Last time I flew from SFO to MUC, we had almost 100mph tail winds, that was already very impressive and shortened the flight time considerably, but nothing compared to this.
Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds
61–70 of 191 posts
Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds
#62Earlier 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.
0.14% is enough to throw the number off by 1mph so it isn’t irrelevant.
lol, ok. Make that 800 MPH then.
Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds
#63Earlier 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.
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.
Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds
#64>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, 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.
Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds
#65So 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?
It clearly must not be a dramatic effect, as the plane must have left the jet stream in order to land :)
Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds
#66This 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…
It's measured by repeatedly taking GPS coordinates divided by time.
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
Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds
#67Earlier quoted context omitted.
Isn’t that the one that truely matters some you want to go from A to B? Genuinely curious I know nothing about aviation.
Yes. Ground speed is basically airplane speed + wind speed.
Technically it would be airplane speed + wind speed vector for the same direction the airplane is heading. :)
Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds
#68This 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,…
You have a typo here (it's "Mach tuck").
Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds
#69So 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?
I would expect the jet stream not to end abruptly, but there is likely a gradient you have to cross before reaching "still" air. When leaving the stream, airspeed will increase, because of inertia, and so will drag. This will quickly bring the plane back to its cruising speed.
I had a debate with a friend on this one the other day, but I am pretty sure something like 80% of the engine power is there to combat drag, which increases with v² (if you discount lift from that, otherwise it' obviously 100%).
Re: Boeing 787 Reaches 801 MPH as a Jet Stream Packs Record-Breaking Speeds
#70This 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…
[deleted]