spring trim metrics

I got the mechanism for my trim system sorted out today. I originally
had a slider block that traveled along a steel rod, now the slider
block is part of the steel rod and the whole assembly travels along
oilite bushings, I am very pleased with the smoothness now, previously
when actuating the trim the slider block would bind due to the force
vector of the knob and the spring/tang locations, not so
now....................so...............here is my
question...............

When the trim is at center postition, is the stick force to move full
forward and full aft, less than the stick force when the trim is full
forward or at full aft position? This is not with the tail on and the
elevator providing any weight.

The way I have it rigged now the force at neutral is even to full
forward and full aft, and maybe about a pound or less of force. I am
finding the selection of spring length, tension force and where it is
pinned is somewhat tricky. The forces when in full forward trim and
full aft trim give me a nice feedback gradient, it just feels a little
weak at center and was wondering what sort of feed back your trim
system may give in the neutral position.

I have a friend with a nice homebuilt that has no trim
system..............but I think I want to have trim.

TIA,
Brad

On Jan 31, 9:38*pm, Brad wrote:

When the trim is at center postition, is the stick force to move full
forward and full aft, less than the stick force when the trim is full
forward or at full aft position? This is not with the tail on and the
elevator providing any weight.

For the Schleichers I have owned, 19 and 28, the trim forces do not
change throughout the trim range. One end of the trim spring is
attached to the stick and the other end to a "fixed" point. The trim
is adjusted by moving the "fixed" point. The trim springs are steel
rods with one turn in them.

The significant difference between the 19 and the 28 is the way the
trim is adjusted. For the 19 it took one hand, for the 28 it takes
2. Not an improvement in my opinion.

The mechanism for the 28 is a bit more complex that I can describe
here but I'll send you a photo if you email me.

Without the trim springs the stick forces would be very light and the
glider would not be easy to fly. I assume the certifications
standards require a minimum stick force gradient but I someone else
can give you a reference.

Andy

On Jan 31, 11:38*pm, Brad wrote:
I got the mechanism for my trim system sorted out today. I originally
had a slider block that traveled along a steel rod, now the slider
block is part of the steel rod and the whole assembly travels along
oilite bushings, I am very pleased with the smoothness now, previously
when actuating the trim the slider block would bind due to the force
vector of the knob and the spring/tang locations, not so
now....................so...............here is my
question...............

When the trim is at center postition, is the stick force to move full
forward and full aft, less than the stick force when the trim is full
forward or at full aft position? This is not with the tail on and the
elevator providing any weight.

The way I have it rigged now the force at neutral is even to full
forward and full aft, and maybe about a pound or less of force. I am
finding the selection of spring length, tension force and where it is
pinned is somewhat tricky. The forces when in full forward trim and
full aft trim give me a nice feedback gradient, it just feels a little
weak at center and was wondering what sort of feed back your trim
system may give in the neutral position.

I have a friend with a nice homebuilt that has no trim
system..............but I think I want to have trim.

TIA,
Brad

Any spring based system will hav no real change in gradient when you
move to a new
trim position, just same force to move it the same amount off trimmed
location.
Suggestion is leave yourself some options to change the spring during
flight test.
Heavy rates required by current standards are actually pretty
annoying, requiring retrimming a lot
in my view.
Also worth noting is a requirement for the glider to return toward
trimmed speed which involves the control
circuit and , in most gliders, adds a cusp on the elevator that acts
like a trim tab to add stick force as speed increases. Don't know if
your tail has this.
FWIW I prefer my stick forces in ounces.
Good Luck
UH