18-09-2012, 11:48 AM
Two-dimensional analysis of tandem/staggered airfoils using computational fluid dynamics
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Abstract
The two-dimensional analysis, using computational fluid dynamics (CFD), of
tandem/staggered arranged airfoils of the canard and wing of an Eagle 150 aircraft and also the
aerodynamic tests conducted in an open-circuit wind tunnel are presented in the paper. The wind
tunnel tests were carried out at a speed of 38 m/s in a test section of size 300 mm (width), 300mm
(height) and 600 mm (length), at Reynolds number 2.25 ¥ 105. The tests were carried out with tandem
and staggered placement of the airfoils in order to determine the optimum position of the wing with
respect to the canard and also to determine the lift coefficient at various angles of attack. The CFD
code FLUENT 5 was used to investigate the aerodynamic performance of a two-dimensional model to
validate the wind tunnel results. The flow interaction was studied in the tandem and staggered
arrangements in the wind tunnel as well as by the computational method. The k-e turbulence model
gave exceptionally good results.
Introduction
Theoretical research conducted by McGee and Kroo [1] has suggested that a tandem
wing configuration can be optimized better than conventional configurations under
all flight conditions of an aircraft. Proper utilization of a canard, however, requires
an understanding of the inflow of the canard and flow behavior over the main wing.
Two-dimensional analysis has been a subject of research for some decades because
of special interest in the structure and dynamics of airflow wakes as a function of
the Reynolds number. Calarese [2] studied close-coupled canard–wing vortex interactions.
The tests were performed at low Mach numbers and various angles of attack.
Three configurations were studied: (1) coplanar, (2) canard placed higher than the
wing, and (3) without canard. The results showed that the flow at the leading edge
of sweptback wings at a moderate or high angle of attack separates and produces
vortex sheets that roll up into vortices on the wing’s upper surface. Interference
changes the turbulence characteristics and the trajectories of the vortices. The influence
of canards on wing aerodynamics can often result in increased lift and
decreased trim drag.
Methodology and experimental setup
The Eagle 150 is a two-seater, single-engine, light aircraft. It is designed to be used
as a trainer as well as a surveillance aircraft. It has a tandem wing design (canard
at the front and wing behind it). The aircraft has a wing span of 7163 mm, chord
736.6 mm, aspect ratio 9.72. The canard span is 4879 mm, chord 736.6 mm, aspect
ratio 6.4. The maximum speed is 240 km/h and stall speed (clean) is 101 km/h. The
front view, plan view and side view of the aircraft are shown in Fig. 1.
The aerodynamic tests were carried out in an open-circuit wind tunnel (Fig. 2)
at a speed of 38 m/s. The test section of the wind tunnel is 300mm ¥ 300mm and
600 mm long.
The two airfoils were made from fibreglass, with semi-span aspect ratios for the
canard of 6.4 and for the wing of 9.2. Wind tunnel tests were carried out for two
separate cases. The airfoils were first tested isolated at 38 m/s to get the coefficient
of lift at various angles of attack. Asecond series of experiments was then conducted
for different tandem/staggered positions of the airfoils at the same wind tunnel speed
and at 0° angle of attack. The wind tunnel test section with the canard and airfoil is
shown in Fig. 3 and the canard/wing configuration is shown in Fig. 4. The turbulent
flow over the airfoil was established by making boundary layer measurements with
the help of a Pitot-Static tube and a special boundary layer probe. This enabled us
to select a k-e turbulence model in computational analysis.
Results and discussion
Fig. 5 plots the coefficient of lift, CL, as a function of angle of attack, a. The coefficient
of lift varies with angle of attack and increases up to the stall angle for both
the airfoils. The stall angle for both the airfoils is 11°. The maximum value of CL
for the canard was 1.32 and for the wing was 0.96.
Fig. 6 shows wind tunnel and computational results for both airfoils. The
computational results coincide with the wind tunnel results at all angles of attack
for the canard. However, there is a difference in the computational and experimental
results for the wing airfoil. The deviation is almost constant at all values of angle
of attack.
Conclusions
The two-dimensional analysis of wing and canard using the k-e turbulence model
has shown excellent results, as validated by the wind tunnel results. The experimental
and simulation results for CL against angle of attack coincide. The wind
tunnel experiments for the tandem/staggered positions of the airfoils have given the
optimum position for the wing and have been validated by simulation. The simulation
results also show that with the tandem position, the wake created by the leading
airfoil disturbs the inflow at the trailing airfoil.