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

Post by ars » Tue, Feb 19, 2019, 10:47 PM UTC

aidenn0 wrote: post #83

MayeulC wrote: post #69

Earlier quoted context omitted.

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, w…

When cruising, in level flight by definition, 100% of the engine power is there to combat drag, right? That is if the thrust vector of the engine is 100% level and the airspeed is constant, then 100% of the engine power applied is offset by drag.

Post by astrodust » Tue, Feb 19, 2019, 10:49 PM UTC

aidenn0 wrote: post #83

MayeulC wrote: post #69

Earlier quoted context omitted.

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, w…

When cruising, in level flight by definition, 100% of the engine power is there to combat drag, right? That is if the thrust vector of the engine is 100% level and the airspeed is constant, then 100% of the engine power applied is offset by drag.

Post by bartread » Tue, Feb 19, 2019, 10:53 PM UTC

FabHK wrote: post #50

BonesJustice wrote: post #29

Earlier quoted context omitted.

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 :-)

Post by bamboozled » Tue, Feb 19, 2019, 10:58 PM UTC

photojosh wrote: post #90

bamboozled wrote: post #76

That is 1289.085 kilometres per hour! The title should be updated on to include a measurement used by the rest of the world including the scientific community.

If you're being pedantic about science-related matters, it's 1290 km/h. Don't give false precision with your conversion; there's no way the original measurement has >3 significant figures. Even better yet, 361 m/s. km/h is a derived unit. ;)

Post by Bendingo » Tue, Feb 19, 2019, 10:58 PM UTC

ozzyman700 wrote: post #16

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)