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Airboat gets unstabel when getting over a certen speed

Old 05-02-2008, 05:16 AM
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kokker
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Default Airboat gets unstabel when getting over a certen speed

Hey i am a novis, in the RC Airboat model building area.

i have build 2 airboats from scratch
bu thay both seems to have the same problem.

when i get over a certen speed it starts to skip... and get unstable (half throttle)

check out my video. it is of my little cat style Airboat(my first one)
but my large flatbottom have same problems, i tryed to make ajustabal Ailrons to correct the problem, but it dont seem to help.

http://www.rcgalleri.dk/html/vid_vis.asp?VideoID=3597

video of the large one, it is a little hard to se, but the when i shoot this video i only sails with half trotle,
http://www.rcgalleri.dk/html/vid_vis.asp?VideoID=4026
it sails with an Brushless Engien running 1000Rpm/V and on 4 LipoCells (14,8V) with an APC 8"x6" prop


hobe some one cant help with some guidelines waight distribution, and so on.

/Jacob Ipsen - Denmark

Old 05-02-2008, 09:34 AM
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Mr Cajun Gator
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Default RE: Airboat gets unstabel when getting over a certen speed

I have not had good luck with any cat style set up on the airboats. They all seem to have control troubles. Many things can cause hopping or skipping. With out having the boat in my hands it is very hard to say what is causing it. Weight, shape, direction of force, water friction, direction of airflow. What makes a boat skip or hop, is that it is trying to fly, but can not. You are getting a force that is causing too much lift on the front but not enough to hold the lift, so it falls back down, in a cycle. The shape of the rake can cause this, on how much angle it has. A defect on the bottom can cause this as the water passes, it will try to follow the shape of the hull. If there is a place that as the water passes and is trying to cause the boat to lift it may not be enough to hold on and again causing hopping. This stuff is very hard for me to explain.
Best I can say is that on the bottom, you want the water to be able to release easy. Cause the best way to give a better idea, run your boat and try to spin it around so that the boat actually slides in reverse, you will see that the back of the boat will raise as the water holds on the the bottom as it follows the rake.
Having the prop airflow pointed towards the water will help to keep the bow down, too much will cause your boat to hook and spin out while trying to make a turn.
CoG can have some effect, but this does not effect the air prop boats as much as the water prop boat. Main reason is that the force point to make the boat move on the air prop is on the top side of the boat vs the bottom side of the boat on the water props.
No 2 home made boats will run the same, so you will need to study your boat and try to picture what happens as the air and water passes across your boat. Check anything that will not allow release, study every thing that changes the flow of the air and water. Some things have more effect than others, but as you study it, you will get a better understanding of just what is happening, making it easer to find the problem.
Old 05-02-2008, 09:56 AM
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kokker
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Default RE: Airboat gets unstabel when getting over a certen speed

tanks for your replay,
the first think i have tryed, was to move the engine futher back to get the tourge as cose to the rudder as posibal,
then i made rc controled Ailrons, ajusting theas wile riding dosent make a differens.

the next thing i where consituring was to try to make the weight center of the boat closer to the front, so se if that makes a diffrence.

image of the bottom


image from the front


image of the new mount and prop design
Old 05-02-2008, 11:53 PM
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crispyspa
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Default RE: Airboat gets unstabel when getting over a certen speed

I think the biggest cause of the "bouncing" is the shape of the bow section (rake). It makes a hard transition. The Rake should have a smooth, gradual transition for water to flow freely from bow to stern. Picture a real flat bottom boat.
Try moving your battery more toward the front of the craft...see how that effects performance.

For the catamaran...I have no idea.

Cool lookin' dog, What breed?
Old 05-03-2008, 05:47 AM
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kokker
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Default RE: Airboat gets unstabel when getting over a certen speed

it is an Kuvasz but it is still a puppy it is only 8 month old
it is an biiig dog, but cute as hell

thanks for the remarks... i will try to fitt some more waight in the front. it makes good speed, but i need som more stability
Old 05-03-2008, 01:38 PM
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mr. ilikehelp
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Default RE: Airboat gets unstabel when getting over a certen speed

my depron boat with a 500 moter on it does the same thing except for it starts to hop out of the water mines kinda a hydroplane.
Old 05-04-2008, 03:43 AM
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kokker
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Default RE: Airboat gets unstabel when getting over a certen speed

i where out testing how it behaved at full speed, and it seemslike it wants to fly, at full speed it leevs the wather, i think it has somthing to do with waight and shape... så i will try to increase the waight.

dos somone know where one could get hands on som blueprints for one that works ?

my other alt.is to build one wigt round shapes, to the sides and to the front and with lower sides.

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