Potential Thrust Loss - Analysis

jstroup1986 · January 28, 2023, 5:29pm

This is an analysis of an occurrence of Potential Thrust Loss warnings I received yesterday. I’d appreciate any comments regarding this incident. Thank you.

Yesterday, I flew a photo survey mission planned for 30 minutes.

About 25 minutes into the mission I received two “Potential Thrust Loss” warnings - so I aborted the mission with a RTL.

This “Potential Thrust Loss” warning occurs when the flight controller commands maximum power (PWM 1950) to a ESC/motor but the motor thrust does not achieve the aircraft attitude the flight controller is seeking. This condition can lead to loss of aircraft control and a crash.

The copter is a 650-class copter. The ESC is a 4-in-1 using BLHeli_32. The battery is a 6S2P 10.5 Ah Li-Ion.

The flight path rows were aligned with and against the prevailing wind - evident by higher amperage and higher RC-OUT when travelling up wind - about 300 degrees.

The low voltage failsafe (18.6V) had not yet tripped (due to the 10 second timer parameter,) but there were dips below the 18.6V failsafe.

After landing, the resting battery voltage was 21.15V. Upon recharge, only 6.5A of the 10.5A had been discharged.

My initial thoughts were that somehow my BLHeli_32 configuration wasn’t enabling enough power to the motors in low voltage conditions. While this still remains a possibility, I no longer think it’s a factor. Here are my BLHeli_32 settings - I’ve reached out to the BLHeli_32 support community for their thoughts and input.

The battery’s label lists specs for “Ideal” and “Max Constant” discharge rates - 270W and 450W respectively.

Graphing the wattage for this flight, the discharge rate is quite high - continuously exceeding the 270W ideal level and getting to peaks of over 500W.

I compared this to a similar 20 minute flight in calm weather - and the wattage in calmer winds is consistently lower - and close to the “Ideal” 270W.

It appears that the low voltage delivered at high wattage loads do not allow this copter to maintain full motor authority in windy conditions.

Flying survey rows cross wind may alleviate this - but I expect for maximum duration with this battery, calm winds are a requirement.

The Li-PO battery I have for this copter is 8.0Ah. Although it’s about 200 grams heavier than the 10.5A Li-Ion, it’s likely to deliver much more than the 6.25A that the Li-Ion was able to deliver in these conditions.

xfacta · January 29, 2023, 4:22am

Good analysis, going over that battery data

Definitely set an action for critical voltage: BATT_FS_CRT_ACT,1

The copter looks a bit overweight, motor outputs consistently around 1800 (equivalent) PWM
If using the LiPo doesnt fix it, then you might have to rethink the motors and props.

In BLHELI settings I would set Motor Timing = Auto and Temperature = 100 (if temp hits 140 the ESC is probably already beyond help)

Other:
The HNOTCH seems a bit misconfigured, I would set:
INS_HNTCH_REF,0.27
INS_HNTCH_FREQ,60
INS_HNTCH_BW,30
INS_LOG_BAT_OPT,4

jstroup1986 · January 29, 2023, 1:23pm

Thank you Shawn - I really appreciate your analysis. I’ll investigate those parameter changes you suggest.

BTW - Funny thing about this copter seeming overweight - with this 10.5A Li-Ion battery it’s 200 grams lighter than with the 8.0A Li-Po battery. I’m guessing that the lower voltage makes the copter appear sluggish. Seems to me there are parameters set initially that corrects or linearizes the voltage range. I’ll have to look it up - do you happen to recall if such a parameter exists?

BTW - I also reached out to Steffen Skaug, the author of BLHeli_32, for advice on my BLHeli_settings. Here’s his response:

xfacta · January 29, 2023, 9:36pm

MOT_BAT_VOLT_* correct the commanded motor outputs for the changing voltage. I think the voltages specified should be realistic and very close to the expected maximum and minimums. Usually Initial Parameters would work that out, but some different Li Ion cells might need some tweaking. Works best if MOT_THST_EXPO is set perfectly too.

Adaptive for Low RPM Power Protect must be a new feature I havent seen before. I have reduced Rampup power in the past, particularly if you get “arm shaking” (yaw oscillations) at arming.

With sine modulation, I can not say I’ve seen longer flight times mainly because we havent done a back-to-back test, but reading the doco it always seemed like a good, appropriate feature, so I have always used it.

I’ve only ever changed:

Low RPM Power Protect = OFF
Low Voltage Protect = OFF
Temperature Protection = 100 (or even 90)
Sine Modulation = ON
Rampup Power = 25% only if required

and never had any desyncs at all, even with heavy payloads, or near-aerobatic activities (considering the copters size) - but I’ve never use Li Ion’s either!
I’m sure Motor Timing used to default to Auto, but anyway I would make sure it is Auto.

jstroup1986 · January 30, 2023, 4:07pm

I began to dig back through my old logs to try to figure out why the RC-OUT values are so low. I suspected that it had something to do with my switch to a Li-Ion battery.

I compared the voltages on two identical missions - one on Li-Po and one on Li-Ion. The voltage difference is apparent.

Digging into the difference I discovered that I had charged the pack to 4.1V per cell, which is the default value for Li-Ion on my Toolkitrc M6D charger. I checked with Titan, the battery manufacturer, and they confirmed that the pack should be charged to 4.2V per cell. They also corrected me - it’s a 6S3P pack.

I’ll repeat this mission on the next fair weather day and report back.