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 Iíd 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
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 doesnít look good. I wanted symmetrical
movement of the rudder halves.
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 Cncíd 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.
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 itís 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