Mounts and Hatches for a Model Aircraft Wing
I have stopped timing the wing construction at this
point. I worked on the hatches at various stages over a couple of days
and lost track of time so often that I couldn't give an accurate estimate.
My intention was to show that building time is not dependent on glue.
It is mostly in parts fabrication, dry-fitting and other preparation.
The hatches turned out to be more complicated
than anticipated. I originally planned to use Futaba S3002 metal-gear,
ailerons. These servos have (depending on who's specs you believe)
52 inch-ounces of torque. They also weigh (again, specs from different
sources vary) 1.27 ounces each. That is over 5 ounces in servos alone
not counting the
hardware, extensions etc. It also is a combined total
of 208 ounce-inches of torque for the ailerons on a .40 size aircraft.
Because of the bearing in the top of these
servos, the arm is slightly higher than a non-bearing servo. The hatch
cover requires a hole that will allow the portion of the servo case around
the output shaft to go through it.
ended up using JR C341 mini servos that do not have a bearing. These
are rated at 31 ounce-inches of torque and weigh .67 ounces — roughly half
as much as the Futaba servos. However, the servo case is flat on top.
Mounting the servo so the shaft sticks through the hatch cover does not give
the necessary clearance between the servo arm and the cover. This left
me in a quandary.
A lot of people mount
their servos on the side with the arm coming out of a slot. I have had
nothing but problems with this set up. First, it is tedious getting
the geometry set up. Second, I have yet to be able to use the full
throw of the servo without it binding. I usually end up having to dial
down the end points to 50-60% which is not good.
I decided I was going to use my preferred setup
even though it made building the hatch covers much more difficult. I
could have cut the plywood
doublers before laminating them to the covers,
but at that time I did not realize there was going to be a problem. I
ended up having to use a router after the fact. If I had cut them out
before hand it would have made the entire job easier.
Although it is not clear in these images, I
located the servos in bays that also have
hinge support blocks. This
decision was made so that the control horns would be located close to a
hinge which reduces
slop in the control setup.