Stability variation

[WingFlaps]

I've flown about 10 different 172's and while they are quite similar
in most respects, but one of them just doesn't seem to trim out nicely
for me. In that particular plane, it is always either climbing or
descending (slowly) if I take my hand off the yoke after I trim at
cruise. Is this normal or is it just me? If it's not my lack of skill,
could it be a rigging problem or what else might be the cause? (I
aways check the trim linkage on preflight and it's firm). A related
question is how long/well should a 172 hold wings level if I let go of
the yoke (ball centered)? I know that the tanks are rarely even but
I'm concerned that the lateral stability that seems less than I would
have expected. (I find that refolding maps in the cockpit difficult as
I keep having to correct).

Cheers

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On Apr 27, 3:20 pm, WingFlaps > wrote:
> I've flown about 10 different 172's and while they are quite similar
> in most respects, but one of them just doesn't seem to trim out nicely
> for me. In that particular plane, it is always either climbing or
> descending (slowly) if I take my hand off the yoke after I trim at
> cruise. Is this normal or is it just me? If it's not my lack of skill,
> could it be a rigging problem or what else might be the cause? (I
> aways check the trim linkage on preflight and it's firm). A related
> question is how long/well should a 172 hold wings level if I let go of
> the yoke (ball centered)? I know that the tanks are rarely even but
> I'm concerned that the lateral stability that seems less than I would
> have expected. (I find that refolding maps in the cockpit difficult as
> I keep having to correct).
>
> Cheers

If a 172 won't hold a trimmed altitude for nothing, then the
elevator system might be sticky. The control column bushings get
gummed up with skin oils, pulleys get dry and stop turning freely,
cables fray and start snagging, elevator hinges get gunked up. All of
it will keep the elevator from floating in the trimmed position.
There's another, much worse possibility. The forward stabilizer
spar in the 172 will crack, due mostly to people pushing the stab down
to lift the nosewheel and rotate the airplane on the ground. The loads
on that spar are far higher in that maneuver than it was designed to
take, and it cracks through the center hole. I have seen a spar broken
completely through, with only the skin holding things together, and
that airplane would not trim out. The stab was flexing too much in
flight and altering its AOA a tiny bit. It could fail altogether in
something like a spin recovery or spiral.

Dan

[WingFlaps]

On Apr 28, 10:13*am, wrote:
> On Apr 27, 3:20 pm, WingFlaps > wrote:
>
> > I've flown about 10 different 172's and while they are quite similar
> > in most respects, but one of them just doesn't seem to trim out nicely
> > for me. In that particular plane, it is always either climbing or
> > descending (slowly) if I take my hand off the yoke after I trim at
> > cruise. Is this normal or is it just me? If it's not my lack of skill,
> > could it be a rigging problem or what else might be the cause? (I
> > aways check the trim linkage on preflight and it's firm). A related
> > question is how long/well should a 172 hold wings level if I let go of
> > the yoke (ball centered)? I know that the tanks are rarely even but
> > I'm concerned that the lateral stability that seems less than I would
> > have expected. (I find that refolding maps in the cockpit difficult as
> > I keep having to correct).
>
> > Cheers
>
> * * * *If a 172 won't hold a trimmed altitude for nothing, then the
> elevator system might be sticky. The control column bushings get
> gummed up with skin oils, pulleys get dry and stop turning freely,
> cables fray and start snagging, elevator hinges get gunked up. All of
> it will keep the elevator from floating in the trimmed position.
> * * * *There's another, much worse possibility. The forward stabilizer
> spar in the 172 will crack, due mostly to people pushing the stab down
> to lift the nosewheel and rotate the airplane on the ground. The loads
> on that spar are far higher in that maneuver than it was designed to
> take, and it cracks through the center hole. I have seen a spar broken
> completely through, with only the skin holding things together, and
> that airplane would not trim out. The stab was flexing too much in
> flight and altering its AOA a tiny bit. It could fail altogether in
> something like a spin recovery or spiral.
>
> * * * * * Dan

Thanks I'll check your ideas out.

Cheers