Stall trigger system

Abstract

A stall trigger system, the stall trigger system comprising: an aircraft wingtip or an aircraft wingtip device; a plurality of sets of stall triggers, the plurality of sets of stall triggers being distributed along the wingspan of the aircraft wingtip or the aircraft wingtip device, wherein each set of stall triggers includes one or more stall triggers that can be activated to trigger local separation of the airflow above the aircraft wingtip or the aircraft wingtip device, and each set of stall triggers has a different activation threshold; and a control system configured to monitor a parameter and activate each set of stall triggers in response to the parameter reaching the corresponding activation threshold of each set of stall triggers.

Description

Technical Field

[0001] The present invention relates to a stall trigger system for an aircraft wingtip or an aircraft wingtip device, and a method for enabling such a stall trigger system. Background Art

[0002] WO 03 / 000547A1 discloses a triggering device which is arranged in one or more leading edge segments and can be deployed from within the winglet cladding so as to protrude above the upper surface of the winglet. The triggering device includes an elongate plate or a series of plates which may be continuous or discontinuous along the wingspan of the winglet and which are capable of moving in suitable guiding means in the leading edge segment. When the plate is lifted, the flow above the upper surface of the winglet separates downstream of the plate, thereby reducing the lift on the winglet. Summary of the Invention

[0003] A first aspect of the present invention provides a stall trigger system, which includes: an aircraft wingtip or an aircraft wingtip device; multiple sets of stall triggers which are distributed along the wingspan of the aircraft wingtip or the aircraft wingtip device, wherein each set of stall triggers includes one or more stall triggers which can be enabled to trigger local separation of the airflow above the aircraft wingtip or the aircraft wingtip device, and each set of stall triggers has a different enabling threshold; and a control system which is configured to monitor a parameter and enable each set of stall triggers in response to the parameter reaching the corresponding enabling threshold of each set of stall triggers.

[0004] Another aspect of the present invention provides an aircraft including the stall trigger system of the first aspect.

[0005] Another aspect of the present invention provides an aircraft wing including the stall trigger system of the first aspect.

[0006] Optionally, the aircraft wing includes a main wing structure, wherein the stall trigger system includes an aircraft wingtip device attached to the tip of the main wing structure, and the stall triggers are distributed along the wingspan of the aircraft wingtip device.

[0007] Optionally, the multiple sets of stall triggers have a wingspan position of 70% of the wingspan or higher, 80% of the wingspan or higher, or 85% of the wingspan or higher.

[0008] Another aspect of the present invention provides a method of enabling a stall trigger system, the stall trigger system including multiple sets of stall triggers distributed along the span of an aircraft wingtip or an aircraft wingtip device, wherein each set of stall triggers includes one or more stall triggers and each set of stall triggers has a different enabling threshold, the method including: monitoring a parameter and enabling each set of stall triggers in response to the parameter reaching the corresponding enabling threshold of each set of stall triggers, thereby triggering a local separation of the airflow above the aircraft wingtip or the aircraft wingtip device.

[0009] Another aspect of the present invention provides a method of enabling a stall trigger system, the stall trigger system including a plurality of stall triggers distributed along the span of an aircraft wingtip or an aircraft wingtip device, the method including enabling the stall triggers in an enabling sequence, thereby triggering a local separation of the airflow above the aircraft wingtip or the aircraft wingtip device in a corresponding sequence of regions of the aircraft wingtip or the aircraft wingtip device.

[0010] The parameter may represent an angle of incidence or pressure.

[0011] Some or all of the stall triggers may have a chordwise position less than 5% of the local chord length of the aircraft wingtip or the aircraft wingtip device.

[0012] Each stall trigger may be configured such that upon enabling it changes shape or moves such that the stall trigger protrudes from the aerodynamic surface of the aircraft wingtip or the aircraft wingtip device, thereby triggering a local separation of the airflow above the aircraft wingtip or the aircraft wingtip device.

[0013] Each stall trigger may be configured such that upon enabling it changes shape or moves such that the stall trigger protrudes from the aerodynamic surface of the aircraft wingtip or the aircraft wingtip device by an amount less than 2% of the local chord length of the aircraft wingtip or the aircraft wingtip device, and preferably by an amount less than 1% of the local chord length of the aircraft wingtip or the aircraft wingtip device.

[0014] The stall trigger system may include three or more sets of stall triggers.

[0015] The stall trigger system may include at least ten stall triggers, and preferably includes at least twenty triggers.

[0016] Each set of stall triggers may include two or more stall triggers.

[0017] The aircraft wingtip or the aircraft wingtip device may include a leading edge and a trailing edge.

[0018] At least a portion of the leading edge may be substantially straight.

[0019] The stall triggers can be distributed along the span of the aircraft wingtip or the aircraft wingtip device on a line having an average sweep angle less than 50 degrees, and preferably less than 40 degrees.

[0020] The aircraft wingtip or the aircraft wingtip device can be a winglet.

[0021] The aircraft wingtip or the aircraft wingtip device can be the tip of the aircraft wing; or it can be a wingtip device attached to the tip of the main wing structure, such as a winglet.

[0022] Multiple sets of stall triggers can be enabled in an enabling sequence to trigger local separation of the airflow above the aircraft wingtip or the aircraft wingtip device in the corresponding sequence area of the aircraft wingtip or the aircraft wingtip device.

[0023] The enabling sequence can be an increasing enabling sequence.

[0024] The increasing enabling sequence can cause the area of the separated flow to become longer in a gradually inward direction. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Embodiments of the present invention will now be described with reference to the accompanying drawings, in which:

[0026] Figure 1 An aircraft is shown;

[0027] Figure 2 is a plan view of the wing of the aircraft;

[0028] Figure 3 is a plan view showing a winglet stall trigger system according to an embodiment of the present invention;

[0029] Figure 4A A piezoelectric stall trigger in a non-enabled state is shown;

[0030] Figure 4B A stall trigger in an enabled state is shown Figure 4A of;

[0031] Figure 5A A "smart material" stall trigger in a non-enabled state is shown;

[0032] Figure 5B A stall trigger in an enabled state is shown Figure 5A of;

[0033] Figure 6 is a cross-sectional view showing flow separation caused by an enabled stall trigger;

[0034] Figure 7Shows a control system for a stall trigger based on the angle of incidence of an aircraft;

[0035] Figure 8 Shows a control system for a stall trigger based on the leading-edge air pressure;

[0036] Figure 9 Shows the first step of an enabling sequence;

[0037] Figure 10 Shows the second step of an enabling sequence;

[0038] Figure 11 Shows the third step of an enabling sequence; and

[0039] Figure 12 Is a graph showing drag reduction versus the angle of incidence of the aircraft. Detailed Description

[0040] Figure 1 Shows an aircraft 10 having a winglet stall trigger system. The aircraft includes a fuselage and a pair of wings. Each wing includes a main wing structure 15 and a winglet 20 attached to the tip of the main wing structure 15. The winglet points upward - in other words, the winglet is angled upward relative to the plane of the main wing structure 15. The port winglet and the starboard winglet are mirror images of each other - only the starboard winglet 20 is visible in Figure 1 Shown. Figure 2 And Figure 3 Show the winglet 20 in a plan view.

[0041] The winglet 20 has an airfoil profile with a leading edge 21, a trailing edge 22, an upper aerodynamic surface 23, and a lower aerodynamic surface 24. The leading edge 21 is generally straight and has a relatively low sweep angle θ (usually θ is less than 40 degrees), and the sweep angle θ is represented in Figure 2 Shown.

[0042] The winglet 20 extends from a winglet root 25 to a winglet tip 26, and the winglet 20 is attached to the tip of the main wing structure 15 at the winglet root 25.

[0043] The aircraft includes Figure 3 The shown stall trigger system, which includes multiple sets of stall triggers 30a to 30d distributed along the span of the winglet 20, and a control system 40. Each set of stall triggers 30a to 30d includes a plurality of stall triggers that can be activated to trigger local separation of the airflow above the winglet 20.

[0044] The span of the aircraft is defined as the distance from the tip of one wing (i.e., the winglet tip 26) to the tip of the other wing. The line from the tip of one wing to the tip of the other wing runs along Figure 2extends in the spanwise direction shown, which is perpendicular to the centerline 29 of the aircraft 10.

[0045] The root of the wing is at 0% wingspan and the tip of the wing (i.e., the winglet tip 26) is at 100% wingspan. % wingspan represents the spanwise position along the spanwise direction. In this example, the winglet root 25 is at approximately 85% of the wingspan, so the multiple sets of stall triggers 30a to 30d have a wingspan position of 85% wingspan or greater. In other words, the multiple sets of stall triggers are located in the outermost 15% of the wingspan of the wing.

[0046] The stall triggers are all the same, and an exemplary stall trigger is given the reference numeral 30, but it will be understood that in alternative examples, each stall trigger or each set of stall triggers may be of different sizes, for example, different in size according to the wingspan position of each stall trigger 30. Each stall trigger 30 is configured such that when enabled, the stall trigger changes shape or moves such that it protrudes from the upper aerodynamic surface 23 of the winglet, thereby triggering local separation 31 of the airflow above the winglet, as Figure 6 shown in, which shows the stall trigger 30 in its enabled state.

[0047] As Figure 6 shown in, the winglet has a chord line 27 from the leading edge 21 to the trailing edge 22. The stall trigger 30 is positioned relatively close to the leading edge 21. For example, the stall trigger may have a chordwise position 28 of less than 5% of the chord (i.e., less than 5% of the local chord length at the wingspan position of the stall trigger).

[0048] Each stall trigger 30 is configured such that when enabled, the stall trigger protrudes from the aerodynamic surface 23 of the winglet by an amount less than 2% of the local chord length, and preferably less than 1% of the local chord length.

[0049] As Figure 3 shown in, the stall triggers are distributed in a generally straight line 32 parallel to the leading edge 21. Thus, like the leading edge 21, the line 32 has a relatively low mean sweep angle (typically less than 40 degrees).

[0050] In this example, most of the leading edge 21 is generally straight and the stall triggers are distributed in a generally straight line 32. However, this is not restrictive, and in other examples, the leading edge 21 (and / or the line along which the stall triggers are distributed) may be continuously curved.

[0051] Figure 4A and Figure 4BA first example of the stall trigger 30 is given. In this case, the stall trigger 30 is a piezoelectric device which, when enabled, changes shape from Figure 4A an unenabled shape in Figure 4A (in which the piezoelectric device is flush with the upper aerodynamic surface 23) to Figure 4B a lengthened enabled state shown in Figure 4B (in which the piezoelectric device protrudes from the upper aerodynamic surface 23).

[0052] Figure 5A and Figure 5B A second example of the stall trigger 30 is given. In this case, the stall trigger 30 is a piezoelectric polymer “smart material” device which, when enabled, changes shape from Figure 5A an unenabled shape in Figure 5A (in which the piezoelectric polymer “smart material” device is flush with the upper aerodynamic surface 23) to Figure 5B an enabled state shown in Figure 5B (in which the piezoelectric polymer “smart material” device protrudes from the upper aerodynamic surface 23).

[0053] The planar profile of each stall trigger 30 is shown as a square in Figure 3 but this is not restrictive (for example, the stall trigger can have a circular or rectangular planar profile).

[0054] In the examples of FIGS. 4 and 5, each stall trigger changes shape when enabled. In another embodiment, each stall trigger 30 can be configured such that when enabled it moves (by translation and / or rotation) to cause the stall trigger to protrude from the aerodynamic surface, thereby triggering local separation of the airflow. In this case, each stall trigger can be driven by, for example, a solenoid.

[0055] In other embodiments, each stall trigger 30 is capable of triggering local separation of the airflow in some other way.

[0056] The control system 40 is configured to monitor parameters during flight of the aircraft and to enable each set of stall triggers 30a to 30d in response to the parameters reaching the respective enable thresholds for each set of stall triggers. Each set of stall triggers 30a to 30d has a different enable threshold, such that as the parameters change, the stall triggers are enabled in an enabling sequence, thereby triggering local separation of the airflow above the winglet 20 in the corresponding sequence regions of the winglet 20.

[0057] Figure 7A first example of a suitable parameter that can be used to control the stall triggers is given. In this example, the parameter represents the angle of incidence of the aircraft. The angle of incidence is measured by the angle of incidence sensor 41 and fed into the control system 40, which enables the respective groups of stall triggers 30a to 30d via the corresponding control lines 42a to 42d.

[0058] Figure 8 A second example of a suitable parameter that can be used to control the stall triggers is given. In this example, the parameter represents the pressure at the leading edge 21 of the winglet 20. The pressure is measured by the local pressure sensor 42 positioned at the leading edge 21 of the winglet 20 and fed into the control system 40, which enables the respective groups of stall triggers 30a to 30d via the control lines 42a to 42d.

[0059] Figure 7 and Figure 8 The examples of and are non - restrictive, i.e., other parameters can be measured during the flight of the aircraft and used to control the stall triggers.

[0060] Figures 9 to 11 An example of the enabling sequence is given.

[0061] In Figure 9 In the first step of the enabling sequence shown in, the parameter reaches a first enabling threshold that causes the control system 40 to enable the first group of stall triggers 30a. This triggers a local separation of the airflow above the winglet 20 in the outer region 45a near its tip 26 of the winglet 20. The region 45a of the separated flow has an inner boundary 46a.

[0062] In Figure 10 In the second step of the enabling sequence shown in, the parameter reaches a second enabling threshold that causes the control system 40 to enable the second group of stall triggers 30b. This triggers a local separation of the airflow above the winglet 20 in the outer mid - span region 45b of the winglet 20. The first group of stall triggers 30a remains in its enabled state, so the outer region 45a of the winglet 20 remains stalled. The regions 45a, 45b of the separated flow have an inner boundary 46b.

[0063] In Figure 11 In the third step of the enabling sequence shown in, the parameter reaches a third enabling threshold that causes the control system 40 to enable the third group of stall triggers 30c. This triggers a local separation of the airflow above the winglet 20 in the inner mid - span region 45c of the winglet 20. The first group of stall triggers 30a and the second group of stall triggers 30b remain in their enabled states, so the outer region 45a and the outer mid - span region 45b of the winglet 20 remain stalled. The regions 45a, 45b, 45c of the separated flow have an inner boundary 46c.

[0064] Thus, in Figures 9 to 11 it can be seen that the three sets of stall triggers 30a to 30c are enabled in the enabling sequence, thereby triggering local separation of the airflow above the winglet 20 in corresponding sequences in three regions 45a to 45c of the winglet 20. In this example, the set of stall triggers 30d at the winglet root is not enabled, but they can be enabled at higher angles of incidence.

[0065] If the parameter used to control the stall triggers is the angle of incidence as in Figure 7 , then each set of stall triggers can be enabled in response to the angle of incidence increasing to reach a corresponding threshold. Thus, as the angle of incidence increases, the length of the separated flow region increases in the inner direction, as shown in the progression from Figures 9 to 11 .

[0066] If the parameter used to control the stall triggers is the pressure as in Figure 8 , then each set of stall triggers can be enabled in response to the pressure decreasing to reach a corresponding threshold. Thus, as the pressure decreases, the length of the separated flow region increases in the inner direction, as shown in the progression from Figures 9 to 11 . In the example of Figure 8 , there is only a single pressure sensor, but in alternative embodiments, multiple pressure sensors can be used, each located near a corresponding set of stall triggers. This enables each set of stall triggers to be independently controlled based on its local pressure signal.

[0067] It should be noted that the number of stall triggers and / or the number of sets of stall triggers can be different from the examples shown in the figures.

[0068] For example, there can be only two, three, or four sets of stall triggers, or many sets of stall triggers.

[0069] The total number of stall triggers can be as low as two or three, but more typically there are at least ten stall triggers, and preferably at least twenty.

[0070] Some or all of the sets of stall triggers 30a to 30d in multiple sets of stall triggers can include only a single stall trigger, but more typically, each set of stall triggers includes two or more stall triggers.

[0071] In the Figures 9 to 11 example, the enabling sequence is a gradually increasing enabling sequence, which causes the region of separated flow to become longer in a gradually inward direction. In other words, the number of enabled sets increases as the angle of incidence increases, and the stall region of the winglet 20 also increases accordingly. In other (non-gradually increasing) enabling sequences, multiple sets of stall triggers can be disabled when the next set is enabled.

[0072] The port winglet and the starboard winglet each carry a respective stall trigger line. A single control system 40 can control both stall trigger lines, or the port winglet and the starboard winglet can be controlled independently by respective different control systems.

[0073] The control system 40 can be housed in any part of the aircraft, including in the fuselage, the wing, or the winglet.

[0074] Figure 12 FIG. is a diagram showing the benefits of the stall trigger system. Diagram 50 shows the drag of the wing without the winglet 20. The diagram with two parts 51a and 51b shows the drag reduction performance of a basic low-swept winglet (i.e., the winglet 20 without a stall trigger).

[0075] As shown in diagram part 51a, the basic low-swept winglet produces a large drag benefit up to a higher angle of incidence, but may then suffer sudden flow separation and stall over a large or entire area of the winglet at the angle of incidence 52 of "sudden stall", so the drag reduction drops sharply. This results in a very large increase in drag and a reduction in lift, as shown in diagram part 51b. Due to the suddenness and sensitivity of the effect, this may occur asymmetrically on the port winglet and the starboard winglet.

[0076] Diagram 53 shows the drag performance of a winglet with a highly swept leading edge. The highly swept winglet produces wingtip vortices at a medium angle of incidence. This reduces the local load in this area and the total drag benefit relative to an ideal winglet (the diagram of the ideal winglet is indicated at 54). As the angle of incidence further increases, the wingtip vortices move towards the wing root, and if the aircraft angle of incidence decreases again, the wingtip vortices move back outwards correspondingly. This behavior produces more benign and symmetric handling qualities relative to the basic low-swept winglet, as indicated by diagram 53.

[0077] Diagram 55 shows the drag performance of the winglet 20 with the above-mentioned stall trigger system. The first activation threshold, the second activation threshold, and the third activation threshold of multiple sets of stall triggers 30a to 30c are indicated at 56a, 56b, and 56c respectively.

[0078] By triggering local flow separation at the wingtip 26 at a medium angle of incidence and then gradually increasing the degree of inboard flow separation as the aircraft angle of incidence increases, the stall trigger system enables the behavior of a highly swept winglet (as indicated by diagram 53).

[0079] The actively triggered winglet 20 will have a lower drag benefit at aircraft angles of incidence below the "sudden stall" angle 52 at which the basic low-swept winglet stalls, but a higher drag benefit at aircraft angles of incidence above the "sudden stall" angle 52.

[0080] The flight vehicle handling quality behavior can also be improved. Due to the rapid activation of the stall trigger, the flow on the winglet can be dynamically controlled to suit the flight vehicle maneuvers.

[0081] In the above-described embodiment, the stall triggers are distributed along the span of the upwardly directed winglet. However, the present invention can be applied to other types of wingtip devices, such as downwardly directed winglets; or wingtip devices that are not winglets. It will also be understood that the present invention is not limited to any particular shape or profile of the wingtip device, for example, the wingtip device can include a continuously curved portion, a straight portion, or any combination thereof.

[0082] The wingtip device can have an in-span position of 70% of the wingspan or higher, 80% of the wingspan or higher, or 85% of the wingspan or higher.

[0083] Generally, the main wing structure does not have a stall trigger.

[0084] In the above-described embodiment, the stall triggers are distributed along the span of the flight vehicle wingtip device attached to the main wing structure. However, the present invention can be applied to a flight vehicle wing without a separate wingtip device. In this case, the stall triggers can be distributed along the span of the flight vehicle wing tip, which can be, for example, an angled wing tip. In this case, multiple sets of stall triggers can have an in-span position of 70% of the wingspan or higher (no stall trigger in the innermost 70% of the wingspan of the wing), 80% of the wingspan or higher (no stall trigger in the innermost 80% of the wingspan of the wing), or 85% of the wingspan or higher (no stall trigger in the innermost 85% of the wingspan of the wing).

[0085] When the word 'or' appears, this will be interpreted as meaning 'and / or', such that the items referred to are not necessarily mutually exclusive and can be used in any suitable combination.

[0086] Although the present invention has been described above with reference to one or more preferred embodiments, it will be understood that various changes or modifications can be made without departing from the scope of the present invention as defined by the appended claims.

Claims

1. A stall trigger system, comprising: An aircraft wingtip device, or the aircraft wingtip of an aircraft excluding said aircraft wingtip device; Multiple sets of stall triggers, said multiple sets of stall triggers being distributed along the span of said aircraft wingtip or aircraft wingtip device, wherein each set of stall triggers includes one or more stall triggers, said stall triggers being capable of being activated to trigger local separation of the airflow above said aircraft wingtip or aircraft wingtip device, and each set of stall triggers having a different activation threshold; and A control system, said control system being configured to monitor a parameter and activate each set of stall triggers in response to said parameter reaching the corresponding activation threshold of each set of stall triggers.

2. The stall trigger system according to claim 1, wherein, Said parameter represents the angle of incidence or pressure.

3. The stall trigger system according to claim 1 or 2, wherein, Some or all of said stall triggers have a chordwise position that is less than 5% of the local wing chord length of said aircraft wingtip or aircraft wingtip device.

4. The stall trigger system according to claim 1 or 2, wherein, Each stall trigger is configured such that when activated, each stall trigger changes shape or moves such that each stall trigger protrudes from the aerodynamic surface of said aircraft wingtip or aircraft wingtip device, thereby triggering said local separation of the airflow above said aircraft wingtip or aircraft wingtip device.

5. The stall trigger system according to claim 4, wherein, Each stall trigger is configured such that when activated, each stall trigger changes shape or moves such that each stall trigger protrudes from the aerodynamic surface of said aircraft wingtip or aircraft wingtip device by an amount less than 2% of the local wing chord length of said aircraft wingtip or aircraft wingtip device.

6. The stall trigger system according to claim 1 or 2, comprising three or more sets of stall triggers.

7. The stall trigger system according to claim 1 or 2, comprising at least ten stall triggers.

8. The stall trigger system according to claim 1 or 2, wherein, Each set of stall triggers includes two or more stall triggers.

9. The stall trigger system according to claim 1 or 2, wherein, Said aircraft wingtip or aircraft wingtip device includes a leading edge and a trailing edge, and at least a portion of said leading edge is substantially straight.

10. The stall trigger system according to claim 1 or 2, wherein, Said stall triggers are distributed along the span of said aircraft wingtip or aircraft wingtip device in a line having an average sweep angle less than 50 degrees.

11. The stall trigger system according to claim 1 or 2, wherein, Said aircraft wingtip device is a winglet.

12. The stall trigger system according to claim 5, wherein, Each stall trigger is configured such that when enabled, each stall trigger changes shape or moves such that each stall trigger protrudes from the aerodynamic surface of the aircraft wingtip or aircraft wingtip device by an amount less than 1% of the local chord length of the aircraft wingtip or aircraft wingtip device.

13. The stall trigger system according to claim 7, comprising at least twenty stall triggers.

14. The stall trigger system according to claim 10, wherein, the average sweep angle is less than 40 degrees.

15. An aircraft wing comprising the stall trigger system according to any one of claims 1 to 14.

16. The aircraft wing according to claim 15, comprising a main wing structure, wherein, the stall trigger system comprises an aircraft wingtip device attached to the tip of the main wing structure, and the stall triggers are distributed along the span of the aircraft wingtip device.

17. The aircraft wing according to claim 15 or 16, wherein, the multiple sets of stall triggers have a spanwise position of 70% of the wingspan or greater.

18. An aircraft comprising the stall trigger system according to any one of claims 1 to 14.

19. A method of enabling a stall trigger system, the stall trigger system comprising multiple sets of stall triggers distributed along the span of an aircraft wingtip or an aircraft wingtip device, wherein, each set of stall triggers comprises one or more stall triggers, and each set of stall triggers has a different enabling threshold, the method comprising: monitoring a parameter and enabling each set of stall triggers in response to the parameter reaching the corresponding enabling threshold of each set of stall triggers, thereby triggering local separation of the airflow above the aircraft wingtip or the aircraft wingtip device.

20. The method according to claim 19, wherein, the multiple sets of stall triggers are enabled in an enabling sequence, thereby triggering local separation of the airflow above the aircraft wingtip or the aircraft wingtip device in corresponding sequence regions of the aircraft wingtip or the aircraft wingtip device.

21. A method of enabling a stall trigger system, the stall trigger system comprising a plurality of stall triggers distributed along the span of an aircraft wingtip or an aircraft wingtip device, the method comprising enabling the stall triggers in an enabling sequence, thereby triggering local separation of the airflow above the aircraft wingtip or the aircraft wingtip device in corresponding sequence regions of the aircraft wingtip or the aircraft wingtip device.

22. The method according to claim 20 or 21, wherein, the enabling sequence is an increasing enabling sequence.

23. The method according to claim 22, wherein, the increasing enabling sequence causes the region of the separated flow to become longer in a gradually inward direction.

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