Thread: Flaps - do they work in model sizes?

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  1. Flaps - do they work in model sizes? 
    Join Date
    Cheshire England
    If you look at the flight performance figures of say a full size Cessna 172 the stall speed is only reduced by 4 or 5 knots (about 10%) with full flap.
    OK when you have an airspeed indicator in front of you but it would be hardly noticeable from the ground.
    To explore what might be possible I made this flapped wing for my highly modified Wing Dragon.
    With a Clark Y section, tapering 2:1 to the tip and no washout it could be expected to have 'aggressive' characteristics with the tip quite likely to stall first.
    The flaps occupy 2/3 of the trailing edge, are fully variable down to a high drag 70 degrees.
    As a result the ailerons are quite small but do have a differential action (more up than down).
    The object was to see how much slower it could fly with the smaller wing (70% of the original parallel wing), was aileron control maintained and what happened at the stall.
    This video gives an idea of how it performs.
    Not really built to be aerobatic it certainly exceeded my expectations. The only drawback being a strong nose up pitch if the flaps are lowered at speed.
    It is quite noticeable that with the flaps down the plane stays at more or less the same attitude even at the slowest speeds so the wing tips do not stall and the ailerons remain effective albeit with their power reducing with declining airspeed.

    The question now was could the wing area be reduced further and the flaps made even more powerful.

    One effect of flaps that is not mentioned is the change in AOA of the portion of the wings with the flaps.

    Dropping the flaps increases the AOA forcing the section(s) with flaps to stall before the section(s) without. Since flaps are typically in the center sections, not the tips this gives a significant "washout" effect that reduces the chance of an unwanted spin.

    For models this is usually more important than any reduction in stall speed.

    Also large flap deflection adds significant drag which is useful for steep descents without building up airspeed.

    Complex flap systems such as Fowler work very well in models also.


    Join Date
    Cheshire England
    The next wing reduced the area to just 55% of the original 'plank' wing.
    To compensate the flaps were made proportionally even bigger and in two sections. To make construction a bit easier it has a parallel chord one piece centre section and tapered dihedral tips.
    With the flaps down it becomes a highly under cambered section.
    To keep weight to a minimum both sections of the flap are driven by a single servo but from different hole positions on the servo arm to give the different movement required. This video shows the action.
    Completed in its 'court jester' colour scheme!
    Despite its small wing area the flaps certainly have a dramatic effect.
    Although the flaps do indeed work these planes are so light and powerful that in reality they do really require a slower speed to execute a safe landing although the extra drag does allow a steeper approach.
    Last edited by quorneng; 10-23-2011 at 06:33 AM.

    Try this method of flap actuation...

    This is my test rig for the way I'm installing scale functioning flaps in a Me-262. Unfortunately it requires 2 servos per flap panel but it does have the flaps move the same as the full scale.

    (slotted flap)
    Attached Images

    Join Date
    Cheshire England
    That a really neat linkage but as you say without a lot of further linkage each hinge requires a servo.

    I did this video to show two extremes of using flaps to make a steep approach landing.
    In both cases the start point would normally be considered much too high.
    The steep fast approach is dramatic but needs care to judge the correct flair point. The slow approach is easier as no real flair is required but as it is flown close to the stall all the way speed control is critical.
    In practise a slightly shallower compromise between the two gives the best margin of safety.

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