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The source of all aerodynamics forces

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Introduction

Previously, we covered the four key aerodynamic flow quantities: pressure (p), density (ρ), temperature (T), and velocity (V), with velocity being a vector. These define the flow field at any point. Now, we shift our focus to aerodynamic force—essential for understanding the forces involved in flight.

Diagram illustrating aerodynamic forces on a wing, showing airflow paths. Faster air and low pressure above, slower air and high pressure below, create lift.

Source of aerodynamics forces

If we were concerned with the flow around a sharp-pointed cone, as illustrated in Fig. 1, then we could define a Cartesian xyz three-dimensional space. Specifically, the free-stream velocity `V_infty` would act in the x-direction, and the cone’s axis would align with this same direction. Consequently, by specifying the relevant flow quantities within this coordinate system, we fully define the flow field.
p=p(x,y,z)
ρ=ρ(x,y,z)
T=T(x.y.z)
V=V(x,y,z)

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.

One of the most practical outcomes of air flowing over an object is the aerodynamic force it generates—like the force you feel when you put your hand outside a moving car window. In later sessions, we’ll explore the nature and effects of these forces. For now, it’s important to note that aerodynamic forces on surfaces like airplanes or missiles arise from just two basic sources.

  1. Pressure distribution on the surface
  2. Shear stress (friction) on the surface

Figure 1

​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 of aerodynamic force is surface shear stress, caused by friction as the flow "rubs" against the body. This shear stress, denoted as `τ_w`, is the tangential force per unit area due to friction. It varies along the surface and, when unbalanced, contributes to the net aerodynamic force on the object.

Regardless of how complex the flow or shape of an object, aerodynamic forces always arise from just two sources: pressure and shear stress on the surface. These are nature’s only means of exerting aerodynamic force—like two invisible hands applying pressure and friction to the body.

In summary, a main goal of theoretical and experimental aerodynamics is to predict and measure aerodynamic forces—primarily by determining pressure (p) and surface shear stress (`τ_w`). Since these often depend on the entire flow field around a body, understanding the flow field is essential for gaining useful, practical insights.

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.

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