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  1. #1
    Balsacutter's Avatar
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    Would Slotted Flaps Really Work?

    I've always been intrigued by the idea of adding powerful flaps to sport planes, to expand the flight envelope and to have something more to play with.

    The late Andy Lennon created some bold designs that promoted slotted flaps (and leading-edge slats) for reducing the landing speed of his small, heavy, high-wing-loading designs.

    But at the modest chord dimensions and low landing airspeeds that we run, would a model-sized slotted flap actually work as expected and generate a lot of extra lift? Will the airflow stay attached to the upper surface of a model flap at deflections of 20% or more? Or are we really just sticking a fancy flat plate out into the wind like a 3D plane's elevator?

    A big increase in the CL at landing would be fun, coming in steep and slow like a hawk. But slotted flaps would be hard to build, especially if you also try to move the surface backwards like a fowler flap. If the slot isn't really going to work, then it would be a lot easier to just hinge a big barn-door surface to the rear of the wing inboard, keep the thrust up through the approach and be happy...

    Anyone have a feel for how well a complex airfoil like a slot would actually work at model dimensions?


  2. #2
    Moderator da Rock's Avatar
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    RE: Would Slotted Flaps Really Work?

    Just look at how any model with light wing loading lands, and you've got your answer. You can land lots of light models at almost walking pace even without flaps. Some of the foam, rtf trainers can be "hovered in" in mild breezes.

    Also, what Andy Lennon wrote about really did happen, so you've got your answer there too.

    Thousands of gliders have used simple flaps for descent control out of thermals and for spot landing control. The flaps work so well, it's really not much of a trick to bring your glider right up to your hand. You simply reach out and take it out of the air. Another answer there.

    Would they work? They have, no reason they won't for you, so the real question is what are you waiting for if you want to have fun yourself.
    Good flying wit ya today

  3. #3
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    RE: Would Slotted Flaps Really Work?

    Da Rock, I see no reason for not to believe Lennon's statements in his book regarding increment of CL (almost twice) by slotted flaps in models.

    The advertisement of this model of him that is offered as a kit currently claims the following:

    (Copied from http://www.mosquitobiteplanes.com/robin.html)

    "The wings feature large slotted flaps that cover 65% of its trailing edge and when extended 40 degrees, virtually double the lift of that 65% of wing area. The wing's airfoil is the Eppler E197. It's aspect ratio (ratio of span to chord) is 6. It has low profile drag and a gentle stall at 20 degrees AOA out of ground effect. Its zero lift AOA is minus 2 degrees."


    Even when the relation inertial forces / viscous forces of the air is less for the dimensions and velocities of our models, the principles behind the slotted flaps and LE slats remain the same:

    A nozzle (slot or slat) accelerates the flow over a surface (wing or flap), increasing the Coanda effect of the air stream and delaying the de-attachment; hence, enhancing the CL of that surface.

    The key question is how complicated a mechanism you want to try for how much lift increment.

    The slot flap is a compromise between plain flaps (minimum effect) and double slotted Fowler flaps (max effect).
    Lnewqban - "God will not look you over for medals, degrees or diplomas, but for scars. He has achieved success who has worked well, laughed often, and loved much." - Elbert Hubbard

  4. #4
    rmh's Avatar
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    RE: Would Slotted Flaps Really Work?

    At what size and at what wing loading is all this data of practical use?
    If anyone knows from actually using these devices.
    Libby is still watching you

  5. #5
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    RE: Would Slotted Flaps Really Work?

    I know they work at 3600 pounds and 100 kts...

    well.. at least they do on a Grey Eagle UAV!!

    lol
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    rmh's Avatar
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    RE: Would Slotted Flaps Really Work?

    How about at 3.6 lbs and 10 knots?
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    RE: Would Slotted Flaps Really Work?

    I've used slotted flaps. Still do. And Miles' all-flying flaps, too.

    They work great. The Miles' type are easy to retro-fit.

    I've used them on models from 6ft span to around 8ft span, without problems.

    Quite possibly the most fun you can have with your clothes on. They can lead to other "problems", though, such as a lack of control at the very low airspeeds which are sometimes possible; easy enough to just fly a little faster, of course.




    I'll see whether I can find a photo.

  8. #8
    Lnewqban's Avatar
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    RE: Would Slotted Flaps Really Work?

    ORIGINAL: rmh

    How about at 3.6 lbs and 10 knots?

    Since lift is proportional to CL and to V^2, if the flap increases CL 1.5 times, the landing and take-off velocities can be reduced to 0.8 respect to non-flap wing.

    ......and that works like that even for ultralight and overpowered foamies.
    Lnewqban - "God will not look you over for medals, degrees or diplomas, but for scars. He has achieved success who has worked well, laughed often, and loved much." - Elbert Hubbard

  9. #9
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    RE: Would Slotted Flaps Really Work?

    Some of this depends on the size of the flaps and how much of the wing area they affect.

    There's no doubt that flaps for models work. But the question is about how well Fowler or other slotted style flaps work and if they can be optimized to work at "our" size. And if so can they provide more lift without a big rise in associated drag. The answer to these questions is "yes... but". The "but" part being that I don't know of anyone who has played with slotted flaps in our size range in either suitable CFD environments or for real in a slow speed wind tunnel.

    Certainly Selig and the others that have partnered with him over the various actual wind tunnel projects have worked with flapped airfoils at various times. But those were all done with simple hinged "plain" flaps. Slotting opens up a whole other side to this.

    I will suggest that it's not realistic to expect fully attached flow over the whole range of travel. Even the big airliners that use Fowler or multi element slotted flaps only use about 10 to 20 degrees of deployment for takeoff.

    Light or heavy wing loadings flaps can certainly help a lot. Keep in mind that allowing a sizeable increase in the lift coefficient isn't a flap's only contribution. Flaps also "twist" the wing to produce an extreme amount of washout. This all but eliminates the possibility of a tip stall and resulting snap roll unless the pilot really doesn't have a clue or wants to deliberately force the stall. This second side effect can be almost as valuable as the extra lift the portion with the flaps is able to produce.
    Witty saying to be plagarized shortly.....

  10. #10
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    RE: Would Slotted Flaps Really Work?

    ORIGINAL: Lnewqban

    ORIGINAL: rmh

    How about at 3.6 lbs and 10 knots?

    Since lift is proportional to CL and to V^2, if the flap increases CL 1.5 times, the landing and take-off velocities can be reduced to 0.8 respect to non-flap wing.

    ......and that works like that even for ultralight and overpowered foamies.
    Well maybe and I saw your "IF".
    The typical really light model uses an almost OR dead flat wing -
    Our newest outdoor injected molded foam model uses an airfoil that is about 5% in thickness - .
    our light stuff is about 2-3% in thickness.
    The benifit of thicker wings is to allow a larger margin of flyability at higher AOA- AT HIGHER LOADINGS.
    So- the increase of 1.5 using flaps simply does not apply to these shapes at the lower loadings
    IF the wing was an old fashioned fatty of 15%- yeh -I can see how the flap would offer a big increase.
    Our "overpowered" models can be brought to a extemely high AOA under power and litterally dropped into a landing
    Done it many times
    Do yo ufly any of this type model?
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  11. #11
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    RE: Would Slotted Flaps Really Work?

    No, I don't, Dick.

    I don't really know how well flaps would work for extremely light wing-load's (that is why the in my post; just to respectfully tease you).

    Could you explain more about how the thickness of the airfoil interferes with the flaps effectiveness?

    I cannot see that, but I can see that light wing-load's mean wing's low pressure differential, which defeats the logic behind the slotted flap (poor nozzle effect at the flap's LE).
    Lnewqban - "God will not look you over for medals, degrees or diplomas, but for scars. He has achieved success who has worked well, laughed often, and loved much." - Elbert Hubbard

  12. #12
    rmh's Avatar
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    RE: Would Slotted Flaps Really Work?

    More relative thickness of the basic wing REDUCES lift-which makes the need for flaps to obtain high lift (and drag)
    Ex: Start with an shape which is simply round - If it is not rotating- it simply will not lift at any angle of attack
    now flatten it some - we get some lift at any + angle of attack -and AOA is not critical
    flatten it some more, etc.. the critical nature goes up as the fineness ratio goes up. OK?
    Now when we get to a very flat airfoil - we can generate huge lift at very low deflection (aoa). the drag also goes up .
    The craft will slow very well and until critical angle occurs - It acts like a flapped, thicker foil. The thicker airfoils don't interfere, they just don't contribute as much lift at low AOA as a very thin one
    Does this make sense to you?
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    RE: Would Slotted Flaps Really Work?

    So, if I made a wing which is paper-thin, it'd be a great lift-generator?

    That is an interesting idea, which you have proposed; explained in that way.

    I'm going to have to dip into Profili and check out some numbers.

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    RE: Would Slotted Flaps Really Work?


    ORIGINAL: rmh

    How about at 3.6 lbs and 10 knots?
    EXACTLY!!! at the lower q... I dunno how aggressive the curvature over the flap leading edge could be....

    Prolly some vortex generators on the leading edge element (of the flap) might be useful.....



    Camber has a pretty significant effect on lift.... a couple percent difference on camber is prolly more significant than a
    couple percent difference in thickness.... Leading edge radius may be as relavent as total thickness (within a reasonable range)
    And at low Rn, (50k to 100k), high curvature on the aft region of the suction side of the wing is a killer....

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    rmh's Avatar
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    RE: Would Slotted Flaps Really Work?

    ORIGINAL: bogbeagle

    So, if I made a wing which is paper-thin, it'd be a great lift-generator?

    That is an interesting idea, which you have proposed; explained in that way.

    I'm going to have to dip into Profili and check out some numbers.
    Profili likely won't show anything helpful
    When I started messing with flat wings I found NO info relevant to extreme low wing loading
    The reason wings are shaped as we typically see on full scale lightplanes and older designs -is the material and power available, dictated a wing thick enough to tolerate faily high AOA and also b strong enough
    crank the calendar ahead fifty plus years and we see super light super strong materials and power plants which are also ligher and more powerful
    It is only logical that the aircraft designs which take advantage of the newpower and material- will have different lifting surface needs
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  16. #16
    Moderator BMatthews's Avatar
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    RE: Would Slotted Flaps Really Work?

    Welllllll..... thin is in for low reynolds numbers. But it tends to fail with higher speeds, bigger wing sections and higher wing loadings. What works for a bumble bee isn't what works for a fancy glassfiber sailplane and it's way out of the league for a Jumbo Jet. And then there's that pesky need for the wing to be thick enough to allow us to install wing spars of sufficient strength.

    The closest we see to thin wings for an all around plane is in the fairly thin sections used on modern jet fighters. The wide chord allows a thin section that still can carry enough spar depth. The interesting thing with these wings is that the lack of high lift is so recognized that when the pilot pulls some amount of positive G that the wing responds by increasing the camber to a value intended to give the best lift with least drag. This means that the leading edge, flap and even ailerons all droop to give the airfoil a rather ugly shape that has more camber than usual for level flying. It's a horid looking segmented shape but it still works better than the thin and rather flat section.

    They even flex to invert the camber for negative G loads.
    Witty saying to be plagarized shortly.....

  17. #17
    rmh's Avatar
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    RE: Would Slotted Flaps Really Work?

    Yes -different tasks=different requirements
    for example the old rule about bankingfor a turn- on our low wingloading stuff with large fuselage side area - the planes turn smoothly perfectly in a dead flat attutide.
    No magic
    The fuselage assumes the load of the turn
    When I was learning to fly RC I had these horrid scale examples of full scale craft that flew badly-were hard to control at low speeds BUT looked like the full scale counterpart.
    The weights and associated problems were simply looked on as being part of learning to fly.
    The criteria for a really good flying small model are simply not the same as full scale - .
    Whe we finally "got it", our models changed completely.
    IF you are into scaling down airfoils in an effort to duplicate flying characteristics- go for it
    However, It does not work
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  18. #18

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    RE: Would Slotted Flaps Really Work?


    ORIGINAL: Balsacutter

    I've always been intrigued by the idea of adding powerful flaps to sport planes, to expand the flight envelope and to have something more to play with.

    Anyone have a feel for how well a complex airfoil like a slot would actually work at model dimensions
    Yes, slotted flaps would really work at and scale that you can build accurately. And that last part is the critical issue. The workings of the slotted flap are easy to understand, but the particulars of the shape of the leading edge of the flap and the trailing edge/slot area of the wing are critical to the lifting ability.

    This has been covered multiple times in the past, and I find that as models get smaller, perhaps some of the difficulty is in holding the dimensions required for the flaps to be effective. I'm saying that at small sizes, even small errors in building the flaps will negate the improvements. I find the folks that have problems getting the flaps to work either don't hold the right tolerances or don't have a good idea of the needed geometry in the first place.

    THe flaps are easiest to build using the offset-hingeline method, and I have used robert hinge points to great effect on the following planes:

    Olympic 2 glider: having used plain flaps, I rebuilt the wing with 2" chord slotted flaps. The slot lip was also hinged to be a spoiler. The flaps could be deployed about 30 deg for a hi-start launch and would get maybe another 100 feet on launch. THe flaps could be hyper-extended to 90 deg, which opened up the slot to a giant gap, turning the flap into a purely high drag device for the landing approach. The slot-lip spoilers could be raised to produce even more drag. Since a higher angle approach tends to bemore precise, that setup was very usefull. Thats as small and slow as I've seen the flaps work.

    I had a GP Trainer 60. Modified that with slotted flaps along the inner 8" next to the fuselage. At 30 deg, it made the takeoff short and smooth. At 60 deg, with about 30% power the plane would do tight lazy-eights within the infield in front of the pilot boxes. Because the flaps were inboard in the prop-stream, they had a blown effect that caused even higher gains in lift with application of power. With one click of power above idle, and 60 deg flaps, the plane would land at a walk and roll maybe 30 feet on smooth asphalt.

    Had a GP Cessna that had the slotted flaps in the scale location as well. This plane was so slick it needed some amount of flaps to get on the ground with the engine still running. It behaved about like the trainer 60, but had even more power coupling with the flaps. At 60 deg flaps, if I went to full power like a touch-n-go, full flaps would require a lot of forward stick to prevent a back loop. But otherwise the flaps were a great use.
    John


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