Speed of Sound

[scrunchthecat]

OK - I have often wondered this....

I was once on a 777 when the in-seat screen shows the speed of the jet to be well over 700mph. We must have hit a tailwind from NRT to EWR and I saw 760mph on the screen. (I was a bit dazed when this happened and did not think to snap a photo until it had gone back down to 715mph)

I thought the speed of sound was 740. I asked the flight attendant if we had passed the speed of sound, and she said she would ask the Captain. She came back and told me he said "no."

Can someone explicate this to me? Is the speed of sound greater the higher up you go? And just how fast can a 777 fly?

[deltayankee]

No, you didn't break the speed of sound.

Traditional airspeed displays measure the speed relative to the air, but the displays you see in the back show the ground speed derived from GPS navigation data, so what you see is the airspeed plus or minus the wind component aligned with the aircraft. If the plane is flying at 700mph compared to the air but the air is blowing at 100mph behind you then the speed over the ground is 800mph.

To break the speed of sound you need to have an actual airspeed in excess of Mach 1. Groundspeed is irrelevant.

[Haven't a clue]

Ground speed = airspeed +/-tail (head) wind

So the moving map may indicate 700mph, however you are actually doing say 550mph in a chunk of air which is already doing 150mph.

Don't worry - your airspeed should never get close the the speed of sound!

[perkin]

Don't worry - your airspeed should never get close the the speed of sound

But surely it does get close to the speed of sound? Don't most airliners cruise at approximately Mach 0.9?

[PA38-Pilot]

But surely it does get close to the speed of sound? Don't most airliners cruise at approximately Mach 0.9?

Between .70 and .85 actually. Only the Citation X gets to Mach .92

[perkin]

Ok, Mach 0.8 it is! I just used 0.9 as a maximum ballpark figure - I'm sure I read somewhere that some freight ops cruise at a higher mach number than pax, but my memory may be serving me incorrectly!

[Wader2]

Is the speed of sound greater the higher up you go?

No, the reverse.

Sound wave propagate through a medium, in this case air. The speed is related to the density of the medium.

Think of a line of people close together. One touches the next who touches the next and the ripple will propagate quickly. Space them out where they have to stretch to get contact, it will take longer. Space them further and they will have to move to touch - slower still.

What we are doing is reducing the density. For sound waves in air the sound is progated molecule to molecule of air. In space, with no atmosphere there is no sound.

As well as reducing density with increasing altitude air also becomes denser as the temperature decreases. When Neville Duke broke the world airspeed record he flew as low as possible to take advantage of the higher pressure and hence denser air which increased the local speed of sound.

If we go into water this is much denser so the speed of sound is much faster.

Sound waves through air at sea level at 0 deg C 331 m/s
At 20 deg C 343 m/s and at 100 deg 386 m/s.

In salt water that is free of air bubbles or suspended sediment, sound travels at about 1500 m/s.

Heidhurtin, thanks for that, got my logic screwed up.

[Heidhurtin]

......As well as reducing density with increasing altitude air also becomes less dense as the temperature decreases. ......

Are you sure? Reduced density with altitude I can understand, but reduced density with reduced temperature? I thought the opposite was true? :confused:

[Two_Squirrels]

Density is proportional to the square root of temperature. Interested to know what your thought process as to why density would increase with reduced temperature?

[Sukhraj]

Two_squirrels: Boyle's law states: Density = (MP)/(RT)

So, density is inversely proportional to T. Hence, reduced temperature (keeping other variables constant) = increased density.

The reason for reduced density at higher altitudes is much reduced atmospheric pressure and vice-versa.

[Sukhraj]

There's an easy to use ICAO Standard Atmosphere calculator here:
http://www.aviation.ch/tools-atmosphere.asp

It gives you the pressure, density, temperature and speed of sound for a given altitude.

[Heidhurtin]

.........So, density is inversely proportional to T. Hence, reduced temperature (keeping other variables constant) = increased density..........

Was just about to say the same thing after looking up my old engineering notes, but you beat me to it. :)

[chornedsnorkack]

Sound wave propagate through a medium, in this case air. The speed is related to the density of the medium.

Maybe - maybe not. That is a matter of definition.


If we go into water this is much denser so the speed of sound is much faster.

Sound waves through air at sea level at 0 deg C 331 m/s
At 20 deg C 343 m/s and at 100 deg 386 m/s.

In salt water that is free of air bubbles or suspended sediment, sound travels at about 1500 m/s.

Sound is faster in water not because it is denser but because it is less compressible.

The speed of sound is about the same in water and in quicksilver, even though quicksilver is denser.

If you change the pressure of air, its compressibility changes together with density, so the speed of sound is unchanged.

You can have -50 degrees at a great height where pressure is low and density is low. Or you can have -50 degrees in winter near ground, in which case the pressure is rather high - and density is extremely high. Sound speed is the same in both cases.

What changes about sound in space is that sound attenuation is strong at low densities. The speed of sound is still unchanged either way.

[scrunchthecat]

OK - so why is not an airplane travelling at over 700mph at 33,000 feet traveling above the speed of sound? According the the calculator, speed of sound kicks in at 669mph.

http://unix.temple.edu/~susanj/lj/2005/china/sound.jpg

[TightSlot]

The screen shows the speed over the ground NOT the speed through the air - as explained above.

[bealine]

So, with the exception of Concorde and its Soviet counterpart Concordski, no commercial airframe has been developed to break the sound barrier?

With today's typical Airbus and Boeing fleets, what speed could they achieve before the airframe started to become damaged? What makes the airframe incapable of withstanding the higher speeds of Mach 1 and above - is it the heat generated by air resistance - or is it the turbulence? (Concorde flew at 60,000 feet - the subsonic fleet cruise at up to 39,000 ft)

Just curious.....

[senshi]

bealine... it isn't the "damage" as such that is the concern, but rather the significant increase in drag penalties that result from cruising at supersonic speeds. compressibility drag forms as a result of shockwaves forming even at subsonic speeds, and continues well through transonic and supersonic speeds.

[scrunchthecat]

Ah, OK I get it now.
I did not understand the diff between airspeed and groundspeed
It's still a little confusing tho. Like the mouse walking along the pipe traveling at the speed of light.

[chornedsnorkack]

With today's typical Airbus and Boeing fleets, what speed could they achieve before the airframe started to become damaged? What makes the airframe incapable of withstanding the higher speeds of Mach 1 and above - is it the heat generated by air resistance - or is it the turbulence?

One concern is Mach tuck. When a plane approaches speed of sound, the centre of lift moves backwards.

This means that if a plane, say, accelerates to speed of sound in a shallow dive then as the centre of lift moves back, it drops nose and is unable to pull out of the dive.

Concorde has fuel pumping systems which pumps fuel backwards and forwards to keep CoG inside limits during crossing speed of sound.

In principle, a plane might have so big an elevator or all-moving tailplane that it simply trims out the Mach tuck. But do commercial subsonic jets actually have them?

DC-8-40 has crossed speed of sound, reaching Mach 1,012 in a deliberate shallow dive. But the test plane had water ballast tanks and pumps in cabin.

Testing of planes generally includes demonstrating their safely in overspeed dives and establishing Mmo. But for example, Airbus 380 was only tested up to Mach 0,96. If it were completely safe to fly Boeing 777 or Airbus 340 at, say, Mach 1,05, why does not anyone demonstrate this by test? Thus, there is probably a real danger of damage above some speed.

[10secondsurvey]

Following the assassination of Concorde by BA in 2003, I keep catching snippets of news, indicating that companies are around working on new types supersonic passenger aircraft. Whilst they are looking to go above MACH 1, none of them seem to be aiming for the dizzy heights of MACH 2 achieved by concorde.

In relation to this, I've always wondered, if an airframe is developed to get beyond MACH1, is it merely down to available thrust as to whether more than twice the speed of sound can be reached. Or is it the case that realistically, it is unlikely that another passenger aircraft will be developed that can equal or beat the speeds set by concorde?

[parabellum]

Decent, proper, well made, economic commercial airliners in the 350 seats plus market fly at between .85 and .86, the rest just follow.;)

[chornedsnorkack]

Whilst they are looking to go above MACH 1, none of them seem to be aiming for the dizzy heights of MACH 2 achieved by concorde.

In relation to this, I've always wondered, if an airframe is developed to get beyond MACH1, is it merely down to available thrust as to whether more than twice the speed of sound can be reached. Or is it the case that realistically, it is unlikely that another passenger aircraft will be developed that can equal or beat the speeds set by concorde?

Well, you have Lapcat A2, at Mach 5.

The Mach 2,04 Concorde, and Tu-144, have certain problems. For example, Concorde variable intake ramps were not used and immobile up to Mach 1,7. Also, as far as supersonic heating went, the Pepsi livery was not allowed above Mach 1,7.

So, if you restrict your speeds to Mach 1,5...1,7, you will save trouble with airflow and supersonic heating and have wider range of materials.

On the other hand, Valkyrie demonstrated flight at Mach 3,08. The engine inlets worked, and cockpit though small enabled shirtsleeve environment. Which means that the materials and engines for Boeing 2707 were possible.

[senshi]

chornedsnorkack... implications of using a mach trimmer to overcome mach tuck in passenger carrying aeroplanes??