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Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds

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Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds

#171

Post by Arnt »

bartread wrote: post #167
ahartmetz wrote: post #116

Earlier quoted context omitted.

Probably because it matters where the air was before. Stuff like: north wind is could, south wind is warm, west wind is humid, etc

Doesn't that very much depend on where you are though? E.g., with respect to the equator, major bodies of water and landmasses, mountain ranges, prevailing weather conditions. Also perhaps on seasons. I can understand that it might be just an ancient tradition originating from a particular part of the world, that we've turned into a convention, but it loses meaning when you go elsewhere.

It rather depends on where you are, but so does language. The words we use were cast locally. If you speak English some words were cast in the Ukraine, some in Germany, some in Rome, some in France, some in Britain, but the people living in those places weren't obliged to make the words globally meaningful.

If you go to Cape Town, a "north wind" presumably doesn't mean cold, because the area north of the city isn't colder than the city itself. This didn't matter to the people who first said "north wind" and I don't think it means much nowadays either. Just another little thing to watch out for in intercultural communication.

Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds

#172

Post by bronco21016 »

abakker wrote: post #20

This makes me think that some time in the distant future, when we're really great a terraforming, we could influence the weather of the planet specifically to create functional jet streams that we could use to make travel faster, and then plan air routes specifically to use them. A more serious question: I assume this kind of event doesn't actually save fuel because the airplane still has to maintain the same airspee…

Where air masses, that are travelling at different velocities, meet you get shearing. Shearing causes turbulence. So even if we could create two jet streams moving in two different directions the turbulence in the zone where you transfer between the two would probably be too severe to actually fly through it.

Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds

#173

Post by NeedMoreTea »

jfk13 wrote: post #23
abakker wrote: post #20

This makes me think that some time in the distant future, when we're really great a terraforming, we could influence the weather of the planet specifically to create functional jet streams that we could use to make travel faster, and then plan air routes specifically to use them. A more serious question: I assume this kind of event doesn't actually save fuel because the airplane still has to maintain the same airspee…

> A more serious question: I assume this kind of event doesn't actually save fuel because the airplane still has to maintain the same airspeed to avoid stalling, right? If a plane maintains the same airspeed, but gets extra ground speed thanks to a tailwind, it'll complete its journey in less time and therefore should save fuel. (Unless it ends up having to circle the destination airport while it waits for its origin…

Where it's possible they use a jet stream route going one way, and a great circle route on the return.

So yes it's a win, but a little more subtly.

Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds

#174

Post by samdoidge »

ChuckMcM wrote: post #3

Note to readers, land speed not equal to air speed. As the article discusses it doesn't break the sound barrier. That said, the news here is that the jet stream is carrying more energy than it has in the past (air mass is moving faster, kinetic energy content is 1/2mv^2). I read a paper a long time ago which talked about the impact of thermal 'tubes' where denser cold air travelling at speed would "punch holes" in hi…

> Note to readers, land speed not equal to air speed. As the article discusses it doesn't break the sound barrier.

Yes, but the first sentence is not why it did not break the sound barrier. They state it is due to being in swiftly moving air. The speed otherwise, even at airspeed (which is a negligible difference to ground speed) is fast enough to break the sound barrier.

Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds

#175

Post by Steve44 »

FabHK wrote: post #44
abakker wrote: post #20

This makes me think that some time in the distant future, when we're really great a terraforming, we could influence the weather of the planet specifically to create functional jet streams that we could use to make travel faster, and then plan air routes specifically to use them. A more serious question: I assume this kind of event doesn't actually save fuel because the airplane still has to maintain the same airspee…

1. The plane has to fly sufficiently fast (to avoid stall), and sufficiently slow (to avoid "Mach tug"). As one flies higher (air density sinks), these two speeds converge, forming the dangerous "coffin corner". Now, in general, planes don't fly as high as possible, I'd say, so that there is some leeway in terms of chosen airspeed. 2. For given airspeed, the plane flies less time with a tailwind than a headwind. So,…

> "coffin corner"

I watched a TV programme a couple of years ago talking about this on the U2 as the speeds were only a few knots apart and also varied with altitude and other factors.

The image at https://i.stack.imgur.com/Lu2Xt.jpg shows the airspeed corridor the pilots have to fly through. It's constantly flying on a knife edge.

Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds

#176

Post by marvin »

aidenn0 wrote: post #118
astrodust wrote: post #93

Earlier quoted context omitted.

Some of that thrust has to be used to generate lift.

If the engines are level, then none of the thrust is generating lift; the wings generate the lift (and with it some corresponding drag).

That's just semantics; you can split the drag generated by the wings and fuselage into different components. Of course, 100% of a constant-speed plane's thrust is necessarily used to counter all the other forces acting on the airplane :)

Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds

#177

Post by Steve44 »

Stratoscope wrote: post #147
TheLoneAdmin wrote: post #18

Earlier quoted context omitted.

The radius of the earth + 30,000 ft is about 0.14% more than just the radius of the earth. Basically ignore it, unless you are programming guidance control systems.

This reminds me of a math puzzle I read many years ago. It went something like this: "Assume the Earth is a perfect sphere, and its circumference is 25,000 miles. You have a 25,000 mile long rope looped all the way around the Equator, so it is tight against the surface. Now you want to raise the rope one foot in the air, all the way around. How much more rope do you need?" I thought about this and said, "You need a l…

A related one is to add a second section to the question asking how much extra rope would be needed to go round a tennis ball the same 1ft further out from the ball.

For someone who doesn’t know, or hasn’t calculated/thought about it that is pretty astounding.

Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds

#178

Post by jsjohnst »

e40 wrote: post #104
ChuckMcM wrote: post #3

Note to readers, land speed not equal to air speed. As the article discusses it doesn't break the sound barrier. That said, the news here is that the jet stream is carrying more energy than it has in the past (air mass is moving faster, kinetic energy content is 1/2mv^2). I read a paper a long time ago which talked about the impact of thermal 'tubes' where denser cold air travelling at speed would "punch holes" in hi…

Do you know if the rides in higher energy jet streams are less comfortable, or is it undetectable by the passengers?

I’ve had a SFO -> London flight once where the pilot intentionally flew into a very strong jet stream current (saved us a lot of flight time, think it was close to an hour). Experienced a short burst of mildly rough turbulence entering and exiting, but otherwise smooth sailing.

Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds

#179

Post by jsjohnst »

brianwawok wrote: post #53
Xcelerate wrote: post #24

I remember once being on an almost empty flight from SFO to ATL a few years ago. I checked the flight information on the seatback screen in front of me and was startled to see we were going well over 700 mph ground speed. That was pretty much the quickest flight I’ve ever been on; I think the whole trip was well under four hours.

For me on a trip right now, I have: 2h drive to ORD 2h airport security Theater 4h Flight to SFO 1h to deplane get an Uber to someplace So 9 hour total journey on a direct flight. A 50% faster flight would make it be 8 hours instead of 9? I don't think I would pay anything for that.

I was in ORD a week ago and went from my hotel (Hilton, connected to the airport) to my gate in under 15m. I think my time going through security was around 45s, give or take. Anecdata caveats apply.
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