How does the altitude of an airplane affect the velocity?

In summary, when using a Pitot tube to measure the speed of an airplane, the altitude affects the velocity measurement due to changes in air density. This is because the decrease in density leads to a decrease in air resistance, resulting in an increase in velocity. However, this also means that the indicated air speed from the Pitot tube may not accurately reflect the actual air speed at higher altitudes. To compensate for this, airplanes must fly faster to maintain the same dynamic pressure on the wings.
  • #1
distalphalanx
3
0

Homework Statement


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.

Homework 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.

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?
 
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  • #2
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.
 
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  • #3
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
fresh_42 said:
What interest me is whether it has to fly faster to keep the aerodynamic working.
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.
 

1. How does the altitude of an airplane affect its velocity?

The altitude of an airplane can affect its velocity in two ways. Firstly, at higher altitudes, the air density is lower, which means there is less resistance or drag on the plane. This can result in the airplane being able to travel faster due to less air resistance. Secondly, at higher altitudes, the air is generally colder, which can increase the engine's efficiency, allowing the airplane to fly faster.

2. Why does the air density change with altitude?

The air density decreases with altitude because the air molecules are more spread out at higher altitudes. This is due to the decrease in air pressure as altitude increases. As there is less air pressure, there are fewer air molecules in a given volume, resulting in lower air density.

3. How does the angle of the airplane affect its velocity?

The angle of the airplane, also known as its pitch, can affect its velocity in two ways. Firstly, if the airplane is pitched too high, it can cause an increase in drag as more air is hitting the underside of the wings. This can slow down the airplane. Secondly, if the airplane is pitched too low, it can cause a decrease in lift, resulting in the airplane losing altitude and potentially slowing down.

4. Does the weight of the airplane affect its velocity at different altitudes?

Yes, the weight of the airplane can affect its velocity at different altitudes. At higher altitudes, where the air density is lower, the airplane's weight has less of an impact on its velocity. This is because there is less air resistance for the weight to overcome. However, at lower altitudes, where the air density is higher, the airplane's weight can significantly affect its velocity as there is more air resistance for the weight to overcome.

5. Can the altitude of an airplane affect its fuel efficiency?

Yes, the altitude of an airplane can affect its fuel efficiency. As mentioned earlier, at higher altitudes, the air is colder and less dense, allowing the engine to operate more efficiently. This can result in less fuel consumption, making the airplane more fuel-efficient. However, flying at higher altitudes can also increase the airplane's drag, which can offset the potential fuel efficiency gains.

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