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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

#131

Post by mc32 »

lisper wrote: post #81
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…

Private pilot here. As others have pointed out, the difference is mostly negligible, but ground speed is the speed of the vertical projection of the aircraft's location on the earth's surface.

So essentially the speed at which the plane's shadow on earth travels while projecting on level ground.

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

#132

Post by lgierth »

mc32 wrote: post #131
lisper wrote: post #81

Earlier quoted context omitted.

Private pilot here. As others have pointed out, the difference is mostly negligible, but ground speed is the speed of the vertical projection of the aircraft's location on the earth's surface.

So essentially the speed at which the plane's shadow on earth travels while projecting on level ground.

Mh no, the shadow's position depends not just on the planes movement, but also on earth's rotation and trajectory.

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

#133

Post by [deleted] »

tzs wrote: post #130
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 It's even worse than not winning. You actually lose if you have to fly in the same wind both ways, once as a headwind and once as a tailwind. The extra time in the into the headwind direction is more than the time saved in the with a tailwind direction. If the ground distance each way is D, airspeed is V, and wind speed relative to…

[deleted]

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

#134

Post by ummonk »

dogma1138 wrote: post #63
ummonk wrote: post #32

Earlier quoted context omitted.

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

It’s likely less than the reading error in most speedometers including those which are used for autonomous vehicles unless you are writing a guidance system for bombs or attempting to land on another planet that’s pretty negligible.

Given that the ground speed was likely drawn from gps data, which should have velocity readings more accurate than 1mph (particularly for an airborne gps unit), the choice of calculation method for ground speed does make a difference.

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

#135

Post by [deleted] »

dingaling wrote: post #129
Kye wrote: post #77

Earlier quoted context omitted.

That would be editorializing.

They have already editorialised by converting the speed from knots Anyway, still doesn't set an unofficial record by variant or type: https://groundspeedrecords.com/wof-top-3/?aircraft-manufactu... Absolute fastest I can find on that site is a 747-400 at 752 knots / 866 mph but I do have a memory of one of that type breaking 800 knots over the Pacific within the last few years.

[deleted]

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

#136

Post by nostrademons »

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?

A lot of the times, when you hit turbulence at altitude in an airliner, it's because of this. If you watch the map display closely on your seatback you can actually see it happening: you'll feel a big bump or jolt and then your speed readout might be 50mph less and your altitude might be 2000 ft less.

As other people mention, it doesn't happen immediately - it's like merging onto a very bumpy freeway.

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

#138

Post by nfc »

tzs wrote: post #130
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 It's even worse than not winning. You actually lose if you have to fly in the same wind both ways, once as a headwind and once as a tailwind. The extra time in the into the headwind direction is more than the time saved in the with a tailwind direction. If the ground distance each way is D, airspeed is V, and wind speed relative to…

Just a small fix for those that were checking as I was

If the ground distance each way is D, airspeed is V, and wind speed relative to the ground is w, total time is D/(V-w) + D/(V+w) = 2DV/(V^2-w^2) = 2D/((V^2-w^2)/V).

For 0 = V: the headwind is so high that you can't make any progress toward the destination).

The last result being somehow obvious ;)

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

#139

Post by [deleted] »

dingaling wrote: post #129
Kye wrote: post #77

Earlier quoted context omitted.

That would be editorializing.

They have already editorialised by converting the speed from knots Anyway, still doesn't set an unofficial record by variant or type: https://groundspeedrecords.com/wof-top-3/?aircraft-manufactu... Absolute fastest I can find on that site is a 747-400 at 752 knots / 866 mph but I do have a memory of one of that type breaking 800 knots over the Pacific within the last few years.

[deleted]

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

#140

Post by FPGAhacker »

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?

Themselves meaning the ground observer as that was the point of view GP was speaking about.
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