> I tried sending this once before, but got no mail for 3 days and never saw
>it when I started receiving it again. I am one of many who have electric
>elevator trim. If you have a stuck switch or relay, in other words a runaway
>trim. I for one would like to be able to reach over and pull the breaker
>before the situation becomes critical. Now I suppose I could wire in a
>cuttout switch if I wanted to, but that would fall under complicating the
>system. I know for a fact if the trim were all the way to one extreme you
>would have a hell of a time bringing the airplane back in one piece. I would
>consider this a saftey of flight item.
>
Thanks for staying with it. The matronics.com URL fell of the edge
of the world a few days ago and everybody's input to the list
was bounced or disappeared. There are still some funny things going
on as I've had to post one or two pieces twice to get them through
the pipe. Computers is wunderful!
Here's a case where I think complicating the system has good foundation
in pilot versus airplane ergonomics.
Pulling a breaker on trim runaway is too time consuming and there's
a lot of window for error. If your trim breaker is located in a whole
nest of other breakers, the possiblity of pulling the wrong one in
time of "tense pilot response" is great. Pulling the wrong one may
no have electrical consequences for flight for inadvertently shutting
down another system but it adds still more delays in perception and
resolution of multiple problems . . . all design and/or pilot induced.
In any case, pulling the breaker is far more time consuming than some
alternatives. During trim runaway, time is of the essence. The longer
you delay in effecting shutdown, the further out-of-trim the airplane
is going to be when the motor stops running . . . we need to shave
tens of milliseconds off of the perception-reaction-action time needed
to bring things under control.
I'm working on an article that suggests powerful electric trim systems
should require two switch actuations for operation . . . not unlike
requiring your starter current to also pass through the battery contactor.
You have a way to deal with a stuck starter contactor by shutting
of the battery master.
Suppose you had a stick grip with the usual coolie-hat trim button and
a second, "arm" button for all electric trims. The arm button would have
positive control over electrical power to all trim systems and would
have to be held closed at the same time you are commanding trim. The
advantage is that you hand is already on the control necessary to
disable the trim when the runaway happens. Even if the trim relay
stuck at the conclusion of your last trim operation (latent, unannunciated
failure) then as soon as you pressed the "arm" button, you'd get an un-
expected trim operation and your reaction to release the button is
a few hundred milliseconds away from successful trim shutdown . . . you
can't do it any faster any other way.
For airplanes with autopilots and powerful servos, the situation
is a bit different. Here we have mulitiple, electrically driven systems
with the ability to drive flight controls. The common wisdom here is
to fit the aircraft with an "autopilot disconnect" button on the wheel
or stick. When you think about it, the pilot's first reaction to
countering
an unexpected input from electrically powered flight controls is to grab
the stick to keep the dirty side down and the pointy end forward. It
stands to reason that all the controls necessary to corral the runaway
system should be on the stick or wheel too . . . the "autopilot disconnect"
or "master trim disconnect" button would have positive control over all
electrical devices connnected to flight controls.
When things are back under control, the pilot may elect to experiment a
bit to see if it was trim or autopilot that caused the upset and shut
down the offending system (perferably with panel mounted, clearly labeled
switches . . . not little black buttons hiding in a flock of other little
black buttons).
>. . . I know for a fact if the trim were all the way to one extreme you
>would have a hell of a time bringing the airplane back in one piece.
If this is true, has your trim system been fully evaluated for both
speed and travel limits? I have several builders I'm working with now
trying to get them to revise the mechanism that connects servos to tabs.
Trims in a mechanical limit on these airplanes produces a barely flyable
airplane.
This conversation goes right to the issue I raised a few weeks ago with
the Great Breaker Debate . . . we've commonly viewed and many of us have
been REQUIRED to compensate for POOR SYSTEM DESIGN by fiddling with
breakers.
IMHO, no matter what fails or how it fails in your airplane, your
reaction should be no worse than, "Oh fudge . . . is that thing
broke again! I think I'm gonna have to find a better part." My brothers
in the certified world would like to design airplanes with that
philosophy but are prevented by a host of regulatory and bureaucratic
walls and mountains. Unencumbered by such obstacles here in
amateur-built world, we're going to build the best airplanes that have
ever existed.
Bob . . .
AeroElectric Connection
////
(o o)
| |
| Go ahead, make my day . . . . |
| Show me where I'm wrong. |
|