Potensky Cobra and Eagle
The two Potensky models proved to be very robust and enjoyable to fly. The choice of picking one over the other would have to lie in the type of flying that you prefer and your piloting skills.
By Greg Covey
Mar 21, 2003, 01:00 AM
||POT 03B (Speed 300 geared 5:1) |
||8amp (14amp burst)|
||7-cell 720mAh NiMH or 2-cell, 2100mAh LiPoly |
||10x4.7 Folding |
||Hitec HS 55 (4)|
The Potensky Cobra is a small 4 or 5 channel, full-house acrobatic 3D parkflyer, made from almost unbreakable EPP foam. The output thrust of the geared Speed 300 motor is greater than the weight of the plane. The fuselage and wings are made from black EPP foam which is a characteristic of its European design. The fins and control surfaces are made from a Depron-like foam called "Potpron". This is a fully acrobatic, symmetrical wing model with a wide range of speeds which is developed for outdoor flying in small fields. The model plane flies for about 10 minutes using a 7-cell 720mAh NiMH battery pack. Optionally, a 2-cell, 2100mAh Lithium Polymer (LiPoly) pack can be used to reduce weight and increase flight time. Static pull is 1.75oz (50g) higher than the mass which allows the model plane "to hang" by the propeller. The plane's 3D dynamics and flight characteristics are exciting to fly, or even just watch.
The kit includes a complete power system and all the parts needed to finish the model. The Cobra design also allows for easy attachment of an optional float set to convert your land flyer into a water acrobat. A custom plastic canopy, long carbon fiber rods, and a steerable tailwheel assembly are also included.
The Hitec 555 micro receiver and HS-55 servos are a great fit for the Cobra To complete the model, you need a 4-channel radio system, 4 micro servos Optionally, a 2-cell, 900-2100mAh, LiPoly pack will reduce weight and add duration.
The basic Cobra parts are all foam (left) and include decals, manual, and a complete power system (right). When I first realized that the kit was made in the Czech Republic, my initial concern was for the building complexity and clarity of instructions. I was relieved to find both a full size drawing with all the components in place, and a manual that consisted entirely of pictures; the universal language. Potensky Microsystems did a great job on both the design and the documentation. After seeing the on-line video of the Cobra performing 3D maneuvers and hovering in place, I was excited to get started! I thought, perhaps this model could be used at my local indoor events for some wild aerobatics in the gym.
The manual is divided into distinct sections and steps, so I have decided to exactly follow the manual with my comments and hints. I hope that you will consider them a helpful aid to the manual.
Cut the 3 circuit board formers free and sandwich them together with the supplied toothpicks. I used CA to glue the assembly together which will be used to support the landing gear. I roughed up the inner side with the supplied sandpaper before gluing.
Cut off the toothpick ends and sand the nubs off the assembly.
A 5th hole (2mm) is drilled near the middle of the assembly for additional support. Again, I used CA to glue the toothpick in place. Do not cut off the toothpick since this will be glued into a slot cut in the fuselage.
To ease the pushing of the landing gear assembly into the pre-cut slot in the fuselage, I rounded the top two corners with sandpaper. I then cut the foam lengthwise at the slot in the fuselage to facilitate the center toothpick. I also sanded the outer sides of the assembly and applied a small amount of white glue before pressing it in place into the fuselage slot. Be sure that the openings for the metal gear rods face the outside.
Rough up the circuit board firewall piece, and glue it to the foam fuselage nose so that the top of the circle on the firewall board is flush with the top of the circle in the foam fuselage nose. I used white glue on this step. The smaller hole in the nose routes through to the top receiver compartment.
Stick four of the toothpicks into the outer holes and glue in place with white glue. This provides additional support for the firewall.
Let dry and then cut the toothpicks off close to the firewall.
This step made me laugh when I saw the need for a hot wire gun because few of us own one. Fortunately, the foam holes needs only to be cut about 1/4" deep to fit the backend of the motor so a sharp razor knife works very well in this step. Trace the firewall circle with your razor knife about 1/4" to 3/8" deep and remove the foam See the "Motor" steps 1-4 for how the motor will eventually be placed.
A 5mm hole is drilled into the side of the fuselage so that the ESC battery connection can be later routed from the inner tunnel to the outside. The hole size here is not critical.
Cut the middle section away from the foam cowl as shown in the manual diagram using a sharp razor knife. I cut my cowl from the back and angled the knife so that the walls became thicker as it approached the nose.
Steps 11, 12
I decided to skip mounting the foam cowl until after the motor was mounted. If the cowl needed more trimming, It would be easier done while still removed from the fuselage.
Cut two pieces of carbon rod about 260mm (10.25") long. Cut each 10" piece from different rods.
Slice a slot into each side of the fuselage rear and glue the carbon rods in place with white glue. I filled the long slice in the foam with glue first and then pressed the carbon rod in place with pliers. After the glue dried, the side-to-side movement of the fuselage tail section was much firmer.
Cut a slot into the bottom tail of the fuselage for the tailwheel assembly.
The pre-assembled tailwheel was excellent quality. I sanded the sides of the assembly and then glued it in place with white glue.
Like the tailwheel assembly, the landing gear wheels and custom hubs were very nice quality. They simply pressed into place onto the gear rod and inserted into the fuselage bottom assembly.
Hold the plastic canopy in place and press the 3 plastic nails into the proper position in the foam. Remove the canopy and nails. Rough up the nail shafts with sandpaper and press into place with white glue.Keep the back two nail heads close to the foam and leave the front nail head
up about 1/16" at an angle. Wipe off any excess glue.
Find the 4 pieces needed to assemble the elevator and stabilizer. The elevator joiner is a piece of the fiberglass circuit board about 1 3/4" long and 3/16" wide.
Cut slots in the leading edge of the elevator halves and glue them together with the joiner using 5-minute epoxy. Use the stabilizer to help you align the elevator and hold it in place with some light weights until the epoxy dries.
Attach the elevator to the stabilizer. The kit comes with some iron-on tape for hinging but I used my own 3M Blenderm tape that has a nice stretchable property to it. I found that the iron-on tape worked best on the EPP foam wing.
Round the leading edge of the horizontal stabilizer with the supplied fine grit sandpaper.
Step 5, 6
Before gluing the elevator assembly to the tail, I decided to first build the rudder assembly and then test fit everything before gluing.
Round the leading edge of the vertical stabilizer with the supplied fine grit sandpaper. The leading edge has the curve in it.
After sanding the leading edge round, I studied the tail assembly again. The toothpicks from the horizontal stabilizer assembly were needed to help mount the vertical stabilizer.First, I re-inserted the horizontal stabilizer assembly into the tail all the way. For the elevator to swing properly, I needed to trim the back of the tail slot a bit. Next, I used a ruler to measure equal distances along the leading edge from tips to fuselage. I then centered the elevator joiner at the rear and double-checked the leading edge again. Lastly, once I was satisfied with the alignment, I stuck two toothpicks down through the top of the fuselage into the stabilizer, locking it in place. It was now easy to pull the assembly apart and then use white glue to secure it back in place permanently.The vertical stabilizer pressed down on the toothpicks so that the trailing edge was flush with the rear of the fuselage. I taped the front down until the glue dried. Be sure to also check for a right angle between the two stabilizers.
Cut a half circle into the rudder so that it clears the elevator joiner. Attach the rudder to the vertical stabilizer using either the supplied iron-on film (it does not shrink with heat) or your own preferred method. I used aileron tape for attaching the rudder to the vertical stabilizer and a Kavan white hinge to solidly mount the rudder bottom to the fuselage.
Use either a small piece of carbon rod or a toothpick to attach the steerable tailwheel bar to the rudder. I CAed two short pieces of toothpick into the rudder.
Slice a line between each servo bay and the receiver bay inside the cockpit.
Install the rudder and elevator servo and route the servo wires into the receiver bay. Hold the servos in place by using the black plastic pins with some white glue.
Step 9, 10
Slice a straight line on each side of the turtle deck and bury the white snake tubing so that it aligns with the outer side of each servo. The bent end of the metal rods should end up about 1/4" above the horizontal stabilizer. Rough up the outside of the white snake tubing and glue in place with white glue.
Cut the white snake tube to the proper length, if needed.
Cut a small slot into the rudder and the elevator to mount the control horns. The left side control rod connects to the rudder and the right side control rod connects to the elevator. I used foam safe CA to glue the control horns but 5-min epoxy would work equally as well.
Whenever you are attaching a control horn to Depron-like foam, be sure to properly spread the glue over a broad area to add strength to the joint. Alternatively, a small piece of balsa can be used around the control horn for adding strength.
Step 1, 2
Do step 2 first, which is to round the leading edge with sandpaper to remove the molded corners. Then use the iron-on tape to attach the ailerons, one to each wing half. The iron-on tape works well on EPS foam. The EPP foam sanded very well using the supplied sandpaper.
Since my ailerons warped after mounting the wing halves to the fuselage, I would recommend waiting to attach the ailerons until later.
Step 3, 4
Slice lines into the top of one wing half and bury the carbon rods after first filling the slots with white glue. The rods are slightly angled as a "V" from the wing tip.
The proper wing alignment and incidence checking are left up to the builder. Please use proper measuring tools to ensure your success. A right angled "builder's triangle" and 36" ruler work well. I felt that this part of the design was rather ambiguous and could benefit from pre-positioned holes in the fuselage and slots in the wing for alignment.
Holes must be poked through the fuselage to fit the carbon rods through. After studying the pictures for a while, I decided to align the center of the aileron servo opening with the center of the black EPS wing (excluding the aileron). This allowed me to hold the wing with the carbon rods adjacent to the fuselage for exact placement to poke holes in the fuselage. The top surface of the wing also aligns with the bottom of the aileron servo opening. I used a spare metal control rod from another kit to poke holes through the fuselage.
After I had finished the Cobra, I was given a tip when building the Eagle that helped with the initial placement of the wing halves. Copy the supplied full size plan and place it directly against the fuselage to mark the correct wing position. For more details, see the Eagle Wing Assembly section.
Step 6, 7
I used 5-minute epoxy to glue the wing half on and held it tight against the fuselage until it dried. I believe that this is how I warped my ailerons since the wing ends may not have fit flush against the fuselage wall. Be sure to epoxy the carbon rod sections that will reside inside the fuselage. I also epoxied the rear toothpick and wing end that pressed against the fuselage. Double-check the wing incidence with the horizontal stabilizer.
This is probably the toughest step of the entire assembly. The manufacturer did not provide a good technique for aligning the wing. Be sure to check the position of the leading edge of both wing halves and the incidence with each other. First, I positioned the free wing half under the carbon rods and marked their positions on the wing after verifying the leading edge alignment.Next, I sliced the slots for the carbon rods and positioned the wing in place. When I was satisfied with the alignment, I marked the trailing edge position on the fuselage and then inserted the toothpick about an inch forward into the fuselage.I then drilled a larger hole than the toothpick width into the mating position in the wing and cut the carbon rod channels a little wider from the wing center to about 1" away from the center. This allowed me some slop to slide the wing half around the exact alignment position.Using 5-minute epoxy, I glued the toothpick area and the entire wing side against the fuselage and held it in place while I triple-checked the leading edge alignment and incidence.The result, although not easy, was not bad! My alignment was sufficiently good for this relatively slow speed aerobatic design.Lastly, I buried the rest of the carbon rods into the wing with white glue.
The HS-55 servos fit very snug without any cutting and really don't require the additional black plastic pins to help hold it in place. This can optionally be added later, if needed.
Step 10, 11, 12
I connected the entire aileron linkage assembly and centered the servos before finding the exact position for the control horns. The control horns ended up right near the leading edge of the aileron which allowed for maximum throw range.
The power connector on the ESC is not a mate for the Dean's 2-pin polarized connector <u>and</u> it is not wired to the US convention. On my battery packs, the male pin is wired to the (+) positive terminal of the battery pack. Check your polarity before connecting the ESC to a battery pack!
Step 1, 2, 3
After roughing the motor case with sandpaper, I glued the mounting bracket into position with 5-minute epoxy. My gearbox, prop, and ESC came pre-assembled so steps 1 and 3 required no work. This was a nice touch!
Feed the controller wire out to the receiver compartment and route the power connector out the hole on the left side of the plane near the wing leading edge. Screw the motor assembly in place with the two provided screws. All the wires were pressed down into the opening in the cockpit and then the receiver was placed inverted on top.
Appling the decals was straightforward and easy. The yellow and black decals enhanced the overall color scheme.After checking the motor for proper operation and setting all the control throws, I glued on my foam nose cowl with white glue and then applied all the decals. To get the decals to properly stick to the EPP foam, I spread a thin layer of white glue on the foam first.
The finished Cobra was RTF at 11.7oz with a 7-cell, 720mAh, NiMH pack. The 7-cell pack was moved just under the CG line for a proper balance The model balanced perfectly with my 3.3oz, 7-cell, 720mAh, NiMH pack from Hobby Lobby after carving the foam out forward of the stock opening.