Friday, May 22, 2020

IoT Platform Endurance

According to the US Department of Energy, , lithium-cobalt battery energy density should be set at 190Wh/kg, with thin-layer electrode construction at about moderate discharging rating, and reserving 10% electrode to prevent damage, equating to 190Wh/kg*90%=171Wh/kg=0.171Wh/kg. With cooling circuits in a passenger electric vehicle, the battery pack energy density is generally 150Wh/kg. The general-purpose IoT forces a landing/termination when the ion reserve is at a 10% level because the 10% level can't sustain lifting the craft, as shown in the video below. So, no protection or cooling circuits are needed, as shown in the following video, where the lift lowers to a landing with a 10% battery level.
The takeoff weight of the craft is 249 grams. License-free. The implication is that the craft should be reproduced for 1 billion units and deployed around the globe for traffic monitoring/roadside assistance applications. Every person should have an assistant drone nearby whenever they step out the door. Think Terminator micro drones but on the human's side. 
The GensAce900 airsoft rifle battery is of thin-layer construction at about 25C discharge rating, with capacity measured between 949 and 998mAh at the first charge cycle, depending on how balanced the cells are when depleted, as shown in the below pictures. 
The below-left picture was with Tattu1000 cells, measurement taken after about 1 dozen charge cycles after this pack was used for the video "World record endurance". The pack gave 870mAh electricity during the mentioned world record mission. The exposed lower-right cell tab is characteristic of the Tattu1000 packs. At the first charge cycle, the Tattu1000 cells have identical charge intake, between 949 and 998mAh, as shown below on the right.  
The pictures together mean that the pack forced a landing with about 90mAh reservation, or 10 percent.

So, overall, the GensAce900mah/Tattu1000mah airsoft rifle battery has a capacity of 970mah. In a 4-S configuration, the endurance time of 35 minutes means power for cruising with HD video is 0.87Ah / (35min/60min) * (4 x 3.7V) = 22.2W. Each cell weighs 18.9 grams. Energy density 0.97Ah x 3.7V / 0.0189kg = 190Wh/kg. Total battery energy is 0.97Ah x 4 x 3.7V = 14.4Wh. Useable energy is 14.4Wh*90% = 12.9Wh.

The endurance build dives in this video,
, which dives the same way as the other builds for adrenaline diving. It just needs extra labor to build. Compared to the general purpose build, the extra 15 grams of battery weight are shaved from (sorted by increasing labor) GPS (5.5 grams absent), the motor(5.9 grams saving), and ESC power system (4.1 grams saving). The HD camera system weighs the same as the general purpose's video system. The weight gain of the long distance video transmission and building custom battery pack is canceled out by lighter shaft and lighter rotor blades. And the custom battery pack itself needs most of the extra labor.

BSR Frame

E-Flight BSR main frame is 23.2 grams and nominally saves 1.1 grams from the 230s frame. The following first 2 photos are of the same frame. Cleaning the frame reducing 0.6 grams (without erroneous trimming of the front battery bay stud) maximum should result in between 20.8g and 22.0g. The hardware screws weigh 1.1x10/9=1.2 grams. So the installation is between 20.8 +1.2=22g and 22+1.2=23.2g , worst case 23.2 grams.
When closing the main shaft cage, the screws go in from the left side of the craft(from the craft's first-person perspective) on the BSR main frame. 
However, the E-Flight BSR anti-rotation bracket is 0.4 grams heavier than 230s' at 0.9 grams. So, the actual weight saving is 0.7 grams when the BSR frame is not further trimmed to save weight. One screw is not used because we use custom M1.7 long screws.

To compare the rear servo geometries, as shown in the picture above on the left, the servo fixture system is forward 1.2mm on the 230s frame. So, the BSR frame's servo setup has a more slanted servo rod toward swash. This can not be improved because we need to prevent servos from caving into the servo cage with a BSR frame, so we can't move the rear left servo forward 1.2mm, related to the cage geometry.

The servo tab location is marked in the picture above on the right and is 10mm behind the BSR frame's servo cage's upper front wall surface, as measured in the pictures below on the left. The placement of the 1.25mm bore holes is 10mm+2mm=12mm behind the upper front wall surface because the struts have a 4mm diameter and a 2mm radius. The counterpart of the 1.25mm half holes on the bottom plate is 12mm+1.4mm=13.4mm behind the lower front wall surface, as indicated in the picture below. The 1.4mm shift is due to the thickness of 4mm of the upper wall. The fixture hole diameter is 1.2mm. The extra thickness from the edge of the fixture hole to the wall's surface is (4mm-1.2mm)/2=1.4mm. The lower Wall's thickness is the same as the fixture hole diameter. The lower wall's surface is tangent to the fixture hole's edge, hence no extra thickness.
The servo fixture hole geometry, namely the width/height/depth of the BSR frames, is congruent to that of the 230s frames. The mentioned 13.4mm distance of the BSR frame is equivalent to having 14.6mm on the 230 frame from the wall to the fastener holes. This distance has a 14.6-13.4=1.2mm reduction in the BSR frame because the 230s frame moves the wall forward 1.2mm, which is the diameter of the servo fastener hole's diameter. In the 230 frame, the wall's inner surface is tangent to the hole; in the BSR frame, the wall is aligned to the hole axis. In either case of BSR or 230s, the axis-to-axis distance between the original fixture hole and the hole for custom struts is 14mm. The math is 13.4+0.6=14=14.6-0.6. 
In the diagram here for the depth dimensions, the curious number 1.25mm of the fixture holes on the anti-rotation bracket are actually half-moon dents of 1.25 mm depth because the screw we use is 1.5mm in diameter, and the bracket's width is 6mm. So, the center of the 2 half moons is 0.5mm from the bracket's side edges. 0.5mm+(1.5mm/2)=0.5mm+0.75mm=1.25mm .

The 230s swashplate fits the 230s main frame unmodified. For the BSR frame, we need to trim the anti-rotation bar, first grind the top to form a flat surface. Use the cutting disk/wheel with the slowest rotary tool speed for this job. Then slice the 2 sides 45 degrees from the top to form a rough final shape.

Once the rough final shape is formed, draw a mental line vertical from the upper left corner down. The plastic material on the left side of the line needs to be preserved. Grind out the lower-right corner material to finalize the trim. 
After the trimming, shorten the bar so it can escape the anti-rotation bracket in the event of a crash.
Unlike the 230s frame, the upper strut for the BSR frame should not be carved to prevent material warping. Instead, for the BSR frame, the anti-rotation bracket's groove needs to be carved, and our custom bracket sits on the flat surface of the anti-rotation bracket, as shown in the picture below on the left. This is because the BSR frame's servo cage ceiling is 1mm lower than the 230 frame's. 
As pictured above on the right, the BSR frame's middle vent window is 1.5mm higher than the 230 frame's. So, the flight computer USB port doesn't need frame carving. The example in the picture needs only a slight scraping of the inner edge with a craft knife.

Battery Pack Build

Use cuticle scissors/cutter and soldering iron at 350C to remove old wiring first, then the GensAce900 pack needs to be trimmed. The rotary tool cutting disk is commonly made of insulating ceramics, and we use it to cut out the first cell with the exposed perforation clamped tab. Cut the glass fiber plate itself so that the cell tabs remain soldered to the terminal. Do not cleave the cell tabs and then attempt to solder the cell tabs because the cobalt alloy does not weld with other metals, even with specialty soldering fluxes such as aluminum flux. 

The Tattu1000 2S cells have the same dimensions with a solid terminal glass fiber plate weighing 0.3 grams heavier per cell. The following illustrations use the Tattu1000. In the pictures, you can see the exposed lower right cell tab that is the middle pole of each pack. This is different from the GensAce900 pack shown in the 250 gram weight breakdown page, where the cell tab is behind in the canal of the carved glass fiber plate.
To start the build, the 2 packs are staggered by 1 inch with a temporary taping. The 22AWG charging wires are bent to the opposite poles and tied to the protruding top pack for the right side of the craft.
After soldering the 5 wires, the additional middle wire is tapped then soldered to the top positive pole of the lower pack. The yellow terminal is what cascades the 2 packs. After the temporary taping are removed, the permanent taping adds 0.47 grams of 35cm clear tape. The XT30 plug is 0.8 grams,  9 soldering joins weight 0.18 grams, and charging wire set weighs 2.4 grams. The Tattu1000 yields a 76.6+2.4+0.8+0.26+0.18=76.6+3.64=80.2 gram pack; The GensAce900 yields a 75.4+3.64=79 gram pack.

Main Shaft Weight Reduction

When measured from the rotor hub bolt hole to the bottom, the carbon fiber main shaft is 3.5mm shorter than the original build's titanium shaft. In the following picture, the rotor hub bolt holes don't appear aligned but are correctly aligned. The tape measure is concaved, so the heavier titanium shaft sinks to the bottom, and the carbon fiber shaft is closer to the camera lens. The camera lens is directly above the 18cm mark, so the hub bolt-hole image is distorted in the picture.
This doesn't mean that the rotor mast will be lowered by 3.5mm and risking tail strikes by the main rotor. Remember that, in the basic build, we had shaft intrusion with the titanium shaft by 2.8mm. Here with the carbon fiber shaft, we push the protrusion up by 2.5mm, leaving only 2.8mm-2.5mm=0.3mm protrusion, and our rotor mast is only lowered by 3.5mm-2.5mm=1mm. So practically, there is no risk of a tail strike.

ESC Build Weight Reduction

The ESC build is modified from the original IoT build with all stranded 20AWG wires replaced with lighter solid 24AWG enameled wires to save 2 and a half grams. The solid wires have slightly larger equivalent cross section when the gauge numbers are the same, so the solid wires can have a slightly larger gauge number. 
The 16 Amp BlHeli products earlier than year 2016 are not to be used because either the dampening can not be disabled or PI loop not available.

20 Amp BlHeli ESCs in the market don't have such problem. Further more, all 20 Amp and 16 Amp BlHeli (not BlHeli_s nor BlHeli_32) have the exact same weight as shown in the above weighing.  Blade Thrust brand has navy blue PCB, DYS uses green, and LittleBee uses black. BlHeli_s or BlHeli_32 are not suitable because neither has the PI loop to provide constant stable RPMs.

For the tail motor, in theory, the 3S 6A ESC can save another 0.9 grams as shown in the picture on the right, but this alternative can only function properly when maneuver is small and smooth. The startup power with 3S is too weak even though the output of the 6A ESC is adequate, as the video below on the left. And this problem persists even after startup boosting is changed to maximum in the ESC's configuration.

HD-FPV Camera

Four different powering options have been tested, similar to the FPV camera setup section of the high-performance build post
  1. The MP1584 chip-based buck converters, such as the Bluesky 5V power supply, are the worst for analog video applications.
  2. The second worst noise interfering with FPV feed is with 12-16V direct battery power supply to the Caddx Turtle cameras (v1, v2, and Baby) in the following video; if you watch the original feed video file (download it then watch it on a video player instead of watching it online with an online player because online video servers process out the noises to save their internet bandwidth).
  3. The FPV feed is best clear of interference lines when VTX's 5V aux power filters out low-frequency ripples. However, the market trends move away from VTX aux power and toward pass-thru powering.
  4. The solution is to share power with servos of the base build, the same as the high-performance build, which has a Wolfwhoop MP2315-chip-based buck converter power supply. 
With an extra alternative 10uH inductor, the FPV feed has the video ripple noise level between direct battery power and VTX's aux power in this solution. The ripple noise can not be completely eliminated with a 10uH inductor LC filter because the ripple frequency is similar to or lower than the 10uH inductor working frequency. The ripple noise can not be eliminated with an electrolyte audio A/C capacitor, either, because the ripple frequency, around 1MHz, is much higher than the electrolyte capacitor's audio working frequency. 
As pictured on the right, a scrap plastic strut fixture is needed to hold the camera board vertically. Use kevlar strings or enameled wires to tie the board to the strut and craft body. And then, CA glues the fixture arm.
Prepare the SD card by sticking it onto a tape with a mounting square, then tear off half of the unused tape. After installing the SD card, fasten the mounting square onto the steel plate of the camera board.
Do not drill holes on the sides of the main frame wall to fasten the camera board because that will weaken the structure and damage the camera wire connectors during a crash.  The experiment shows a complete cleavage of the upper main frame when the left side wall is weakened with fastening holes. 
The alternative Eachine VTX03 has AUX power at 360mA, as shown in the specification on the right. It can give the best FPV feed with Caddx Baby Turtle. However, as seen in the following specification comparison, the alternative Runcam Split has a much larger power consumption than the VTX03 can support.
Runcam Split series tapping AUX power of VTX03 results in video freeze with high G maneuver power surges/dips. One incidence of such video freeze is below on the left during punching, where the drone is lost from a considerable altitude.
A second video freeze incident with Split is above the right. If you watch the video to the end, the frozen last frame stays indefinitely after the drone touches the ground.


Velcro strips use are split in the middle to cut the weight to half to bring down the craft under 250 grams.

Notice that the 250g weighing uses alternative grips(1.5g extra), alternative servo arrangements (1.2g+ 0.4g esc tabs extra but 0.6g solder reduction), CC3D+RX wire(0.6 reduction), alternative tail ESC(0.9g reduction), alternative tail boom(0.4g reduction), alternative RX kit wires (0.3 grams extra), and alternative tail motor(0.3 grams reduction). Net weight gain 0.6 grams.

Maintaining optimal airfoil angle 
The well-known helicopter dynamic says that cruising enhances lift. So, RPM needs to be lowered for the world record endurance to keep the cruising airfoil at 6.2 degrees at 100/200 throttle level. This is done by the transmitter RPM tuning knob to lower throttle 5 points from 85 to 80. The knob is multiplexed into throttle channel in the transmitter.

Motor Pairing With Blade Span For Highest Lifting Efficiency

Cargo and passenger transportation often aims at 5kg/kW=5g/W lifting efficiency with disk loading, as seen in this paper,

The overall efficiency has 2 peaks as shown in the video, and we use the first peaks of motor-propeller combinations for the highest efficiency. 

The first hump (the highly high peaks) is dominated by the intrinsic efficiency of the propeller fluid dynamics curve because the motor electrical current is low and generates little waste heat; The second hump (very small, almost insignificant) is the high electrical efficiency given by the motor's tendency to need high RPM. Also, the second hump is not always present prominently.

The 4004 kv400 motor has an efficiency of about 14.7 g/W for 250g lifting force can be extrapolated by the statistical curve beneath the manufacturer data sheet, as well as experimentally tested in the first paragraph of this chapter page, servo/RX/FC 120mA, Baby turtle 380mA, VTX 200mW, (0.12A+0.38A+0.2A)*5V=3.5W, tail 1.7W with 20g thrust at 11.8g/W, gross power is 22.2W, 250g/(22.2W-3.5W-1.7W)=14.7gW.

The manufacturer's published efficiency data is for blades with curved propeller edges that increase the air downwash velocity near the propeller's root, giving the propeller a higher effective disk area. The correlation between the curved propeller diameter and the effective blade diameter for straight-edged blades is 150%. So, as shown in the above table, the 13-inch propeller's efficiency data is effective for helicopter blades with a span width of 13 inches * 25.4 mm/inch *150% = 495 mm.

According to , disk loading is reciprocally correlated to lifting power consumption efficiency. Disk loading is inversely proportional to propeller width squared for the same craft weight. So, the Phython code of calculating the efficiency of the most efficient motor-propeller combinations for 249 gram lifting force is directly proportional to propeller width squared, 
lifting_weight_per_watt = 14.7 * (propeller_span_width/479.0)**2;
This formula is for blades with straight rectangular edges. The coefficients are calibrated by this experiment.

This generalized formula can be applied to heavier motors. For example, the 4004 kv300 motor has an efficiency of about 16.2 g/W for 250g lifting force by the previous extrapolation curve with the following manufacturer data.
And the matching blade span width is sqrt(16.2/14.7)*479 = 502 mm, or 502 / 1.5 / 25.4 = 13.18 inches. But the manufacturer doesn't have data points between hole inches.

The generalized formula can be applied to lighter, smaller motors, such as T-Motor AS2306 1500kV at 8.3 g/W efficiency and a 9-inch propeller. The manufacturer does not have a testing data point for between whole inches.

14.7 * (9 * 25.4 * 1.5/479)**2 = 7.5
14.7 * (9.45 * 25.4 * 1.5/479)**2 = 8.3
14.7 * (10 * 25.4 * 1.5/479)**2 = 9.3
The generalized calculation blade span for the highest lifting efficiency agrees with the data and gives more detailed data points and trends.

The 1303.5 motor's efficiency is shown in the product specification,

The endure time calculation is as the following graphing calculation. The closest blades in the market to make the optimal blade span width of 475mm are 210mm blades in combination with a root span of 57mm. This overshoots the blade span width from the optimal width. But the endurance is nearly identical to the optimal endurance by calculation. 
The phython code of the calculation is 
craft_construct_mass = 111.0; #change it for a particulr craft constructions endure time curve
propeller_span_width = 479.0; #change it to slide the rotor width with in a endure time curve
motor_propeller_mass = 59.9 * (propeller_span_width/479.0)**2 + 2.6;
battery_cells_weight = 249.0 - craft_construct_mass - motor_propeller_mass;
battery_energy = 0.171 * battery_cells_weight;
lifting_weight_per_watt = 14.7 * (propeller_span_width/479.0)**2;
power_tran_power_consume_rate = 249.0 / lifting_weight_per_watt;
total_craft_power_consume_rate = power_tran_power_consume_rate + 5.2;
endure_time_minutes = battery_energy / total_craft_power_consume_rate * 60.0;
print (endure_time_minutes);

Alternative Swashplate Weight Reduction

The BSR swashplate weighs 1.5 grams less than Blade230s's. But the ball link EFLH1151 for the BSR swashplate is too short when pairing with Oxy2's DFC arm. The solution is the Align250 aluminum DFC arms. With Align250's DFC pins, the DFC arm and pin weigh 2.2g, 2.2g-(0.3g Oxy2 DFC arm + 1.2g Oxy2 DFC pins)=0.7g heavier. So, this alternative setup saves 1.5g-0.7g=0.8g. The Align300's DFC arm is identical to the Align250's DFC arm, same length, the same bearings, and the same ball socket. The ball socket diameter of the Align DFC arm kit is 3.5mm, too tight for BSR's swash's 3.6mm balls.
When the alternative Align DFC arm is used, the arm is about 2mm longer, and the swash is overall lowered by 3mm, including the mandatory lowering of 1mm with the shorter carbon fiber main shaft. To avoid the swashplate "twitch" and colliding with the shaft collar during the F411 flight computer startup, we need to use the alternative flight computer that doesn't have the twitch problem. 

So, overall the alternative Align DFC arm needs the EFLH 1151 ball socket and M1.4 threaded rod, such as the Lynx Blade 180 CFX DFC arm kit rod. It also needs the M2 0.2mm flange ring, such as the ones in the Align 250 grip kit, a 1mm nylon spacer, and 2 M2 0.3mm metal washers. The overall spacing is 1.8mm for each of the 2 DFC arms.

The combined 3.6mm spacing is the width difference between Align250's rotor hub and Blade180's. When using the 1.4mm rod of the Lynx kit, 2-5mm of the rods need to be trimmed out. The flange ring is needed so the spacer only presses onto the inner tube of the DFC arm's bearing, not rub on the DFC arm itself. The metal washers are needed to sandwich the nylon washer-spacer to prevent deforming the nylon.

The plastic Align DFC arms are not suitable here because its 1.4mm rod is one-way installed into the DFC arm. As shown in the picture on the right, I tried to turn and pull the 1.4mm rod out with pliers to no avail. It left a dent on the hexagonal part of the 1.4mm rod.

In summary, the dimensions of all component options are in the following table. And the reference component diagram beneath the table. 

Tail Weight Reduction Alternatives

The original Lynx Blade 230 tail boom is 255 mm long. The replacement 8x7x500mm carbon fiber tube from ebay xzw791 can produce 2 250mm booms. The weight of the replacement is expected to be 4.8g x 250mm / 255mm = 4.7 grams. But the actual weighing has 17.6g / 4 = 4.4 grams. So, the replacement has 0.4 grams weight reduction.

Main Rotor Alternative Weight Reduction

The alternative Align250 carbon-fiber blades are 0.2 grams lighter than Oxy2 nylon blades, and the blade root thickness fits the aluminum Align250 grips. The Align250 aluminum grips are incompatible with Oxy2 blades due to the narrower jaw width. 
In the picture below on the left, the upper blade has a pronounced sagging due to a previous experimental test with Align aluminum grip. In that experiment, the blade root was sanded thinner, which softened the material surface. A rapid pitch-up flip at 1:08 in the video below on the right caused a tail strike. Also, the test showed that generic translucent 18-lb zip ties in the tail are not strong enough and break with the tail strike. The part listing's zip tie is stronger than the generic product's nominal tensile strength of 18 lb. 
The sagging and tail strike signifies that a blade surface should not be reworked outside the factory to force compatibility. 

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