Techpod

In the last post we installed the pod cover.  In this post we will install the rudder servo and rod. We have used the Hitech 82MG servo for the rudder, the same as the elevator servo.

The servo is located in the cavity below the boom under the pod.  Insert the long control rod in the boom from the small side hole at the rear of the plane. As it approaches the side hole near the front of the boom, push it through so it feeds through the side.

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Install the servo arm on the control arm, drill the servo arm hole as required. Dry fit the servo with the control arm in place and centered. Test the control arm and ensure that it can move freely. Note that the arm is located inside the boom. The 82MG fitted perfectly requiring no modifications.   Using a glue gun focusing on the foam sides and corners, install the servo.

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With the servo installed, we can clean up the wiring and install the foam cover. We pushed both wires through the bottom foam join below the boom.  In addition, the elevator servo wire was hot glued to the rear of the boom hole (opposite side of the servo) to keep it well clear of the rudder servo arm.

Our foam cover interfered with the servo arm’s movement and accordingly we cut a section out to resolve this. In addition, we trimmed a little around the ends for the JB Weld glue residue on the boom and also for the elevator servo wire. After these changes, the foam cover fitted well.

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The cover was glued with only a little hot glue, a stripe along each side to allow it to be removed easily if required. In any event, we plan to tape the bottom of pod with reinforced tape to protect the foam on landing.

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If you have completed the soldering of your tail servo plug leads, now is a good time to fit the clear plastic ‘NADIR’ cover. After carefully roughing the very edges of the clear plastic cover it is glued in with a narrow bead of got glue place along the recessed area of the foam.  Not that the plastic is shaped with the higher ‘pointy’ side placed at the rear.  You will need to hold the cover down firmly in position whilst the glue sets.

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We also took this opportunity to tape the bottom of our plane. One strip down to the end of the foam. Three strips from the nose to just behind the servo cover minus the clear NADIR cover. We also placed a thin half size strip around the edges of the NADIR cover.

In the last post we completed the two halves of the foam pod.  In this post we will detail the construction of the pod cover.

The cover is effectively held in place by two plastic bolts which are recessed into the cover.  The bolts bind to the two timber plates in the fuse which have sprung clip on nuts.  The alignment of the bolts is critical to the success of the task. However, the use of the nylon washes greatly assists with this alignment process. The image below identifies the pieces, in addition to the cover, that are required to complete the task.Techpod Build-90

The two longer ply pieces are fitted to the pod whereas the two short pieces are fitted to the cover. Don’t be tempted to clip the nuts as there is a risk they will be contaminated with glue whilst installing.

Assuming they ply holes look well centered, glue the long ply taps to the pod with CA.  The rear tab has a slot in the pod whereas the nose tab is merely a recess.  Once the glue is set, run a narrow bead around edge with a glue gun (particular the front ply tab), where the ply is in contact with the foam. You can also access the underside of the front tab through the front nose. Once the glue has dried, install the clip nuts to the ply holes, they merely slide and snap into position.  Be careful not to unseat the tabs from the pod.

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With the nuts installed, partially screw the bolts into the nuts backwards, in other words, from the button up. The bolts should only be exposed few millimeters on the top side. Now fit the cover pressing the cover firmly against the bolts such that the bolts leave a small indent in the foam of the cover.  This indent will provide a good guide for the bolt hole we will make later.

If you dry fit the cover tabs, you will note that they do not line up well with the small pilot holes in the cover and also extend beyond the edge. Using the pilot holes as a centre guide, mark any ply or foam that requires trimming. Make sure that the indent we made a little earlier is comfortably within the hole, Whilst indents which are closer to the centre are  better, it does not have to be in the very centre.

Once the cover ply tabs are trimmed, install with CA and then run a bead around the edge with the glue gun where the ply is in contact with the foam. The bead creates a lip against the ply being pulled out vertically. Don’t place any hot glue on the outside edge as we will cut this shortly to fit the nylon washer.

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Using the indents we made earlier, make a hole that just fits the nylon bolts, but not the bolt head. We used an old soldering iron to make a nice clean hole.  

Before making the hole, you can double check the alignment with a straight thin rod placed through the cover pilot hole. Whilst the pilot hole may not be the planned centre of your expanded hole, it will however give you an idea if you are way off.

Fit the foam cover and bolt into place. With the bolts fitted, mark the circumference of the foam with a thin pen. This marking will be your guide to cutting the foam cover to recess the bolt head in the foam.

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With bolt hole now complete we need to ensure that it will hold securely.  If you had not observed already, the holes in the ply tabs are quite large relative to the size of the bolt head.  This is effectively secured with the nylon washers. With sharp knife, cut a very narrow slot into the foam cover just above the ply tab.  This hole will be used to slip the nylon washer into place. Once the washer is in place, dry fit the cover and bolts to ensure the alignment is correct. Once correct, close the hole with a narrow bead from the glue gun.

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The final product looks like this:

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In the next post, we will detail the installation of the rudder servo and cover.

In this post we will mount the motor and glue the pod to the frame.

The foam pod half is glued to the ply skeleton and the other half in various places.We principally used Goop with some CA. Care must be taken to ensure that the no glue is applied to the moving parts of the clamp.  You can choose to glue both halves in one go, however, we elected to do each half individually as we weren’t confident of the glue coverage before the glue set.

Before you can glue the halves, mount the motor and run the motor cables into the plane fuse. We split the cables across the spine to minimize the cable bundle size given this is a narrow part of the foam fuse. Our motor cables were too short and were given a short extension. As we were planning to mount the ESC in the rear of the fuse, the new extension leads were just long enough to allow a quick connection to the ESC plugs.  Any shorter and the available space for your finger to access the plugs would have been difficult.

Once the leads were in place, we used a glue gun to hold the leads in place. With the leads in place we completed a dry fit which highlighted that the fuse did not mount flush as there is no cavity for the motor leads. Using a sharp craft knife we cut a small channel down and around the motor mount.

Once everything was dry fitted, we applied Goop to all of the contact points except the clamp. As some gaps remained, we used CA to fill the gaps and tape to hold the pieces tightly together.  Remember to also apply Goop to the front and back of the two ribs where they slot into the foam. Once the first half was set, you joined the second foam half.

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In the next post we will fit the rudder servo and the pod access cover.

 

In the last post we completed the assembly of the wing spar clamp.  In this post we assemble the remaining pod ply pieces join the ply skeleton to the boom. As usual we dry assemble repeatedly and analyze the build from every angle to ensure the parts are correctly aligned.

Start by dry fitting the skeleton inside both sides of the foam pod. The correct position is the where the spine aligns with the rear of the pod and also the edge of the boom at the bottom (see red lines). You may need to remove the boom to check the bottom alignment is correct.

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The first challenge to achieving a good fit was the extra thickness of the ply spine resulting from from the fibreglass and our addition of carbon fibre strips. Using a very sharp craft knife, we removed the necessary foam to achieve a neat fit.  Depending on the fibre glass finish , you may need sand any raised areas. The image below, shows the before and after the removal of the extra foam around the motor mount.  We also took the opportunity to make a small recess in the foam for our motor mount  nuts. These small recesses may not be required dependent on your motor mount system.

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With the excess foam removed, we can revisit the fit of ply skeleton. In the case of our plane, the rear alignment was great wheres the bottom did not. The bottom of the spine appeared straight, however, the spine was just under a 1 mm too long.  Using a ruler, we drew a dark line the full length of the spine bottom marking the correct alignment. Using a long sanding block, sand the length of the spine until the spine is flush with the line.

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In addition, our two boom holes were just a little tight for comfort. These were improved with sandpaper and a file.  Although not a big issue, try to preserve the small wooden alignment pin in the front rib boom hole.

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Once all of the parts are ready for fitting, we are ready to glue the rear rib, with clamp installed, to the spine.  Use a square to hold the two pieces at 90 degrees whilst the epoxy sets. A drop or two of CA is useful as a instant position holder. Alternatively, you can tape the square in place. IMPORTANTLY, ensure that you have the rib facing the correct direction.

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We are now ready to glue the boom to the ply spine. Dry fit the boom and mark the area where the boom will be in contact with the spine. As my white marker was dry, we used tape. Give a light sand to the boom in this area and clean with a damp cloth. Apply a narrow strip of JB Weld to the boom and gently fit the ply spine to the boom. Try not to smear the JB Weld with the spine until its in place as we want to ‘plant’ the spine into the JB Weld forming a small ridge of glue on each side of the spine.

Now the spine is in place, we can focus our attention on the alignment.  Start by placing a strip of tape over the boom and attach to the two sides of the spine. The tape should be tensioned to support the weight of spine and also limit the movement of the spine until the glue drys. To get the alignment correct, we are going to use an ‘eyeball’ approach.  Whilst this may seem rough, we have special gun sights for the job!

Turn the tail upside down and rest the elevator stab on the bench. Place something under the motor mount to support it at roughly level. Stand around a metre back from the ply skeleton, stretching out your arm, and using one eye, look down the length of the boom through the motor mount and also the centre hole, just above the bottom of motor mount.  The spline will be perfectly aligned when it lines up with the centre carbon rod rod in the tail stab. The first image below identifies where the boom is incorrectly aligned whereas the second image has near perfect alignment.

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In our next post we will finalise the motor and servo wiring before joining the two foam pod halves together.

 

During the last post we attached the ply motor mount to the spine of the pod frame.  In this post we will prepare and assemble the wing clamp.

The clamp is used, via a clamp screw to hold the wing spar  in place whilst flying thus allowing for easy dis-assembly during transport.  The images below detail the end result of this post:

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To complete the clamp you will need the following key pieces. In addition, you will require the two 14 mm self tap screws and the washer for the clamp screw.

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Start by dry fitting all of the ply pieces and insert the Delrin tube. Take particular note of the direction of the Clamp Rib. The Clamb Rib fits properly only in one direction, see how the bottom tab of the rib fits neatly into a recess in pod foam. Once you feel comfortable with how the pieces fit together, remove the ply spine.  Now squeeze the clamp by hand and test resistance of the Delrin tube being pushed out sideways.

You will likely note that the pressure of the two clamps is restricted which hinders their ability to grip the delrin tube. This is due to the extra ply layer at the top of the rib not aligning correctly. Inspect the  rectangular slots where one layer is higher than the other. Using a flat file or sand paper, sand down this excess until you get similar movement in both clamps. The image below identifies the latitude of movement you are seeking.  Mark each of the clamps with a pencil as to their fitting location, ie.  T(op) and B(ottom).

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The clamp mechanism uses two self tap screws as additional protection should the clamp not grip securely during flight.  Drill two 2 mm pilot holes in the top clamp for the tap screw. Dry fit and remove the screws. 

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Top Clamp

As the clamps perform the challenging work, we will add extra strength by dousing both clamps, on both sides, liberally with thin CA. Once the CA has hardened, add the hidden nut with a hammer to the lower side of the bottom clamp. You will need to drill out the existing pilot hole to around 4-5 mm before installing.

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Bottom Clamp

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Dry fit the clamp and Delrin tubing again but this time use the clamp bolt to hold the clamp in place.  The kit includes a washer which should be installed at the top (head) of the bolt. Apply just enough pressure to observe the Delrin tube deform slightly. However, you will likely observe that the movement of clamps is now limiting by the Clamp Guide preventing the Delrin tube from being firmly clamped (refer red arrows in image below).

Using a flat file or sand paper, progressively remove enough of the clamp guide slots to allow the clamp to squeeze further to a point where the Delrin tube starts to deform slightly. You will want around 1 mm of additional gap in the clamp guide slots. Do not remove too much as you will risk the clamp being over tightened and potentially snapping a clamp     

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Dry fit all of the clamp parts again, this time with the ply spine attached. You will likely observe that the clamp bolt tip collides with the ply spine. Use a file to remove a little allowing the screw to extend as required. Ensure that the removed area does not interfere with the recess that holds the Clamp Guide in position.

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Delrin Tube

In the next part of the post we will modify the Delrin tube to function.  Join the two wings together with the delrin tube as the joiner. Don’t stress the joint as we do not have the internal carbon rod installed.  The objective of this exercise is to ensure the Delrin tube is not to long. In our case, the length was perfect or we would be required to trim any excess. Note the front of the wing join is flush whilst the back maintains a gap of several millimeters. Once you are comfortable with the length, cut the Delrin tube into two equal halves. Clean up any burr from the cut with sandpaper or file.

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Once cut, dry install the Delrin tube on each wing spar and then slot the spars into the clamp. Ensure the tap screws are well raised with the tip not exposed on the underneath side of the top clamp. Push the wings firmly together and tighten the clamp to a gentle holding pressure (as the carbon rod is not installed). Gently tighten the tap screw until the screw tip cuts a small mark in the Delrin tube. Disassemble and use these marks to drill a hole of 3.5 mm in the Delrin halves.

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At this stage, the tap screws are too long and will extend through the Delrin tube wall (3 mm) into the carbon wing spar. If the screws were to strike the spar, the result could be severe. Accordingly, the tap screws will be shortened where they protrude only 2mm through the clamp giving a 1 mm buffer.

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After grinding the screws down, this completes the clamp mechanism. In our next post we will start to assemble the various pod components together.

During this post we will commence the construction of the pod.

Essentially the pod comprises a ply skeletal frame which is then surrounded by foam. The boom is glued to the bottom of the frame and the wings are held in place with a wood clamp.  Also attached to the frame is the motor mount.

Motor  Mount

The supplied motor mount slots and glues into the main large central spine of the frame.  The top of the mount sits flush with the foam and someway forward of the rear of the pod. Essentially, this requires your motor mount to be resolved early in the build as any retrospective changes are possible but challenging.

The ply mount holes are 25 mm x 25 mm. If your motor mount holes are different, say 25 mm x 20 mm as is our motor, additional holes are required to be drilled into the mount. Alternatively, a custom modification based on the following will provide more flexibility for motor changes down the track:

http://fpvlab.com/forums/showthread.php?13707-TechPod-motor-mount-upgrade

Essentially it uses two motor mount plates, one with 25 mm x 25 mm and is bolted to the Techpod ply mount. The second plate, which specifically fits our motor, is bolted to the first plate with stand off spacers between. Our motor can easily be bolted onto the second plate later in the build. Also, by changing the rear plate, we can install different motors as required.

Which ever mount system you adopt, ensure that the propeller will extend past the rear of the fuse.

Installing the Ply Motor Mount

To install the ply motor mount, insert the ply mount fully into the slot. The mount is held in place with with fiberglass matt and epoxy.  The kit includes the matt, which is cut in half  and applied to both sides.

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In the images above we have elected to apply two short carbon fibre strips to each side.  These are not required or part of the kit.  We have used 30 min epoxy to wet the cloth. As the fibre glass is quite thick, it does not bend around corners without air bubbles.  Accordingly, we have used a large freezer bag over the whole piece and then removed the air with a vacuum cleaner. We  recommend gloves whenever working with epoxy.

This is the end result which requires minimal sanding.

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In the previous post we installed the servos and wiring along the wing. In this post we will install the the carbon spars. and foam cover for the spar.

Techpod Build-47Start by lightly sanding the spar all the way along, wiping any dust with a damp cloth. Test fit the spar and foam cover making sure no cables interfere. Use a small dab of low temp glue gun to hold any stray servo wires in place.  If you have not already done so, trim the foam cover around the servos as required.

You will note that the spar is a tight fit and accordingly it takes a little effort to ensure that there are no raised edges around the edge of the foam cover. Any gap weakens the wing and may disrupt the airflow across the wing.

To glue the spar and cover in place in we have used Goop, CA and ultimately reinforced tape.

Start by covering all areas of the foam slot (bottom and sides), spar and, the underneath and sides of the cover with Goop. Given the extent of area required to cover, this task is better done by two people.  Use gloves to enable you to quick spread the glue. Once the glue is in place, install the spar and cover paying particular attention to ensure the cover edges are flush with the wing.

Once you have done the best you can, run a bead of CA along the cover join. Systematically work along the join with accelerator holding the cover firmly in place and flush with the wing.  Once this task is completed, run two strips of reinforced tape along the length of the wing covering each side of the foam  join.  Apply pressure ahead of the tape to help ensure the cover is flush. Place a strip or two over each servo cover.

In addition to a better finish, the tape helps further stiffen the wings and hold the servos firmly in place.

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Other Comments

Please note that we wont be installing the spar joiner tube and rod until later during the pod assembly.

 

In this series, we will be constructing the two wings for the Techpod.  The first step is to install the servos followed by the wing spars

Install Servos

This build uses Hitech 65HB for the ailerons and HiTech 65MG for the flaps.  These have been chosen as the HB are very durable which given the planned use of an autopilot, get a constant feed of little adjustments.  We have gone with metal gears for the flaps as they are more resilient when the ‘brake’ is applied hard.

The Techpod wings are quite long by foam plane standards. Accordingly, servo cable extensions are mandatory. You can use pre-manufactured  extensions or custom lengths. As we are yet to finalise the fuse layout, we have gone with customised cable extensions and left around 15 -20 cms spare cable.

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Once the cables are done, we are need to cut the foam to fit the servos.  Dry install all of the items to determine the appropriate location for the servos.   You start from the horn as a constant and work towards to the servo. Connect the  rod to the horn and ensure the servo rods lie perpendicular to the aileron/ flap hinge line.  You may need to drill the horn to fit. The rods should be screwed in so they just poke through the other side of the clevis hole.

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With the servo centred, mark the location of the servo case on the foam. We want the servo fit flush on the base/bottom and also at trailing edge side.  Once marked, cut the foam to suit. You will also want to remove some of the raised features from the floor to ensure a flat fit. 

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Once everything is perfectly aligned use a low temp glue gun to install the horn and servo.

For the horns, a squirt in each hole plus a good dab on the surface foam. In regards to the servo, apply mostly to the sides and tab slot with a some on the base. Press the servo in firmly and hold down for around 20 seconds until it firms up.  Wipe any excess immediately as you install the servo as is easier before it firms up. Be careful to wipe clear any glue pops through the other side of the wing when adding glue to the horn holes.

The cables need to be carefully retained flush with the corner. There is plenty of space for the cables however the cables need to be kept in place or they will interfere with the installation of the rod and foam cover strip. The cables can be retained  by placing 1-2 match head blobs of low temp glue every 4-5cms. Use just enough as it adds unnecessary weight. Hold the cable down firmly until the glue drys and pull the cable tight for the next location .  You will need add more locations in and around the corners of the servo housing.  Use a screw driver of similar to press the cables into the corners.

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In order for the aileron cable to get passed the flap servo, we have cut a small amount of foam from beneath the housing screw tab to enable us to run the cable below the tab otherwise our foam cover was obstructed. Alternatively, you could cut the tab off.  Also note that we have stayed well clear of the servo arm.

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In this post we will attach the vertical stab to the boom and then join the two stabs.

Attach Vertical Stab to Boom

Attaching the vertical stab to the boom is quite straight forward. The three support rods fit neatly in the grooves and hold everything in place and aligned.  Make sure that your servo extension wires are properly fed through first as this will be difficult to install afterwards.   Practice installing the vertical stab a couple of times as you won’t have much time play when you go ‘live’. You slide the stab in from the top to avoid breaking the the rods. It will get fairly tight the last few centimeters, a little force will overcome the friction.

If you haven’t already, the top surface of the boom where it attaches to the stab should be lightly sanded, wiping off any dust with a damp cloth. Also lightly sand the vertical support rods.  The vertical support rods will be glued with CA whilst the stab will be glued to the boom with Goop.   Apply the Goop first and then run a health bead of CA down the three foam groves. Install the stab moving quickly to ensure that the rods don’t prematurely adhere to the stab prior to getting the stab in place. Pull the servo wire tight  as you go to remove any slack.

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Once the stab is fully installed, run a small bead of CA around the join between the foam and the boom. Wipe off any excess glue and give a quick spray with accelerator. Rub the edge of the foam down to reduce any gaps in the join.

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Join the Stabs

Joining the stabs is not difficult. It requires careful planing to ensure there is proper alignment of the yaw or roll.  The two halves are glued with Goop and then a bead of CA around the join edge.  I prefer to install whilst the tail is upside down. You will however require an object of similar height to hold up the other end of the boom.

To ensure there is no roll in the elevator stab, we use a square on both sides, checking repeatedly until we are conformable it is all square. A piece of string taped on both sides and over the top of the boom can be used to hold the vertical in place. The string does not have to be tight, just enough to hold the stab in place.

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Once happy with the roll alignment, we need to ensure that there is no yaw. We do this tape a length of string to the front of the boom and measure by hand to the two corner edges of the elevator stab making sure that each corner is exactly the same length by the string.

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Recheck everything again and again. Once in place, place tape on the join to hold  in place.Wait around 30 mins before you apply the CA in cracks so not to disturb the alignment. After several hours, you are good to go!

4. Install Elevator Servo

In this post we will install the servo and related wires for the elevator.  You will notice that there is a recessed servo area located on the top side of the horizontal stab. The servo recess fits a Hitec 82MG perfectly.

There are no holes to trace the servo wire from the horizontal stab servo recess through to the other side and then down the vertical stab to the boom. Accordingly, in order to install the servo wire we will need pop a hole to the other side of the servo recess and then another hole down the length of the vertical stab.  

Vertical Stab

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Lets start with the vertical hole. This is clumsy exercise without too much precision. The key to the job is too:
a) Be patient, go slowly and carefully.
b) Start with something that is long and very thin to create a pilot hole. eg. a narrow screw driver. If you use a screw driver, spin the driver slowly as you move forward.
c) Hold the stab up to a very bright light intermittently in order to track how you are going.
d) Once you have a pilot hole through, you can move to something larger in diameter.

The hole runs parallel to the vertical rod recesses. Also ensure that the hole on the bottom of the stab lines up with the rear circular hole on the boom (servo wire will feed through here).

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Once your hole is created pull your servo extension wire through the hole.

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As the servo wire is required to feed through to the servo recess, line up the two stabs and and mark where the hole should be on the bottom side of the horizontal stab. ie. make sure the hole lines up with the vertical and horizontal stabs. Make the hole through to the servo recess on the other side and pull your servo extension wire through. The hole will need to be angled at around 45 degrees as the servo recess is located off centre.  

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Now dry fit the servo. Unfortunately, the location of the servo wire on the housing of the 82MG servo wire is not located near our new hole and furthermore, the hole comes through exactly where the servo arm operates.  It is therefore necessarily to remove foam from the bottom and or corner edges to feed the servo wire around to the hole we have just made.

Essentially we want to make sure the servo sits flush and square on all sides without interference from the servo wires. The servo arm must also operate without interference.  The hole and rebate were made with an old soldering iron. A bit rough but effective and it is hidden in any event.

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After everything dry fits nicely we join our wire extension with our servo and glue in the servo.  The wire joins are not common lengths to minimise risk of shorting but also to keep the diameter down to fit through the vertical hole. 

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After you have centered the servo and attached the arm, the servo can be installed with low temp glue gun. Most of the glue is focused on the sides and a little on the bottom. Stay clear of the servo arm. You can also apply a small blob on the wire on the reverse  side of the stab, this is to keep the wire taught and away from the servo arm. To ensure minimum movement, two strips of reinforced tape are applied staying clear of the arm.    

 

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