CA2718026A1

CA2718026A1 - Vibration-based ice protection sleeve

Info

Publication number: CA2718026A1
Application number: CA 2718026
Authority: CA
Inventors: Jean Perron; Caroline Laforte; Peter Quinlan; David G. Zimcik
Original assignee: National Research Council of Canada; Universite du Quebec a Chicoutimi UQAC
Current assignee: National Research Council of Canada; Universite du Quebec a Chicoutimi UQAC
Priority date: 2010-10-19
Filing date: 2010-10-19
Publication date: 2012-04-19
Also published as: EP2630423A1; WO2012051717A1; CA2770812C; EP2630423A4; CA2770812A1

Abstract

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.

Description

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 -[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 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 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 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 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 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 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.

[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 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 [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-

Claims

1. An ice protection sleeve for protecting a structure against icing, the sleeve comprising:

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 sheet for transmitting mechanical vibrations to said sheet.

2. The sleeve as claimed in claim 1, wherein all said at least one actuator are located outside a portion of the structure that is subject to icing such that there is no actuator on said portion of the structure that is subject to icing.

3. The sleeve as claimed in claim 1 or 2, wherein said actuator is a piezoelectric actuator.

4. The sleeve as claimed in claim any one of claims 1 to 3, wherein said gap is a void.

5. The sleeve as claimed in any one of claims 1 to 3, wherein said gap is filled with an elastomer material which allows vibration of said vibrating portion of said sheet.

6. The sleeve as claimed in any one of claims 1 to 5, wherein said sheet is made of a rigid material having a rigidity allowing transmission of mechanical vibrations in elastic regime.

7. The sleeve as claimed in any one of claims 1 to 6, wherein said sheet is made of a metallic material.

8. The sleeve as claimed in claim 7, wherein said sheet is made of aluminum.

9. The sleeve as claimed in any one of claims 1 to 8, further comprising an anti-icing coating on an outside surface of said sheet.

10. The sleeve as claimed in any one of claims 1 to 9, wherein said structure is a blade.

11. The sleeve as claimed in claim 10, wherein said blade is a rotorcraft blade.

12. The sleeve as claimed in any one of claims 1 to 11, further comprising a driving module for generating drive signals for said actuator, said driving module having a function generator with a frequency sweeping mode for sweeping a frequency of said drive signals between vibration modes of said sleeve.

13. A method for protecting a structure against icing, the method comprising:
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.

14. The method as claimed in claim 13, wherein said transmitting mechanical vibrations comprises actuating a plurality of actuators located outside a portion of the structure that is subject to icing and transmitting said mechanical vibrations toward said a portion of said sheet that covers said portion of the structure that is subject to icing.

15. The method as claimed in claim 13 or 14, wherein said transmitting mechanical vibrations comprises generating vibrations at a sonic frequency.

16. The method as claimed in any one of claims 13 to 15, wherein said transmitting mechanical vibrations comprises sweeping a frequency of said mechanical vibrations between a plurality or vibration modes of said shielding sheet.

17. The method as claimed in any one of claims 13 to 16, wherein said transmitting mechanical vibrations comprises sequentially actuating a plurality of actuators distributed longitudinally on said structure.

18. The method as claimed in any one of claims 13 to 17, wherein said transmitting mechanical vibrations comprises actuating at least one piezoelectric actuator coupled to said sheet.

19. The method as claimed in any one of claims 13 to 18, wherein said forming a gap comprises leaving a void between said sheet and said structure.

20. The method as claimed in any one of claims 13 to 18, wherein said forming a gap comprises inserting an elastomer material which allows vibration of said vibrating portion, between said sheet and said structure.