Flapper thread (not about lofting)

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    Rene Limberger

    on the german fai forum a thread about current flappers has started. most of the writing is in german (try google translate), but there are quite a few pictures already and i expect more to be posted soon.


    anyone on this forum planning to build a flapper for the 2009 season? seems that in europe a flapper building frenzy has broken out, everyone is building one now. what will the americans do? 🙂


    A preferred nomenclature for these exciting models are Variable Camber (VC) models. Flapper models only raise or lower their flaps, while the rest of the wing remains fixed. So, to retain the wing-stab decalage, the stab requires a suitable offsetting. And because the fixed portion of the wing is attached to the fuselage, modifying the wing’s camber considerably changes the stab’s location within the wing’s down-wash.

    In VC models, both portions of the wing rotate, held together by the hinge between the wing’s leading and trailing portions. Consequently, the wing-stab decalage is roughly unchanged as the wings are cambered or de-cambered, so no adjustments are required for the stab. In fact, most the new F1A are VCs, so lets refer to them that way.

    Rene Limberger

    Aram, your point is noted. However, fyi, there are quite a few models already being used in contests this year that are not VCW’s as per your definiting. most notably Allard’s amazing ExSpan2008 has the wing fixed at the fuselage and only moves the flap. Victors is similar, only the flaps move.

    It seems that with modern, servo articulated stab drives, there is enough range to compensate for the change in angular difference when flaps are up or down.

    also, some pilots have recently experimented with radically different hook positions to further compensate the problem. some with success.


    Glad to hear that servos are the life-lines for flappers. I’m also aware of Allard van Wallen’s older work through Gregorie Martin’s web site. Although, at that stage he only flapped the main panel.

    The ’08 Symposium has an article on Variable Camber F1A Models which I happened to have written. Besides giving a history and technical information about VCW models, it also considers the optimal wing section during the OLA. The calculations, with the assistance of Brian Eggleston, show that the highest L/D (or on-line acceleration) is achieved when the flaps are partially or fully down. However, this is very difficult to implement and M&K actually use a second best solution which is to keep the wing de-cambered (flaps up) during the OLA.


    Rest assured, my 2007 and 2008 flappers were (are) full span. A new one is being built as we speak/write: ExSpan 2009 also equipped with the specially designed all new Smoo11 airfoil (stay tuned for the next FFQ!).
    The concept of flappers was already disclosed in an article in the 2005 Symposium report which contains more info than Martin’s internet pages. (Aram I believe you were involved in the creation of this magnificent publication).

    Rene Limberger

    Allard, you joined this forum!? Wow, welcome to the 21st century – buddy 🙂


    About 30 years ago I built a whole series of v-dihedral flappers. They were easy to trim, had a thin, flat bottom section with lower surface hinges and flew really well……………. Of course they were HLGs.

    The flapper thread that is mentioned is very good and has some good images. What strikes me is that few people have got to grips with the structural issues (hinge qualities and flap performance) but many are looking at airfoils. I think the empahsis is wrong.

    I well remember the Stamov flaper of many years ago (anyone remember the date ?); three elements and quite flexible yet mechanically very clever. Victor’s new model has obviously been influenced by F1Cs.

    I’ve seen M&K fly (superb height gain) and Findahl (superb glide) but sadly not seen what Allard has been doing. All have promise.

    What are the views on upper versus lower surface hinges – both are out there ? Then how about fixed front versus front/read pivoted sections ?

    So many questions ………


    A flapped wing is probably is the simplest mechanically, as its flap’s hinge is fixed and the flap is rotated up and down from the pod. A variable camber (VB) wing has two counter moving parts, the leading portion pivots on the main rod. The flap is connected to it by a series of hinges and pivots at its mid point around a short ~1/16″ rod. And since the main rod is fixed, the rear pivot points of the flap have to move back and forth when the wing is cambered and de-cambered. (One exception is Aringer’s F1A which pivots on the rear rods).

    The big structural issue of flappers/VC F1As is how to keep the wing’s section from distorting during the OLA. Although an upper flap hinge disturbs the upper air flow to a lesser degree, the flap can only be anchored at the pylon. So, flap distortions during the OLA are considerable. A lower hinge (used by M&K) buts the flap against the wing’s leading portion, locally transferring its lift as a twisting moment and sheer forward. And, if the hinge is tight, the flap will hardly distort during OLA. Obviously, the gaping “V” hole when the wing is cambered in glide should be glossed over. (More details can be found in the ’08 NFFS Symposium: “Variable Camber F1A’s”).

    The highest on-line acceleration (in terms of the section’s L/D), is reached when a VC wing is cambered or equivalently, when a flapper’s flap is down (ibidem ). But this is the crux of the matter. If the M&K flaps (with a lower hinge) are lowered during the OLA, they will only be anchored at the fuselage’s pod. Consequently, they will distort just like the flaps with an upper hinge. The problem of upper hinged flap distortions has led to en efforts to torsionally re-enforcing the flaps by either egg-box CF capped structures (Aringer/Findhal) or V-boxing them with a CF sheets. (An alternalive may be to D-box the flap’s leading portion).

    Che’s observations about M&K (enormous altitude/ so-so glide) and Findhal (so-so altitude/ superb glide) correspond to having a lower and upper flap hinge respectively. M&K have a “V” upper surface indentation in glide when the flap is down but rigid flaps during the OLA, while Findhal has cleaner upper surface in glide but twisting flaps (when the flap is up) during the OLA.

    Conceptually, a new sort of sophisticated hinges, effectively moving the (imaginary) hinging point above the upper surface, while cleverly morphing the wing could solve the problem. Until then we are stuck with repertoire of upper/middle/lower flap hinges.


    Chris, did you have a look at the 2005 Sympo? There you will find the concept of bottom hinge (and why is is preferred) as well as the problem of flap distortion explained.
    Flight tests have shown that the gap at the top surface does not influence the glide. This is very likely caused by the gap being well within the turbulent boundary layer.
    As to specially developed airfoils, stay tuned for the coming FFQ! It will show a radically new airfoil specially developed for F1A flappers. I have flown it against Koglot’s standard big model in the last Favli Italy fly off and its rate of descent was better (lower that is 😉 ) so it shows great potential.
    In my opinion a full carbon skinned flap is the only way to get sufficient flap stiffness. I have experimented with different flap settings during OLA and yes there are big differences, indicating that the flap does not distort all the way to its upper surface blocking position, unlike other constructions (eggbox, torsion tube etc.).



    Interesting points about the flap hinge position. Is this why M&K changed from upper surface hinge (long flapper @ LH 2008) to lower surface (short model @ Euro Champs) ?

    I actually think M&K get higher launches than Findahl purely down due to the point at which they start their acceleration. M&K do something called ‘The Arc’ as you know whilst Per get the model low and straight into wind before he starts accelerating; if he used a similar approach to M&K then he get higher launces but no doubt there are good reason why he doesn’t (he did tell me that rudder position is critical during OLA).

    Allard : Yes I have the 2005 sympo report but need to re-read it.

    My view is that you need three distinct flap positions :

    Straight and circle tow – slight camber for stability
    OLA + cruise – flat bottomed
    Glide – full camber

    Anyway, this ‘variable camber’ definition is too long winded. I now declare the method to be called ‘Partick’.


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