Commercial Jets and The Sound Barrier

[Ecko]

Hi guys!


One of my dads friends, who has no aviation knowlegde whatsoever, told me today that commercial jets often pass the speed of sound whilst cruising. He explained this was because of the high tailwinds they can get from the jetstream.

I know that commercial jets* cannot pass the speed of sound safely, but I don't know how to explain it to him.


Quite frankly, it's got me a bit confused too..


The mach speed will go up when you get a tailwind, since the mach speed is related to the groundspeed, correct?
-Then how can you prevent an aircraft from going past mach 1 if you get a really really strong tailwind?

I know the design of modern day carriers greatly limits the speed near mach 1, but can it really prevent it?


I'm a little confused, help would be greatly appreciated. I don't need hour-long technical explanations, but a short, packed version would nice.

[C]

The mach speed will go up when you get a tailwind, since the mach speed is related to the groundspeed, correct?
-Then how can you prevent an aircraft from going past mach 1 if you get a really really strong tailwind?

I know the design of modern day carriers greatly limits the speed near mach 1, but can it really prevent it?


I'm a little confused, help would be greatly appreciated. I don't need hour-long technical explanations, but a short, packed version would nice.

Simple. It isn't. Mach number is derived from the Indicated Airspeed or True Airspeed of an aircraft, not its Groundspeed. Hence the aircraft is not supersonic - ie the relative airflow into which the aircraft is flowing is not above the speed of sound.

[Tweek]

Think about it, if it was true, all that matters is the indicated airspeed. I don't know myself, but like you said, it would break the sound barrier, if talking about the relative ground speed.

What would rip an airliner apart from overstressing (in this case, from speed), would be because of the sheer air resistance, caused by great speeds. If there is a strong tailwind, this will, in effect, blow the air away from the aircraft, and thus decreasing air resistance (and also air flow over the wings - indicated airspeed).

Think of it this way:

If you had a 100mph tailwind, and the plane was travelling at a actual airspeed of 150mph (not talking about airliners specifically here), then the indicated airspeed would be 50mph, and therefore the aircraft would more than likely stall, due to the lack of airflow over the wings, in the direction of the tail.

In other words, I don't think that the Mach gauge in an airliner would pass Mach 1, even though they could be travelling faster than the speed of sound.

...I think! At least that's my take on all this scientific malarky. Please correct me if I'm wrong!

[Ecko]

Simple. It isn't. Mach number is derived from the Indicated Airspeed or True Airspeed of an aircraft, not its Groundspeed. Hence the aircraft is not supersonic - ie the relative airflow into which the aircraft is flowing is not above the speed of sound.

Thanks a lot Charlie, it made me that much smarter!

Thank you too Tweek, although you're pretty much saying the same as myself.

[Hagar]

If you had a 100mph tailwind, and the plane was travelling at a actual airspeed of 150mph (not talking about airliners specifically here), then the indicated airspeed would be 50mph, and therefore the aircraft would more than likely stall, due to the lack of airflow over the wings, in the direction of the tail.

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.

In the same way a balloon has no airspeed at all. If there was no wind it would rise & fall vertically.

PS. The only supersonic commercial airliners to date were Concorde & the Tu 144.

[C]

PS. The only supersonic commercial airliners to date were Concorde & the Tu 144.

...and the DC-8!

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

M1.012 at 41,000ft!

[Hagar]

Ah, I meant straight & level c/w passengers.

After climbing to an altitude of 52,090 feet, the DC-8-42 series aircraft attained a maximum speed of Mach 1.012 or 660 mph while in a controlled dive through 41,088 feet.

[Mobius]

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.

[Hagar]

We're in danger of getting too technical here. Going back to Ecko's original question I'll try to explain this as simply as possible. It might be easier to think of a boat on a fast flowing river. The water is the air & the river bank is the ground.

Let's say the river is flowing at 10 mph & our boat can do a maximum of 5 mph. If the boat is going flat out downstream it will be travelling at 15 mph (5 + 10) in relation to the river bank. Now turn the boat round & go upstream & it will actually be going backwards at 5 mph (5 - 10) in relation to the river bank - but it's still going forward at 5 mph through the water.

[TacitBlue]

To add to Dougs post:

If the airspeed indicator reads 600 knots, and you have a tailwind of 100 knots (like the jetstream) then your ground speed would be 700 knots. If sound travels at 661 knots, then yes, your ground speed would be above the speed of sound, but your airspeed would not.

P.S. I'm not that famillier with airliners, is 600 Knots a normal speed for one?

[beaky]

Here's a simple take on this: all the drama of exceeding the speed of sound is about going at that speed relative to the air around you. That's what really counts; a groundspeed of 800 knots or so is just a number, but "breaking the barrier" (which is really a barrier created by the airplane itself, as it deals with the peculiar effects of moving through air faster than sound waves) is something a little more interesting, and challenging.
And no, since the Concordes were retired, airliners do not "go supersonic". Almost, but not quite.

[Ecko]

I get it now, easy to explain really.


You're going faster than the speed of sound, but you don't "hit" the sound barrier cause the air around the aircraft is travelling fast also. That way the complications of supersonic flight doesn't affect the aircraft. The aircraft is over the speed of sound, but not over mach 1.

Thanks for the help guys!

[Chris_F]

I get it now, easy to explain really.


You're going faster than the speed of sound, which is 340.29 meters per second, but you don't "hit" the sound barrier cause the air around the aircraft is travelling fast also. That way the complications of supersonic flight doesn't affect the aircraft. The aircraft is over the speed of sound, but not over mach 1.

Thanks for the help guys!

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.

[Nexus]

Yeah, over the ground you're going faster than the speed of sound, but like several posters said, the airflow around the aircraft you're flying in does not.
Remember what Rotty said: Groundspeed is just a