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Comment by vanous | Hacker News Reader

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?

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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%).

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.

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".

There are two types of drag, parasitic drag and lift-induced drag, but they are both drag.

Some of that thrust has to be used to generate lift.

If the engines are level, then none of the thrust is generating lift; the wings generate the lift (and with it some corresponding drag).

That's just semantics; you can split the drag generated by the wings and fuselage into different components. Of course, 100% of a constant-speed plane's thrust is necessarily used to counter all the other forces acting on the airplane :)

The engines provide the energy to keep the plane flying. It must overcome two forces: Drag and gravity.

The energy requirements are split between those two.

If the plane is not generating lift then it's on the ground or about to end up in the ground.

After some clarifications from replies to my sibling post, the answer is "definitely more than 50%" because the lift-induced versus parasitic drag curve is at its minimum when the two are equal, but the efficiency curve is a relatively flat-bottomed bathtub shape, so the cruising speed is selected to be faster than where this point is as you get significant reductions in travel time for relatively small reductions in total efficiency when you are near the 50% point and increase airspeed.

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?

But doesn't the plane have any inertia?

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.

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!

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

It clearly must not be a dramatic effect, as the plane must have left the jet stream in order to land :)

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.

Google for Clear Air Turbulence.

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