For vertical diving proximity, use 3-D modeling of the mission scenario sketch. The example with Blender is in the picture on the right. The small matchbox at the lower left corner is the base station. And, based on the mission scenario, the RF-control/FPV antennas are arranged to prevent internal interference and obstructions.
For surface proximity, ground obstacles were viewed in mapping software.
An example here is the low-tide exposed rock obstacles that is not shown in map satellite images.
Zooming in reveals an extra outcrop of rocks blocking LOS of the 1km proximity site.
The land survey revealed a radio LOS dead zone packet during low tides.
The aerial survey avoided the dead zones with large margin accordingly.
However, during the actual proximity mission, the high tide prompted acute steering toward the land. Without an on-location survey, the proximity mission during high tide failed (drone loss in the sea) with altered path obstacles.
The high-tide view of the crash site immediately after the craft loss shows no sandy shoreline.
The rendezvous plan record was photographed immediately after the crash. Actual dive at 6336, 6090 by memory.
On-location investigating the next morning during low tide to walk the failure path. First, the obstacle path based on landmarks (wind generator and distant house), marked red, is plotted.
The red lines and red curves rest on the obsticale rock outcrop that blocked RF signals.
The view of the ground station is used to identify the on-location perspectives.
On-location perspective and tracing the path of the blocking obstacle were identified at the location of the crash.
The failed mission path and the original planned path were plotted.
The off-course path on-location perspective is identified next to the original planned (good LOS throughout) one. The doggy footprint trace is identified as the crash location.
The large rock is a reliable visual landmark (unblocked) after a storm eroded sand and displaced pebble stones.
Then, the proximity mission succeeded in the next attempt.
The in-car battery and VRX were under a white food bag to cool them in the sun during the mission.
RF Transmitter Setup
The alternative micro sized transmitter module wiring pictured here.
The alternative Frsky R9M module can be set to output 500mW using alternative Taranis QX7's transmitter's regular power pins without external power source, as shown in this video,
This flight sequence rehearses the mission plan, diving from 330 feet east, 440 feet north, ground distance 550 feet, air distance 1000 feet, relative to ground station. 3 flips above the landing patch area. Level traversal between 330 feet east, 440 feet north, and 330 feet straight east.
Video Receiver Setup
The gaming monitor automatically adjusts its refresh rate to the highest the VRX can generate, as pictured here. There is no obvious way to change this.
The brand-new VRX cost is here.
In-Car Battery Charging
Fight batteries should be warmed/cooled with car vent before charging in cold/hot temperatures. When the car engine is offline, the car battery voltage has a slight drop, so the flight battery charger needs to be set to output below 1 amp to prevent low low-voltage cut-off during charging. The charger wires are hung by velcro stripes without a sticky backing.
TalentCell power bank YB1206000-USB has overheating protection. Keep it on the dashboard and plug in the charging plug year-round. If it is overheated by the sun, wait until it cools down to power on.
Walksnail 1S Lite USB Adapter Compatible With HD V2 Cameras.
As long as the plug is aligned with the socket to the left most pin when viewing from the right side of the craft toward the left side of the craft.
Box 17.5 Inches Long And 3 Inches Thick Or Larger
Cut the cardboard by the green line and the seam line pointed by the arrow, cutting the box in half while keeping a lid.
When viewed in Android MX Player or the FPV headset, the last 3 frames used to track down the wreckage is not available frame-by-frame. A laptop or Chromebook with Avidemux.AppImage is needed to step in the last 3 frames.
Avsaase/walksnail-osd-tool From Github
Compatible with Chromebook for crash GPS searching with frame-by-frame video of a crash.
Anthwlock/untrunc From Github
Compatible with Chromebook for crash-interrupted VRX .mp4 file un-corrupting. The code is based on original ponchio/untrunc that has bug compatibility with Chromebook.
248 git clone --recurse-submodules https://github.com/ponchio/untrunc
249 cd untrunc
250 vim ../untrunc-master/README.md
251 ls
252 cd libav/
253 ls
254 ./configure
255 sudo apt install yasm wget
256 ./configure
257 make FF_VER=3.3.9
258 vim ../untrunc-master/README.md
259 cd ../
260 vim ../untrunc-master/README.md
261 grep g++ ../untrunc-master/README.md
262 vim ../untrunc-master/README.md
263 g++ -o untrunc -I./libav file.cpp main.cpp track.cpp atom.cpp codec_*.cpp codecstats.cpp codec.cpp mp4.cpp log.cpp
Product source is ebay seller amain. Item "OXY2-081 OXY Heli Plastic Main Blade 210mm (Orange) (2)"
The airfoil profile is (NASA predecessor)NACA0015. The symmetrical airfoil is generally known to have the center of pressure CP at quarter blade width from the leading edge during cruise flight when the attack angle is smaller than 2-3 degrees in the research paper. As shown in the picture below, the CP point is at 17.5mm/4 = 4.375mm from the leading edge but the bolting is at 4.3mm, so, when the attack angle is very small, it has perpetual pitch suppression. The research measurement also shows that the airfoil has perpetual up-pitching torque at a cruise attack angle of 6 degrees because of the "center of pressure moving forward of the quarter chord".
When the mechanical margin of error is added between the actuator servos and the actual blade position, the blade angle shifts as the following timeline.
This means that a large slop gap can result in an unstable craft. And in the above example depiction, the slop should be improved from 3.2 degrees to 3 degrees to escape the perpetual pitch suppression slump. And to avoid over-correction, the slop needs to be smaller than 2.1 degrees.
Balancing With Feathering Shaft Slop Larger Than 0.2 mm
Going from 2650 to 3150 and higher RPM requires fine-tuning the rotor with hand-tethered hovering. Here the R/D test retained the original decal of the blades. And the tuning took 3 tries in the research process, as pictured below. First, I fixed the tremoring by cutting out a corner of a decal. When aggressively trimming the corner, it was over-corrected, and tremoring reoccurred. The balance was restored by cutting the corner of the other decal. Once the amount of taping was determined, all decals were removed, and the clear tape was put in place for the required balancing. The balancing progress during the research is the 4 pictured below from left to right. The conclusion is that one square centimeter of tape makes all the difference because the rotor is stretched by centrifugal force like a musical instrument wire, and the 3150 RPM is on one of the resonance frequencies of the device. The thickness of tape 0.002-0.003inch = 0.005-0.0075cm, weight (assuming density 1g/cm-square) 0.005-0.0075 grams. This means RPM3150 needs a precision balance of 1/100th of a gram.
However, during multiple test builds, it was discovered that most builds could not achieve smooth balance and that blade swapping was needed. One of the successful blade pairings was pictured above on the right with a green checkmark. The 2 distinct symmetries, rotation and mirror, for balancing the rotors is diagramed here.
The swapping needs to restore the rotational symmetrically of curved blades because our rotor hub does not have a cord-wise balancing bolting like the Bell 222 rotor, as highlighted in the picture on the right. The 2 blades with up-down warpings can be none-mirroring as the right side in the above diagram when subjected to 3250+ RPM's centrifugal force. The solution is to pick and match a pair that dynamically match the warping to each other.
A new blade with natural backward curving when spun up (not the same curving at rest) can be matched to a old blade with crash-induced backward bending, as shown in the above example with a green checkmark; the older blade's color fades, but it is the best match picked out from 3 different sets of new purchases. The at-rest warping mismatch shall be disregarded. At rest, one may warp up, and the other down, but the forces may balance out when spun up to 3250+ RPM, depending on the material's internal stress. All the other blades from the 3 sets fail to match the lower blade in the picture. The only way to check the pairing is to spin them up to full speed. And each pairing test needs to be checked with 2 alternative orientations of the grip installation. In the pictured example, only one orientation can produce a smooth spin. The other orientation always has vibrations that can't be tuned out with scotch tape.
Before the pairing process of a purchase, mark each blade with a unique serial number you manage, then weigh each blade individually and note the heaviest blade. The following pictures show an example of the heaviest blade, blade 3, of 7.301 grams. Then use scotch tapes spread evenly on each blade (except the heaviest one) to patch all the blades in the purchase to the heaviest weight. This ensures that the pairing process focuses on matching the the warpings of the blade material.
So, what happens if the rotor is a non-symmetrical mismatch? RPM detection with Engine RPM Android App can not lock and detect the RPM.
Use the Android app Engine RPM to measure the rotor's RPM with the settings in the screenshot below. The app automatically logs the detected RPM every 3 seconds, as seen in the screenshot below. There is a 1.5% discrepancy between the PWM control signal and the actual measured value. The PWM controls are pictured with the respective actual RPM logs below. We use the actual measured RPM as standard.
The ultimate smooth balancing has a lateral (on the side) ballast, in addition to the axial balancing, as follows with 24 AWG copper wires.
The first loop has the crossing beneath the DFC bolting and excess wire trimmed. Then, pinch and turn the wire for a few turns. In the example on the right, the 7 loops on one side are the correct balancing because 14 loops were tried on one side, and zero loops on either side were also tested to be lesser balance. This example ballast weighed 0.2 grams with 7 loops. However, this level of ultra balance is not required for smooth HD video shooting.
The flight computer can have 3 operating modes: pass-thru (no computer stabilization), accelerometer (leveling attitude), and gyro (holding attitude). But the pass-thru(manual) mode is not operafble for the main rotor as shown in this video.
tilt right and backward - 0:01
attitude mode to rescue - 0:02
tilt right and forward - 0:03
to attitude mode - 0:04
tilt left straight - 0:05
attitude mode - 0:06
tilt backward - 0:08
attitude mode - 0:09
tilt forward - 0:10
attitude mode - 0:11
landing approach too high - 0:25
backing off far and fast - 0:35
good landing approach - 0:40
touch down - 0:53
So, the OpenTX transmitter should engage in a stabilization mode before liftoff.
The problem of erratic, forceful PID Integral over-correction (winding) is common throughout nearly all helicopter flight computers. An example of the same problem is with Blade230's AS3X; in the video on the right, during the spool-up, the error rolling to the right is at 0:14. This is because the PIDs are tuned for flying in the air when rotor RPM is sufficiently high, but the ground contacts force the craft attitude out of the planned flight envelope when the spooling-up rotor RPM is too low to correct in time. To overcome this problem, we cancel the PID process and use pass-thru-manual mode during rotor spool-up. The spool-up RPM should be controlled directly by the pilot by the throttle proportion on channel 5 to ensure safety, and the pilot's control with the stick should only be relinquished and set by the fixed RPM with the pilot's explicit input on channel 6 switch, even when the throttle position reaches full speed. The spool-up options note discusses the reason for this channel switch assignment and the substitutes. The spool-up throttle channel setup is pictured here on the right.
To solve the problems of the previous two paragraphs together, we configure the flight computer to give the pass-thru mode a channel switch, and this channel switch for the pass-thru mode is tied to a constant negative collective pitch. The negative pitch prevents a takeoff while the pilot spools up the rotor with the CV1 curve. In blue bars, we highlight the disabling of the accelerometer and/or gyro sensors. The flesh color underneath all flight modes is for the same set of PID coefficients throughout all modes.
To convert the fraction system to the flight computer's PWM timing system, Betaflight/F7 needs to anchor the endpoints to the nearby 25 points of the 1000us-to-2000us range, which is the range the OpenTX can produce after 98% scaling of 989-2011us.
To reduce pilot/operating error, we should consistently use all these modes on both the flight computer and the three modes on the transmitter. There is no such thing as a reliable, field-improvised easy transmitter setup, as the following 2 paragraphs discuss.
When the first mode does not set the collective pitch to -23(that prevents liftoff) via an extra channel one mixer page in a faulty improvised setting, the pilot may forget to switch channel six to the middle position when attempting to reduce time to takeoff to save battery life in the midst of enabling video recordings and operating other equipment. The result is forceful crashes due to the high power applied for takeoff and the high-speed drift of the manual mode. The video of such a crash is on the right.
When the transmitter and flight computer setups are improvised out of sync, the takeoff can happen in gyro-only mode. This happens even after a hand-tethered hovering test because the gyro-only mode hovering can be successful without the pilot knowing that the modes are out of sync, for that gyro-only mode does have stabilization characteristics. Then the actual takeoff will, again, crash badly due to the high craft momentum nature of the gyro-only mode. The video of this gyro-only mode takeoff is on the right. This field fault occurs when we are tempted into setting a single 2-position switch on Aux2-channel6 for a cruise-only mission. In the field, the PWM for profile switching appears to need more than 33%, say 40% or 50%, weight to expand over the 1/3 spool-up region at a low position. And that is true for a 2-position switch, but Betaflight would interpret the high position (2000-1500)*0.4+1500=1700 in the gyro-only setting because the weight scaling is centered at 1500, not 1000, as depicted in the diagram on the right.
How about tying stabilization mode to arming the craft in flight software? The problem with the tying is that abruptly powering off the RF transmitter (to conserve battery or to avoid accidentally activating stick commands) prevents returning to manual mode for swash plate level checks. Checking swashplate leveling is needed after each hard landing with servo clutch slip, which can be needed frequently, and hence repowering up the RF transmitter just to disarm complicates the work procedure and frustrates operators.
Betaflight is not replaceable with iNav flight software: Our production build of the converted IoT platform has a servo order per iNav flight software, pictured here.
The HGLRC F722 Zeus Mini (20x20) can run either custom iNav or official iNav 8.01 software. The Walksnail OSD data works correctly with iNav 8.01 official software, as pictured here.
However, neither software allows "resource remapping," assigning the LED strip signal to the main motor ESC, as iNav is well-known for not allowing remapping. The iNav is not a viable substitution because there are no 5 physical signal outputs with the Zeus Mini board. As shown in the screenshot above of iNav 8.01 official software's resource section, the 4 servo ports B00, B01, B04, and B05 have physical solder pads, but the MOTOR1 at B06 pin does not. The pin B06 has no physical output as evident as motor 7 in default configuration in the default Betaflight software installed in the Zeus Mini because the physical output bus connector only allows 4 motor signal wires. The reason the motor is at the 7th output signal wire is because just above the first picture of this section is the plane's engine signal 7, depicted as s7. But, even if it were programmed as signal 5, it sill won't fit into the 4-signal-wire bus connector.
The custom iNav flight software should not have any servo output. The heavier 30x30 HGLRC F722 Zeus might have 5 physical signal outputs, but its weight approaches 9 grams.
The ground preparation includes calibrating RF control sticks at the correct temperature. With the Radiomaster Pocket handset's factory calibration in a cool room, the thermal expansion of the stick components raises the PWM of the rotor RPM setpoint, as in the video on the right, resulting in failed arming with a spinning rotor and non-silent gyro IMU. The subsequent takeoff resulted in a crash (lost drone to the Atlantic Ocean). There is a prototype experimental flight software that prevents crashes with arming failure, and there are specialty handset hardware that has extremely high thermal stability, but not the Betaflight v4.5.2 and mass production software/hardware. The correct temperature to calibrate the RF control sticks for the Radiomaster Pocket handset is the hot car temperature after the handset is under sustained sun exposure. The first segment of the video on the right was taken after the crash on the same day in the afternoon, with the handset still in factory calibration. During the crash takeoff, the throttle stick was 4 points higher than calibration and shown as -93 in the LCD display on the negative-100-to-positive-100 scale when the handset was subjected to sun exposure. The correct set point is -97 during idle and the boot process of the handset. The second segment of the video was taken after calibration in a cool room and subjected to another sun exposure. The set point varies by 11 set points of temperature fluctuation with sun exposure at an end point of the negative-100-to-positive-100 scale (it varies by 2 points in the very similar sun exposure) and can not be remedied by software scaling under 3%.
At the center region of the stick, when the craft is in the air, the variation is negligible because the thermal expansion occurs in the stick components, both top and bottom, and cancels each other. This is illustrated in the pictures on the right when the throttle stick made PWM 1645 for RPM set point immediately after a cool room treatment, and then the handset was subjected to hot car and direct sun exposure for half an hour, but the PWM only shifted 1us to 1646, no set point changes in the negative-100-to-positive-100 scale. On the reverse testing of setting PWM 1645 in a hot car under direct sun exposure, then the handset is cooled in a cool room, the RPM set point barely moved from the original 28 to dithering between 28 and 27 with PWM 1643, pictured below. This means that the calibration doesn't have to be redone for winter.
Motors: Use Multi-Rotor ESC Software For Simple, Quick Response
Prototype build
Production build
Main motor size
4004 or 2808
2806
Main motor RPM
2640 or 3100
3250 or 3430
Main rotor diameter
18.6 in/472mm
16.1in/410mm
Li-ion Battery
3 or 4 in series
4 in series
Main motor software
MAIN
MULTI
Tail motor software
TAIL
MULTI
Main motor governor
Compensation with collective pitch
BLHeli governor or Betaflight Vbat compen.
Betaflight version
4.2.5
4.5.2
The converged IoT platform includes 9 computers (9 CPUs), 1 Raspberry Pi CPU, 3 servo PID control CPUs, 2 ESC motor control CPUs, 1 GPS CPU, 1 F722 flight computer CPU, and 1 RF control receiver CPU, each with its own software. A comparison of the prototype and production builds is in the table on the right. The endurance optimization chapter optimized the prototype power plant for racing with a 2806 motor with a 4-series (13.6+ volts) battery and 180 mm blades. According to airfoiltools.com data of the Joukowsky airfoil that has motor heating inline with the prototype NACA0015 blades, the production build uses the optimized setup. The motor heating at an extraneous collective pitch is just as good as the prototype power plant.
In prototype builds, it was not obvious whether fixed RPM will be required during takeoff and landing. The CV1 spool-up throttle curve picture on the right is super-imposed by collective pitch curve, and the 2 curves were simultaneously in effective with build test hovering. The test showed that the region between the 2 red dash lines had tail punches (yaw turn to the right), indicating insufficient RPM brought on by variable RPM configuration. So, the production build is determined to require fixed-RPM with ESC governor for both takoff and landing, not with the spool-up throttle curve. The question was which software among MAIN, MULTI, and TAIL to use with variable RPM in-flight. And, MAIN code with the RPM governor disabled, resulted in a RPM change delay of roughly 3 seconds whenever throttle-up and resistance were detected. That meant when the protype craft descends too fast approaching an obstacle, I couldn't punch up the craft immediately because the ESC senses blade foil drags with throttle-up and gives small increments of torque over 3 seconds to prevent damage to the helicopter pinion/gear. There is no resistance reduction gearing in the gearless helicopter setup of our converged IoT platform, so the ESC needs to apply the torque regardless of resistance, just like a multi-rotor system. The answer is shown in the BLHeli ESC assembly code, which indicates that the MULTI code is simpler/cleaner, without torque detection, and should be used in our IoT production build.The assembly code shows that MULTI code has all the MAIN code functionalities, including fixed-RPM governor, without the unwanted spool-up torque detection, and that MULTI code has all TAIL code functionalities except the unneeded idle spin, and hence the MULTI code should be used for both the primary and tail rotor of converged IoT builds. To configure the ESC, we use the generic F722 Betaflight 4.2.5's passthrough USB-to-UART connection program with BLHeli Configurator Chrome WebApp. Betaflight versions newer than 4.2.5 often have bugs in the PWM protocol ESC passthrough, and version 4.5 completely removes the passthrough. Betaflight 4.2.5 binary hex file for the F722 CPU based flight computer boards is betaflight_4.2.5_STM32F7X2.hex, downloadable here.
For the pass-thru program to work, the MOTOR 1 pin needs to be temporarily assigned to the SERVO pin of the tail motor(SERVO 4 in CLI), while the 4.2.5 software is not supported by the latest Betaflight configurator GUI. This means loading the dump_all config file into the CLI terminal (which is supported despite the GUI being unsupported) to do the "resource" reassignment commands and the "save" command. This also means forcefully running a "save" command after loading the dump_all config because there are command features of the version 4.5.2 software that are not supported by 4.2.5, and multiple errors occur during the loading of dump_all. In the above screenshot, the Betaflight software version is reloaded for configuring the ESC properly, but the resource reassignment is wrong, servo 5 instead of servo 4, and the result is the failure in flashing the ESC software, screenshot here. This means that the signal wire needs to be resoldered when flashing ESC software to the servo 4 lead. Safety precaution dictates that motor power with ESC battery should be applied last after all connections and software are set up, and motor power should be the first to be torn down after any configuration/software flashing. With that in mind, power only the flight computer by connecting a micro USB cable, start the BLHeli app, and click "Connect" with the default baud rate and auto-detected tty port. The computer now reboots into the USB-to-UART adapter program. But, wait, there is sometimes a 1-minute delay for the reboot to occur if the BLHeli program determines that the Internet connection is available but the software website is unreachable, so the best is to ensure Internet access of the laptop PC. Now connect ESC's craft battery power, then click "Read Settings" to connect to the ESC computer. If the "Read Settings" button is unavailable, it is still in the 1-minute period. After flashing ESC with motor software, remove ESC's craft battery power plug first as a safety precaution. Then tear down software/other connections. If you don't tear away the power, when the program switches back to BetaFlight software, the motors will spin up to compensate for craft orientation or to match any unintended RF transmitter command signal, posing a great danger. The prototype "compensation" throttle setting for a specific motor, case-by case, is not applicable for large-scale deployment with varying hardware. The solution is to use ESC's RPM control PID governor or (future)flight computer automatic Vbat compensation. I-Gain needs to be high at 3.00 to track RPM precisely. P-Gain at 0.5, to prevent motor governor oscillation. The governor in the MULTI software doesn't detect torque and has a much shorter acting time than 3 seconds. The entire configuration is as screenshot here. The latest, and last BLHeli version is v14.9, and the correct main motor RPM control gains (P=0.5, I=3), startup power (1.25), and damped off were determined during the endurance optimization building to reduce heating and prevent RPM collapse.
The production build uses Favorite LittleBee 20A Opto because the substitute DYS SN20A, as rebranded as Blade-Thrust, has multiple issues, such as this screenshot here, that need software overwrite.
With a software overwrite with DYS SN20 MULTI software version 14.9 (latest and last BLHeli version from opensource github.com, also downloaded here https://drive.google.com/file/d/1VR40QiSTiCpBNoLuRfzF2tX-r1SYDhEg/view?usp=sharing, https://drive.google.com/file/d/1VRZuM9waIJCqShnFd6Oaq3NoW2o6_C_5/view?usp=sharing), the ESC's life span is shortened to about 5 power cycles, resulting in short circuit (danger) or MOSFET failure.
The 4.2.5 software does not have the one-click-installation feature to allow the installation of our production build's 4.5.2 software.
Flashing the flight computer software after downgrading to Betaflight 4.2.5 requires putting the flight computer into DFU mode with the depressed physical boot button before plugging in the USB cable to the configurator computer. And the DFU mode requires removing the RF control receiver according to the following video, or face USB failures as pictured here.
So, once you finish using 4.2.5 software and are ready to move back to 4.5.2, use the bootloader button on the Zeus board to enter DFU mode, and then install the 4.5.2 software with the "No reboot sequence" option, as pictured here.
With the ESC motor RPM PID control software component, called the "governor" for short, the craft with the 12-pole production motor needs 3250 RPM / ( 50,000RPM / 7 ) x 200points - 100points= -9 points level in the -100-100 scale of the transmitter. The 50,000 max RPM is seen in the above GitHub page of the ESC source code. 1/7 fraction is attributed to the multiplication of the 14-magnet brushless motor compared to the elementary 2-magnet motor. The main motor ESC calibration is performed in the Betaflight configurator GUI. Channel 5 is mapped to "Servo 6", counting up from 3 main rotor servos from 2, "bypass" checked in the GUI. In the CLI's servo section (not the resources section and not the smix section), the fifth channel is specified as the channel "4", counting from 0th in the last parameter. In the GUI, the 5th checking column is checked for "bypass," for Servo 6 only.
Calibration, configurator needs tentative(first click Live mode and don't click Save in the GUI) Servo 6's MIN set to the high position of 2000. The tail ESC is "Servo 5" in the GUI.
The tentative MIN position only takes effect after you click Enter or click on a different field in the GUI as shown in the screenshots above. The scaling is finalized when setting back to 1000 to save the calibration.
The motor is not configured as a motor in the mmix section of the dump_all. The motor can not be controlled with Motors GUI because Betaflight motor mmix is tied to throttle channel T, a collective pitch channel in our IoT build, after all other substitutes were ruled out.
Here the 200-point RF control (2000-point ESC control) calibration is assumed to perfectly corresponds to 1000us to 2000us RF transmitter PWM output, and if the RF transmitter uses the 989us to 2011us range, such as Frsky's X9 Lite or QX7, the RF transmitter needs scaling the channel by (2000-1000)/(2011-989)=97.8%.
A momentary tail reset became frequent after changing the motor from a prototype test build's 1306 (with oblong-shaped props) to 1303.5 and 1106, and also the change of ESC software with a different default startup power, as shown in the video on the right. Notice that the heading is the same at 0:07 and 0:12 . The tail motor spins down at 0:07 when torque suddenly lowers down or when the wind blows from the side onto the tail fin/prop creating artificial counter torque. At 0:08, the tail motor can not spool up fast enough, and the deviation between the expected and actual heading is too large. At 0:09, the tail PID resets, allowing a (unintended) 360-degree turn. At 0:12, the tail spools up successfully.
Research by SiieeFPV here shows that there is no definitive trend of startup power and stuttering.
So, the solution for the tail reset is not a particular startup power. But it was the most documented configuration with the production 1303.5 motor duringextreme robotics build with tail ESC's startup power at 0.5 for startup without delay and without stuttering.
The configuration for racing for the tail ESC is the rightful solution for the converged IoT, shown in the following screenshot and video. Damped off as required to prevent ESC overheating in summer, and is the only tested and documented setting.
This also enables the Low RPM Power Protection feature to protect the ESC in case of a crash when the propellers are stopped by the ground or other solid objects.
With the production dump_all PID configuration, the production build's tail reset is infrequent and needs deliberate efforts to reproduce the tail reset problem, as in the following 2 videos.
Yet, another question is whether the Low RPM Power Protection feature is detrimental to the quick spinup. And our testing in the following 2 video footage showed that the Low RPM Power Protection feature is irrelevant when the propellers don't hit a solid obstacle, only spinning up the motor mid-air.
Collective Pitch Curve For Fuel Economy
All credit and copyright of airfoil data belong to airfoiltools.com , NASA, and Xfoil , for the following picture.
Prototye rotor RPM for level hovering is 2640/minute = 44/s . Rotor diameter 0.48m , speed at 3/4 blade span is 0.48m x 3.14 x 44/s x 3/4 = 50m/s . Our Reynolds number is 50m/s x 18mm / 1.46 x 100000 = 62,000 . So, between the light brown and green curves, but closer to the light brown curve, is our curve for attitude-accelerameter mode.
For simplicity of collective pitch angle calculation, we choose servo arm length identical to blade pitch lever length, in the prototype build, 12 mm pictured above and on the right. The Oxy2 Sport grips and the Align 250 grips have congruent geometry as shown in the picture above. When all 3 pillars of the swash plate are raised evenly, the blade lever and angle are congruent to the servo arm and angle. The angle of travel of the RF control PWM 1000-2000 µs is 83 degrees in the above picture and 84.6 degrees in another test with the flight computer PWM 1000-2000 µs, averaging 83.8 degrees.
As tested by FlightPoint, the angle of travel of PWM 989-2011 µs is 85.4 degrees, as pictured here.
For the prototype optimal lift-to-drag ratio blade pitch 6.25 degrees, servo arms also rotate 6.25 degrees. The full range angle of the EMAX 9501 for 1000us to 2000us PWM is 83.8 degrees. 6.25 degrees / 83.8 degrees x 200 points = 14.9 points. To fit the PWM with our swash plate mixers here, we double the hovering points because most flight computers allocate 50% swash travel for collective mixing. So, the prototype used a straight line for the collective pitch curve from 0 to 14.9x2=29.8=30 points at mid throttle stick. This angle and set point are optimal for cruising for the production build's Joukowsky 10.5 blades. We stipulate the collective pitch to 3/4 of the throttle stick to avoid straining and overheating motors that have mislabeled KV. We use the servo sequence, the first servo on the front, the second on the right, and the third on the left, when looking down on the craft from above the craft. With Betaflight, servo 0 and servo 1 are reserved as pan-tilt servos for camera, so, the servo numbers start from 2 for the first craft servo.
Also in the -50 mix lines, the source 0 and 1 are stabilized roll and pitch respectively. Source 7 is throttle channel 1, not an aux(source 8+). The problem of assigning collective pitch control channel to Aux is that flight computer TPA(Throttle PID Attenuation) and TPS(Thrust PID Scaling) only corresponds to throttle, not AUX. The consideration with throttle input channel 1 in TAER being recognized as the throttle means arming for throttle position should not near 1500us to avoid stick function commands. After considering all substitutes, the arming is set to have a large negative collective pitch for low throttle. This large negative collective pitch is set as the inverted hovering pitch angle for racing RPM, which is -23 points, or 1500-23/200*1000=1385us. The F7's min_check=1385us can mislead us to think that a hypothetical RF calls for -57 point of 1215us collective signal will be conflated to 1385us in F7 setup, but F7's signal 7 is raw RF signal, not processed nor conflated.
The production craft's servo mixing is larger than the prototype craft's because the servo arms are shorter relative to the blade pitch arm width, the p-value in the following diagram. The high mixing also allows the overshooting of the servo-throttle to avoid flight software startup cyclic "rapid twitch" discussed in the build notes https://nocomputerbutphone.blogspot.com/2018/09/converged-iot-platform-build-notes.html.
The relationship between servo mixer attenuation and servo/swash/pitch/DFC arm spans is illustrated here.
Because the prototype's servo mixing used standard CC3D 50%/50% collective/cylic mix when the p value was 1, the extra attenuation of production build's p value at 1.5 means the production build's collective mixing, if and we do want prototype's throttle curves reused in production build, needs to compensate the 50% mix by the factor of 1.5, 50% x 1.5 = 75%.
Because the prototype's cyclic mixing occurred with p*(d/u)=1*(1.833/0.916)=2 while production build's p*(d/u)=1.5*(2.25/1.1875)=2.842. The extra attenuation of 2.84/2 = 1.42 is compensated as 50% x 1.42 = 71%, and (50*sqrt(3)/2)% x 1.421 = 62%.
Because we compensate the mixer difference in software, the pitch curve calculation in RF hardware controller based on the 50%/50% mix does not need recalculation.
The endurance build notes show the measurements of p, d, and u, where lightweight substitutes were considered
All these production-improved settings are part of the dump_all file in the operating notes.
The prototype's optimal lift-to-drag ratio pitch angle doesn't mean the overall motor-propeller-battery combination is the most fuel-efficient, but just the propeller optimal. The production build is optimized for overall fuel efficiency and uses a 6.0-degree collective pitch for cruising. The production build's servo arm length is 2/3 of the grip pitch arm width, which attenuated the collective pitch by the same fraction, so the dump_all has an allocation of the swash movement needs to be exaggerated by the inverse of the fraction, 50% * 3/2 = 75%, as the following 3 lines,
smix 1 2 7 75 0 0 100 0
smix 4 3 7 75 0 0 100 0
smix 7 4 7 75 0 0 100 0
. The production build has collective pitch curve points 0, 15, 30, 45, and 45, same as the prototype build. The production build's first servo is on the back, the second servo on the left, and the third servo on the right, by the craft's own perspective.
The production build's RPM 3250 is inherited from the prototype(II) build of endurance build note with 193 mm blades for neutral buoyancy at 6.2 degrees of collective pitch that is optimal for in-place hovering, which coincidentally is the optimal collective pitch for cruising of the production build at mid throttle stick. But RPM 3250 itself is not optimal for cruising. Instead, RPM 3250 coincidentally is optimal for production build neutral buoyancy at 5.47 degrees optimal collective pitch. What this means is that when the craft reaches neutral buoyancy after takeoff, it also reaches the optimal fuel efficiency for in-place hovering. When you raise the throttle collective pitch stick to mid-stick, the pitch is at optimal cruising fuel economy, but you need to lower the RPM to prevent climbing.
This also means that if you want to experience mid-throttle-stick-means-neutral-buoyancy, simply swap the blades to 193 mm with NACA0015 airfoil and 18 mm chord length of the prototyp(II) build.
Swashplate Setting
Insert a 2mm wide tape strip to fill the slop gap between the swash ball and servo rod socket, as indicated by red arrows in the below picture, to prevent pitch trap instability, discussed here https://nocomputerbutphone.blogspot.com/2018/10/oxy-210-blade-characteristics.html. When clipping on the link, the swash should be tilted at a large angle to avoid cutting the tape strip by the socket because the sockets have a sharp-edged rim. If, after checking that the tape inserts are working correctly, the craft still suffers from unmanageable altitude swing, replace the swashplate because an old swashplate with many crashes is loose and has an unacceptable slop gap.
Remember, our spool-up is performed in manual mode. If the swash banks to one side of the craft, there is no PID correction by the computer and no heavy battery or long tail lever on the sides of the fuselage to counter the tipping. Use a zip tie with the half-cut end, as shown in the below picture, to check that all 3 pillars nearly touch the servo linkage. This zip tie should only be used after checking for enough room between the swash and the hub after booting up the flight computer.
It is well known that servos accept PWM range 1ms to 2ms, or 1000us to 2000us. But, the consensus is that manufactures specify margin of error of 150us between flight computer and servos when viewed in BLHeli configurator. So, it is safe to give servos PWM between 1000-150=850 and 2000+150=2150. The spline count of the servo arm is 15; the servo arm travels 83 degrees for 1000us. So, each spline dial notch is equivalent of 1000.0*(360.0/15)/83.0=289us PWM change. The actual testing result is that 1 spline dial is equivalent of 310us PWM at the near center. That means that when attempting to set the high-end PWM to larger than 1660us, you could dial the arm position up 1 spline and increase PWM on top of 1660-310=1350us, which will set the high-end PWM closer to the central 1500us in the tolerated range.
When changing the servo PWM center micro-second value, the change should be multiples of 10us because it doesn't have the resolution of the full 1000 PWM points; instead, it only changes with 1/100 of the full range. Considering that the full range of the servo arm is 83 degrees, 1/100 of the range is 0.83 degrees. To center the swash plate, check the top of the blade boltings pictured on the right. Change all the 3 PWM center neutral values 10us at a time.
Indoor Hovering With RF Receiver With Proximity Cutoff
In the following video, XM+ receiver's LED starts out green, the transmitter antenna is shifted to create stronger radio coupling with the receiver at 0:01. The cutoff occurs at 0:02, the LED turns red, fail safe takes effect. The camera moves forward to zoom in to the receiver. The movement inadvertently shifts the transmitter antenna again , weakening the proximity coupling. The cutoff is released at 0:07, and LED returns to green.
To avoid mid-air failure, the FrSky XM+ receiver requires putting transmitter on "range testing" mode before hovering in close proximity. This is not needed with Crossfire or R9M receiver, which does not have proximity cutff. In the pre-prototype, the receiver sits on the pedestal as appropriate for the small, geared motor under 22mm diameter of stater, spaced across the cage. In production, when 4004 direct drive motor is used center in the cage, the receiver signal is severely jammed by the spinning motor. So, production build does not use the pedestal.
Takeoff Procedure With Safety Checks
Motor bolting failure in-flight has the same characteristic left turn, resulting in twisted tail motor wires and missing bolts after recovering the wreckage. The tail wreckage of such a case is pictured here on the right.
The takeoff procedure for the prevention of mechanical failures follows.
1. DFC bolts marking on the installed position. Twist the tail motor and tail props to see no loose screws.
2. Plug in the craft battery. Check the rough zero pitch and level of the swashplate. If the servo arm clutch is sprained during the last mission or handling, dial it to correct it.
3. Position the craft on the takeoff surface with friction that prevents it from sliding. Remember, during the spool-up of the main rotor, there is no counter-torque compensation unless the tail yaw is applied manually. The blades should not scrape grass or thin twigs on the ground because the initial spool can bend the foldable hinges, throw the rotor off balance, and sprain the servo arms.
The following steps show the OSD percent display of channel 1 throttle in a video clip for illustration only. Our base build does not require OSD; RF control transmitter LCD is. The corresponding RF control transmitter LCD point displays are next to the OSD percent.
4. Leave the craft and power on the RF control transmitter. The video above shows that the collective pitch changes from neutral to -4.8 degrees at 0:00 when the LCD display point of channel 1 throttle changes from 0 to -22.5. The throttle percent shown at OSD goes from 18 to 0. The 0 percentile display refers to the PWM signal at min_check from the RF transmitter, which is a negative collective pitch. The 18 percentile display refers to the actual neutral zero collective pitch at 1500us in the scale from min_check = 1386us to 2000us.
5. Arm the craft. This occurs at 0:00 of the video.
6. Spool up the main rotor slowly to the hovering RPM at 2/8 stick, as seen in the above video from 0:05 to 0:17, reaching the top red line of the soil color region in the table on the right. Slowly make the spool up so that the craft doesn't capsize due to an unbalanced rotor with folded blades. Then the craft should have no movement after the shaking because the friction of the ground overcomes the main rotor torque, and the negative pitch lift force presses down the craft firmly on the ground.
7. Switch on stabilization attitude mode with the throttle stick at 2/8 position, as seen in the above video at 0:18. The collective pitch will suddenly become positive 3.4 degrees, as shown as 14-15 LCD throttle point, also shown as 30 percent in the upper right OSD of the video. The craft will swing for a few seconds for computer stabilization to settle.
8. As shown in the video, raise the collective pitch to 3/8 stick from 0:18 to 0:24. The craft will stay on the ground.
9. Raise collective pitch toward 1/2 stick slowly; craft will reach neutral buoyancy between 3/8 and 4/8 stick and lift off at 25.5 throttle LCD display points or 39 percentiles shown in OSD, as shown in the video from 0:24 to 0:27.
10. If the drift is acceptable, raise the collective pitch to mid stick 4/8 at 29.3 throttle LCD display points, or 42 percentiles in OSD. The climb starts. At this point, the collective pitch is at the most fuel-efficient angle for level cruising as verified in the endurance build. To benefit from the fuel economy, start the forward movement of the craft to reach the optimal configuration.
Overall, the craft flies in the blue sky color region of the table on the right and stays on the ground in the green grass color region.
Reduce Trimming From RF Control
The Takeoff Procedure shows rolling to the right when switching to Attitude mode due to the required 4-degree tilt (align_board_roll = 4 in dump_all config) of the helicopter hovering physics. The RF control has a small right-roll trim. The rolling and self-correction don't need pilot input to correct. However, some builds may need a 5-degree tilt. If the flight computer configuration is off and the roll trim is large, the RF control trim will cause more rolling than shown in the video and can be a risk for tip-over.
Accidental Manual Spool-Up Mode
The negative collective pitch of the spool-up mode with tailspin somehow results in near-perfect craft inversion. In the footage on the right, the accident occurred at 0:05 during hovering with a hovering throttle (collective pitch) in attitude mode. The recovery is flipping the mode switch to attitude mode, keeping the same throttle (collective pitch) at a hovering level. The altitude drop from the corrective action of flipping the switch to attitude mode to stabilizing altitude is 6 meters.
Failsafe Setup
With all-default settings, when more than 0.15 seconds of RF signal loss, the procedure of DROP the craft enters. The craft will drop to the ground if nothing else happens. When the RF signal resumes, the first 0.05 seconds of the signal is considered dirty and discarded. If the failsafe sets arming status to disarmed, it would require lowering channel 1 to lower than min_check when flipping on the rearming switch. So we set the arming status to armed during a failsafe. So the craft recovered successfully without the main rotor's slow spool-up nor flipping any switch on the RF control transmitter, as tested in the first video footage, which disconnected the ribbon cable between the RF control transmitter and the TBS Crossfire Mini module.
Summary RF Control Setup
In gyro-only mode, the accelerometer is not used, and the outer loop calculation of PID in flight software is zeroed, so the RF control's trim from the transmitter handset is inappropriate. And so, the roll and pitch RF control mixer should have 2 separate pages for each axis, one with trim, another without trim.
To summarize, with all substitutes rejected, the montage of 25 screens of created/edited pages are here below. Navigate-thrugh pages without needing to modify the page are skipped here.
The above cell avoids mixing disarming channel 7 into a motor cut so that accidental disarming does not cause motor jerking when rearming
The 26 pages don't need to be set up at once. For a hand-launched hovering test, one single mixer page on the left is all that is needed to map an RPM curve and a throttle pitch curve, and add an arming channel mix. As pictured on the left, the bare minimum pages serve as the first page for each channel's mixer, allowing the craft to continue building to its full capabilities. In the absence of channel 6 flight modes, the default middle 1500us is presumed in the flight computer, which assumes the accelerometer-stabilized mission.
Balance The Rotor Or Tip Over On Takeoff
As seen in the video explanation on the right, vibration can result in seemingly deliberate mechanical movement that produces erroneous attitudes by the micro-electro-mechanical gyroscope during takeoff.
The building step of trimming the feathering shaft discovered that the feathering shaft slop contributes the most to a rumbaling unbalanced rotor. When the slop is larger than 0.2 mm, the infinitesimal imbalance shifts the feathering shaft to either side of the blades after spooling up, and the direction of the shift is unpredictable and only observed after spooling down. When the feathering shaft's centering slop is less than 0.2 mm, the balanced rotor with smooth hissing can easily be achieved with thin, clear tape squares of 1cm sides.
Change Rotor RPM With Locale/Weather
According to the NASA education page screenshot below on the left, density affects blade lift linearly; in the screenshot below on the right, the lift is proportion to RPM to the square power. Using binomial approximation, blade airspeed RPM should be adjusted by half the percentage of air density change with weather reports.
Overclocking RPM can cause craft instability and/or vibration because the flight computer PID gain is tied to rotor angular momentum and proportional to rotor RPM.
Our reference condition is the middle of the Fahrenheit scale of 0-100, 50 degrees Fahrenheit, 10C temperature for RPM 2643 at 27 throttle points in the -100-to-100 points scale of the 1000us-2000us PWM signal range. Weather temperature and pressure can change air density from normal conditions by 11%. For example, a typical winter temperature weekly low of -10C produces air density 293.0/(293-20-10)=1.11 times the reference air density around Boston suburbs. Barometric pressure generally does not affect air density as much as temperature; for example, a 13% pressure change from 30 inHg to anything below 26 inHg would approach world records for the eyes of an intense cyclone. On the other hand, locale consideration for elevation, many cities above the elevation of 3000 feet, such as Tucson, northeast of Los Angeles County, El Paso, Denver, can reduce air pressure by 11% by altitude. However, the simple square relation doesn't consider turbulence effects with Reynold's number, and the simple square relation consistent for both drag and lift in response to blade speed change would contradict the deviation of Cl/Cd ratio curves when Reynold's number changes. The effect of blade speed is more powerful than the second power proportion because the higher the blade speed, the higher Reynold's number and the higher the Cl/Cd ratio. So, we will use a spreadsheet table on the right for RPM adjustment, approximating the blade speed effect to the 2.5th power.
IoT Video Monitoring
The AIY Vision kit with Raspberrypi image aiyprojects-2021-04-02.img allows setting up Raspberry Pi without any HDMI monitor or FTDI terminal connection. Instead, an Android app Google AIY Projects gives your WIFI hotspot's password to the Raspberry Pi with Vision Bonnet board, as shown in the following screenshots.
Once the Raspberry Pi Linux system receives the WIFI password, it connects to the WIFI hotspot that assigns a DHCP IP address to the Linux system. From then on, the Raspberry Pi Linux system's pre-running SSH server accepts connection from any computer on the WIFI network.
Raw video transmission data has too much delay and is impractical. Instead, the H264 compression is a good balance between processing delay and transmission delay. The setup in the craft follows (the AIY Linux system image has a default video transmission service, "joy_detection_demo", incompatible with the 1.6-gram, 160-degree FOV camera and needs to be disabled).
The App in Android is Raspberry Pi Camera Viewer by BigDotSoftware. Above is the setup, with the Android phone as the WiFi hotspot, itself at IP address 192.168.1.1, as the example, which is the target of h264 video streaming.
The Viewer app must start receiving the video data stream before the video transmission starts because the app can not decode the video data stream in the middle. In the video, the Viewer app starts at 0:02, and you can hear the craft powering up at 0:04 with ESC's beeping sequence. The receiving of the video data stream starts at 0:09 before video data transmission starts at 0:48.
This video also reveals the landing process with a line of sight, heading away from the pilot while descending.
For vertical diving proximity, use 3-D modeling of the mission scenario sketch. The example with Blender is in the picture on the right. The small matchbox at the lower left corner is the base station. And, based on the mission scenario, the RF-control/FPV antennas are arranged to prevent internal interference and obstructions.
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