Trim Question--Long

[Bill Glaze]

Troy:
Excellent explanation.  In fact, on full size aerobatic planes, the 
"spades" that can be seen attached to the control surfaces are to take 
some of the load off the stick.  If improperly adjusted, they can 
actually "grab" the stick from the pilot, who can then be involved in a 
wrestling match with his own airplane.
Probably a little more serious than with a model, I would guess. :-(

Bill Glaze

Troy Newman wrote:

>Dan,
> since the plane has a counter balance.......I would say I have seen some
>funny things happen with the counter balanced rudders that so many planes
>have today.
>
>Not totally sure why its happening but probably do in some part to control
>Snatch. This happens when the control surface tab grabs the air and reaches
>a point where it overrides the flight forces normal on the surface. What
>happens is when you reach a certain point in rudder deflection the tab gabs
>and all of a sudden the load on the servo reverses from pulling the left
>cable to hold left rudder...to the result of the tab...trying to get more
>left rudder than commanded. The servo is then driving right to hold the
>commanded position left rudder. Not sure if I'm explaining it correctly or
>not....
>
>Where I have seen this happen most is rolling from inverted to a knife or an
>8th point position.  The coupling in the model is mixed perfectly for knife
>edge flight even to the point of extreme rudder throws and knife edge
>loops....but when the model is rolling in certain orientations the coupling
>changes. I have seen this tendency with almost all counter balanced rudder
>models I have flown. The counter balance works very well at making the
>rudder servo work less....However in these certain conditions its putting
>bigger loads on the servo in the snatch condition...
>
>
>My theory on this as rolling from inverted the rudder and specifically the
>tab is in clean air below the model....it digs in and has some more
>effective responses than we are used to. Since you are flying in a normal
>almost no rudder or no rudder situation perhaps the tab is catching in the
>clean air....How is your rudder trim? Is the rudder straight to the
>fuse....not the tab but the rudder...Look from the belly of the model while
>inverted back at the rudder and see if its tracking straight...If it is here
>How is the tab centered on the fin....Basically is the rudder straight, and
>tab sanded correctly to track straight... Next rudder cable tension? Tighter
>cables may help.....Next geometry on your pull pull system? Can you get more
>leverage for the servo.....Sometimes crossing the cables can do this.
>Sometimes not.
>
>Solutions I have found work.....Better rudder servo. If you are not using an
>8411SA this is what you need...I even know of some very loyal Futaba flyers
>that found the 8411SA to be a solution to their rudder problems. Not bashing
>any servos here just stating the facts I have experienced. I minimized this
>on my Smaragd with a rudder tab using the 8411SA....then I tested an 8611 on
>the rudder and the issue went away...This proved to me it was servo power
>and precision related.
>
>Better geometry on the linkage. Final thing is the knife and put the rudder
>tab on top of the fin with some clear tape and see if it solves it. We have
>found that it can be minimized using better servo setups...and eliminated
>with the removal of the tab. This is one really good reason to have a
>removable rudder...You can just take the rudder off...remove the tab and
>then attach it temp style with tape to the fin and fly it....
>
>Another solution to minimize the problem is CG....Change the way the model
>is flying....Maybe add some nose weight and see if the problem minimizes And
>I don't mean a 1/2 oz....Like 4-6-8ozs and see what happens. As the guys on
>this list gasp for air....I'll say that big CG changes can tell you a  ton
>about the way the model is flying. If you make a huge shift forward like
>4ozs on the nose. Most models will tend to chage the little settle down
>things like you are dealing with. If the 4ozs helps a littl...then go
>another 4ozs. I hate to say move the CG back as I feel many pilots fly their
>CG too far aft as it is......
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>  
>


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