Originally Posted by
jeff4912
Gray Beard said:
"If the outer tube isn't glued in tight all along it's length about every 6 or 8 inches it will allow the rod to shift and it will change the trim "
Is that an in flight issue, because my trim changes are occurring on the ground prior to the flight when my Sig is sitting in the sun.
When I get my Sig home and inside the garage(out of the sun), the rudder and elevator go back to center.
Here is the paragraph from the Sig 4 Star 60 building manual:
Before flying, check and adjust all the pushrod linkages so that the control surfaces are in neutral position when the transmitter sticks and trim levers are centered. When you get to the field, don't be surprised if the elevator and rudder are slightly misaligned. Temperature and humidity changes can cause the nylon pushrod tubes to expand or contract slightly. Use the trim levers on the transmitter to return the control surfaces to neutral, and do the final trimming in the air.
Jeff
what you are experiencing is very common with most any push rod set-up. The nylon push rods that glide inside outer sheaths are more prone since they expand more as temp increases. But all materials (nylon,wood, carbon steel, ally) expand to one degree or another. Throw in the fact that the wood fuses also move with temp and you have a recipe for exactly what you are experiencing.
It's physics; every material has a COE or coefficient of thermal expansion. In Pattern and IMAC where precision set-ups are of paramount importance, we try to match the materials of the pushrods to those of the fuse. Many also will use servos as close to the surfaces as possible to minimize push-rod runs.
What I often do for elevators and rudders is to use pull-pull set-ups. There is no discernible difference in control cable lengths since the cables driving either side are made from the same stuff.
BTW- this is an age old situation, having been around since the start of model control.