The aileron servo installation (HiTec HS-5245MG) differed slightly from the instructions in that the servo was mounted in the inboard wing section rib and not to the plywood cover plate provided. This had the advantage of not have to make an electrical break each time the outer wing panel was removed. A new horn made of a simple ¼ dowel epoxied into the aileron root and a 4-40 ball link was installed.
The 2 7S-5000 (made from two 3s and two 4s) 25C Enerland Li-Poly packs reside behind the nose cap in a sliding tray box fabricated by the author. The nose cap is held on internal rubber bands attached to wood hooks. Hook and loop fasteners straps in the front of the box keep the batteries from shifting forward. This 4 lb battery assembly was required to achieve proper balance. A glow powered version could probably deal with only 3 lbs of lead as 90 4 stroke motors are only slightly heavier than the Axi Motors.
A vented sheet Lite ply battery box was constructed to contain the 2 7s Li-poly battery packs. The two Castl;e Phoenix HV45 ESC’s were temporarily mount to the box. The ESC’s actually reside on each side of the Pilot in the open sliding windows for cooling.
The completed battery box and sliding tray. The tray is epoxied into the nose of the model. The sliding box is retained by a single 8-32 screw. The batteries are secured inside the box by hook and loop fastener straps.
Foam and plywood scale 250KG bombs were fabricated to hang and drop from the provided centerline bomb rack fairing. The bombs weigh 3 ounces each but consists of approximately 75% lead to achieve the proper flight path. The bombs are secured to the rack through an eyelet on the top of the bomb. A pushrod passes through the eyelet actuated by a micro servo inside the fairing. One bomb can be dropped at a time or both salvo’ed at the same time.
The bomb rack was made removable by added 6-32 screws to the rack and threaded inserts in the lower wing skins. The process started by adding 3/8 diameter hollow carbon tubes a the screw locations. |
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Holes were drilled into the lower wing skin to accept 6-32 threaded inserts. |
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The screws were coated with soap and “potted” into the tubes with an epoxy/micro-balloon mixture. When the epoxy dried, the holes were opened to the clearance drill size.
The removable bomb rack also hides the aft wing bolts, enhancing the exterior appearance of the model.
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A micro servo was installed in the forward section of the fairing to drop the bombs individually or together. Mounted to a 3 position switch, this picture shows the servo in the “bombs attached” position.
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One bomb away! The servo actuates in the middle position of the three position switch to release the left bomb (shorter pushrod). |
Second bomb away! The servo actuates in the third position of the three position switch to release the right bomb (longer pushrod). |
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The servo lead from the bomb servo passes through to the radio compartment through a cardboard tube mounted in the wing.
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A Castle Creations 10 amp external BEC unit converts the 8V (2 cell Li-poly) 2000mah to 5 volts to provide power to the Rx. Test showed that with many of the servos in motion showed only a 3 amp spike in current from the pack.
Castle Phoenix HV45 ESC”s provide control to the AXI 4130/20 motors. The ESC’s reside abreast of the pilot with both side cockpit windows modeled in the open condition to provide cooling air to the ESC’s.
The flap servo (HiTec HS-5245MG) was mounted in the nacelle where the throttle servo (for the glow engine) would normally reside. The model is provided with a cover just inboard of the nacelle on the lower wing skin if you chose not to hide the servo/pushrod as the author did. This arrangement did require a new horn and pushrod to be fabricated.
The installation of the remainder of the radio equipment in the voluminous fuselage was as per instructions. The tail wheel, retract valve servos, RX (FASST 7C 2.4ghz), retract tank and valve and RX battery reside here. There is no fear here of affecting the CG with placement. None of these items weigh enough to affect the CG of the model.
The author chose not install landing gear doors. Simple cut-outs in the provided lower nacelle cover were made to pass the strut and wheel. You can see the flap servo mounted in the glow throttle servo mount (in picture 2).
The Axi 4130/20 outrunner motor was mounted on an extender made from ¼” birch ply. A hole was drilled in the top of the nacelle to pass the motor wires to the wing and finally into the fuselage were the two Castle ESC’s reside.
The author desired to hide the cowl retaining screws inside the cowl. Access will be from the front through the spinner opening. 4 aluminum angle brackets (per nacelle) were made and epoxied to hardwood blocks. The blocks were shaped to approximate the inside surface of the cowl.
The brackets with the hardwood attached were epoxied to the inside of the cowl (make sure to rough-up and clean the inside of the cowl) with the aft face of the aluminum bracket flat against the provided firewall.
The cowl brackets were drilled on assembly with the nacelle to accept 4-40 screws. T-nuts were installed behind the firewall.
Aluminum tubes (four 3/8 OD upper, two ½ OD lower) to emulate the forward firing machine guns and 20mm cannon were installed in the fiberglass nose cap for a “scale look” and to provide cooling air to the batteries.
The tubes were “rough shaped” off the model and then installed with a slurry of epoxy. Painters tape was applied over the factory paint to protect it while sanding the tubes to match the shape of the nose cap. The tubes were later painted to match the local color and dummy machine gun barrels installed in the inboard tubes as they are the only ones visible on the full scale aircraft.
Though the fiberglass parts provided with the model have panel lines molded in, the flying surfaces (made from wood and covered) do not. The author inked-in the remaining panel lines from a 3 vu drawing. The existing panel lines were also enhanced with the fine tip permanent marker.
KMP pilots (optional) are installed in the cockpit, but slightly below the deck provided for a more scale-like look. The Castle ECS’s are just to the left and right of the pilots bust. The switch for the RX is also hidden inside the cockpit and access through the open sliding windows the author provided in the stock canopy.
The model is provided with scale-like glycol wing radiator vacu-formed parts. The author modified them slightly to allow the air to flow through the parts and glued them to the wing using “canopy” glue. This white glue dries clear and a little flexible.
The model arrives and rather large box, well packed with most parts individually wrapped in thin foam and bagged.
PLANE: Bf 110C Messerschmitt Zerstorer ARF
DISTRIBUTOR: Kondor Model Products (www.kmp.ca)
TYPE: Nearly scale German WWII fighter/bomber (approx. 1/6.7)
FLYING SKILL: Experienced
FLYING WEIGHT: 24 lb. electric, 23 lb. glow (estimated)
LENGTH: 71.5 in.
WINGSPAN: 95 in.
WING AREA: 1,343 sq. in.
WING LOADING: 41 oz./sq. ft. (39.5 oz./sq. ft. glow estimated)
RADIO: Futaba FASST 7C, Hitec HS-5245 servos on flaps/ailerons (2 each), Hitec HS-5085MG on retract valve, Hitec HS-5085MG on rudder (2), Hitec HS-5485HB on elevator and tailwheel, 4 channels minimum required; flown w/aileron, elevator, throttle, rudder, flaps & retracts. RX power: Castle 10A external BEC w/2-cell Kokam 2000mAh
POWER SYSTEM: 2 Master Airscrew 16x10 3-blade props, AXI 4130/20 external rotor brushless motor, Castle 45HV speed controller, 7S 5000mAh Enerland 25C Li-Po
GLOW ENGINE RECOMMENDED: .91 4-cycle or equivalent 2-cycle
FULL THROTTLE POWER: 42 amps, 1100 watts/engine
POWER LOADING: 92 W/lb.
TOP RPM: 6,100 (static)
DURATION: 8-10 min. on 2 Enerland 7S 5000 packs
TOP SPEED: 83mph
CRUISE SPEED: 65mph
PRICE: $679.95 (www.kmp.ca)
COMPONENTS NEEDED TO COMPLETE: Motor (engine), props, ESCs, radio, motor battery, RX battery, retracts (offered as an option)
SUMMARY
The KMP BF-110 ARF is an exceptional looking and flying machine when converted to e-power. The ability to swing large near-scale props translates into a superior climb and a more realistic speed despite the high gross weight. The lack of cylinder heads protruding out the cowl also enhances scale appearance. The changes required to accept the electric power system are minor.
The BF-110 Zerstorer (destroyer) was referred to (even by the Germans) as the bomber escort that needed a fighter escort. Though the BF-110 enjoyed much success as a “fighter” during the early days of WWII in Poland, Norway and France, when the aircraft had to fight against the likes of Hurricanes and Spitfires, it was immediately outclassed. BF-109’s had to protect the BF-110 that was sent along to protect the Ju-88’s and HE-111’s during the Battle of Britain! Though the BF-110 with its two Daimler-Benz 601 engines was faster than many of the single engine fighters of the late 1930’s, it’s turning radius was abysmal. The aircraft was relegated to NachtJäger squadrons (night hunters) and sent off to the Russian Front where its rugged nature kept it fighting throughout that campaign. The particular aircraft KMP chose to model is a BF-110C from Stab II/ZG 1 'Wespen Geschwader' (wasp squadron), stationed on the eastern front near Bagerowo, Russia in July 1942.
Power System Selection
I made the arrangements to do this review at the 2009 WRAM show. While I was waiting for the model to arrive, I used the time to determine how much power I needed to fly the model convincingly and to acquire the power system components. I first needed to assess how much I thought the model would weigh. There is some conflicting data on the flying weight of the BF-110. The KMP website and magazine advertisements list the model as flying at 17 lbs. The specifications on the box states a flying weight of 20 lbs. I was originally going to be slightly pessimistic and expect the flying weight to be near 22 lbs. I will discuss the final flying weight a bit later.
I decided I wanted a power loading of around 100 watts/lb but would have been happy with slightly less. My previous experience with twin engined models showed that they fly very well at lower power loadings than typically chosen for a single engine aircraft. A 22lb model will need 2200 watts or 1100 watts per motor. Though there are probably a lot of brushless motors (I never, ever considered anything but a brushless motor) out there capable of absorbing 1100 watts, I had one other requirement; swing a near scale 3 blade prop! This predicated the use of a large outrunner or an in-runner with a gearbox.
1100 watts input divided by 40 amps is 27.5 Volts and at Li-poly battery nominal voltage of 3.7V/cell, this translates into a 7 cell battery pack. The Electri-Calc program gave me an answer pretty quick using this data as a starting point; the AXI 4130/20 outrunner was the only motor with a low enough Kv to swing a large 3 bladed prop and stay within the current limits of that class motor. The current realized (after a quick bench test) was 42 amps on a 7S Li-Poly pack and a 16 x 10 3-bladed prop (18 inches is actually closer to scale diameter.) I just happened to have a Master Airscrew 16 x 10 3 blade prop on hand. With the motor decision out of the way, I ordered two Axi 4130/20’s, two 3-cell and two 4-cell packs of Enerland 25C 5000mah packs. The motors were to be driven by two Castle Creations 45HV ESC’s.
My original assessment that this power system would fly a 22 lb fighter-bomber convincingly began to fall suspect as the weight crept up. Do not fear. Read on. The story has a happy ending!
The ARF
When the HUGE model box arrived, I could not wait to get started. The model was well protected in the box. Parts are individually wrapped in foam and bagged in clear plastic. The model is a mixture of glass and wood construction. The fuselage and nacelles are fiberglass and the wings and tails expertly built up from laser cut balsa and plywood parts. A complete hardware set (screws, pushrods, tail wheel, quick-links etc.) is included. Hardware is also provided in the basic kit for fixed landing gear. The model is completely painted with beautiful decals already in place. Because of the sensitivity in some countries to the Swastika, these decals are left off the vertical tails. I made a set from trim black and white MonoKote®. A clear painted framed canopy is also provided. The entire model is painted/covered in colors that approximate the RLM (German wartime standards) dark gray-light gray upper and light blue lower camouflage scheme, typical of many 110’s on the Eastern Front. Panel lines are molded into the fiberglass parts but none are found on the built-up flying surfaces. The model comes with a few painted aluminum and vacuum-formed parts to add some external detail to the model. These include the under wing glycol radiators/flap fairings and lower fuselage antennas. Scale-like fiberglass exhaust stacks are also provided. The cockpit area is quite stark, but can be dressed up with pilot busts available from KMP. These busts are listed as being 1/6 scale, but they are probably closer to 1/7th scale and just about right for this model.
Assembly
The instructions provided are adequate to assemble the ARF into a fine flying scale model and I will not bore you with the little details. The “translated” English does leave a lot to the imagination though. Experienced modelers will have no trouble just using the pictures as a reference. I will, however, discuss some of the things that are required to convert the model over to e-power and point out a few things that can be done to ‘dress-up” the model further.
Nacelle rework
I first began the electrification by removing the wooden fuel tank mount in the forward part of the nacelle. At this point, I did not yet know whether the batteries were going to be installed in each nacelle or centrally located in the fuselage. I have converted enough models from glow to electric to know that many ARF’s need the motor battery pack as far forward as possible to achieve a reasonable balance without adding lead (especially when using Li-Poly battery chemistry). The forward part of the nacelles are no farther forward of the CG than the nose of the fuselage. After removing this wood from the nacelles, I decided to make things easier for me to remove the batteries to charge them by putting them behind the removable nose section (more on this later). The only unknown at this point was if I would have to add any additional weight to the nose to get the model’s CG in the correct location.
The Axi motors need to be mounted very far forward of the supplied firewall. I made a plywood extender made from 1/4 plywood in the form of a “+” and glued a 1/4” plywood base plate and motor mount plate onto each end. All the pieces were glued together using “Gorilla Glue” (a polyurethane glue). A hole was drilled in the motor mounting plate to let the back of the Axi shaft pass though. The extender/motor assembly was then screwed to the existing firewall using the screws provided with the model. A hole was cut into the top of the firewall to allow the ESC wires to pass into the aft section of the nacelle and down the wing to the fuselage.
I really don’t like external screws to hold parts onto a scale model. I try to hide these attachments as often as possible. The cowl is intended to be held onto the nacelle with screws around the outside at the mating lap joint. I decided that there would be plenty of room to secure the cowl through the spinner end of the cowl. I enlarged the opening in the front of the cowl to within ¼” of the edge. This not only allows me to remove the Axi motor without removing the cowl, but allows access to the inside of the cowl all the way to the firewall. I made a set of 4 brackets from aluminum extruded angle stock and glued them to basswood blocks which in turn were glued to the inside of the cowl. Make sure you rough-up the inside of the cowl and clean it with alcohol before gluing to it. The aluminum angle brackets lay flat against the forward face of the firewall. Holes drilled through the bracket and into the firewall accept a 4-40 socket head cap screw and blind t-nuts behind the firewall.
The “Zerstorer” has a great scale “scoop” in the lower cowl to help keep the motors cool. Cooling air exits either the aft part of the nacelle over the flap, or through the cut-out in the lower nacelle for the retracting landing gear should you decide this option and not make working doors (as I did).
The flap servo is intended to be mounted in the wing under the cover provided between the nacelle and the fuselage on the lower wing skin. I decided to hide this servo and pushrod inside the nacelle. The space where throttle servo would normally be installed was a great place to do this. A new .030 sheet fiberglass horn was epoxied to the flap inside the nacelle trailing edge fairing. A ball link on the horn end keeps thinks aligned during flap deployment.
Wing Assembly
The wing assembly went basically as outlined in the instructions. The nearly 8 ft wing is assembled from 4 panels per side, three wing sections and the nacelle section. The manufacturer includes templates to drill locating holes in each section to aid in installing alignment pins during gluing. The main spar is a nearly 1” diameter thin wall aluminum tube that passes through each of the four wing/nacelle sections. A thick plywood dihedral brace is provided to mate the right half with the left using epoxy. The outer most panel is intended to be made removable, sliding on the provided tube-spar. I chose not to use the supplied rubber bands/hooks to secure the outer panel to the main section and instead I use a single screw through the lower wing skin. A hardwood block was glued into the wing between the inside surface of the wing skin and the outside of the fiberglass tube in the panel that accepts the aluminum tube-spar. This block (and aluminum tube) was tapped to secure the single screw.
I also chose not to install the aileron servos in the bay provided in the outer most panel under the lower Balkenkreuz (German cross). Instead I decided to mount the servo in the end rib of the inner panel assembly. This removes the requirement of electrically connecting the servo each time the wing is assembled but now creates the requirement of moving the aileron control horn and connecting the pushrod each time the outer wing panel is installed. To simplify this, I substituted a ball link for the control horn. A wooden dowel, the end tapped for a 4-40 screw ball link was glued into the root of the aileron.
Fuselage Mods and Battery Box
The stock KMP pilot busts sit a bit too high if glued to the deck provided under the canopy. To rectify this, I cut away the fiberglass deck under the canopy and installed a false floor of 1/8 balsa about ½” lower than the provided deck. On each side of the pilot rests one of the Castle 45HV ESC’s. Since the full scale aircraft had sliding side windows opposite the pilot, I cut away that window from the provided canopy and installed a .030 thick sheet window propped open a little more than half way to provide cooling air to the ESC’s.
A fiberglass nose cap completes the fuselage length. I suspect it was only there to keep the box from getting any longer, as there would be no reason for it in a “glow” version. It became a “feature” for an E-powered version however. It is extremely convenient to have the two 7S-5000 packs come out of the front of the model after the nose cap has been removed. Once the model was fairly complete of all structure and equipment that would NOT reside near the CG, I placed the two battery packs in the nose and to my surprise the model balanced at 25% of the MAC (more about that later). I now needed to make a quick and convenient way to get the nose off without visible screws and a proper way to secure 4 lbs of batteries!
I started by making a 3mm lite-ply box (with cooling holes) that would just fit the two 7S packs (actually made by putting a 3 cell and 4 cell pack in series). Added to this box were a pair of hardwood rails that mate a lite-ply tray that would be permanently attached to the floor of the nose of the model. Also attached to this battery box would be the retract air supply valve and the two APP (Anderson Powerpole, available from West Mountain Radio) connector blocks to the Castle ESC’s. The box is retained in the tray by a single nylon 8-32 screw.
Before I was to install this box in the nose, I needed to remove the fiberglass and wood bulkhead installed by the factory. I was certainly wondering why a (heavily epoxied) 5mm plywood bulkhead was in the front of a fuselage that neither supports an engine or a nose gear. Then, it dawned on me. If I needed to add 4 lbs of batteries to the nose of this model to get it to balance, how would a modeler ever get this model to balance with a pair of .90 4-stroke engines that only weigh slightly more than the electric motors? I did some quick calculations based on the difference in weight (minus batteries) between a glow and electric power system and realized that one would probably have to add 3 lbs of lead to the front side of this bulkhead to get a glow version to balance. This model could NEVER weigh 17 lbs (as the website states) as this is probably just the weight of all the parts in the box. Even 20 lbs is unreasonable now knowing that nearly 3 lbs of lead would be needed to achieve the proper CG. I have estimated that the best anyone could do with a 90 4-stroke engine version is approximately 23 lbs.
Once the fiberglass/wood nose bulkhead was removed, the battery box and tray was installed so that it is flush with the old bulkhead. Hook and loop straps keep the batteries from shifting forward during flight.
The nose cap was modified to accept aluminum tubes that emulate the 4 upper machine gun ports and the two lower 20mm cannon ports. Only two of the four machine guns are exposed on the full scale aircraft. The other two barrels reside just inside the tubes. I used this opportunity to use these openings to feed ram-air cooling to the batteries. Dummy machine gun barrels were fabricated and installed in the two inboard upper tubes. The entire nose cap assembly is retained to the fuselage using internal rubber bands and hooks. The hooks are mounted to the left and right side of what is left of the nose bulkhead. A single wooden “key” was made for the top of the nose cap to align it with the fuselage using the “Wespen” (Wasp) paint scheme as a guide.
Radio Installation-Fuselage
The installation of the radio equipment in the midsection of the fuselage was as per instructions. The only minor deviation was that I installed a separate 2s-2000 LiPo battery pack and a Castle 10 amp BEC in the forward section to provide power to all the HiTec digital servos installed in the model. I used the HiTec HFP-20 digital servo programmer to reverse the direction of one of the flap and rudder servos to effect the proper device deflection. I also tested the complete system servo current draw at both idle and with many servos moving and found that the total maximum current observed was only a little over 3 amps.
The elevator and two rudder servos are mounted in the horizontal/vertical tail assembly. The leads have to travel a long way to get to the Futaba FASST 7channel Rx I chose to use. I made my own extensions using 20 gauge wire and did not add any additional connectors. The connector was removed from the servo and long leads soldered to the existing servo leads near the case. New connectors were “crimped” at the Rx end. This was not done for the aileron and flap servos, though 20 gauge extension leads or y-harnesses were used. It is important, not only when long distances are traveled, but especially when this is combined with higher-current digital servos, that the wire size of the extensions be properly chosen.
Flight Tests
The model tipped the scales at a few ounces over 24 lbs without the bombs installed (see sidebar). This gave me a 41oz/sqft wing loading and just over 90watts/lb power loading. Neither really concerned me in this large-scale model. At 90watts/lb, many would consider this “underpowered”. The model is actually “nicely powered” and very scale-like.
The CG was another issue. The instructions quote a CG location of 85mm (3.35”) back from the LE of the wing at the aircraft centerline. This seemed very far forward (and almost unobtainable without buying every bag of lead shot on Long Island!). I laid-out the wing planform shape in my CAD system and determined that 25% of the MAC (mean aerodynamic chord) is 5.8” (147mm). With the fairly large horizontal tail and the tremendous moment arm, I decide to ignore the instructions and use my number.
It was a rather humid mid-spring morning for the first test flights. I decided not to carry the bombs, even though the weight increase was a mere 1.5%. I did a thorough check of the radio system, landing gear and power system. I also checked the RPM and input wattage to each motor one last time. I buttoned up the model and taxied out onto the “active”. Expecting a strong swing to the left (with both props spinning CW as viewed from the cockpit), I added the power slowly and held some right rudder. The tail came up almost immediately but the model had no tendency to continue up onto its nose. The model rose off the ground in probably less than 150 ft without any input from me and climbed out a rather shallow rate. I increased the climb with a little elevator and it responded nicely. I tucked the gear just as it left the boundary of the field and the model began to accelerate quickly and climbed even steeper.
After leveling the model out at a few hundred feet, I started to adjust the trims. To my surprise I need none! None in any axis! It pays to set-up a true aircraft, even when the model is mostly prefabricated! My CG was correct! The model handled very well in the first short flight. I dropped the flaps and landed the model. The landing speed is reduced considerably with the flaps down and no re-trimming is required in the pitch axis. Just drop the flaps and let the model “float” a bit, then it will settle into a nice glide slope. Adjust your land spot with power just like the big boys.
On subsequent flights, I continued to expand the flight envelope, pushing the model harder and harder. I did some basic aileron rolls and found the model responds like most fighters: with a slight “barrel” motion. It also likes to enter the roll from a slight climb or a lot altitude will be lost on the return to level. I also made some abrupt pull-ups, looking for a snap roll, but I could not induce one with the prescribed 20mm elevator deflections (and my CG position). Stall turns are fun, but you cannot do them to the right with both motors turning in the same direction. (MAS does make an opposite 16 x 10 3 blade if you would like to make the props contra-rotating and negate torque and more importantly; P-effect). Stalls will fall off on one wing (usually the left) but recovery can be quick (relatively for such a large model) if one removes the elevator and adds power slowly. I did not sense that the model would precipitate into a spin from a “normal” stall, but I have not had the courage to intentionally spin the model.
On the fourth flight I added the bombs to the centerline rack. Take-off and climb-out seemed unaffected by the additional weight and drag. After a circuit around the field I made a pass down the runway centerline at about 200ft and let the first bomb go. Of course I could not watch it fly, but my spectators said they flew straight and true and very scale-like. I dropped the second one on the next pass and my camera man Joe Cabana captured the release.
On the fifth and sixth flight I installed an Eagle Tree Airspeed Micro-sensor (V3) logger. As a stand-alone unit, the logger will record the highest airspeed achieved since the unit was powered-up. The airspeed is displayed on an LED one digit at time. The fifth flight was to see just how fast the model would go in level flight. The BF-110 surprised many of us when I landed and queried the unit to find it topped out at 83 MPH! The sixth flight was to asses the “comfortable” cruise speed. I took off the model and carefully accelerated (after the gear was tucked) to what I considered a comfortable cruising speed. I dropped the flaps and landed the model. The Eagle Tree showed a max airspeed of 65 MPH.
The model is very enjoyable to fly, and actually less stressful than some of my other large single engine warbird e-conversions.
Making a scale-like bomb drop
Eastern Front BF-110C’s were modified to carry two ETC-250 bomb racks housed in a single centerline fairing capable of carrying two 250kg bombs (551lbs). KMP provides a scale-like fiberglass bomb rack fairing, but no bombs or mechanism to drop them.
I built a pair of scale bombs (from pictures on a website) from foam and plywood and installed a small eyehook in a hardwood block epoxied into the bomb halfway between the two hangar hooks on the full scale bomb. I added lead to the nose of the bombs until the bombs felt slightly nose-heavy when hung from my single hook. The boobs weighed just under 3 ounces each RTF, mostly due to the lead to make them fly properly. Anti-sway pads were made from 1/16 plywood and installed on the rack to keep the bombs from shifting during flight.
The bomb release mechanism consists of a single micro servo mounted to the inside of the bomb rack in the forward section. A single pushrod, split into two segments, one slightly longer than the other, pass through the screw-eyes mounted to the bombs. The servo is plugged into the channel 7 of the Futaba 7C radio which has a three position switch. The first position captures both bombs, the second position moves the servo just enough to drop the left bomb, and the third position will drop the right side bomb.
The bomb rack is intended to be glued to the bottom of the wing. With a servo that may need servicing inside the rack, I decided to secure the rack to the lower wing skin with six 6-32 screws. I taped the rack in place and drilled holes through both the rack and the wing for 6-32 threaded inserts. I installed some scrap carbon fiber tubes (3/8” diameter) with epoxy at the screw locations. I then filled the bottom of the tubes (against the wing skin) with epoxy and re-drill clearance holes for the screws. The rack now also hides the two ¼-20 nylon wing bolts at the TE of the wing.
The unit has worked flawlessly and drops the bombs in very scale manner. The CG of the bombs creates a straight and true trajectory after release.
Links
Castle Creations, www.castlecreations.com, (913) 390-6939
Eagle Tree Systems, www.eagletreesystems.com, (425) 614-0450
ElectriCalc, distributed by MaxCim Motors, www.maxcim.com/ecalc.html, (716) 662-5651
Enerland Batteries, www.enerland.com, www.a123systems.com, (617) 778-5700
Futaba, distributed exclusively by Great Planes Model Distributors, www.futaba-rc.com, (800) 682-8948
Hitec RCD USA, www.hitecrcd.com, (858) 748-6948
Kokam America, Inc., www.kokamamerica.com, (816) 525-1153
Kondor Model Products, www.kmp.ca, (888) 968-7251
Master Airscrew, distributed exclusively by Windsor Propeller Company, www.masterairscrew.com, (916) 631-8385
West Mountain Radio, www.westmountainradio.com, (203) 853-8080
Note: All flying shots and shots of the assembled model taken by Joe Cabana.
All construction photos by the author.
