Nothing to do with intelligent design, as such, but Sal Cordova brought up the issue of how wings produce lift in this thread.
hmmm.
[Edited to add xkcd, which my son just reminded me of….]
Like most people, I bought the airfoil myth for many years until I actually started to think about designing a kite (it was a fun first-year architecture project). And then I didn’t think much more about it until an interminable thread about a wind-powered cart that could travel downwind faster than the wind arrived at Talk Rational, in the course of which I discovered something I hadn’t appreciated about air, which is that it isn’t readily compressible unless in a confined space (I’d always imagined propellers in water behaving rather differently to propellers in air, but it turns out they don’t). Anyway, thought it might make a change from ID, and maybe Gil Dodgen might like to weigh in on Something Completely Different :). I’ll invite him.
I don’t think this is true. Pressure ports will measure the contribution from dynamic pressure depending on orientation.
Pressure is the force per unit area. When you put your hand out of the window of a moving car you will feel force on your hand. That force per unit area is physically measurable pressure (the dynamic pressure).
We could in principle resolve the argument with experimentation with a barometer. I don’t have one handy, but suffice to say, the Pitot Tube wouldn’t work unless there were differences in physically measurable pressure depending on orientation.
The reason I say this, pressure is force per unit area. Force is change in momentum per unit time. Momentum is velocity times mass. A mass of moving air creates dynamic pressure when it hits against a surface (like your hand extended out of a car window). You feel the force of the wind because your hand changes the momentum of the moving air. That is a physically measurable quantity, thus dynamic pressure IS physically measurable. The reason the Pitot tube also measures static pressure is so that it can estimate airspeed.
But, in any case, thank you for the discussion.
From NASA website:
http://www.grc.nasa.gov/WWW/k-12/airplane/dynpress.html
Thus not only is static pressure physically measurable, so is dynamic pressure.
The experiment I suggested illustrates the directionally dependent features of pressure when moving air masses are involved.
A barometer in principle ought to be able to measure dynamic pressure. That is what the forward part of the Pitot tube does.
I’m afraid we are miscommunicating. Yes of course pressure is force per unit area. Of course the force that is exerted against your hand when you stick it out a car window is due to the change in momentum of the dynamically moving air as it rebounds from your hand. That’s not the point. My point is that the terminology “static pressure” and “dynamic pressure” does not mean what you think it means (nor does it mean what 90% of the internet thinks it means).
I’ve worked with wind tunnels, including teaching university engineering students how to take pressure measurements in them. If you want to measure the pressure distribution around a test object, whether an airfoil, an airplane model, or a model of a high rise building, you create the model with pressure taps (holes) distributed all over the surface. You connect those taps to some type of pressure measuring instrument. Those taps are called “static pressure taps” because no matter where they are located relative to the flow direction they only measure static pressure. But that static pressure will be higher at a tap facing the airflow because of the momentum of the air impinging on it. We can calculate the dynamic pressure at each location by taking the difference between the measured static pressure and the total pressure we measured with a Pitot tube. Dynamic pressure can only ever be determined by taking the difference between a static pressure measurement and a total pressure measurement (which is also a static pressure).
I don’t disagree with anything written on the NASA webpage, and everything on it is consistent with what I’ve written.
Here is another page from that same website showing static pressure ports all around a turbine blade wind tunnel model. It confirms that they only measure static pressures. The surface of an object with air flowing over it can only “feel” static pressures.
If you carefully read the passage you quoted, it says that the measured pressure depends on the flow direction, which leads to the definition of dynamic pressure. It doesn’t say that the measured pressure, which depends on flow direction, is the dynamic pressure. In the final paragraph of that page it says that the dynamic pressure is a defined property of a moving flow of gas. By “defined property” they mean that it is a property that was defined into existence rather than a physical quantity that exists independently.
From Anderson’s book Introduction to Flight page 114:
If it is measuring total pressure, that means measured pressure is not to be equated with static pressure unless there is no flow.
Where we disagree is whether measured pressure is always static pressure.
Of course, you could say inside the tube and at the opening, the air is stationary, hence it is static there, but that is not usually meant by static pressure.
Clearly we are miscommunicating. I respect that you have taught at university, but apparently my teachers taught differently than you and so did my textbook which says total pressure is physically measurable.
Thank you for the discussion. It was informative to me to be made aware of the differences in usage of terminlogy. Thank you again.
There is no flow inside a Pitot tube. A wall separates the air in the two sections of the tube. A manometer measures the pressure difference (not “total pressure”, whatever that means) between the two sections. The tube is oriented in such a way that the pressure difference Δp = ρv2/2 according to the Bernoulli equation. The manometer reading Δp is converted to airspeed v.
Mutual thanks for the discussion.
As a point of clarification, nowhere have I said total pressure is not physically measurable. To the contrary, two posts ago I said that total pressure is what is measured at the end of a Pitot tube. What I’ve said is that dynamic pressure isn’t directly measurable. The only way to “measure” dynamic pressure is to measure total pressure (which is the static pressure value when the dynamic pressure is zero) and to measure static pressure and take the difference. Static pressure is the only kind of pressure that can be directly sensed. This is because dynamic pressure isn’t a real pressure that acts on anything in a way we can sense directly.
If you would humor me for just a short while longer, I think I can attempt to resolve our miscommunication with a fairly simple question.
You have several times referred to the example of sticking your hand out the car window. If you do that, and you point your hand and forearm forward, palm down, then tilt your hand and arm upward, you will obviously feel a large lift force wanting to further rotate your arm back. In that case, do you believe that the dynamic pressure is greater on the underside (windward side) of your arm/hand or on the topside (leeward side) or your arm/hand?
Not necessarily. But for the sake of argument, assume it is lower, what matters is the force created by the dynamic pressure and that force depends on the orientation of the hand. Dynamic pressure may exist, but it is not realized as force unless the flow is deflected.
But before we go further there has been a miscommunication here about what one means by “static” pressure. If the palm faces the wind, there is a spot where there isn’t flow of air, much like at the opening of the Pitot tube. It is “static” pressure in that sense, but this static pressure is higher than the barometric pressure of the air the car is slicing through, and this barometric pressure is what is often being referred to when one uses the word “static” pressure. So using your terminology, one is comparing static pressures at one point with static pressures at another point. That is not the convetion I was using, and hence the miscommunication.
For example, if you have a stream of air (like coming from a blow drier), you can state the dynamic pressure of that stream. If we put a pitot tube infront of the stream, and merely because the flow stops at the opening of the pitot tube, we ususually don’t say static pressure is what is measured at the opening of the pitot tube, but rather total pressure. Yes, of course one could say the total pressure at the opening is static in that it is stationary relative to the tube, and in that sense the “static” pressure is equal to the total pressure, but that’s not usually what one means by static pressure, at least in the context of Pitot tubes.
In the context of Pitot tubes the term “static pressure” refers to the ambient air pressure, not the pressure of the static (stationary) air inside the closed tube that faces the wind. I understand the convention you are using, and that is the source of the miscommunication, and hopefully this clarifies where we are using different conventions in our terminology.