Physics of flight

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An airplane wing shaped object is in the water.

When does a water current behave like air on an airplane wing (wing moves both linearly & perpendicularly to the current) and when does it not, meaning it simply drags the wing shaped object linearly in the same direction as the current.

Is it the density of the water, the viscosity, the velocity....all three, and if so at what value of these?

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Dear guskz:

Add> An airplane wing shaped object is in the water.

It is best to ask this in a mechanical engineering newsgroup. Poutnik is correct, but there are several dimensionless groupings (besides Reynolds number which is most important) that correlate behavior of a shape in water, to a behavior in air. IN the right newsgroup, you might input from those that do that analysis... If they can see your post through the spam.

David A. Smith

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And what do you suppose a course in physics is about?

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I'm sorry, I thought you were asking about modelling wing shapes in two different fluids, which Science only revisits when they analyze he laminar-flow flight of bees. They passed this up hundreds of years ago, having left "dimensional analysis" behind for us engineers. And is certainly not a topic germane to relativity, as you should know.

David A. Smith

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Water behaves like subsonic air in the sense that the equations are identical. Drag, for example, is a function of the coefficient of drag (which is in large part a function of shape and Reynolds number Re), fluid density, cross sectional area, and velocity squared. Lift works exactly the same way, same formula, but the coefficient of lift is different for a given shape.

If you hold Re constant, the drag and lift will be different in air and water, scaled by the density difference, but the drag to lift ratio will be the same in both cases.

For further elucidation, I would recommend (remember I'm an engineer) R.D. Blevins, _Applied Fluid Dynamics Handbook_. It contains much practical information on solving problems in the area of fluid drag and associated issues.

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Water behaves like subsonic air in the sense that the equations are identical. Drag, for example, is a function of the coefficient of drag (which is in large part a function of shape and Reynolds number Re), fluid density, cross sectional area, and velocity squared. Lift works exactly the same way, same formula, but the coefficient of lift is different for a given shape.

If you hold Re constant, the drag and lift will be different in air and water, scaled by the density difference, but the drag to lift ratio will be the same in both cases.

For further elucidation, I would recommend (remember I'm an engineer) R.D. Blevins, _Applied Fluid Dynamics Handbook_. It contains much practical information on solving problems in the area of fluid drag and associated issues.

• posted

Water behaves like subsonic air in the sense that the equations are identical. Drag, for example, is a function of the coefficient of drag (which is in large part a function of shape and Reynolds number Re), fluid density, cross sectional area, and velocity squared. Lift works exactly the same way, same formula, but the coefficient of lift is different for a given shape.

If you hold Re constant, the drag and lift will be different in air and water, scaled by the density difference, but the drag to lift ratio will be the same in both cases.

For further elucidation, I would recommend (remember I'm an engineer) R.D. Blevins, _Applied Fluid Dynamics Handbook_. It contains much practical information on solving problems in the area of fluid drag and associated issues.

• posted

Water behaves like subsonic air in the sense that the equations are identical. Drag, for example, is a function of the coefficient of drag (which is in large part a function of shape and Reynolds number Re), fluid density, cross sectional area, and velocity squared. Lift works exactly the same way, same formula, but the coefficient of lift is different for a given shape.

If you hold Re constant, the drag and lift will be different in air and water, scaled by the density difference, but the drag to lift ratio will be the same in both cases.

For further elucidation, I would recommend (remember I'm an engineer) R.D. Blevins, _Applied Fluid Dynamics Handbook_. It contains much practical information on solving problems in the area of fluid drag and associated issues.

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