When the aerodynamic forces are examined in 2 dimensions the force in the z direction, with respect to the aviation axis system, is called the lift force, and the force in the x direction is called the drag force. In this article, we will focus on the lift force in the z direction.
Before talking about lift force, if we examine how aerodynamic forces occur for a 2-dimensional airfoil. These are two simple natural resources;
a. Shear stresses (Friction)
b. Pressure distribution over the surface
The current flowing around the object creates friction. Shear stress is defined as the force per unit area acting tangentially to the surface due to friction. It is a point feature, it changes along the surface, and the unbalance of the surface shear stress distribution creates an aerodynamic force on the body.
When the velocity profile on the wing is examined, positional velocity changes create pressure distributions in 2 dimensions. When examined for an airfoil, the difference between the upper wing pressure and the underwing pressure provides the formation of aerodynamic forces. This pressure difference, which occurs mostly, has been tried to be explained through the Bernoulli equation. Air accelerating on the upper surface of the wing creates a low pressure area. As a result of this difference created by the high pressure area on the lower surface of the wing, the lift force is formed. However, this does not happen in symmetrical profiles if there is no angle of attack. For this reason, no lift force is produced for 0 angle of attack in symmetrical wing profiles.
When looking at the formula of the lift force:
It is expressed by the formulation. In this context, we can say that the lift force varies depending on the dynamic pressure, reference area and lift coefficient. The lift force coefficient is expressed by the formulation given below:
When we look at the formulation, we see that the carry coefficient depends on the angle of attack, the angle changes on the control surfaces, the alphadot, q and Mach numbers. All the derivatives and effects seen here also affect the lift force. Here α is called the angle of attack. It is the angle that the airfoil makes with the flow and is critical for the lift coefficient. δ_f and δ_e represent the displacements of the elevator and flap elements in radians. q represents the angular velocity on the y-axis. The expression c/(2V_a ) is the expression that provides the alpha derivative and q terms as nondimensional expressions. where c is the chord length and V_a is the wind speed.
Considering the change of the lift coefficient with the angle of attack, a linear change is seen for the small angle of attack range (α<8^°), while a more parabolic increase is seen at the high angle of attack. After a certain angle of attack, flow separations begin to appear and the lift coefficient decreases, a phenomenon called a stall, and stall angles are available for certain airfoils.
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Author: Berkay Çakır
Editor: Halit Yusuf Genç
Engineer For Space 