Why are trailing edge flaps used for landing? [duplicate]

Question

Why are flaps used when landing? Doesn’t a flap increase lift?

Edited: why are there so many different flaps on the back of a wing? Isn’t one just used when taking off and landing? Are the others backup?

Answer 1

Flaps increase the stalling lift coefficient of a wing. They can also change the angle of attack for a given lift coefficient.

By allowing the wing to operate at a higher lift coefficient without stalling, the airplane is allowed to fly slower (without stalling).

$C_L = \frac{W}{0.5\,\rho\, V^2\, S_\mathrm{ref}}$ can be rearranged and made specific to stall:

$V_\mathrm{stall} = \sqrt{ \frac{W}{0.5\, \rho\, S_\mathrm{ref}\, C_{L,\mathrm{max}}}}$

So, a wing that previously stalled at CL of 0.8 might instead stall at a CL of 1.2 with flaps. This is a 50% increase in CL -- which will reduce Vstall to 81% of the un-flapped value.

By deploying flaps for landing, an aircraft can fly slower. This makes for a safer landing and also for shorter runways.

Flaps also increase drag -- which makes it easier for an airplane to slow down. So, the landing configuration of flaps is usually the maximum possible -- increase lift as much as possible and increase drag while you're at it.

In contrast, during takeoff, you want the reduction in takeoff speed -- but you don't want a bunch of extra drag. Consequently, most aircraft use a moderate setting of flaps for takeoff.

Flaps can be deployed fully or partially. Most flaps rotate. Some rotate and translate -- these are called Fowler flaps. Even for flaps that have combined translation and rotation, the pilot's controls are still marked just in terms of the angle. So, you might deploy flaps 10, 20, or 30 degrees. A takeoff setting might be 20 degrees with a landing setting 30 degrees.

If an airplane has multiple spanwise flaps installed, you will use them all at once, they are not just redundancy, they give a full capability. You want to reduce the landing speed as much as possible.

If you're wondering why a particular aircraft has many flap segments across the wingspan? This is because wings are actually quite flexible. The flap is also flexible -- but not exactly the same flexibility as the rest of the wing. Consequently, when the wing bends, the flap also bends, but not the same amount. In order to keep the mechanism (hinge, track, etc) from binding, control surface segments are kept short. That way, the deflection difference over any segment can be accounted for, but it is not as large as it would be for a full-span control surface.

If that doesn't answer your question, please clarify and we'll try some more.

Answer 2

The difficult part of landing an aircraft isn't losing height — it's losing speed. Losing height without losing speed is called falling (or diving, when done deliberately) and coming into contact with the ground while falling at high speed is, in layman's terms, commonly described as crashing.

Specifically, to land an aircraft, you want to lose as much speed as possible while the aircraft can still maintain horizontal flight and then gently touch the runway and brake to lose any remaining horizontal speed ASAP before you run out of runway. In fact, if it was possible, the ideal landing would be one where you'd come to a full stop in the air before even touching the ground and then just gently float straight down to land.

Unfortunately only helicopters (and a very small number of other VTOL aircraft like the V-22 Osprey) can actually pull off a landing like that. The rest, including most fixed-wing airplanes, have a minimum forward speed that they need to maintain in order for their wings to generate enough lift to counter their weight and maintain level horizontal flight.

Flaps divert the air flowing around the wings downwards and thereby generate extra lift, which allows the aircraft to fly slower and still maintain horizontal flight. Conveniently (for landing) this also causes flaps to generate extra drag, which helps slow down the aircraft. That, in simple terms, is why flaps are used for landing.

(The reason flaps are not used all the time is that during high speed cruising flight the extra lift is unnecessary and the extra drag is unwanted. So the flaps are retracted during cruise to reduce drag, which allows the plane to fly faster while using less power from the engines — and thus less fuel — to maintain its speed.)

Answer 3

The questioner's confusion is understandable, because pilots often loosely use the word "lift" in a way that is not really accurate.

In the rigorous sense of the word-- an actual force, measurable in pounds or Newtons-- lift does not vary nearly as much as one might think. In linear flight, lift must equal weight * cosine (climb or descent angle), so for shallow climbs and glide angles, lift must be nearly equal to weight. (And note that lift is actually less in a steep climb than in horizontal flight.)

Lift is proportional to lift coefficient times airspeed squared. When we talk about "high lift" devices, what we are really talking about is a way to increase the lift coefficent, which means that for any given airspeed, the lift force will be increased. This also means that for any given required lift force, airspeed is decreased. That's the key to understanding why we use flaps for landing-- we can fly slower without stalling.

An additional benefit of flaps is that they decrease the ratio of lift coefficient to drag coefficient, which is also the ratio of Lift to Drag. The glide angle is equal to the arctan ((D-T) / L), where T is thrust, or if thrust is zero, the glide angle is simply the arctan (D / L). So decreasing the L / D ratio makes the glide path steeper (for any given Thrust setting), which makes it easier to judge the final approach. With jet engines that have some delay in powering up, there's an advantage in being able to operate at a higher power setting for any given descent path, and decreasing the L / D ratio facilitates this.

For take-off, the situation is different-- we want to be able to support the plane's weight at the lowest possible airspeed, so we want a high lift coefficient, but we also want to keep Drag as low as possible, because we need to accelerate throughout the takeoff run, and in many cases we also want to continue accelerating well into the climb. (Also, in the case where the airspeed is constant so all the excess thrust is used for climbing rather than accelerating, the climb angle that we can achieve is equal to arctan ((T-D) / L).) So we have a trade-off -- we want to increase the lift coefficient, but without increasing the drag coefficient too much. Therefore we'll use a more limited deflection of the various different flaps than we would for landing.

Some of the moving surfaces that you see on wings are called "spoilers". Spoilers decrease the lift coefficient and increase the drag coefficient. On a large airliner where minimizing the landing speed is important, spoilers would not be deployed until the wheels touch the ground. On the other hand, sailplanes (gliders) will often deploy spoilers (also called "dive brakes") throughout the final approach and landing because the slight increase in touchdown speed is not really a problem, and the steeper descent path is very beneficial-- with spoilers closed, the final approach would be so flat that it would be very difficult to accurately hit the desired touchdown spot. Yet some sailplanes use flaps rather than spoilers to steepen the glide path, just like most powered airplanes. And some use both.

No aircraft is designed with extra moving surfaces "just for backup", at least in the civilian world. Each part of the flap system is there for a specific reason and will be used whenever that will help optimize the aircraft for the task at hand. Note that especially with lighter aircraft, different amounts of flap deployment are optimal for different tasks, even in the realm of approach and landing-- e.g. short field landings, soft field landings, landings in strong gusty crosswinds, etc. And likewise for takeoff -- a different flap setting would often be selected for a short, soft field with no obstacles at the far end, than for a paved runway with tall trees or hills at the far end.

Note that with multi-engine jet aircraft with wing-mounted engines, the flaps are often split into different sections simply to avoid the jet wash. In such a case the pilot does not have the option to deploy only the inboard flaps, or to deploy only the outboard flaps. (Another answer has addressed this in more detail-- avoiding the jet wash is not the only reason for splitting up flaps into segments.)

Some related questions and answers--

Does lift equal weight in a climb? (see various answers)

Is excess lift or excess power needed for a climb?

Why is the L/D ratio numerically equal to the glide ratio? (see various answers)