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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

#51

Post by subhro »

BonesJustice wrote: post #29
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)

I’m a bit under the weather, so my comprehension may be lacking, but I think you’ve got your signs reversed (or your head/tail terminology flipped). If tail wind (moving with the plane) were negative, then air speed would exceed ground speed. Intuitively, that doesn’t make sense. Air speed can only exceed ground speed if the wind is blowing in an opposing direction, i.e., the plane must travel faster against the wind…

No you are right. In case of tailwind, it is KIAS + WS whereas headwind is KIAS - WS

KIAS: Knots Indicated Airspeed (i.e. the speed at which air is hitting the pitot tube) WS: Wind speed

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

#52

Post by subhro »

logifail wrote: post #39
jfk13 wrote: post #23

Earlier quoted context omitted.

> 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…

> Of course, planes going the other direction will use extra, so overall we don't win Aren't some(all?) long-haul routes chosen on a flight by flight basis to take advantage of favourable winds where possible? Compare yesterday's eastbound BA11 (LHR-Singapore) vs the westbound BA12 (Singapore-LHR) flights.[0] Neither route looks like a great circle. Eastbound routing: London - north of Berlin - Minsk - Voronezh - Vol…

Yes, most of the time. There is actually Winds aloft data that is made available to pilots to select routes. And in lots of cases IFR routes are designed keeping these speeds in mind.

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

#53

Post by brianwawok »

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.

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

#54

Post by danepowell »

rconti wrote: post #46

>The ordinary cruising speed of a Dreamliner is 561 mph, with a maximum propulsion of 587 mph. Any speed gained on top of that is thanks to Mother Nature's helpful boost. Wow, cruise speed is that close to 'max' speed? Or maybe that's just a max suggested cruise speed.

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, controllability, and structural integrity ("mach buffet").

I think cruise N1 is something like 80% (that is, of full power). If you pushed the engines to 100% power at cruise altitude, they could probably get you past 587 mph / mach 0.85, but you'd be having a really bad day.

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

#56

Post by cyberferret »

borkt wrote: post #19
AnimalMuppet wrote: post #10

Earlier quoted context omitted.

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.

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

#57

Post by borkt »

BonesJustice wrote: post #35
vanous wrote: post #28

So what if they suddenly left the jet stream, for example flew a bit below or besides... If that's possible... and happened to be in normal wind speed, the plane would disintegrate due to sonic boom?

It wouldn’t disintegrate due to a sonic boom, because this extraordinary speed is due to the plane being helped along by tailwinds. If the tailwinds disappear, the plane slows down; it doesn’t speed up. What I’m not clear on is, would the passengers feel like they just decelerated by 200+ mph in an instant?

The jetstream, like all weather and winds, is on a continuum. There is no barrier where outside of the jetstream the air is static. See the principles of flight and how air functions VERY close to a wing for an example. This is actually a good example, though a bit unrelated.

https://news.ycombinator.com/item?id=13829625

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

#58

Post by subhro »

ilovecaching wrote: post #55

Is this dangerous for the plane if the wind hits it sideways or something? Does this mean the flights will be extremely bumpy?

No it's not. The airplane just has to do wind correction by either crabbing into or away from the wind depending on the wind direction.

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

#59

Post by cyberferret »

ilovecaching wrote: post #55

Is this dangerous for the plane if the wind hits it sideways or something? Does this mean the flights will be extremely bumpy?

If you stay within the jetstream, it is a surprisingly smooth ride - a bit like being in a canoe in a middle of a fast flowing current in a river. Jetstreams tend to be fairly long and continuous.

Where it gets bumpy is if you are transitioning in or out of the jetstream zone - a bit like the abovementioned canoe hitting some rapids in a shallow part of the river.

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

#60

Post by FabHK »

cm2187 wrote: post #45
BonesJustice wrote: post #35

Earlier quoted context omitted.

It wouldn’t disintegrate due to a sonic boom, because this extraordinary speed is due to the plane being helped along by tailwinds. If the tailwinds disappear, the plane slows down; it doesn’t speed up. What I’m not clear on is, would the passengers feel like they just decelerated by 200+ mph in an instant?

But doesn't the plane have any inertia?

Yeah. Good question. It would really seem to depend on how fast the air speed changes.

I mean, the opposite effect is well attested to - you fly along with a headwind (and sufficient airspeed), then comes along a sudden tailwind, and your airspeed drops sufficiently that you stall, or at any rate descent. Wind shears, often associated with cold fronts or thunderstorms.

Maybe a) mach 1 is further from cruise speed than cruise speed from stall speed, and b) drag slows down the plane more effectively and quickly than the engines can accelerate it.

That could explain why wind shear has and does lead to stalls, but not to sonic booms/structural disintegration.

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