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

71–80 of 191 posts

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

#71

Post by cyberferret »

cm2187 wrote: post #30
mertd wrote: post #7

Earlier quoted context omitted.

Yes but it won't make a sonic boom.

If the wind is unstable, and slows down, can't the plane break it just out of inertia?

Not really - the airplane would still be doing around 500 knots (mph) relative to the air around it. The only way the scenario you mention could happen is if the airplane hit a sudden headwind of around 300 knots in an instant.

Fluid dynamics basically means that it is impossible for any parcel of air to have an instant demarcation from nil wind to 300 knot wind without going through a (very very turbulent) transition layer. And 300 knot winds?? Maybe routine on Venus or Jupiter, but very very rare if not nonexistent on Earth.

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

#72

Post by theothermkn »

cyberferret wrote: post #56
borkt wrote: post #19

Earlier quoted context omitted.

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...

Yes, for those readers who may not be familiar - the speed of sound changes with altitude. At sea level, it is around 750mph but at 40,000 feet it is around 620mph IIRC. It is affected by the density of the air (which is a combination of the air pressure and temperature) - hence why they use Mach numbers to denote airspeed above certain altitudes as that takes into consideration the air density to give you a speed re…

> [The speed of sound] is affected by the density of the air (which is a combination of the air pressure and temperature)...

This is incorrect. The speed of sound in an ideal gas is a function of temperature only, and not its density. Namely, it is sqrt(gamma x R x T), where R and gamma are unchanging properties of the gas.

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

#73

Post by TomVDB »

cyberferret wrote: post #43
jfk13 wrote: post #34

Earlier quoted context omitted.

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.

My standard way to think about stuff like that is to extrapolate to infinity: even if your tail wind were infinite and you’d arrive in 0 time, your way back would have infinite headwind and you’d never make it back.

Conclusion: increased wind increases the round trip time.

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

#74

Post by thecopy »

brianwawok wrote: post #53
Xcelerate wrote: post #24

I remember once being on an almost empty flight from SFO to ATL a few years ago. I checked the flight information on the seatback screen in front of me and was startled to see we were going well over 700 mph ground speed. That was pretty much the quickest flight I’ve ever been on; I think the whole trip was well under four hours.

For me on a trip right now, I have: 2h drive to ORD 2h airport security Theater 4h Flight to SFO 1h to deplane get an Uber to someplace So 9 hour total journey on a direct flight. A 50% faster flight would make it be 8 hours instead of 9? I don't think I would pay anything for that.

2h airport security? In Zürich im usually done in 10 or 15 minutes...

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

#78

Post by heavenlyblue »

snowwrestler wrote: post #66
btian wrote: post #27

Earlier quoted context omitted.

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

>> GPS measurements are comprised of a 2D surface coordinate system

GPS is inherently 3 dimensional.

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

#79

Post by aeijdenberg »

subhro wrote: post #51
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…

No you are right. In case of tailwind, it is KIAS + WS whereas headwind is KIAS - WS KIAS: Knots Indicated Airspeed (i.e. the speed at which air is hitting the pitot tube) WS: Wind speed

Slight, but important (if you don't want to run out of fuel) nit, indicated airspeed (KIAS) is not the same as true airspeed (KTAS).

To calculate ground speed (as required for navigation and fuel planning) you need true airspeed (not indicated) as well as wind direction and speed.

See some discussion here: https://www.quora.com/In-aviation-what-is-the-difference-bet...

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

#80

Post by microcolonel »

ummonk wrote: post #32
TheLoneAdmin wrote: post #18

Earlier 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.

That's less error than slopes and oblateness introduce, so I think it is acceptable.
Post reply on HN