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

latimes.com

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

#101

Post by Johnny555 »

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…

This makes me think that some time in the distant future, when we're really great a terraforming

We're great at accidental terraforming now, we're actually warming the temperature of the planet.

But based on global reaction to the current terraforming I'm skeptical that we'll ever reach the point to where we can intentionally terraform the planet.

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

#102

Post by matwood »

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.

Yeah. I flew from JFK to Glasgow in April of last year and IIRC we arrived around 2 hours early. It was early enough that there were no passport control agents around, and they had to be called in.

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

#103

Post by theothermkn »

markab21 wrote: post #64
danepowell wrote: post #54

Earlier quoted context omitted.

I'm not an expert. But my understanding is that the "maximum propulsion" speed (587 mph) is limited by the "critical mach number" of the airframe, which for jetliners is something like 0.85. Faster than that, air around portions of the plane may start moving faster than the speed of sound (even though the airframe as a whole is going slower than the speed of sound), and this wreaks havoc in terms of lift, drag, contr…

When I was getting my pilots license I stumbled into a really cool video NASA did showing exactly what happens when you reach a fluttering condition like what you described. This is a link to the NASA recording if anyone is interested. https://www.youtube.com/watch?v=pEOmCkZyXzk https://www.youtube.com/watch?v=OhwLojNerMU (Older with music, kinda funny old-school science video from NASA) Edit: formatting.

In practical terms, flutter due to approaching transonic conditions is probably pretty rare. That kind of flutter is (mostly or most often) due to the interaction of normal shocks with the control surfaces, or coupling with an elastic mode of the wing or tail surface.

The flutter in the first video is occurring at very much subsonic speeds, and looks to be either the result of flying a purposely underdesigned tail surface, or flying a properly built one beyond its rated flight envelope. The second video contains a wide variety of flutter instances, some of them aeroelastic, some of them transonic, and so on.

One way to get into a transonic flutter, however, is to be a hotshot business jet pilot who flies higher and higher and faster and faster. The higher you go, the lower the density, so your minimum speed increases. Also, the temperature goes down, so the speed of sound decreases. Where these two meet is called "coffin corner," and you don't always have to fly yourself into it by increasing your speed and altitude; you can fly close to coffin corner, and then fly into colder air or less dense air. No matter how you get there, you're stuck. Slow down, and the wings stall, the nose drops, you pick up speed, and hit transonic flutter. Speed up to stay in the air, by dropping the nose, and you hit transonic flutter directly.

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

#104

Post by e40 »

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?

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

#105

Post by snowwrestler »

heavenlyblue wrote: post #78
snowwrestler wrote: post #66

Earlier quoted context omitted.

GPS measurements are comprised of a 2D surface coordinate system (e.g. lat/long) and an altitude above mean sea level. So, "speed" as measured by GPS is speed along the Earth's surface. EDIT: assuming a constant altitude flight

>> GPS measurements are comprised of a 2D surface coordinate system GPS is inherently 3 dimensional.

> and an altitude above mean sea level

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

#106

Post by ChuckMcM »

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?

As far as I have been able to tell in flying across the continent is the change in flight time (arriving early or late after departing on time).

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

#107

Post by kens »

heavenlyblue wrote: post #78
snowwrestler wrote: post #66

Earlier quoted context omitted.

GPS measurements are comprised of a 2D surface coordinate system (e.g. lat/long) and an altitude above mean sea level. So, "speed" as measured by GPS is speed along the Earth's surface. EDIT: assuming a constant altitude flight

>> GPS measurements are comprised of a 2D surface coordinate system GPS is inherently 3 dimensional.

There's some interesting stuff going on behind the scenes. GPS inherently gives you a 3-dimensional coordinate, which is useless for most applications. So the coordinate gets converted to a 2D surface coordinate (latitude/longitude) plus an altitude, which is what you generally want. However, since the Earth isn't a perfect sphere, this conversion is not well-defined.

The solution is a datum, a model of the Earth's surface that is used for the conversion. WGS84 is the datum used by GPS, and approximates the Earth as a specified ellipsoid. However, maps can use use different datums (data?), so there can be a discrepancy of many meters between what GPS says the location is, and what the map says the location is, and they can both be right relative to their datum.

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

#108

Post by ummonk »

microcolonel wrote: post #80
ummonk wrote: post #32

Earlier quoted context omitted.

0.14% is enough to throw the number off by 1mph so it isn’t irrelevant.

That's less error than slopes and oblateness introduce, so I think it is acceptable.

That is also an interesting question. If they’re measuring speed as if the airplane were at the surface rather than at 30,000 feet, is it speed along the WSG84 earth model?

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

#109

Post by [deleted] »

heavenlyblue wrote: post #78
snowwrestler wrote: post #66

Earlier quoted context omitted.

GPS measurements are comprised of a 2D surface coordinate system (e.g. lat/long) and an altitude above mean sea level. So, "speed" as measured by GPS is speed along the Earth's surface. EDIT: assuming a constant altitude flight

>> GPS measurements are comprised of a 2D surface coordinate system GPS is inherently 3 dimensional.

[deleted]

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

#110

Post by jazzyjackson »

Bendingo wrote: post #99
ozzyman700 wrote: post #16

Earlier quoted context omitted.

https://www.grc.nasa.gov/www/k-12/airplane/move.html ground speed is the speed of the plane, relative to an observer on the ground, air speed is how fast the people on the plane are going relative to themselves. Airspeed = Ground Speed - Wind Speed (wind blowing towards front of plan is positive, tailwind is negative)

> air speed is how fast the people on the plane are going relative to themselves That makes no sense. How can people be moving relative to themselves?

relative to their own location Δt ago?
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