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The DLR Project LamAiR: Design of a NLF Forward Swept Wing for Short and Medium Range Transport Application

Authors:

Abstract

The LamAiR (Laminar Aircraft Research) project deals with the design of a laminar wing for short and medium range transport aircraft operated in the transonic regime. It is well known that extensive laminar flow on wings of such aircraft still can be achieved by natural means, i.e. solely by contour shaping of the airfoil sections. But with Reynolds numbers being in the order of 25 millions in cruise condition the leading edge sweep of the wing should not be higher than approximately 20deg in order to limit the growth of cross-flow instabilities and, hence, prevent early transition. Consequently, the design cruise Mach number for laminar wings of conventionally aft-swept configurations cannot exceed values of about 0.75 and it is expected that the high-speed off-design performance is rather poor. Within the DLR project LamAiR it is therefore investigated if these aerodynamic shortcomings can be overcome by employing forward sweep in combination with aeroelas-tic tailoring using CFRP (Carbon Fiber Reinforced Plastics) materials. In particular the goal is to design a forward swept laminar wing having a design Mach number of 0.78 and the capability of reaching Mach 0.80 in high-speed off-design. The present paper gives an overview on the current status of the project as well as prospects for future work.
American Institute of Aeronautics and Astronautics
1
The DLR Project LamAiR: Design of a NLF Forward Swept
Wing for Short and Medium Range Transport Application
Arne Seitz
1
, Martin Kruse
2
and Tobias Wunderlich
3
DLR, Institute of Aerodynamics and Flow Technology, 38108, Braunschweig, Germany
and
Jens Bold
4
and Lars Heinrich
5
DLR, Institute of Composite Structures, 38108, Braunschweig, Germany
The LamAiR (Laminar Aircraft Research) project deals with the design of a laminar
wing for short and medium range transport aircraft operated in the transonic regime. It is
well known that extensive laminar flow on wings of such aircraft still can be achieved by
natural means, i.e. solely by contour shaping of the airfoil sections. But with Reynolds
numbers being in the order of 25 millions in cruise condition the leading edge sweep of the
wing should not be higher than approximately 20deg in order to limit the growth of cross-
flow instabilities and, hence, prevent early transition. Consequently, the design cruise
Mach number for laminar wings of conventionally aft-swept configurations cannot exceed
values of about 0.75 and it is expected that the high-speed off-design performance is rather
poor. Within the DLR project LamAiR it is therefore investigated if these aerodynamic
shortcomings can be overcome by employing forward sweep in combination with aeroelas-
tic tailoring using CFRP (Carbon Fiber Reinforced Plastics) materials. In particular the
goal is to design a forward swept laminar wing having a design Mach number of 0.78 and
the capability of reaching Mach 0.80 in high-speed off-design. The present paper gives an
overview on the current status of the project as well as prospects for future work.
Nomenclature
ALT = attachment line transition
C
L
= lift coefficient
CFI = crossflow instability
CFRP = carbon fiber reinforced plastics
C
P
, C
P
*
= pressure coefficient and critical pressure coefficient
c = chord length
c
l
= section or local lift coefficient
c
df
= section friction drag coefficient
c
dw
= section wave drag coefficient
c
m25
= pitching moment about quarter chord point
dc = drag counts
NLF = natural laminar flow
N
CF
= crossflow N-factor
N
CF
= crossflow N-factor
M
= Mach number
Re
AMC
= Reynolds number based on aerodynamic mean chord
1
Research Scientist, DLR Braunschweig, Lilienthalplatz 7, 38108 Braunschweig, arne.seitz@dlr.de.
2
Research Scientist, DLR Braunschweig, Lilienthalplatz 7, 38108 Braunschweig, martin.kruse@dlr.de.
3
Research Scientist, DLR Braunschweig, Lilienthalplatz 7, 38108 Braunschweig, tobias.wunderlich@dlr.de.
4
Research Scientist, DLR Braunschweig, Lilienthalplatz 7, 38108 Braunschweig, jens.bold@dlr.de.
5
Research Scientist, DLR Braunschweig, Lilienthalplatz 7, 38108 Braunschweig, lars.heinrich@dlr.de.
29th AIAA Applied Aerodynamics Conference
27 - 30 June 2011, Honolulu, Hawaii
AIAA 2011-3526
Copyright © 2011 by DLR Deutsches Zentrum fuer Luft- und Raumfahrt e.V. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
... Effective measures to delay transition are also explained in an earlier work by Seitz et al. [65]. The specific design of the forwardswept wing is directly derived from previous results obtained in dedicated DLR studies, i.e., projects LamAiR [66] and TuLam [67]. Seitz et al. [66,67] demonstrated promising benefits in terms of fuel-saving but at the cost of additional weight and adding complexity in the structural design of the wing and the design of the high lift devices. ...
... The specific design of the forwardswept wing is directly derived from previous results obtained in dedicated DLR studies, i.e., projects LamAiR [66] and TuLam [67]. Seitz et al. [66,67] demonstrated promising benefits in terms of fuel-saving but at the cost of additional weight and adding complexity in the structural design of the wing and the design of the high lift devices. A lower fuel consumption also means lower emissions but might come at the cost of higher aircraft weight and reduced takeoff and landing performance increasing noise. ...
... It is noticeable that the V-2 design has a lighter wing than the V-R and V-3, which is the result of the smaller wing span. The heavier wing of the V-3 is the result of the forward sweep and larger wing span, requiring the wing to sustain higher bending moments [66]. The fuselage of the V-2 and V-3 are longer than the V-R's (39.90 m/40.25 m vs. 37.70 m) in order to reduce the empennage size and reduce trim drag, again leading to a heavier fuselage compared to the V-R. ...
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... To tackle this problem, a key step forward was taken by the LamAiR [18][19][20] and Tulam project [21] of the German Aerospace Center, DLR, (hereafter, DLR) in which NLF forward-swept wings (FSW) were designed for short-and medium-range transport aircrafts operated in the transonic regime. The advantages of a FSW have been explained and successfully demonstrated by the DLR Institute of Aerodynamics and Flow Technology [18][19][20][21][22]. First, the local sweep angle near the shock-wave position (around midchord) is increased due to the wing taper, when the leading-edge sweep angle is equal to that of a BSW. ...
... To tackle this problem, a key step forward was taken by the LamAiR [18][19][20] and Tulam project [21] of the German Aerospace Center, DLR, (hereafter, DLR) in which NLF forward-swept wings (FSW) were designed for short-and medium-range transport aircrafts operated in the transonic regime. The advantages of a FSW have been explained and successfully demonstrated by the DLR Institute of Aerodynamics and Flow Technology [18][19][20][21][22]. First, the local sweep angle near the shock-wave position (around midchord) is increased due to the wing taper, when the leading-edge sweep angle is equal to that of a BSW. ...
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... ALLEGRA is a mid-range transport aircraft configuration for 150 passengers which was investigated in the DLR project ALLEGRA (AeroeLastic stability and Loads prediction for Enhanced GReen Aircraft, 2012-2016) [59]. Its distinguishing features are the forward swept wing that enables natural laminar flow at a cruise Mach number of 0.78 [88] and the T-tail. The design originates from the DLR project LamAiR (Laminar Aircraft Research, 2009-2012). ...
... The design originates from the DLR project LamAiR (Laminar Aircraft Research, 2009-2012). Its structure is made of composite materials [88]. Figure 2.2 shows the geometry of the ALLEGRA configuration. ...
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This thesis develops and demonstrates an aircraft pre-design process for loads analysis, load alleviation, structural optimization and fatigue analysis. It is shown that the consideration of maneuver and gust load alleviation in early design stages is a promising concept to reduce wing bending moments, structural mass and extend the fatigue life. The reference aircraft considered are two mid-range configurations: one with a backward and another one with a forward swept wing, respectively. In the loads analysis, quasi-steady maneuvers and dynamic 1-cos gusts are considered. For the load alleviation during maneuvers, the ailerons are deflected symmetrically with precalculated amplitudes. For the gust load alleviation, a feed-forward, proportional control algorithm is set up and the main input for the controller is the gust angle of attack. Analogous to maneuver load alleviation, the ailerons are deflected symmetrically. With the post-processed loads from the simulations, the structure of the wing and horizontal tailplane (HTP) is optimized toward mass minimization. The constraints considered are material strength, buckling stability and static aeroelastic requirements. The steps loads analysis and structure optimization of the developed design process are conducted iteratively until the wing box mass converges. For the reference aircraft, the load alleviation yields a reduction of wing box mass by 2.8% and 6.1%, respectively. Beyond that, a qualitative fatigue analysis is carried out to compare the fatigue behaviors of the active and passive aircraft (with and without load alleviation). In this step, loads due to continuous turbulence and ground-air-ground cycles are considered. For the reference missions, the fatigue life of the active aircraft is improved by 28% and 12% respectively, on top of the mass benefit. However, these numbers of fatigue life improvement are only valid for the considered loads and selected positions. If more loading conditions or structure elements are taken into account, the fatigue benefit may vary. As a conclusion, the proposed process can serve to gain an insight into the benefits of load alleviation for a given aircraft in the pre-design phase, before it advances to the next design stage.
... The NLF technology maintains a long laminar-flow extent through a favorable pressure distribution by shaping the wing platform and airfoil geometry [4][5][6]. The HLFC technique combines shape optimization [7,8] and boundary suction at the leading-edge region to delay transitions [9]. ...
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... A forward swept wing offers many aerodynamic advantages which are briefly explained here. A detailed description can be found in references [18][19][20][21][22]. ...
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