CA1078358A - Propeller hub and blade crack detection - Google Patents

Abstract

Abstract of the Disclosure Propeller hubs and the hollow inner ends of propeller blades are filled or substantially filled with a liquid so that cracks in the hub wall or the blade wall leading from the interior to the exterior surface provide a pathway for the liquid to flow out to the exterior thereof, thus providing a visual indication of the presence of a crack prior to a catastrophic failure of the part.
The liquid may be sealed in the blade or hub or may be carried within the hub as a lubricating fluid, partially or substantially filling the hub. The liquid may contain a dye to enhance visibility even of very small amounts which flow to the surface when the propeller is subjected to centrifugal force. Maintenance personnel and/or operators may then periodically inspect the exterior of the hub and/or blade 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 fatigue failure.

Description

~0'78358 ~ ,-Background of the Invention 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 suffered cata- ~ ;
strophic failures due to metal fatigue. Commonly, when such failure has occurred, it may be shown that a crack had pre-viously developed, usually at a point of stress concentration.
This crack will have then progressed slowly through the cross-sectional 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 eye.

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 pro-peller off of the engine or cause the propeller to separate, ~078358 :. .
or they may break the engine supporting structure 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 dev~ce 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 I7, I956; 3,765,124 issued October 16, lY73;
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 des-cribed in U.S. patent No. 3,667,862 issued June 6, 1972.
While such differential pressure systems may be useful for helicopterrotor 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 1~7~3~
.
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 ~.S.
Patent No. 2,758,659 issued August 14, 1956. Insofar as applicant is aware, such fluid was never employed for crack detection purposes.

Summary of the Invention . . .
The present invention is directed to aircraft propellers and to a method of detecting fatigue cracks in propeller hubs and 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.
The invention resides in placing a substantial quantity of indicating fluid in the hub and/or in the hollow blade shank and sealing the same therein so that the appearance of any of this liquid on the exterior surfaces is an immediate indi-cation of a possible incipient crack.

In one aspect of the invention, there is provided ~*
the method of detecting fatigue cracks in an aircraft propeller hub and blades in which the propeller blades are par~ally hollow at the butt end thereof with hollow ends opening into a generally hollow blade supporting hub having a movable pitch changing mechanism therein, comprising the steps of partially filling the hub with a quantity of combined lubricating oil and dye, the quantity of said oil being suffi-cient to at least fill said hollow blade butt ends during rotation of the propeller and providing lubrication to the ., , pitch changing mechanism, but insufficient to interfere with the proper operation of such mechanism, and periodically inspecting the exterior of said hub and blades and adjacent aircraft parts after use in service for any evidence of said dye on the exterior surfaces thereof indicating ~he presence and location of an incipient crack in either said hub or blades.

In another aspect of the invention, there is provided a controllable pitch aircraft propeller including a generally hollow sealed propeller hub supporting thereon a plurality of propeller blade~ 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, the improvement comprising a substantial quantity of combined lubricating oil and dye in said hub 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 providing an internal air space to prevent hydraulic interference with the operation of said pitch adt~sting mechanism, said dye providing a distinct visual surface indication of the location of an incipient crack in the wall of said hub or in the inner ends of said blades.

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.

It is therefore an important object of this invention to provide a fail-safe means for detecting cracks in rotating propeller components, specifically including propeller hubs and blades. ;
Another important object of this invention is to provide a propeller hub and blade which has a hollow interior containing ;
a quantity of lubricating oil and dye which provides a visual means of detection in case of the development of a crack in the wall thereof.
A further object of this invention is to provide a -~
hollow propeller hub partially or substantially filled with a lubricating liquid capable of providing a visual indication of the presence of a crack in the hub.

These and other objects and advantages of the invention will be apparent from the following description, the accompanying drawings, and the a~pended claims.

Brief Description of the Drawinas Fig. 1 is a partial vertical section, with some of the parts being broken away, of a propeller blade and hub asse.~bly in accordance with this invention;
Fig. 2 is an enlarged fragmentary section of a portion of Fig. l; and Fig. 3 is a vertical section through a propeller blade butt end in accordance with this invention.

~' , ~0'7835~
: ~` '' ,'' Description of Preferred Embodiments Referring to Figs. 1 and 2, a propeller hub 10 is shown as being moùnted to the conventional flange 12 of an engine drive shaft 14. The hub 10 supports for rotation, two or more metal bl~des 15, only one of which is shown in Fig. 1. The hub ~!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 20 opposite direction. While a constant speed non~feathering r 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 construction such as used .,~ .
` for controllable pitch, constant speed and feathering, or reversible pitch propellers.

In the hub and propeller assembly in Fig. 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.

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~C~'7835~ :
, , In accordance with the present invention, 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 addi- -tional function of providing lubrication for the pitch change mechanism and the prooeller 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 pre-sent 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 ; 20 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 i9 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.

10';~358 , , 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 circumferen-tially. 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 w;ll flow to the outer surface and provide a visual ~indication of the crack.
In the case of the crack S2, the liquid S0 will flow along the mating threads 54 of the blade retaii~ng 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 pro~ress 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 thr ~hreaded 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 secured 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 ind~cator 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.

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7~358 . - - , .
~` The liquid 50 may be treated with a dye to make its appearance more immediately ~istinctive and noticeable.
Red petroleum dye may be added to an SAE lOW30 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 as~ociated 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 Fig. 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 add~d 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 li-quid 50~ In a typical installation the air annulus in the 20 rotating propeller would have a radius of approximately 2.5 ~;
inches when the piston in the dome 18 ia 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 posi-tion, and in intermediate positions such as during cruise l~t7~35~ .:

there will be approximately a 6-7 psi increase in pressure.
Therefore, the pressure due to centrifugal force inherently aids the crack detection method of this invention and assures that fluid will flow through even the smallest of cracks which develop through the propeller blade wall or the hub ~;
wall.
Referr;ng to the embodiment of Fig. 2, a typical blade butt l5a is provided with a hollow interior 36a having the inner end closed by a plastic plug 70. The interior space may be substantially filled with the liquid 50 which, in this case, may be any suitable non-corrosive or îhhibited indicator liquid having a sufficiently low freezing point and high boiling point as to rema~n liquid throughout service conditions of the propeller. The lîquid 50a will be subjected to the same centrifugal forces as described above and will provide the protection in the same manner as des-cribed in connection with the embodiment in Fig. 1. The filled blade technique may thus be used where it is not desirable to seal and apply fluid to the lnterior of the hub. It may also be used in replacement propeller blades.
While the methods herein described and the forms of apparatus for carrying these methods into effect consti-tute preferred embodiments of this invention, it is to be understood that the~invention i~ not 11mit~d to these precise methods and forms of apparatus, and that changes may be made in either without departing from the scope of the invention.

Claims (2)

WHAT IS CLAIMED IS:

1. The method of detecting fatigue cracks in an aircraft propeller hub and blades in which the propeller blades are partially hollow at the butt end thereof with hollow ends opening into a generally hollow blade supporting hub having a movable pitch changing mechanism therein, comprising the steps of partially filling the hub with a quantity of combined lubricating oil and dye, the quantity of said oil being suffi-cient to at least fill said hollow blade butt ends during rotation of the propeller and providing lubrication to the pitch changing mechanism, but insufficient to interfere with the proper operation of such mechanism, and periodically inspecting the exterior of said hub and blades and adjacent aircraft parts after use in service for any evidence of said dye on the exterior surfaces thereof indicating the presence and location of an incipient crack in either said hub or blades.

2. A controllable pitch aircraft propeller including a generally hollow sealed propeller hub supporting thereon a plurality of 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, the improvement comprising a substantial quantity of combined lubricating oil and dye in said hub 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 providing an internal air space to prevent hydraulic interference with the operation of said pitch adjusting mechanism, said dye providing a distinct visual surface indication of the location of an incipient crack in the wall of said hub or in the inner ends of said blades.