GB1568455A

GB1568455A - Propeller hub and blade crack detection

Info

Publication number: GB1568455A
Application number: GB478577A
Authority: GB
Original assignee: Cessna Aircraft Co
Current assignee: Cessna Aircraft Co
Priority date: 1976-02-05
Filing date: 1977-02-04
Publication date: 1980-05-29
Also published as: CA1078358A

Description

(54) PROPELLER HUB AND BLADE CRACK DETECTION (71) We, THE CESSNA AIRCRAFT COMPANY, a corporation organised under the laws of the State of Kansas, United States of America, of 5800 East Pawnee Road, Wichita, Kansas 67201, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement: It is well known that metal aircraft propeller blades and their mounting hubs are subject to centrifugal forces, bending moments and other stress inducing forces. It is also well known that alternating stresses in metal have a cumulative effect as regards the structural change of the metal and possible failure of the part due to fatigue failure.

Aircraft blades and/or hubs have suffereed catastrophic failures due to metal fatigue. Commonly, when such failure has occurred, it may be shown that a crack had previously developed, usually at a point of stress concentration. This crack will have then progressed slowly through the crosssectional structural area of the hub or the blade, and will commonly lead to a surface thereof, even though the crack may be invisible to the unaided eve.

Once such a crack has developed it will commonly progress more and more rapidly as the crack itself increases the stress concentration and as the cross-sectional area of the remaining material is reduced, until the point is reached where the remaining supporting material has such a small area that the part will fail from overload. If the blade fails it will result in blade separation while a failure of the hub will often result in a blade loss.

The very large unbalanced centrifugal forces of the remaining blade or blades of the propeller, following blade separation, are so high that they can break the propeller off of the engine or cause the propeller to separate, or they may break the engine supporting and isolation mounts and displace the engine in the nacelle. In some cases the engine can be pulled out of the nacelle. As is evident, such failures are of a catastrophic nature, they generally occur suddenly without prior warnings, generally cause a complete loss of power and subsequent forced landings. and in isolated cases cause an uncontrolled descent of the aircraft.

It has been known to apply air under pressure to the interior of a blade, specifically a helicopter rotor blade, and employ a pressure responsive device such as a gauge to indicate loss of pressure due to cracks or the like, as shown in United States patents Nos. 2,754,918 issued July 17, 1956; 3,765,124 issued October 16, 1973; and 3,768,922 issued October 20, 1973. Alternatively, a vacuum has been applied to the interior of helicopter rotor blades and loss of such vacuum is detected as described in U.S. patent No. 3.667,862 issued June 6, 1972.While such differential pressure systems may be useful for helicopter rotor blades which have large internal open spaces and which may be provided with a visual gauge or the like to sense a loss of pressure, this method of crack detection does not lend itself for use on propeller blades or propeller hubs and blade combinations where the internal volume available is relatively small, where either a positive or negative gas pressure could interfere with the proper operation of the pitch changing mechanism, and where there would still remain a necessity for providing a suitable gauge to detect loss of pressure. Further, such gas systems do not provide any indication of the location or position of the crack.

It has also been known to apply hydraulic fluid to the interior of the hub in certain controllable pitch propellers for lubrication purposes, such as shown in U.S. Patent No.

2,758,659 issued August 14, 1956. Insofar as Applicant is aware, such fluid was never employed for crack detection purposes.

The main object of the present invention is to provide a method of detecting fatique cracks in aircraft propeller hubs and/or blades which provides an early warning to pilots, owners, operators and maintenance personnel of incipient failure before the crack reaches a critical length and before it is capable of causing a catastrophic failure.

To this end, and from one aspect, the present invention consists in a method of detecting fatique cracks in an aircraft propeller hub and/or blades in which the hub and/or the propeller blades at least at the butt ends thereof are hollow, said method comprising placing a quantity of visual indicator liquid into the interior of the blades and/or hub, periodically inspecting the exterior of said hub and/or blades and adjacent aircraft parts after use in service for any evidence of said liquid on the exterior surfaces thereof indicating the presence of an incipient crack in said blades and/or hub.

Partially hollow interiors are now commonlv provided in adjustable pitch propeller blades. The indicator liquid. may contain a dye to enhance visibility even of very small amounts of liquid which flow to the surface when the propeller is subjected to centrifugal force. The open butt ends of the blades may be closed and sealed, encapsulating the liquid.

The hollow interior of the blades may open into the hollow interior of the blade supporting hub. with the hub itself being at least partially filled with the visual indicator liquid which then flows outwardly into the hollow portions of the blade by reason of centrifugal force. Where the hub and blade are thus supplied with indicator liquid. the common liquid interior then provides an indication of either hub or blade failure, and the quantity of liquid provides a substantial amount of leaking through a crack to assure its early detection. Such leakage will visually appear not only on the hub and blade but will also run back in the air stream to cause a tell-tale streak on the spinner. nacelle. or adjacent shroud.Maintenance personnel and/or operators may then periodically inspect the exterior of the hub and/or blades to check for any evidence of the indicator liquid on the exterior surfaces or on the adjacent aircraft surfaces as a warning of possible incipient crack or fatique failure.

Advantageously. the liquid may consist of a lubricating oil which will then lubricate the propeller thrust bearings and the pitch change mechanism. eliminating the need for special greases or lubricants at these points.

When the propeller is operating in service, a substantial centrifugal force is applied to all of the rotating parts including the encapsulated liquid so that the liquid will be forced through a newly formed crack at a relatively high pressure, thus enhancing early detection before the crack has progressed to a critical point.

From another aspect, the invention consists in a method of detecting cracks in a hollow aircraft propeller hub, comprising placing a substantial quantity of liquid dye in said hub, and periodically inspecting the exterior of said hub and adjacent aircraft parts after use of said hub in service in which said hub has been subjected to centrifugal force for any evidence of said liquid dye on a surface thereof indicating the presence of an incipient crack in said hub.

The invention also consists in a controllable pitch aircraft propeller including a generally hollow sealed propeller hub supporting thereon a plurality of at least partially hollow propeller blades and containing a pitch adjusting mechanism for said blades, in which the inner surfaces of the inner ends of said blades are exposed to the interior of said hub, said hub having therein a substantial quantity of combined lubricating oil and dye in contact with the exposed inner surfaces of said hub and said inner blade surfaces when said propeller is rotated and is subjected to centrifugal forces in service, said combined lubricating oil and dye partially filling said hub thereby leaving an internal air space to prevent hydraulic interference with the operation of said pitch adjusting mechanism, said dye providing a distinct visual external surface indication of an incipient crack in the wall of said hub and/or in the inner ends of said blades.

In order that the invention may be more readily understood, reference will be made to the accompanying drawings, in which: Figure 1 is a partial vertical section, with some of the parts being broken away, of a propeller blade and hub assembly constructed in accordance with this invention; Figure 2 is an enlarged fragmentary section of a portion of Figure 1: and Figure 3 is a vertical section through a modified propeller blade but end.

Referring to Figures 1 and 2, a propeller hub 10 is shown as being mounted to the conventional flange 12 of an engine drive shaft 14. The hub 10 supports for rotation, two or more metal blades 15, only one of which is shown in Figure 1. The hub 10 may contain the usual pitch change mechanism, and for this purpose a hydraulic dome 18 includes a conventional piston 20 (only partially shown) which is connected through a toggle link 21 to engage an actuating pin 22 forming a part of the propeller blade mounting ferrule or retainer 25, to effect pitch change movements of the blade 15 in a conventional manner. The retainer 25 supports the blade 15 on the hub 10 for pitch change movement through a thrust bearing 28.Oil under pressure from a governor may be applied to the piston dome 18 through the hollow piston rod 30 to move the propeller blades in one pitch changing direction while the rotational centrifugal twisting movement of the rotating blade and a compression spring 32 telescoped about the rod 30 is used to move the propeller blades in the opposite direction. While a constant speed non-feathering type propeller has been shown herein for the purpose of illustration, it is understood that the invention may be applied to any type of hollow hub constriction such as used for controllable pitch, constant speed and feathering, or reversible pitch propellers.

In the hub and propeller assembly in Figure 1, the blade 15 is shown as having a partially hollow shank or butt end portion 35 defining an open space 36 therein. This open space communicates into the interior of the hub 10.

The hub 10 is partially filled with an indicating liquid 50. Preferably, this liquid may take the form of a light lubricating oil.

The hub is not completely filled as some air space is desired to avoid lockup of the pitch change mechanism. The use of a lubricating oil serves the additional function of providing lubrication for the pitch change mechanism and the propeller thrust bearings.

The propeller hub is suitably conventionally sealed so that the indicator liquid 50 is retained therein. A substantial quantity is provided so that when the propeller is being rotated by the engine shaft 14, there is a sufficient quantity to fill the openings or recesses 36 in the propeller blades and preferably to cover the thrust bearing 28, and to wet the major inside surfaces of the hub housing 10.

Types of cracks which are detected by the present invention are normally fatigue type failures. If a crack develops it will usually occur at a point of high stress concentration and will progress slowly through the cross sectional structural area of the hub or the blade. Such a crack, once it progresses through the wall of the structure, will progress more and more rapidly as the crack itself increases the stress concentration and as cross sectional area is reduced, until the remaining supporting area of material is so small that the part will fail from overload.

The primary purpose of the present invention is that of detection of cracks just as soon as they have progressed through any wall portion of the hub or blade.

A crack in the hub 10 is illustrated at 52.

A hub crack will generally start on an inside surface and will progress slowly outwardly while expanding circumferentially. Such a crack will generally intersect the outer surface relatively quickly in its growth period and will then grow circumferentially.

However, just as soon as such crack has surfaced, indicator liquid 50 will flow to the outer surface and provide a visual indication of the crack. In the case of the crack 52, the liquid 50 will flow along the mating threads 54 of the blade retaining nut 55 and the housing 10.

In the case of the blade 15, a typical crack is illustrated at 60. A blade crack will generally start on the outside surface and progress slowly inwardly and will expand circumferentially until it intersects the opening 36 inside the butt end 35 of the blade 15.

As soon as intersection has occurred, the liquid 50 will flow through the crack 60 and then along the threads between the blade and the threaded retainer 25 and will be pulled into the air stream and deposited on the adjacent aircraft surface, such as the propeller spinner, the engine cowling, etc., providing an indication of failure. An annular weather ring or shield 62 may be seucred to the outer surface of the blade 15 by a sealant 63 to prevent intrusion of water into the underlying threads when the propeller is at rest. It also operates to provide an open flow path for the indicator liquid in the event of an inwardly located crack and allows it to flow out over the adjacent nacelle where its presence is detected as a possible crack.

The liquid 50 may be treated with a dye to make its appearance more immediately distinctive and noticeable. Red petroleum dye may be adeed to an SAE 10W30 oil as the liquid 50, or a turbine oil or hydraulic fluid may be used with or without the dye. It is preferred to incorporate a dye of distinctive color to assure more rapid detection and to distinguish the presence of the liquid 50 from that of engine oil or other fluids which are associated with aircraft engines and engine installations. By reason of the earlier detection provided by this invention, a crack can be found before it has reached a critical length.

In the operation of the embodiment of Figure 1, the hub would be suitably drilled and tapped for a pipe plug 64 in the wall in the housing, and the integrity of the hub seals are thus checked by air or nitrogen pressure. Then a specific amount of the indicator liquid 50 is added to the interior of the hub 10. As an example, if the air volume within the assembled propeller hub is approximately 2,000 cc it would be preferred to add approximately 1500 cc of the liquid 50.

In a typical installation the air annulus in the rotating propeller would have a radius of approximately 2.5 inches when the piston in the dome 18 is in the low pitch position and 2.2 inches when in the high pitch position.

When the propeller is rotated in the operating range of between 2,000 to 2,850 rpm, a 30-60 psi pressure will be developed within the hub and a somewhat higher 35-70 psi pressure will be developed within the blade opening 36. A total pressure increase of from 12-14 psi will occur when the piston moves inwardly from a low to a high pitch position, and in intermediate positions such as during cruise there will be approximately a 6-7 psi increase in pressure. Therefore, the pressure due to centrifugal force inherently aids the crack detection method and assures that fluid will flow through even the smallest of cracks which develop through the propeller blade wall or the hub wall, thereby providing a fail-safe means for detecting cracks.

Referring to the embodiment of Figure 3, a typical blade butt 15A is provided with a hollow interior 36A having the inner end closed bv a plastics plug 70. The interior space may be substantially filled with the liquid 50 which, in this case. may be any suitable non-corrosive or inhibited indicator liquid having a sufficiently low freezing point and high boiling point as to remain a liquid throughout service conditions of the propeller. The liquid 50A will be subjected to the same centrifugal forces as described above and will provide the protection in the same manner as described in connection with the embodiment in Figure 1. The filled blade technique may thus be used where it is not desirable to seal and apply fluid to the interior of the hub. It may also be used in replacement propeller blades.

WHAT WE CLAIM IS: 1. A method of detecting fatigue cracks in the aircraft propeller hub and/or blades in which the hub and/or the propeller blades at least at the butt ends thereof are hollow, said method. comprising placing a quantity of visual indicator liquid into the interior of the blades and/or hub, and periodically inspecting the exterior of said hub and/or blades and adjacent aircraft parts after use in service for any evidence of said liquid on the exterior surfaces thereof indicating the presence of an incipient crack in said blades and/or hub.

2. A method as claimed in claim 1, in which said liquid includes a light lubricating oil.

3. A method as claimed in claim 1 or 2, in which said liquid also includes a dye to enhance the visibility thereof upon seepage thereof through a crack.

4. A method as claimed in any one of claims 1 to 3, comprising closing said blades at said inner ends to capture said liquid in the hollow interior thereof.

5. A method of detecting cracks in a hollow aircraft propeller hub. comprising placing a substantial quantity of liquid dye in said hub, and periodically inspecting the exterior of said hub and adjacent aircraft parts after use of said hub in service in which said hub has been subjected to centrifugal force for any evidence of said liquid dye on a surface thereof indicating the presence of an incipient crack in said hub.

6. A method as claimed in claim 5, in which said liquid includes a light lubricating oil.

7. A controllable pitch aircraft propeller including a generally hollow sealed propeller hub supporting thereon a plurality of at least partially hollow propeller blades and containing a pitch adjusting mechanism for said blades, in which the inner surfaces of the inner ends of said blades are exposed to the interior of said hub. said hub having therein a substantial quantity of combined lubricating oil and dye in contact with the exposed inner surfaces of said hub and said inner blade surfaces when said propeller is rotated and is subjected to centrifugal forces in service, said combined lubricating oil and dye partially filling said hub thereby leaving an internal air space to prevent hydraulic interference with the operation of said pitch adjusting mechanism, said dye providing a distinct visual external surface indication of an incipient crack in the wall of said hub and/or the inner ends of said blades.

8. A controllable pitch aircraft propeller constructed and adapted to operate substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

9. A controllable pitch aircraft propeller constructed and adapted to operate substantially as hereinbefore described with reference to Figures 1 and 2 as modified by Figure 3 of the accompanying drawings.

10. A method of detecting fatique cracks in an aircraft propeller hub and/or blades, substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. When the propeller is rotated in the operating range of between 2,000 to 2,850 rpm, a 30-60 psi pressure will be developed within the hub and a somewhat higher 35-70 psi pressure will be developed within the blade opening 36. A total pressure increase of from 12-14 psi will occur when the piston moves inwardly from a low to a high pitch position, and in intermediate positions such as during cruise there will be approximately a 6-7 psi increase in pressure. Therefore, the pressure due to centrifugal force inherently aids the crack detection method and assures that fluid will flow through even the smallest of cracks which develop through the propeller blade wall or the hub wall, thereby providing a fail-safe means for detecting cracks. Referring to the embodiment of Figure 3, a typical blade butt 15A is provided with a hollow interior 36A having the inner end closed bv a plastics plug 70. The interior space may be substantially filled with the liquid 50 which, in this case. may be any suitable non-corrosive or inhibited indicator liquid having a sufficiently low freezing point and high boiling point as to remain a liquid throughout service conditions of the propeller. The liquid 50A will be subjected to the same centrifugal forces as described above and will provide the protection in the same manner as described in connection with the embodiment in Figure 1. The filled blade technique may thus be used where it is not desirable to seal and apply fluid to the interior of the hub. It may also be used in replacement propeller blades. WHAT WE CLAIM IS:

1. A method of detecting fatigue cracks in the aircraft propeller hub and/or blades in which the hub and/or the propeller blades at least at the butt ends thereof are hollow, said method. comprising placing a quantity of visual indicator liquid into the interior of the blades and/or hub, and periodically inspecting the exterior of said hub and/or blades and adjacent aircraft parts after use in service for any evidence of said liquid on the exterior surfaces thereof indicating the presence of an incipient crack in said blades and/or hub.