Tuned engine mounting system for jet aircraft

An aircraft assembly includes a wing, a pylon structure attached to the wing, an aft engine mount attached to the pylon structure, and an engine attached to the aft engine mount. The aft engine mount includes a pivotal attachment to the pylon structure and first and second spring beams operatively connected to the pylon structure at opposing sides of the pivotal attachment for damping pivotal movement of the engine with respect to the pylon structure to enable tuning of the natural frequency of the engine to avoid wing flutter.

BACKGROUND OF THE INVENTION
 1. Field Of The Invention
 The present invention relates to a tuned engine mounting system for a jet
 aircraft, and more particularly to an engine mounting system in which an
 aft engine mount is provided with spring beams for damping pivotal
 movement of the engine.
 2. Background Information
 It is a design goal that when an aircraft wing is subjected to a momentary
 force in flight which causes the wing to oscillate between a bent state
 and an unbent state, that in the absence of this force, the oscillations
 will damp out and the wing will return to a steady, unbended state. On the
 other hand, "wing flutter" refers to a phenomenon in which the wing
 oscillations between the bent state and the unbent state do not damp out.
 Rather, the amplitude of these oscillations either remains constant or
 increases over time.
 Wing flutter is an aeroelastic instability produced by the coalescing and
 proper phasing of two or more structural vibration modes of an aircraft in
 flight. A flutter mode usually involves both bending and torsion-types of
 motion in which the torsional motion extracts energy from the airstream
 and drives the bending mode to increasingly higher amplitudes. In other
 cases, these oscillations are lightly damped, but stable, within the
 operating speed envelope of the aircraft and can cause a reduction in
 riding comfort of the aircraft.
 The location of the engine nacelle relative to the wing, the mass
 properties of the engine, and the stiffness of the strut which attaches
 the nacelle to the wing are factors which influence the flutter
 characteristics of the wing. More specifically, the natural frequency of
 the nacelle and the manner of strut installation can influence the mode
 and air speed at which the wing oscillations become unstable (flutter).
 Conventionally, in order to avoid wing flutter, the natural frequency of
 the nacelles and nacelle struts are restricted within a narrow range. For
 example, in earlier models of the Boeing 747 aircraft, the outboard engine
 nacelles are permitted to oscillate at a natural frequency of about 2
 cycles per second in a lateral direction. If the outboard engine nacelle
 lateral frequencies are significantly above or below 2 cycles per second,
 then wing flutter can result at an unacceptably low air speed.
 However, in some newer aircraft which feature stronger but less stiff
 lifting surfaces, flutter can occur at air speeds below that required by
 government regulations. In this case, the avoidance of wing flutter
 requires the unsatisfactory solution of reducing the maximum operating
 speeds of the aircraft.
 U.S. Pat. No. 4,917,331 discloses a method for preventing wing flutter in
 an aircraft, wherein the lateral natural frequencies of the left and right
 engines are sufficiently different so that when subjected to a time
 varying disturbance in flight, the flutter speed of the aircraft is
 increased. In this design, spring beams are attached at the interface
 between the pylon structure and the wing for damping movement of the
 engine. The spring beams may be tuned to provide the desired lateral
 frequency.
 FIG. 1 shows a prior art engine and wing structure, wherein a wing 10
 supports a pylon structure 12, which supports the engine 14. The pylon
 structure 12 is attached to the wing 10 at the upper link interface 16 and
 mid-spar fittings 18. The spring beam structure described in U.S. Pat. No.
 4,917,331, referenced above, would be attached at the interface between
 the wing 10 and the pylon structure 12, such as in the areas of the upper
 link interface 16 or mid-spar fittings 18 shown in FIG. 1. The spring beam
 attachment structure would replace such attachment devices 16,18. Because
 the spring beams are spaced substantially from the center line 20 of the
 engine (the center of mass), the placement of such spring beams is
 limited, and the size of the entire spring beam attachment structure is
 significant as a result of strength requirements. Accordingly, the spring
 beam attachment structure generally defines the placement of the engine
 14. In other words, as a result of the strength limitations of the
 attachment, the engine 14 may not be positioned where desired to optimize
 efficiency.
 SUMMARY OF THE INVENTION
 The present invention provides a significant improvement over the prior art
 described above by providing a spring beam attachment located at the aft
 engine mount, such as aft engine mount location 22 shown in FIG. 1. The
 aft engine mount 22 shown in FIG. 1 is replaced by a spring beam structure
 on opposing sides of a pivotal connection. Because the aft engine mount
 location 22 is substantially closer to the center line 20 of the engine 14
 in comparison with the attachments 16,18, the moment arm length is reduced
 by approximately 3 feet and the size of the spring beam attachment
 structure may be substantially reduced. This smaller structure is
 therefore not limiting upon the placement of the engine 14, thereby
 allowing optimization of engine placement for improved efficiency. Also,
 the placement of the spring beams at the attachment between the pylon
 structure 22 and the engine 14, as opposed to the attachment between the
 pylon structure 12 and the wing 10, provides the same advantage of
 enabling lateral frequency tuning to minimize flutter.
 More specifically, the present invention provides an aircraft assembly
 including a wing, a pylon structure attached to the wing, an aft engine
 mount attached to the pylon structure, and an engine attached to the aft
 engine mount. The aft engine mount includes a pivotal attachment to the
 pylon structure and first and second spring beams operatively connected to
 the pylon structure at opposing sides of the pivotal attachment for
 damping pivotal movement of the engine with respect to the pylon structure
 to enable tuning of lateral frequency of the engine.
 Objects, features and advantages of the invention are readily apparent from
 the following detailed description of the best mode for carrying out the
 invention.

DETAILED DESCRIPTION OF THE INVENTION
 The present invention provides an engine mounting hanger system which has a
 tunable dual spring beam arrangement that can be altered slightly to
 change the mount stiffness and dramatically change the overall lateral
 natural frequency of the below-wing propulsion package. Essentially, the
 dual spring beam pylon/wing attachment has been relocated from the
 pylon/wing interface to the aft engine mount where the desirable lateral
 damping feature is preserved while allowing the ultimate strength of the
 pylon/wing interface to be increased and uncompromised. This invention
 allows the airplane to meet flutter avoidance criteria with maximum cruise
 speed, fuel load and range, and minimum structural weight in the wing.
 With the stiffness tuning element moved to the aft engine mount where the
 loading is much lighter, the spring elements required are much smaller and
 lighter, and the pylon-towing attachment may be a conventional design.
 Additionally, the dynamic pylon-towing loading is significantly reduced,
 thereby allowing further growth in engine size and improvements in
 airplane performance.
 As shown in FIG. 2, the improved aft engine mount 30 attaches the engine 14
 to the pylon structure 12. The pylon structure 12 includes a pylon
 bulkhead 32, and pylon lower spar chords 34,35. A hanger center pivot 36
 pivotally attaches a hanger-shaped fitting 38 to the pylon structure 12.
 Accordingly, the hanger-shaped fitting 38 is pivotable about the hanger
 center pivot 36. The opposing ends 40,42 of the hanger-shaped fitting 38
 are connected to the spring beams 44,46 by the flanges 48,50 and the pivot
 pins 52,54.
 Each of the spring beams 44,46 is connected at first and second ends 56,58
 thereof to the respective pylon lower spar chord 34,35. The first end 56
 of each spring beam 44,46 is movably attached within the journal 60,62 to
 allow fore and aft movement as the spring beam 44,46 flexes. The second
 end 58 of each spring beam 44,46 is pivotally connected to the respective
 pylon lower spar chord 34,36 by the forward beam pins 61,62.
 Accordingly, as the engine 14 pivots laterally on the hanger-shaped fitting
 38 about the center pivot 36, the spring beams 44,46 will flex between the
 first and second ends 56,58 thereof as a result of forces applied through
 the pivot pins 52,54. As bending of the spring beams 44,46 occurs, the
 first end 56 of the respective spring beams 44,46 will travel forward
 within the respective journal 60,62, and the second end 58 of each spring
 beam 44,46 will pivot about the respective forward beam pin 61,62, thereby
 allowing flexing therebetween.
 The hanger-shaped fitting 38 is pivotally connected to the engine turbine
 case 66 by the first and second end links 68,70. The first and second end
 links 68,70 are connected, respectively, at opposing ends 72, 74, 76, 78
 between the hanger fitting 38 and engine case 66 by the respective pins
 78, 80, 82, 84. The end links 68,70 also include secondary catchers 86,88
 loosely pivotally connected to the hanger-shaped fitting 38 as a stop gap
 measure. The catchers 86,88 only bear load if the pins 80,88 are not
 torqued down properly. A middle link 90 is also pivotally connected
 between a middle portion 92 of the hanger-shaped fitting 38 and the engine
 case 66 via the pins 94,96. The links 68, 70, 90 have spherical bearings
 at each attachment point, thereby allowing cocking of the links, as
 illustrated in FIG. 3.
 An applied side load will cause the engine 14 to rotate about a line
 between the forward engine mount and the aft engine mount hanger center
 pivot 36. This rotation is limited by the flex of the two spring beams
 44,46. The size and material of the spring beams 44,46 can be tailored to
 produce the desired reduction in overall lateral natural frequency of the
 underwing propulsion system.
 FIGS. 5 and 6 show side views of the aft engine mount 30 of the present
 invention attaching the pylon structure 98 of a wing 100 to the core cowl
 102 of an engine 104.
 While an embodiment of the invention has been illustrated and described, it
 is not intended that this embodiment illustrates and describes all
 possible forms of the invention. It is intended that the following claims
 cover all modifications and alternative designs, and all equivalents, that
 fall within the spirit and scope of this invention.