Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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I'his invention rela-teS to propeller fan blades of con-
stant inclination angle, such as are used, for example, in
cooling fans.
Typically propeller fans are composed of two or more
blades rigidly attached to a central hub for rotation. The
inclination angle of the blade is the angle between the ro-
tational axis of the hub and a tangent to the surface of the
blade. Blades of constant inclination angle along their
length are,essentially, flat, and are also known as "non-
twisted" blades.
It is the object of -this invention to provide an improved
blade for a propeller fan which blade has a constant inclina-
tion angle along its entire length, and which blade presents
tabs along its trailing edge, which tabs have suitable dimen-
sions and orientation, particularly in the axially inward
blade zones, which zones have, during rotation, a lower
circumferential speed. In this way an advantageous increase
in the effective displacement of air of a propeller fan
having blades of constant inclination can be obtained, in
particular at the blade zones with lower circumferential
speed.
At present the increase of a fan's effective displacement
of air at the blade zones with lower circumferential speed is
achieved by means of one of the following methods:
(a) Increase of the blade's chord (width) proceeding
from the distal (axially outward) end to the blade's proximal
(axially inward) root.
~; ~
~:a~
(b) Blade's twist l=increase of the geometrical incid-
ence angle of the bladels sections, proceeding from the distal
end to the paddle's root).
(c) Combination of the methods mentioned in (a) and (b).
Such techniques, which are used at present, exhibit the follow-
ing difficulties:
(1) The chord change of the blade's profile is not
feasible on massive paddles, manufactured by extrusion, and
is complex with hollow paddles particularly if reinforced.
(2) The twist of blades made of plastic material and
obtained by extrusion is not feasible.
(~) The twist of blades made of light alloy and obtained
by`extrusion, shows the following inconveniences:
-- necessity to exceed the yield point of the material;
-- practical impossibility to reach suitable twist values
(particularly on blades having shell section and low
elongation ratio, defined as the ratio of the blade's
length to the chord) without causing damage to the
paddle itself.
The present invention avoids the above mentioned diffi-
culties and provides other advantages, which will be appre-
ciated by those skilled in the art, as specified in the
following description.
According to the present invention, an increase of the
lift coefficient of the blade profile (and consequently an
increase of the effective displacement of air~ of a propeller
fan, whose blades have a constant inclination angle, can be
achieved by means of tabs having suïtable wIdth and inclina-
tion, fastened at the trailing edge of the blades themselves.
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Such increase depends on the width of the tab and on its
inclination in relation to the profile chord
Cornbining suitably the width of the tab with its inclina-
tion, increments of the effective displacement of air by the
blades can be achieved at the blade zones with lower circumfer-
encial speed, which are equal or higher in comparison with
the increments obtained by means of blade twist.
Under the same performance, the overall efficiency of the
propeller fan equipped with the proposed devices (tabs) is
equal or even slightly higher than known propeller fans with
twisted blades.
According to the invention there is provided a propeller
fan blade having a trailing edge and a substantially constant
inclination angle along its entire length comprising a rigid
tab secured to the trailing edge at an incidence angle to
the blade of between 10 and 70, said tab having a maximum
width at the proximal end of the blade and a minimum width at
the distal end of the blade, said maximum width being not
greater than 30~ of the width of the blade.
Figure 1 is a graph showing the change of the lift co-
efficient CL of a blade profile with and without tab,
respectively A and B curves, as a function of the aerodynamic
incidence angle of the profile.
Figure 2 is a graph showing the change of the mentioned
coefficient CL as a function of the tab's width expressed as
percentage of the profile chord.
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llti~
Figure 3 is a perspective view of one embodiment of blade
according to the present invention.
Figures 4a and b are perspective views of further embodi-
ments of the present invention.
Figures 5a and b are plan views of two further embodiments
of the present invention.
Figure 6 is a representative cross-section of the tabs
of the preceding Figures secured to the blade.
Figures 7a, b and c are cross-sectional views showing
alternative contours which the tab may take.
Figure 8 shows in cross-section an example of a method
of securing the tab to the blade.
Figure 9 is a perspective view of a further embodiment
of the present invention.
Figures lOa and b shows a further example of a blade and
tab, with figure lOa being a plan view thereof, and Figure lOb
being a cross-section taken along line A-A of Figure lOa.
Figure 11 is a graph showing the results of experimental
tests conducted on the blade of Figure 10 and two prior art
devic~s.
As can be seen in Figures lOa and lOb, blade 20 is secured
to hub 22.
Referring to Figures 3 through 9, the blade, generally
designated 20, is provided with a leading edge 24 and trailing
edge 26, and with proximal end or root 23 and distal end 25.
The leading and trailing edges are joined by an upper surface
28 and a lower convex surface 30. The lower surface 30 has a
greater curvature than that of the upper surface 28. Tab 32
is secured to the trailing edge 26, and extends upwardly and
outwardly from the trailing edge.
In the following description the alternative structures
of the tab will be examined.
The tab can be applied to the trailing edge 26 of the
blade 20 along its total length as in Figure 4a or along part
of its length as in Figure 4b.
The inclination of the tab 32 in relation to the profile
chord can be constant or variable along the blade's length.
The tab's width along the blade length can be variable
in a linear relationship (Fig.5a), in a non-linear relation-
ship (Fig.5b~ or in a combination thereof.
The tab can be formed simply by a bent plate (Fig.6).
The mean line of the tab can be straight (Fig.7a) or can
have negative or positive curvature (Figs.7b-7c), or can be
a combination thereof.
The constraint between tab and blade can be constructed
in such a way that the following options are possible-
(a) the incidence angle between the tab and the bladeis fixed and not adjustable;
(b) the incidence angle between the tab and the blade is
adjustable when the fan is not running;
(c) the incidence angle between the tab and the paddle
is adjustable when the fan is running.
The tab 32 can be fastened to the blade 20 by means of:
(a) glueing, rïveting, bolting, welding, dovetailing
and similar methods (see, for example, Fig. 8);
(b) by a combination of two or more of the above mentioned
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methods;
(c) the tab 32 can be formed also by removal of part
(shown in phantom lines1 of the trailing edge 26 of a blade
20, in which case the tab is an integral part of the blade
itself (Fig. 9).
The inclination angle of the tab 32 in relation to the
profile chord can vary between 10 and 70; the desired effect
is not achieved when the angle is less than lO and more than
70, while optimum results are obtained around 40.
The width of the tab can vary from 0% to 30% ( and above,
in particular cases) of the blade's width.
EXPERIMENT
The following description, with reference to Figs. lQ and
ll further illustrates a particular embodiment of the invention
and describes an experiment carried out with this embodiment.
Figure lO gives a schematic representation of the hub 22
and of one of the four blades 20 of the propeller fan used in
the experiment.
The characteristics of the fan are as follows:
r = length l,65 m
d = diameter of the hub 0,3 m
c = blade chord 0,3 m
s = maximum width of the tabs 0,070 m
l = length of the tabs l,0
B = connection angle of the tabs 45
n = revolutions per minute 3~2
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The results of the experiment are reported in the graph
of Fig. 11 with the radius r as abscissa and the effective
displacement of air per opening unit (kg/m) as ordinate.
In the graph three curves are illustrated, which curves
show the trend of the effective displacement per opening unit
along the radius r, in the following conditions:
curve A : blade not twisted, without tab;
curve B : twisted blade, without tab;
curve C : blade not twisted, with tab.
The increase of the effective displacement shown by
curve C, which represents the blade of the present invention,
is evident.
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