How do Flaps Work? | How to Use Flaps During Landing | Flaps Up Landing

[0:00]If you're in a window seat on an airliner looking out during landing, you'll notice part of the wing moving like this with the back surface extending downwards, more and more as the airplane gets closer to land. These are flaps. Not every aircraft has flaps. They are a secondary control surface. But even many of the smaller general aviation aircraft, like the Piper Arrow here, will have flaps which raise and lower, somewhat like they do on the airliners. What flaps do for us can be explained by looking at how the wings keep the aircraft in the air. If we look at the plane from the side here, we'll be able to get a good look at the shape of the wing. The wing is designed so that it has a relatively flat lower surface and a somewhat more curved upper surface. The curvature of the upper surface is known as camber and it helps the wing keep the plane aloft. If we draw a black dashed line from the front or leading edge of the wing to the back or trailing edge, we have what's called the cord line. Now, if we draw a curve which splits the wing equally between the upper and lower halves, we have the mean camber line. The distance between the two lines gives us an expression of the camber of the wing, the curvier the upper surface, the greater the distance between the lines and the greater the wing's camber. Why we care about camber is because it's one of the factors that goes into how well the aircraft keeps us in the air. The wing produces lift which opposes the weight of the aircraft. If the lift equals the weight, the aircraft is in equilibrium and so the aircraft can, for example, remain at a constant altitude. The wing produces lift by deflecting air downwards. The more air it's able to deflect, the greater the lift it can produce. One of the ways we can deflect more air is by flying faster at a higher velocity V. But we can only fly so fast with a given engine. Another factor is to increase the angle the wing makes with the air we're flying through. This angle, angle of attack, can be increased to increase lift. This too has a limiting factor though. If we pitch up too much and raise that angle of attack too high, the air will no longer flow downwards across the wing and the wing will stall. Finally, a wing with a higher camber or more curved upper surface will be more effective at deflecting air downwards. If we could somehow swap out our wings mid-flight for a curvier one, we could change camber, or we can change the shape of our wing using flaps. A typical aircraft wing can have a hinged surface like this at its trailing edge. This is the flap and a basic type of flap design like the one we saw on the small Piper arrow is the plain flap. This type of flap rotates on a hinge to extend and retract the flap. Extending the flap in this way will affect both the cord line and the camber line. The cord line will have more of a slant, and the camber line will drop off through the extended flap, increasing the wing's camber. So now, by extending the flap, we're increasing lift, both because the angle of attack is increased, the angle the cord line makes with the air we're flying through is greater. And by increasing the wing's camber. All this extra lift means that if we're trying to keep the aircraft in level flight, we can reduce lift and still have enough to counteract weight. We could reduce lift by pitching down and lowering our angle of attack. First of all, this will give us a better view out the cockpit on a descent to land. But more importantly, at a lower angle of attack, we're further away from stalling the wing. We can also fly at a lower air speed. So with the use of flaps, we've allowed ourselves to maintain equilibrium at a lower angle of attack and air speed, and we're also more protected from a stall. This part is crucial because it means that we can continue to slow the aircraft down and compensate for reduced lift by increasing the angle of attack, and we won't find a stall until we've reached a very low air speed. indicated by the bottom of the white arc on the air speed indicator. Being able to approach a runway at slower air speeds is crucial. Here, we're on an approach to land. With our flaps up, if we begin a descent, the aircraft picks up speed just like a roller coaster would as it goes down a track. This speed has to be bled off in landing and so it means we'll use a precious runway getting ourselves slowed down. But with flaps, the aircraft can be slowed down. Not only do we produce the same lift at a lower air speed, but the added drag from throwing these barn doors out into moving air slows us down too. What this means is that we can descend at an even steeper rate. And not only do we not pick up the same speed as the aircraft with no flaps, but we could actually be slower and able to land and get stopped in a shorter distance. Let's compare two aircraft landing on a runway with and without flaps. The speed of each aircraft is the same, 80 knots, just for comparison purposes. Only the aircraft up top isn't using flaps while the aircraft at the bottom has flaps extended for landing. First of all, the aircraft with flaps up, up top has to come in at a shallower angle to maintain such a slow speed, so the view isn't as good. Without the added drag from the flaps, it takes longer to bleed off the extra energy and we'll need to stand on our brakes just to bring the aircraft to a stop. Using up almost all of the short runway. In addition to landings, aircraft use flaps on takeoff and sometimes during other phases of flight, when a change in camber is needed for an increase in lift, without sacrificing angle of attack or air speed. 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