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?
> 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 original landing slot!)
Of course, planes going the other direction will use extra, so overall we don't win.
> Of course, planes going the other direction will use extra, so overall we don't win
Aren't some(all?) long-haul routes chosen on a flight by flight basis to take advantage of favourable winds where possible?
Compare yesterday's eastbound BA11 (LHR-Singapore) vs the westbound BA12 (Singapore-LHR) flights.[0] Neither route looks like a great circle.
Eastbound routing: London - north of Berlin - Minsk - Voronezh - Volgograd - cross Caspian sea - Turkmenistan - Lahore - New Delhi - KL - Singapore.
Westbound: Singapore - KL - south of Jaipur - Iran - just touched Turkmenistan - south of Baku - Tbilsi - Turkey (just) - south of Prague - south of Dortmund - LHR
Typically yes. Right now there is a general avoidance of Ukrainian (Crimean and nearby) and Syrian airspace so the flight plans will take that into account, the winds aloft, and the airways available.
Yes, most of the time. There is actually Winds aloft data that is made available to pilots to select routes. And in lots of cases IFR routes are designed keeping these speeds in mind.
> 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 the ground is w, total time is D/(V-w) + D(V+w) = 2DV/(V^2-w^2) = 2D/(V-w^2/V).
For 0 < w < sqrt(V), this is more than the 0 wind case. (For w >= sqrt(V), the headwind is so high that you can't make any progress toward the destination).
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 < w < V: this is more than the 0 wind case.
For w >= V: the headwind is so high that you can't make any progress toward the destination).
Or just fly with the jetstream in all cases, all the way around the earth if necessary. Some trips would be quite long, but think how much fuel they'd be saving!
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.
Reading your comment, I thought "how bad is a high-altitude stall anyway? it's obviously undesirable, but what makes it dangerous? possibly entering a flat spin?"
Doing a little digging, it seems that there's also a significant risk of entering an overspeed condition during recovery, and inducing a structural failure! Makes sense considering that we're talking about speeds near Mach 1 to begin with.
That's... a bit different? AF447 is a classic case of human factors in aviation accidents. The stall would have been completely recoverable if the pilot had nosed down.
I watched a TV programme a couple of years ago talking about this on the U2 as the speeds were only a few knots apart and also varied with altitude and other factors.
The image at https://i.stack.imgur.com/Lu2Xt.jpg shows the airspeed corridor the pilots have to fly through. It's constantly flying on a knife edge.
No. When you are flying high (at the route flight level), air becomes less dense and the stall speed increases a lot (expressed in true air speed). When a flight is at its highest available flight level, the range of available speeds is very narrow.
Since they arrive faster they will spend less time in the air. Less time in the air means less fuel burnt. They are probably still burning fuel at a similar rate though.
If I'm in a tailwind of 100mph and have a cruising speed of 200mph, then I'm essentially travelling at 300mph relative to the ground, but using as much fuel as if I were travelling at 200mph. On the return leg, I would hit a head wind, and be travelling at 100mph while still using as much fuel as if I were travelling at 200mph.
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 minutes (200 miles at 300mph), but the return takes 2 hours (200 miles at 100mph) for a total cruising time of 2:40, and a significantly greater total fuel burn.
(In practice, airlines may route the two legs quite differently to optimize better, taking advantage of the tailwind in one direction while avoiding it as much as possible when going the other way.)
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.
On a round trip, with a constant wind velocity, you use extra fuel overall - consider the extreme case where the wind speed is equal to the cruising airspeed.
Where air masses, that are travelling at different velocities, meet you get shearing. Shearing causes turbulence. So even if we could create two jet streams moving in two different directions the turbulence in the zone where you transfer between the two would probably be too severe to actually fly through it.
This makes me think that some time in the distant future, when we're really great a terraforming
We're great at accidental terraforming now, we're actually warming the temperature of the planet.
But based on global reaction to the current terraforming I'm skeptical that we'll ever reach the point to where we can intentionally terraform the planet.
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?