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1970-01-01

Something that was never clear to me at this level of detail is how a tailwind enables an airplane to move faster. In other words, if the airflow is coming from behind, the lift equation should fall apart and the airplane should fall out of the sky.

https://www.skytough.com/post/tailwinds-make-plane-faster

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Our intuitive experience with wind on the ground is wrong. Next time it’s windy outside imagine the entire volume of air stretching out for miles and miles moving across with the wind speed, we’re just standing at the bottom of this vast air ocean. It will blow your mind and you’ll think about wind differently from then on. So with that in mind, once the airplane is in the air, it doesn’t “know” if there’s a headwind or a tailwind at all, unless you have a way to reference the ground somehow (for example, with a GPS) - just like a boat doesn’t “know” it’s carried by a current downstream. If you are still on the ground, it is very possible that the tailwind is strong enough for you to not be able to takeoff in the available runway - but then you would go in the opposite direction or more likely sit the storm out :)

I’ve noticed this effect while diving. When you’re in a current, you’re basically the same density so you’re moving with the water. In mid-water with poor visibility, this is really freaky, because you have no way of telling in what direction you’re moving, and how fast. If you “forget” your orientation, you can’t really recover it. Thankfully, you always have highly accurate depth gauge, but as for lateral movement, it’s an eerie feeling. You could just pop up anywhere.

Yeah, I wish meteorologists explained this concept better to the general public, since they're basically poised for it. One day I was just wondering where wind _started_ from, and started digging deep into the topic, but essentially we're all just standing on the bottom of a roiling ocean floor that is very sensitive to heat changes from the sun.

> So with that in mind, once the airplane is in the air, it doesn’t “know” if there’s a headwind or a tailwind at all

This stops being true for quick changes - because there's still the inertia of the aircraft. So if the wind speed changes quickly, the aircraft can't immediately move along with it.

This is why gusts are so dangerous to landing aircraft. A strong gust from behind can cost you all your lift, and a strong gust from the front can temporarily stall your wing.

Airplanes are always traveling forward relative to the wind, at some angle of attack. Tailwinds don't work by blowing against the airplane's surface and pushing it forward. Since the airplanes are themselves traveling at, say, N kt forward relative to the wind, then if they are inside a 10 kt tailwind, they'll be doing N+10 kt over the ground, if they are inside a 50 kt tailwind, they'll be doing N+50 kt over the ground. If they are inside a 25 kt headwind, the'll be doing N-25 kt over the ground.

The plane is just up in the air moving relative to the air around it, it doesn't care how the air it's in is moving relative to the ground.

A tail wind is just saying that the air is moving in a certain direction with respect to the ground (the same direction the plane is flying). The plane doesn't give a shit about that.

Yes that makes sense at a high level, but there must be a point of transition between calm air and a jet stream that makes the wings useless to the airplane for at least a few seconds.

Indeed it does, that's effectively what turbulence is.

If you took a plane flying in still air and magically, instantaneously replaced all the air around it with a tail wind equal to its velocity then yes the plane would stall and fall out of the sky.

Fortunately that kind of instantaneous change doesn't happen in real life.

GO AROUND, WINDSHEAR AHEAD!

It's relevant in practice when landing against head wind. You need to have extra speed to not stall when you enter the slower air near ground.

Do you mean landing with a tailwind? A headwind should allow the plane to create the same amount of lift it needs to avoid stalling at lower ground speeds.

Yes, that’s correct, but the headwind stops being so headwind-y near the ground, so your plane needs to go a bit faster to compensate for the loss of headwind-ness in the seconds before touchdown.

On the flip side you also get ground-effect when you are low to the ground where the high-pressure underneath the wing gets trapped against the ground creating a cushion of pressure increasing lift.

Which can be a bit of a challenge when trying to land, especially for aerodynamically efficient aircraft.

Presumably the plane would accelerate as it climbed through the velocity gradient, never falling to a point of negative air-relative airspeed.

Yes, the wings are useless once the air is moving close to the speed of the plane. Thankfully, we have jet engines that help planes move a lot faster than the 100-200 knots that jet streams can reach. They'll still affect the flight but only temporarily.

These sudden changes do indeed happen in stormy weather, as adjacent layers of air can move with different velocities relative to the ground (the technical term is “wind shear”). If an airplane climbs or descends through those it will look like your speed (relative to air) is suddenly increased or decreased by some amount and you would have to compensate. It’s also a bigger problem for large, heavy airplanes as you have more work to do to accelerate for a given amount of speed loss.

Jet stream boundary is usually not this sharp, and the airplane would fly much faster than the difference anyway.

It also changes a lot for sailboats, and even more for faster sailing craft (windsurfers, etc.).

You can feel a much stronger pressure in the sail when moving towards the wind on a fast windsurfer/windfoil as you can do 15-20kts 45deg towards the wind, giving you an apparent wind that is 10-14kts stronger than the true wind.

On the same craft, going away/downwind, you will feel the apparent wind at a similar angle 10-14kts less. In fact, because of the change in drag and forces, you'll probably be going faster and feel even less wind on the downwind leg.

When you turn, this can be a big benefit for going downwind (jibing) as at some point the sail feels zero apparent wind (your motion cancelling out the true wind), feels very light in your hand, and easy to rotate to face the other way. Even knowing the physics of it, the timing and execution is still something that takes a lot of practice...especially on big race gear with a 9.0m2 sail.

The bottleneck when it comes to a plane going faster is drag which increases with the square of velocity relative to the air. More drag means the plane has to consume more fuel to stay at its current velocity. So if a plane normally goes 600 mph with no wind then a 100 mph tailwind will allow that plane to go 700 mph relative to the ground to experience the same amount of drag as if it were flying at 600 mph on a day with no wind.

It's like swimming in a stream. Even if you did nothing you would move basically at the rate that the water is moving.

Lift only comes from the interaction of the air and the wing, so if there's zero relative motion then you will fall out of the sky, regardless of if you have a 200 knot groundspeed.

This also means that, if the wind at altitude is above your plane's stall speed, you can hover in place by flying straight into the wind! (example here: https://www.youtube.com/watch?v=n_e6ijREScE)

Similarly, if you are in a packet of air that is moving at 200 knots, the fact that you are moving at 500 knots indicated airspeed does not mean you are flying supersonic from an aerodynamic perspective, despite having a groundspeed of 700 knots.

Yes airplanes have to travel faster (in terms of ground speed) to not stall. This is why head winds are preferred for landings and take offs as it allows ground speed to be lower. But during cruise you want a tailwind to reduce the amount of drag for a given ground speed.

This is a kind of relativity thing.

The only connection a plane has to the universe is the air around it. It simply does not know or care what the ground is doing until the ground is quite close.

A small plane in a very high wind is perfectly happy having a "backward" ground track.

Same thing as if you were trying to swim upstream in a fast river. How fast you move through the water doesn't have anything to do with how fast the water is moving across the land.

It works by reducing the amount of CSS the plane needs to carry.

Both drag and lift depend on the speed of the airplane relative to the wind, not relative to the ground. So, if the maximum efficiency dictates that the airplane should travel at speed X relative to the wind, and the wind is flowing at speed Y in the same direction as the airplane needs to travel, then the airplane, flying at maximum efficiency, will be travelling at speed X+Y relative to the ground.

Planes move through the air, and relative to the air mass.

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