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On a commercial airliner, what is the purpose of the flaps? I understand that depending on which direction they move in, they can be used to decrease airspeed, but I don't know when flaps are used. Are flaps used before takeoff, after takeoff and on approach?

rbp
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spike.y
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    Have you reviewed the other questions tagged [tag:flaps]? – Greg Hewgill May 21 '14 at 03:02
  • @GregHewgill I was out on my phone when I wrote this question. I did look for the flaps tag but I must have missed it. I will look again. – spike.y May 21 '14 at 04:20
  • Hmm, when you say "which direction they move in," do you mean the 'flaps' you see after an airliner touches down on the runway that stick up from the wings? I believe those are not flaps, but rather spoilers, or air brakes. – CGCampbell May 21 '14 at 11:51
  • Also, spoilers DO directly affect (are designed to) airspeed and are used to slow the plane's speed down. – CGCampbell May 21 '14 at 11:52
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    @CGCampbell Not true at all. Spoilers affect rate of descent, and attitude affects airspeed. – tar May 21 '14 at 14:59
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    Ooof, must always research. Spoilers reduce lift. – CGCampbell May 21 '14 at 15:49

3 Answers3

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Flaps change the camber of the wing's airfoil. This in turn changes the zero-lift angle of attack, increases the maximum lift potential and, in case of fowler-type flaps, the wing's area.

Airliners use flaps on both take-off and landing, and most of the time they are only partly deflected. Full deflection is only set on final approach to reduce the landing speed as much as possible.

When simple camber flaps are used, the drag increase is negligible for small deflections (±10° for a 20% flap), and they can be used for both take-off and landing. Other types of flaps are designed to increase drag (split flap, Zap flap) and should be used only for landing. If flight speed stays constant, a flap deflection will only change induced drag if it redistributes lift over the wingspan. Modern gliders use camber flaps inboard and flaperons (a combination of aileron and flap) outboard, and changing flap settings will not affect induced drag.

Generally, the purpose of flaps is to shift the minimum airfoil drag to the desired lift coefficient $c_L$. Especially laminar airfoils have a pronounced range of lift coefficients with low drag. The pilot should attempt to stay within this region of minimum drag, and flaps give him/her the means to achieve this. See below for an example of an airfoil at a Reynolds number of 1 Mio.

flap polar

Flaps for lift increase work the same way, but change camber much more. Normally, they need to be combined with leading edge devices (slats) which change camber at the airfoil's nose to unfold their full potential. Below are some examples for flaps, in combination with proper leading edge devices. Note that the solid outline of the fowler flap is the position for landing, while the dashed outline is the position for take-off. $\delta$ is the relative chord of the flap.

enter image description here

Junkers flaps were the best way to control big aircraft in the days before more elaborate force compensation mechanisms were developed, and split- and Zap flaps were very popular in the 1930s. They are still used on smaller aircraft today. The simple Fowler flap is used on the C-130, and airliners use slotted flaps like the ones in the lowest two rows of the table above.

Besides increasing drag, powerful flaps change the pitching moment of the wing by shifting the center of lift backwards, which requires sufficient tail size and control power. On airliners, the incidence of the full horizontal tail can be adjusted in order to create enough downforce to counteract the pitching moment. Note on the picture of the An-70 below that the stabilizer has a slat which operates in the opposite direction to that of the wing.

enter image description here

Peter Kämpf
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  • I wish that I could give you 10 up-votes. Great answer! :) – Lnafziger Nov 01 '14 at 15:26
  • @Lnafziger: Thanks for the kind words! I hesitated first if I should post a long answer to an old question, but the line "Flaps increase both lift and induced drag" in the only answer so far bugged me. – Peter Kämpf Nov 01 '14 at 16:08
  • @Peter Kämpf, can we say that flaps are also deflecting the airflow downwards and increasing lift this way? In other words they introduce some sort of Newton's 3rd law principle? – Electric Pilot Dec 05 '17 at 23:04
  • @ParadigmPilot: Not only the flaps, the whole wing is doing this constantly while in motion. The flaps only increase this effect. – Peter Kämpf Dec 06 '17 at 00:16
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Flaps increase lift and drag by changing the camber of the airfoil, and allow lower airspeeds. There is also an effect of shifting the center of pressure (C of P) aft. There may be an immediate decrease in airspeed due to the increase in drag (see the Wikipedia article linked below) until the aircraft regains stability by nosing down. There are some incorrect discussions online, such as one answer on this page.

Flaps are frequently used when landing because they lower the stall speed and increase the angle of descent. The pilot will also have better visibility due to the lower nose position caused by the aft C of P.

Flaps increase both lift and induced drag, but a small amount (e.g. 10 degrees) of take-off flaps may provide enough extra lift to allow the plane to become airborne sooner, without a large increase in drag, allowing for a shorter takeoff roll. Whether takeoff flaps can be used is declared in each aircraft's standard operating procedures.

I found these resources that I like:

tar
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  • The wikipedia article pretty explicitly states that flaps do in fact affect airspeed... When they are deployed they do increase the drag of the wing, which does slow the craft down. I doubt it's as effective as an airbrake, but still... Everything else you said appears correct though. – Jae Carr May 21 '14 at 13:40
  • @JayCarr it is a delicate subject, if you increase drag, you need more thrust to maintain airspeed and altitude. Flaps are usually extended during descent, or while standing still on the ground and not in level flight, so the effect on airspeed is not immediately apparent. – falstro May 21 '14 at 14:51
  • Updated, thanks Jay Carr – tar May 21 '14 at 14:52
  • @tar; some aircraft will return to trimmed speed all by them self, no manual pitch down required; and I imagine autothrust will counter to the drag with thrust, so it's not that simple :) – falstro May 21 '14 at 14:54
  • @tar It would be better if you just moved your "update" into the main answer. Readers that come here tomorrow or next year won't need to know what was original content and what was an update. –  May 21 '14 at 14:58
  • @falstro It is that simple. If you extend flaps, all other things being equal, airspeed will decrease. –  May 21 '14 at 14:59
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    @Articuno; I suppose that depends on your definition of being equal. Autothrust still enabled? Trimming? The DA20 Katana will happily maintain airspeed when you extend the flaps. (The immediate effect is airspeed decrease, sure, until it settles in with an increased descent rate and the airspeed returns to the trimmed speed, thus extending flaps all other things being equal increases the descent rate and affects pitch attitude) Of course the DA20 is no airliner, but the point remains, it's not that easy unless you define what should remain equal. – falstro May 21 '14 at 15:07
  • @falstro - "All other things" are to remain equal in the example. The overall effect of flaps on the system is to increase drag and lift. Yes you can, and probably should, compensate. But the overall effect of the flaps, and the flaps alone, is drag + lift. – Jae Carr May 21 '14 at 15:18
  • @JayCarr; exactly, they do not necessarily decrease airspeed. – falstro May 21 '14 at 15:19
  • @tar - No problem. Flipped my vote from down to up, good answer :). – Jae Carr May 21 '14 at 15:19
  • Lower pitch attitude is not due to aft CofP, but due to increase in lift. Because in stable flight the airplane needs lift exactly to oppose weight, the increase in lift at given AoA is compensated by flying at lower AoA and provided the glide slope remains the same lower pitch. – Jan Hudec May 21 '14 at 17:41
  • Induced drag from deploying flaps is inversely proportional to airspeed (see the Di curve here: http://qph.is.quoracdn.net/main-qimg-3a303d1f0dcee8a6ffef124a9f8aecb6). At low (landing) airspeeds, the amount of drag from flaps in the landing configuration is near its maximum.

    If you trim the aircraft for hands-off flight, then deploy landing flaps without touching the yoke or the throttle, the pitch of aircraft will change to keep the aircraft flying at same speed.

     – rbp Oct 12 '14 at 16:53
  • @rbp It's true that the induced drag drops with increased airspeed (it's actually inversely proportional to the square of the airspeed,) but it's not true that total drag is at a maximum at low speeds. The opposite is usually the case, as parasitic drag eventually dominates as airspeed increases and it is directly proportional to the square of the airspeed. In the graph you linked, note that both Dp (parasitic drag) and Dt (total drag) increase with airspeed after the point where Di equals Dp. – reirab Oct 26 '14 at 05:30
  • OK, well I prefer to land at or near 1.3Vso, not at Vne – rbp Oct 26 '14 at 12:13
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The purpose of trailing edge flaps and leading edge slats is to alter the wing profile in such a way that it generates more lift at low airspeeds, while lowering the stalling speed. This makes it possible for the aircraft to take off and land at lower speeds than would otherwise be necessary. From the design perspective the inclusion of flaps makes it possible to optimise the basic wing section for the best performance in cruising flight.

J. Southworth
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