How dynamic and static pressures are related

[cjl]

As I said earlier, "Bernoulli" vs "Newtonian" aren't two different things. They're two different explanations for the same phenomenon, and both can explain it in full. You can explain the entirety of the lift in any configuration through a Newtonian momentum transfer and associated downwash, and you can also explain the entirety of the lift by looking at local velocity distributions and using the bernoulli relation. Bernoulli never "tapers off to nothing", and the reactive Newtonian component is there from the instant the airfoil starts making lift.

 

[russ_watters]

God, I hope that’s right.

Good so far!

I suppose what’s left that I need to clarify is...

Why Bernoulli doesn’t apply below the wing.

Bernoulli's principle most certainly does apply below the wing.

At a zero lift AOA the relative wind produces as much lift above as it does below.

This is poorly worded. Lift is an upward force. It is improper to say that lift from the lower surface pulls the airfoil down. I know what you meant, but that's not the right way to say it.

But in either case the air flows over the top and bottom of the wing and produces a positive or negative lift respectively by the same method... the acceleration of the flow as it passes across the surface. Bernoulli is working both above and below.

Yes. Acceleration or deceleration.

As the AOA is increased positively more lift is produced above than below and as the relative wind begins to strike the bottom of the wing Bernoulli tapers off to nothing and we begin to see a reactive Newtonian force adding to the total lift...induced lift on top and Newton on the bottom.

No. As the AOA rises, the airflow under the wing decelerates more and more, causing the lift produced to increase more and more, as Bernoulli tells us.

 

[cjl]

No. As the AOA rises, the airflow under the wing decelerates more and more, causing the lift produced to increase more and more, as Bernoulli tells us.

It's worth noting that, while this is correct, most of the additional lift as you increase AoA actually comes from the suction peak on the upper surface increasing in magnitude, and not from the increasing pressure on the underside.

 

[rcgldr]

It's worth noting that, while this is correct, most of the additional lift as you increase AoA actually comes from the suction peak on the upper surface increasing in magnitude, and not from the increasing pressure on the underside.

True for a conventional airfoil, but in the case of the M2-F2 and M2-F3 prototype reentry vehicles, which would have to operate at reentry speeds, it appears that most of the lift is due to the bottom surface. The shape is similar to a cone split in half with a tapered tail. The trailing edge is blunt since that where the rocket engines went on the M2-F3.

Getting back to the main topic, think of wings as being designed to curve (divert) the relative airflow downwards, and the coexistent differences in speeds and pressures above and below a wing being consequences of that design (due to pressure gradients perpendicular to flow being coexistent with curvature of flow).

 

[cave man]

Seems to me that this is similar to potential and kinetic energy. Simply-a stretched rubber band contains potential energy. When you release it it becomes kinetic. Could you explain dynamic verses static in this way?

 

[Lee Duke]

The lift on an aircraft (wings, body, tail) is a force that is normal to the aircraft velocity vector. No velocity no lift.

The key to measuring the dynamic pressure on a vehicle is a Pitot tube which returns STATIC and TOTAL presure. The Pitot tube should be reasonable distant from the wings. On research aircraft, you often see a nose boom. Besides weather vanes for measuring angle of attack and angle of sideslip (yes, I know the measure isn't of sideslip exactly, but I wanted to give all the trivia nerds a place to show their "amazing brilliance.") Anyway, a Pitot tube is generally located at the tip of that nose boom to get it as far away from the aircraft disturbances in the air. It's called the freestream (or more correctly, tstagnation) pressure.

I think boneh3ad tried to explain all this and, to mind, did a credible job.

The pressure profile at some location on the wing is measured using either piezoelectric sensors or holes on the wing surface containing the equivalent of Pitot tubes. If an aircraft is going to fly their needs to be lower pressure beneath the "lifting portion" of that vehicle be it wings, the body, or thousands of unseen angels holding the aircraft aloft.