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I am trying to wrap my head around the effect of the jet stream on energy consumption as well as flight time.

My understanding is that everything that has to with the flying part (vs navigation) depends on airspeed. If a plane is flying westbound it experiences the jet stream as headwind and thus more airspeed and free lift? Are the engines then (almost) only used to move forward (which also increases lift - why isn't the plane climbing even higher then?) and does the jet steam produce more friction than "free" lift is worth? Obviously flying eastbound is faster, does the jet stream push the plane with equal speed forward and the engines are only used to produce the required lift as well as a few hundred km/h extra and thus safe time and energy?

Or does all this boil down to some complicated numerical analysis and there is a net gain in doing this which might not be totally obvious?

VeNETHER
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  • " If a plane is flying westbound it experiences the jet stream as headwind and thus more airspeed and free lift? " --this is a deeply flawed assumption, and thus the question is basically unanswerable. – quiet flyer Mar 11 '21 at 20:20
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    @quietflyer: The question already highlights that it's their understanding, not a fact, which makes it answerable, by tackling the misunderstanding -- we've covered this point before. –  Mar 11 '21 at 20:23
  • The "free lift" provided by a headwind is only relevant when there is a headwind relative to the motion of the air mass around the plane, as Bianfable points out in their answer. When the aircraft is at 0KT and wants to be going faster (takeoff), or is at some fast speed but wants to be going 0KT (landing), then the headwind is relevant. At altitude the aircraft only cares about its own movement relative to the air mass, and the fact that the air mass happens to be moving relative to the ground does not affect the aerodynamics. – randomhead Mar 12 '21 at 04:22
  • @randomhead -- re "when there is a headwind relative to the motion of the air mass around the plane" -- does this phrase really make sense? – quiet flyer Mar 13 '21 at 14:19
  • @quietflyer no it doesn't, not really, but I hope my point comes across. I suppose I could say the headwind is only a factor when it is sufficiently unaccounted for in the plane's current momentum: gusts are gusts, whether at 10AGL or 35,000MSL, but when the plane's airspeed is low and particularly when it is trying to connect with or depart from the ground is when the windspeed makes a difference. – randomhead Mar 13 '21 at 15:20

4 Answers4

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As far as aerodynamics is concerned, there is no jet stream1. The aircraft flies at a fixed airspeed with respect to the surrounding air2. How much thrust it needs, how much lift it experiences and so on, only depends on the airspeed w.r.t. to the surrounding air. How fast that air is moving across the ground (wind speed) is irrelevant for aerodynamics.

The ground speed is however affected by the wind. With a strong headwind, the aerodynamics is the same, but you need longer to reach your destination airport (which is usually fixed on the ground). Likewise, a tailwind will get you to your destination faster. There is no "complicated numerical analysis" required to calculate the effect on ground speed:

$$ \text{Ground Speed} = \text{True Air Speed} \begin{cases} - & \text{Headwind} \\ + & \text{Tailwind} \end{cases} $$

Therefore, fuel flow per time will be identical in jet stream east- and westbound. But the total time for the flight (and therefore the total fuel required) will depend on it. Calculating the extra fuel required is not necessarily so straight-forward, since carrying extra fuel makes the aircraft heavier and therefore consume more fuel per time. A heavier aircraft also has a lower optimum cruise altitude, where the wind could be different.

1 Not entirely true, since aircraft can definitely feel the jet stream when entering, leaving or passing through it. This often causes clear air turbulence.

2 This assumes the aircraft flies at the same fixed airspeed of course, but this is usually the case for jet aircraft flying in the jet stream, which fly at a fixed Mach number. Actual airspeed will depend on flight level and temperature.

Bianfable
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  • Thanks for the equation. The distinction between air speed and ground speed isn't always intuitive if you have only dealt with it for a short time. I now understand that I had some wrong assumptions, this happens if your lectures on this topic are still a few semesters away, and you overthink the question. – VeNETHER Mar 11 '21 at 21:01
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Best way to conceptualize it is to just analogize it to a ship navigating ocean currents. If a ship going 15 knots through the water is riding with a current going 3 knots, it's speed over the ocean bottom is 18 knots (call it a "tail current"); if it's going into current, it's going 12 knots (call it a "head current"). If it's going at 90 degrees to the current, it's a wash, and its speed over the bottom is the same as its speed in the water, 15 knots.

Normally the ship is in a large mass of water moving in one direction, an ocean current. But maybe near the edge of that mass of water you might have a small area where the current speeds up to 8 knots, because of some phenomena, maybe a rip tide or something. If you are in the 8 knot current, your over-the-bottom speed goes to 23 knots if it's going your way, or 7 knots if going against you. You might call this extra fast local current an "ocean jetstream".

If you were crossing the Atlantic, say from the Caribbean to the UK, in the Gulf Stream and taking advantage of its 1 or 2 knot "tail current" to get you there a bit faster, but you knew that somewhere off to the left or right there was a narrow local current going 4 or 5 times faster than that, you might head over to get that that current to really speed up your trip. On the way back though, you will avoid that current at all costs. You will also find that the water at the edge of the fast current can be pretty rough from shearing turbulence.

Airplanes are pretty much the same deal, except the currents are gaseous, the ocean bottom is the ground, the speeds are 20-30 times higher, and instead of rough waves at the edge of the fast current, you have Clear Air Turbulence at the edge of the jet. The jet streams reside in the little wedge of air on the warm side of frontal boundaries, just under the Tropopause, where the air gets squirted along at 80-140 knots in a little ribbon, like a pipeline, a few miles wide. If you get in it, you want to be going in the same direction or at least just going across, and will move elsewhere to avoid going against it.

John K
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  • Nice answer. Should be upvoted. – Mike Sowsun Mar 12 '21 at 13:37
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    Even the pure "side current" will slow the ship down because it will need to change course in order to sail into the correct direction, slowing it down a bit along the desired track. – Peter Kämpf Mar 13 '21 at 07:28
  • Well, I'm talking stead state conditions. In any case, that only applies if there is a lag in the correction so the ship proceeds in a semi teardrop pattern and increases the distance travelled. If the current was 0 at point x and increased to 3 knots directly abeam at point y and I adjusted heading to stay on track in concert with the current change, the effect is negligible. Crossing jet streams, if you are more or less perpendicular, you will see the AP point the nose to the upwind side, than back after you leave the jet. Thrust untouched, no change in CAS, ground speed while this happens. – John K Mar 13 '21 at 15:47
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There are some excellent answers here already, and I won’t try to compete directly with their explanations. However, your comment about “free lift” from a headwind was an important clue about your thinking, and I wanted to take a slightly different approach in my answer. (A little more “touchy-feely” perhaps...)

What is apparent is that you are looking at this from an earth bound frame of reference. Because a kite, tethered to the earth, DOES extract energy the energy from, and experience wind as “free lift”. In response it climbs.

I know that intellectually you understand that airplanes are not tethered, but try to find a way to untether your mind from this groundling paradigm. Aircraft are literally “craft of the air”. They don’t know or care which direction or how fast the surface of the planet below them might be moving, they only respond to and interact with gravity, plus the aerodynamic effects of movement through the viscous three dimensional medium that contains them - The airmass.

Only the pilot cares about the surface of planet below, and only to the extent that he/she desires to interact with it. (I.e. navigate to a place on it, touch the wheels to it, avoid hitting protrusions of it, etc.)

No “complicated numerical analysis” is required, just a gentle shift in mindset. Augmented by thoughtful observation of the world around us. Do you have a river nearby? Go to it and quiet your mind... Watch objects float by. (If nothing is floating by, throw a stick in). Observe how floating objects are one with the current.

If it is safe to do so, go swimming. Notice the effort needed just to keep position with a spot on the bank? Relative to the shore, can you swim faster upstream or downstream? Do you get any “free” energy swimming upstream as you presumed an airplane might get from flying into a headwind?!

Maybe you don’t even need to go swimming, hopefully you “get it” now, so I won’t belabor any further... ;)

Good luck with your upcoming lectures.

Michael Hall
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You are correct that airspeed is key to aerodynamics and flight performance. Whereas groundspeed is key to navigation. Neither really affects fuel consumption rate as far as endurance.

Fuel endurance is measured as fuel quantity over time (ex.gallons or pounds per hour). Fuel endurance is a function of engine power. Fuel Range is measured as fuel quantity over distance (miles per gallon or pounds). Groundspeed in relation to engine power (or really fuel endurance) will affect fuel consumption rate as far as range. Fuel range and fuel endurance are separate but interrelated quantities of fuel consumption.

An aircraft at a set engine power, altitude, barometric pressure, temperature, humidity, and other atmospheric conditions will travel at the same airspeed regardless of the wind speed or direction. Headwinds, tailwinds, or crosswinds will not affect the aircraft airspeed. Therefore, they will not affect the aircraft fuel endurance. However, they will greatly affect the aircraft groundspeed. Which in turn affects the aircraft fuel range.

Also, for the same or similar airspeeds, tailwinds give aircraft faster groundspeeds, and headwinds give aircraft slower groundspeeds. But, the amount of lift produced is a function of airspeed, and not of groundspeed.

Think of it this way. Aircraft fly in and with the airmass as the airmass itself is in motion. Airspeed is the speed relative to the airmass. If you threw a ball from the back to the front of a bus at 60 mph (airspeed) while the bus was moving forward at 60 mph (groundspeed), the ball would be traveling at 120 mph (groundspeed). On the other hand, if you threw a ball from the front to the back of a bus at 60 mph (airspeed) while the bus was moving forward at 60 mph (groundspeed), the ball would be traveling at 0 mph (groundspeed). In both scenarios, the strength/effort it would take to throw the ball would be the same. In this example, the bus is the jetstream (the airmass). The aircraft is the thrown ball. The person doing the throwing is the engines. And, the fuel flow rate is the strength/effort producing the power.

Dean F.
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