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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

#41

Post by mannykannot »

benj111 wrote: post #26
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…

If I'm in a tailwind of 100mph and have a cruising speed of 200mph, then I'm essentially travelling at 300mph relative to the ground, but using as much fuel as if I were travelling at 200mph. On the return leg, I would hit a head wind, and be travelling at 100mph while still using as much fuel as if I were travelling at 200mph. So yes this does save fuel.

On a round trip, with a constant wind velocity, you use extra fuel overall - consider the extreme case where the wind speed is equal to the cruising airspeed.

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

#42

Post by rconti »

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…

No, it saves fuel. They're probably going faster AND using less throttle. Cruise speed is WAY above stall speed.

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

#43

Post by cyberferret »

jfk13 wrote: post #34
benj111 wrote: post #26

Earlier quoted context omitted.

If I'm in a tailwind of 100mph and have a cruising speed of 200mph, then I'm essentially travelling at 300mph relative to the ground, but using as much fuel as if I were travelling at 200mph. On the return leg, I would hit a head wind, and be travelling at 100mph while still using as much fuel as if I were travelling at 200mph. So yes this does save fuel.

It saves fuel for the downwind leg of the trip. However, it's worth noting that if the outward (downwind) and return (upwind) legs are affected by the same wind, the outward savings are more than offset by the extra fuel needed for the return. Suppose the journey in your example is 200 miles. With no wind, that's an hour each way, so 2 hours total cruising time. With the 100mph wind, the outward journey only takes 40…

I distinctly remember my CFI in flying school brow beating it into us rookie pilots that "what you lose in a headwind, you never regain in the tailwind on the way back", and explaining it to us long handed on the blackboard. Seems illogical, but the math stacked up as in your illustration above.

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

#44

Post by FabHK »

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, this does save fuel, I'd think.

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

#45

Post by cm2187 »

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?

But doesn't the plane have any inertia?

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

#46

Post by rconti »

>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.

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

#47

Post by jessriedel »

JoblessWonder wrote: post #36
kgermino wrote: post #6

This is probably a stupid question, but how is "ground speed" measured? Is it how quickly the airplane moves past a fixed point at it's altitude, or along the earth's surface? I.e. the plane is effectively moving along the surface of a sphere, but if the plane is 30,000 ft up, then the radius of the plane's sphere is 30,000 ft greater than the radius of the "ground" so traveling at the same angular velocity will requ…

Ground speed is what most people would think of when they think of a speed. How fast is it moving from Point A to Point B. Airspeed takes into account how fast you are going you are going AS WELL as how fast and in what direction the air around you is going. Another example would be a person walking on a motorized walkway (like at an airport.) Person A is going WITH the direction of the walkway like someone normally…

kgermino is asking about the difference between (1) the actual speed (relative to the Earth) at the altitude where the plane is flying and (2) the speed of the path of the airplane when it is projected onto the ground. These are extremely close but not the same (~0.15%) because the Earth is a sphere and the airplane is at a slight further distance from the center of that sphere than it's projected point on the ground.

You are explaining the difference between airspeed and ground speed, which can be much more substantial (e.g., ~10%, or ~%100 in extraordinary situations).

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

#48

Post by AnimalMuppet »

jfk13 wrote: post #38
GaryNumanVevo wrote: post #31

Earlier quoted context omitted.

Just make a jet stream going the other way at a different altitude

Or just fly with the jetstream in all cases, all the way around the earth if necessary. Some trips would be quite long, but think how much fuel they'd be saving!

A 200 MPH tailwind doesn't compensate for a 20,000-mile-longer flight.

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

#49

Post by cyberferret »

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?

Fluid dynamics would spread that deceleration over a short period of time, so it wouldn't be like 'hitting the brakes in a car' type sensation. Passengers may feel a slight shift, but overall, as you explained to the previous poster, the speed is all relative to the air around the airplane.

More likely the turbulence and vortices in the demarcation zone between the jetstream and the different air current the plane enters would jostle and toss the passengers around more than the deceleration would.

From the article, it said the 787 was cruising at 35,000 feet. Seeing as the 787 was built to cruise close to 40,000 feet (above most traffic and weather) I daresay the pilot or company flight planners deliberately asked for a lower altitude based on expected wind conditions. I bet many other airlines were doing the same, leading to a very busy jetstream highway that day!

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

#50

Post by FabHK »

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…

Wind is "named" after where it comes from. If you fly north, and the wind is a north wind, you fly slower. (This terminology is in line with what GP said.)

Then, ground speed = air speed - wind speed.

Thus, GP seems to have either the sign wrong, or ground and air speed mixed up (but not both :-)

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