Friday, May 22, 2020

IoT Platform Endurance

The Lithium-Cobalt cells are generally rated at 230-240Wh/kg maximum theoretical energy density. Actual driving adds cooling/control and discharge limit, and the packs have equivalent 150 energy density for Tesla battery packs. The lithium manufacturing distillation electricity/fuel cost grows hyperbolically in relation to purity and energy density. For an all-around IoT platform, the cell energy density should be set at 190Wh/kg , and reserving 10% ion for extended battery life equates to 170Wh/kg. The general purpose IoT forces a landing/termination when ion is at 10% level because the 10% level can't sustain flight as shown in videos below. So, no protection or cooling circuits needed.

The GensAce900mah/Tattu1000mah airsoft riffle battery has capacity of 970mah. 4-S configuration. Flight time is 0.97Ah x 90% x 4 x 3.7V / 22.2W(power for cruise with HD video) = 0.582 hour = 35 minutes. The implication is that the craft should be reproduced for 1 billion units and deployed around the globe for traffic monitoring/road side assistance applications. Every person should have an assistant drone nearby whenever they step out of the door. Think terminator micro drones but on human's side. Each cell weighs 18.9 grams. Energy density 0.97Ah x 3.7V / 0.0189kg = 190Wh/kg. Takeoff weight is 250 grams. Total battery energy is 0.97Ah x 4 x 3.7V = 14.4Wh.

The DJI Mavic mini has 46% less flight time of 24 minutes with HD video while carrying 20% more "fuel" with 17.33Wh battery energy, as this video shows, .
The reason DJI's drone is much less fuel efficient is because the disk area of our IoT platform is much larger. DJI's 4 4-inch props have a combined 50 square inch area, which can be covered by a single 8 inch prop. Our IoT platform has a 19 inch prop with 280 square inches area, but the straight blades give a sloped wing loading halving the effected area to 140 square inches, equivalent of being covered by a 13.4 inch prop. We can calculate that our IoT platfrom's fuel efficiency is (17.33Wh/0.4h) / (14.4Wh/0.582h) = 43.3W / 24.7W = 1.75 times that of DJI's efficiency, or 1.68 times considering DJI's electronic navigation power consumption of 10% is absent in this IoT build but the DJI test has omitted 6% power consumption of full HD recording. So, the 140/50=2.8 disk area ratio is roughly the square of fuel economy ratio because 1.68x1.168=2.8. And fuel economy is proportional to prop diameter ratio, 13.4 / 8.0 = 1.68. This is expected Newtonian physics.

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.

Battery Pack Build And Measurements

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 then attempt to solder the cell tabs because the cobalt alloy does not weld with other metals even with specialty soldering flux such as aluminum flux. 

The Tattu1000 2S cells have the exact 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.

The GensAce900 capacity is measured between 949 and 998mAh at first charge cycle, depending on how balanced the cells are when they are depleted.
The Tattu1000 cells have the exact same capacities. The following picture's measurement was taken after about 1 dozen charge cycles and taken after this Tattu1000 packs were used for the video "World record endurance". You can also see the exposed the lower right cell tab.
The pack gave 870mAh electricity as shown in the picture of charging after the endurance flight video was taken. That means that the pack forced a landing with about 100mAh reservation.

Carbon fiber main shaft

The carbon fiber main shaft is 0.05mm smaller in diameter than titanium shaft. The short bell shaft amplifies the wabbling 5 times to 0.5mm as shown in thie video,
, and the solution is to insert a 2mm wide tape on the wall of the bell shaft hole. But the tape can be easily cut by the sharp edges of the bell shaft whole and the shaft itself. So, the rim of the shaft whole needs very subtle, delicate scraping, and the shaft's end needs grinding where the grinding grain runs from the shaft bottom to the shaft side. And, also, when closing the bell with the stater, the bell needs to be lowered with an angle in a controlled manner to prevent snapping the bell on to the stater. The snapping would cut the tape. 
When the main shaft is driven into the bell, friction draws the tape into the shaft hole leaving a flap of tape outside. You can feel the consistent, strong resistance when installing if the tape insert does not break, and and friction should be too strong for removingt shaft by hand without using a screw driver to push out the shaft from the bottom . For this reason, the fuselage needs a 3mm hole drilled on the bottom plate beneath the shaft. The tape insert between the motor bell and the shaft can be verified unbroken after uninstalling the motor and removing the shaft. The picture below shows that the tape is inserted into the shaft hole and not merely cut by the shaft during installing the shaft and extruded out of the motor.

Main Rotor Build Alternatives 

In the picture above you can see the upper blade has a pronounced sagging due to a previous experiment with Align aluminum grip, which has a slightly narrower jaw width. In that experiment, the blade root was sanded thinner, which softened the material surface. The rapid pull back caused the tail strike. The above test did damage the tail. Also the above test shows that generic translucent 18-lb zip ties in the tail is not strong enough. The part listing's zip tie is stronger than the nominal tensile strength of 18-lb of the generic product. 
         The solution is to avoid the aluminum grips except with the lighter alternative Align 250 carbon-fiber blades.

Also the grip setting with Align plastic grips is to adjust the bolt to drape the blades the same as noted in the Operating Note, then tighten the bolt for an additional 90 degree turn of the hex driver. On paper, a 90 degree turn of a screw driver may not seem a lot, but in practice, the extra clamping results in significant blade settling resistance. This procedure produces the same maximum blade bending as with the Oxy2 grips. 

ESC Build

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 Camera Mounting

A scrap plastic strip is needed to hold the camera board vertical as pictured on the right.

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 crash.  Experiment shows a complete cleavage of the upper main frame when the left side wall is weakened with fastening holes. 

Instead, use enameled wires to tie the board and tie it to fixture arm. And use glue to glue the fixture arm. 

The camera taps its power on the transmitter's 5V output or directly on the battery as seen in previous post . FPV feed has slightly more interference when tapping the 5V on the buck converter's output, though not as bad as tapping directly on the battery. The transmitter taps its power on the buck converter output with the Picoblade connector socket.

Substituting Caddx Baby Turtle with Runcam Split: the Split series needs additional, dedicated power ripple filter to prevent video freeze with high G maneuver power surges/dips. Incident of such video freeze here during punching, which the drone is lost from a considerable altitude.

Runcam Split 3 Nano manual
VTX03 manual
Notice that this is not the only incident of video freeze. If you search "250-gram helicopter cloud surfing", you will see another video freeze at the very end.


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.

16 Amp ESC Substitutes

Further fuel economy optimization

The overall efficiency has 2 peaks as shown in the video

The first hump (the extremely high peaks) is dominated by 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. 

Here below, the manufacturer's spec states that both 4004 300Kv and 400Kv motors weigh 52 grams, but, during our build note, we found that the 400Kv motor weighs 17.6g(bell) + 32.3g(stater) + 1.3g(without trimming/carving) = 51.2 grams. The peaks and humps are marked in the drawing.

We will optimize toward the first hump because the second hump's efficiency can only be, at best, 70% of the first hump's efficiency. We will not optimize with the thumbnail/front chart of the video because it only optimize the motor electrical input/output power ratio for the second humps, not the overall efficiency.

Based on manufacturer's specs, we can plot the trend of the efficiency of producing 250 gram thrust in relation to the motor's weight.
Bisection approximating the optimal fuel economy steps:

      1. Trading 7 grams of fuel weight for 7 grams addition of the motor weight for higher efficiency
efficiency is 15.4g/W , 15.4/14.3 =  1.077 , or 7.7% better efficiency
fuel quantity is (80-7)/80 = 0.9125, or 8.8% less fuel
so it actually reduces flight time by about  1.1%

      2. Trading 4 grams of fuel 
          efficiency is 15 , 14.9 / 14.3 = 1.049, or 4.9% more efficiency
          fuel quantity is 76/80 = 0.95, or 5% less fuel on board
          so, the trading reduces flight time by about 0.1%

      3. Trading 2 grams of fuel
          efficiency is 14.7 , 14.7/14.3=1.028 , or 2.8% more efficient
          fuel 78 / 80 = 0.975 , or 2.5% less fuel on board
          overall increase of flight time by 0.3%

      4. Trading 1 grams of fuel
          efficiency is 14.5 , 14.5/14.3=1.014 , 1.4% more efficient
          fuel 79 / 80 = 0.9875 , or 1.25% less fuel
          overall increase flight time 0.15% 

     So, the optimal weight of motor should be between 52 and 53 grams. The propeller's intrinsic efficiency is assumed to be 16g/W . Because this intrinsic value is fixed for the physical propeller, but motor weight can go up indefinitely, the assumed value should not be too far off from the real value, and the motor weight needs to be stipulated to achieve the maximum flight time.
     Optimizing tail motor fuel economy should not produce any significant improvement because the "first hump" is intrinsic of the propeller. Further hindering reinforcing tail motor efficiency is the fractioning of the tail power to the overall power ratio. The tail only consumes a small fraction of the overall power, so during the optimization steps, the overall electrical efficiency gain needs to be a small fraction of calculated motor efficiency gain, however, the weight taken out of fuel is directly taken out of flight time without fractioning. On the other direction of reducing tail motor weight, the current motor weight is only 2% of craft weight, so it is not possible to gain any flight time of 2% or higher by tail weight reduction.

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.

Align DFC Arms

The BSR' swashplate weighs 1.5 grams less than 230S's . But the ball link EFLH1151 for the BSR swashplate is too short when pairing with Oxy2's DFC arm. The solution is Align DFC arms.

The alternative Align DFC arm needs the EFLH 1151 ball socket and M1.2 threaded rod, such as the rod in the Lynx Blade 180 CFX DFC arm kit. It also needs the 2mm flange ring, such as the ones in Align 250 pro grip kit, and it also needs a 1mm nylon spacer and 2 M2 0.3mm thickness washers.

When using the 1.2mm rod of Lynx kit, 2-3mm of the rods need to be trimmed out. The flange ring is needed so that the spacer does not rub on the DFC arm itself. The metal washers are needed to sandwich the nylon washer-spacer to prevent deforming the nylon and also to add 0.6mm spacing. When the alternative arm is used, the arm is about 2mm longer, and either the main shaft needs to be adjusted upward or the swash server arm need to be lowered. 

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