Motor Setup
To prepare the motor for gearless direct drive, remove the C-clip of the shaft to remove the bell and clamp the shaft up with a wrench socket spacing above the motor bell until the shaft's bottom is flush with the bell bore's bottom plane. Use the broad open side, not the narrow wrench shank side, on both the prototype T-Motor 4004 400kv motor and the production Sunnysky 2806 400kv motor because the narrow side can inadvertently catch the collar of the shaft. Then, place a #6 flat-head 1/2 inch machine screw as a pusher on the clamp. Clamp the shaft up the second time with the push rod assembly, as shown in the picture, and remove the stock shaft of the motor. 
Our direct drive main shaft is not pressure-fit to the motor but is slightly thinner than the original motor shaft. The video shows that the short bell shaft hole amplifies the thinned diameter, resulting in wobbling in a pre-prototype build.
Our direct drive main shaft is not pressure-fit to the motor but is slightly thinner than the original motor shaft. The video shows that the short bell shaft hole amplifies the thinned diameter, resulting in wobbling in a pre-prototype build.
The solution is to use a Kevlar thread (dental floss) as an insert. However, the Kevlar material can be cut by the sharp edges of the bell shaft hole and the shaft itself. So, the rim of the shaft needs very subtle, delicate scraping. Carbon nanotube inserts should eliminate this hassle, but the nanotube tow delivery has a lead time of at least 1 week and shipping time. The Sunnysky motors already have a 45-degree opening of the shaft borehole, but they need to be polished, and the burs still cut the Kevlar strand, so the scraping needs to be done twice on both the top rim and the lower rim of the 45-degree
The last picture above on the right shows 3/4 of Kevlar filaments severed without rounded work surfaces with the prototype 4004 motor. The kevlar string had to be tied to form a loose loop that avoids the sharp edges of any setscrew or other boreholes of a prototype motor bell. Consistent, substantial friction occurs when the main shaft is driven into the motor bell from the craft top, even if just 1 Kevlar strand is inserted. However, a single insert strand is only suitable for preventing motor wobble. For the prototype, Sunnysky 4004 400kv motors, one and a half strands of the insert are the solution for the titanium shaft with friction-only production installation for the particular batch of shafts I received. It has been tested with 2 strands with different angles with the specific batch of shafts pictured with a green cross-out, but they are either too tight for even strong hands, to insert the shaft or get cut during inserting the shaft. Splitting the Kevlar string, as pictured below, is not hard because the fibers are held with wax.
For the production v2806 motor, however, 2 full strands of kevlar are the right fit, and the pullout of the shaft shows largely intact 2 strands, as shown in the pictures below. So, the product has half strands of Kevlar thread error. It is not the more, the better when it comes to the inserts because the pull force is exerted on the motor bearing during maintenance of removing the shaft. Too strong friction can damage the bearing because there is no axial bearing, only radial bearings that can't handle too much axial force.
Trim the motor stater base as mandatory for friction fitting because the servo tabs are slightly longer than the bearing block height and protrude beneath the lower bearing block. During the pull-out of a shaft, the pressure exerted on the servo tabs and tab bolts can damage them and destabilize the servos if the motor stater base is not trimmed. As in the middle picture above on the right, all the black marked lines in the weighing picture are trimming lines. All rotary cuts are pre-cuts when trimming the stater base, leaving 1mm for safety from damaging the stater winding. Then, plier bending the material causes metal fatigue that actually makes the breaks.
The bottom retaining borehole of the shaft with sharp edges is dealt with by simply trimming the shaft from the bottom borehole down, and then the edges need to be ground to a smooth 45-degree rounded surface. The direction of the grinding sanding is to grind from the bottom of the shaft up so that any burs don't cut Kevlar. To install the shaft, use both hands to clamp. It has intense friction and only healthy and strong hands can do it. It can be verified that the insert is unbroken after driving in the shaft and the string still retains the looping, as pictured below. Cut Kevlar is pictured below with a red cross. Use the prototype endurance build's 4004 motors with setscrew boltings if you don't have strong hands for the friction-only installation.
Sticky motor bells can occur when the clamping by hand is too strong and harsh, or it can be due to shorted motor terminals by the PWM frequency off option of the ESC or just the ESC's unmodifiable programming, as discussed in the video linked by the right picture. If not carefully monitored, clamping too firmly is common in friction fitting, such as in our production build. The motors are rated at 1 kg lift, so there is no thrust/axial bearing. Strong hands can easily clamp at 100kg, such as in bar exercises and chin-ups. When it occurs, it has the same sticky feel as the ESC shorted terminal feel when turning the main rotor with fingers. But, no worries, there is no damage with clamping by hand. Just pull out the shaft a bit to loosen it up.
Electrical Setup
For the 5-V power supply, the 1.25mm-pitch connector that comes with the buck converter has a current rating of 1A, sufficient with power tapping by servos plus a Full HD camera system (either Raspberry Pi or Runcam Thumb consumes less than 0.38A or 0.28A, respectively, and an analog camera with its video transmission have 0.5A extra current). The vendors often mistakenly label the Molex Picoblade 1.25mm-pitch connectors as JST connectors. Here, the actual trade name is used throughout this build. In the following video, the current is measured on the power output of the buck converter, which powers 3 servos, a 70mA pre-prototype flight computer, and the 30mA RF receiver.
When the servos are idle, the specification says each servo consumes 15mA, and indeed, the current reading is 145mA with the flight computer consuming 70mA and receiver consuming 30mA, 70+30+15*3=145. During bursts of heavy loading of servos, the reading is slightly higher than 250mA, an average of 220mA. The production flight computer and RF receivers do not use the 5-V power supply. So the unplanned maximum current usage is 220mA - 30mA - 70mA + 380mA + 500mA = 1A.
When the HGLRC FD411 flight computer is used, it taps power directly on the battery terminal on the tail ESC, and the RF receiver and flight computer don't consume current from the DC buck converter, either.
To start the dual ESC build, remove the original wraps using cuticle scissors to open them from the sides of the ESC circuit board. Then shorten the tail ESC's battery wires to 1 inch so that they reach the main ESC. The two short tapping wires should not be shorter than 1 inch each because that would impregnate the entire wire with solder during soldering and solidify the wire, leaving no flexibility. As picture below on the left, pre-tin the short battery wires and tie the battery wires as shown in the lower corner of middle picture for the triple-junction soldering jobs. The battery-wire-to-battery-wire ties are tentative scaffolding and can be any string or scrap wire; the buck-converter-wire-to-battery-wire ties are permanent and should be Kevlar strings. Solder the short wires to the sides of the main ESC's battery terminals. It is OK for the short wire leads to touch the capacitor between the 2 electrical poles because the capacitor is electrically connected to the 2 poles. To save about 1 gram of weight, tail ESC build needs desoldering the original thick motor wires and replacing them with 4cm of tail motor's lead wires. The soldering iron temperature for ESC working is 350 degrees. Any lower temperature lengthens contact time. The newer BLHeli_S or BLHeli_32 with protruding motor solder pads, as seen in the crossed-out in the picture in the middle below, can not substitute the original BLHeli because the newer ESCs of any model have no RPM governor.
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The ESC and DC buck converter tap onto the power distribution point from the battery output, that is the scrap from the tail ESC battery wire pair, as in the lower corner of the middle picture. For weight accounting, there is no added weight of solder to the dual ESC build because the factory pre-tins the leads and the soldering pads. Here the scrap battery wires, from cutting tail ESC battery wires, join XT30 connectors and the power distribution point, so that the power distribution point doesn't need re-pre-tinning. This weight-saving cancels out the weight of the added pre-tin solder at the beginning of this dual ESC build.
The picture above on the right is the threading of the Kevlar string onto the pseudo guide needle of enameled copper wire, which can be used in places that the Kevlar tie needs piloting during building.
As pictured below on the left, when tying a knot, make 2 same-orientation loops first because the 2 same-orientation loops allow adjusting length with slip. A 3rd loop should be reverse-orientation to lock the length, and then a 4th knot reverses the orientation again to double-secure the tie. The Kevlar string should snag the ESC at the many-solder-points MOSFET transistors.
In the picture above on the right, the main motor and ESC motor wires are tentatively wedged in the main frame in alignment. In the picture below on the left, the motor wires are soldered, and they suspend themselves. Then clear mounting tape is weaved between the 3 poles to create insulation in the middle picture. One layer of clear mouting tape is added to the back of the weaving, and then the insulation closes itself as pictured below on the right.
However, for the tail motor, the ESC wires are too thin and soft to self-suspend in place. The ESC wires were attached to the clear mounting tape on the sidewall, the soldering jobs done, followed by closing the insulation with one larger, top layer clear mouting tape, as pictured below on the left. The ESCs need to be naked for maximum cooling. Without this maximum cooling, ESCs overheat in summer as shown in the video below on the right.
Flight Computer Setup
To wire up the flight computer, use the following diagrams.
This general layout and orientation is applicable to multiple brands and substitute products because the manufacturers generally use the same orientation of quadcopter components, namely the ARM chip on top for better cooling away from one-piece quadcopter bottom plate and motor signal wires face forward in line with intuition and away from bottom plate to avoid short circuiting.
The fixture wholes allow 4 1A(AWG 26) wires thru. And the bottom side of the FD411 has the soldering scheme when flipped.
The 3-pin PicoBlade wire set is spliced for the RF receiver and for powering and signaling Raspberry Pi or for video OSD.
The male half, 9 cm without the plug, is for the RF receiver connection. The female half with the male plug is for (Raspberry Pi or analog) camera powering/signaling.
With all choices of the flight computers, the original M1-M4 signal socket faces rearward in our craft, and signal wires point out and rearward for accessible soldering jobs. The array of pins provides an even mounting surface.
Rotor Setup
For the prototype build, keep the original 210mm blades; purchase the original Oxy2 193mm (nominally 190mm) blades for the production build, or trim the 210mm blades to 193mm with a pair of office scissors. To eliminate tail strikes, the rotor is raised from 180 smart fuselage original positioning. The anti-rotation/GPS mount raises the rotor blades with this mount to match the raised rotor. The 1cm x 3cm Kydex sheet is sheered by a pair of scissors because Kydex is flexible enough at 1.5mm thickness. Then the sheet is CA corner bonded to anti-rotation bracket. The drill hole is marked 4mm below the original mounting bore hole and drilled. Then use a soldering iron at 300F beneath (but not touching) the Kydex sheet at pictured below to make a thermal forming bend of 90 degrees. Then the mount is installed and GPS attached with clear mounting tape.
The height from the upper edge of the upper bearing block to the hub bolting hole axis shall be 30mm to prevent a tail strike during forceful maneuvering. As pictured below on the left, for the 30 mm height rotor setup, the shaft dent of the V950 shaft coincides with the upper bearing block, and the collar's screw wedges to the dent's upper corner in the picture after installing the collar. When friction-fitting the main shaft, the retaining collar's bolting needs dents because the extra friction used to fit the main shaft above the friction resistance can be greater than an undented collar bolting's friction can handle. More importantly, during the pull-out of the main shaft, the entire main shaft friction on the motor bell is countered by pulling the collar with a plier, as pictured in the middle. The stater rotation stopper is an M3 on the remaining stater base arm after trimming out 3 other arms. The bolt head is CA-glued and bolted on half-way. Then, the bolt head is trimmed out to save about 0.2 grams. But be careful not to wedge the bolt in between the strut for the fuselage and the strut for the servo because that space is not enough for free-vertical pressing of the stater onto the bearing block. The bolt should be between the 2 struts toward the two sides of the fuselage plates.
The DFC pin requires a bushing section without threads with the DFC arm. Factory partial threaded M2-15mm bolts with 3mm end threading take multiple weeks to deliver, and even then, the non-threaded section isn't a high-quality fit because a bolt is never meant to be a bushing axix. For fast production, use copper tape of 1/4 inch to wrap the section in contact with DFC arm bushing. The copper foil tape and M2 fully-threaded bolts are part of the essential supplies of the science and art industries and so the delivery is the fastest among all parts of the build. As pictured here, the 1/4 inch copper tapes generally have a similar thickness as very thin clear stationary tape. The example needs 2 layers of the tape, so the length to use is 2mm x 3.14, about 6 mm. The product description of the copper foil tape often indicates "double-sided" adhesion, but, depending on product picture, the adhesive is double-sided while the copper foil facing outside can not be double-sided with adhesives, so the outside face is without adhisives and with low friction. The wrapping doesn't need to be very neat or tight because it can always be compacted after inserting into a DFC arm, just by use a screw driver to tun and rub the foil against the DFC arm's inner wall for a couple minutes.
In the example picture, the wrapping of the copper foil may seem very tight and not increase the bolt's thickness at all, but the actual installation is very tight and requires the technique of turning the screw against the DFC arm inner wall for a few minutes to compact the wrap to make a proper fit. When the DFC pin is correctly made, the DFC arm can fall by itself when held and released, as shown in the video.
Tail Build
We need 2 nylon washers as spacers to give room to the protrusion of the motor shaft. As shown in the picture here, the space is tight between the shaft and the tail fin board. It is OK to mill a slight concave of the fin plate at the tight spot, but we shouldn't drill a hole through because that would weaken the fin structure. Depending on the bolt length and fin plate thickness, we can use another 2 nylon washers to cushion between the brittle acrylic and the hard M2 screws' head, and also to stop the bolting from crushing the stater winding. The motor's 2400-2750 KV range requires Y termination of the motor stater's wiring, so there should be a solder stub as circled out green in the picture.
Mislabeled sales of the 2400-2750kv motors with delta winding (missing the Y center termination) don't have any noticeable working characteristics differences, as shown in the video on the right.
The diagram above of the fin has the coordinates of the corners, such as (3.4 , 8) and (17 , 11.6), unit mm. It is to the scale when printed out with 60 pixels/cm. The tight corner in the middle is hard to cut with a rotary disk, and it will be easier if following the arrow in the diagram of cutting sequence to make 2 cuts meet in the middle. Use rotary 1mm drill to make 2 adjacent holes on the 4 corners of the attachment square for the zip ties, then break the 2 holes to form a oblong whole. Use CA glue sipping between the fin and the carbon tube. Pinion gear is meant to be a fitting spacer and has a very tight fit on the motor shaft, so you need to place the gear on a surface then press onto the motor shaft. CA glue is applied to prop's lip before attaching it to the motor bell.
Over tightening the tail motor bolts and/or the fin zip tie pre-load the fin with stress, resulting in premature break of the fin , as shown in the picture. The PVC insulated, low-voltage wires is not used because the low-voltage, 30V, light wires are not available for retail from digikey.com and other vendors. Above picture has 60cm of it cut from a Picoblade 3-pin connector set. It is the lightest in the market but still weighs 0.88 grams extra for 111cm. 0.8x(111/60) - 0.6 = 0.88.Mounting Radio Receiver For Reduced Interference
Without proper spacing between main motor and radio receiver, the interference can disconnect the radio locking with Flysky XM+ when the craft is well within the expected range, resulting in "Failsafe Mode free fall", when the receiver was mounted on the rear pedestal of the main frame. The spacing is sufficient about 3 inches behind the pedestal. If the receiver is further extended to the rear, the receiver is struck and destroyed by tail strike in a crash, as occurred multiple times on the 2 tail booms pictured here.
, and the 3rd case,
. In the last picture of the 3rd case, the tail strike also detached the receiver's antenna but no damages, which means that the mounting tape should be placed as far as the last picture's, at 85mm or less from the front end of the boom.
The 2 antennas of MX+ are encased by the mounting tape square.
The tail boom insert and close-up of the mounting and the option of TBS Crossfire Nano receiver follow.
Servo Setup
To make the servo arms resilient to crash, cut the spline and partially split the arm to make a torque clutch. That way, the clutch slips whenever the impact force is larger than the gears can handle, relieving the gears from damaging forces. When dialing the spline position, loosening the arm's fastener screw is unnecessary with such a servo clutch modification.
When making the slit, liberally apply the cutting, and do not be conservative. As shown in the picture below on the left, when one of the servo's cutting was very conservative, the clutching was overly strong, and a crash damaged the servo itself. The correction was liberal cutting to thin the remaining material and make the opening approach the servo rod, as pictured below on the right.
The servo power tapping loom should not have twisted conductors because twisted solder makes replacing wires hard.
The raised rotor requires raised servo arms, which are done by rotating servos to make arms on the upper side of the servos. This raises the arms by about 10mm, and then they need to be lowered by 5mm with trimming of the servo rods. Here, pictured below, the servo rod is marked with the new servo arm position, and then the original banding (2 sections) is trimmed, and a new banding is made with a plier.
The trimming of the 5 mm excess needs to be conservative because of a trick we use to mitigate a bug unique to the FD411 flight computer. With the prototype fuselage build, it was discovered that when the FD411 was booted up without RF control turned on on the transmitter side, a pulse of the lowest collective pitch signal, as stipulated by the servo range configuration, was sent to the 3 servos. That made a rapid twitch down and up motion of the rotor blades, which added unwarranted wear and tear to the servos, as shown in the video on the right. But there was a solution discovered subsequently.
It was accidentally discovered during the production fuselage build that when the servo upper range was higher than 2080 us, the twitch motion disappeared, which meant that the servos ignored PWM commands higher than (include) 2080 us. In the production build, the servo range direction is in inversion, so the lowest collective pitch signal pulse translates to the highest PWM signal to the servos. To ensure that the high-end signal is higher than 2080 us, the servo arm rods need to be tall enough so that the high end needs to be raised 80 higher than (include) 2000 to oppress the tall servo arms. To keep the servo arm rods tall enough, the trimming of the 5mm excess needs to be conservative. The first servo arm hole, at 8 mm, is used with the servo rods. M1.5x6mm screw (#0 American gauge screw) fastens the servo arms to the end turn and the arm turns. Then, back off a half turn. The thread width is 1/32 inch, so half a turn allows about half a mm for servo clutch relief and ease of clutch slip.
Summary of the torques used in the build.
Battery-Camera Mount
6cm long hook velcro without backing sticks to the overhang, and it goes around the battery pack, wrapping the front bottom as a cushion for landing on battery.
The holes on the battery tray avoid studs as pictured below. The holes for camera zip ties is 15mm from the front edge. The side zip ties require drilling of the carbon fiber side wall beneath the original bolting hole because the original bolting hole is too close to the side wall's material edge.
Raspberry Pi Setup
A generic zip tie goes around the right servo and wire lead is preserved for a quarter inch on the lower part. Similarly, the battery tray's zip tie on the right aft also has the wire lead. Raspberry pi's fixture hole goes to these 2 leads.
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