How much do you trim?

On 2006-10-10, Mortimer Schnerd, RN mschnerdatcarolina.rr.com wrote:
Heh heh... I must have done it four or five times before the pilot caught on.
G Lucky for me, there was no ejection seat in the C-172.

You are only a student in evil tricks, I'm afraid :-)

When doing hood time for my instrument rating, I tended to do all the
training at night, since it simulates IMC better when under the hood -
no sunlight cues to tip you off.

At this point I had ILS approaches nailed - I could keep the needles in
the donut in the middle of the instrument. I was really proud on how I
could make it look like the instrument wasn't even functional.

My safety pilot - let's call him Paul because that's his real name and I
don't want to protect the guilty - gave me lots of aggro that night in
the name of training, but his best one was on the ILS.

Passing the outer marker, I made the customary power and pitch changes
and began to follow the glideslope. Then the plane started drifting
high. So I corrected. Then it drifted low. I just couldn't nail the
glideslope whatever I did. I was getting more and more distracted by
this, when two lights shone through that bit of the windscreen the hood
doesn't quite block, convincing my sense of balance that we were now in
a 60 degree bank. Not only was I fighting spatial disorientation, I was
all over the place on the glideslope, and now started to go all over the
place on the localiser. At the decision height I was so glad to whip the
hood up - I was starting to get overloaded.

We did the missed approach, and flew home.

When we were taxiing in, I grumbled about how terrible the ILS was - and
how I couldn't figure out how I'd flown it so badly. Paul did not say a
word. He just slid his seat back then forwards! The seat rails in a C172
are very long, so he could move an awful long way fore and aft - and
he's quite a big guy...

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Dylan Smith writes:

When we were taxiing in, I grumbled about how terrible the ILS was - and
how I couldn't figure out how I'd flown it so badly. Paul did not say a
word. He just slid his seat back then forwards! The seat rails in a C172
are very long, so he could move an awful long way fore and aft - and
he's quite a big guy...

So what was his purpose in messing up the approach?

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Robert M. Gary writes:

It totally depends on the plane. I used to fly a Globe Swift and I
never found any reason to need to touch the trim. The plane flew hands
off at whatever you pointed it to. I currently own a Mooney and I spend
more time in the pattern moving the trim than holding the throttle. The
entire downwind of a Mooney is rolling the trim back, slowing the plane
down.

Interesting. So what is it that's different between the two planes?
If the aircraft requires no trim, does that mean that control surfaces
stay where they are put, or what?

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Mxsmanic wrote:


Interesting. So what is it that's different between the two planes?
If the aircraft requires no trim, does that mean that control surfaces
stay where they are put, or what?

My guess... its the lever difference between the CG and center of
pressure. My guess is because the C-150 and the Swift are 2 seat planes
that the moment between CG and center of pressure is small so the plane
does not have the same degree of stability and therefore, wants to stay
at the pitch you put it at. This is not generally considered a good
thing, most designs keep those distances larger such that letting go of
the controls will cause the plane to seek its trimed airspeed right
away. However, since the trim in the Swift was in the back behind my
head, I didn't complain that I never needed to get to it.

-Robert

Mxsmanic wrote:

Also, I presume that most autopilots use trim for pitch control. If
you shut off the autopilot, does the trim remain whereever the A/P set
it? If so, do you change it? Is it hard to remember that the A/P has
probably changed it?


It seems that nobody has responded to this yet ...

The autopilot controls pitch with the elevator servo,
much in the same way the roll is controlled with the
aileron servo. The elevator servo can measure the force
needed to keep the proper attitude, and after a small
delay, it trims the force off with the trim servo.

When the autopilot is released, it leaves the airplane
in trim to the pilot - there's no need to remember the
trim position.

HTH

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tauno voipio (at) iki fi

Mxsmanic wrote:
What I see in the literature seems to vary between warnings against
spending too much trim or flying with trim tabs, and not ever trimming
the aircraft at all. So how much and when should I trim?

I understand trim to be a convenience, so that a pilot doesn't have to
constantly maintain force against the controls for long periods. Thus
it should never be dangerous not to trim, except insofar as it can be
tiring to hold an untrimmed aircraft in a given attitude for long
periods.

The thing I wonder about is the possible distraction of trimming the
aircraft. It looks like trim controls are often in spots like the
pedestal or throttle quadrant, where presumably one must direct one's
attention in order to adjust trim. It's hard to imagine doing this
during critical phases of flight such as take-off or landing, and yet
I read recommendations for trim in both cases. Where do you draw the
line between trimming unnecessarily and not trimming enough? How
often do you actually reach for the trim controls?

A one-hour introductory flight (in a real airplane) would answer
so many of your questions. Why not do it?

Dan

writes:

A one-hour introductory flight (in a real airplane) would answer
so many of your questions. Why not do it?

If you don't want to answer my question, why waste my time and yours?

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Mxsmanic wrote in
:

writes:

A one-hour introductory flight (in a real airplane) would answer
so many of your questions. Why not do it?

If you don't want to answer my question, why waste my time and yours?

The above answer is as good as it gets.

Why wast our time asking questions when all your answers can be answered as
suggested above.

Or better yet, just go to a sim group and ask your questions since you
don't want to experience flying a REAL PLANE.

Allen

On Mon, 09 Oct 2006 19:11:06 +0200, Mxsmanic wrote:

writes:

A one-hour introductory flight (in a real airplane) would answer
so many of your questions. Why not do it?

If you don't want to answer my question, why waste my time and yours?

Because no matter who responds, you're getting the answer filtered through their
senses and experience. It's like asking someone what an apple tastes like,
rather than biting into one yourself. Everyone's experience differs; you will
not gain a consensus on which to base an opinion. Five minutes in an aircraft
would allow you to understand the issues of trim, from both a conscious and
muscle-memory perspective.

Ron Wanttaja

Ron Wanttaja writes:

Because no matter who responds, you're getting the answer filtered through their
senses and experience. It's like asking someone what an apple tastes like,
rather than biting into one yourself. Everyone's experience differs; you will
not gain a consensus on which to base an opinion. Five minutes in an aircraft
would allow you to understand the issues of trim, from both a conscious and
muscle-memory perspective.

Why ask what it's like to be President, when five minutes being
President can answer all your questions?

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