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High performance jets that use hydraulics to move their control surfaces always use multiple actuators with independent
hydraulic systems. This is done for redundancy and safety, as well as to increase the control precision.
The "safety" benefit is obvious...but "increase precision" - what's that all about? You see, they use linear linkages
too, and they realized early on that EVERY linkage has inherent slop. To compensate, they came up with a little trick
to "take up the slack" in the linkage.
Since they have two actuators on each surface, what they do is to set them so they do not center the same. In other
words, each actuator seeks a different position, so they constantly work against each other - one is always pulling
a little against the other one that is pulling back. The end result is that the slack in each actuator linkage is taken
up, so the control has no "dead spot" at the neutral position. Pretty neat trick!
So, by now you are naturally thinking, "great, but how does that help me?" Well, if done with care, we
can do the same thing to our planes.
See, with Direct Drive you have no linkage slop, but we always have a little unavoidable slop in the servo gear
train. If you have multiple servos on a control, you can set the center points of the servos off - ONLY A DEGREE - and
take up the slack between the two servos! Talk about cool!
Now there is a real danger to this, if it is not done exactly right. Hydraulics can work against each other all
day long and it doesn't cause a problem. Unfortunately, electric motors cannot. They generate heat when stalled,
which quickly cooks them. This is especially true with modern digital servos, that have incredible torque built into
such tiny packages.
The key to this technique is to set one of the servos on your surface in the normal way. Using the radio - or MatchBox
- set the center and both end points where you want them. Now, when you set the center point of the second servo, don't
set it exactly to the first, but off...just a little. When done correctly, one servo will always hum, but
not both servos. If you push the surface with your hand, one servo will hum when you push it one way, and
the other servo will hum if you push in the other direction. When you let go, it goes back to only one servo humming,
since that servo is carrying the weight of the surface. Once again, only one servo should
hum when the surface is at center with no loads. In this way you make sure the servos have taken up their
mutual slack, but they are not fighting each other.
Finish setting up the second servo by setting the end points in the usual fashion. You only need to "take up the
slack" at neutral. Off center - towards the endpoints - the air loads take up the slack for you, so you
don't need to offset the servos there. That also makes sure both servos work pretty evenly in powering the control away
from center. You have your cake and eat it too!
When you are finished, your control will have zero play at center. It will be rock solid, and the difference is
very noticable in the air. If done properly, there is actually less wear on the equipment, since the control is not
constantly rattling against the servo gear trains when the engine is running.
And by the way - "yes" - this technique will work with linkage setups too, for those of you that were wondering.
You can control the linkage play, but you still have the geometry and other problems a linkage gives you.
Lest someone missed the disclaimer:
Only try this if you completely understand what you are doing! Do it
wrong and you will fry your servos as a minimum, and loose you plane at the worst. But for those serious about precision,
this is a trick that's hard to pass up.
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