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Comment by boyter | Hacker News Reader

Isn’t that the one that truely matters some you want to go from A to B?

Genuinely curious I know nothing about aviation.

Yes, correct!

And because the wind is not constant, airline dispatchers will generally try to pick the route and altitude with the most favorable wind direction and speed.

Yes. Ground speed is basically airplane speed + wind speed.

<pedant mode>Technically it would be airplane speed + wind speed vector for the same direction the airplane is heading.</pedant mode> :)

Hey, I said 'basically'! :P

But yes, you are correct. However, the person I was replying to already said they were ignorant of this stuff, so I didn't want to get too crazy.

Yes but it won't make a sonic boom.

If the wind is unstable, and slows down, can't the plane break it just out of inertia?

Not really - the airplane would still be doing around 500 knots (mph) relative to the air around it. The only way the scenario you mention could happen is if the airplane hit a sudden headwind of around 300 knots in an instant.

Fluid dynamics basically means that it is impossible for any parcel of air to have an instant demarcation from nil wind to 300 knot wind without going through a (very very turbulent) transition layer. And 300 knot winds?? Maybe routine on Venus or Jupiter, but very very rare if not nonexistent on Earth.

I suspect that a 787 traveling at 801 MPH airspeed might suffer some damage to the control surfaces. 787s aren't designed to break the sound barrier.

And I would argue that not damaging the plane matters more than how long it takes to get there.

Just an interesting fact that the 787 has been tested to m0.97 in a dive, pretty impressive! But not near 801 MPH airspeed.

https://web.archive.org/web/20110408073905/http://www.flight...

Yes, for those readers who may not be familiar - the speed of sound changes with altitude. At sea level, it is around 750mph but at 40,000 feet it is around 620mph IIRC. It is affected by the density of the air (which is a combination of the air pressure and temperature) - hence why they use Mach numbers to denote airspeed above certain altitudes as that takes into consideration the air density to give you a speed relative to the speed of sound at that altitude.

> [The speed of sound] is affected by the density of the air (which is a combination of the air pressure and temperature)...

This is incorrect. The speed of sound in an ideal gas is a function of temperature only, and not its density. Namely, it is sqrt(gamma x R x T), where R and gamma are unchanging properties of the gas.

Thank you. I stand corrected.

It's travelling at 801mph relative to the ground.

If I'm swimming down stream at 1 mph in water moving at 4mph, relative to the bank I'm doing 5mph relative to the water 1mph.

No sonic booms where involved.

I'm quite well aware of that. I'm not sure how you read my post as saying anything different.

boyter asked how anything but time from A to B could be relevant. Well, not damaging the plane is relevant. And therefore the airspeed not being 801 MPH matters.

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