The net lift and drag force acts at the center of pressure of the airfoil. The net vertical force is termed the lifting force and the net horizontal force is termed the drag force. It is important to remember that the above result is true irrespective of the shape of the surface in question the net aerodynamic force acting on any body in a free stream of air will always be the sum of the pressure and shear distributions acting along the body. The resultant aerodynamic force acting on the airfoil is therefore the sum of the pressure and shear contributions. Similarly, adding the shear contribution along the airfoil surface results in a net shear force. Taking the local pressure contribution at each point along the surface and adding each contribution together (integration) results in a net pressure force acting on the airfoil. The shear distribution acts locally parallel to the airfoil surface. The pressure distribution acts locally perpendicular (normal) to the airfoil surface. Pressure (normal) and Shear (parallel) distribution on an airfoilĮxactly the same thing happens when we consider an airfoil subjected to a flow of air over its surface: a pressure and shear distribution are present acting over the entire airfoil surface. Here the force being exerted on your hand is being generated by two force distributions acting on your hand: a pressure distribution and a shear distribution. In this case the lift force tends to push your hand upward while the drag force pushes your hand backward. If you have ever stuck your hand out of a moving vehicle and felt the force of the air pushing on your hand you should intuitively have a pretty good idea of the concept of lift and drag. We are going to specifically focus on the wing for the rest of this tutorial but the concept behind aerodynamic loading can just as easily be extended to any other component of the aircraft such as the fuselage, an engine cowling or even a canopy. In this post we will examine how and why aerodynamic forces are generated as the airplane moves through the air, and introduce a method to non-dimensionalize the forces such that aircraft of various shapes and sizes can be directly compared to one-another. If you have read the previous post you will understand that lift must be produced by the airplane wing in order to act as a counter-force to the total flying weight, and that as a natural consequence to the motion of the aircraft through the air, a drag force that opposes this motion is also present. Two of the four fundamental forces acting on an aircraft during flight come about as a result of the aerodynamic loading on the body as it flies through the air. We will look at the relationship between the two forces, study how they interact with one another, and learn how to non-dimensionalize the resulting forces. We are now going to look more closely at the two aerodynamic forces Lift and Drag. In the previous post we introduced the four fundamental forces acting on an aircraft during flight: Lift, Drag, Thrust and Weight and examined how they interact with one-another.
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