Re: air speed measurement above mach 1



Raul Ferreira wrote:
> >
> >The speed of sound in air depends on the pressure, temperature, and
> >humidity of the air.
> >
> >An airplane moving at Mach 1 at 5000 feet is not going the same speed
> >as an airplane moving at Mach 1 at 20,000 feet.
> >
> >Airspeed indicators don't measure the speed of sound, they measure the
> >speed of the airplane relative to the air by measuring air impact pressure.
> >
> >Sound has nothing to do with it.
> >
> >The speed of anything is measured directly by measuring how long it takes
> >to go between two fixed points a known distance apart.
> >
> >How do you observe two fixed points if you are moving?
>
>
> When you are talking about airspeed you are talking about pressure. A
> "static" pressure against another static and different pressure.
>
> When you talk about sound you talk about vibration. The pressure is
> not static but it is vibrating. There is also sound pressure.

There is quite a bit jumbled up here. Let's clarify a little bit.

If you push on a bit of air from one side, this will increase the
pressure in that bit of air, which will push on the neighboring bit of
air, and so on. The speed that this pressure disturbance propagates
through the air is the speed of sound.

Sound is typically but not necessarily a periodic repetition of such
pushes, and in such cases there will be a sequence of disturbances
propagating through air, like waves on a beach. However, it is the
speed of *one* of these disturbances that defines the speed of sound,
not the speed of the repetition. The speed of the repetition is a
different quantity altogether, the frequency of vibration.

The best way to measure the speed of sound is to initiate a pressure
disturbance at point A and wait for it at another point B. The speed is
then the A-B distance divided by the wait time. Since the pilot in the
plane is at point A, he will need a cohort away from the plane at point
B. A pilot in a plane can't measure the speed of sound as he goes, any
more than you can measure the speed of sound of your voice while you're
riding a bike.

>
>
> For instance, the sonic boom, it happens when the plane reaches sound
> speed. I was told that it happens because the pressure of the air
> against the fuselage is "converted" into sound ( correct me if i
> wrong). This sound has a frequency or it is just a sudden change in
> pressure?
>
> After sonic boom, the plane generates loud noise sounds, so there are
> vibrations everywhere. What kind of sound does a fighter pilot hear
> in the plane above Mach 1 if he could hear it? I suppose that it is
> something so loud that he has to use some protective device.
>
> By the way, the fighter pilot hears the sonic boom has we (on the
> ground) do? Does he feel any kind of reaction on the plane at Mach1?

The best thing here is an animation, such as what can be found at
http://www.kettering.edu/~drussell/Demos/doppler/doppler.html

A sonic boom is simply the piling up of successive pressure
disturbances. The noise the pilot hears is the same as always, though
there is a transition that a pilot hears that has to do with the
shockwave at Mach 1 passing behind the pilot. The piling up happens
away from and behind the plane, where it eventually spreads and reaches
the ground. This piled-up sound appears as a "boom" to ground
observers. After the piled-up sound front passes the ground observer,
the ground observer hears the plane in the customary way, though
Doppler-shifted down in frequency.

>
> When i wrote sound speed barrier i think i mean sound speech barrier.
> Do you agree?

There is no "barrier" either way. Note that the sound inside the plane
is not moving relative to the pilot(s), so two pilots in the plane can
talk normally, the same way that passengers on the Concorde could talk
with each other normally. The so-called sound "barrier" simply
expressed the erroneous belief that planes would not survive the shock
wave encountered at Mach 1. This belief was dispelled in the 50's.

PD

.

Relevant Pages

  • Re: Wiring for the stage
    ... As the outside air pressure is different from ... The ribbon mic is still dealing with pressure changes, which is what sound ... itself reacts to the sound waves. ...
    (alt.guitar.bass)
  • Re: Hansen discovers how to reason.
    ... stationary field then travels in vacuum (a reasonable presumption since ... air, only the frequency. ... > airhorn in plane A. The sound proceeds to the front of plane A at 343 ...
    (sci.physics)
  • Re: Hansen discovers how to reason.
    ... stationary field then travels in vacuum (a reasonable presumption since ... air, only the frequency. ... > airhorn in plane A. The sound proceeds to the front of plane A at 343 ...
    (sci.physics.relativity)
  • Re: air speed measurement above mach 1
    ... I was told that it happens because the pressure of the air ... > "When an airplane travels through the air, it produces sound waves." ... Engine noise has nothing to do with it. ...
    (sci.physics)
  • Re: air speed measurement above mach 1
    ... >>humidity of the air. ... >>Airspeed indicators don't measure the speed of sound, ... >>speed of the airplane relative to the air by measuring air impact pressure. ... > For instance, the sonic boom, it happens when the plane reaches sound ...
    (sci.physics)