# Help with static and stagnation pressure!

by Graviman
Tags: pressure, stagnation, static
 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
 Sci Advisor P: 5,096 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.
 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?
PF Patron
HW Helper
P: 2,794

## 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.