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Help with static and stagnation pressure!

by Graviman
Tags: pressure, stagnation, static
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Graviman
#1
Nov25-07, 07:45 AM
P: 19
I am currently very confused about how an altimeter works. I understand that stagnation pressure is the pressure of bringing the flow to rest, and that static pressure is the pressure flowing with the fluid (air in this case). What i don't understand is that the static port is used for altimeter, while pitot tube is used for air speed indicator.

I've always understood that the venturi effect is explained because the total stagnation pressure remains constant, while dynamic pressure changes - thus the static pressure changes. If that is so why doesn't altimeter alter with speed? Alternately what is to stop the static port being put on the wing? Ahhh!

This has come about because of a discussion with some helicopter test pilots about how ground effect reduces hover power. I thought i understood this stuff but i'm coming unstuck. Pan! Pan! Pan!

Mart
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FredGarvin
#2
Nov25-07, 09:32 AM
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Altimeters are basically barometers. Are you familiar with the operating procedures for an altimeter? An altimeter has to be constantly adjusted for varying pressure changes. That is why altimeters have a hand dial that sets your current location's barometric pressure, i.e. sea level is 29.92. You have to constantly adjust altimeter settings when traveling long distances as well.

The pitot tube uses, essentially, Bernoulli's theorem for calculating air speed. It needs the difference between the local static and stagnation pressures to calculate the airspeed.

Static port location is chosen to eliminate as much of the aircraft influence as possible. This is in an effort to get a pressure measurement as close to the actual surrounding atmosphere as possible. It should be apparent that the static pressure on a wing surface will not be the same as the surrounding atmosphere.

And yes...hover power required decreases in ground effect.
Graviman
#3
Nov25-07, 12:40 PM
P: 19
Fred, thanks for the reply. Yes, i am familiar with the use of altimeters - but, only from a pilot's perspective. I appreciate that my question appears as if from someone with no understanding. It is more that i thought this made intuitive sense to me, but now have my doubts.

My confusion comes from how the static port is located. The ideal position is that it is in a position where local velocity is that of the free stream. On a fixed wing fuselage this is on the flatest portion possible. So my understanding is that if the local velocity is higher, such as it would be on a wing, then the altimeter would read high (ie pressure low). So this means that on a helicopter the altimeter only gives the right reading when the rotor downwash is not passing the static - such as in forward flight. Again it will read altitude high, pressure low, in the hover.

Perhaps i understand this better than i thought. Let me explain the reason for my doubt:
The engineering understanding of why ground effect reduces power is that the downwash wake contraction is reduced, and in fact becomes wake expansion near enough to the ground. The wake expansion results in lower flow velocities, hence a lower rotor induced power (pitch, torque, etc). The pilot understanding is that the helicopter near the ground builds up a region of increased static pressure, which increases the rotor thrust for a given power.

I used the analogy of putting an altimeter under the hovering helicopter, but even as i reasoned i began to doubt my own words. I can't believe that for incompressible flow (ignoring tip speed effects) that the static pressure would be different anywhere in the flow field. The altimeter at the stagnation point at hover centre ground level should read unchanged from when the helicopter was not there. Anywhere else and the flow field would cause it to read altitude high, pressure low.

Any thoughts?

Q_Goest
#4
Nov25-07, 05:13 PM
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Help with static and stagnation pressure!

Hi Graviman,
I think where you're getting confused is in thinking that - stagnation pressure is dependant on how fast air is going across some port whose axis is perpendicular to the direction of air flow (ie: this is incorrect).

Here's a nice picture of a typical pitot tube we can use as an example:
http://www.centennialofflight.gov/es...Flow/TH7G3.htm

The pitot tube is a combination of two tubes taking two different pressure measurements as seen in this picture. Those two tubes are labeled (a) Pitot tube and (b) Static tube. The static tube has air flow perpendicular to the axis of the holes in the side of the tube. This is actually measuring 'stagnation pressure' of the air, which can also be thought of as the air pressure which would be read if there were no air flow whatsoever.

As pointed out by Fred, this stagnation pressure measurement must be placed where there is the least influence (ie: turbulance) from the aircraft as possible.

The same thing holds for the altimiter, which is going to be a port where there will be a flow of air which is perpendicular to the opening such that the pressure measured would be the same pressure measured if the aircraft were stationary. This is the same as stagnation pressure.

Note also as pointed out by Fred, this pressure must be referenced to some ground pressure so an altimeter must be adjusted periodically to account for this difference.
FredGarvin
#5
Nov26-07, 06:41 AM
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Quote Quote by Graviman View Post
So my understanding is that if the local velocity is higher, such as it would be on a wing, then the altimeter would read high (ie pressure low). So this means that on a helicopter the altimeter only gives the right reading when the rotor downwash is not passing the static - such as in forward flight. Again it will read altitude high, pressure low, in the hover.
In most cases, the error introduced by this is going to be rather small. For example, if you look at the static port locations on a CH-47, you'll see a great example of what we are discussing. There are 4 static ports on the chin bubbles up front and two on the fuselage facing to the left and right sides. Again, these are the best locations that are available to the designer to locate them.

Quote Quote by Graviman View Post
The engineering understanding of why ground effect reduces power is that the downwash wake contraction is reduced, and in fact becomes wake expansion near enough to the ground. The wake expansion results in lower flow velocities, hence a lower rotor induced power (pitch, torque, etc). The pilot understanding is that the helicopter near the ground builds up a region of increased static pressure, which increases the rotor thrust for a given power.
Honestly I have never heard the pilot version of that explanation. I don't subscribe to that one bit. The only thing you didn't mention is the interruption to tip vorticies For a static pressure increase that you are describing to happen, there would have to be some sort of back pressure. Since the complete area around the aircraft is open, I see no way for that static pressure build up to happen. I have never heard of anyone doing static pressure measurements under an aircraft at a hover. I am going to have to look through some references to see if I can find anything on that.
Graviman
#6
Nov26-07, 01:08 PM
P: 19
Fred, i wouldn't take the pressure bubble explanation too seriously. When you transition from hover to forward flight the downwash can roll up to produce a partial vortex which is reingested (similar to vortex ring state). Basically you need to briefly raise the collective before forward inflow reduces power again. Pilots are taught that this is the helicopter flying off the pressure bubble, but as an engineer i'm trying to develop a better understanding. I'll keep an eye out for those chook static ports.


QG, i think i might need a bernoulli equation 101 lesson. A thin film of oxide has developed over my understanding. Looking at wikipedia:
http://en.wikipedia.org/wiki/Stagnation_pressure

Stagnation (Total) Pressure = Dynamic Pressure + Static Pressure

or

Pstagnation = 1/2 Rho V^2 + Pstatic

Now i'm puzzled here, since i though that the pitot measured stagnation pressure, while static port measured static pressure. The difference between the two gives you dynamic pressure, which thus gives you airspeed? Part of my confusion is that from the venturi effect the stagnation pressure remains constant, so that the static pressure alters.

It sounds like i'm in need of zinc chromate...
Graviman
#7
Nov27-07, 09:23 AM
P: 19
OK, i think i have found what i am looking for on this site:

http://www.av8n.com/how/htm/airfoils.html

Consider the following line of reasoning:

The airplane’s altimeter operates by measuring the pressure at the static port. The static port is oriented sideways to the airflow, at a point where the air flows past with a local velocity just equal to the free-stream velocity.
In accordance with Bernoulli’s principle, this velocity must be associated with a “lower” pressure there.
You might think this lower pressure would cause huge errors in the altimeter, depending on airspeed. In fact, though, there are no such errors. The question is, why not?
The answer has to do with the notion of “lower” pressure. You have to ask, lower than what? Indeed the pressure there is 1 Q lower than the mechanical energy (per unit volume) of the air. However, in your reference frame, the mechanical energy of the air is 1 Atm + 1 Q. When we subtract 1 Q from that, we see that the pressure in the static port is just equal to atmospheric. Therefore the altimeter gets the right answer, independent of airspeed.

Another way of saying it is that the air near the static port has 1 Atm of potential energy (pressure) and 1 Q of kinetic energy. The altimeter is sensitive only to pressure, so it reads 1 Atm — as it should.

In contrast, the air in the Pitot tube has the same mechanical energy, 1 Atm + 1 Q, but it is all in the form of potential energy since (in your reference frame) it has no kinetic energy.

The mechanical energy per unit volume is officially called the stagnation pressure, since it is the pressure that you observe in the Pitot tube or any other place where the air is stagnant, i.e. where the local velocity v is zero (relative to the airplane).

In ordinary language “static” and “stagnant” mean almost the same thing, but in aerodynamics they designate two very different concepts. The static pressure is the pressure you would measure in the reference frame of the air, for instance if you were in a balloon comoving with the free stream. As you increase your airspeed, the stagnation pressure goes up, but the static pressure does not.

Also: we can contrast this with what happens in a carburetor. There is no change of reference frames, so the mechanical energy (per unit volume) remains 1 Atm. The high-speed air in the throat of the Venturi has a pressure below the ambient atmospheric pressure.


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