Why can headwind assist planes when taking off and landing? Why does tailwind not help with takeoff?
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Planes generate an upwards force using to effects. One is the Bernoulli effect where the faster moving air abow the wing creates a pressure difference which provides an upwards force on the wings. The other effects is air resistance by flying into the air at an angle the air will push the plane up. Its like sticking your hand out from the car window, if its at a 45-ish° angle to the oncoming wind your hand will try to rise.
Now planes when taking off will use a lot of that second effect but they need to get into the air first. Thats where the Bernoulli effect comes in. Now hom much lift comes from that is a function of the velocity of air moving over the wings. Since all kinds of lift comes from the relative air speed for a plane it isn’t the velocity relative to the ground thats important its their air speed. If there is headwind that increases air speed. So the plane has to accelerate less to take off.
Same for landing. The safest landing is a landing preformed with high air speed and low ground speed. You want to have a low velocity relative to the ground so you can stop before you reach the end of the runway. But if the air speed is too low you will stall, an fall. Head wind keeps air speed high to allow the plane to approach much slower.
The way to think about this is that a plane uses fast moving air to stay in the sky but from the perspective of the plane it doesn’t matter whether its the plane moving fast through still air or the wind is blowing fast across the stationary plane. The plane will have lift either way.
Lift is created by the speed of the air relative to the wing. If I am going 50mph on the runway in still air, I get exactly as much lift as going 25mph on the same runway with a 25mph headwind.
If I have a 25mph tailwind, I would need to go 75mph on the runway to get the same amount of lift.
What creates lift is the amount of airflow over the wings. Think like holding your hand out a car window. The faster the car goes the more lift you can create regardless of wind direction. If the wind I blowing head on it increases the airflow relative to the wings and thus increases lift meaning the aircraft needs less runway to takeoff. The opposite is also true. Tail wind might be ideal once airborne but on takeoff it would actually reduce the airflow under the wings and make it harder to achieve the required lift for takeoff (longer runway).
On landing the same is true but in reverse. You want to be able to approach the runway at the minimum possible speed while still having enough lift to stay airborne. If you were coming in with a tail wind you would need more speed and a longer runway to slow down.
For planes it doesn’t matter how fast they are going compared to the ground when it comes to when they can lift off/need to land. They only care about how fast the air is moving over the wings when it comes to flying
Let’s say they need to reach 100mph through the air to lift off. If they have a 20mph headwind they only need to gain 80mph before they can lift off. If they have a 20mph tailwind then they need to gain 120mph to lift off.
On landing they care about how much runway they need to land. If they have a 20mph headwind they only need to bleed off 80mph to land. The same wind in the other direction would have to bleed off 120mph. Imagine stopping at 40mph vs 60 mph.
Sometimes the best way to imagine this is extremes. It is possible (and actually midly frequent) for small planes like Cesnas to fly at 0mph. If the wind is as fast or faster than the flight speed then you can fly in one direction while going nowhere or even backwards!
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