2 Aerodynamics
161
maximum lift coefficient of the wing, but also reduces the efficiency
of the aileron and other control surfaces, as shown in Fig. 2.92. The
stall characteristics of swept wing are similar to those of trapezoidal
wing.
It can be seen that the elliptical wing not only has a good lift–drag
characteristics at medium and small angles of attack, but also has good
stall characteristics at high angles of attack. The lift–drag characteristics
of the rectangular wing are not as good as those of the elliptical wing
at medium and small angles of attack, and the C Lmax at high angles of
attack is also small. However, the separation of the wing root first will
not cause the deterioration of the aileron characteristics, which can give
the pilot a warning of impending stall. Because the lift–drag characteristics of trapezoidal wing are similar to those of elliptical wing at medium
and small angle of attack, the structure weight of trapezoidal wing is
also lighter, so it is widely used. However, the separation first occurs
near the wingtip, causing the wingtip to stall first. So in terms of stall
characteristics, trapezoidal straight wing is the worst of the three wings.
Especially, the reduction in aileron efficiency caused by the first separation
of wingtip may lead to serious flight safety problems, which is a serious
and even inadmissible disadvantage in terms of aerodynamics. But as has
been pointed out before, the plane shape of trapezoidal wing is the closest
to the optimal plane shape, so trapezoidal wing is often used, but measures
should be taken to improve its stall characteristics. Common methods are
as follows:
(1) geometric torsion, such as external wash torsion, is adopted to reduce
the angle of attack in the wingtip area to avoid the wingtip reaching
the stall state too early. The value of torsion angle is ϕ = −2° ∼−40°.
(2) the airfoil with a high stall angle of attack is adopted near the wingtip.
(3) the leading-edge slat is used in the outer part of the wing, which
makes the airflow with high pressure flow from the lower wing surface
to the upper surface through the leading-edge slot, accelerating the
airflow on the upper wing surface, thus delaying the separation of the
boundary layer of the outer part of the wing.
2.10 Interaction Between Shock Wave
and Boundary Layer in Supersonic Flow
1. Overview
When the flight speed of the aircraft exceeds the critical speed or supersonic speed, the supersonic flow will appear when the airflow bypasses
161
maximum lift coefficient of the wing, but also reduces the efficiency
of the aileron and other control surfaces, as shown in Fig. 2.92. The
stall characteristics of swept wing are similar to those of trapezoidal
wing.
It can be seen that the elliptical wing not only has a good lift–drag
characteristics at medium and small angles of attack, but also has good
stall characteristics at high angles of attack. The lift–drag characteristics
of the rectangular wing are not as good as those of the elliptical wing
at medium and small angles of attack, and the C Lmax at high angles of
attack is also small. However, the separation of the wing root first will
not cause the deterioration of the aileron characteristics, which can give
the pilot a warning of impending stall. Because the lift–drag characteristics of trapezoidal wing are similar to those of elliptical wing at medium
and small angle of attack, the structure weight of trapezoidal wing is
also lighter, so it is widely used. However, the separation first occurs
near the wingtip, causing the wingtip to stall first. So in terms of stall
characteristics, trapezoidal straight wing is the worst of the three wings.
Especially, the reduction in aileron efficiency caused by the first separation
of wingtip may lead to serious flight safety problems, which is a serious
and even inadmissible disadvantage in terms of aerodynamics. But as has
been pointed out before, the plane shape of trapezoidal wing is the closest
to the optimal plane shape, so trapezoidal wing is often used, but measures
should be taken to improve its stall characteristics. Common methods are
as follows:
(1) geometric torsion, such as external wash torsion, is adopted to reduce
the angle of attack in the wingtip area to avoid the wingtip reaching
the stall state too early. The value of torsion angle is ϕ = −2° ∼−40°.
(2) the airfoil with a high stall angle of attack is adopted near the wingtip.
(3) the leading-edge slat is used in the outer part of the wing, which
makes the airflow with high pressure flow from the lower wing surface
to the upper surface through the leading-edge slot, accelerating the
airflow on the upper wing surface, thus delaying the separation of the
boundary layer of the outer part of the wing.
2.10 Interaction Between Shock Wave
and Boundary Layer in Supersonic Flow
1. Overview
When the flight speed of the aircraft exceeds the critical speed or supersonic speed, the supersonic flow will appear when the airflow bypasses