I used home-made servo-savers on my Ducks back in the 90's. These
worked well but required a bit of fabrication. Note this is for
bottom-horn, bottom servo-arm installations only. It will stretch upon
being pulled, so if you are using bottom servo / top flap-horn you
need a stretch-when-pushed geometry instead.
Each servo-saver consists of a pushrod, two clevises, two brass
tubes, and a spring. The brass tubes are two sizes. An "inner" tube
fits over the pushrod and the outer tube fits over the inner (.128
OD ) tube. The inner tube's ID (0.01) is the same as the pushrod's OD
(0.08) and the outer tube's ID (0.13) is the same as the inner tube's
OD.
Note that you COULD use just one tube instead of the inner/outer, but
the latter works better. The first ones I built had a "Z" bend instead
of a clevis on the flap horn end, but the unit with clevises on both
ends works better using the inner/outer tube approach.
Cut a steel, threaded pushrod or piece of all-thread to 1- 2 inch
length and thread the clevis onto it. Cut the inner brass tube to the
length between the servo and flap horns with the flap at neutral and
servo arm in corresponding position.
Cut the outer tube to the same length as the inner tube. Solder a
clevis into one end or the outer tube. Trial fit and cut the inner
and outer tubes blank ends so that the collapsed assembly fits between
the flap horn and servo arm, less about 1/4" or so for clevis
adjustments. Remember, this is the collapsed length.
The assembly is held together using a spring which stretches then
retracts (pulls) the unit back to the adjusted length. When the flap
contacts the ground, or your shop door, etc, the unit stretches, and
is then is pulled back, acting like a shock absorber.
The spring must easily slide over the outer brass tube. You can find
one with the correct ID (.156) at ACE hardware, etc. a 2" spring will
do in most cases. It will be cut to length in any event. As for
tension, I don't have any data, but you should be able to judge the
correct amount of tension. The 3" springs I selected had a wire
diameter of 0.02". Just remember that the tension needs to be enough
to hold the flaps in place during launches but still allow flap
deflection upon contact. Buy several so you can experiment to find
the best for your specific application.
The last step is to attach a spring. This may be accomplished by
soldering one end of the spring to the outer brass tube where it is
soldered to the clevis, and bending the last coil on the other end of
the spring, cutting and bending the last coil so that it may be
hooked into the clevis' "Y".
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