GB2269573A - The reduction of configuration buffet on transport aircraft by flap shroud extension. - Google Patents

Abstract

An arrangement for substantially reducing configuration buffet on a transport aircraft including a wing (1) (Fig 1) having a trailing edge including a spanwise extending shroud (8) and two or more trailing edge flaps (3, 4) (Fig 1) mounted on the wing. The flaps are translatable rearwardly and downwardly to create a predetermined gap and overlap relationship between the leading edge of the flaps (3, 4) and the trailing edge of the shroud (8) to define an aerodynamic slot. The shroud (8) incorporates a spanwise chordal extension (7) over at least that portion lying immediately adjacent the ends of each trailing edge flap to give a reduction in area of separated flow on the flap upper surface in the vicinity of the adjacent flap sections whilst maintaining aircraft lift coefficient with a consequential reduction in vibration and buffet forces. Fig 3 shows shroud extensions (13, 14) carried by a spoiler (15) and an airbrake (16) respectively, and the shroud portion (17) at the wing kink has its own extension (18). <IMAGE>

Description

THE REDUCTION OF CONFIGURATION BUFFET QE TRANSPORT AIRCRAFT BY FLAP SHROUD EXTENSION This invention relates to means for reducing configuration buffet on transport aircraft.

When flow separates from the surface of an aircraft, an unsteady aerodynamic force may be induced on the aircraft structure which can cause considerable and undesirable airframe response. This unsteady aerodynamic force is defined as buffet and the subsequent uncoupled structural response as buffeting.

Aircraft are often equipped with multiple leading and trailing-edge devices to achieve the high lift requirements at take-off or landing. The extension of these devices may, in certain circumstances, cause flow separation and induce a buffet force. This may cause heavy aircraft buffeting if its frequency coincides with one or more structural modal frequencies. Since it is related to a specific aircraft configuration it is often referred to as "Configuration Buffet", a phenomenon which has resulted in failures of the trailing edge flaps on several known transport aircraft.

It is known that, in the early development phase of one transport aircraft, severe buffeting was first experienced in initial flight tests on a low speed approach, resulting from local flow separation on the flap surface. This configuration occurred during the approach for landing with flaps extended to the maximum allowable angle. This resulted in an unacceptable level of buffeting vibration in the cockpit and passenger cabin together with large amplitude vibration of the flaps and an associated vibration on the tailplane. This was clearly unacceptable not only for reasons of crew and passenger comfort, there being an additional risk of fatigue damage to the flaps and the tailplane.

In this particular instance the wing was designed with full span leading edge slats and trailing edge flaps. It was thought that the flow on the flap upper surface was separated due to the three-dimensional flow interaction at the "Kink" joint of inboard and outboard flaps at a large flap angle. It was also discovered that the tailplane was submerged in the wake of the separated flap flow, which accounted for the vibration.

The problem was to some extent alleviated by retracting the flaps to a reduced angle and modified in such a manner that levels of vibration on the aircraft were greatly reduced whilst maintaining the required approach and landing performance. Thus the buffeting vibration on the aircraft was no longer a concern for crew and passenger comfort or for the structural fatigue of the airframe and its components.

Whilst the use of a reduced landing flap angle proved adequate to achieve the required airfield performance, this could comprise the potential development of the aircraft or adapt and/or extend its operating role etc. Thus, it was considered desirable to provide a solution to the "configuration buffet" phenomenon without compromising the potential operating performance of the trailing edge flap system.

Accordingly, it is the object of the present invention to provide means for substantially reducing the degree and effects of "configuration buffet" on a transport aircraft.

According to the present invention there is provided an arrangement for substantially reducing configuration buffet on a transport aircraft, said arrangement including: a wing having a trailing edge including a spanwise extending shroud and two or more trailing edge flaps mounted thereon, each of said trailing edge flaps deployable from a stowed position in cruise where, in conjunction with said shroud they conform to and define the wing aerodynamic profile to a maximum landing setting or intermediate settings thereof, said flaps being translated rearwardly and downwardly such that at a specific setting there is a pre-determined gap and overlap relationship between the leading edge of said trailing edge flaps and the trailing edge of said shroud upper surface to define an aerodynamic slot, the invention characterised in that said shroud incorporates a spanwise, chordal extension over at least that portion lying immediately adjacent the ends of each trailing edge flap, so that the aerodynamic slot is convergent, the arrangement being such that, in flight, preferably though not exclusively at maximum flap settings, said spanwise chordal extensions give a reduction in area of separated flow on the flap upper surface in the vicinity of adjacent flap sections whilst at least maintaining aircraft lift co-efficients with a consequential reduction in vibration and buffet forces.

One embodiment of the invention will now be described, by way of example only and with reference to the following drawings in which: Figure 1 illustrates schematically an aircraft wing with high lift devices deployed.

Figure 2 illustrates relative positions of a wing to a trailing edge flap in a wing cross-section.

Figure 3 illustrates, schematically, in plan view, a wing trailing edge arrangement incorporating the present invention.

Figure 4 illustrates graphically contours of the buffet force parameters for variations of "overlap" and "gap".

Figure 5 and 6 illustrate graphically a comparison of flap buffeting vibration in cases with and without "shroud extension" as respectively applied to outboard and inboard flaps.

Figure 7 illustrates a comparison of cockpit vibration with and without "shroud extension".

Figure 8 illustrates a comparison of tailplane vibration with and without "shroud extension".

Figure 9a and 9b respectively illustrate the spectra of wing-tip acceleration with and without "shroud extension" at 8" incidence.

Figure 10 illustrates a cross section through a wing with deployed high lift devices view in direction of arrows 10-10 in Figure 1.

Referring to the drawings, Figures 1 and 2 illustrate a typical transport aircraft wing 1 incorporating high lift devices, namely leading edge slats 2 and two trailing edge flaps 3 and 4, these devices illustrated in their deployed configuration. In this embodiment, the wing is "kinked" at the trailing edge, and the trailing edge flaps 3 and 4 each terminated adjacent that "kink" 5.

As previously stated, one known transport aircraft experienced severe buffeting during approaches for landing with flaps extended to the maximum allowable angle resulting in unacceptable levels of buffeting vibration and large amplitude vibration of the flaps was clearly visible. It was thought that this problem arose due to flow separation on the flap upper surface and that such separation arose as a consequence of three-dimensional flow interaction at the "kink" joint 5 of inboard and outboard flaps. Flow visualisation using tufts revealed that upper surface flow was indeed separated at the joint region, resulting in the tailplane being submerged in such separated flap flow and accounting for the vibration.

It was also found that the area of separation on the flaps could be reduced by partial flap retraction to the smaller angle whilst maintaining the required landing performance but this was considered an undesirable expedient. By reference to Figure 2 it will be seen that there is a "gap" and "overlap" relationship between the leading edge 6 of the flap and the 'beak' 7 of the flap shroud 8 shown as "gap" lOa and "overlap" 9a in the maximum flap setting position 11 and "gap" lOb and "overlap" 9b at the second largest flap setting 12 (shown in broken line). The setting of "overlap" and "gap" for an aircraft flap system is optimised in the design stage by taking the performance parameters at high and low incidence into consideration. However, an optimised setting for the performance may not result in the lowest buffet.It was considered. possible that a certain compromise between the concerns of performance and buffet would lead to a dramatic reduction in the level of buffet whilst still maintaining the desired performance.

Figure 4 is a contour of the amplitude at the critical frequency of the flap accelerometer response for the grid of "overlap" and "gap" tested at the aircraft maximum flap angle.

It shows that, the minimum buffet force can be achieved by setting the "overlap" in the region of 1 - 2% and the "gap" in the position of one percent of the chord. Outside this region, the buffet force can increase dramatically. For the equivalent setting on the aircraft at the flap joint, an average of 0.5 percent of "overlap" and 1.5 percent of "gap" was selected. This setting in the two-dimensional case would give rise to a buffet parameter of about 40% more than the minimum.

In wind tunnel tests aimed at a better understanding of buffet phenomena, the model was tested over a range of incidences from -40 to that at stall. Most modal responses showed a steady variation with model incidence except that the mode at 145Hz became dominant in the incidence range from 7" - 140. the aerodynamic excitation becoming much greater than elsewhere.

This was thought to be the result of a sudden bursting flow separation.

It was subsequently determined that 145Hz is the bending mode of one of the flap supporting tracks and that the flap was experiencing severe buffeting due to a large excitation at about 145Hz. Furthermore, separated flow on the upper surface at the inter-flap joint was revealed by flow visualisation as subsequently confirmed in flight tests.

Referring to Figures 3 and 3a, a marked reduction in the level of buffeting was achieved by means of localised spanwise chordal extensions 13 and 14 to the shrouds, effectively extending the trailing edge of the wing over the flap leading edge 6 and forming a convergent aerodynamic slot. At a model incidence of 8 , the dominant mode at 145Hz was significantly reduced as will be seen by reference to Figures 9a and 9b as was the wing bending mode at 32Hz.

In the arrangement of Figure 3 the shroud upper surface comprises, in the vicinity of the "Kink", twin spoilers 14 and 15 and inner airbrake 16 with only a central portion 17 of fixed shroud. Thus the "shroud extensions" 13 and 14 are attached to, or are preferably integrally formed as part of the spoiler 15 and airbrake 16 and the central portion 17 of the shroud includes its own element of "shroud extension" 18, providing continuity across the kink.

Supplementing wind tunnel tests, subsequent flight tests demonstrated that at various flap settings, several approach speeds and a wide range of incidences, the "shroud extensions" had brought about a broad-band reduction in the buffeting at all measured locations and components of the aircraft.

Figures 5 and 6 show that a broad band reduction in flap vibration was achieved at a typical approach configuration with maximum flap angle. Similarly, Figures 7 and 8 show similar significant reductions in the level of vibrations at the cockpit and the tailplane, being about 40% at the cockpit.

In each of Figures 5 - 8 inclusive, the reference plot, that is the unmodified condition is marked 19 and that arising from the "shroud extension" marked as 20. Thus the present invention, a "shroud extension modification effectively changing the wing and flap relative geometry resulted in a significant reduction in the overall level of aircraft buffeting.

Claims (5)

1. An arrangement for substantially reducing configuration buffet on å transport aircraft, said arrangement including: a wing having a trailing edge including a spanwise extending shroud and two or more trailing edge flaps mounted thereon, each of said trailing edge flaps deployable from a stowed position in cruise where, in conjunction with said shroud, they conform to and define the wing aerodynamic profile, to a maximum landing setting or intermediate settings thereof, said flaps being translated rearwardly and downwardly such that at a specific setting there is a pre-determined gap and overlap relationship between the leading edge of said trailing edge flaps and the trailing edge of said shroud upper surface to define an aerodynamic slot, the invention characterised in that said shroud incorporates a spanwise, chordal extension over at least that portion lying immediately adjacent the ends of each trailing edge flap, so that the aerodynamic slot is convergent.

the arrangement being such that, in flight, preferably though not exclusively at maximum flap settings, said spanwise chordal extensions give a reduction in area of separated flow on the flap upper surface in the vicinity of adjacent flap sections whilst at least maintaining aircraft lift co-efficients with a consequential reduction in vibration and buffet forces.

2. An arrangement according to claim 1 or claim 2 in which said shrouds, over at least a part of their spanwise upper surfaces include lift spoilers and/or airbrakes which in the vicinity of immediately adjacent trailing edge flap sections incorporate spanwise chordal extensions.

3. An arrangement according to claim 1 or claim 2 in which said spanwise chordal extensions are localised extension plates mounted off said shroud upper surfaces or said lift spoilers and/or said airbrakes.

4. An arrangement according to claim 1 or claim 2 in which said spanwise chordal extensions are integrally formed as part of said shroud upper surfaces or said lift spoilers and/or said airbrakes.

5. An arrangement for substantially reducing configuration buffet on a transport aircraft substantially as hereinbefore described and with reference to the accompanying drawings.