Dave I would check the linkage from the elevator servo motor to the to the
bellcrank, for slack in the linkage. I would also check the linkage from the
bellcrank to the anti servo trim pins, If you find none there I would then
check the elevator push rod for slack in that system. Have somebody hold the
elevator stable, and then try to move the control stick to see if slack is in
that system.
One other thing I would check is the autopilot. We just had a S-Tec
autopilot installed in a Cessna 172. Once we engaged the autopilot the plane
would start to "Dutch roll" at speeds less than 90 mph. Above 90 mph there was
no problem. Our problem was speed related and it turned out that the factory
gain control ,was incorrectly set by the factory. Once the unit was reprogramed
the problem went away.
Jim Brown
N398JB.
Dave DeFord wrote:
> Hi Folks,
>
> N135TD tends to oscillate slowly in altitude when the stick is left free.
> This is most noticeable when the Navaid autopilot is on, since there is a
> natural tendency to keep the airplane level, without really noticing it,
> when the pilot's hand is on the stick. Typical parameters are speed
> variation +/-5 KIAS, altitude +/-50 feet, VSI +/-300 fpm, and a total cycle
> time of about 1 minute.
>
> The only significant play in the pitch control system comes from the
> (normal) loose fit of the hinge pins in the antiservo/trim tabs. There is
> no deadband in the pitch control during flight, and the response to stick
> inputs is quite positive. Though we haven't measured stick force gradient,
> it feels very reasonable, with a solid push or pull required for a change in
> pitch attitude.
>
> The only deviation we can see from the specifications is the width of the
> Flettner strips. By the time we got done filling and finishing over them,
> they measure about 3/4" to 7/8" wide from the trailing edge, to the point
> where they disappear into the tab surface. The manual specifies 1/2"
> maximum. Thickness at the trailing edge is 3/8", which is the maximum
> recommended. If we need to make the slope to the trailing edge more abrupt,
> we will grind them down appropriately. I would just hate to tear up the
> nice painted surface, only to find that it made no difference! Anyone with
> experience or some solid theory on this?
>
> Dave DeFord
> N135TD
>
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