Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

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

latimes.com

Post reply on HN
Print view View original

11–20 of 191 posts

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

#11

Post by dragonwriter »

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…

The Earth has almost a 4000 mile radius; 30,000 feet is less than 6 miles. The difference between movement over ground and movement with respect to the top of a 30,000 foot pole stuck in the ground is pretty negligible.

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

#12

Post by [deleted] »

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…

[deleted]

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

#13

Post by kevmo314 »

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…

30k ft is nothing compared to the radius of the Earth which is 21M ft. The measurement instrument has larger error sources, so the answer is it doesn't actually matter.

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

#14

Post by dlb_ »

boyter wrote: post #4
gok wrote: post #2

Ground speed

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, correct!

And because the wind is not constant, airline dispatchers will generally try to pick the route and altitude with the most favorable wind direction and speed.

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

#16

Post by ozzyman700 »

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…

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)

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

#17

Post by frankchn »

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…

The difference is about 1.1 mph (edit: out of 800 mph) at 30,000 ft compared to ground, so I don't think it matters too much.

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

#18

Post by TheLoneAdmin »

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…

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.

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

#19

Post by borkt »

AnimalMuppet wrote: post #10
boyter wrote: post #4

Earlier 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.

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...

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

#20

Post by abakker »

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 airspeed to avoid stalling, right?

Post reply on HN