Note: Descriptions are shown in the official language in which they were submitted.
CA 02718026 2010-10-19
05200801-19CA
VIBRATION-BASED ICE PROTECTION SLEEVE
TECHNICAL FIELD
[0001] The invention relates to the protection of structures against icing.
More
specifically the invention relates to the protection of structures against
icing using
mechanical vibrations.
BACKGROUND
[0002] Icing of structures, i.e. the setting of freezing water or other
contaminants
such as snow or slush under icing conditions has been a pervasive problem in
cold
climates. It adds unbalanced structural weight to the iced structure which may
cause damage or collapsing of the structure. Such ice is also prone to
detaching at
unpredictable times which, when falling, may harm people, properties or other
surrounding structures or equipment. Icing of vehicles typically presents the
most
hazardous effects. Icing of land vehicles adds weight and may reduce
visibility or
affect control. Ice accumulation on sea vessels adds weight which may cause
listing or capsizing if ice is unbalanced across the ship. Icing of aircraft
can lead to
a catastrophic failure of control or flight abilities due to increased weight,
airflow
disruption, and detached ice impacting on critical surfaces or equipment. In
all
cases there would be a definitive improvement in safety and efficiency if the
structures and vehicles were protected from icing.
[0003] The issue of ice protection is best illustrated in the case of small
rotorcrafts. Small rotorcrafts are structurally complex aircrafts which
require ice
protection to avoid catastrophic failure when flying under icing conditions.
Furthermore, small rotorcrafts have highly restrictive limits on available
weight and
power to accommodate any type of ice protection devices.
[0004] Many technologies have been applied in an attempt to protect structures
against icing. Electrothermal de-icing technology is widely used against
icing.
Electrical power is used to heat resistive wires which cover the surface
subject to
icing and melt the ice. Electrothermal ice protection requires large amounts
of
electrical power to protect any significant surface area. For small rotorcraft
applications, the power required to protect the rotor blades is prohibitive.
DOCSQUE:89138618 - 1 -
CA 02718026 2010-10-19
05200801-19CA
[0005] An ice protection device with relatively low power consumption using a
light and simple device would more effectively protect most ice prone surfaces
for
structures and vehicles. The use of such an ice protection device on small
rotorcrafts would also allow opening their flight envelope to use in icing
conditions.
SUMMARY
[0006] There is provided an ice protection sleeve for protecting a structure,
such
as a rotorblade, against icing. The sleeve comprises a shielding sheet to be
mounted and attached to cover at least a part to be protected of the
structure, the
leading edge of the rotorblade for example; a gap between the sheet and the
structure to allow vibration of the sheet; and actuators, such as
piezoelectric
actuators, coupled to the sheet for transmitting mechanical vibrations to the
sheet.
[0007] In accordance with one aspect, there is provided an ice protection
sleeve
for protecting a structure against icing. The sleeve comprises: a shielding
sheet to
be mounted and attached to cover at least a part to be protected of said
structure;
a gap between said sheet and said structure to allow vibration of said sheet;
and at
least one actuator coupled to said vibrating portion for transmitting
mechanical
vibrations to said sheet.
[0008] In accordance with one aspect, there is provided a method for
protecting
a structure against icing. The method comprises: attaching a shielding sheet
over
said structure to cover at least a part to be protected of said structure;
forming a
gap between said sheet and said structure to allow vibration of said sheet;
and
transmitting mechanical vibrations to said sheet to at least one of prevent
icing and
brake ice formed on said structure.
[0009] It is noted that while methods and devices are described herein in the
context of small rotorcraft blades, the provided methods and devices also
apply to
any other types of structures to be de-iced. For example, the provided methods
and devices may apply to other vehicles such as aircrafts, unmanned aerial
vehicles, ships, trains and other ground vehicles. It may also apply to
buildings,
infrastructures such as bridges, communication towers, wind turbines, power
line
towers, transformer boxes, satellite dishes and function specific structures
such as
oil platforms, drilling stations construction equipment, etc.
DOCSQUE: 89138618
-2-
CA 02718026 2010-10-19
05200801-19CA
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] Fig. 1 is a cross-sectional view of a blade shown with one embodiment
of
an ice protection sleeve mounted thereon wherein a gap between the sleeve and
the blade is empty;
[0011] Fig. 2 is a partial cross-sectional view of a blade shown with another
embodiment of an ice protection sleeve mounted thereon wherein the gap
between the sleeve and the blade is filled with an elastomer material;
[0012] Fig. 3 is a partial cross-sectional view of a blade shown with another
embodiment of an ice protection sleeve mounted thereon wherein the sleeve is
mounted in a hem-like manner on the blade; and
[0013] Fig. 4 is block diagram of a driving system for activating the sleeve
of
Fig. 1, Fig. 2 or Fig. 3, in accordance with one embodiment.
[0014] It will be noted that throughout the appended drawings, like features
are
identified by like reference numerals.
DETAILED DESCRIPTION
[0015] Now referring to the drawings, Fig. 1 shows a cross-section of a blade
10, such as a rotorcraft blade, shown with one embodiment of an ice protection
sleeve 12 installed thereon. It is noted that Fig. 1 and others are schematic
only
and that the relative dimensions of the various elements of the sleeve 12 are
exaggerated for better illustration.
[0016] The ice protection sleeve 12 is typically installed on the portion of
the
blade 10 that is the most subject to icing, i.e. the leading edge portion 14
of the
blade 10. The ice protection sleeve 12 comprises a shielding sheet 16 which is
a
sheet of a rigidly elastic material, such as a metal sheet, that is bent or
otherwise
molded to conform to the underlying shape of the blade 10. The shield sheet 16
is
mounted and attached to the blade 10 using fastening means 18 such that it
covers the leading edge portion 14 of the blade 10, from the bottom to the top
of
the blade 10 and over most of the length of the blade 10. The shield sheet 16
has
an outer surface 20 which is exposed to icing conditions while protecting the
DOCSQUE: 89138618
-3-
CA 02718026 2010-10-19
05200801-19CA
leading edge 14 of the blade 10 against ice formation thereon, and inner
surface
22. As described hereinafter, de-icing of the shield sheet 16 is made using
vibration. The shield sheet 16 is mounted to the blade 10 in a manner to leave
a
gap 24 between the inner surface 22 of the sheet 16 and the blade 10 to
mechanically isolate the sheet 16 from the blade 10 to allow vibration of the
sheet
16. In this embodiment, the gap 24 is empty, i.e. filled with air and thereby
allows
for free vibration of the sheet 16. Piezoelectric actuators 26 are bounded or
otherwise coupled to the inner surface 22 of the sheet in order to induce
vibration
to the sheet 16.
[0017] The sheet 16 has two attaching portions 28 and a vibrating portion 30.
One attaching portion 28 is located on each of both longitudinal edges 32 of
the
sheet 16 that extend longitudinally to the blade 10, i.e. a first attaching
portion 28
attaches to the bottom side of the blade 16 while the other attaches to the
top side
of the blade 16. The vibrating portion 30 extends between the two attaching
portions 28 and covers the leading edge portion 14 of the blade 10.
[0018] The sheet 16 is mounted to the blade 10 at discrete points using bolts
or
other fastening means 18 on the attaching portions 28 only of the sheet 16 in
a
manner to form a gap between the vibration portion 30 of the sheet 16 and the
blade 10, to allow free movement of the vibrating portion 30 relative to the
blade
10. In this embodiment, the sheet 16 is fastened using a line of bolts that
are
uniformly distributed along each attaching portion 28. The fastening means 18
comprise spacers 34 inserted between the blade 10 and the sheet 16 in order to
space the sheet 16 from the blade 10 and form the gap 24.
[0019] The gap 24 should generally be sufficiently large to allow the sheet 16
to
vibrate without reaching the outside surface of the blade 10 in its full
vibration
amplitude. Accordingly, the gap 24 typically has a width of about half the
vibration
amplitude of the sheet 16. A gap 24 of about 1 to 2 mm is typically
sufficient.
[0020] In order to induce vibration to the vibrating portion 30 of the sheet
16, the
actuators 26 are positioned on the vibrating portion 30, next to the attaching
portion 28. At both ends of the vibrating portion 30, actuators 26 are
uniformly
distributed along a line which extends longitudinally to the blade 10. In this
DOCSQUE 89138818
-4-
CA 02718026 2010-10-19
05200801-19CA
configuration, the actuators 26 are all outside the portion of the blade 10
that is the
subject to icing. This provides a proper mechanical transmission of the
vibrations
along the vibrating portion 30. It also avoids melting of ice caused by
piezoelectric
activation. Such ice melting is undesirable because it may lead to runback
freezing
or may impede de-icing due to a layer of water which then forms between the
layer
of ice and the sheet 16. Other arrangements of the actuators are also
possible. For
example, in another embodiment, additional actuators are distributed all over
the
inner surface 22 of the vibrating portion 30.
[0021] The piezoelectric actuators 26 are driven using drive signals generated
by a driving system as described hereinafter. Frequencies in the sonic range
are
used to drive the actuators 26 in order to produce a vibration which transmits
in the
vibrating portion 30 of the sheet 16 to induce a ripple-like strain capable of
braking
ice formed or in formation on the outside surface 20 of the sheet 16.
[0022] The sheet 16 is typically made of a metallic material. Metallic
material
has elasticity properties which allow proper transmission of vibrations along
the
surface. The material of the sheet should be selected according to its
rigidity. If the
shield is too rigid then effective de-icing modes occur at high vibration
frequencies
which increase the electrical power requirement on the driving signals of the
actuators 26. If the shield has a very low rigidity then vibration is not
efficiently
transmitted along the sheet 16 and strain is induced only on areas very close
to
the actuators 26. A rigid material in elastic regime allows for transmission
of
mechanical vibrations throughout the vibration portion 30 of the sheet 16,
with
effective de-icing mode frequencies that remain sufficiently low for
acceptable
power requirement. It is noted that the rigidity of the sheet 16 may be
tailored by
varying its thickness or by choosing material with varied ductility and/or
hardness.
A Young modulus that is similar to or higher than that of ice is typically
suitable.
The use of a sheet over the structure to be protected against icing allows for
protecting structures made of any material, irrespective of their elasticity.
[0023] For the piezoelectric actuators 26 to be efficient at generating
vibration of
the sheet 16 at effective de-icing mode frequencies, the selection of the
piezoelectric actuators should consider their material and their thickness.
The
piezoelectric material should have a high coupling factor and a high charge
DOCSQUE: 891380
-5-
CA 02718026 2010-10-19
05200801-19CA
constant in order to produce large amplitude vibrations and high strains at
effective
de-icing mode frequencies. The loss coefficient should also be minimized to
reduce the conversion of electrical energy into heat which may create
localized
melting of ice at and around the actuators 26. Lead zirconate titanate
piezoelectric
materials known under the name PZT-5 offer a good balance of effective factors
and may be used, among others, in the actuators 26. The thickness of the
piezoelectric actuators 26 across the electrodes should be balanced with the
capabilities of the electrical system. A thicker piezoelectric actuator 26
generates
more force for actuating the vibration of the sheet 16, thereby providing more
effective de-icing. However, increasing the thickness of the piezoelectric
actuator
26 increases the voltage requirement to achieve the required displacement.
[0024] In one embodiment, the required actuation voltage level is reduced by
the stacking piezoelectric devices in each piezoelectric actuator 26. Large
vehicles
or structural applications which have more available power may use thicker
piezoelectric devices or larger piezoelectric stacks to create a more robust
de-icing
system while more optimization may be required on the piezoelectric actuator
thickness and sheet material and thickness for small vehicles and applications
with
power restrictions.
[0025] Fig. 2 shows a partial cross-section of a blade 10 shown with another
embodiment of an ice protection sleeve 38 mounted thereon. The ice protection
sleeve 38 is similar to that of Fig. 1 but has a layer of elastomer material
40, such
as rubber or polyurethane for example, filling the gap 24. The layer of
elastomer
material 40 is bonded to the outside surface of the blade 10 and the sheet 16
is
also bonded on top of the elastomer material 40. In this embodiment, no
further
fastening means is being used. However, the sheet 16 may also be fastened to
the
blade 10 as described herein with reference to Fig. 1. Yet in another
embodiment,
the layer of elastomer material 40 may also be bonded to an additional surface
which is detachable from the blade 10 for service of the sleeve 38 or for
removal
when flying in non-icing conditions.
[0026] At least one open space 42 is typically provided within the elastomer
material 40 in order to accommodate the piezoelectric actuators 26 such that a
void remains between each piezoelectric actuator 26 and the outside surface of
DOCSQUE: 891386/8
-6-
CA 02718026 2010-10-19
05200801-19CA
the blade 10 such that the actuators 26 are free and in no contact with both
the
layer of elastomer material 40 and the blade 10. The layer of elastomer
material 40
allows movement of the sheet 16 without significant constraint.
[0027] Fig. 3 shows a partial cross-section of a blade 10 shown with another
embodiment of an ice protection sleeve 50 mounted thereon. The ice protection
sleeve 38 is also similar to that of Fig. 1 except for the fastening thereof
on the
blade 10. In this embodiment, the edge of the attaching portion 52 of the
sleeve 16
is curved inwardly and back under the sheet 16 and toward the blade 10 so as
to
form a hook-like hem 54. The section of the attaching portion 52 which is next
its
edge thus defines an inner panel 56 which is used to bolt or otherwise
fastened
the sheet 16 on the blade 10 using fastening means 18. The hook-like hem 54
also
spaces the sheet 16 from the blade 10 so as to form a gap 24 in-between. The
sleeve 50 further comprises an anti-icing coating 58 on the outside surface of
the
sheet 16 to slow down the icing of the sheet 16 under icing conditions. Of
course,
such an anti-icing coating may also be provided on the sleeve 12 of Fig. 1 or
the
sleeve 38 of Fig. 2.
[0028] Depending of application needs, suitable coatings may include low
adhesion coatings such as Wearlon , nanostructured superhydrophobic coatings
and chemically semi-active coatings such as PhasebreakTM.
[0029] Fig. 4 shows a driving system 60 for generating the drive signals to
drive
the actuators 26. The driving system 60 has a function generator 62 for
generating
the alternative signal of various frequencies used to drive the actuators, a
power
amplifier 64 to amplify the alternative signal to the required power and an
output
transformer 64 to multiply the voltage of the amplified alternative signal and
generate the drive signals for the actuators. In one embodiment, the function
generator 62 comprises a function generator of the model 33220A by AGILENT,
the power amplifier 64 comprises an amplifier of the model AL-1000-HF-A by
AMP-LINE and the output transformer 64 is a 14:1 transformer including a power
source of the model AL-10ODC by AMP-LINE for offsetting the signal.
[0030] The function generator 62 is capable of generating proper alternative
voltages with the frequencies in the acoustic ranges and comprises a frequency
DOCSWE: 89138618
-7-
CA 02718026 2010-10-19
05200801-19CA
sweeping mode for sweeping the frequency of the drive signals between de-icing
vibration modes of the sleeve.
[0031] Other driving systems may be used but it should be kept in mind that
sufficient voltage amplitude should be generated in order to obtain the
required
strain amplitude in the sheet 12 at the de-icing frequency modes, for de-icing
its
outside surface 20. The driving system 60 generates high enough voltage across
the whole frequency range.
[0032] It is noted that the effective de-icing mode frequencies vary with the
configuration of the sleeve and with the various effects of ice accretion on
the
sleeve. The frequency sweep range is thus typically chosen so as to cover the
variation range of the effective de-icing modes. The effective de-icing mode
frequencies are those which are shown to be effective on testing. Modal or de-
icing tests are generally performed on the sleeve in operation before mass
production. The effective de-icing modes are generally those past the first
two
vibration modes since complex modal motions that include all three surface
movements of bending, extension and torsion are more effective at de-icing.
The
effective frequencies for the ice protection sleeves described herein
typically fall in
the sonic range, i.e. from about I to 20 000 Hz. It is noted that de-icing is
also
possible with frequencies above 20 000 Hz with a compromise on an increased
power required to drive the actuators. It is also noted that it is also
possible to
make modifications to the effective de-icing mode frequencies of a sleeve by
changing the geometry of the sleeve such that the frequency range remains
within
the capabilities of the driving system. For example, the effective de-icing
mode
frequencies may be lowered by increasing the thickness of the shielding sheet.
[0033] The frequency sweep of the driving signals is made at a slow enough
rate to allow modal resonance to be activated before the frequency is swept to
far
away from the resonance frequency. The ideal frequency sweep rate of a
particular sleeve may be determined using tuning procedures. A sweep rate of
about 1-Hz over a 10-kHz bandwidth typically provides enough time for the
modal
resonances to be activated while keeping the de-icing time below one second.
DOCSQUE 8913864
-8-
CA 02718026 2010-10-19
05200801-19CA
[0034] In one embodiment, all actuators are activated at the same time.
However, in another embodiment, the actuators are activated in sequence to
limit
the peak power requirement on the driving system 60. An example of such a
sequence starts from the inboard portion of the blade 10 and progresses down
the
blade 10 in an over lapping leapfrog type pattern, i.e. one actuator 26 keeps
sweeping while the one behind it is deactivated and the one ahead of it is
activated. Such a sequence is generally paired with the same sequence on the
opposing rotor blade for balanced ice shedding. The sequential operation
provides
smooth de-icing progression along the blade while keeping electrical power
requirements within small rotorcraft capabilities.
[0035] An example design of an ice protection sleeve adapted for small
rotorcraft blades of the type NACA 0012 is now described in more details. This
example is based on the embodiment of, and is therefore described with
reference
to, Fig. 2.
[0036] In this example, the sheet 16 is a sheet of aluminum having a thickness
of 0.5 mm. The sheet is shaped to conform to the shape of the profile of the
leading edge portion 14 of the blade 10 such that it covers the ice prone area
of
the blade 10. The sheet 16 covers the upper and the lower surface of the blade
10
from the leading edge portion 14 back to at least 20 % of the total chord
length of
the blade 10. The layer of elastomer material 40 is a 1 mm thick layer of
supple
rubber. A plurality of piezoelectric actuators 26 are bonded on the inside
surface of
the sheet 16 at evenly spaced intervals of 200 mm as far back from the leading
edge portion 14 as possible on both the upper and lower surface of the blade
10.
Rubber is removed at actuator positions for free mechanical vibration of the
actuators 26. Each actuator 26 is a PZT-5 piezoelectric wafer with dimensions
of
50 mm x 30 mm and a thickness of 0.5 nom. The actuators 26 are connected to a
driving system such as the one described herein with reference to Fig. 4. In
this
case, the frequency of the driving signals sweeps from 1 kHz to 10 kHz at a
sweep
rate of 1 Hz. In such an example embodiment, strain amplitudes of up to 0.0005
are produced within the sheet 16 at effective de-icing modes within the sweep
range, which is typically sufficient to achieve effective de-icing.
DOCSWE: 89138618
-9-
CA 02718026 2010-10-19
05200801-19CA
[0037] It should be understood that many changes may be made to the devices
and methods described herein. For example, the gap 24 may be filled with a
fluid
instead of an elastomer material. Also, while the embodiments described herein
use piezoelectric actuators for generating mechanical vibrations in the sheet,
other
types of actuators may be used such as magnetostrictive actuators for example.
The embodiments described above are intended to be exemplary only. The scope
of the invention is therefore intended to be limited solely by the appended
claims.
DOCSWE 89138618 -lo-
