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By Sam Parfitt
Page 6
Retract system
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Cut a new servo tray to lay over the old one in the main
fuse. In the middle will be the standard nose steering servo. Flanking
the nose servo will be two micro servos (Hitec HS-81) to control the retracts
and brakes.
I'm using the micro servos so I can get 3 servos in the same place as
the normal two servos.
Top left is the Spring air control valve.
Below left is the Ultraprecision brake valve.
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Gluing in the air tank and the new servo tray.
I moved the tank back as far as it would go to make room for the new servo
tray.
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Something to do while watching the game:
There weren't enough decals cut on the sheet to do the whole canopy, but
there's enough decal left on the sheet that you can just cut some more
strips and use those.
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I'm putting all my air and charging outlet in the nose
of the fuse.
Along the inside is black plastic used for auto wire protection.
It has a slit on one side to put all the wires/lines in.
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The fill valve and robart's pressure gauge.
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I had to cut 1/2" off the top of the strut so the wheel would fit inside. |
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Retracts up. |
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Retracts down. |
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Setting up the nose strut is presenting some problems.
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As you can see in the picture, the masking tape (the edge
away from the wheel) is where I have to cut the strut so the plane sits
level to the ground. the black line above it is where the internal cylinder
is to put pressure for the oleo action of the spring. As you can see, we
have to cut below that black line.
Presently, I'm figuring I can push that internal cylinder down about 1"
so we can connect the strut to the retract. I'm also going to have to cut
the spring shorter so it still works on the oleo. |
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I'm setting up two nose struts using the 7/16" that
came with the P-61 ARF (top one in picture) and a 3/8" strut that
I had as a spare (bottom one in the picture).
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There's only a 1/16" difference in the diameter but
as you can see in the picture, the overall size and appearance of the 7/16"
strut is much beefier that his smaller sister. The nice thing about the
3/8" is no cutting is required. Comparing real pictures, the 3/8"
strut and wheel appears more scale with the 2 3/4" wheel. It may not
hold up as well though if you start going off the runway and into grass.
The bulk of the weight is definitely on the mains but being out front, severe
turns at a high speed or heavy grass as high speed may do it in. |
Working on the 3/8" strut first:
I using my vise, versus using a hammer, to push the 'plug' into the oleo.
(the 'plug' being the sleeve with a hole in it that makes a perfect fit
to the male end of the retract).
I've taken the oleo apart so not to damage the offset weld (it can't take
this kind of pressure). I also put a piece of wood at the other end so the
end is not damaged by the vise. I rounded (beveled) the end of the 'plug'
so it is easier to align and get it to enter the strut.
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I male end of the nose retract is a skoch larger that 1/4"
in diameter so I used a 17/64" bit to drill out the plug in the
strut.
(I used a small square first to make sure the strut is perpendicular
to the drill table).
As usual, I used cutting oil to keep friction/heat to a minimum.
After drilling the hole, all that was needed next was to put the strut
back together again. (holes and tapping will be done later to hold the strut to the retract).
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Nose gear: 7/16" diameter strut modifications.
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To get the plug out that holds the spring against the bottom of the
strut,
I had to use a center punch to knock it out. |
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I was initially worried that it may be welded in there,
but my fears soon faded when the plug came out very easily. We won't be
using it again because the bottom of the plug that attaches to the retract
will now be compressing the spring. We have to do this due to the amount
of the top of the strut that is cut off. I cut off 2 3/4" off the top
of the strut with a dremel cut off disk |
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The plug (sleeve) that comes with the 7/16" strut is
too large in diameter to go into the strut (at least, on mine it was).
I had some pieces of a 3/8" strut laying around and this is about
the ID (inside diameter) of the 7/16" strut.
I used a 1" piece of the 3/8" strut and two of its plugs (sleeves)
to put into the 1" piece of 3/8" strut.
Again, I used the vise.
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Here is the 1" piece of 3/8" strut with the two
plugs in them. We now have to drill out a 17/64" hole in the center
as we did for the 3/8" strut. I had to sand the paint and some of the
metal off the outside of the 1" piece of 3/8" strut (we shouldn't
really call it a strut since it is now just a larger plug (sleeve). To make
sure that this new plug is not too large to go into the 7/16" strut,
keep sanding the outside until it fits 3/8" hole in a drill gauge (
a drill gauge just has a bunch of holes in it indicating what size hole
each one is). Once the outside diameter fits through a 3/8" hole, I
can now press fit the plug into the top of the 7/16" strut. |
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NOTE: since I had to sand the 3/8" piece of strut
to get it to fit, you might as well just use the original plug (sleeve)
that came with the 7/16" strut and sand that one down until it's 3/8"
in diameter. I did the sanding by putting the sleeve on a drill bit so it
could move freely and then held it against the disk portion of my table
belt sander. The sleeve rotates when it meets the sanding disk but this
way, it evenly sands the entire surface.
A little care must be taken when putting metal to a fast moving disk! I
used a small chisel to keep the sleeve from coming off the drill bit and
pushed the sleeve lightly against the disk platform to help slow it down
a little bit while the sanding disk does its job (this was the only way
I could think of evenly sanding the surface of the sleeve: I'll do stupid
things like this around sanders where the worst I'll get is an abrasion
on the skin, Never around cutting blades!). Always wear eye protection.
Since I moved the plug lower to where it used to be to put pressure
on the spring, I had to do some additional cutting so things wouldn't
bind. The lower part of the strut that holds the wheel, I cut off 1/2"
off the top male part that slides into the upper female half of the strut.
I also cut 1" of the spring off.
This seems to be about right as the oleo still functions (amazing, luck
perseveres again!). |
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3/8" Nose strut.
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Here we have the 3/8" and the 7/16" nose struts
made up. If one doesn't work, I'll have the other for a backup. The 3/8"
is easier to make. The 7/16" can handle more stress. It's 'your call'
on which is better for the parameters of your field and flying experience.
Both oleos from the bottom to the top (not counting the wheel) is 6 3/4"
long. The larger wheel on the 7/16" strut made it a 1/4" longer |
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7/16" Nose strut
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For the nose strut, we need to drill and tap for two set
screws (two to make sure if one fails, we have a backup,
Freely translated: lets not hose up our 2000 buck plane)
Here's the pilot holes drilled on the nose strut for the set screws.
(This tapping not needed for the mains since the whole strut goes into
the retract).
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After using a #40 drill bit to drill the final holes in the
strut, we tap the two holes.
(A well stocked hobby shop should have packages of a drill and a tap as
a set).
(remember:make sure you use some oil when drilling and tapping).
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After drilling and tapping, the set screws can now be put
in.
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I just noticed that the main wheel sticks out.
Those black plastic pieces in the ARF are going to be functional.
Looks like we'll later cut a hole in our gear doors and glue the plastic
covers over the hole to cover the wheels.
(hmmm: picture 38 in the manual shows the procedure)
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The nose retract and strut are installed.
The retract must meet the back of the retract wooden mount.
This is necessary so the steering control arm has room when the retract
is up (you'll see this in the second picture).
I also had to trim the one former holding the back of the wooden mount.
You have to cut the side off so the control arm can move by it, and also
so the back of the retract sits all the way back on the end of the wooden
mount.
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Nose strut up.
Here you can see how far back the retract is from the front former. You
can also see the room needed for the steering arm as the arm goes below
the top level of the retract wooden mount.
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Note that all 3 struts have the oleo scissors toward the front of
the plane, as with the prototype.
If your retracts bind: check that you have the set screw all the way
in. Also, loosen (one at a time) each of the four screws that hold the
retract to the wooden mount until the retract moves freely. The last screw
loosened is causing a bind. You'll have to shim that corner up before
tightening the screw. |
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| Left main up. |
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Right main up. |
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