Airboat Rudder

Hi again everyone,
I am back with another question. We are designing appropriate rudders for our 'boat' on wheels. We thought about using airfoil design, however the lifting force produced by that desng is too much and is thus unsafe.
Now we are thinking about using just simple plates for the rudders. I wa wondering if anyone has done calculations on this sort of problem. We are trying to calculate that at a certain speed, how much does the boat turn if a plate rudder is rotated a certain angle.
Please do let me know any site or any formulas that can help me in this calculations.
Sorry I forgot to ask another question. I have also seen boats online that have short rudders. Does anyone know what is the deal with that sort of design. If is it more stable or easier to manuver.
Thanks again
Your help is always appreciated.

Biomechanist.

 

I don't know any formula 4 your problem, sorry. You can calculate the lifting force (in this case the side force) of the rudder at a given speed and the theorical torque created (= side force x distance of the rudder from CG or CF).
Short rudders give more speed (=less drag), but not good manouvrability.

 

i want go into design and construction of airboat and i need general information on following; 1. resistance 2. thrust and speed 3. working principle 4. hydrodynamic behaviour.

 

i want to go into design and construction of AIRBOAT. i am requesting for general information on the following; 1. resistance 2. thrust and speed 3. working principle. 4.hydrodynamic behaviour.

 

Biomech, you might be interested in using a symmetrical (zero-lift) foil. A true foil shape tends to give better steering response at sharp angles than a plate, and it doesn't stall as quickly.
As far as plates go, you could probably estimate the numbers you're looking for with a bit of basic fluid mechanics and a helping of second-year physics. In streamline coordinates it would be pretty easy to find the force normal to the boat's trajectory (the force exerted by the air on the rudder), then translate that to a moment about the boat's CM and a force on the boat. You could then compute a radius of curvature if you know how much boat is in the water, its speed, etc. but you'll probably end up with a couple of nasty integrals. If you go with plate I'd be very tempted to just start big, and if it's too big cut a bit off until it's right.
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Plates can be fine if the rudder area is about 1/2 of the prop area. Then the angles of the rudder are small enough to avoid " stalling " and the resulting loss of control, till the boat is delibertly slowed down enough to regain control . Florida Fish and Game has great airboats. Check with them.

 

You'll find that the amount of lift per unit of deflection is nearly independent of the choice of airfoil sections. Thin plates will produce essentially the same lift for small to moderate deflections as carefully crafted NACA airfoil shapes. The theoretical lift coefficient for a thin flat plate is two-pi per radian of angle of attack. When you include the effect of the boundary layer, the slope is more like 0.1 per degree.

Planform has a far greater effect on the lift curve slope. The lift coefficient slope for the full planform will be approximately 0.1 / (1 + 5.73 / (pi AR)), where AR is the aspect ratio (span^2 / area) of your rudder.

If the force is too much, you have too much area! Make the rudder chord shorter and keep the span the same.

 

Such a boat will have much in common with a Jet Ski.

Close the throttle and you LOOSE the steering.

Only the air accelerated by the engine will have sufficent force and velocity to actually steer the unit.
At least thats what happens on our Hovercraft.

FAST FRED

 

Air boats usually have 2 speeds. Displacement and fast.

 

Here is a air boat company. www.airboatfl.com