Electric Conversion of Rich Urevitch's OV-10 Bronco Design

By Jerry Hamm

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Specifications:

* Wingspan: 58" (lengthened wing)

* Wing Area: 588 square inches

* Overall Length: 51"

* Power: Two AstroFlight 05G motors, in series

* Battery: 16 cells, 1700 mah * Speed Control: AI Robotics MX35D

* Props: 11 X 9 Master Airscrew Electric

* Motor mounts: Stitzler Aero-Vee * Plastic Parts: Rich Urevitch

* Flying Weight: 123 oz.

* Available From: originally published in Model Airplane News, November 1995. Plans available via the MAN plans service. Visit Model Airplane News

Introduction

This model was originally designed for glow power and was published as a set of plans in the November, 1995 issue of Model Airplane News. Though it was designed for internal combustion engines, in the MAN article Urevitch suggested that it was adaptable to electric power. My project was begun several years ago, shortly after the article and plans appeared in Model Airplane News, but completion and flight were delayed by a residence and shop move. Since that time, the model has been kitted by Hobby Hanger. I have not seen their kit or a model built from it; however, since I recommend making certain modifications from Urevitch's plans for electric flight, I would suggest building from the plans and incorporating the modifications as listed in this article.

Construction

Cutting out the parts is really quite easy, so a kit is not at all necessary.

I made extensive modifications to the fuselage and nacelles to reduce weight for the electric version. The wing structure was not altered from the plans, but I used very light balsa for the ribs, sheeting, trailing edge, and ailerons. I will not go through step-by step construction since that is both fundamental and well covered by Urevitch's article. Construction is very simple and straight-forward and should present no difficulty to any modeler who has completed several kits or plans-built projects. The modifications that I made to reduce weight and to provide installation of electric motors, battery pack, and components are as follows:

1. The wing was extended one bay on each side, outboard of the nacelles. (span 58", total wing area 588 sq in). This gives a nice reduction in wing loading without significantly altering the scale appearance.

2. All light-ply fuselage and nacelle formers were replaced with 1/8" balsa except for landing gear bulkheads.

3. All aircraft-ply fuselage and nacelle formers were replaced with 1/8" light-ply.

4. An additional 1/8" aircraft-ply former (do not use light-ply here) was added between N-1 and N-2 as a firewall/motor mount. The exact location is determined by placing the assembled motor and mount combination over the plans. This former is the same width as N-1 and N-2 and the height is cut to fit. A hole was cut in the center for the motor wiring to pass through. The Aero-Vee motor mount is bolted to this former.

5. The center of former N-1 was cut out to allow the motor to project through.

6. The wing tip blocks were hollowed out extensively after shaping.

7. All fuselage and nacelle sheeting (except the fuselage floor) was replaced with 1/16" light balsa.

8. The vertical stabilizer sheeting was replaced with 1/32" light balsa.

9. The horizontal stabilizer was built from 1/8" medium balsa strip stock rather than light-ply. Sheeting was omitted. (Although no problems have been encountered with this light construction, if I were to re-build, I would sheet the stab with 1/32" light balsa.)

10. The elevator was replaced with 1/8" medium balsa (3/16" if you sheet the stab.)

11. Rudders were replaced with built-up structures made of 1/8" balsa strip stock similar to vertical stabilizer construction.

12. The entire cockpit area was made removable to facilitate battery replacement. A 1/32" ply floor doubler was installed in the battery area. I used a dowel to secure the aft end of the cockpit section and a BVM canopy latch for the forward end to make the section easily removable.

13. The horizontal stabilizer has 1/8 " dia. alignment dowels into the vertical fins as per the plans. After fitting and drilling, glue the dowels into the horizontal stab. Harden the dowel holes in the vertical stabs with thin C/A and re-drill. The removable stab sits on 1/4"balsa tri stock which is glued to the vertical stabilizers. The stab is held in place by four #2 X 3/8 sheet metal screws into fiber inserts which are glued into the tri stock with thin C/A. Use thin C/A to harden the balsa around the screw holes and place washers under the screw heads.

14. The receiver and receiver battery (110 mah) are in the right nacelle.

15. Standard servos for rudder and elevator are in the left nacelle.

16. Micro servos for the ailerons are installed in the wings. They were installed through a rib. 1/4 " square spruce rails glued to the rib for the servo attachment screws.

17. A micro servo was gooped to the fuselage floor for nose wheel steering. A Y-connector was used to connect it to the rudder channel along with the rudder servo. Make sure that your nose wheel servo linkage provides the same turn direction as the rudder!

18. Flight battery, switches, and speed control are in center fuselage.

19. Extensive lightening holes were cut in the ailerons and elevator.

The aileron servo installation (far left). Close-up of a nacelle, showing the motor mount (left). You can see the model's framework in this picture, which was taken before the sheeting was applied (right). This bare bones shot shows the completed model, sans convering. Note the many lightening holes (far right).

Install rolled paper tubes in the wings for servo and motor/battery/speed control wiring. I used a sharpened brass tube to cut the holes in the wing ribs before building the wing. Don't skimp on the tube diameter. There is a lot of wiring on this airplane! I strongly suggest making a complete wiring diagram before you start soldering any connections. Use one set of tubes for the servo wiring and another set for the power system wiring.

I found that the plane rested a bit too tail-high using the landing gear dimensions as per the plans. This, in addition to the horizontal stabilizer incidence problem mentioned elsewhere, contributed to difficulty in taking off. I bent the nose gear downward somewhat to increase the take off attitude. I would suggest making the nose gear about 1/2" longer than the plans show and adjusting it as necessary to achieve the desired attitude as determined by testing.

I used the excellent plastic parts offered by Rich Urevitch for the nose and tail cones, canopy, and cowls. I covered the model with red, white, and olive drab MonoKote. Plastic parts were painted with a light coat of matching LusterKote, which matched the MonoKote perfectly. Stripes were cut from MonoKote. Decals from Rich Urevitch were used for the "Stars-and-Bars" and the lettering.

Equipment Installation

All power and radio items were installed per the comments above. The battery pack used is two 8-cell 1700 mah packs connected in series and placed side-by-side. The packs are secured to the fuselage floor with Velcro straps which are glued to the battery compartment floor. Space is not a problem, so equipment installation is easy and adaptable to the builder's needs. The tiny 110 mah receiver battery pack is feasible because of the feature of the AI Robotics MX35D controller which charges the receiver pack in flight.

Flying

Control surface throws were set up as recommended in the article. No dual rates or exponential were used. I am normally an exponential flyer, but found that to be unnecessary on this model.

My first attempt at flying the model was remarkably unsuccessful! I used 11 X 7 props, which are probably the optimum choice for the Astroflight 05G motors. The grass was long, the field soggy, and the nose was too low. The model simply would not attain flight speed. Since I do not have easy access to a paved runway, I had no choice but to wait for totally different conditions. Many months later, I tried again. This time the field was short-cut, and very hard and dry. I went better prepared, with props from 11 X 7 up to 12 X 10! I figured that if the grass was still a problem, the 12 X 10's would develop enough thrust to get it off of the ground, although flight times would suffer. I also increased the "up" elevator throw slightly and bent the nose gear to increase the take-off incidence.

For the first attempt I tried 11 X 9 Master Airscrew electric props. After a run of approximately 150 feet, given full "up" elevator, the model rose gracefully into the air! What fun and excitement! The only trim adjustment required during the flight was elevator, and quite a bit of "up" trim was needed. The plane flew extremely well and was remarkably solid on the controls. I did a number of orbits of the field, and tried a few basic maneuvers including rolls and split-S's. No problem! At full throttle the plane moved quite briskly, but could be throttled back to a very slow and stable cruising speed that looked quite authentic. For a first flight, this was total joy! Here was a great looking model that flew as well as it looked. When landing time came, I misjudged the crosswind a bit and missed the narrow runway, landing in some slightly taller grass. It landed fairly well, but I did seem to run out of "up" elevator before reaching a full flare.

The first flight had been performed in a crosswind of approximately 10 knots, which offered no problem at all to this plane. For the second flight, I chose to stick with the 11 X 9 props. I am sure that on a paved runway, 11 X 7's would be the best choice and would offer the ideal compromise between performance and duration; however, on grass the additional thrust of the 11 X 9's was welcome. The model flew great again, but rolls seemed more sluggish. After landing, I found that one aileron servo was not properly plugged in and I was flying on only the right aileron!

I have subsequently corrected the elevator effectiveness problem by adding 1/8" of negative incidence to the horizontal stab. I did this by slotting the forward dowel holes in the vertical stabs and shaving the tri-stock that supports the horizontal stab by about 1/8" at the front.

Recommendation

Yes! Build it and fly it. This is a fun plane which looks great and is easy to build. Electric twins are neat!

This is one great looking, great flying airplane. It makes a perfect multi-engine electric project. Flight characteristics are unusually smooth and solid with no bad habits. It will take off from a grass field if the grass is short and the field dry and hard. Paved runway takeoffs should be a snap. Landings required all of the available "up" elevator for flare (see further comments in flight discussion).

I store the model fully assembled. It can be transported in a minivan without disassembly. To fly, all I have to do is charge the battery packs and go! Plans for this model can be downloaded at Scale Aircraft

The plans come with a .dxf file extension. To open this file you'll need to download "Turbo Cad LE" freeware which can be downloaded at : IMSI's Free Download Center. Download the LE version (Learning Edition) and under "select your download", select "never expires", then "GO".

 

The K&A Models OV-10A Bronco

By Terry McGill

Browse Articles by Terry McGill

Specifications:

* Wingspan: 43.1 inches actual (Manuf: 42.5 in)

* Wing Area: 335 Sq In

* Length: 37 inches actual

* Flying Weight: Actual: 44.3 ounces (Listed: 38-42)

* Wing Loading: 18.8 oz/ sq. ft.

* Motor used: 2, 6-Volt Sp400

* Props: 2, GR6070P (6.5x4) I used 6x4 and 7x4, now will try 6x5 for more speed

* Cells: Kit recommends 7or 8, 1700 mah (parallel wiring); I used 16, 800 mah (series)

* Radio: Futaba FP-T8UAF

* Speed Controller: Mini-Max 40 * Kit Price: $79.99 plus shipping

* Manufacturer: K&A Models (505)836-3681

Seduced again! One look at the K & A Models ad in the magazine, showing a picture of a twin-motored Bronco, and I had to get one. I remember that ‘other’ Bronco kit – the one that could be converted to electric. This one has nearly the same wing area, but was designed as an electric for two Speed 400’s. It just couldn’t be as heavy as that other one, I thought. ..and it’s not.

Built pretty much ‘stock,’ the ready-to-fly weight is 44.3 ounces (1255 grams). That gives a wing loading of 18.8 ounces per square foot of wing area, with two props spun by 750 SHP turbos pulling this little beauty into the wild blue yonder …oops…that’s the prototype specs! Imagine, though – you are in a de-militarized bronco: No gun pods, no combat radios, no armor, nothing on the hard points so your flying weight is around 10,000 pounds – and you have 1,500 shp on command. "Hot" seems a fitting description of this Fireboy’s Flyer!

Flying qualities

So how does it fly? Quite well! Very stable, and the control throws recommended are tame so that there were no surprises. Well, except for the speed. It is slower than my Zagi 400! Now, that is not a problem - it moves around the sky with authority, and with good control. But this is not a highly-loaded model (under 19 ounces per square foot) and with the recommended 6x4 props and 16 cells in series, it just isn’t a speedster. Hey, on the first flights – slower is good! I intend to try different props, though. I want just a bit more speed.

Incidentally, landings are like a fast walk – the Bronco did not slide even 20 feet on our slick nylon-belt runway, at the end of the flight. Oh – the 6x4 props, even windmilling, didn’t touch the runway on landing due to the angle between the bottom of the fuselage, and the wing-tips. No broken props upon landing!

Color and Markings

The color scheme I chose is one used by the California Department of Fire, where they use a fleet of 14 Bronco’s for fire spotters. The scheme is well-documented by Bob Banka, of Scale Model Research, with the best set of photos I have yet seen for a scale modeler.

I first saw this color scheme in the back of the "OV-10 Bronco in action" book from Squadron Signal, #154. If you don’t like the CDF color scheme, take a look at the Squadron book, as it has ten military color schemes – all of them eye-catching. The Squadron Signal book also has great detail shots of the cockpits, weapons systems, cargo area, and gear shots – and the only photos I have seen of the OV-10B(Z). This last variant was developed by North American for the Germans, who wanted more power to tow target tugs. They mounted a jet engine on top of the wing! No color shots of these, but several black and whites.

A great site for Bronco info, or PC wallpaper, is: OV-10Bronco.Net Gentlemen, click your mice!

Building the Kit

You will like this kit – premium balsa, selected for each application. And the canopy! Very scale, clear, and thick! The plans are minimal, but since it is a war-bird (well, depending on the color scheme) and a twin-boom model, chances are than anyone building it will have plenty of building skills. If you don’t, hook up with someone who does in your club.

If you have any questions while building the model, you will appreciate the customer support from K & A Models. I emailed Ken Williams at K & A Models several times during my building of the kit, and he always responded immediately to my questions. After completing the model, I emailed him the below issues and comments. He notes that over 120 kits are now amongst us, and most of the issues have been addressed in the later runs of the kit. Your plans and kit should be great, but you might like to see how another modeler went about building an early kit. If so, read on.

Wing

Reading between the lines of an email from the manufacturer, I guess I spent unnecessary time on this mod, but I did not want to have two wing-spar glue-joints in the center of the wing – even on a 44-ounce model. So, I took the 4 spar pieces and cut two of them in half, and scarf-joined two pieces to each end of the other two pieces, creating two spars. That gives three pieces for the top spar, and three for the bottom spar. I had to be very careful to keep the scarf cuts accurate (sand carefully with 250 grit paper and an aluminum sanding bar), but I got what I wanted: no center glue joints on the spars, and no need to fiberglass the middle of the wing (and have covering hassles from that). It should be obvious, then, that I built the entire wing in one piece. I always prefer that when the wingspan will allow it, since it reduces the sanding I always seem to have to do to get the trailing edge even.

The ribs for the part of the wing between the two booms have a hole drilled in each one, permitting passage of the servo and motor wires. Before gluing any wing ribs I drilled another hole in each rib, about an inch closer to the spar, for the motor wires. I wanted some separation (even one inch is enough with Speed 400’s) between servo wires and motor wires.

Wait - don’t glue any wing ribs, yet. Check carefully to ensure that each rib seats completely over the bottom spar, touching the bottom sheeting. Some of my ribs had to have the slots enlarged. Do a similar check with the top spar, before gluing. Ensure that the spar seats fully into each rib - check with a straight-edge (ruler) to be sure that the top spar is flat across the top.

This item seems critical. It sure changed how I assembled the tail booms, much later in the building process: The original plans and instructions say to plank between two ribs on each side of the wing, at the area where the tail booms will be glued. When I did that, I found that there was no wood extending out past the sides of the tail booms (on the bottom of the wing) to which I could iron on the covering. So - cut the planking for the bottom of the wing about ½ inch wider, where the tail booms will later be attached, so that the planking will extend about a quarter inch wider on each side of the ribs than the plans show.

You can save possibly 5 grams each side of the wing, if you hollow the wing tips. I was amazed to find that so much sawdust lightened the tips so little. This kit has good wood! Same result for the motor wire holes I drilled into the inner wing ribs: Not even 6 grams total savings.

I chose to block sand the wing before cutting the ailerons free, instead of following the instructions and doing it after the ailerons were free. I have had more consistency, this way, on my wings.

The plans did not show where the aileron servo wires run, or the motor wires, either – and more importantly, the plans did not mention to run the wires through the wing, before one is instructed to glue parts (miss this and you’ll have to fish the wiring through, later).

The two pictures of the fuselage pod sitting on the wing (with the tail-booms already glued on) show the orange and blue Sermos motor wire plugs, which will plug into the ESC in the fuse pod. The ESC is Velcroed to the side of the fuse pod, above. The blue blob with the white rod going to the right is the Dean’s antenna; the white rectangle above the blue blob is the Rx battery. The Rx in hidden inside the round cross-piece which the wing will butt against – you can see all the servo wires disappearing into the recess where it, too, is Velcroed. The vacu-formed rear of the fuselage is screwed onto four pieces of ply, one of which is visible just at the rear of the fuselage by the antenna, and another on the opposite of the rear of the fuselage.

Before covering the wing, ‘dry fit’ your motors and radio system. Ensure that your elevator, aileron and motor wiring has been completely thought out, extensions checked, and run all your wiring inside the tail boom and wing – to the center section, and out some holes – before you cover the wing. The plans don’t prompt one to do that, and I trust plans to tell me these things! (Check to ensure that your wiring comes out holes you cut in the bottom of the wing.)

Tail Booms

I had never built a twin-boom model, and I only decided to do the Bronco because a flying buddy has done twin-booms and I could always plead for help if my fears of misaligning overcame my desire to see the Bronco fly. (Translation: if I chickened out and quit.) Surprise for me: Although there was a lot of fiddling, blocking, weighing parts so they didn’t move, and measuring with my Robart incidence gauge – the booms came out so perfect that I can’t see any angular difference between them. All that worry for nothing!

I turned the elevator servo upside-down relative to the plan, for ease of future maintenance. I would rather cut in through the bottom of the boom, if I have to, than have to take the (glued) wing off the boom. I also used a Futaba S133 for that servo, instead of one half the price, because that elevator servo is buried in that boom.

Fins and Stab

I noted that the pieces for the fins are of different thickness, and emailed Ken Williams at K & A Models about that – he noted that the differences between the wood sizes creates a ‘step’ between the fin and the rudder, which he says looks great after covering – just be sure to get the Monokote sealed tight at that step. I chose to sand the step off, and the ‘rudder line’ you see in the photos is created by a 1/32 inch piece of Monokote…easy to do all the panel lines that way, and the color lines too. (Use a metal ruler and a new blade to cut the small pieces, and just cut carefully, holding pressure on the covering with the metal ruler so it doesn’t move. Scissors won’t give you as straight a cut. Tack the strips down with the iron on medium, with a sock on the iron. If you want them to stay on forever, put a little thin CYA on the edges.)

NOTE: There are no incidence references for the wing or the elevator. I measured carefully, and the horizontal stab to wing difference is 2 degrees, with the front of the stab being negative (down) relative to the wing. Because of the height of the stab relative to the wing, I thought it was much more, but an email to K & A confirmed this measurement. I used a Robart Wing Incidence rig to measure this, too.

Attaching the Wing to the Fuselage

This is not a hard step – but: The only error I made with this model was in the attachment of the wing to the fuselage. I decided that gluing the wing to the fuselage would save me half an ounce…and now if I ever want to get inside and swap ESC’s or Rx’s or anything else, I have to hire an elf to crawl in there – no room for even small hands. So - follow the instructions here, and make the wing removable, or you’ll have to pay the same price I will, in the future.

Overall Impression, and Ordering Instructions

What a model! I am going to order another Bronco from K & A Models, so I have one on hand should something nasty happen some flying day. This model is even nicer than my Kyosho Agwagon – since it has two motors.

If you like the looks of a Bronco, and like smaller models – you will love this kit. Order yours from your local hobby shop, or through K & A Models. Email: kenwood@nmia.com.

These articles reprinted courtesy of the Ezone.

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