Sunday, January 18, 2015

Nova Rebuild - Small problems - THR_MID

Here are a few small problems I noticed. Since quad is flying so good now, these are easy to spot.

THR_MID Seems Off
Still working on why THR_MID value has to be so high. 700 seems high for this minimal payload. I have a basic stock Nova (with stock 8045 props) with the following installed:
FrSky X8R Receiver
Stock camera Mount
Mobius camera
Prop Guards (stock - 4)
HK 915Mhz radio transceiver (a small radio module)

Can 700 be right? That's currently what it needs to be for me to hover at 50% stick. Is thrust low?



Also, I'm getting errors like this, each time I run a Auto Analysis on a log file (other lines Pass Good):

Test: Thrust = FAIL - Avg climb rate 38.36 cm/s for throttle avg 712
Test: Thrust = FAIL - Avg climb rate -0.30 cm/s for throttle avg 771


This parameter is used to set the throttle (between manual-control and other Flight Mode transitions) and the stick's 50% mid-point. Any time the quad's weight or payloads change, you should probably re-set it.

To get an accurate reading, you do a very simple and basic flight (only):
  • Arm quad and gently hover at 2-meters.
  • Land, dis-Arm and read THROTTLE_TRIM Parameter
  • Set THR_MID parameter the same and Write params back


I can see on the Taranis display that my Throttle Ch-3 goes from -100 to +100 when I move the stick through it's full range (just like the other sticks).
I can see in Mission Planner/FailSafe that my (calibrated) Throttle CH-3 goes from 990 to 2015 pwm, and 50% mid-point travel is around 1502 (again, similar to the other sticks).

I have gone through the Parameters and they look ok (mostly stock). The numbers here are proportions based as percentages (x10) of the PWM values.  I use WinMerge Compare. It works for both text files (nice for parameter files) and comparing directories. LiPos are not at fault either.

My Logs all look ok (I think) . This log is without props, running at 50% and then 100% throttle.
- Left side is CH-3 RC-IN/RC-OUT. This appears to be in PWM values. RCIN from 1000-2000 is correct with 50% being 1500.
- Right side is ThrIn and ThrOut and appears to be ESC values. ThrIn is 1-1000 with 500 being 50% and 1000 is 100%. ThrOut 50% (hover stick) is 700 because that was my THR_MID at the time (also makes sense why 500 is default value ... being half of 1000). Notice ThrOut is hitting 1000 or 100% which is what I was wondering about. Now, what ESCs do with that control-data is another story.

To the untrained eye, RCOUT is what looks to be "off" but it must be ok because it's hitting the proper targets on the right-side graph?

Of course my ESCs are calibrated, but I was wondering if I could see the table that matches PWM values to ESC range. ModellerMark (from says that the calibration of each ESC itself is held within the firmware of each ESC rather than as a parameter within APM .. and is not easy to view. I suppose it might be an APM algorithm and not really a "table" at all.

For Logs, I discovered that MOTORS is now RCOUT (so that's on, the PWM values). I think RAW is now IMU and/or COMPASS (again, both currently Enabled). In fact, pretty much everything else is Enabled for logging as well. This is a good intro to APM Logging (also here and here).

I just wanted to make sure that nothing was broken or mis-set, causing thrust issues.

Resolution/ Conclusion:

By removing the Prop-Guards, I got THR_MID down to 630 and no more Auto-Analyze Fail messages. It seems the 17g * 4 = 68grams weight is not the only penalty to be paid for installing them. They appear to be disturbing the air-flow around the props and further affecting the thrust of the quad. This makes the "weight penalty" for the Prop-Guards higher ... around 90 it seems. It looks like my idea of keeping them installed always as standard equipment (to avoid occasional prop-strikes due to tipping on landing) is not realistic .... not with a fully payload-loaded quad anyway. And finally, since they affect the airflow around the props/ overall thrust ... basically performance of the propulsions system ... they seem to make the aircraft more unstable (therefore counter-productive).

Other Thoughts:

Comparing my payload and THR_MID to other in the RCGroups forums, there still seems to be a discrepancy of sorts. While I have v1.0 ESCs, we have seen gradual newly revisioned ESCs being released from assembly line (v1.4, 1.6, and now even 2.1). We all know the ESCs aren't changed for no reason. While I'm sure they are making dependability changes, I wonder if the newer ones can handle more Amps, and are therefore delivering more power to stock motors (resulting in more thrust).

The quad is still mostly default with 8 inch 8045 stock props, stock motors, and revision v1.0 ESCs. I think (even slightly) different props will change thrust as well. Might be time to experiment with some alternate ones.

Edit Feb.01,2015 ... been working on it. Not sure what I changed (Parameters should be back to default) but this is now at a reasonably low level. Got it down to THR_MID=570 (still with no Prop-Guards). The Nova All-Up-Weight is 1002g.

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