The source of all aerodynamics forces

Introduction

In previous session of The Importance of Aerodynamic Flow Quantities in Engineering Design We have just discussed the four basic aerodynamic flow quantities: p, ρ, T, and V , where V is velocity, which has both magnitude and direction; that is, velocity is a vector quantity. A knowledge of p, ρ, T, and V at each point of a flow fully defines the flow field.Central to this science is the concept of aerodynamic force,which is essential for the forces needed for flight, as we investigate in this blog post.

Source of aerodynamics forces

In practice, the flow field about a right circular cone is more conveniently described in terms of cylindrical coordinates, but we are concerned only with the general ideas here.

Theoretical and experimental aerodynamicists labor to calculate and measure flow fields of many types. Why? What practical information does knowledge of the flow field yield with regard to airplane design or to the shape of a rocket engine? However, the roots of the answers lie in the following discussion.

Probably the most practical consequence of the flow of air over an object is that the object experiences a force, an aerodynamic force, such as your hand feels outside the open window of a moving car. In Subsequent session we discuss the nature and consequences of such aerodynamic forces. The purpose here is to state that the aerodynamic force exerted by the airflow on the surface of an airplane,missile, or the like stems from only two simple natural sources.

Figure 2

Figure 3

We have already discussed pressure. Referring to Fig. 2,3.we see that pressure exerted by the gas on the solid surface of an object always acts normal to the surface, as shown by the directions of the arrows. The lengths of the arrows denote the magnitude of the pressure at each local point on the surface. Note that the surface pressure varies with location.The net unbalance of the varying pressure distribution over the surface creates an aerodynamic force.

The second source, shear stress acting on the surface, is due to the frictional effect of the flow “rubbing” against the surface as it moves around the body. The shear stress `τ_w` is defined as the force per unit area acting tangentially on the surface due to friction, as shown in Fig. 2,3.It is also a point property; it varies along the surface;and the net unbalance of the surface shear stress distribution creates an aerodynamic force on the body.

No matter how complex the flow field, and no matter how complex the shape of the body, the only way nature has of communicating an aerodynamic force to a solid object or surface is through the pressure and shear stress distributions that exist on the surface.These are the basic fundamental sources of all aerodynamic forces. The pressure and shear stress distributions are the two hands of nature that reach out and grab the body, exerting a force on the body is the aerodynamic force.

Finally, we can state that a primary function of theoretical and experimental aerodynamics is to predict and measure the aerodynamic forces on a body. In many cases, this implies prediction and measurement of p and `τ_w` along a given surface.Furthermore, a prediction of p and `τ_w` on the surface frequently requires knowledge of the complete flow field around the body.This helps to answer our earlier question about what practical information is yielded by knowledge of the flow field.

Conclusion

In the field of aerodynamics, aerodynamic forces originate from interactions between solid objects and the air. Lift, weight, thrust, and drag influence aircraft behavior in flight, impacting their performance. Understanding aerodynamics enables humans to overcome gravity and achieve greater heights in aviation. Let us admire the intricate aerodynamic principles that enable flight as we advance in aviation technology.