Wing root aerofoil for forward swept wings

A swept forward wing for aircraft comprising an inner wing portion and an outer wing portion in which the upper surface curvature of the inner wing portion is designed to create three dimensional flow thereover to manipulate the sweep of the isobars and prevent desweeping thereof, the inner wing portion including a wing root section (4) having a far aft maximum thickness position (14) coupled with high camber in the region of said maximum thickness position, said wing root section (4) further including a negatively cambered leading edge portion (10) and a nose-down twist configured to suppress high leading edge velocities, the combination of thickness and camber forms aft of the leading edge region (10) causing the flow to accelerate until a maximum velocity is reached relatively far back on the wing.

BACKGROUND TO THE INVENTION 
This invention relates to the design of the inner wing region of a forward 
swept wing for transonic flight regimes. More particularly it relates to 
the design of the wing sections towards the wing root which suppresses the 
tendency of the isobars to desweep over the inner wing, so maintaining 
shock wave sweep far inboard. 
Forward swept wings have some potentially attractive aerodynamic features 
including a higher geometric sweep at the shock position, `good` stalling 
characteristics due to the more highly loaded inner wing, and lower wing 
root bending moments leading to a lighter wing structure than the 
`equivalent` aft swept wing. They do have some disadvantages, not the 
least of which is the aerodynamic design of the inner wing region. The 
outer wing design is fairly straight forward and it is not difficult to 
maintain well swept isobars. Over the inner wing, however, there is a 
tendency for a very strong, unswept shock to form at high subsonic Mach 
numbers and at high lift coefficients. 
DESCRIPTION OF THE PRIOR ART 
Various attempts have been made in the past to control the flow over the 
inner wing. These include, 
(i) planform modification to increase the local Doc:US6432/03/03/92 chord 
over the inner wing so reducing the loading intensity in this region. 
(ii) fuselage shaping to provide a more tolerant boundary for the inner 
wing flow 
(iii) using the downwash field generated by lifting foreplane to control 
the onset flow on the wing. 
Although these methods may reduce the strength of the shock wave over the 
inner wing, it still remains largely unswept. It therefore generates 
significant amounts of wave drag and, behind the shock, experiences flow 
breakdown at relatively low lift coefficients compared with wings of 
equivalent aft sweep. 
SUMMARY OF THE INVENTION 
It is the object of the present invention to provide an improved aerofoil 
in which the isobars are forced to remain swept over the inner wing at the 
design condition, so avoiding the large wave drag penalties due to an 
unswept shock. 
According to one aspect of the present invention there is provided a swept 
forward wing for aircraft comprising an inner wing portion and an outer 
wing portion, said inner wing portion including a wing root section having 
a far aft maximum thickness position coupled with high camber in the 
region of said maximum thickness position said wing root section further 
including a negatively cambered leading edge region and a nose-down twist 
configured to suppress high leading edge velocities; 
the combination of thickness and camber forms aft of the leading edge 
region causing the flow to accelerate until a maximum velocity is reached 
relatively far back on the aerofoil and whereby steep recompression is 
alleviated by significant three dimensionality of the flow over the inner 
wing. 
According to a further aspect of the present invention said inner wing 
portion includes at its intersection with said outer wing portion an 
aerofoil section of advanced transonic form and the wing root section 
aerofoil is extrapolated across the inner wing span to the said transonic 
section and the position and strength of the shock wave is controlled.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
The basic approach for the design of the inner region of a forward swept 
wing for transonic flight regimes is outlined below. 
The design of the inner wing poses a problem on a forward swept wing 
because the isobars tend to desweep towards the wing root (especially when 
the wing is at a significant incidence). Referring to the drawings this 
tends to occur irrespective of the wing planform shape as shown in FIGS. 
1a, 1b and 1c illustrating respectively a straight-tapered wing, one 
having an extended leading edge region 1 over the inner wing and one 
having an aft swept leading edge region 2 over the inner wing. Each 
indicates the position of the shock wave 3. This is in respect of a wing 
having a Mach number=0.78 and a typical thickness/chord=12.8%. 
The approach outlined by this invention is to use the geometry of the 
sections towards the wing root to control the flow over the inner wing. As 
shown in FIGS. 3 and 4, this requires a radical change in the section 
geometry of the wing root 4 compared with the typical advanced transonic 
aerofoil section geometry 5 employed over the outboard wing shown in FIG. 
2. Across the inner wing there is an interpolation between the two 
sections 4 and 5. 
In FIG. 3 the wing root section 4 is depicted as comprising a leading edge 
6, an upper surface 7, a lower surface 8 and a trailing edge 9. 
In FIG. 4 the section 4 is shown with a leading edge 10 with a fairly small 
radius of curvature R. This is blended rapidly into the upper surface 11 
and to the lower surface 12 where the curvature is larger and there is a 
considerable lower surface "thickness," i.e.: the lower surface is a 
considerable distance from the chord line. The upper surface region 13 aft 
of the leading edge has a large radius of surface curvature with curvature 
increasing towards a maximum position at 14. The region 15 aft of this 
point has a gradually reducing curvature to the trailing edge 16. The 
design of the lower surface 17 is less critical and could include a 
reflexed region 18 at the rear of the lower surface. An example of the 
type of thickness and camber form for the wing root section is illustrated 
respectively in FIGS. 5a and 5b. The maximum thickness 19 is far aft on 
the section (typically aft of 50% of the chord). The camber form of FIGS. 
5b and 5d show the negative camber 20 at the leading edge increasing to a 
maximum 21 far aft, on the section. The combination of these features 
produces the required upper surface curvature conditions to control the 
flow over the section as illustrated in FIG. 5c. 
FIG. 6 shows the variation in inner wing thickness by means of contours 33 
which indicate distance from a wing axis system where the x axis is 
parallel to a fuselage centre line. To form the inner wing design the wing 
root section 4 is twisted nose down as also shown in FIG. 6a and then 
interpolated across the span to the section geometry 5 required for the 
outer wing. This involves the gradual `pulling forward` of the maximum 
thickness position 14 of the root section 4 to match the maximum thickness 
position 22 of the outboard wing section. The nose down twist 23 of the 
root section 4 is also reduced fairly rapidly to match the twist 24 of the 
outboard sections 5. The rapid change in section twist at the trailing 
edge of the inner wing is shown in the region 34. 
The isobar distribution over this inner wing is shown in FIG. 7a. The 
geometry of the wing root section modifies the flow over the inner wing, 
forcing the isobars to remain swept far inboard. At high transonic Mach 
numbers this means the shock wave 25 over the wing also remains swept 
inboard as shown in FIG. 7b. Typical upper surface chordwise pressure 
coefficient distributions at three spanwise positions over the inner wing 
are shown in FIG. 8. Sample distributions are detailed for a section at 
the wing root section 4 (FIG. 9), a typical outboard wing section 5 (FIG. 
11), and an intermediate inner wing section 26 (FIG. 10). The pressure 
distribution over the upper surface of the wing root section shows that 
the negatively cambered leading edge region, coupled with the overall nose 
down twist of the section is effective in minimising the large leading 
edge peak velocities 27. The gradual increase in upper surfaces curvature 
across the wing root section forces the flow 28 to accelerate until a 
maximum in curvature is reached and the local flow 29 velocity becomes a 
maximum also. This is followed by a recompression 30 to the trailing edge. 
This recompression is mild compared to that expected by looking at the 
two-dimensional geometry of the root section, and it is the strongly 
three-dimensional nature of the flow over the inner wing regions which 
allows the use of these extreme section geometries. The pressure 
distribution over the typical outboard section shows the shock wave 31 aft 
on the section. This form of pressure distribution extends over a large 
proportion of the outboard wing producing a well swept isobar pattern. The 
pressure distribution over the intermediate section ranges between the 
two, the leading edge peak reducing and the amount of supercritical flow 
aft of the leading edge increasing for progressively more outboard 
sections. 
The operation of the present invention is now believed apparent. To achieve 
the desired characteristic of well swept isobars over the inner wing 
region of a forward swept wing, the flow over the wing root region must be 
controlled by using novel section geometries. 
At the root of a straight, planar, forward swept wing there is a tendency 
for a large peak in local velocity to develop close to the leading edge of 
the section (as was seen in FIG. 1a). This dominates the flow over the 
inner wing, pulling the isobar pattern forward on the sections. By a 
combination of negative camber over the leading edge region and the 
overall nose down twist over the inner wing these local velocities can be 
suppressed. Reducing the severity of the leading edge velocity peak alone 
is not sufficient to introduce any significant sweep to the isobars. This 
invention therefore performs a second function by using the geometric 
shaping of the inner wing aft of the leading edge sections (particularly 
the curvature of the upper surface and the thickness distribution) to 
accelerate the flow at the wing root to a maximum aft on the section. 
Having forced this characteristic at the wing root section, the 
intermediate sections over the inner wing, by gradually moving the maximum 
thickness position forward on the chord to match that of the outboard 
wing, also encourages the flow to accelerate aft on the sections. This 
creates the well swept isobar pattern over the outboard wing to be 
`carried over` onto the inner wing region. 
The third feature of this inner wing design is the exploitation of the 
three-dimensional nature of the flow to alleviate the steep pressure 
recovery over the rear inner wing sections. This means that the `extreme` 
geometries of the sections close to the wing root can be tolerated by the 
three-dimensional wing flow because the flow direction over the aft 
position of the inner wing is far from chordwise. 
The inner wing design resulting from this approach can be thicker than 
present forward swept wing designs especially around the 70% chord 
position. This facilitates the incorporation of trailing edge high lift 
systems as well as generally increasing the volume within the sections, 
potentially increasing the fuel volume and leading to a lighter wing 
structure. 
This inner wing geometry can be achieved by modifying the geometry of the 
wing root section 4 itself, as shown in FIG. 12, or by the addition of a 
wing/fuselage fairing, fillet or shaped body 32, as shown in FIG. 13, 
having the same general shape (and function) over a conventional wing root 
section design 35. 
Other inner-wing shapes, slightly modified from that described above are 
possible in the light of the underlying design principle. It is primarily 
the concept of the control of the flow over the inner wing, particularly 
using the upper surface curvature to `manipulate` the sweep of the 
isobars, and the three-dimensionality of the flow to alleviate the 
pressure recovery over the rear of the sections, which comprises the 
novelty of the invention. The detailed description of a specific example 
set forth above is by way of illustration only and is not to be taken as 
limiting on the invention.