f
flung outwardly and rearwardly of the tip end
A similar composite airfoil is disclosed in Fig.
only to minimize drag at critical compressibility
17, combining the wedge shaped entering edge
speeds attained byV said tip, means controlling
50
portion H9, the flared rear edges of which form
the open trailing- edge and responsive to blade
slots H8 and |29 relative to the open front of
movement to regulate the said open trailing edge.
the main airfoil element |2|. Sources of energy
CHARLES TOWNSEND LUDINGTON.
in either or both of the confronting portion will
ROGER W. GRISWOLD, II.
cause such jet or slot emission as to secure
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