# How does the altitude of an airplane affect the velocity?

1. Dec 15, 2015

### distalphalanx

1. The problem statement, all variables and given/known data
How does the altitude of an airplane affect the measurement of the velocity, when using a Pitot tube to measure the speed? The Pitot tube is horizontal, with one end open and another closed. The stagnation pressure is measured at the end of the Pitot tube, and the static pressure is the surrounding pressure.

2. Relevant equations
Bernoulli, reduced to v=√((2*(p_t - p_s)) / ρ_air) , where p_t is the stagnation pressure of the Pitot tube, p_s is the static pressure of the Pitot tube and ρ_air is the air density.

3. The attempt at a solution
It is common knowledge, that air density decreases as altitude increases. As the density decreases, the air resistance (friction) decreases. This will lead to an increase in velocity. Is this a sufficient answer?

2. Dec 15, 2015

### CWatters

Your answer could be taken as explaining why an aircraft can fly faster at high altitude. The question asks about the "measurement" of velocity. Not the same thing.

Last edited: Dec 15, 2015
3. Dec 15, 2015

### Staff: Mentor

What interest me is whether it has to fly faster to keep the aerodynamic working. Or which are the minimum velocities depending on altitude for a common commercial airliner, e.g 737?

4. Dec 15, 2015

### insightful

Yes, it has to fly faster to produce the same dynamic pressure on the wings. Indicated air speed from the pitot tube is a good indication of dynamic air pressure on the airframe. It is not a good indication of actual air speed, reading too low at higher altitudes. See Wikipedia articles.