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The Measurement of Air Speed

Published online by Cambridge University Press:  14 September 2016

A. P. Thurston
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Extract

It is a matter of extreme importance to be able to measure accurately the speed of the air in any situation and under any condition, because all our knowledge of the dynamical properties of the air is dependent upon a correct measurement of its velocity, and because the safety of a pilot depends upon knowing accurately the speed at which he is flying. He is then able to guard himself against the risk of stalling his machine or of attaining too great a speed. An air speed indicator is really as indispensable to a pilot as a foot rule is to a carpenter or a “hooter” to a motorist.

The velocity of the air may be determined by three main methods.

In the first method the velocity is measured directly by the time taken by a particle, body or substance floating in or dragged along by the air to travel from one point to another.

Type
Research Article
Information
The Aeronautical Journal , Volume 18 , Issue 71 , July 1914 , pp. 245 - 271
Copyright
Copyright © Royal Aeronautical Society 1914

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References

Note on page 246 * The theory of the Pitot lube is as follows :—

The total energy E in unit mass of a fluid = the potential energy + the pressure energy + the kinetic energy.

Where (in suitable consistent units) h = potential height of fluid.

p = pressure.

w = weight of a column of the fluid of unit cross section and height.

v = velocity.

Now in the case under consideration the potential energy is not available and therefore h = 0.

If the velocity and the kinetic energy are assumed zero, as is the case at the mouth of the Pitot tube,

then

(1)

And if all the pressure energv is converted into kinetic energy

then

(2)

But the total energy is constant.

Eliminating E from (1) and (2)

then

Let h′ = the “head” of fluid corresponding with the pressure p

(3)

If h˝ = head of water in tilting gauge corresponding to the head h′ of the fluid. ρ= density of fluid.

Substituting this value in (3).

The density of air is generally taken as 0.0012

If h˝ is measured in feet and g is taken equal to 32.18, then the velocity given by the above equation will be in ft. per sec.

Note on page 249 * “Aëronautical Journal,” July, 1913.

Note on page 249 † “Philosophical Magazine,” Vol. XXI., Fry and Tyndall “On the Value of the Pitot Constant.”

Note on page 249 ‡ Proceedings, Institution of Civil Engineers, Vol. CLVI.

Note on page 249 § “ Aëronautical Journal,” April, 1911.

Note on page 249 ∥ “Aëronautical Journal,” July, 1913.

Note on page 249 ¶ “Aëronautical Journal,”July, 1913.

Note on page 256 * “ Engineering,” December 27, 1912.

Note on page 257 * * “ Engineering,” August 8, 1913.

Note on page 260 * * “ Engineering,” September 20, 1912.