eli5 why are flaps extended during takeoff if they slow down the plane?
In: Engineering
In modern aviation, the wings are designed optimally for (fuel) efficient flight at high speeds and high altitudes – since that’s where they spend the vast majority of the flight. So, they tend to have a thin cross-section and be long and ‘narrow’, and that generates enough lift but relatively little drag.
But the above type of wing would be far from ideal for low-speed take off and landings. So, most aircraft have slats on the leading edge of the wing and flaps on the trailing edge which can be lowered to *effectively* turn the wing into a ‘deeper’ aerofoil shape which generates a lot more lift at low speeds – even if it is more draggy. Requiring a high take-off speed is dangerous, especially in the even of an aborted take-off.
Flaps convert forward motion into lift when deployed.
However that means they sacrifice some forward speed.
But when you’re taking off, you’re generally going very fast already, and what you really want is lift.
Equally, when you land, you’re going fast and what you want is to slow down and touch down gently (i.e. slowly move down), so flaps assist in turning forward motion into giving some “free” lift.
They increase the surface area of the wing which allows the airplane to take off at a lower speed, which is safer and more practical since this means that on the ground airplanes are rolling much slower than what they would have if they didn’t use flaps and that also means runways don’t have to be twice as long. It also makes the aircraft more stable and controllable at slow speeds. Once in the air they can simply raise flaps and increase their speed. It’s worth noting that the flap setting for take off is much smaller than for landing, and it creates less drag.
Flaps are for extra lift so the airplane can take off easier. This is because they make the wing “bigger”. Depending on how long the runway is and the weather/temperature outside, an airplane may need to use a higher flaps setting. They also allow a plane to fly slower while maintaining lift when approaching a runway.
Now, they technically do slow the plane down due to drag but it’s more like they impose a maximum speed it can fly when extended. Spoilers on the other hand would be what actually slows the aircraft down.
Because the plane is designed that way.
Whys the plane designed that way?
Because the wing shape that is efficient for high speed flight is not the same as the wing shape for low speed flight. You’d have to fly faster, and use more runway, to takeoff without the flaps.
Why not use them all the time then?
At high speed, the wing generates lift easily, and the flaps would generate additional drag, costing efficiency – and that inefficiency has a real dollars per mile cost, in the form of fuel.
Lift is proportional to the square of your airspeed, if nothing else changes. Slowing down costs lift. You can gain it back in a variety of ways – tilting the plane a bit more nose up, so the wing attacks the air a little more aggressively, or extending the flaps – modern airliner flaps both increase the wing area, and increase wing camber, and both of these things improve lift. Effectively they allow you to become airborne at lower speeds, and to remain airborne at lower speeds.
That’s good for tyres, and great for airliners, as there are plenty of short runways, and the less runway you need, the safer it is.
You can build an aircraft without flaps. Many aircraft with them, can takeoff without using them.
Flaps slows down the plane, but also make it easier to be airborne.
During takeoff the plane need to be airborne even at slow speeds, so it uses flaps.
During landings, the plane will use again flaps because they allow it to fly at lower speeds.
To just slow down the plane, there are other components like flaps but just to brake in the air. They are called “air brakes”.
They make the wings bigger so they push the plane upwards harder.
Try this: Get a friend or family member to drive you, and open the window. Use a couple regular pieces of paper, but cut on in half cross-wise so it’s 8.5×6.5 inches. Glue each one to some cardboard. Then go drive around somewhere safe holding one of those out the window tilted slightly up. Then do it at the same speed with the other one. You’ll notice that the air pushes up harder against the bigger piece. It also pushes back, against the direction the car is going. The more you tilt, or the bigger the piece, the harder it pushes up and back.
What flaps do is make the wing bigger, so it pushes up harder. That also means it pushes back more, too. In fact, if you go too fast then you aren’t allowed to use the flaps at all because they’ll break from how hard the air pushes them back.
That’s why we use flaps just at takeoff and landing. We want to be going ever slower to land, so we don’t have to roll so far to stop once we’re on the ground. So flaps help with that. And on takeoff we start from 0, so we use flaps until we’re in the air and going pretty fast. Then we pull in the flaps so the air doesn’t push back so hard, and we can go even faster.
You extend the flaps so that the plane will fly at lower speeds. Thus, when you want to go as slow as possible and still fly, you put the flaps down. You want the airplane to fly as soon as possible on takeoff because runways are a finite resource, they only go so long. So the quicker you can get in the air the better, even if it means you aren’t achieving full speed to do it. Once you are up and your airspeed has risen enough, you can raise the flaps because the plane no longer needs them to fly and would just slow it down. The, when you are landing, you want to slow down as much as you can while you are still in the air in order to land in the shortest distance possible, because, once again, runways are finite resources.
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