Static and dynamic pressure.

[Sandesh]

Dear members and fellow aviators,

I was reading the asa ground school book for private pilot and I stumbled upon the concept of static and dynamic pressure and how this is the same as the high and low differtianl pressure on the camber. Can anybody explain to me what static and dynamic pressure and how it relates to lift?

Thank you for all of your help.
Sandesh.

[meritflyer]

Static is the thing that shocks you after you shuffle your feet on the carpet and touch the door knob.

Dynamic is when you...


I've gone cross eyed.

[Sandesh]

Hmm not exactly the answer I am looking for but thanks anyway, anybody else like to elaborate.

[tgrayson]

Originally Posted by Sandesh

Can anybody explain to me what static and dynamic pressure and how it relates to lift?
Sandesh.

A volume of air contains energy in two forms (for the sake of this discussion), kinetic and potential. Static pressure is a measure of the potential energy per volume air and dynamic pressure is a measure of the kinetic energy per volume air. You can convert kinetic energy into potential and potential into kinetic, but the sum total remains the same.

If you use the potential energy contained in the airflow to accelerate it, kinetic energy increases. Due to Law of Conservation of Energy, the same airflow will experience a loss of potential energy, meaning the pressure will drop.

An airfoil is designed to accelerate the airflow over the top, and hopefully slow it on the bottom. Due to the above effect, the pressure on top will fall and the pressure on the bottom will go up. The net result is a force in the upward direction.

[fish314]

Tgrayson did an awesome job explaining static versus dynamic pressure. Here is how it relates to lift. Like he said, the airflow on the top of the wing has sped up a little, which converted some of the potential energy into kinetic. Or in other words, some of the static pressure was converted into dynamic pressure, and the static pressure on the top of the wing decreased.

On the bottom of the wing the airflow may have sped up, slowed down, or stayed the same, depending on the design of the wing, but it doesn't really matters. All that matters is that compared to the top of the wing, the airflow on the bottom of the wing didn't speed up as much. So more of the potential energy on the top was converted to kinetic compared to the bottom. Or in other words, static pressure decreased on the top of the wing more than it did on the bottom of the wing.

Now you can kind of think of dynamic pressure as acting along the direction of the moving air, and static pressure acting in every direction. Since the air is moving parallel to the surface of the wing, the dynamic pressure isn't really affecting the wing. (For the purists out there, this is a technically incorrect explanation, but I'm using it as a simplification). But the static pressure IS pushing on the wing, because it acts in all directions, not just in the direction that the air is moving.

So how does this generate lift? Well on the bottom of the wing, the static pressure is higher than on the top of the wing, and this pressure on the bottom is pushing UP with more force (or more pressure, actually) than the static pressure on the top of the wing is pushing DOWN. Hence, lift--- or at least a layperson's explanation that should work for the level of the private pilot...

[ananoman]

It would probably be better if you post this stuff in the Tech forum.

[Sandesh]

OK, From now on these stuff will be kept in the tech forum, sorry about that.

[JaceTheAce]

When you have low dynamic pressure you will have higher static pressure. Then when you hvae high dynamic pressure you have lower static pressure. The forward and aft stagnation points on the wing has low dynamic pressure and high static pressure... the opposite is true for the divergent flow of air (high dynamic, low static).