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I'm giving her all she's got Captain!/Problem with weight, balance or something else?

#1
Hi there!

I built a flamewheel using real DJI parts and an "E300 Tuned Propulsion System" (full specs below) and it flew OK, but has been under powered; it was at around 70% throttle to hover but never any issues with maneuverability, recovery etc. I am using a 2200 3s 20-30c lipo. But I am starting to think that, that is not sufficient for the setup. At max everything at once I need around 65-70A and my current setup only covers about 66A at burst and 44A sustained.

This battery may be the source of the issue here (at least I hope so).

But anyway, I thought, since this thing still flies I should put a gimbal on it ... So I strapped on some upgraded props (DJI 9450 up from 9443) and bought the Turnigy mobius gimbal from HK and a mobius.

While mounting that I added a small fiberglass plate and about 10 zip ties in addition the the cam and gimbal.

When I first mounted everything I had a balance issue because I stuck the gimbal out front too far. It would not even get off the ground because it seemed like there just was not enough power to lift the "low side" once the first half lifted. Tried full throttle and it never would lift.

I have since moved the gimbal back to a normal COG with the battery countering any forward placement, and can get it off the ground, but if there is even the slightest wind it starts descending (awkwardly) and I have to land it (awkwardly).

I am thinking about going up to a 4s and adding a BEC in to power the gimbal, to get more voltage and hopefully more power, but don't know if the added weight would just negate it. (ecalc tells me my flight time would be similar if I didn't change anything else other than more weight and a 4s 2200)

So I have some questions for the awesome people of flite test land.

Is my battery not supplying enough constant amperage (too low C)?

Is there enough thrust here to lift this type of a setup if I provide the right C rating with a 3s or do I need more voltage?

Would a similar capacity 4s (with sufficeint C rating) solve my issues, or would just a 3s with a better C rating and maybe more capacity solve it?

Any input you have is greatly appreciated. I am learning as I go, and would like to get this thing somewhat stable!

I have not been able to get an accurate weight on it. Here is a parts list and weights.

~ = estimated
sans ~ = actual (as reported)

DJI Flamewheel F450 frame 282g
Landing gear (plastic) 72g
DJI 15A opto escs ~150g
DJI 920kv 2212 motors 200g
DJI 9450 props ~40g
generic gps mast ~40g
3DR ublox GPS 16.8g
3DR Pixhawk 40g
wires/connectors ~20g
Turnigy Mobius Gimbal 250g
mobius 40g
reciever ~5g
glue ~5g
zip ties ~20g
extra fiberglass plate ~20g

battery 3s 188g
OR
battery 4s 242g

1388g total 3s
OR
1442g total 4s
 

ZoomNBoom

Senior Member
#2
Your battery C rating is definitely too low (battery C ratings are always exaggerated) , and your 15A ESC's are exceeding their rating.

Googling around, those motor/props should deliver ~630gr each, x4 = 2.5Kg. Thats only marginally enough to fly 1.4 Kg. Factor in a losses due to the frame, a battery that sags and off center cg, and you're no longer flying Also note in that test they draw <10A each on 3S. IF you are drawing substantially more, thats probably because your battery voltage sags.

4S and higher C rating are the solution. On 4S your setup should deliver close to 4Kg of thrust, and still be below 15A per motor.
 

Cyberdactyl

Misfit Multirotor Monkey
#3
Another important check you should make initially when doing a maiden or if you have changed any components like ESCs, motors, props, batteries, etc., is to check temps on the motor stator, ESCs and battery. Once the FETs on the ESCs or the motor stators get uncomfortably warm >120F, or the battery >105F, you need to look at the setup more closely for inadequate sizing.

If you're merely hovering and tooling around and need 70% (actual full) throttle, I suspect one of the components, in this case it sounds like the battery, is getting toasty.
 
#4
It may also be your vertical gain setting. You are supposed to increase that until it hovers at ~50%. If raising that makes it jitter then you may have some other problems.
 
#5
Thanks!

ZoomNBoom - thanks for the info. I think I will give the 4s route a try! That additional thrust should be enough to cover the heavier battery! Do you think the 15A ESCs will be OK on the 4s setup? Also, do you think the flight time will suffer or improve if I keep the same capacity on the 4s?


Cyberdactyl - Thanks! I sincerely think that this battery is the source, but I never had any heat on it with or without the gimbal.


Darkback2 - I had adjusted that in mission planner to get match the level I was seeing but it was never enough to fully get it to half position. :(
 
#6
It may also be your vertical gain setting. You are supposed to increase that until it hovers at ~50%. If raising that makes it jitter then you may have some other problems.
Any more info on this? I have a similiar issue and have set up quite a few rigs with missionplanner but don't recall this setting anywhere?

Like I said I'm having a very similiar issue with a new quad I just finished. The new craft was built around the idea of making an effecient 3S AP rig. Initially I thought the craft was just underpowered/overweight but since I have reviewed the DVR footage of the OSD I see the rig is hovering at 20A and at full throttle is only pulling 28A. Hover is ~60% throttle. Everything is stone cold upon landing (which would be expected if everything is sized for 30A and your pulling 1/3 of that)

Now according to the motor spec sheets 3s/12" should pull 1100g @ 17A per motor. I'm running 13 Props and each motor is only pulling 7A at Full throttle. Its enough to hover but climbs so slow the OSD cant even register it as 1m/s.

The craft's AUW is 2200g which easily puts the power to weight ratio above 2:1.

Like i said I just realized this issue so going forward im gonna just throw a servo tester on it to either elimanate or prove that the flight controller is the problem and go from there. As well im gonna pull the logs from the first couple test flights and see what the motor raw out actually is.

Thoughts on why I'm being limited on power? I've attached the spec sheet for the motors im using. Note it only shows 12" props but I assumed that if it can handle 12" on 4s that 13" on 3s would be okay in a non acrobatic multirotor aplicication.


 
#7
Ahh...I'm dumb...I for some reason thought you were using the DJI Naza-M V2 flight controller. I didn't catch that you were using the Pixhawk. My bad. Sorry for any confusion this may have caused.

DB
 

zenguerilla

Senior Member
#8
I'm pretty sure if you are just 'under powered' you want to go for bigger motors, lighter weight, higher voltage, or bigger props, if possible. If your craft is having operational problems regarding battery, it may then be appropriate to increase Amps capacity (C rate x mAH).
 
#9
Gonna update this and say I found my issue. Turns out I had a defective ESC on one corner. Found out by testing each individually with a servo tester and wattmeter. 3 motors pulled 19.5amps. the defective one was only pulling 9amps. This ESC was already suspect as it would not always initialize upon first power up but just flash a red led a few times first.
 
#10
Glad you found the problem! I would look at maybe some 30 amp ESCs that are at least 4S capable to future proof your rig. If your motors are pulling 19.5 amps already, those 15 amp ESCs are going to be sketchy. Also look into a higher C rated battery. All 4 motors at max throttle will be pulling nearly 80 amps. That 2200 at 20C is only able to supply 44 amps constant, or 66 amps at 30C in bursts. I have heard that battery manufacturers also overstate the discharge ratings a bit, so you want to shoot a bit over your max amperage draw. Always better to be safe than sorry when you have equipment flying high in the air.