96
P. Liu
will expel the water body to obtain the reverse force to leave the water
body again, and the aircraft pushes the air downward continuously during
the flight, so as to obtain the lift according to the reaction force. It is
theoretically believed that the lift is mainly generated by the lower surface
of the airfoil, and the contribution of the upper surface of the airfoil
can be ignored. It is concluded that if the lower surface of the airfoil is
unchanged, the shape of the upper surface will not change the lift, which
is obviously wrong. A typical example is the spoiler. When the spoiler on
the upper surface of the wing is opened, the lower surface does not change.
It can be said that the shape of the upper surface does not change much,
but it has a great impact on the aerodynamic force.
For the real airflow around the airfoil, there is a viscous boundary layer
near the airfoil wall, which leads to the flow velocity along the airfoil.
There are different sizes in the vertical direction of the airfoil. The closer
to the object, the greater the difference.
3. Flow tube theory. When the air flows through the upper and lower
surfaces, due to the bulge of the upper surface, the spacing between the
upper streamlines becomes narrow, while the lower part is relatively flat,
and the spacing between the streamlines becomes wider. According to the
continuity theorem of fluid, when the fluid continuously flows through a
pipe of different thicknesses, since the fluid in any part of the pipe cannot
be interrupted or stacked, the mass of the fluid flowing into any section
and the mass of the fluid flowing out of another section are equal at the
same time, resulting in a greater flow velocity on the upper surface than on
the lower surface, then according to Bernoulli’s theorem, the lift is generated. The doubtful point is that this theory can only be established in a
two-dimensional environment. A large amount of airflow around the real
wing is affected, and the contraction deformation of the flow tube is not
obvious.
4. Downwash airflow theory. The flow direction around the airfoil tends to
deviate downward, and at the same time, a reaction force is generated to
provide lift. This part of the lift does exist, which is called “impact lift”,
but it accounts for a relatively small proportion of the whole airfoil lift.
For the “supercritical airfoil” used in large aircraft, the loading effect after
an airfoil depends on the downwash generated by the downward bending
of the trailing edge of the wing to provide lift.
All the above theories or viewpoints are obtained under the condition
of inviscid flow. The author thinks that it is difficult to explain the whole
region of airfoil flow with any theory or viewpoint. In fact, they are only
P. Liu
will expel the water body to obtain the reverse force to leave the water
body again, and the aircraft pushes the air downward continuously during
the flight, so as to obtain the lift according to the reaction force. It is
theoretically believed that the lift is mainly generated by the lower surface
of the airfoil, and the contribution of the upper surface of the airfoil
can be ignored. It is concluded that if the lower surface of the airfoil is
unchanged, the shape of the upper surface will not change the lift, which
is obviously wrong. A typical example is the spoiler. When the spoiler on
the upper surface of the wing is opened, the lower surface does not change.
It can be said that the shape of the upper surface does not change much,
but it has a great impact on the aerodynamic force.
For the real airflow around the airfoil, there is a viscous boundary layer
near the airfoil wall, which leads to the flow velocity along the airfoil.
There are different sizes in the vertical direction of the airfoil. The closer
to the object, the greater the difference.
3. Flow tube theory. When the air flows through the upper and lower
surfaces, due to the bulge of the upper surface, the spacing between the
upper streamlines becomes narrow, while the lower part is relatively flat,
and the spacing between the streamlines becomes wider. According to the
continuity theorem of fluid, when the fluid continuously flows through a
pipe of different thicknesses, since the fluid in any part of the pipe cannot
be interrupted or stacked, the mass of the fluid flowing into any section
and the mass of the fluid flowing out of another section are equal at the
same time, resulting in a greater flow velocity on the upper surface than on
the lower surface, then according to Bernoulli’s theorem, the lift is generated. The doubtful point is that this theory can only be established in a
two-dimensional environment. A large amount of airflow around the real
wing is affected, and the contraction deformation of the flow tube is not
obvious.
4. Downwash airflow theory. The flow direction around the airfoil tends to
deviate downward, and at the same time, a reaction force is generated to
provide lift. This part of the lift does exist, which is called “impact lift”,
but it accounts for a relatively small proportion of the whole airfoil lift.
For the “supercritical airfoil” used in large aircraft, the loading effect after
an airfoil depends on the downwash generated by the downward bending
of the trailing edge of the wing to provide lift.
All the above theories or viewpoints are obtained under the condition
of inviscid flow. The author thinks that it is difficult to explain the whole
region of airfoil flow with any theory or viewpoint. In fact, they are only