GB2189204A

GB2189204A - Titanium alloy erosion shield for aerofoil

Info

Publication number: GB2189204A
Application number: GB08708226A
Authority: GB
Inventors: John Thomas Macgregor; Jack Spooncer
Original assignee: Westland Group Plc
Current assignee: Westland Group Plc
Priority date: 1986-04-17
Filing date: 1987-04-07
Publication date: 1987-10-21
Also published as: FR2597508A1; DE3712705A1; IT8747853A0; GB2189204B; GB8708226D0; GB8609355D0; IT1205818B

Description

GB2189204A 1

SPECIFICATION

Erosion shields for aerofoil surfaces THIS INVENTION relates to erosion shields for aerofoil surfaces and is more particularly con- 5 cerned with erosion shields formed from titanium alloy and with a method of manufacturing such erosion shields.

It is known to protect the leading edge surfaces of aerofoil members, such as wings or helicopter rotor blades, against erosion by bonding onto the leading edge surfaces erosion shields formed from a suitable metal such as stainless steel or titanium. G13A-1585130 dis- 10 closes a helicopter rotor blade manufactured from fibre reinforced plastics material fitted with such an erosion shield. For composite rotor blades we have, in practice, used commercially pure - titanium having a mean ultimate tensile strength in the order of 570 Mega Pascals (MPa) to form the erosion shield protecting the outboard or tip end length of the blade where it is required to make little contribution to the structural stength of the blade, whereas, along the leading edge at 15 the inboard or root end length of the blade, where the erosion shield makes a significant contribution to the structural strength of the blade, we have used an erosion shield which is hot formed from a titanium alloy containing 6 per cent aluminium and 4 per cent vanadium (hereinafter referred to as Ti-10 alloy) and having a mean ultimate tensile strength in the order of 900 MPa. 20 Ti-10 alloy is both expensive and difficult to obtain because at the present time the main source of supply is in the U.S.A. and there is considerable demand for the alloy within the U.S.A.

Also, it is important that the cross-sectional shape of the erosion shield be accurately formed so that it can be bonded to the leading edge surface of the blade without excessive pressure 25 having to be applied to negate the presence of voids between the erosion shield and the surface to which it is bonded. In order to obtain the cross-sectional shape of the erosion shield with the required degree of accuracy in Ti-10 alloy it must be hot formed.

Furthermore, Ti-10 alloy has a relatively high modulus of elasticity compared with the compo site material of a composite helicopter main rotor blade or other composite aerofoil member to 30 which it is bonded and under load this causes high strains at the bond line between the erosion shield and the surface to which it is bonded.

It is an object of the present invention to provide an erosion shield manufactured from a cold formable titanium alloy which is less expensive and more readily obtained than Ti-10 alloy whilst having a modulus of elasticity closer to that of a fibre-reinforced composite material aerofoil 35 surface to which the erosion shield is to be bonded.

Accordingly, in its broadest aspect, the present invention provides, for an aerofoil surface, an erosion shield manufactured from a cold formable titanium alloy comprising substantially 15 per cent vanadium, 3 per cent chronium, 3 per cent tin and 3 per cent aluminium with the remainder being substantially titanium. 40 Percentage figures used herein are all percentages by weight.

Whilst an erosion shield in accordance with the present invention may be manufactured by bolster press forming it is preferably manufactured by a method which includes one or more stretch forming operations.

In another aspect the present invention provides a method for manufacturing by cold forming 45 an erosion shield for an aerofoil surface from a sheet of titanium alloy comprising substantially per cent vanadium, 3 per cent chromium, 3 per cent tin and 3 per cent aluminium with the remainder substantially titanium comprising. the steps of, a. cutting to a required length a strip of said titanium alloy sheet; b. press forming the strip to a channel-section in transverse cross- section, the channelsection having a base portion with flanges upstanding therefrom; c. overbending the flanges beyond an angle of ninety degrees with respect to the base portion so as to allow for spring back and cause the flanges to be upstanding substantially normal to the base portion; d. turning over the top edge portion of each flange so that it projects inwardly towards the 55 opposite flange and is substantially parallel to the base portion; e. stretch forming the base portion to the desired erosion shield transverse cross-section; f. trimming off the flanges to obtain the finished erosion shield.

Preferably, said stretch forming comprises a first stretch forming operation in which said base portion is formed to an intermediate cross sectional shape and a second stretch forming 60 operation in which said base portion is formed to a finished cross sectional shape.

The titanium alloy may be cold formed in a solution annealed condition and the properties of the finished erosion shield may be enhanced by age hardening at a temperature between 400 and 600 degrees Celsius. Preferably, age hardening is accomplished by heating the erosion shield to a temperature between 533 and 543 degrees Celsius and holding at this temperature 65 2 GB2189204A 2 for eight hours followed by air cooling. The erosion shield may be supported in a support tool during age hardening so as to prevent distortion.

An erosion shield manufactured from said titanium alloy in accordance with the method of the present invention finds particular application in protecting the leading edge of a helicopter main rotor blade or tail rotor blade. 5 Accordingly, the present invention further provides a helicopter rotor blade manufactured from fibre-reinforced composite material having an erosion shield bonded to the blade so as to extend in a span wise direction along at least a part of the length of the blade leading edge characterised in that the erosion shield is manufactured from a cold formable titanium alloy comprising substantially 15 per cent vanadium, 3 per cent chromium, 3 per cent tin and 3 per cent 10 aluminium with the remainder substantially titanium.

In protecting the leading edge of a helicopter rotor blade manufactured from fibre-reinforced composite material a plurality of erosion shields may be bonded to the blade in end-to-end relationship throughout substantially the entire span-wise length of the leading edge.

The erosion shield or shields extending span-wise along an inboard region of the leading edge 15 may be age hardened after manufacture so as to increase its tensile properties whereby the erosion shield or shields contribute significantly towards the structural strength of the rotor blade.

The erosion shield or shields extending span-wise along the outboard region of the leading edge may have lower tensile properties than those in the inboard region and may be manufac- 20 tured from said cold formable titanium alloy in a solution treated condition.

The invention will now be described by way of example and with reference to the accompany ing drawings in which:

Figure 1 is a transverse cross-section through a strip of titanium alloy after a first pressing operation in a method for manufacturing an erosion shield in accordance with one embodiment 25 of the present invention; Figures 2 to 5 are transverse cross-sections after subsequent forming stages in the method of manufacturing the erosion shield; Figure 6 is a transverse cross-section through the finished erosion shield; and Figure 7 is a perspective view of a helicopter rotor blade having erosion shields manufactured 30 from titanium alloy bonded to the leading edge of the blade.

The titanium alloy used in the manufacture of erosion shields according to this invention has a chemical composition substantially within the limits set out in Table 1.

The alloy, which will hereinafter be referred to as Ti15-3 alloy, may be procured in sheet or strip form in a solution treated condition. In a solution treated and aged condition Ti-15-3 alloy 35 has tensile properties within the ranges set out in Table 2 below. Solution heat treatment is carried out by heating to a temperature within the range 790 to 815 degrees Celsius, holding at the required temperature within 10 degrees Celsius for 3 to 30 minutes, and cooling at a rate equivalent to air cooling.

Ageing is carried out by heating for 8 hours in an electric aircirculating furnace or salt bath at 40 538 degree Celsius 5 degrees Celsius and then cooling in air to room temperature.

TABLE 1

ELEMENT WEIGHT % Min Max 45 Vanadium 14.0 16.0 Chromium 2.5 3.5 Aluminium 2.5 3.5 Tin 2.5 3.5 Iron - 0.25 50 Oxygen - 0.13 Carbon - 0.05 Nitrogen 0.05 Hydrogen - 0.015 Residual Element (Each) - 0.10 55 Residual Elements (Total) - 0.40 Titanium REMAINDER 3 GB2189204A 3 TABLE 2

Heat Nominal Tensile Properties Treatment Thickness 0.2% Proof Stress-Tensile Strength-Modulus Condition (mm) (MPa) (GPa) 5 Min Max Min Max Min Max Solution treated:53.2 690 800 720 850 93 10 Solution treated & -53.2 970 1100 1050 1200 93 aged 15 A method of manufacture in accordance with one embodiment of the invention of an erosion shield for attachment by bonding to the leading edge of an aerofoil member, for example a helicopter rotor blade having an asymmetric aerofoil shape in transverse cross-section, will now be described with reference to the accompanying drawings.

In a first step of the manufacturing method a strip of Ti-15-3 alloy in the solution treated 20 condition and cut to required width and length dimensions is placed in a suitable tool and formed by an upstroke pressing operation to a transverse cross-section shape approaching that shown in Fig. 1 which comprises a channel-shaped section having a base portion 10 and flanges 11 and 12 extending normal to the base portion 10. However, due to spring- back exhibited by the Ti-15-3 alloy the flanges will tend to spring outwardly away from each other so that in a 25 second step of the method they are over bent beyond an angle of ninety degrees with respect to the base portion, as shown by the broken lines in Fig. 2, in a brake press operation to obtain the channel section shown in Fig. 1.

In a third step of the method top edge portions 13 and 14 of the respective flanges 11 and 12 are turned over by a press bending operation so as to project inwardly towards each other 30 and be substantially parallel to the base portion 10, as shown in Fig. 3.

Next, the strip formed to the section shown in Fig. 3, is placed in a holding tool (not shown) which grips the section by the flanges 11 and 12, and the inturned portions 13 and 14. The holding tool is loaded onto the bottom platen of an hydraulic stretch form press (not shown) and a tool 15 attached to the top platen of the press is brought into contact with the surface 35 16 of the section base portion 10. Load is applied and the tool 15 stretch forms the base portion 10 downwardly into the holding tool to the intermediate cross sectional shape shown in Fig. 4, the holding tool being designed to permit the flanges 11 and 12 together with their inturned portions 13 and 14 to rotate through an angle of ninety degrees during this stretch forming operation. 40 Using a tool 17 having a peripheral profile corresponding to the internal shape of the finished erosion shield, the strip formed to the section shown in Fig. 4 is stretch formed to its final cross sectional shape as shown in Fig. 5. This operation is carried out using the same holding tool as is used for the first stretch forming operation but at a slower rate than that which is applied in the first stretch forming operation. 45 The flanges 11 and 12 are then trimmed off to leave the finished erosion shield 10 as shown in Fig. 6.

If desired the erosion shield may then be age hardened as hereinbefore described to increase the tensile properties of the Ti-15-3 alloy. To prevent distortion during ageing the erosion shield must be supported in a suitable tool. 50 A helicopter main rotor blade 19, as shown in Fig. 7, is manufactured from fibre-reinforced composite materials and has a span-wise length in the order of 9.15 metres. Erosion shields 20, 21, 22 and 23 are bonded to the leading edge portion of the blade so as to extend span-wise in end-to-end relationship along a major portion of the blade length. The erosion shields are formed from Ti-15-3 alloy in accordance with the present invention, the shields 20 and 21 55 which are bonded along the inboard length of the blade being of lengths in the order of 1.0 metre and 2.15 metres, respectively, and being of Ti-15-3 alloy in the solution treated and age hardened condition. The two shields 22 and 23 which are bonded along the outboard length of the blade are lengths in the order of 2.6 metres and 2.65 metres, respectively, and are of Ti 15-3 alloy in the solution treated condition. 60 Erosion shields manufactured from Ti-15-3 alloy have been found to provide adequate protec tion against erosion of the blade leading edge and, in different conditions of treatment, has the required tensile strength properties to enable use throughout the blade span. Furthermore Ti-15 3 alloy can be cold formed and has a modulus of elasticity to that of the fibre-reinforced composite blade material to which the erosion shields are bonded so as to minimise strains at 65 4 GB2189204A 4 the bond line between the blade and the erosion shields.

Whilst several embodiments have been described and illustrated, it will be understood that many modifications may be made without departing from the scope of the invention as defined

Claims

in the appended Claims.

5 CLAIMS 1. An erosion shield for an aerofoil surface is characterised in that it is manufactured from a cold formable titanium alloy comprising substantially 15 per cent vanadium, 3 per cent chrom ium, 3 per cent tin and 3 per cent aluminium with the remainder substantially titanium.
2. An erosion shield as claimed in Claim 1, further characterised in that at least one stretch 10 forming operation is used in the manufacture of the erosion shield.
3. A method for manufacturing by cold forming an erosion shield for an aerofoil surface from a sheet of titanium alloy comprising substantially 15 per cent vanadium, 3 per cent chromium, 3 per cent tin and 3 per cent aluminium with the remainder substantially titanium, comprising the steps of, 15 (a) cutting to a required length a strip of said titanium alloy sheet, (b) press forming the strip to a channel section in transverse cross section, the channel section having a base portion with flanges upstanding therefrom.

(c) overbending the flanges beyond an angle of ninety degrees with respect to the base portion so as to allow for spring back and cause the flanges to be upstanding substantially 20 normal to the base portion, (d) turning over the top edge portion of each flange so that it projects inwardly towards the opposite flange and is substantially parallel to the base portion, (e) stretch forming the base portion to the desired erosion shield transverse cross section, (f) trimming off the flanges to obtain the finished erosion shield. 25
4. A method as claimed in Claim 3, further characterised in that said stretch forming com prises a first stretch forming operation in which said base portion is formed to an intermediate cross sectional shape and a second stretch forming operation in which said base portion is formed to a finished cross sectional shape.
5. A method as claimed in Claim 3 or Claim 4, further characterised in that said titanium alloy 30 sheet is in a solution annealed condition.
6. A method as claimed in any one of Claims 3 to 5, further characterised by the additional step of age hardening the erosion shield at a temperature between 400 and 600 degrees Celcius.
7. A method as claimed in Claim 6, further characterised in that age hardening is accom- 35 plished by heating to a temperature of 540 degrees Celcius for a period of eight hours followed by air cooling.
8. A helicopter rotor blade manufactured from fibre reinforced composite materials having an erosion shield bonded to the blade so as to extend in a span-wise direction along at least a part of the length of the blade leading edge, characterised in that the erosion shield is manufactured 40 from a cold formable titanium allay comprising substantially 15 per cent vanadium, 3 per cent chromium, 3 per cent tin and 3 per cent aluminium with the remainder substantially titanium.
9. A rotor blade as claimed in Claim 8, further characterised in that the erosion shield comprises a plurality of erosion shields bonded to the blade in end-to- end relationship through out substantially the entire span-wise length of said leading edge. 45
10. A rotor blade as claimed in Claim 9, further characterised in that the erosion shield or shields in an inboard region of said leading edge are age hardened after manufacture so as to increase the tensile properties whereby the said erosion shield or shields contribute towards the structural strength of the rotor blade.
11. A rotor blade as claimed in Claim 10, further characterised in that the erosion shield or 50 shields in an outboard region of the leading edge have lower tensile properties than those in said inboard region.
12. A rotor blade as claimed in Claim 11, further characterised in that said outboard erosion shield or shields are manufactured from said cold formable titanium alloy in a solution treated condition. 55
13. A method for forming an erosion shield substantially as hereinbefore described and with reference to Figs. 1 to 6 inclusive of the accompanying drawings.
14. A helicopter rotor blade substantially as hereinbefore described and with reference to Fig.

7 of the accompanying drawings.
15. Every novel feature and every novel combination of features disclosed herein. 60 Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon) Ltd, Dd 8991685, 1987. Published at The Patent Office, 25 Southampton Buildings, London, WC2A l AY, from which copies may be obtained.