Pressure and Lift around a Wing

[russ_watters]

I think the splitting of pressure, defined as the normal compressive force per unit area on an infinitesimal surface, into static and dynamic terms is confusing and confused me.

Some people find it useful or even prefer to consider these things as not being different types of pressure, probably for that reason (only wanting there to be one thing called "pressure"). Rather, they are different types of energy. Dynamic pressure can then be thought of as being the kinetic energy contained in a moving volume of air. The units (when simplified) actually match. Here's an example with Bernoulli's equation:
http://hyperphysics.phy-astr.gsu.edu/hbase/pber.html

 

[russ_watters]

Having been 200 feet behind a tow plane and getting bounced around by the extreme turbulence my take is that it is the air coming off the bottom of the wing and being forced down that develops a upward force on the plane.

Many peoples' first thought on lift comes from sticking their hand out a car window. You can feel the air hitting the bottom of your hand, lifting it up, and you feel nothing on the back of your hand. So it's all a push from below, right?

You will see some confusing and inconsistent perspective on the issue. Depending on who and how you ask, that interpretation is either correct or incorrect - or even both at the same time.

On the one hand, people will point out that all pressures are absolute and can only "push". That is indeed what you feel on your hand when you stick it out the car window. But what gets missed in that answer is the nuance that it is *changes* in pressure (from normal atmospheric) that result in an inequity between the top and bottom surface that you then feel as a "push". Due to the shape of the wing, the pressure on the bottom surface goes up and the pressure on the top surface goes down.

Very interesting comment from pilot experience. But I think it would be practically impossible for a pilot to distinguish between air pushed down by impacting the bottom of the wing versus air directed down after going above the wing. In fact, calculations of force from air hitting the bottom of the wing usually come up way short of the actual lift force. So the complete story is more complicated than that.

Interesting. Where can i find some of those calculations.

Calculations are hard. Measurements are easy and the results easy to visualize:

This is a graphical representation of the static, gauge pressure distribution around a wing in flight. The short arrows on the bottom show pressures above atmospheric pressure and the longer arrows on top show pressures below atmospheric. In this case - as is common in level flight - the pressure reduction above is greater than the pressure increase below.

Engineers quantifies this in wind tunnel testing or CFD analysis by putting pressure ports all around the wing to measure those pressures. For actual measurement and calculation, you plot the results on a graph:

Note that the graph is inverted to match what is going on with the wing: "Negative" pressures are up and positive pressures are down. Again, by adding together (integrating) the pressure differential, you get the total lift.

You might look for a gliding club and ask about it or even take a ride.

I have! Lots of fun!

 

[FactChecker]

Calculations are hard. Measurements are easy and the results easy to visualize:

That is very true -- especially the visualizing part. But the wind tunnel time and making the model can be expensive. And it is a whole different ball game if the plane is just one of several conceptual designs. The experts have a reasonably good idea early on, but there can be nasty surprises. The bottom line is that even in programs with very good funding, the real numbers may not be adequately known till flight tests are done.

 

[arydberg]

Many peoples' first thought on lift comes from sticking their hand out a car window. You can feel the air hitting the bottom of your hand, lifting it up, and you feel nothing on the back of your hand. So it's all a push from below, right?

You will see some confusing and inconsistent perspective on the issue. Depending on who and how you ask, that interpretation is either correct or incorrect - or even both at the same time.

On the one hand, people will point out that all pressures are absolute and can only "push". That is indeed what you feel on your hand when you stick it out the car window. But what gets missed in that answer is the nuance that it is *changes* in pressure (from normal atmospheric) that result in an inequity between the top and bottom surface that you then feel as a "push". Due to the shape of the wing, the pressure on the bottom surface goes up and the pressure on the top surface goes down. Calculations are hard. Measurements are easy and the results easy to visualize:
View attachment 227024
This is a graphical representation of the static, gauge pressure distribution around a wing in flight. The short arrows on the bottom show pressures above atmospheric pressure and the longer arrows on top show pressures below atmospheric. In this case - as is common in level flight - the pressure reduction above is greater than the pressure increase below.

Engineers quantifies this in wind tunnel testing or CFD analysis by putting pressure ports all around the wing to measure those pressures. For actual measurement and calculation, you plot the results on a graph:

View attachment 227025

Note that the graph is inverted to match what is going on with the wing: "Negative" pressures are up and positive pressures are down. Again, by adding together (integrating) the pressure differential, you get the total lift.

I have! Lots of fun!

One very important issue here is angle of attack. It shoud be included.

 

[russ_watters]

One very important issue here is angle of attack. It shoud be included.

It is.