Speedometer Principle: Measuring Air/Water Speed

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In summary, ships use turbine flow meters to measure speed relative to the water, and can also use GPS or corrections for current to determine true speed referenced to the Earth. Airplanes use pito-static tubes to measure airspeed, which is affected by density and altitude. Bernoulli's equation explains the relationship between airspeed and altitude, and corrections must be made for wind.
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pixel01
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Hi there,

I ask about the speedometer of a ship or an airplane. The water (or air) flows so how do they know the absolute speed of the ship (plane)?
 
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  • #2
There is no such thing as absolute speed.

Ships use turbine flow meters. They are basically a paddle-wheel sticking through the hull. A sensor measures the rate of rotation and converts that to speed in knots. If the navigator wants "true" speed (referenced to the Earth instead of the water), s/he uses GPS or adds a correction based on known currents.

[subsonic] Airplanes use pito-static tubes that measure the velocity pressure of the air and convert to speed via Bernoulli's equation. That gives airspeed. If they want ground speed, they can use GPS or make corrections to airspeed based on altitude and wind (airspeed drops with altitude, according to Bernoulli's equation).
 
  • #3
russ_watters said:
There is no such thing as absolute speed.
that is to say; all speeds must be measured relative to something else. Ships and aircraft measure their speed relative to the fluid through which they travel.

(airspeed drops with altitude, according to Bernoulli's equation).
Please doublecheck my reasoning on this, but Pitot tubes measure both ambient air pressure and velocity air pressure, displaying the difference between the two. I believe this means that no "correction for altitude" is required for giving ground speed; only a correction for wind.
 
  • #5
LURCH said:
Please doublecheck my reasoning on this, but Pitot tubes measure both ambient air pressure and velocity air pressure, displaying the difference between the two. I believe this means that no "correction for altitude" is required for giving ground speed; only a correction for wind.
Jeff said:
Indicated air speed. Affected by density; essentially reports based on mass flow, at any given IAS (Indicated Air Speed), the relative mass flow is the same regardless of altitude (density or temperature).
Sorry Jeff, I guess I got my aviation terms wrong (or just not specific enough). Affected by altitude is what I meant, so IAS, it is.

Bernoulli's equation has only three terms here: velocity, pressure, and density. Density drops with altitude, so IAS drops with altitude. What it means for a pilot is that for, say, 100 kts ias at 5,000 ft, the actual speed of the air over the wings is higher than for 100 kts ias at 1,000 ft. But the lift of the wing is the same because, as Jeff said, the mass flow rate of air over the wing is the same.
 

FAQ: Speedometer Principle: Measuring Air/Water Speed

What is the speedometer principle?

The speedometer principle is a method for measuring the speed of air or water. It works by using a rotating disc that is placed in the path of the air or water flow. The rotation of the disc is directly proportional to the speed of the air or water passing through it.

How does the speedometer principle work?

The speedometer works by using a magnet and a coil. The magnet is attached to the rotating disc and the coil is fixed in place. As the magnet rotates with the disc, it creates a changing magnetic field that induces an electric current in the coil. This current is then measured and used to calculate the speed of the air or water.

What are the advantages of using the speedometer principle?

One advantage of using the speedometer principle is that it is a non-intrusive method of measuring speed. This means that it does not require direct contact with the air or water flow, which can be beneficial in certain applications. It is also a relatively simple and cost-effective method compared to other types of speed measurement.

Are there any limitations to the speedometer principle?

One limitation of the speedometer principle is that it may not provide accurate measurements in turbulent or non-uniform flows. The accuracy of the measurement can also be affected by factors such as the size and shape of the rotating disc, and the distance between the disc and the air or water source. Additionally, the speedometer principle may not be suitable for measuring extremely high speeds.

What are some common applications of the speedometer principle?

The speedometer principle is commonly used in various industries such as aerospace, automotive, marine, and meteorology. It can be used to measure the speed of air or water in wind tunnels, engines, pipelines, and watercrafts. It is also used in weather stations to measure wind speed and direction.

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