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Thread: How I used to think airplane flaps worked- any scientfic basis?

  1. #1

    Default How I used to think airplane flaps worked- any scientfic basis?

    I've been playing flight sims for 30+ years and flaps to me were simply things to make the plane easier to land. I understand the purpose, to increase lift and drag resulting in a slower landing speed.

    The scientific explanation of how they worked though wasn't a big issue for me. A few days ago though I read about how they increase the camber of the wing as long as they're extended. Increased camber means more lift.

    However for many years I thought the way flaps worked was this- as they are extended they force the oncoming airflow to be be deflected downwards and according to Newton's Third Law "for every action there is an equal an opposite reaction". So basically I thought that the force of the air being deflected downwards acted in accordance with Newton's Third Law pushed the aircraft upwards, giving it lift. And the flaps themselves had a small speedbrake effect, slowing the plane.

    I suppose it "sort of" makes sense but is there any real logic behind my long held view of aircraft flaps or is time I wore a dunce cap?

  2. #2
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    Cause and effect can sometimes be confusing to the non-scientist/engineer. There is indeed deflection of the air from extended flaps, and there is indeed a "speedbrake effect" (usually known as drag) from flaps, just as you can get lift in a similar fashion with a barn door laid flat (or a paper airplane, or stick-your-hand-out-the-car-window) and tilted upward. But that barn door-style lift isn't very efficient and has a LOT of drag, so the airfoil shape was developed (modeled after birds) to get the effect with less drag and weight, and more predictably.

    Today the camber model seems to be preferred, both for wing lift and for flaps, and it is certainly accurate, as far as it goes. The real question (to me) is which model is more accurate, or what interaction there is between the two models or what is cause and what is effect? In regular lift (wing only, no flaps) air is thrown downward also, thus using Newton's third law too. Bernoulli is thrown in there, too.

    So if the "3rd law" way of looking at it works for you while simming, have at it. But note that there is more and more of a push in real world aviation to reduce/eliminate LOC* (Loss Of Control) crashes, partly by educating pilots in the aerodynamics of how these LOC things occur, to a much greater extent than has been the case in past training. So more theory (and a different way of practicing stalls at altitude) is likely on its way for future (and recurrent) training.

    I'm going to suggest that you read the book Stick And Rudder by Wolfgang Langewiesche, an excellent book written from the pilot's viewpoint of how things work, rather than the engineer's viewpoint.



    * LOC: These are often stalls in the traffic pattern, usually after overshooting final, for example, and trying to turn more sharply without increasing the bank angle, thus causing a skidding turn, something that can quickly become a stall/spin without sufficient altitude for recovery.

    Larry N.

    As Skylab would say:
    Remember: Aviation is NOT an exact Science!

  3. Default

    Here's what I know about flaps, and this is my basic definition. LOL

    When your angle of attack increases, your flaps correct that so you don't stall. At the same time the flaps WILL add drag.

    I have also learned that you should NEVER add flaps and speed brakes at the same time. Doing so means you've created an aerodynamic rock.

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    When your angle of attack increases, your flaps correct that so you don't stall...
    Sorry, that's not quite right -- lets not confuse things. The flaps don't "correct that," rather, as cel70 mentions "they increase the camber of the wing," which does increase the lift (and drag). The angle of attack may get reduced as a by product, to some degree or other, but that reduction isn't what causes the lift and drag changes. Part of the increased drag is from the increased lift (induced drag), and part of it is pure parasite drag.

    How much lift and drag is dependent on the specific design, as well as how much the flaps are deployed. Split flaps, for example, aren't generally as effective at generating lift as Fowler flaps. On the typical single engine Cessna, those big barn doors mostly increase lift for the first and second notches (up to 20º or so in other words), while the third (and on older ones 4th) notch(es) (30º and 40º) add mostly drag. The flaps on most of the Grumman American singles don't do a lot, by comparison, adding only a small amount of both lift and drag. Most other aircraft are somewhere in between those two.

    The Fowler flaps on most jet airliners (that I know of) are similar, in most ways, to those of the Cessnas, adding mostly lift with the initial extensions (where they extend mostly back) and then a lot of drag with further extensions (as they start to angle downward as well as backwards).

    I've only scratched the surface here, but this Wikipedia article will explain a lot more, including discussing flap types I didn't mention, even getting into the equations should you care that much.

    Larry N.

    As Skylab would say:
    Remember: Aviation is NOT an exact Science!

  5. Default

    Quote Originally Posted by lnuss View Post

    So if the "3rd law" way of looking at it works for you while simming, have at it. ---.
    If that was all there was to it, we would be waterskiing, not flying.

    Oncoming air in the subsonic regime sees the airfoil general camber quite a distance in front of the airplane and starts to deviate in a smooth pattern to pass both above and below the wing. How it diverges is dictated by what it perceives as it approaches, including flap deflection, etc.

    Increased lift at a positive Angle of Attack increases induced drag, and flap deflection increases parasite drag due to increased camber profile. Inumerable wind tunnel experiments show this.

    This "dirty drag combination" is especially helpful in jet aircraft because you have to carry higher power settings, which improves small power change response times for any jet engine.
    Last edited by mikeandpatty; 10-25-2018 at 09:27 PM.

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    If that was all there was to it, we would be waterskiing, not flying.
    Read again what I said. After a more proper explanation, I suggested that, for simming, if that mental image works for him then go for it. I was NOT stating that it was a correct explanation, nor that you should use it for real world flying.

    That drag reduction is, of course, useful in jets, but it's also useful in lighter aircraft for an increased approach angle, often a major help in clearing obstacles, among other things.

    There's lots more that could be said, but I'll let the Wiki article I linked above say a lot of it.

    Larry N.

    As Skylab would say:
    Remember: Aviation is NOT an exact Science!

  7. Default

    Quote Originally Posted by lnuss View Post
    Sorry, that's not quite right -- lets not confuse things. The flaps don't "correct that," rather, as cel70 mentions "they increase the camber of the wing," which does increase the lift (and drag). The angle of attack may get reduced as a by product, to some degree or other, but that reduction isn't what causes the lift and drag changes. Part of the increased drag is from the increased lift (induced drag), and part of it is pure parasite drag.

    How much lift and drag is dependent on the specific design, as well as how much the flaps are deployed. Split flaps, for example, aren't generally as effective at generating lift as Fowler flaps. On the typical single engine Cessna, those big barn doors mostly increase lift for the first and second notches (up to 20º or so in other words), while the third (and on older ones 4th) notch(es) (30º and 40º) add mostly drag. The flaps on most of the Grumman American singles don't do a lot, by comparison, adding only a small amount of both lift and drag. Most other aircraft are somewhere in between those two.

    The Fowler flaps on most jet airliners (that I know of) are similar, in most ways, to those of the Cessnas, adding mostly lift with the initial extensions (where they extend mostly back) and then a lot of drag with further extensions (as they start to angle downward as well as backwards).

    I've only scratched the surface here, but this Wikipedia article will explain a lot more, including discussing flap types I didn't mention, even getting into the equations should you care that much.
    LOL


    Just admit that when you add a flap your AoA goes up from where it was on speed reduction. The technicalities are not really necessary when you just want to fly the aircraft. All I know is that when I'm happily flying along in say my F-22 and I start reducing speed I see my AoA increase, and while I don't know what the flap schedule is for the F-22, when I see the AoA go to about 2.7-3.0 I add a flap and then my AoA goes back to normal. I continue the process all the way down to my last flap which takes me down to about 130 knots depending on fuel load. I probably could lower the speed more, but the AoA increases exponentially even though the F-22 has a high angle of attack. I chose to land my plane with an AoA of about 2.2. And looking outside at the aircraft I see that is about right looking at its profile on landing. Well, in the Sim at least...

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  8. #8
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    Just admit that when you add a flap your AoA goes up from where it was on speed reduction.
    Did you consider that this all came up because of a question asked by the OP, not because of your opinion? You didn't answer his question -- hopefully the rest of us did.

    Larry N.

    As Skylab would say:
    Remember: Aviation is NOT an exact Science!

  9. Default

    Quote Originally Posted by lnuss View Post
    Did you consider that this all came up because of a question asked by the OP, not because of your opinion? You didn't answer his question -- hopefully the rest of us did.
    So I can't respond to you after you quoted me?

    My forum project. Click me
    OOM errors? Read this.
    Pick an Amazon wish list, help a child out
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  10. Default

    Quote Originally Posted by Cel70 View Post
    I've been playing flight sims for 30+ years and flaps to me were simply things to make the plane easier to land. I understand the purpose, to increase lift and drag resulting in a slower landing speed.

    The scientific explanation of how they worked though wasn't a big issue for me. A few days ago though I read about how they increase the camber of the wing as long as they're extended. Increased camber means more lift.

    However for many years I thought the way flaps worked was this- as they are extended they force the oncoming airflow to be be deflected downwards and according to Newton's Third Law "for every action there is an equal an opposite reaction". So basically I thought that the force of the air being deflected downwards acted in accordance with Newton's Third Law pushed the aircraft upwards, giving it lift. And the flaps themselves had a small speedbrake effect, slowing the plane.

    I suppose it "sort of" makes sense but is there any real logic behind my long held view of aircraft flaps or is time I wore a dunce cap?
    Cel70 Alas Newtons laws have little directly to with what flaps do. Flaps are technically LIFT augmentation devices. Flap extension or deployment changes the shape of the wing (camber) and changes the lift/drag ratio, you get more lift but you get more drag. The key point is that it changes the centre of pressure of the wing (the point where lift acts upwards) moving it back and giving you a slight nose down pitch as a result. Add more flap same changes occur again. The increased lift reduces the stall speed of the aircraft hence you can fly slower and land at a slower speed. The change in the camber of the wing delays the separation of airflow at the rear of the wing (the turbulent part after about 30% of the leading edge. Really flaps are just a clever way of changing the shape and size of the wing to give you more lift for a given weight and speed as a bonus the pitch change that results gives you a lower nose attitude and hence better view forward on landing. Different flaps do it the same way.

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