4
$\begingroup$

An excerpt from the BBC article here https://www.bbc.co.uk/news/uk-england-london-51433720

"Despite travelling faster than the speed of sound the plane would not have broken the sonic barrier as it was helped along by fast-moving air."

Could someone please elaborate on how that is possible?

$\endgroup$
4
  • 2
    $\begingroup$ Ground speed vs air speed... $\endgroup$ – jcaron Feb 9 '20 at 21:27
  • $\begingroup$ @jcaron, I see, learnt something there. The article doesn't mention anything. $\endgroup$ – Samer Tufail Feb 9 '20 at 21:34
  • 3
    $\begingroup$ The airplane never achieved a supersonic airspeed per se. It was a flying a normal crew speeds in a very strong tailwind, which yielded a ground speed which would’ve been greater than the speed of sound had it been achieved in still air. $\endgroup$ – Carlo Felicione Feb 9 '20 at 21:49
  • $\begingroup$ It was a British Airways flight and their callsign is Speedbird for a reason :-) $\endgroup$ – PerlDuck Feb 10 '20 at 8:32
5
$\begingroup$

The sound barrier is a function of airspeed and air density. If you have a strong enough tailwind, your ground speed would be above what most lay people would consider the speed of sound without breaking the actual sound barrier.

Aircraft (or any object moving through the air alone) are governed by airspeed. Ground speed is only relevant when traveling on the ground or transitioning to that state (approach to landing).

$\endgroup$
6
  • $\begingroup$ They do not make that differentiation or even a mention of groundspeed / airspeed in the article. $\endgroup$ – Samer Tufail Feb 9 '20 at 21:36
  • 2
    $\begingroup$ @SamerTufail - That’s the problem with mass media. They are either uninformed, or believe the right answer too complicated to be digestible to the uninformed. They are still in the business of entertainment after all. The news gets aviation information wrong all the time due to their misinterpretation of technical terms as keywords and sound bites. But, those keywords and sound bites sell. They buy our attention. $\endgroup$ – Dean F. Feb 9 '20 at 21:39
  • $\begingroup$ Absolutely. I knew ground speed / airspeed but then I read this could not add it up, it didn't light up for me and posted a question. Now I feel bad. $\endgroup$ – Samer Tufail Feb 9 '20 at 21:44
  • 2
    $\begingroup$ Why feel bad? There are no stupid questions. Just stupid people who don’t ask questions. Like the mass media. Someone once told me that knowing the answers is not as important as knowing what questions to ask. $\endgroup$ – Dean F. Feb 9 '20 at 21:50
  • 1
    $\begingroup$ "The sound barrier is a function of airspeed and air density." Actually, speed of sound in air only depends on temperature. $\endgroup$ – Bianfable Feb 10 '20 at 7:35
1
$\begingroup$

The speed of sound is measured in relation to the medium an object (or sounds waves) are moving. The actual speed is dependant on the tempreture and some other factors of the medium. Let's take for example the the information here : From the EngineeringToolbox We can see that in dry air, 20 celsius, the speed of sound is about 340 m/s.

In adtition, if we measure the aircraft's velocity with relation to the ground, we have to take in account the velocity of the wind.

In the example noted in the question, if we have lets say an aircraft travelling at a ground speed of 380 m/s, but it has a tailwind of 80 m/s, it means that by the ground track- it was supersonic if it would have traveled in relation to the ground. Meaning it does travel faster than the the speed of sound, if the sounds moves with no tailwind. But- in relation to the actual air (enclosed system), it travels in a velocity of 300 m/s, which is about 0.88 mach.

And that means- that a sound wave, in the same air mass- launched from the same origin with a base speed of 80 m/s (the tailwind)- would travel faster than the aircraft.

$\endgroup$

Not the answer you're looking for? Browse other questions tagged or ask your own question.