In addition to utilizing active control surfaces to control LCO and control reversal, AWT
has researched active control surfaces for the use in increasing the maximum lift
coefficient of an aircraft. The desire to increase the maximum lift coefficient stems from
the desire to decrease the stall speed of the aircraft. Typically, the minimum airspeed of an
aircraft is limited by the stall speed. As the aircraft approaches the stall angle, the airflow
over the top of the wing detaches. The detached flow leads to a drastic loss in lift causing
the aircraft stalls. The stall speed is expressed by the following equation
has researched active control surfaces for the use in increasing the maximum lift
coefficient of an aircraft. The desire to increase the maximum lift coefficient stems from
the desire to decrease the stall speed of the aircraft. Typically, the minimum airspeed of an
aircraft is limited by the stall speed. As the aircraft approaches the stall angle, the airflow
over the top of the wing detaches. The detached flow leads to a drastic loss in lift causing
the aircraft stalls. The stall speed is expressed by the following equation
[3.0.1]
The weight of the aircraft is given by W; the planform area of the wing is given by S; r
is the density of the air; and is the maximum lift coefficient of the airfoil. In most cases,
the density of the air is constant with altitude and the planform area of the wing is as small
as possible to minimize the weight. The maximum lift coefficient, , is the ideal parameter to
optimize in order to reduce stall speed. This section discusses how can be increased by
controlling the airflow about the wing. Leading and trailing-edge flaps and leading edge slats
are the traditional way to increase the lift coefficient. Oscillating flaps are new technique to
used to achieve higher lift coefficients.
is the density of the air; and is the maximum lift coefficient of the airfoil. In most cases,
the density of the air is constant with altitude and the planform area of the wing is as small
as possible to minimize the weight. The maximum lift coefficient, , is the ideal parameter to
optimize in order to reduce stall speed. This section discusses how can be increased by
controlling the airflow about the wing. Leading and trailing-edge flaps and leading edge slats
are the traditional way to increase the lift coefficient. Oscillating flaps are new technique to
used to achieve higher lift coefficients.
3.1 Leading-Edge Devices
Nose flaps, Kruger flaps, and slats are several types of leading edge devices used to
increase the maximum lift coefficient of the aircraft. A slat is shown figure 4. The system
has an opening at the leading edge of the airfoil allowing high pressure air under the airfoil to
pass. As a result, the high pressure air mixes with the air at the top surface, and increases
the energy of the boundary layer at the surface. "By increasing the energy of the
boundary layer the wing can sustain higher angles of attack and a higher maximum
coefficient of lift." [3]
increase the maximum lift coefficient of the aircraft. A slat is shown figure 4. The system
has an opening at the leading edge of the airfoil allowing high pressure air under the airfoil to
pass. As a result, the high pressure air mixes with the air at the top surface, and increases
the energy of the boundary layer at the surface. "By increasing the energy of the
boundary layer the wing can sustain higher angles of attack and a higher maximum
coefficient of lift." [3]
3.2 Trailing Edge Flaps
Flaps change the pressure distribution around the airfoil by changing its chord length
and camber. There are several types of trailing edge flaps. A plain flap is the most common
type of flap. When flaps are deflected downward, they increase chord length, planform area,
and camber of the airfoil. As shown in equation 3.0.1, increasing the planform area of the
airfoil results in a decrease in stall speed. Moreover, a cambered airfoil inherently has a
higher lift coefficient.
and camber. There are several types of trailing edge flaps. A plain flap is the most common
type of flap. When flaps are deflected downward, they increase chord length, planform area,
and camber of the airfoil. As shown in equation 3.0.1, increasing the planform area of the
airfoil results in a decrease in stall speed. Moreover, a cambered airfoil inherently has a
higher lift coefficient.
Figure 5 shows the double slotted flaps mounted on a wing. Slotted flap systems are used to
achieve higher maximum lift coefficient than the plain flap system. "Slotted flaps achieve
higher lift coefficients than plain or split flaps because the boundary layer that forms over
the flap starts at the flap leading edge and is healthier than it would have been if it had
traversed the entire forward part of the airfoil before reaching the flap [5]."
achieve higher maximum lift coefficient than the plain flap system. "Slotted flaps achieve
higher lift coefficients than plain or split flaps because the boundary layer that forms over
the flap starts at the flap leading edge and is healthier than it would have been if it had
traversed the entire forward part of the airfoil before reaching the flap [5]."
3.3 Oscillation excitation
The oscillating flap is usually located near the leading edge of the wing to control the
separation of the airflow from the wing. As mentioned before, the separation of the airflow
induces stall. The purpose of the oscillating flap is to delay the separation or reattach the
separated flow onto the surface. The fact that separation occurs at the top surface of the
wing means that the airflow does not have enough energy to withstand the adverse
pressure gradients. The loss of energy of the flow can be overcome by vortices created by
the oscillating flap. Vortices are energetic rotating fluid cells. Studies have found that "the
vortices shed from the oscillating flap enhance the momentum transfer between the free
stream and the boundary layer, which makes the reattachment of vertices occur more
upstream [6]." The further upstream the vertices are, higher maximum lift coefficient can
be achieved. Moreover, the maximum lift coefficient was further increased when the
excitation frequency of the flap corresponds to the vortex shedding frequency [6].
separation of the airflow from the wing. As mentioned before, the separation of the airflow
induces stall. The purpose of the oscillating flap is to delay the separation or reattach the
separated flow onto the surface. The fact that separation occurs at the top surface of the
wing means that the airflow does not have enough energy to withstand the adverse
pressure gradients. The loss of energy of the flow can be overcome by vortices created by
the oscillating flap. Vortices are energetic rotating fluid cells. Studies have found that "the
vortices shed from the oscillating flap enhance the momentum transfer between the free
stream and the boundary layer, which makes the reattachment of vertices occur more
upstream [6]." The further upstream the vertices are, higher maximum lift coefficient can
be achieved. Moreover, the maximum lift coefficient was further increased when the
excitation frequency of the flap corresponds to the vortex shedding frequency [6].
The applications of the oscillating flaps have been shown to be effective in controlling
the separated airflow over the wing and increasing the maximum lift coefficient at certain
excitation frequencies. However, there still are problems for this technique; there is no
conclusive result as regards to the oscillating mode shapes of the flap motion. Furthermore,
this technique is only successful for limited airflow conditions. AWT believes that further
studies of increasing the lift coefficient by oscillating a flap are in order. Bolding's wing-
stabilator model can be modified for simultaneous studies in LCO suppression, control
reversal, and increasing the lift coefficient. The implementation of active control surfaces
into the current wing-stabilator model applies to all three areas.
the separated airflow over the wing and increasing the maximum lift coefficient at certain
excitation frequencies. However, there still are problems for this technique; there is no
conclusive result as regards to the oscillating mode shapes of the flap motion. Furthermore,
this technique is only successful for limited airflow conditions. AWT believes that further
studies of increasing the lift coefficient by oscillating a flap are in order. Bolding's wing-
stabilator model can be modified for simultaneous studies in LCO suppression, control
reversal, and increasing the lift coefficient. The implementation of active control surfaces
into the current wing-stabilator model applies to all three areas.
|
|
