Malcomfrary,
Not sure there is any mileage in bringing aircraft ideas into airboat handing. Aircraft operate in a 3D environment whereas airboats operate in 2D. Or, at least they ought to unless you want them to become airborne or dive under water a'la submarine?
The side forces on a deflected air rudder, being above the models Center of Gravity, will always cause a model airboat to lean out of the turn. Assuming the "fixed fin in front of the air rudder" you mention is to be on the hull bottom, then being below the C of G, this could well do the same thing.
Redpmg
Yes, a bodies Inertia is what makes it resist any change in Velocity (Velocity describing both speed and direction of motion). You can work all three of Newton's Law's of Motion into steering an airboat but his First Law might be most applicable here.
Once the motor has converted electrical energy in to the Kinetic energy, the models Inertia wants to keep it going in a straight line. The only way to change this is with a sideways push towards the center of the turn you desire (i.e. centripetal force). In a displacement hull this sideways force can be created by water pushing on the side of the hull (if you think about it the water rudder in this situation is actually pushing out of the circle, bit like a conventional aircraft where the elevator pushes down to make the aircraft go up).
In this respect an airboat has a problem, the deflected air rudder has a force acting on it out of the circle, not the direction we want. The propellers thrust vector is now however angled inwards and the radial component of the thrust can create the desired centripetal force. I suspect that at small rudder deflections the two forces could well balance out, this explaining why the model is reluctant to turn even though it is clearly slewed away from the direction it is traveling. It might also explain DM's (and mine) observation that easing off the throttle before applying the rudder helps. The "wrong way" sideways force on the rudder ought to be reduced and, traveling slower, could put more of the hull into the water to create a "useful" force towards the center of the circle? There again it could just be slower so less tendency for the hull to "dig in"?
To be honest, I'm not sure if twin air rudders would improve things on model airboats. There may be good reasons why the full-size use them though. Also, the full size need to consider ride comfort and safety.
tonyH,
Ah....... Vic Smeed's "Spurtster" model! Must try and find the original article. I've only built one fast waterjet model, the electrically powered Scudder ( Model Boats Winter Special 2013). It's handling was much the same as the airboats , appearing to skid around in turns with the bows pointing inwards but it didn't show the tendency to "dig in" during tight turns.
And finally, I could remove the air rudder and replace it with a water rudder, but that would make it hard to sail on ice, snow and damp grass. Anyway, I quite enjoy sailing models that "keep me on my toes!".
Glynn Guest
