Thread Tools
Jan 07, 2013, 04:59 PM
Registered User
Thread OP
Question

Center of Gravity (Balance) on a Outrigger Airboat?


This is the "Predator Outrigger" that I am making into an airboat. I have the bottom and top pieces of the main sponson laying on the supports (I still have to build them). The motor will mount on the top plate of the motor stand. My question is where should the center of gravity (balance) of this boat be?
Sign up now
to remove ads between posts
Jan 07, 2013, 10:38 PM
Steps? What steps?
zozer's Avatar
Typical water-prop outriggers have the CG around 15% of the afterplane length behind the sponson transoms, but it actually depends on the breakdown of lift from the front and rear sponsons. An airboat makes it more difficult to predict. Build it so you can adjust the CG from 10% to 20% or so, then you have room to tune the boat.



.
Jan 07, 2013, 11:34 PM
Registered User
Thread OP
Quote:
Originally Posted by zozer
Typical water-prop outriggers have the CG around 15% of the afterplane length behind the sponson transoms, but it actually depends on the breakdown of lift from the front and rear sponsons. An airboat makes it more difficult to predict. Build it so you can adjust the CG from 10% to 20% or so, then you have room to tune the boat.



.
Do I have this right? The afterplane on this boat would be the distance from the transom of the front sponson to the transom of the rear sponson correct? Then the 10% to 20% of the afterplane is measured from the transom of the front sponson towards the rear of the boat correct?
Jan 08, 2013, 01:13 AM
Nitro User
1/2A Nut's Avatar
If you use a front turn fin it should be in the center of the tub if you desire good left and right turns at full throttle. If you want to race right hand turns then the fin is located normally on the right sponson rear ward as possible to the static cg. CG should be in the normal rigger location as this is a rigger not a flat bottom swamp boat mono hull nor deep V. Your prop will pull the rigger as it lifts at a angle that produces forward thrust. I would sand / round off the flat fronts of the motor tower for better airflow and less noise but mainly to induce some vertical stab yaw stabilization capabilities by creating a airfoil.

Munch on this too and digest on the water with your "Air Rigger"

This original rigger was designed to lift up and ride on to the strut and prop hub to be in tune with the AOA angle of attack of the sponson. With your current set up you will end up with the back end a hair down this will have your motor stand a few ticks degree from a right angle to the water. If the motor is mounted at a right angle from the motor stand the air prop in motion will tend to lift the back end and push on the front sponsons ride pads. You should have adjustable in the field thrust angle if your setting up for high speed and for prop torque you need to offset with a tad right thrust so your rudder trim stays true when trimmed for straight running.
Jan 09, 2013, 06:26 PM
Steps? What steps?
zozer's Avatar
Quote:
Do I have this right? The afterplane on this boat would be the distance from the transom of the front sponson to the transom of the rear sponson correct? Then the 10% to 20% of the afterplane is measured from the transom of the front sponson towards the rear of the boat correct?
Yes, that is right. I see that you have an adequate rear sponson to maintain the correct angle of attack when running fast, and to hold up the transom when the boat is running slow or is stopped. You will have to tune the exact CG position depending on the amount of lift you get from both the front and rear sponsons. Make sure you can adjust the motor angle to change the thrust line - this is usually done by placing washers between the motor mount and the firewall. You can angle the thrust to counter-act prop walk. This will also let you adjust the dynamic weight on the front sponsons, like tuning a water-prop rigger with the water prop depth and angle.

I've built and raced a lot of riggers, set SAW records with them too. But I've only played with air-prop riggers a little, much more experience with tunnel hulls and air motors. It will be fun to see what you can do with this one.



.
Jan 09, 2013, 08:53 PM
Registered User
Thread OP
Thanks for the help both of you!! I need to move the motor stand forward to get in the 10% to 20% of the afterplane range. This would be 15%. Battery is in the middle of it's mounting plate (so I have some adjustment) and ESC is as far forward as it can go. I don't have the motor or ESC yet, so I am using weights instead of them (Hobbyking tells the weight on their site). This is not an exact science, so I won't mount motor stand until I get all the parts. I doing a build log of this boat https://www.rcgroups.com/forums/show....php?t=1754379


Quick Reply
Message:

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Build Log Predator Outrigger Airboat Dave8888 Racing Boats - Electric 43 Jun 18, 2013 08:58 PM
Rant Center of Gravity vs. Balance Mark Wood Electric Plane Talk 108 Dec 12, 2007 07:08 PM
Discussion How to balance the Center of gravity jdhuntnfish Micro Helis 3 Jan 02, 2007 10:06 AM
How do check balance and center of gravity? us_matrix Micro Helis 2 Jun 27, 2005 08:07 PM
corona : whole balance around gravity center machbiole Electric Heli Talk 7 Dec 23, 2004 11:51 AM