What’s the absolute minimum speed a plane has to go for it to stay in the air?
In: Physics
It’s not really possible to give a simple answer to this – it depends on the plane, and on whether you mean air speed or ground speed. Against a strong enough wind, a light plane could fly while going backwards relative to the ground.
For light prop planes, it’s around 45 knots IAS, and it goes up from there. You can look this up for almost any plane, and it’s called the *stall speed*. If you go slower than the stall speed, your wings lose laminar flow and stop acting like airfoils.
The [Gossamer Condor](https://airandspace.si.edu/collection-objects/maccready-gossamer-condor), in 1977, flew a figure 8 course of a little over a mile and averaged between 10 and 11 mph on the course. I would guess that, at liftoff, it was going at a jogging pace, maybe 6 mph or so.
So, it can be very slow, but this is getting down there at the minimum air-speed, I would think. You have to have some airflow over the wings to create lift.
It depends on the aircraft. This one was human-powered. [You could ride a bike at a leisurely pace and keep up.](https://www.youtube.com/watch?v=sp7yv67B5Sc)
They later created the Gossamer Albatross that flew across the English Channel powered by a single human
Aircraft like the F35 and Harrier can hover, so 0 km/h
Gossamer Albatross is a human powered plane with a top speed of 29 km/h.
The Wright Flyer averaged about 11 km/h.
Planes with custer wings can fly very slowly since they use the engines to blow air over the wings to create more lift. They can fly at 18 km/h, but become dangerous if an engine fails. https://en.wikipedia.org/wiki/Custer_CCW-5
There’s no theoretical minimum…if you have a big enough wing with a light enough aircraft it can be single digit miles per hour (relative to the air). Like /u/HappyHuman924 aid, for small prop planes it around 50 mph. Human powered aircraft like Gossamer Albatross had *top* speeds below 20 mph but it could fly considerably slower than that.
For a really good paper airplane, you get can get it down to a brisk walk (~4 mph). If you purpose built with exotic materials you could go even lower.
This is called stall speed because in order to generate enough lift to stay in level flight the wings need so much angle of attack that they are about to stall and loose lift. There are a lot of variables going into calculating the stall speed of an airplane. For example the type of aircraft, the weight of the aircraft, configuration, the altitude, temperature, pressure, etc. So the stall speed might be anything between 35 knots and 350 knots, sometimes even for the same aircraft.
To stay airborne, an aircraft has to produce enough lift to offset weight. Lift increases with airspeed, and the angle of the aircraft to the airflow. If speed decreases, the angle must increase. If the angle increases too far, the wing stops producing lift, and the aircraft falls. This is called Stall. The speed at which is this happens can be determined from a pretty simple equation:
V = sqrt( 2 * aircraft weight / (air density * wing area * maximum lift coefficient))
Where the maximum lift coefficient is the lift coefficient that occurs right before the aircraft stalls. Using this equation, you can calculate the minimum speed of any aircraft given it’s weight, and the density of the air around it. I know equations aren’t exactly ELI5 material, but that equation is easy to use, and answers the question for every aircraft, regardless of it’s loading, altitude, and local weather.
Though it really depends on a lot of factors, checkout this video about the [Short Take Off and Landing (STOL)](https://youtu.be/hPakbghLe38) competition held in Alaska each year.
It’s a really cool example of the ‘very slow’ end of the extremes.
Well, you see, Bobbie, a plane is always feeling a downward acceleration due to gravity. And the plane needs to be generating enough lift to not fall.
A lot of planes do this by having the wind push up on their wings more than it pushes down. If there’s a strong wind pushing against them, they go up! This is how a kite works.
You know how when you put your face out the window, you feel a lot of wind? Most planes move fast enough that they have enough wind hitting their wings to stay aloft. If they didn’t move fast enough, they’d fall. That’s why pilots have a lot of training and that’s why we should be quiet on the plane to not distract them.
How fast does the plane have to go to not fall out of the sky? Well that depends on the plane and the amount of wind it is flying into. It’s probably called “minimum airspeed” and I’d guess that each airplane has a different minimum speed.
Depends on the plane (faster planes usually have higher stall speeds), whether or not the flaps are deployed (flaps reduce stall speed) and how much it weighs (more payload and fuel increases stall speed).
Typically I believe it to be (flaps down) about 50kn for a small plane, around 80kn for a commercial jet and about 120kn for a fighter jet.
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