Airspeed/pressure/pitot-static system?

[VNugget]

I have just realized I don't know the first thing about the pitot-static system.

Alright, so according to Aerodynamics for Naval Aviators, as the speed of the free airstream increases, the dynamic pressure increases and the static pressure decreases, as per Bernoulli's law; they always add up to the same number, which is the total pressure. So far so good.

Furthermore, at the stagnation point in front of the pitot tube, the local flow (and therefore the local dynamic pressure) drop to zero. The local static pressure is equal to the total pressure, and the difference between the free-stream static and local static pressures is equal to the free-stream dynamic pressure. So far so good.

So the pitot tube measures that total pressure, and the static port measures the free stream static pressure, which decreases with velocity. As velocity increases and static pressure decreaess, the difference between the total pressure (from the pitot tube) and the static pressure (from the static port) increases, and this difference is equal to the dynamic pressure, and is measured by the airspeed indicator. So far so good?

Not so fast!

If these inferences are carried out to the ir logical conclusion, the altimeter reading, which is done from the static port, would decrease as soon as the plane starts moving (even on the ground when there's a headwind) due to the drop in static pressure, so the altimeter couldn't possibly work offf the static port. The altimeter would actually have to work off the pitot tube, which measures total pressure and stays constant throughout all velocity changes.

Obviously, that is not the case.

What gives?

 

[mar]

What's wrong with this picture?

Whoa! 'Not so fast' is right.

Son, you've come to at least a *few* false conclusions.

Let's not try to reinvent fluid dynamics, ok?

It's pretty simple.

It's true the pitot tube measures Total Air Pressure.

It's true Total Air Pressue = Dynamic Air Pressure + Static Air Pressure.

It's true the Altimeter measures only Static Air Pressure.

But this statement is false: <<As velocity increases and static pressure decreaess, the difference between the total pressure (from the pitot tube) and the static pressure (from the static port) increases, and this difference is equal to the dynamic pressure, and is measured by the airspeed indicator.>>

First of all, not to be too pedantic, but the term velocity (when speaking about physical mechanics) always includes a direction. For instance, Wind Velocity: Southeast at 15kts.

In this case it's probably just easier to use the terms 'speed' or 'rate'.

At any rate what's occuring in the magical bellows of the Airspeed Indicator is pretty simple. First of all, as you know, you have Total Air Pressure being crammed into the pitot tube. But all you're interested in having displayed on the instrument face is Dynamic Pressure. Since the little mechanism needs to know what the Static Pressure is in order to deduct it from Total Pressure, there's a line from the static port plumbed into the Airspeed Indicator.

That's it.

Now the Altimeter, as you know is plumbed with only Static Air Pressure and that is compared mechanically to the "aneroid wafer" (a bellow with a fixed and known air pressure) to display our altitude--with all of it's associated errors which we're all very familiar with.

I think where you're encountering some muddiness is in the relationships between pressure, density and temperature.

Specifically, I think you're confusing Bernoulli's principle with the fact that atmospheric air pressure decreases with altitude. Not speed.

It's true that localized flow (over the wing) will experience a decrease in air pressure due to Bernoulli. But what happens in flight is the exact same phenomenom that causes your ears to pop. That is to say, when you effect a change in altitude, you also effect a change in density (which is one element that affects pressure, the other being temperature) all things being equal, of course.

Aero for Naval Aviators is a classic and it should *definitely* be part of your library but man I have a hard time will all the math.

Another book I *highly* recommend is Flight Theory for Pilots by Charles E. Dole. Published by IAP, Inc. ISBN 0-89100-338-X. It has lots of good theory but it's a much easier read for those of us who don't sleep with a slide rule under their pillow.

I hope this helped. Keep asking questions.
And fly safe.

 

[VNugget]

Thanks for the reply.

First of all, not to be too pedantic, but the term velocity (when speaking about physical mechanics) always includes a direction. For instance, Wind Velocity: Southeast at 15kts.

I actually know this, and have been known to chastise people for the same mistake, but here I am making a slip-up and ending up on the receiving end


At any rate what's occuring in the magical bellows of the Airspeed Indicator is pretty simple. First of all, as you know, you have Total Air Pressure being crammed into the pitot tube. But all you're interested in having displayed on the instrument face is Dynamic Pressure. Since the little mechanism needs to know what the Static Pressure is in order to deduct it from Total Pressure, there's a line from the static port plumbed into the Airspeed Indicator.

That's pretty much what I said in my paragraph that you said was wrong, but maybe I crammed too many thoughts into too few sentences at the expense of clarity.

So we agree that total pressure (which is constant) = static pressure + dynamic pressure.

We also agree that dynamic pressure is what correlates with vel...speed, and we can measure it by subtracting the static pressure (static port) from total pressure (pitot tube).

(Let me clarify that in this whole discussion, I am talking about a constant altitude, so let's put aside any changes in total pressure due to altitude change, and all of our beloved altimeter errors. When I was talking about the altimeter changing reading, I was talking about my hypothetical assumption about an error due to speed change. Obviously speed does not give you an altimeter error, but I am trying to figure out where I am erring in my deduction that it should.)

Anyway, back to the actual discussion.

Since an increase in airspeed implies an increase a dynamic pressure, it also necessarily implies a decrease in static pressure. (again, static=total-dynamic follows from dynamic=total-static, which you have acknowledged.)

We thus have a decrease in static pressure (agreed?) and the altimeter is hooked up only to the static port. Why does its readout stay constant?

 

[mar]

We're gettin' somewhere now!

Ah ha. I see what your question is now.

For a constant altitude, an increase in airspeed will mean an increase in dynamic pressure *only*.

Static pressure *for a constant altitude* will remain the same.

That's why the altimeter doesn't change when you increase airspeed.

I'm not sure I can be any clearer. Static pressure is by definition atmospheric pressure. That is affected by changes in temperature and density.

As I said, the airplane will experience *localized* changes in pressure (increases and decreases), but the designers deliberately choose the location of the static ports to give the *best* indication of static atmospheric pressure.

How are we doing now?

 

[VNugget]

Well at least we're zeroing in.

For a constant altitude, an increase in airspeed will mean an increase in dynamic pressure *only*.

Static pressure *for a constant altitude* will remain the same.

...

tatic pressure is by definition atmospheric pressure.

OK, I thought that total pressure is atmospheric pressure, and is a consant at a given altitude.

You're saying that with an incrase in airspeed, static pressure stays constant and dynamic pressure increases, thus total pressure increases.


As I said, the airplane will experience *localized* changes in pressure (increases and decreases), but the designers deliberately choose the location of the static ports to give the *best* indication of static atmospheric pressure.

Unless otherwise specified, all of my references are to free stream pressure.

 

[mar]

Almost there!

---OK, I thought that total pressure is atmospheric pressure, and is a consant at a given altitude.

---You're saying that with an incrase in airspeed, static pressure stays constant and dynamic pressure increases, thus total pressure increases.

---Unless otherwise specified, all of my references are to free stream pressure.

---No sir. Total Pressure = Dynamic (Ram/Pitot) + Static (Atmospheric). Static *is* Atmospheric.

---That's correct.

---Ok. But the term 'free stream' is closely related to 'relative wind' and that has certain *dynamic* properties. I'm just trying to be clear about what we're talking about.

I think you got it now. Thanks for exercising the old muscle.

 

[VNugget]

Alright, I think I have it after digging around some Bernoulli info on the net. Total pressure stays constant, yeah, but only along a given streamline. That's a new one to me, but it seems to mean that it applies only to a closed system, such as the venturis described in textbooks.

The whole time I was missing the obvious stipulation that in my discussion of changing airspeed, the two airspeeds were NOT in such a closed system, but were supplied by external factors, and were therefore representative of two different energy states and total pressures.


Thanks for the patience.

 

[mar]

No sweat.