A number of items went by on this topic recently
and several have statements/questions that I'd like to
comment on. The following mini-diatribe is a response
to a mixture of original posts:
One gentleman asked what the the big deal was about trim
runaway and allowed as how stick forces shouldn't become
so great as to make things unmanagable . . .
I'll suggest that stick forces alone, while they may not
be great for a particular situation, there may be elements
of surprise or distraction that precipitate the mishap.
Grahm's situation is a good example. His airplane may indeed
have been quite flyable in the condition he discovered but
as soon as he sensed something wrong, his modus-operandi
became one of aborting a takeoff within the spaces which
remained in front of him.
Had he KNOWN exactly what the problem was, elected to deal with
it and continued the take-off, the whole thing might well have
ended more happily. But he DIDN'T know and elected to reject
the takeoff. The damage to his airplane was not a direct result
of trim runaway but from a secondary set of circumstances that
might have been triggered by any number of other malfunctions.
Let's consider the issue of stick forces for trim extremes.
How many of you already flying KNOW how manageable your airplane
is with trim set to either limit? This experiment should be
part of your flyoff plan. Go around the patch with increasing
degrees of mis-trim in both directions. Do the same exercise
at cruising speeds as well. Land, and note the position of your
trim tab or make careful note of what your trim indicator meter
says (I like them MUCH better than three-light systems). Do this
exercise for both forward and aft C.G. locations . . . some
airplanes handle very differently at their C.G. limits.
If you find that your system as currently configured is
CAPABLE of presenting you with uncomfortable stick forces,
you should take steps to physically limit the tab's travel.
This should be accomplished by reconfiguration of horn
and linkage geometry. I've seen builders add auxiliary
hard mechanical stops which caused damage to their actuators
because the internal limit switches were set OUTSIDE the
travel of the hard stops.
Many homebuilts have far too much trim authority both in travel
-and- speed. I'm building trim controllers right now for
a group of TA-16 Trojan builders (a big, all metal, 4-place
anphib). We've discovered in early flight testing that the
MAC servos (it takes two per airplane) are much too fast
with a full 14 volts applied in cruise. However, if we
slow them down with voltage adjustment for cruise, then
the system is frustratingly slow during approach. The answer
in this case is a two-speed switch build into the trim controller.
For the Lear 35's I used a switch that actuated at 10
degress of flap or greater to initiate high speed operation.
On the Lear 55 we had a microswitch on the stabilizer
actuator to select trim speed . . . the stab jack was
aways set for a LOT of up-trim in the approach configuration.
I proposed a later design for the Lears that would use
air data values for IAS and adjust trim speed accordingly
to give the pilot a fixed perception of trim action irrespective
of IAS. They never did act on that proposal. A similar
system is still quite applicable to airplanes like the
Lancair or Venture. Ships the size of Europa and Kitfox
can probably get by with a single speed but you still need
to decide WHAT the most comfortable speed is and design that
into your system.
On big airplanes, trim speed was limited by the pilot's
ability to react to a trim runaway. Hands in lap and
a/c trimed for level flight - trim runaway initiated.
Pilot had to wait 3 seconds before reacting. Trim
excursions were not allowed to be so fast that he
could not regain control of the airplane. The 3-second
wait was to allow for the "surprise" factor I spoke
of above.
Has anyone seen the trim used on the Rans S6?
It's looks very much like the Cessna type . . .
. . . wheel for an adjuster. The wheel is part of a worm drive
hooked to a solid wire/cable back to the tab. Very simple,
light weight and all mechanical, no electrics, easy to
see indicator. . . .
There are many variations on the theme out there. The most
important design consideration is slack and/or deflection
loading that can occur in the system when new, when at end
of service life -AND- when some part of it becomes disconnected.
The reason I'm doing this electric trim for the Trojan is because
one builder experience a very dynamic, low speed "flutter" that
drove the elevators stop to stop at about a 1 cycle per second
rate. Very violent but fortunately it occured at low speeds
(approach) and didn't overstress anything. They're not sure what
combination of things brought it back under control (the yolk
was untouchable during the event) but recovery and landing was
made.
The problem proved to be slack generated in the mechanical trim
system when pitch loads deflected structure between cockpit
and tail. After all things were considered, a very short
coupled electric system seemed to be in order. We've begun
flight testing and have yet to develop limits and speeds but
it WILL be done and SHOULD be done on every new airplane.
Seems like we (meaning most kitbuilders and manufacturers) spend
a tremendous amount of time trying to make things more complex
in the quest of convenience. Perhaps we should just look around
more at what has already been done by others so that we can improve
on good sound concepts. I don't mean that we shouldn't try to
come up with something new; there's nothing wrong with that.
It's just that new isn't necessarily always better. I think that
whenever we can serve our purpose with something that is simple
and mechanical, meaning non-electrical, (no offence Bob!) . . .
No offence taken . . .
. . . and does the job that's intended, then we've
accomplished a lot.
But take care that "simplification" doesn't negate some very
important aspect of system performance that the original designer
worked very hard to insure. When it comes to structures and
aerodynamics, not everything is plug-n-play. Amateur builders
have enviable opportunities to explore and incorporate
improvements but there are sound engineering and flight test
principals that have kept test pilots alive for a lot of years.
In the Trojan project alone, I came close to loosing two clients
in two different incidents in a half a year. All attributable to
deficiencies in ORIGINAL design that came to light long after the
first airplane had flown.
Rigidity, multiple load paths, travel limits, speed characterization,
satisfactory failure mode effects analysis, etc. are not just buzz
words. Before you consider any modifications, talk to the kit
designer. Bring your ideas to every forum that will discuss it with
you. Stand up before the world and defend your approach. Only after
you can field all the rocks thrown are you ready to build. Then,
seek the advice of an experienced test pilot to build a flight test
plan that allows you to sneak up on potential problems and perhaps
tickle them just enought to make 'em giggle . . . before they
turn around and bite.
There's nothing wrong with looking at other approaches . . . for
airplanes like Kitfox, may I suggest you look over the system
on the early Pipers? The PA-22 has a mini-stabilizer jack
screw driven by a crank and cable arrangement from the cockpit
overhead. The aerodynamic rigidity of this particular system
is excellent. Lot's of it's pieces can come unhooked without
causing a hazardous condition.
I'm all for simplification whether elecrical or mechanical but I'd
also like to read contributions to these forums that
speak of carefully explored successes; not of smoking holes in
the ground.
Regards,
Bob . . .
AeroElectric Connection
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