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Split Rudder Airbrake
My latest version of the Midge has made use of a split rudder as an airbrake to assist in slowing the model down on landing. There has been quite a bit of interest in this, so I thought Id provide you with the info so you can make your own.

The main components for this are two rudders made half the thickness of the original one piece rudder. These need to be operated independently from two different servos either directly from the Rx with some electronic mixing, or via a mechanical mixer. I use electronic mixing through connecting one rudder channel as normal and the other servo by program mixing through an Auxiliary channel (Aux 1) to be controlled from the rudder stick on the Tx. (Rudder (primary), Aux 1 (slave)). Ensure that the servos work in unison and by the same amount pushing an pulling the rudder.  For the airbrake function, these are then both mixed with the throttle stick (primary) to Rudder (slave), which when operated, pull both rudder controls which opens up the two rudder halves. Note, the rudders should also work even when the airbrake is open. You may have to set up the end travel in the Tx mixing to ensure that the rudders do not try to travel too far.

Two methods can be used for manufacturing the Rudders / airbrake and for connecting these to the rudder post.


 

Method 1 - Hinge on the outer edge

Two rudder halves chamfered from the outer edge and are hinged to the outer edge of the rudder post. Use of thin mylar hinges work well. However, this method does give some asymmetry to the installation given that when rudder is input (both rudders move in the same direction) due to the hinge not being in the same place the rudder halves slide against each other. Not a major problem, in fact this is probably the easiest method to use. It is functional, but for me, it just doesnt look good. I wanted symmetrical movement of the rudder halves.

 

 

 

Method 2 - Centre Hinge.

By using cad and a cnc router machine, I have produced some hinges on epoxy board that allow for symmetrical movement. These use a central hinge system that pivots around a single piece of thin piece of piano wire inserted through all three hinges from the bottom. The hinges consist of left and right plus centre (rudder post) components. It took a few attempts to get the geometry right but it was worth the effort.

I also Cncd the balsa rudder halves complete with the cut outs already for the hinge components. I sanded these to a tapered shape and also chamfered the hinge line from the centre, then vac bagged the spearate rudder halves with 25g cloth and resin to stop the balsa warping. Once cured and trimmed, the hinges were glued into the slots. The rudder halves were then assembled with the rudder post hinge component and glued this into place in the rudder post.

 



 

 

 

 

 

 

A second servo was installed into the fuselage servo plate alongside the original rudder servo. I had to open up the original servo hole to allow for this. I also moved the Elevator servo off centre so that the rudder piano wire snakes (x2) did not interfere with the elevator control.

The thin piano wire snakes were cut to length and installed terminating at the rudder integrated control horn with a 90 deg bend and pushed through the horn on both sides of the rudder. These are secured in place by a small plastic ball with a hole in it. Actually its a fishing accessory designed to hold swivels in place. Remember to glue the snake outers to the inside of the fuselage though, otherwise these will flex too much when push control is input.



The Rudder airbrake works well on its own to slow the model down but on some slope sites a little more lift reduction is also required. So as you will see on the youtube video, I have also couple up going aileron (Aileron Crow) in addition to the airbrake. This is an additional function of the throttle lever but only when a specific switch is also selected on the Tx, some down elevator compensation is also required for this function. This works really well to bring the model down in a flat attitude and at a relatively slow speed - be ready to cancel the airbrake just before landing to prevent any damage to the rudder though.

See my youtube video for a demo of this working.        Midge SR on Youtube

 

 

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