6 ms·

So 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 plan

by vanous 8y ago

So 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?

BonesJustice 8y ago
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?
- cm2187 8y ago
  But doesn't the plane have any inertia?
  - FabHK 8y ago
    Yeah. Good question. It would really seem to depend on how fast the air speed changes. I mean, the opposite effect is well attested to - you fly along with a headwind (and sufficient airspeed), then comes along a sudden tailwind, and your airspeed drops sufficiently that you stall, or at any rate descent. Wind shears, often associated with cold fronts or thunderstorms. Maybe a) mach 1 is further from cruise speed than cruise speed from stall speed, and b) drag slows down the plane more effectively and quickly than the engines can accelerate it. That could explain why wind shear has and does lead to stalls, but not to sonic booms/structural disintegration.
- cyberferret 8y ago
  Fluid dynamics would spread that deceleration over a short period of time, so it wouldn't be like 'hitting the brakes in a car' type sensation. Passengers may feel a slight shift, but overall, as you explained to the previous poster, the speed is all relative to the air around the airplane. More likely the turbulence and vortices in the demarcation zone between the jetstream and the different air current the plane enters would jostle and toss the passengers around more than the deceleration would. From the article, it said the 787 was cruising at 35,000 feet. Seeing as the 787 was built to cruise close to 40,000 feet (above most traffic and weather) I daresay the pilot or company flight planners deliberately asked for a lower altitude based on expected wind conditions. I bet many other airlines were doing the same, leading to a very busy jetstream highway that day!
- borkt 8y ago
  The jetstream, like all weather and winds, is on a continuum. There is no barrier where outside of the jetstream the air is static. See the principles of flight and how air functions VERY close to a wing for an example. This is actually a good example, though a bit unrelated. https://news.ycombinator.com/item?id=13829625 https://news.ycombinator.com/item?id=13829625
- lostapathy 8y ago
  It clearly must not be a dramatic effect, as the plane must have left the jet stream in order to land :)
MayeulC 8y ago
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%).
- aidenn0 8y ago
  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.
  - ars 8y ago
    Depends on how you define drag. If you define it as friction due to air against the plane then no, because a lot of the drag is due to sending air downward to keep the airplane up. The distinction is that, that that part of the drag is impossible to reduce, while friction you can work to reduce. That's what the GP meant by "discounting lift".
    - aidenn0 8y ago
      There are two types of drag, parasitic drag and lift-induced drag, but they are both drag.
  - astrodust 8y ago
    Some of that thrust has to be used to generate lift.
    - aidenn0 8y ago
      If the engines are level, then none of the thrust is generating lift; the wings generate the lift (and with it some corresponding drag).
gHosts 8y ago
Google for Clear Air Turbulence.
nostrademons 8y ago
A lot of the times, when you hit turbulence at altitude in an airliner, it's because of this. If you watch the map display closely on your seatback you can actually see it happening: you'll feel a big bump or jolt and then your speed readout might be 50mph less and your altitude might be 2000 ft less. As other people mention, it doesn't happen immediately - it's like merging onto a very bumpy freeway.

by vanous 8y ago

So 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?

BonesJustice 8y ago

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?

cm2187 8y ago
But doesn't the plane have any inertia?
- FabHK 8y ago
  Yeah. Good question. It would really seem to depend on how fast the air speed changes. I mean, the opposite effect is well attested to - you fly along with a headwind (and sufficient airspeed), then comes along a sudden tailwind, and your airspeed drops sufficiently that you stall, or at any rate descent. Wind shears, often associated with cold fronts or thunderstorms. Maybe a) mach 1 is further from cruise speed than cruise speed from stall speed, and b) drag slows down the plane more effectively and quickly than the engines can accelerate it. That could explain why wind shear has and does lead to stalls, but not to sonic booms/structural disintegration.
cyberferret 8y ago
Fluid dynamics would spread that deceleration over a short period of time, so it wouldn't be like 'hitting the brakes in a car' type sensation. Passengers may feel a slight shift, but overall, as you explained to the previous poster, the speed is all relative to the air around the airplane. More likely the turbulence and vortices in the demarcation zone between the jetstream and the different air current the plane enters would jostle and toss the passengers around more than the deceleration would. From the article, it said the 787 was cruising at 35,000 feet. Seeing as the 787 was built to cruise close to 40,000 feet (above most traffic and weather) I daresay the pilot or company flight planners deliberately asked for a lower altitude based on expected wind conditions. I bet many other airlines were doing the same, leading to a very busy jetstream highway that day!
borkt 8y ago
The jetstream, like all weather and winds, is on a continuum. There is no barrier where outside of the jetstream the air is static. See the principles of flight and how air functions VERY close to a wing for an example. This is actually a good example, though a bit unrelated. https://news.ycombinator.com/item?id=13829625 https://news.ycombinator.com/item?id=13829625
lostapathy 8y ago
It clearly must not be a dramatic effect, as the plane must have left the jet stream in order to land :)

MayeulC 8y ago

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%).

aidenn0 8y ago
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.
- ars 8y ago
  Depends on how you define drag. If you define it as friction due to air against the plane then no, because a lot of the drag is due to sending air downward to keep the airplane up. The distinction is that, that that part of the drag is impossible to reduce, while friction you can work to reduce. That's what the GP meant by "discounting lift".
  - aidenn0 8y ago
    There are two types of drag, parasitic drag and lift-induced drag, but they are both drag.
- astrodust 8y ago
  Some of that thrust has to be used to generate lift.
  - aidenn0 8y ago
    If the engines are level, then none of the thrust is generating lift; the wings generate the lift (and with it some corresponding drag).

gHosts 8y ago

Google for Clear Air Turbulence.

nostrademons 8y ago

A lot of the times, when you hit turbulence at altitude in an airliner, it's because of this. If you watch the map display closely on your seatback you can actually see it happening: you'll feel a big bump or jolt and then your speed readout might be 50mph less and your altitude might be 2000 ft less. As other people mention, it doesn't happen immediately - it's like merging onto a very bumpy freeway.