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Airplane fishtailing?
  • I have an aerodynamic problem that may be interesting to a wider audience.I have a Chinese friend who is testing some very large scale (9 ft wingspan, 120 cc Zenoah engine)  models of an amphibian that looks very similiar to the Icon A5. He's on his third iteration and it now handles very well on water and in the air but it has one aerodynamic quirk that he is struggling to understand/cure: fishtailing. This is a back and forth yaw that developes when the model is in a shallow dive and the speed increases. He's tried increasing the vertical stabiliser - no real improvement, even changed the wing planform (had a slight sweepback on the outer panels - straightened the wing so the leading edge is a straight line with some taper on the trailing edge on the
    outer panel - just a slight improvement in the fishtaling.It makes no difference if the engine is on or off so it does not appear to be a thrustline, propwash/vortex (it's a pusher and the fuselage is short coupled) or torque effect. In all other respects the flight envelope is excellent - stalls, recovery from unusual attitudes, even spins are easily recovered. It's probably a form of Dutch roll although there is no obvious pitch change when it happens.

    From my own reading I have two hypotheses:

    1) it is vortices breaking off the vertical stabiliser/rudder - when it reaches a certain speed these are large enough to push the tail one way at which point the vortex breaks off the other side and the process reverses. Cure might be to thin out the stab. section and/ or thicken the trailing edge of the rudder (some 3 D models have very thick/square edges to the trailing edge perhaps for this reason?)

    2) it is something to do with the high wing being well above the CG. I gather large cargo planes have anhedral to help minimise this effect (and many large jets have automatic yaw dampers but I assume this is related to swept back wings).   Anhedral on a amphib would be

    Maybe it's a more complex interaction between the fuselage (which is an odd shape , cut away to allow the pusher prop, and short coupled), CG, centre of pressure and the T tail.   That's beyond my limited knowledge. 

    I gather this phenomena has been a problem in some full size (e.g.the V tailed Bonanza) and some models have this problem - even those with standard planforms. I see Parkzone
    have a Icon A5 model - I wonder if any of your readers have observed similiar fishtailing on that?


    Anyway, thought you might be able to shed some light on this curious phenomenon.

    (via email from Paul)
  • Stability problems that couple the axes can be very tricky to resolve. Both of your explanations sound plausible. I put in my vote on the first one for a couple of reasons:

    • From my experience, stability problems affect an airplane across a wide range of airspeeds.
    • Thickening the trailing edge of the rudder is trivial to do.

    I have also seen models experience a type of control surface flutter. Having lighter control surfaces, balanced control surfaces, or stronger servos helps with that.


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