S-Tec altitude hold with Century IIB

[Bartman]

My 77J has the original Century IIB with added Stec Alt Hold. When I first got her the wings would rock back and forth and it was really really bad. After seeing a post here about adjustments we have cured that and I am very pleased with the the steady performance. It tracks great in both heading and NAV mode and as a matter of fact, over the past year it seems that the more I use it the better it gets. 

However, when I turn on the altitude hold I get this proposing osscillation. If it is turbulent you cannot tell it so bad.  If it is smooth air it is up and down about 100ft and so annoying you cannot stand it. 

Where to start ? 

Edited July 15, 2016 by Bartman

[jetdriven]

The breakout friction may be too high. Make sure the elevator system is free and the bridle cable is tight. then check the pulleys in the bridle cable. Then check the startup voltage on the servo. Then also check the static lines are sound. When I bought my plane, the static line inside the tail cone, the tygon tube part was disconnected, the tygon was soft and bad. the two pieces right near the altitude sensor, which is located between the static ports. A new piece of tygon fixed it. What was happening was the increased airspeed would cause a change in pressure in the tailcone, low i think.  Then the STEC 30ALT would try to descend to recapture its set pressure. then the speed wound increase, and it would hunt altitude a bit. 

[ArtVandelay]

If you are perfectly trimmed, try adjusting trim slightly one way or another.

[Ron McBride]

10 hours ago, teejayevans said:

If you are perfectly trimmed, try adjusting trim slightly one way or another.

My Stec 30 system does the same thing, I just adjust the trim a little and it stops.

Ron

[Bartman]

Byron, I will have the static lines checked. May be time to go ahead and install new if we find something. After all, she just turned 39 this year. 

TJ and Ron, I have always turned on the Century IIB, trimmed for level, and then turned on the S-Tec Alt Hold. Next flight I will add some up or down trim to see if it changes. Other than that I will have the static lines checked, but I don't think I will throw a lot of money at it. It works great in turbulence when I need it, just not when it smooth when I don't really use it anyway. 

[Bartman]

Ok so I flew home this evening in still air without a bump even on climb or decent. I trimmed perfectly level and turned on the Stec Alt Hold and it did as always oscillating up and down about 50 ft and very uncomfortable. I added a little down trim and it did as TJ and Ron said. She flew perfectly straight and level with no oscillations whatsoever. 

Thank you !!!

[carusoam]

Funny stuff with the old non-digital computer things...  They may work best in a relatively narrow range.  Or work best under a set of specific conditions.

Go MS!

Best regards,

-a-

[jetdriven]

36 minutes ago, Bartman said:

Ok so I flew home this evening in still air without a bump even on climb or decent. I trimmed perfectly level and turned on the Stec Alt Hold and it did as always oscillating up and down about 50 ft and very uncomfortable. I added a little down trim and it did as TJ and Ron said. She flew perfectly straight and level with no oscillations whatsoever. 

Thank you !!!

That's still not normal. I always trim it deadass on and it never porpoises.  It actually doesn't porpoise at all, even with the UP or DN trim beeping.  

[OR75]

Not sure it will solve the porpoising but with those s-tec units - I have a 60PSS which plays very nicely with the century IIB. Also have auto trim. -  make sure you start the unit well before engaging the altitude hold mode. (I am guessing  for the pressure sensor to stabilize) 

Edited July 19, 2016 by OR75