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Flaps increase lift during landing and T/O. But when retracted, they do nothing. The space needed to stow the common fowler flaps can't be used for anything else - fuel or structure. Extended flaps are thin, making them need more material to maintain stiffness. Moreover, they transfer their loads through the wing, instead of directly to the fuselage.

So, why do we use flaps instead of just scaling the clean wing up?

  • Is it because of concerns over cockpit visibility during landing? Can't we use video cameras do display the bottom during high AoA?

  • Is it because the extra lift - and drag - is unwanted? We can simply increase the cruising altitude without changing the cruise speed.

  • Is it because (for fowlers) the chord reduction really is that beneficial? The larger wing volume for fuel leaves more space in the fuselage for everything else. And since the span does not change, induced drag does not increase.

  • Is it because slotted airfoil, as in many modern flaps, are not good for cruise? Surely we can add slots to the airfoil, and add mechanisms to seal them for cruise.

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    $\begingroup$ related: For a large commercial plane on landing, does the L/D ratio increase, decrease, or not change much?, this answer, and this answer (this last one also include speed polars for a specifi aircraft at different flaps settings). $\endgroup$ – Manu H May 6 at 11:09
  • $\begingroup$ Another closely related, if not a dupe: Why and when to use flaps? $\endgroup$ – Manu H May 6 at 11:11
  • $\begingroup$ "Close the slots for cruise" would take additional, heavy mechanisms that would need to be dragged along for the entire flight and take up space in the wing that couldn't be used for anything else. i.e. the same issues you complain about for flaps, therefore, even if lighter and requiring less space than flap-retract mechanisms, not that much of an improvement over the current situation of retractable flaps. $\endgroup$ – FreeMan May 6 at 12:11
  • $\begingroup$ "even if lighter and requiring less space than flap-retract mechanisms" That is my point. Besides, we don't seem to be designing A/C's so much with simplicity as efficiency in mind. $\endgroup$ – Abdullah May 6 at 12:30
  • $\begingroup$ This question completely overlooks the effect of flaps on the drag coefficient. $\endgroup$ – quiet flyer May 6 at 12:32
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Your concerns about heavy flaps are well founded. The designers try to get away with as few high-lift devices as they can afford to. But not fewer!

If you observe the trend over the years, flaps became more complex with every new airliner generation, starting from simple split flaps in the 1930s to triple-slotted flaps on the Boeing 747 in the late Sixties. But then things reversed a bit. Now double-slotted flaps are standard and lighter versions of the same type (think A318 versus A321) get away with simpler flaps.

One reason is wing tank volume. In order to cross the Atlantic, the first generation of jets needed large wing tanks which were made possible by a high wing area. Simple, single-slotted flaps were sufficient for the desired landing speeds. With the much lower fuel consumption of high-bypass engines we now can afford smaller wings with less chord, but now the flaps have to make up for what is lost in area. High-lift devices are a major part of the aircraft development effort and a lot of work goes into reducing the complexity of flaps and slats. The variable-camber Krüger flaps of the 747 are great, but were never repeated on newer designs.

Even simpler wings would be possible if the cruising altitude were higher. But there is not much benefit from climbing above the tropopause (except for strategic bombers, but their development effectively ceased half a century ago), so that is where the installed thrust is optimized for. If you want to fly higher, you need larger and more expensive engines but gain little in cruise efficiency.

And to not retract the flaps is not an option. The larger area means that gusts can potentially put larger loads on the wing and the increased surface area would cause more friction drag. Reducing wing area saves fuel, even though the wing becomes heavier. On top, a heavily cambered wing would be completely unsuitable for transsonic flight.

Flaps have become much thinner in the last half century, and for good reasons. Yes, you need a complicated load path through flap rails and into the main wing, but that is where the stiffness is for carrying large loads. That should not be duplicated in order to keep structural mass low! The effort to reduce flap complexity has led to ever thinner flaps, and the development of transsonic airfoils with their high rear camber has allowed to put more camber on the flaps as well which improves their effectiveness. Note that the fairing of the flap tracks is used for area ruling and helps to limit the transsonic drag increase.

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When flaps are retracted they do nothing, which is the whole point. The byproduct of lift is drag, a larger wing will create more lift, but more drag as well. More drag equals a slower cruising speed, or bigger engines to power past the drag along with higher fuel consumption. Flaps let airplanes cruise faster by getting out of the way.

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    $\begingroup$ Great @ABJX, so you cruise slowly at a higher altitude, which it takes longer to climb to and descend from. As a regular passenger on commercial flights I don't call that a win. On a light aircraft a higher cruising altitude has no benefits, I can already cruise high enough to need an oxygen system. $\endgroup$ – GdD May 6 at 9:57
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    $\begingroup$ You are assuming higher altitude is a good thing @ABJX. I spend my flying time in a light aircraft below 5000ft, I can't fly higher for the most part because of controlled airspace above. I wouldn't want to trade no flaps for a slower cruising speed. $\endgroup$ – GdD May 6 at 10:10
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    $\begingroup$ Uh, no @ABJX, it doesn't work that way. Higher drag at lower altitudes means higher drag at higher altitudes, you aren't going to achieve the same cruising speed just by going higher. Yes, air resistance is lower but you still have to move through it. $\endgroup$ – GdD May 6 at 13:45
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    $\begingroup$ @ABJX: "Higher alt = less air" = having to pressurize your airplane & turbocharge your engines, and spend more time climbing to cruise altitudes. At which your jet engines may not even work... $\endgroup$ – jamesqf May 6 at 16:39
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    $\begingroup$ @ABJX: I don't happen to own or fly a #%@! jetliner. I'm not sure that my Cherokee would even climb with full flaps, or a wing equivalent to them. $\endgroup$ – jamesqf May 7 at 2:57
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  • Climb to cruise burns fuel.
  • Adding additional drag burns fuel.
  • Adding retractable mechanisms adds weight that burns fuel.
  • More drag, even at higher cruise altitudes, requires larger engines for the same cruise speed. Larger engines burn more fuel (despite increases in modern engine fuel efficiency).
  • Retracting high-lift, high drag devices reduces fuel burn (even though they add weight, thus drag which burns fuel).
  • Carrying the fuel necessary to carry that additional fuel burns fuel (several questions here about that, I invite you to look to see just how much it costs).
  • Increased fuel capacity reduces paying cargo (both boxed and self-loading) capacity.

Just like everything else in aircraft design, there's a trade off between the full-time lift/drag of a high-lift wing vs the weight/complexity of retractable high-lift devices on an otherwise low-lift wing.

Designers have decided that the reduction of fuel load in the wing and the additional weight and complexity of retractable flaps and slats to generate the lift necessary for safe and sane take off and landing speeds and runway length is a better bet than adding additional lift and drag, but additional fuel capacity, by designing a higher-lift wing.

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  • $\begingroup$ "More drag, even at higher cruise altitudes" what? $\endgroup$ – Abdullah May 6 at 12:34
  • $\begingroup$ @ABJX a fixed high-lift wing will generate more drag (even at a high altitude) than a flaps & slats wing. Guess that wasn't clear, edit coming. $\endgroup$ – FreeMan May 6 at 12:37
  • $\begingroup$ @ABJX however, given any 2 aircraft at the same cruise altitude, the one with the higher Cd will require more power for the same speed, will it not? So, if you have a craft with a fixed, high-lift wing 5 miles ahead of, and at the same altitude as, a craft with a low-lift wing, but otherwise identical, the fixed high-lift winged craft will require more energy to maintain the cruise speed and stay ahead. $\endgroup$ – FreeMan May 6 at 12:40
  • $\begingroup$ Aha, I get your problem. "a fixed high-lift wing" that is, "a wing with non-retractable slats and flaps" would certainly not work. But that is not what I was talking about. I was talking about eliminating the flaps and scaling the airfoil to compensate. $\endgroup$ – Abdullah May 6 at 12:42
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    $\begingroup$ As so often, the best answer languishes at the bottom. +1. $\endgroup$ – Peter Kämpf May 6 at 20:18
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The questioner seems to have noted that the basic wing with flaps retracted provides a high ratio of L/D (or Cl/Cd). Where L denotes lift, Cl denotes lift coefficient, D denotes drag, etc.

We can certainly scale up the basic unflapped wing to provide as low a landing speed as we wish, although landing will be tricky due to the flat glide path. Flaps help with landings by increasing the drag coefficient as well as the lift coefficient, making the glide path steeper.

The main problem with this approach is that for cruising flight, not only do we wish to achieve a high L/D ratio, we wish to achieve it at a high airspeed. Lift is proportional to lift coefficient times airspeed squared, and in cruising flight, lift cannot be larger than weight. If the wing is too large, it will be optimized to deliver its peak L/D ratio at a much lower airspeed than we wish to cruise at. In such a case, at our intended cruise speed, if we increased the angle-of-attack to the max L/D angle-of-attack, the wing would be making way too much lift, and we'd pitch up into the start of a loop. To keep the flight path level, we'd have to trim for an angle-of-attack far to the right side of the peak L/D ratio, as portrayed on the polar curve of L/D ratio versus airspeed. In other words, we'd have to trim to an angle-of-attack much lower than the angle-of-attack that delivers the max L/D ratio. We'd end up with more drag than we'd have if the wing were smaller.

The situation is not unlike that of a glider pilot wishing to achieve a flat glide at a high airspeed. The glider gets a lower sink rate and a better glide ratio at that high airspeed when the wing loading is high than when the wing loading is low, because the wing may be operated at the angle-of-attack that yields the best L/D ratio, rather than at some much lower angle-of-attack. So water ballast is carried.

In powered flight, the equations are a bit different, and simply adding weight to the aircraft never improves high-speed cruising performance. But if the wing were designed to be large enough to give an acceptably low landing speed without using flaps, then scaling down that wing to a smaller size certainly would improve high-speed cruising performance. And that's why it's worth carrying around the weight and internal volume of a complicated flap system-- because we can make the wing smaller, so that when the flaps are retracted, it is optimized to deliver its peak L/D ratio at a high airspeed.

The basic thrust of this answer remains the same regardless of whether we are trying to achieve a low landing speed by scaling up the wing in all dimensions, or by only increasing the chord. In general, a high peak L/D ratio is associated with a high aspect ratio, and therefore a small wing chord. However, if in cruising flight we know we must fly our scaled-up wing at some airspeed that is much higher than its maximum L/D airspeed, it's possible that we'll have a better L/D ratio at that high airspeed if we've scaled up the wing by expanding the chord alone than by expanding all dimensions. Because the curve of L/D versus airspeed may be less "peaky" with the lower aspect ratio than with the higher aspect ratio. But the better solution is to keep the high aspect ratio, and keep the wing small enough so that it can actually be flown at its maximum L/D ratio at the intended cruising speed. Then we "scale up" the wing for landing by extending the flaps.

Of course, extending the flaps does much more than just "scale up" the wing. At full extension, the designer's goal is to minimize the stall speed, so maximizing the lift coefficient is the priority. He or she is free to choose a configuration that maximizes the lift coefficient, with no concern for minimizing the drag coefficient to maximize the L/D ratio. As noted above, an increase in drag is actually helpful during final approach-- it is easier to guide the aircraft to the intended touchdown zone if the power-off glide path is not too flat. (And no, despite the questioner's suggestion, we cannot eliminate the extra drag created by the extended flaps in cruising flight simply by "closing the slots".) The flaps are designed purely to make the wing's lift coefficient as high as possible, while the airfoil of the clean wing is designed to optimize the ratio of L/D or Cl/Cd. Therefore, to achieve the same low stall speed simply by scaling up the unflapped wing -- either chordwise, or in all dimensions -- would require a much greater increase in wing area than the area that is actually added by the deployed flaps.

Related --

(Q) Why would a glider have water ballast? If it is trying to stay aloft without an engine, wouldn't it be better to be as light as possible?

(A) For a large commercial plane on landing, does the L/D ratio increase, decrease, or not change much?

(A) For a large commercial plane on landing, does the L/D ratio increase, decrease, or not change much?

(A) Why and when to use flaps?

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  • $\begingroup$ Ohhhhh sorry. I am learning not just aviation, but how to ask questions. " we cannot eliminate the extra drag created by the extended flaps in cruising flight simply by "closing the slots".)" I'm gonna try to clarify more in the next edit. By the way, I see that even this answer ignored the option of higher cruising altitude. $\endgroup$ – Abdullah May 6 at 18:07
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    $\begingroup$ @ABJX -- "By the way, I see that even this answer ignored the option of higher cruising altitude." -- yes, for once, I guess I figured that one of my answers had already gotten long enough. $\endgroup$ – quiet flyer May 6 at 18:13
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    $\begingroup$ @ABJX If you expect a higher cruising altitude to reduce drag then a higher cruising altitude will get a bigger drag reduction if you add flaps. Then you say "higher" to which the flapped wing can also go higher and have less drag. Continue until you reach service ceiling $\endgroup$ – slebetman May 6 at 22:27

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