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Canard configuration airboat CG ?

Old 01-10-2013, 12:21 PM
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critx
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Default Canard configuration airboat CG ?


Hi everybody,

I am working now to design a canard type airboat that I want to build from scratch. I have faced already many chalenges but CG location is the bigest one. I know canard is a different aproach but I would really like to get as much I can from this configuration.
Can enyone suggest what would be best pozition for the center of gravity on this airboat?

I have searched a lot on internet but information I have found was just for water propulsion canard boats.

The boat will be 24" long 11" wide, with back sponsons of 11" long.

Thanks,
Old 01-10-2013, 01:01 PM
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pinky_d_brain
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Default RE: Canard configuration airboat CG ?

What motor and prop?
For the dimensions you stated I would recommend a 6" prop.
Remember the 3 hulls don't have to be big but they do have to be spread out.

The CG will be somewhere around the same as the water propped ones but there are other problems to work out

The main thing to remember is that airboats are reveres of water propped boats in terms of where your thrust is coming from. Your prop is pushing down on the front of the boat instead of lifting. The Canard type hull in my opinion is the hardest to get working well. I recommend making it longer and wider than normal. turning is also tricky with the force up high pushing down on the front in a turn there is nothing to stop it from rolling in the terns. to help with this get your rudder below the CG as much as possible this will help lean the boat into a turn keeping it upright.
Also the smaller the hull up front the longer you will have to go to counteract the down force on the front

Anyway hope this helps and give you a bit of a heads up on some of the problems to expect
Old 01-10-2013, 01:32 PM
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critx
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Default RE: Canard configuration airboat CG ?


@ pinky_d_brain thanks for the answers

So the CG should be located at about 1" back from the main hull's step? Is this corect?

The power plant is at the moment a 33A brushless motor at 1250kv and I was thinking to use a 10" prop. As you suggest an only 6" prop I asume I should think to make the boat wider to make sure does not screws in the water when accelerates.
I am interested to get maximum speed on streight line so manouvers are not a priority. But it will stil need to be stearable.

As you said the thrust line concerns me also. For the water propped boats if the prop gets out of the water - because of too much speed and lift off - the boat will lose speed and will go back in the water.This is like a negative closed loop that stabilizes the boat on water. In our case if the boat takes off, it will loose the drag created by the water friction and will acctualy get airbone. So I need to make sure that at high speed the wing will push the boat to keep the contact of the three points with the water - minimal contact if possible.

Regarding the rudder the plan was to have an air rudder - two control surfaces in the props airflow but I may change it to a water rudder. A suggestion I have received was to have a fin in the main hull positioned where the step is.
Old 01-10-2013, 03:45 PM
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Altered1
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Default RE: Canard configuration airboat CG ?

If you are buiding a 3 point hydro, a wet rudder will provide positive directional control. Air rudders can be a bit of an issue with this style of boat. If you go a fin, then wet rudder definitely.

Prop Thrust line is not such an issue ...... as such...... - set her up straight down the middle, allow a couple of degrees up and down adjustment for fine tuning later. Tilt up for more down force on the front, down for less.

I'd make it 12 inches wide.....
Old 01-11-2013, 02:54 PM
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pinky_d_brain
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Default RE: Canard configuration airboat CG ?

I would think the CG should be somewhere around the step of the outside hulls but not sure on this.
Old 01-11-2013, 11:46 PM
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critx
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Default RE: Canard configuration airboat CG ?

@pinky_d_brain I apologize I have missread your message regarding the CG. It looked strange to me also anyway but a picture just striked me later where a canard boat is on its stand but with the back part of it. If the CG would be where I have told in the earlier message it would fall in it's nose.

Regarding the thrust,would be best to have the prop close or far back from the CG? To me it looked it not makes to much diference as @Altered1 said I will need to be able to adjust the thrust angle.
Old 01-12-2013, 03:19 AM
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Altered1
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Default RE: Canard configuration airboat CG ?

For my flatty, the CG (or more correctly) balance point with a full load of batts and fuel, is 1/3 of the hull length from the rear. The prop is close to this point too.

A 3 pointer is tentatively my next build, so I cannot comment exactly , but the closer the prop is to the balance point will make largest differences to any trim modifications, as you are pivoting the prop around the balance point too.

So you are going with the sponsons to the rear ? Several successful designs have the prop up front in puller configuration, down the back puller or pusher as well.... All have different CG requirements.



Old 01-12-2013, 03:36 AM
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critx
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Default RE: Canard configuration airboat CG ?


@Altered1, thanks for the feedback.

This boat will have rear sponsons, with puller prop in the back of the CG. I will post a sketch in the next few days.

But you say something that makes sens. As I am mostly interested in maximizing speed in streight line, having the CG upper, close to the prop axis point, will increase efficiency of the power plant and will make the adjustments easier. I will need to make the boat wider for lateral stability, but is already 12" as you suggested right? May be installing the battery in the back of the engine, but I need to analize if this really gives benefits?
Old 01-12-2013, 05:11 AM
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Altered1
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Default RE: Canard configuration airboat CG ?

Critx,

You are going to have to do some anaylsis, yes, but thats part of the hobby

Have a look around the forums. there are plenty of boats built in many configurations. This will give you pointers on what your scale should be. With the trailing sponsons, you are going to have to consider turning carefully.

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