There are no springs involved with the 3-line throttle control system. Correctly set-up, the mechanism provides for positive control over the full range of throttle travel.

The inverted bellcrank unit is probably best for an underwing mount.

When you're looking at it, consider that the thing is set up so that the tension on all three lines must be equal, meaning that if you hold the elevator at neutral and pull the throttle cable one inch out of the plane, the 'up' and 'down' cables will draw in by 1/2 inch each.

So the force for the throttle actuation comes from line tension, just as the force for elevator actuation does.

"Upright" or "Inverted" means will it sit on top of the bellcrank platform(upright), or hang beneath the bellcrank platform (inverted). Besides that, there choices about the elevator bellcrank part of the system. You can get a 2 1/2" span bellcrank, or a 3 1/2" span bellcrank. You may also choose whether the up line is the front leadout or the rear leadout.

There are no springs used in a typical 3 line system. The tension of the throttle line opposes the tension of the elevation lines.  When you pull the throttle leadout, the up and down lines "retract" into the wing by the same amount you pulled out the throttle line. Properly set up, you can release the trigger on the handle and the throttle stays put.

Several caveats: The bellcrank and handle work as a system. There are some home made systems out there, and it is possible to mix and match parts, but do yourself a favor and get the things that are meant to work together. Second, there is no provision for line length adjustment on the handle, and getting the throttle line just the right length is paramount, so you may have to fine tune things with different size line connectors. Third, expect to do some fiddling to make everything work, so it's better if the control system is all external.

I would consider a different airplane, in all honesty. My first 3 line trainer was a slab wing model, similar to the Streak Trainer. The fun part of flying this type of airplane is touch and go's, so you want a robust airplane with stout landing gear. A higher wing loading is a good thing for this! You also want enough room for an RC clunk tank, or at least a tank with the pickup at the bottom rather than on the centerline. Adjustable leadouts would also be helpful. I think everyone in the club flew mine, and it finally died when the threads on the elevator pushrod wore out after hundreds of flights.

Once you are successful, you will be able to adapt just about any model to throttle control. Go ahead, pull the trigger.....

I recently noticed that Brodak has a DVD about 3 line bellcranks,for $20.00. I have not seen it but it might be worth it if you are starting from ground zero!   Mike

Thanks Bill!  Very similar to what I've found ... but more descriptive.  I'll save this one ... :-)