The CF6-50 (Figure 2.1) is a high bypass fan engine which is used in primarily commercial applications, al-though it is also used on the 747 Flying Command Posts that are in the Air Force inventory. In the diagram it is shown with its full nacelle, some parts of which are sup-plied by the airframe manufacturer. Forward (on the left in GE engine drawings) is the inlet, made of aluminum sheet frames and skins with thinner barrel lined with acoustic treatment. The inlet and cowls, which form the outer fiowpath for the fan and core discharge, are air-frame designed and supplied. The fan and core thrust re-verser are GE designed and supplied. The first engine static structural parts we see, as we follow the flow of air through the engine, are the fan cases which are bolted to the outer end of the fan frame struts and provide the sup-port for the inlet. The inner part of the fan frame, or hub, provides the gooseneck shaped fiowpath from the booster into the inlet of the high pressure compressor.
The hub also supports the tt\ bearing by means of a large aluminum cone and the ffl roller bearing by a cast steel cone. These two bearings hold the fan shaft and the front end of the low pressure shaft. The inner surface of the thrust reverser, while it provides the fiowpath and acous-tic treatment for the fan discharge, is not part of the basic structure of the engine. From the fan frame hub the main structural loadpath is the casing of the high pressure compressor, the compressor rear frame, a turbine mid-frame, and finally, the low pressure turbine casing. The compressor rear frame is also the combustor casing and the high pressure turbine casing. The turbine mid-frame carries the aft mounts. The low pressure turbine casing provides the support for the primary nozzle and, in this diagram, a turbine reverser.
Note that the fan frame hub carries the 03 ball bearing at the front of the high pressure compressor, and the com-pressor rear frame carries the #4 roller bearing between the rear of the compressor and the front of the high pres-sure turbine. The turbine mid-frame carries two roller bearings, the #5 bearing at the rear of the high pressure turbine and the #6 bearing at the forward shaft of the low pressure turbine. At the rear of the basic engine, just ahead of the turbine reverser, is the turbine rear frame.
STATIC STRUCTURES 2-1
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Figure 2.2 F101-GE-102 Augumented Turbofan
STATIC STRUCTURES 2-3
which supports the #7 roller bearing behind the low pressure turbine. Here, we have examples of a number of ways to support components. The fan is supported by the fan frame and is basically overhung from it on two bearings. The high pressure compressor and the high pressure turbine are both straddle mounted with bearings at the front and the rear, and the low pressure turbine is straddle mounted with bearings at the front and rear.
This bearing arrangement on the rotors gives us a four-sump engine: A-four-sump in the fan frame, B in the com-pressor rear frame, C in the turbine mid-frame, D in the turbine rear frame. On the turbine mid-frame are the mount elements which are supported by links to the air-craft pylon. The forward mount is right behind the fan frame outside the forward flange of the compressor, but is not visible in this view.
While looking at the overall structure shown in Figure 2.1, note that in the A-sump, between the tfl and #3 bearings, there is a set of bevei gears connecting the high pressure rotor with a radial drive shaft at the 6 o'clock position. This shaft connects to a bevel gear transfer gearbox and then to a shaft running forward to the acces-sory gearbox, which on this engine is located in the fat lower lip of the nacelle.
The lower half of Figure 2.3, which shows the CF6-50 in more detail. (A little later on, we will study the top half of this figure, showing the -80A, which evolved from the -50.) On the fan frame hub, can be seen the bleed doors and bleed door mechanism between the acoustic panel and the hub outer wall. The doors or vari-able bleed valves (VBV) open in order to bypass excess booster flow, so that we do not overload and stall the high pressure compressor. The containment ring over the fan blades can also be seen. This stainless steel ring is sized to contain the energy that would be released if a fan blade should fail in the shank, just above the dovetail to the fan disk. Forward of the fan blades and between the fan blades and the outer guide vanes is the acoustic treatment in the fan flowpath. Just aft of the fan blade, is a deep stiffener ring that was added at the bolted joint be-tween the containment ring and the aluminum midfan case. This stiffener ring raises the vibrational frequency of the containment system, so that it is not resonant with any vibrational mode of the fan rotor in the operating range of the engine. Just aft of the OGVs is the bolted joint between the aluminum fan mid-case and the
alumi-num aft fan case, which goes over the fan frame struts.
Two deep rings over the fan frame struts distribute the reaction of those struts into the shell of the casing.
Recognize that the lower half of Figure 2.3 is really the 12 o'clock section of the -50 engine. The thrust mount is really pointing upwards since the engine is always
mounted either on the wing or in a tail engine position on a DC 10. The mount is shown just outside the inlet guide vanes (IGV) of the high pressure compressor. The plat-form has a short fat pin, which takes the thrust in shear to the pylon foot; the platform is clamped to the pylon foot with two 7/8" bolts that belong to the airframe de-signer. The main thrust connection between the mount platform and the 12 o'clock strut of the fan frame is the short link between the two pins. This link or dogbone carries about 95% of die thrust load. The other 5% is carried through the bolted flange between the compres-sor and the fan frame. This bolted joint also carries verti-cal load and a slight amount of torque and sideload. This is "slight** because the forward pylon connection, to which it is attached, is quite flexible in torsion. The rear mount is shown at the 12 o'clock position on the turbine mid-frame. The mounting span of the engine between the front mount and the three rear mount links includes the compressor, the compressor rear frame combustor case, and the high pressure turbine section. Notice that the fan module is completely overhung from the front mount and the low pressure turbine, nozzles, and core reverser (if there is one) are overhung from the rear mount. This makes the -50 interesting from a dynamics point of view. The engine, which has some of the struc-tural character of a dumbbell, is actually mounted near the nodal points of its fundamental bending mode, and therefore, the mount reactions are small under large un-balanced loading. Dynamically, the difference between that engine and the one shown at the top of Figure 2.3 is quite marked. Note that the front mount of the -80A is at the same axial location (the station of the shear pin is identical to that of the -50). However, the mount does not appear to be the same, and it is not. The differences in mose mounts will be discussed in a later section of this chapter. The turbine mid-frame has been removed and the high pressure turbine of the -80A is cantilevered from a larger, heavier sump in the compressor rear frame. The low pressure turbine is supported by only one roller bearing in the turbine rear frame. You will note that the rear mount has now been moved aft of the low pressure turbine to the outer shell of the turbine frame. This increases the span between the mounts, and since it moves the mounting points away from the natu-ral nodes, the reaction forces for the -80A under vibra-tion are somewhat larger than for the -50 with the same unbalance in any one of the rotors.
Going back to the forward end of the engine, notice the difference in the containment systems for the fan blades.
The -50 systems shown in the bottom half of Figure 2.3 is a steel containment ring, or shell, that runs from the front flange to the beginning of the acoustic panel behind the fan blades. This steel ring is designed to resist pene-tration by pieces of the fan blade and to absorb the total
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Figure 2.3 CF6-80 and CF6-50
STATIC STRUCTURES 2-5
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energy of a fan blade released due to a shank failure at the disk dovetail. In the -80A containment system, shown atthe top of Figure 2.3, the steel ring has been machined down until it is just a very thin shell, except for the section for about 2 inches behind the front flange where the steel itself must provide all of the containment capability. The containment of fan blade particles or the energy of an entire blade are absorbed by a Kevlar belt over the fan blades. The basic system consists of the thin steel shell, a deep section of aluminum honeycomb, and a graphite-epoxy outer band forming a sandwich that gives an equivalent stiffness to that achieved by the steel shell and the stiffener ring of the -50. The Kevlar con-tainment consists of many layers of Kevlar cloth, in this case about 65 layers, that are woven to have the pocket shape that you see in the diagram. This containment sys-tem design is equivalent to the magician's trick of catch-ing a bullet in a silk handkerchief.
Moving aft from the fan, the booster and the gooseneck passage into the high pressure compressor look very similar to that of the -50. The compressor flowpath con-tour appears to be quite similar, though the detailed aero-dynamics of the airfoils is somewhat different.
However, in the rear section of the -80A compressor the casing goes all the way to the 14th stage. There is no in-termediate bolted joint like there was on the -50, which originally had a titanium forward case and an Inconel 718 aft case. The -80A has a steel case of M152 over the entire compressor. Extra length in the compressor flow-path has been removed and the CRF struts have been moved forward. The fuel nozzles and the combustor have been moved closer to the aft edge of the struts. All of these changes have significantly shortened the -80A engine. In the absence of a turbine mid-frame, with a gooseneck to the stage 1 low pressure turbine blades, the LPT of the -80A now has a near cylindrical inside diam-eter and a conical outside shape as opposed to the -50 which has the cylindrical outside diameter on the flow-path and a conical inside shape. The turbine rear frame serves as the support for the low pressure turbine as well as for the mounting of the engine to the pylon. Finally, the turbine frame has much longer struts in the axial
di-rection because they serve as airfoils straightening the flow entering the exhaust nozzle.
Figure 2.2 is a supersonic, low by-pass, augmented tur-bofan for the military and powers the BIB bomber.
Again, starting on the left at the forward end of the en-gine, there is a front frame. It supports a small sump and bearing so that this fan is what is known as "straddle mounted," with a bearing forward and aft. The aft bear-ing is carried in the fan frame. This engine does not have a booster or low pressure compressor. The small diame-ter basic structure of this engine is quite different from
the high by-pass commercial engines. There is a long fan duct that carries the flow from the fan back to the mix-ers, so that the core stream and the fan bypass stream are mixed just ahead of the augmentor. The duct that sur-rounds this fan flow is quite large in diameter and stiff.
Consequently, the structural characteristics of this en-gine with the forward mount on the front frame and the aft mount on the mount ring outside the turbine frame are quite different from the structural characteristics of the large fan engine where the mounts are on the basic core structure. For this engine, the entire augmentor and nozzle assembly is cantilevered from the rear frame, which is just behind the low pressure turbine. There is no sump and bearings inside the combustor on this en-gine. The connection between the compressor and the high pressure turbine is a large tube or shaft. The bear-ing that supports the back end of the high pressure rotor is an intershaft bearing between the high pressure tur-bine aft shaft and the low pressure shaft. The LP shaft, in turn, is supported by a bearing from the turbine frame. This makes a very stiff HP rotor, but a somewhat softer support for the HP rear shaft than if there were a frame and bearings between the turbines. There are both advantages and disadvantages for an arrangement of in-tershaft bearings supporting a high pressure turbine as opposed to a sump and a high pressure turbine overhung from a compressor rear frame.