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On the Most Common Misunderstandings of High-Lift Flows > AEROSPATIAL 2016 > Wild > Oct. 26, 2016 DLR.de Chart 1 AEROSPATIAL 2016, Bucharest, 26-27 October 2016 J. Wild DLR Institute of Aerodynamics and Flow Technology

On the Most Common Misunderstandings of High-Lift …elib.dlr.de/109654/1/On the Most Common Misunderstandings of High... · On the Most Common Misunderstandings of High-Lift Flows

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Page 1: On the Most Common Misunderstandings of High-Lift …elib.dlr.de/109654/1/On the Most Common Misunderstandings of High... · On the Most Common Misunderstandings of High-Lift Flows

On the Most Common Misunderstandings of High-Lift Flows

> AEROSPATIAL 2016 > Wild > Oct. 26, 2016 DLR.de • Chart 1

AEROSPATIAL 2016, Bucharest, 26-27 October 2016 J. Wild DLR Institute of Aerodynamics and Flow Technology

Page 2: On the Most Common Misunderstandings of High-Lift …elib.dlr.de/109654/1/On the Most Common Misunderstandings of High... · On the Most Common Misunderstandings of High-Lift Flows

• We are facing 100 years of high-lift systems • Slotted wing patent application by G. Lachmann (1917/1918) • Slat patent by F. Handley Page (1921) • Flap patent by Harlan D. Fowler (1927)

• Well known (and cited) summaries on high-lift system aerodynamics

• W. Pleines, Wing Brake Flaps. Aircraft Engineering 7(9) (1935) • A.R. Weyl, High-Lift Devices and Tailless Aeroplanes, Aircraft Engineering,

17(19) (1945) • A.M.O. Smith, High Lift Aerodynamics. Journal of Aircraft 12(6) (1975)

• Nevertheless, some basic principles are still misunderstood in the wider

community and still present in text books and recent publications

Motivation for this talk

> AEROSPATIAL 2016 > Wild > Oct. 26, 2016 DLR.de • Chart 2

Page 3: On the Most Common Misunderstandings of High-Lift …elib.dlr.de/109654/1/On the Most Common Misunderstandings of High... · On the Most Common Misunderstandings of High-Lift Flows

• Slots A second method [ to delay stall] is to supply kinetic energy to the boundary layer on the upper surface, making it, at the same time turbulent. This can be done by the provision of appropriately shaped air ducts (slots) leading from high pressure regions of the under surface to low-pressure regions on the upper surface, through which a flow of air is sustained. When this flow of air is by means of convergent walls (nozzle) transformed into one of low pressure and high velocity and then added to the boundary layer, it ,"re-vitalizes" the stale boundary layer, enabling it to follow the contour of the wing surface without separating from it. The powerful downwash which is created by the strong circulation over the forward slat acts in the same way. Slots thus delay the stall; in this way, considerable values of maximum lift are obtained. But slots do not improve the slope of the lift curve, dCL/da.

A.R. Weyl, High-Lift Devices and Tailless Aeroplanes, Aircraft Engineering, Oct. 1945

> AEROSPATIAL 2016 > Wild > Oct. 26, 2016 DLR.de • Chart 3

Page 4: On the Most Common Misunderstandings of High-Lift …elib.dlr.de/109654/1/On the Most Common Misunderstandings of High... · On the Most Common Misunderstandings of High-Lift Flows

• Principle. Boundary-layer control by means of slot (such as in slotted trailing -edge flaps) is based on the concept of injecting momentum into a „tired“ boundary layer. […] Still another important property of slots is [...] that is in a converging slot (as in a nozzle) an equalization of total pressure takes place […]. As a consequence […] such slots are a suitable means of feeding momentum into the boundary layer at the upper side of flaps or airfoils

Remark: The original book from Hörner dates back to 1967. The sections on slotted airfoils have not been edited in the current edition.

S.F. Hörner & H. V. Borst, Fluid Dynamic Lift, 1985

> AEROSPATIAL 2016 > Wild > Oct. 26, 2016 DLR.de • Chart 4

Page 5: On the Most Common Misunderstandings of High-Lift …elib.dlr.de/109654/1/On the Most Common Misunderstandings of High... · On the Most Common Misunderstandings of High-Lift Flows

• energy conservation along stream tube: Bernoulli‘s equation

• without viscous losses the local kinetic energy is

• the kinetic energy of the slot flow is (at maximum) exactly the same as the outer flow at the boundary layer edge

„Slots supply kinetic energy to the boundary layer“

> AEROSPATIAL 2016 > Wild > Oct. 26, 2016 DLR.de • Chart 5

212 .tp p constρ= + =v

2 21 12 2p U pρ ρ∞ ∞ ∞= + −v

Page 6: On the Most Common Misunderstandings of High-Lift …elib.dlr.de/109654/1/On the Most Common Misunderstandings of High... · On the Most Common Misunderstandings of High-Lift Flows

• Separation criteria by Stratford

• shortening the length increases allowable adverse pressure gradient

• for the same pressure rise, shortening the length increases the adverse pressure gradient

• the remaining benefit on separation is the reduction of x0

Lift potential of „fresh boundary layer“

> AEROSPATIAL 2016 > Wild > Oct. 26, 2016 DLR.de • Chart 6

Stratford, B. S., "The Prediction of Separation of the Turbulent Boundary Layer," Journal of Fluid Mechanics, Vol. 5, 1959, pp. 1-16

( )110

2

max

60

00 0

1

10

; : start of pressure rise

p

pp

ucu

dcc x Re S

dxU xRe xν

= −

=

=

Fig.: © AMO Smith, High Lift Aerodynamics. Journal of Aircraft 12(6) (1975) 501-530

Page 7: On the Most Common Misunderstandings of High-Lift …elib.dlr.de/109654/1/On the Most Common Misunderstandings of High... · On the Most Common Misunderstandings of High-Lift Flows

[Slots work] by means of convergent walls (nozzle)

• consequence (of this idea): • narrowing the gap increases the

local flow velocity at the slot exit

• experimental observation • slot velocity decreases with

decreased slot width

• explanation • other than in pressurized ducts,

there is no defined mass flow to go through the slot

x/c [-]

c p[-]

-0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25

-4

-2

0

∆gapS = -1%∆gapS = -0.5%∆gapS = 0%∆gapS = 0.5%∆gapS = 1%

DLR-F153-element airfoil

wing leading edge pressureM∞ = 0.15, Re∞ = 2.0x106

slot width decrease

> AEROSPATIAL 2016 > Wild > Oct. 26, 2016 DLR.de • Chart 7

Page 8: On the Most Common Misunderstandings of High-Lift …elib.dlr.de/109654/1/On the Most Common Misunderstandings of High... · On the Most Common Misunderstandings of High-Lift Flows

• slot effects (Smith, 1975)

What really happens

> AEROSPATIAL 2016 > Wild > Oct. 26, 2016 DLR.de • Chart 8

Fig.: A Betz, Theory of the Slotted Wing. NACA TN-100 (1922)

cp: -13 -11 -9 -7 -5 -3 -1 1

p < p∞

Fig.: © AMO Smith, High Lift Aerodynamics. Journal of Aircraft 12(6) (1975) 501-530

Fig.: © AMO Smith, High Lift Aerodynamics. Journal of Aircraft 12(6) (1975) 501-530

Page 9: On the Most Common Misunderstandings of High-Lift …elib.dlr.de/109654/1/On the Most Common Misunderstandings of High... · On the Most Common Misunderstandings of High-Lift Flows

„low-speed (high-lift) flows can be regarded incompressible“ common assumption for incompressible flows:

> AEROSPATIAL 2016 > Wild > Oct. 26, 2016 DLR.de • Chart 9

0.3M∞ ≤

Page 10: On the Most Common Misunderstandings of High-Lift …elib.dlr.de/109654/1/On the Most Common Misunderstandings of High... · On the Most Common Misunderstandings of High-Lift Flows

• potential theory • incompressible flow equation (linear potential equation)

• compressible small disturbance equation

• Prandtl-Glauert compressibility correction

• incompressible flow assumption

„low-speed (high-lift) flows can be regarded incompressible“

> AEROSPATIAL 2016 > Wild > Oct. 26, 2016 DLR.de • Chart 10

0xx yyφ φ+ =

( )21 0xx yyM φ φ∞− + =

,

2,

11

p k

p ik

cc M∞

=−

,

2,

1 1.0483 5%1 0.3

error in p kp

p ik

cc

c= = ⇒

Page 11: On the Most Common Misunderstandings of High-Lift …elib.dlr.de/109654/1/On the Most Common Misunderstandings of High... · On the Most Common Misunderstandings of High-Lift Flows

„low-speed (high-lift) flows can be regarded incompressible“ • what‘s a small disturbance?

• let‘s assume 5% error in pressure

coefficient for local M>0.3

> AEROSPATIAL 2016 > Wild > Oct. 26, 2016 DLR.de • Chart 11

( ) ( )( )( )

1 1

1

2 21 12 2

2 212 2

1 1

1p

M Mc

M M

κ κκ κ

κκ

κ κ

κ κ

− −

− −∞

−∞

+ − +=

+

0,0001

0,001

0,01

0,1

1

10

100

0 0,1 0,2 0,3

-cp

M∞

cp ( M=0.3 )

Page 12: On the Most Common Misunderstandings of High-Lift …elib.dlr.de/109654/1/On the Most Common Misunderstandings of High... · On the Most Common Misunderstandings of High-Lift Flows

„low-speed (high-lift) flows can be regarded incompressible“ • other characteristic pressure values

for compressible flow

• critical pressure coefficient for local M=1

• 0.7 vacuum rule (Mayer, Research Memo. L8F23, 1948, NACA)

> AEROSPATIAL 2016 > Wild > Oct. 26, 2016 DLR.de • Chart 12

1* 2

22

1 2 111 1pc M

M

κκ

κ

κκ κ

∞∞

− = − + + +

2,0.7 1p vacc M∞ ≤ −

25,00 16,31

1,21

0,00

0,00

0,01

0,10

1,00

10,00

100,00

0 0,2 0,4 0,6 0,8 1

-cp

M∞

cp,0.7vac

cp(M=0,3)

*pc

Page 13: On the Most Common Misunderstandings of High-Lift …elib.dlr.de/109654/1/On the Most Common Misunderstandings of High... · On the Most Common Misunderstandings of High-Lift Flows

„low-speed (high-lift) flows can be regarded incompressible“ • example: slat pressure minimum

> AEROSPATIAL 2016 > Wild > Oct. 26, 2016 DLR.de • Chart 13

-30,0

-25,0

-20,0

-15,0

-10,0

-5,0

0,0-10,0 0,0 10,0 20,0 30,0 40,0

cp,min

α [°]

F15 - 3eRef (VG) Re=7x106

M=0.13M=0.15M=0.2M=0.25

Page 14: On the Most Common Misunderstandings of High-Lift …elib.dlr.de/109654/1/On the Most Common Misunderstandings of High... · On the Most Common Misunderstandings of High-Lift Flows

„low-speed (high-lift) flows can be regarded incompressible“ • example: slat pressure minimum

> AEROSPATIAL 2016 > Wild > Oct. 26, 2016 DLR.de • Chart 14

-30,0

-25,0

-20,0

-15,0

-10,0

-5,0

0,0-10,0 0,0 10,0 20,0 30,0 40,0

cp,min

α [°]

F15 - 3eRef (VG) Re=7x106

M=0.13M=0.15M=0.2M=0.25 cp*

cpM2 = -1 M=0.2

Page 15: On the Most Common Misunderstandings of High-Lift …elib.dlr.de/109654/1/On the Most Common Misunderstandings of High... · On the Most Common Misunderstandings of High-Lift Flows

„low-speed (high-lift) flows can be regarded incompressible“ • example: slat pressure minimum

> AEROSPATIAL 2016 > Wild > Oct. 26, 2016 DLR.de • Chart 15

-30,0

-25,0

-20,0

-15,0

-10,0

-5,0

0,0-10,0 0,0 10,0 20,0 30,0 40,0

cp,min

α [°]

F15 - 3eRef (VG) Re=7x106

M=0.13M=0.15M=0.2M=0.25

cp*

cpM2= -1

M=0.25

Page 16: On the Most Common Misunderstandings of High-Lift …elib.dlr.de/109654/1/On the Most Common Misunderstandings of High... · On the Most Common Misunderstandings of High-Lift Flows

„low-speed (high-lift) flows can be regarded incompressible“ • example: 2-element clean leading

edge

> AEROSPATIAL 2016 > Wild > Oct. 26, 2016 DLR.de • Chart 16

α

CL

CD

0 5 100

1

2

3

0

0.1

0.2

0.3

0.4

0.5

0.6

CLCD

2-element airfoilM∞=0.18, Re∞=22.6x106

Page 17: On the Most Common Misunderstandings of High-Lift …elib.dlr.de/109654/1/On the Most Common Misunderstandings of High... · On the Most Common Misunderstandings of High-Lift Flows

• high-lift system aerodynamics often described in text-books in a very simplified way

• the idea of boundary-layer control as the central mechanism survived nearly 100 years of better explanation

• explaining gap flows as „nozzles“ gives the wrong direction for design • high-lift system flows are a very complex flow structure that may contain very

different features • compressible flow including shocks • viscous flow including boundary layers, free shear layers, separated areas

• the right understanding is necessary to enable highly targeted designs to

reduce the complexity, weight, effort

Conclusions

> AEROSPATIAL 2016 > Wild > Oct. 26, 2016 DLR.de • Chart 17

Page 18: On the Most Common Misunderstandings of High-Lift …elib.dlr.de/109654/1/On the Most Common Misunderstandings of High... · On the Most Common Misunderstandings of High-Lift Flows

Thank you for listening

Jochen Wild DLR Institute of Aerodynamics and Flow Technology Transport Aircraft Branch High-Lift Group [email protected]

> AEROSPATIAL 2016 > Wild > Oct. 26, 2016 DLR.de • Chart 18