- #1
Michael1160
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I have read a number of the forums on lift generation, and I would like to add some comments for feedback.
I read the forum describing lift as the centripetal force required to curve the airflow (streamlines), and I must say this makes absolute sense, I myself have always believed that concept. However, I would like to add that this concept of lift has been around for a long time, since 1889, when the German engineer Otto Lilienthal first proposed it.
There is no doubt that pressure distribution is the source of lift. However, one must remember that air (gas) pressure has its origins in Newtons Laws. Air pressure is caused by the incessant impacts of the constituent molecules. The molecules impact the surface (and each other) and cause a force to be applied to that surface, the surface in turn applies an equal and opposite force (Newtons third) to the molecule. In reality the impact "forces" are caused by the electrostatic forces between the eletrons in the molecules.
A fluid element has mass to it, for instance at sea level a cubic foot of air has .0023769 slugs to it (standard day). This gives the fluid element a physical weight of .08 pounds. However, a cubic foot of air at sea level exerts a pressure of 2116 PSF (standard day) due to the translational kinetic energy (temperature) of its molecules, as described above.
Since the fluid element has mass, in order for it to move in a curved path (centripetal acceleration) there must be a net force acting on it. This force is generated by the vertical pressure gradient in the flow. This requirement is Newtons second law for curved motion.
Circulation is also required in order to move the stangnation point from the top surface of the airfoil to the trailing edge. Even if there was no circulation, the streamlines are going to curve, requiring vertical pressure gradients, but with the stagnation point on top of the airfoil, the resulting pressure distribution is symmetrical, hence no lift. So circulation is also required to move the stagnation point. So, you can see you have a combination of physics laws all working together to produce lift. I am just surprised that there is so much disagreement on this concept.
Michael 1160
I read the forum describing lift as the centripetal force required to curve the airflow (streamlines), and I must say this makes absolute sense, I myself have always believed that concept. However, I would like to add that this concept of lift has been around for a long time, since 1889, when the German engineer Otto Lilienthal first proposed it.
There is no doubt that pressure distribution is the source of lift. However, one must remember that air (gas) pressure has its origins in Newtons Laws. Air pressure is caused by the incessant impacts of the constituent molecules. The molecules impact the surface (and each other) and cause a force to be applied to that surface, the surface in turn applies an equal and opposite force (Newtons third) to the molecule. In reality the impact "forces" are caused by the electrostatic forces between the eletrons in the molecules.
A fluid element has mass to it, for instance at sea level a cubic foot of air has .0023769 slugs to it (standard day). This gives the fluid element a physical weight of .08 pounds. However, a cubic foot of air at sea level exerts a pressure of 2116 PSF (standard day) due to the translational kinetic energy (temperature) of its molecules, as described above.
Since the fluid element has mass, in order for it to move in a curved path (centripetal acceleration) there must be a net force acting on it. This force is generated by the vertical pressure gradient in the flow. This requirement is Newtons second law for curved motion.
Circulation is also required in order to move the stangnation point from the top surface of the airfoil to the trailing edge. Even if there was no circulation, the streamlines are going to curve, requiring vertical pressure gradients, but with the stagnation point on top of the airfoil, the resulting pressure distribution is symmetrical, hence no lift. So circulation is also required to move the stagnation point. So, you can see you have a combination of physics laws all working together to produce lift. I am just surprised that there is so much disagreement on this concept.
Michael 1160
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