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

  1. #11
    Join Date
    Mar 2005
    Westminster, CO


    So I can't respond to you after you quoted me?
    Of course you can. I was trying to point out that you didn't really answer the OP's question.

    Just admit that when you add a flap your AoA goes up from where it was on speed reduction.
    But I can't "admit" that. AoA does NOT equal lift, in and of itself. Adding flaps will actually lower the AoA. It actually "redesigns" the wing, adding camber and, in many cases, wing area as well. Note that a cambered wing (which most are these days -- symmetrical airfoils are different) produces lift even at a 0º AoA.

    What you've deduced from years of simming isn't necessarily correct when applied to real world flying, and can even get you in trouble from a wrong understanding of aerodynamics.

    The technicalities are not really necessary when you just want to fly the aircraft.
    Actually, some of the "technicalities" ARE necessary when you fly real aircraft, because an understanding of what's going on can, under certain circumstances, save your life because they allow you to do the right thing in certain unusual circumstances. In simming there are no consequences for poor flying or making a serious mistake. In the real world, as you certainly should know (and this is what the OP wanted to know about), consequences can range from minor damage to an aircraft up to loss of life.

    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,
    That isn't the same as adding flaps. Certainly as you reduce speed, all else being equal, the AoA will increase, but adding flaps means that all is no longer equal, since you've "redesigned" the wing by changing its camber (reshaping the airfoil, in effect).

    As an example, in a Cessna 150 (or 172 or most other aircraft) the white arc (allowed flap deployment speed range) covers a portion of the speed range where flaps are not required, so you can slow from cruise to 70 mph (let's assume we're holding constant altitude) with the AoA increasing as you slow. Fly for a little bit at 70 mph, then ease in some flaps, lowering the nose (thus reducing AoA) and adding power (to overcome the drag) to maintain 70 mph. You're now at a lower AoA. Now add more flaps, requiring you to lower the nose some more and add more power to maintain 70 mph and the same altitude. Your AoA is lower yet.

    Let's look at a case that has killed people: Someone makes a low pass down the runway at high speed, then abruptly pulls back too hard on the stick, causing a stall at high speed, well above the published stall speed (which is only accurate on slow deceleration, not when pulling Gs), and crashing because there was insufficient altitude to recover. A proper understanding of the aerodynamics involved would have allowed one to avoid doing that.

    Here's a case where understanding saved MY life. I was doing some mild aerobatics in a Stearman some years back, and in one of the loops, while pulling out from the backside of the loop, my pitch attitude suddenly wasn't changing -- I was aimed about a 30º angle from straight down -- I immediately knew that I needed to relax my back pressure on the stick, and did so, whereupon the pitch started increasing again. I knew I needed to relax the back pressure because I understood that you can stall an aircraft at any speed and at any attitude when you add G-loading, and was able to recognize that this was a stall, thus applying the remedy. If I'd continued to hold that back pressure, mistakenly thinking that was what was needed to recover, then I wouldn't be writing this.

    There have been other times, too.

    Larry N.

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

  2. Default Not into the newtonian thing

    Quote Originally Posted by Cel70 View Post
    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".
    When it comes to flying, I certainly wouldn't rely on, or even refer to anything pertaining to the alchemist, Isaac Newton. Certainly when it comes to aerodynamics. As far as I'm concerned your third law does nothing but impose some bizarre pseudo-science on what should be and is described as something as a kid flying a kite. Of course you have drag introduced as well when you do increase (flaps1,.. flaps2) the wing area: As per most aerodynamic sites, excluding the whole Bernoulli thing: "The pressure distribution acts locally, perpendicular (normal) to the airfoil surface. The shear distribution acts locally parallel to the airfoil surface.
    Taking the local pressure contribution at each point along the surface and adding each contribution together (integration) results in a net pressure force acting on the airfoil. Similarly, adding the shear contribution along the airfoil surface results in a net shear force."

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