Aircraft fan containment structure restraint

Woven fiber ballistic fabric (22) of multiple layers surrounds an isogrid support structure (20). A cuff portion (38) has shorter warp threads than the major portion (3) and also is impregnated with epoxy resin. A diameter restrains the fabric from aft movement during a blade ejection event.

TECHNICAL FIELD 
The invention relates to structures for containing fractured blades of a 
turbofan gas turbine engine, and in particular, for the restraint of the 
ballistic fabric thereof. 
BACKGROUND OF THE INVENTION 
Turbofan aircraft engines have large fans at the forward end. They rotate 
at a high speed at about 4000 revolution per minute. 
Foreign objects such as birds, hailstones or debris ingested from the 
ground occasionally strike the fan blades. It is possible for this to 
cause the fan blades to fragment. These fragments may be on the order of 7 
kilograms traveling at about 930 meters per second. It is essential to 
contain the blade fragments and also to retain the casing. 
A typical containment structure is shown in U.S. Pat. No. 4,490,092 
entitled "Containment Structure" and issued to Emile J. Premont. A support 
structure has "c" shaped stiffeners between inner and outer sheets. This 
structure surrounds the fan and has multiple layers of woven KEVLAR.RTM. 
ballistic fabric (trademark of DuPont Corporation). This fabric is wound 
under tension and serves to resiliently contain blade fragments passing 
through the support structure. 
FIGS. 10 through 15 of the above-cited U.S. Pat. No. 4,490,092 illustrate 
the general track of a blade fragment passing through the support 
structure and retained by the ballistic fabric. The blade fragment has an 
aft component and moves in the aft direction pulling the ballistic fabric 
with it. The ballistic fabric is pulled downstream with the fabric from 
the forward location coveting the opening. The fabric on occasion is 
pulled into the hole by the rotor during this failure event. Interaction 
of the fabric with the blades causes additional damage. 
This has been avoided in the past by making a large honeycomb structure 
positioning the cloth well away from the rotor. This however increases the 
diameter required. An alternate approach would be to use mechanical 
fasteners to keep the fabric in place, but this could lead to concentrated 
loading and tearing of the fabric. 
SUMMARY OF THE INVENTION 
The cylindrical case surrounding the fan blades has a large diameter 
containment portion throughout the major part of the length. It has a 
reduced diameter neck portion at the forward end of the case. 
A multiple layer winding of ballistic fiber surrounds both the large 
containment portion and the smaller restraint portion. The fabric has warp 
yarns extending circumferentially of the case. 
The warp yarns in the portion of the winding that surround the reduced 
diameter neck portion form a cuff by being shorter than the warp yarns in 
the portion of the winding surrounding the large diameter portion, when 
these yarns are at equal strain. This is established by specifying such 
weaving for the fabric to be used. 
The portion of the fabric in the neck portion is saturated with a cured 
epoxy resin throughout the circumference, this being throughout all of the 
layers rather than just a surface coating. 
The step between the large diameter and reduced diameter is preferably 
should be greater than 2% of the reduced diameter.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, the turbofan aircraft gas turbine engine 10 has a 
plurality of rotating fan blades 12 at the forward end. A case 14 of about 
2.4 meters diameter surrounds these blades being supported by struts 16. 
The case has an isogrid structure 20 with aluminum metal sheet plate 18 
bonded thereto. This is surrounded by multiple wraps of ballistic woven 
fabric 22 such as KEVLAR fabric. The casing also contains on a radially 
inside surface a wear strip 24 and a sound deadening honeycomb 26. 
The casing has a flange 28 at the forward end for securing an inlet airflow 
cowl (not shown). 
FIG. 2 is a section through casing 14 showing along with fan blade 12. 
Throughout the major pan 30 of the length of the casing there is a large 
diameter 32 of 2.941 meters. Multiple layers of woven ballistic fabric 22 
form the surrounding restraining structure, there being in the order of 
140 wraps of the woven fabric. The reduced diameter neck portion 34 is 
located at the forward end of the case with a diameter 36 of 2.877 meters. 
The diameter and therefore the circumference of the major part is 
therefore 2.22% greater than the neck portion. A cuff portion 38 of the 
ballistic fabric is established in contact with the reduced diameter 
portion of the case. 
In specifying the weave of the fabric to be applied, the portion which will 
be applied at the forward end is specified to have warp yarn 2.17% shorter 
than the warp yarn of the aft portion. This is accomplished by the 
manufacturer by weaving the fabric on a curved mandrel. Such fabric may be 
obtained from Fabric Development Inc. of Quakertown, Pa. Accordingly, the 
warp yarns are more than 2% shorter at the forward end with the fabric at 
equal strain. In other words, with no tension of the fabric these yarns 
are shorter, and remain shorter as tension increases to whatever level is 
used in application of the fabric to the casing. 
During installation of the fabric on the casing, uniform tension is applied 
with the shaped mandrel. An epoxy resin is applied during the application 
of the fabric with this saturating the entire thickness of the multiple 
wraps of fabric throughout the cuff portion 38. The applied resin is 
cured. 
The step 40 adjacent the cuff is produced by a circumferentially running 
flange 42 which has at its forward facing outer comer 44 a radius greater 
than 0.25 millimeters approximately 0.277 millimeters. This forms a sharp 
edge which is not likely to cut the fabric. An under cut 46 on this flange 
is supplied to reach the thickness required for strength, while saving 
weight. The broader surface in contact with the fabric spreads the fabric 
load, lowering the bearing stress.