Observation On BLDC Hard Starts

[scallybert]

Every now and again, I get a harsh start - in spite of the ESC being reasonably carefully tuned for soft starts. I have some theories about why this happens - but that's not the point.

When this happens, the blades partially fold back, and it gets a bit messy.

You can tighten the grips to prevent this - but that's not ideal in itself.

What I seem to have noticed, is that if you start the motor at full -ve collective (-14 deg, in this case), it seems far less likely that the blades will fold.

I'm guessing this is mainly because the blades would need to rise (against gravity) to fold back.

Once it starts to spin up, I move to slightly <0 collective; but the apparent advantage is at the start.

Anyone else tried this , etc ?

[tomatwalden]

I've never had an issue with soft starts ?! All my (Castle) ESCs are rigged for soft start and auto-bailout. Having never experienced a "hot start", I've never attempted a work around.

What ESC are you using?

[scallybert]

I've never had an issue with soft starts ?! All my (Castle) ESCs are rigged for soft start and auto-bailout. Having never experienced a "hot start", I've never attempted a work around.

What ESC are you using?

There's a slight irony that this is largely a number of Talon 90s on Goblin 500s [as you have one of these.] Though mine are currently the Quantum 4120-1200; rather than the 4115-1200.

I didn't want to get bogged down in why I see occasional harsh starts, as it's not the point of the thread; however, I have a guess as to why they happen.

My guess is that it's because the Goblin's have two reasonably heavy duty belts (primary and tail). The nature of belts is that you get tight and loose spots; so my guess is that there's a slight lottery as to what the ESC has to deal with at startup [bearing in mind that the startup is open loop - the ESC has no idea what the motor is doing, until it's spinning at several hundred RPM.] ie, it's potentially presented with a different drag to overcome at each start.

It's also possible that my helis are more firmly planted on the ground at startup. Hence, the harshness is transmitted to the head; rather than causing a small rotation of the heli.

Most of the time, I get nice start-ups; but, maybe 1 in 10, are a little harsh. Possibly I could tune this out - but I suspect I'd have to re-do this each time I did anything that affected belt tension.

However, this wasn't the point. Particularly as I know there are other configurations (maybe non-CC), where people have been unable to tune out the harshness - and blade-folding is an issue then.

I could also make the grips a little tighter - but I think I'd rather have them just tight enough so they don't drop under their own weight.

So, it seems starting with a lot of -ve collective helps, if you're experiencing this. It might also enable running lighter grip (lead/lag) damping, if that was desirable. [I believe, that once the head is at operating speed, the only function of the grip damping is to provide friction damping. On small helis, I'm not sure it's helpful at all.]

[brin]

Maybe the fact that you always start in flight mode 1 or 2 and maybe the esc and motor are already warmed up and that could have an effect.

There's a slight irony that this is largely a number of Talon 90s on Goblin 500s [as you have one of these.] Though mine are currently the Quantum 4120-1200; rather than the 4115-1200.

I didn't want to get bogged down in why I see occasional harsh starts, as it's not the point of the thread; however, I have a guess as to why they happen.

My guess is that it's because the Goblin's have two reasonably heavy duty belts (primary and tail). The nature of belts is that you get tight and loose spots; so my guess is that there's a slight lottery as to what the ESC has to deal with at startup [bearing in mind that the startup is open loop - the ESC has no idea what the motor is doing, until it's spinning at several hundred RPM.] ie, it's potentially presented with a different drag to overcome at each start.

It's also possible that my helis are more firmly planted on the ground at startup. Hence, the harshness is transmitted to the head; rather than causing a small rotation of the heli.

Most of the time, I get nice start-ups; but, maybe 1 in 10, are a little harsh. Possibly I could tune this out - but I suspect I'd have to re-do this each time I did anything that affected belt tension.

However, this wasn't the point. Particularly as I know there are other configurations (maybe non-CC), where people have been unable to tune out the harshness - and blade-folding is an issue then.

I could also make the grips a little tighter - but I think I'd rather have them just tight enough so they don't drop under their own weight.

So, it seems starting with a lot of -ve collective helps, if you're experiencing this. It might also enable running lighter grip (lead/lag) damping, if that was desirable. [I believe, that once the head is at operating speed, the only function of the grip damping is to provide friction damping. On small helis, I'm not sure it's helpful at all.]

[tomatwalden]

I know you say you don't want to get bogged down in the "why" - but I would be interested in seeing your Talon settings. My soft-starts never so much as glitch, and take a good 10-seconds to reach full ramp-up ... you can see for yourself here : https://www.youtube.com/watch?v=1MxcRWsVNbo

[scallybert]

I know you say you don't want to get bogged down in the "why" - but I would be interested in seeing your Talon settings. My soft-starts never so much as glitch, and take a good 10-seconds to reach full ramp-up ... you can see for yourself here : https://www.youtube.com/watch?v=1MxcRWsVNbo

Looks like mine.

The start-ups are great - except occasionally a blade folds. [at start-up; not spool-up.]



///

################################################## #####
# Castle Link Data File
# Created: Wednesday, September 23, 2015
# Do Not Edit This File By Hand
################################################## #####
Auto Rotate Enabled: False
Auto-Lipo Volts/Cell: 3.2 Volts/Cell (*)
BEC Voltage: 6.0V (*)
Brake Delay: .6 sec (Delayed) (*)
Brake Ramp: Immediate (*)
Brake Strength: 0%, Disabled (*)
Current Cutoff Type: Soft Cutoff (*)
Current Limiting: Insensitive
Cutoff Voltage: Auto Li-Po (*)
Desired Head Speed 1: 2455
Desired Head Speed 2: 2455
Desired Head Speed 3: 2455
Direction: Forward (*)
Governor Gain: Low (15) (*)
Governor Mode Type: Fixed
Head Speed Change Rate: Medium (5) (*)
Hex55: 85
Initial Spool-Up Rate: Medium (5) (*)
Link Live Enable: Disabled (*)
Motor Start Power: Medium (59) (*)
Motor Timing: Normal (5) (*)
Power-On Beep: Enabled (*)
PWM Rate: 12 Khz (*)
Throttle Response: Medium (5) (*)
Throttle Type: Governor Mode
Vehicle Setup - Battery Type: LiPo
Vehicle Setup - Battery Voltage: 22.2000
Vehicle Setup - Gear Ratio: 9.000
Vehicle Setup - Motor Kv: 1200
Vehicle Setup - Motor Number of Poles: 8
Vehicle Setup - Number of Cells: 6
Vehicle Type: Helicopter
Voltage Cutoff Type: Soft Cutoff (*)



///

[scallybert]

Maybe the fact that you always start in flight mode 1 or 2

Now, you know I always start in flight-mode 8 or flight-mode 9..!

On all the models except the 3-blade one, the throttle output is toggled from -105% to +100% by the motor on/off switch.

It's completely independent of anything else on the model.

[3-blade, that switch toggles between -105% and a value set by a knob.]

and maybe the esc and motor are already warmed up and that could have an effect.

This is true, as I regularly run packs through back to back. It might be more common on the subsequent packs.

This *is* veering off-topic, though...

[kenny d]

I think the reason the startup workes better in -pitch is because the wind against the blade is jamming them more in negative pitch than 0 degrease pitch , it acts like a damper , as a plumber I am fully aware of this , on balkcocks you sometimes get a vibration when they are closing , if I twist the arm to the side it then James slightly
& acts as a damper , I don't know why you are having a problem with your start up but I would tighten them a bit more , I've had 2 g500's with talon90's & never experienced a problem with start up

[scallybert]

I think the reason the startup workes better in -pitch is because the wind against the blade is jamming them more in negative pitch than 0 degrease pitch , it acts like a damper , as a plumber I am fully aware of this , on balkcocks you sometimes get a vibration when they are closing , if I twist the arm to the side it then James slightly
& acts as a damper , [...]

Interesting point wrt the ballcocks.

I think the inertia of the blade has more effect than wind drag, at start-up.

For the blade to fold back, this inertia would need to lift the blade against gravity; which is harder than if the blade was at zero pitch, or +ve pitch. This may also increase any 'jamming' affect in the grip.

This may be even more of a benefit on small helis, where often the blades are quite loose.