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Coefficient of lift

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Coefficient of lift

Old 16th Oct 2020, 15:35
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Coefficient of lift

Can someone explain how the coefficient of lift can be greater than one. My understanding (clearly incorrectly) was that the coefficient of lift was the proportion of the dynamic pressure contributing to the lift force. How can you have more force than the dynamic pressure that is available.

If you can help me with the answer Iíll consider ending lockdown!

regards Dan
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Old 16th Oct 2020, 20:33
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The coefficient of lift, which is a dimensionless number, expresses the ratio of the lift force to the force produced by the dynamic pressure times the area.

Cl = L / (q * A)

Cl = coefficient of lift
L = Lift
q = Dynamic pressure
A = Wing area
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Old 20th Oct 2020, 08:06
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Keep in mind that total energy available with the air mass isn't just dynamic pressure. Think of static pressure & temperature for example.
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Old 23rd Oct 2020, 13:52
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Lift will equal the pressure differential between the top and bottom of a wing only if the wing passes through the volume of air and leaves it undisturbed.

At a positive angle of attack the volume of air the wing passes through is forced downwards. The reactive force to this acceleration of air downwards is additional lift.
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Old 25th Oct 2020, 07:15
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Can someone explain how the coefficient of lift can be greater than one.

Megan provided the equation. It's really just a matter of when the particular section plus high lift bits and pieces lets go of the air and it all turns to turbulence. The numbers come from the measurements. No reason I can see why it ought not to be greater than one if that's what the section does ...

The reactive force to this acceleration of air downwards is additional lift.

It really is a matter of how you want to go about explaining stuff.

There is a pressure difference above and below - easy to show experimentally.

There is upwash and downwash - again, easy to show.

The two are not different/unrelated animals, though. To get the flow changes we need the pressure difference and the associated pressure gradients from the near-surface flow out to the free stream - one needs to recall that the only way to exert a force on a fluid flow is by the influence of pressure gradients - otherwise, the flow sort of just slips left, right, up, or down, as the case may be and continues to do its own thing. One needs the other and I don't think it is valid to try to separate them ?

only if the wing passes through the volume of air and leaves it undisturbed.

Now, that's an interesting concept. You don't happen to have a convenient video showing this happening in a tunnel, do you ?



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Old 26th Oct 2020, 04:39
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The reactive force to this acceleration of air downwards is additional lift
There is no "additional lift". Best think of a propeller, it's just an airfoil that gets its airspeed by rotating, the pressure difference between the forward and rear faces causes the airflow to flow in a rearwards direction at considerable speed at the beginning of take off, so providing thrust (lift as far as the airfoil is concerned).

Force(thrust or lift) = mass X acceleration
= d(mV)/dt

where d/dt = derivative with respect to time eg rate of change with time, mV = momentum, product of mass and velocity

Lift is often explained as throwing air at the ground, stand beneath a hovering helicopter for an example, or behind a stationary prop driven fixed wing running at high power.
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