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K & A Models OV-10 Bronco Build

The CAL FIRE OV-10 Bronco.

By Steve Belknap

Back in the early 2000s, our club (SEFSD) hosted a yearly event that we called the Mid-Winter Electrics.  It was a 2-day event that showcased the latest and greatest in electric powered RC planes.  There was open flying, demonstrations, etc.  We also had vendors that would come a long way to attend.  One of the regular vendors was K & A Models of Albuquerque, NM (no longer in business).  Ken Williams would showcase his expertly crafted kits.  I bought a few of them and recently completed one, the OV-10 Bronco.

It was a pleasure to build such a well made kit.  Everything fit perfectly.  The fiberglass work was second to none.  I made many modifications to mine mainly because I wanted more power, retractible landing gear and functioning rudders.  More details about these mods later.

I chose the CAL FIRE markings because I liked the look better than the typical camouflage you see on most military aircraft.  It turns out CAL FIRE has a whole fleet of these stationed all around California.  CAL FIRE uses the OV-10s as the primary command and control platform on wildland incidents.  I found there was one stationed at the Ramona airport so I modeled mine after that one.

Please see the video of the first flight.

The specs are:

Wingspan:  43.5″
Weight:  64oz
Power:  (2) Cobra C-2808-26
Prop:  (2) APC 8x6E & 8x6EP
Battery:  4S-3000 mAh Li-Po

The power system chosen makes about 600Watts.  This gives the plane 150W/lb.  During the first flight I was mostly using less than half throttle.  Performance was excellent.

WING

The kit was ordered with the optional pre-sheeted wing (balsa over foam).  All cutouts are made by the modeler.  Careful planning, measuring & cutting are very important.

 

The ailerons are cut from the wing.

 

The LE of the ailerons and one end of the ailerons have been trimmed to accept the balsa blocking and droop angle.

 

The aileron and wing have been blocked with vertical grain 1/16th balsa.

 

Openings were created for balsa blocks.  Then slots were made in the blocks to glue in the 1/16″ plywood control horns (after covering).

 

Pockets were opened up in the underside of the wing to accommodate the aileron servos.  1/32″ plywood used to make the covers.  The servos are attached to the covers.

 

The holes where the nylon hold-down screws go are reinforced by 1/2″ dowel.  This keep the wing from being crushed when tightening the screw.

 

The leading edge has been installed on both wing halves.  The one on the right has been shaped by sanding to conform to the airfoil of the wing.

 

Cut-outs have been made in the leading edge to accept the two tail booms.

 

One eighth balsa blocking used in the cut-out.  A block of balsa is used on the back of the blocking to accept the dowel that will be used for mounting on the tail boom.

 

Blocking finished on both cut-outs.  Just need the dowels.

 

The hole for the wing hold-down dowel is created with a right-angle Dremel attachment, then a hand drill for exact size.

 

Test fit on one boom.  For proper wing seating, the dowel hole in the boom should be a close fit.  The hole in the wing is a bit larger for a sloppy fit.  When ready to glue the dowel into the wing, the three parts are all assembled with epoxy used in the wing hole only.  Make sure the wing is fully seated.  Point the nose up so epoxy does not run out of the wing hole.  When cured the wing should fit perfectly.  Fill in the remaining gaps in the wing hole around the dowel.

 

Joiners were created to securely attach the two wing halves.  Foam was removed in three places.

 

Access holes are opened up on the wing bottom to gain access to all the wiring.

 

Test fit for joining the wing halves.  None of these joiners came with the kit so I made them from balsa.  They are joined using 30-minute epoxy.

 

Test fit.  All looks good.

 

The wing tips are vac-u-formed plastic.  They are cut loose from the flashing then a rib-shaped balsa stiffener is glued in.  Later the tip assembly is painted and taped in placed after covering the wing.

 

Horizontal Stabilizer

 

Test fit for horizontal stabilizer.  The stab is attached with small nylon screws driven through the vertical stabilizer.  The thin balsa stabilizer will not hold a screw so a 1/2″ wide piece of bass wood is glued to the outboard ends of the stabilizer with grain going for and aft.  Holes were then drilled and threaded for the screws.  The bass wood was shaped to fit the curve of the vertical stabilizer

 

Rudders

 

The model was not intended to have working rudders.  I wanted rudders in case one motor failed.  They were carefully cut from the fiberglass tail booms.

 

At this point it was a good time to add some glue to fill in-between the inner and outer layers of the tail section where the horizontal stabilizer screws were to go.  This keeps the sides from collapsing when the screws are tightened.  Hysol Loctite EA 9462 is a non-sagging epoxy perfect for the job.  White pillars of glue were created to join the inner and outer halves.

 

The screw holes were then drilled through the fiberglass sides and the epoxy.

 

The blocking are strips of balsa that need to be formed to fit.  Then they are epoxied in place.  Since the booms are hollow, I needed to add blocking to give both the booms and rudders form and stiffness.  Balsa blocks were shaped and added to achieve this.

 

Test fit for the stabilizers.

 

Tail Booms

The fiberglass work on the booms is outstanding but they were never intended to have retractable gear and support a 4 lb plane.  To give the booms a little help I added a layer of 6 oz fiberglass to the area inside where the gear will be mounted.  West Systems Laminating Epoxy is used.

 

An access door is cut out of the bottom of the boom where the gear will mount.  This makes it easier to install the retractable gear and mount assembly.

 

The gear mount plate and attachment rails are created for each boom.  The mounting plate is removable after the rails are glued in place.  Also shown is the 1/8″ plywood mount plate for the wing screw located at the rear of the opening.  It is epoxied in place then drilled and tapped for the wing screw.  This was done in four locations.  

 

The gear mount assembly is glued in as a whole.  Zip-Lock bag (Halloween themed) is used to keep the Hysol from sticking to the plate.

 

The mounting plate is removed after the epoxy on the rails is cured.  This system allows for the fore and aft re-location of the gear plate if needed.

 

The 3mm wire gear struts that come with the retracts is all wrong for this application.  I needed to make new ones.  Normally I would use 3mm music wire but this time I had another idea.  I used 3mm aluminum wire instead of steel.  This allowed me to make prototype struts that could be easily bent and re-bent.  Once I got the final configuration for all three gear struts I re-made them from music wire.  The wire bender shown above is home-made.

 

One of the main gear with aluminum strut is shown extended, in place on plans.

 

Shown retracted, in place on plans.

 

One main gear installed in a tail boom, shown retracted.

 

Shown extended.

 

Aluminum support plates are made to support and strengthen the retracts.

 

Sitting on the mains.

 

I made individual servo mounts for the rudders and elevator that were glued inside the booms.  The arm sticking out holds the push/pull cable sheath.

 

The elevator and rudder servo mounts are glued in.  The retracted main gear is shown just between them.

 

The motor mounts are made from 1/4″ plywood and shaped to fit inside the curvy nose of the boom.  A template was made to locate the center and mounting screw hole placement.  On the left, the hook-eye screw was used to hold the mount for fit check.  On the right, the motor was bolted to the mount and glued in place with a spinner mounted on the shaft for proper alignment to the boom.  After the motor was removed a steel tube, with a set screw added, was used to hold the motor shaft for easier re-installation.

 

The mounts, motors, props, backplates & screws.  The center holes are opened up to accommodate the hub on the back of the prop adapters.

 

The Castle 25A ESCs are installed in a very crowded boom.

 

Installing the nose gear followed the same process as the mains.  Open a large hole for access, make a plate for the retract and the steering servo, add two rails and glue in place.  Again, the strut is aluminum.  Good thing too, I had to re-bend it at least once.

 

This shows the nose gear, steering servo and linkage all integrated into one assembly.

 

Finally, it has all 3 legs to stand on.

 

The cockpit.  Details were printed and taped on painted balsa.  Just needs pilots.

 

There they are!

 

The large cut-out doors on the bottoms of the booms are replaced with openings cut just large enough for the gear to pass through.  After painting, these doors will be taped in place.

 

Painting has begun.  A primer was used.  The base color is white with red on the tails.  There will be black as well.  LVP Paints can make custom colors for you in spray cans.

 

The tails have been painted and pin striped.  The black and white pin striping is made from thin strips of 3M Oracal 751 cast vinyl, cut on my Wife’s Cricut.  The cockpits uses the same white vinyl strips.

 

Wing Finishing

 

Using a Chinese version of MonoKote, the wing covering begins.

 

The red is more Chinese iron-on covering.  The pin stripe is vinyl.  The aileron link is simple easy to use.  Just two wires, heat shrink & medium CA.

 

This shows how a strip of white vinyl is used to attach the pre-assembled wing tips.

 

A lot of wire is passed through the wing from the center fuselage to the booms.

 

All the Rest

 

The bottom of the horizontal stabilizer and side of the right boom.

 

The gear door was reinforced with carbon tow.

 

The main gear door shown taped in place.

 

Displayed outside.  All the markings were created and supplied by the wife of a modeling friend.  Thank you again Jeri, they are perfect!

 

Prior to maiden flight.