Simviation Forum - Commercial Jets and The Sound Barrier

Reply #15 - May 11^th, 2006 at 8:58pm

Ecko Offline
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Kinda.

340m/s is only the speed of sound at sea level/standard temperature. The speed of sound changes with pressure, and thus changes with altitude. You can be traveling over 340m/s without traveling over the speed of sound even in still air. You just have to do it at altitude.

So that may be the confusion. Airliners do sometimes have a ground speed higher than 340m/s yet they do not exceed the speed of sound.

I'll leave out the 340 m/s then.. Wink

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Reply #16 - May 11^th, 2006 at 9:17pm

Mobius Offline
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Remember also, that the airplane doesn't care in the least where the ground is and it doesn't too much (some though) which way is down. Everything would work right as long as the wind is coming from straight ahead. So, like Rotty said "relative to the air around you" is one of the most important phrases in aviation, along with "stick forward, houses get bigger, stick back, houses get smaller", but they kind of contradict each other, don't they. Sorry Tongue

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Reply #17 - May 12^th, 2006 at 7:59am

town Offline
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Are you gentlemen saying that sound travels slower in a head wind ergo faster in a tail wind ???

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Reply #18 - May 12^th, 2006 at 8:11am

Nexus Offline
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Compared to speed over the ground, yes!

Flying at mach 1 with a tailwind will generate a higher groundspeed, than with a headwind component.

But groundspeed is irrevelant in this matter, since it has no influence over wing performance.

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Reply #19 - May 12^th, 2006 at 10:57am

town Offline
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Ah so

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Reply #20 - May 12^th, 2006 at 11:16am

Tweek Ex Member

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I thnk you're a tad confused there Tweek. The tailwind could be 500 mph & the airspeed would still be 150. In this case the groundspeed would be 150 + 500 = 650 mph but the aircraft would not be supersonic.

Not confused, but I probably explained it the wrong way.

I know exactly what you're saying, and that's what I was trying to get at... it's one of these things which can be a little difficult to explain!

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Reply #21 - May 15^th, 2006 at 11:26pm

cheesegrater Ex Member

A 747SP went supersonic with 274 people oboard and that particular aircraft is still being flown today.

http://www.aviation-safety.net/database/record.php?id=19850219-0

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Reply #22 - May 16^th, 2006 at 12:39am

expat Offline
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...and the DC-8!

http://www.dc8.org/library/supersonic/index.php

M1.012 at 41,000ft! Grin

In the words of General Cecil Hogmanay Melchard, "Well bugger me with a fish fork"

Matt

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Reply #23 - May 17^th, 2006 at 5:19am

chornedsnorkack Offline
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Not to mention conventional turbo-jets and turbo-fans cannot work when the incoming airflow is supersonic. Supersonic jets have to use a system of ramps and moving panels to slow the speed of the air down. That's all because the air in front of a supersonic shockwave is going supersonic, and behind it's subsonic, I can't quite remember exactly how it works, but airliners don't have that built into their engine intakes. Wink

I think this is not quite the case. Concorde intake ramps are only moved past Mach 1,7. A plenty of fighters with turbojets and low bypass turbofans go supersonic in the region of Mach 1 to 1,5 without complicated variable intakes.

However, there are several problems with going supersonic:

Transonic drag rise. Well, this might break up the plane in air, or simply slow the plane down so it does not sustain level supersonic flight.

The shift of the centre of lift. It moves back, so the plane tends to drop nose down into dive and go even more supersonic, until it meets ground or breaks up in air.

Concorde has elaborate fuel pumps to move weight backwards when going supersonic. Yet the DC-8 somehow also was able to recover from dive.

Also: the speed of sound is NOT dependent on pressure - only temperature.

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Reply #24 - May 17^th, 2006 at 7:05am

Nexus Offline
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That's what I thought, but some people on this forum stated otherwise in another thread.
I've been learned that the speed of sound is only dependant of temperature, but fellow members stated that was not the case and they had good arguments for it.

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Reply #25 - May 17^th, 2006 at 8:15am

expat Offline
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I think this is not quite the case. Concorde intake ramps are only moved past Mach 1,7. A plenty of fighters with turbojets and low bypass turbofans go supersonic in the region of Mach 1 to 1,5 without complicated variable intakes.

EDIT..Just found this, so ignore my ramblings below, this website explains it all very well:

http://selair.selkirk.bc.ca/aerodynamics1/High-Speed/Page7.html

Rule of thumb (and not some NASA out there experimental stuff) is that air intake speed MUST be below supersonic speed. For subsonic aircraft, the air intake to a jet engine presents no special difficulties, and consists essentially of an opening which is designed to minimise drag. However, the air reaching the compressor of a normal jet engine must be travelling below the speed of sound, even for supersonic aircraft, to sustain the flow mechanics of the compressor and turbine blades. At supersonic flight speeds, shockwaves form in the intake system and reduce the air speed at inlet to the compressor. Some supersonic intakes use devices, such as a cone or ramp to augment this, to slow the air down by varying the intake shape. If you where to cut the intake vertically in two, you would see, if memory recalls a divergent duct shape, allowing velocity to decrease, pressure increase and temperature to also increase.
As for Concord, I did read once that the air speed in front of the engine intake was only about 300mph.

Matt

PETA People Eating Tasty Animals.

B1 Boeing 737-800 and Dash8 Q-400

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