Breakaway fan blade

A fan blade having a breakaway leading edge positioned on the spar and shell that constitute the main structure of the blade, the leading edge being adapted to break away, in part at least, upon being struck by a large foreign object, with the shell providing an acceptable leading edge after the breakaway.

BACKGROUND OF THE INVENTION 
Fan blades are subject to damage particularly on the leading edge in the 
event of impact by foreign objects, with resultant damage to the 
effectiveness of the leading edge and thus to the functioning of the 
blade. Small foreign objects may do little damage but large objects may 
damage the blade so much as to require engine shutdown, and may break away 
such a piece of the blade as to cause serious engine damage when ingested. 
It is desirable to have a blade construction in which the damage to the 
blade may be limited and in which the damaged blade may still be operable. 
SUMMARY OF THE INVENTION 
One feature of the invention is a blade construction in which the main 
structure of the blade has a breakaway leading edge mounted thereon, this 
leading edge having a predetermined frangibility, greater than that of the 
main structure. Another feature is a secondary leading edge, exposed when 
the breakaway leading edge is damaged which provides an adequate 
aerodynamic shape for continued functioning of the blade. Another feature 
is a fan blade leading edge with controlled breakaway locations so as to 
control the extent of damage to the blade in the event of impact with a 
foreign object, with this leading edge of more frangible material than the 
main blade structure. 
According to the invention, the blade which is made up of a spar and a 
shell mounted thereon and forming an acceptable airfoil, with the trailing 
edge portion forming the finished blade contour, and with the leading edge 
portion located inwardly of the preferred leading edge location to receive 
thereon a breakaway leading edge portion cooperating with the remainder of 
the shell to form the desired blade airfoil shape. This breakaway leading 
edge is more breakable than the shell and is secured to the shell so that 
in the event of foreign object impact near the leading edge, a length of 
this breakaway structure will break from the remainder or separate from 
the shell without damage to the shell. 
The foregoing and other objects, features, and advantages of the present 
invention will become more apparent in the light of the following detailed 
description of preferred embodiments thereof as illustrated in the 
accompanying drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The invention is shown in a built up blade construction in which a spar 2, 
extending lengthwise of the blade structure has mounted thereon a shell 4 
which, of itself, forms an acceptable airfoil at each section of the 
blade. As shown, the spar is hollow and approximately a flattened ellipse 
in section and the shell is also hollow, being made of opposed plates 
securely attached to the opposite surfaces of the spar in contact 
therewith and secured together at the edges to form leading and trailing 
edges 5 and 6. The trailing-edge portion 7 of the shell is shaped to the 
desired shape for this part of the airfoil shape of the completed blade 
and the outer surface of the shell is the finished surface of the blade. 
Forwardly of the spar the leading edge portion 8 of the shell forms a 
leading edge airfoil shape, but the shell leading edge 5 is spaced from 
the blade axis a shorter distance than that of the finished blade shape so 
that the leading edge portion 8 combines with the trailing edge portion 7 
to form a narrower blade structure than the desired blade design shape. 
The spar and shell may be either metallic, such as titanium, or may be a 
composite. In either event, these two elements form the main structure of 
the blade. 
To produce the desired blade leading edge contour a separate primary 
leading edge element 12 is built up on the leading edge portion 8 of the 
shell. This leading edge element is constructed of a material that is more 
frangible than the shell, as for example, layered fiber cloth mesh 
embedded in a resin and cured as in a suitable mold to produce the desired 
airfoil shape for the finished blade. The resin also holds the breakable 
leading edge element securely to the shell. With such a construction, an 
impact by a large object will break away at least a portion of this 
breakaway portion without damage to the shell and spar, and normally will 
break away at the attachment to the shell to expose the secondary leading 
edge so that the blade is still operable. 
The breakaway portion may be covered at and near its leading edge by a 
sheath 14 of high-impact material, such as a thin layer of steel to 
protect against small object impact. This sheath normally protects only a 
part of the breakaway leading edge element and terminates preferably 
forwardly of the secondary leading edge 5 as shown in FIG. 1. This sheath 
may terminate just short of the tip of the blade as shown if desired. 
To control to some extent the extent of breakaway in the event of impact, 
the sheath may have chordwise notches 16 cut in from the back edge toward 
the leading edge. These notches define zones of breakage for the sheath 
and thus of the underlying element 12. If desired, the element 12 may also 
have lines of weakness formed therein, either coincident with the notches 
16 or at additional locations for determining the extent of the element 12 
that will be broken away. It will also be desirable to assure that the 
attachment of the element 12 to the shell is such that broken portions of 
the element 12 will separate from the shell without damage to the 
configuration of the shell thereby having the secondary leading edge 
intact. The extent of these weakened or breaking zones is preferably 
selected so as to control the size of broken pieces of the element 12 and 
the sheath 14 so as to limit the size of these pieces that may be ingested 
by the engine. 
Although the invention has been shown and described with respect to a 
preferred embodiment thereof, it should be understood by those skilled in 
the art that other various changes and omissions in the form and detail 
thereof may be made therein without departing from the spirit and the 
scope of the invention.