7" drone with 3s battery having trouble lifting off the ground

[solid anvil]

I recently built my first fpv drone and everything seems good. in betaflight configurator, the motors spin in the motors tab, the directions of the motors look good. The build ended up weighting 1.9lbs. However, when I took it out for its first flight, It would not lift off the ground unless I went full throttle. After I raise the throttle, the motors would not slow down unless I disarmed it. I checked in betaflight configurator, and the controller signal seems fine, and the motors seem fine still. My soldering work seems a little shoddy, but I don't see any obvious shorts, and I think if it was a short, there would be bigger problems.

does anyone have any clues to whats going on? Is this a battery issue?

[plush mango]

what motors?

[solid anvil]

Ah my motors were 3115 900kv 3-6s brushless motors

My full build list is as follows:
Motors: 3115 900KV 3-6S Brushless Motor
Props: HQPROP 7X4X3 7040 3-Blade PC Propeller for RC FPV Freestyle 7inch Long Range LR7 Cinelifter Drones DIY Parts
Battery: CNHL black series 2200 mah 3s 30C lipos
Flight Controller/ PDB 4 in 1 ESC : F4 V3S 60A ESC
Video Transmitter: RUSHFPV RUSH TANK SOLO VTX 5.8G 48CH PitMode Pitmode 25mW 400mW 800mW 1.6W Adjustable 2-6S Microphone
Receiver: JHEMCU ExpressLRS RX24T 2.4G ELRS Radio Nano
Frame: Mark4 Mark 4 7inch
Camera: RunCam Phoenix 2

[livid oracle]

i did read that youre running 3s, on a 7", with 3115 motors

that sounds like something id dream up when im on pain meds in hospital

something tells me that even though those motors are rated for 3-6s, they arent gonna enjoy 3s, especially 3s on an 860g quad

60a esc should be able to handle 6s, why dont you run 6s on it?

or even 4s?

I think ive found your problem, according to chatgpt (take it with a grain of salt). 4 3115 900kv motors, running 15 volts, should put out 2.2kg of thrust at full throttle. That gives you a little less than 3:1 thrust to weight, which isnt great. That would be a 4s battery. A 3s, at full charge, is 12.6 i think, which gives a total thrust of 1.8kg, which is a little over 2:1 thrust to weight. Consider using 4s or 6s. Usually those motors would be paired with bigger props.

either way, you need more voltage, or a bigger frame for bigger props

The not slowing down part is weird, but i think it might be linked to the quad being confused about the gryo data, since it thinks it should be flying at mach ridiculous at full throttle

[mighty vessel]

I hope you are aware how useless chatgpt is and that it can't actually exercise any kind of reason and is essentially just glorified predictive text, and whilst the idea is correct, the numbers aren't since you essentially asked "how fast can a tire go" then just clicked on what autocorrect has in the little bar above the keyboard
And 4s will be really low on those, those motors are pretty much 8s kv for 7" props, 6s will lift off but will still be lethargic, and a big part of the issue is also the terribly undersized lipo that probably also sagged so much the effective voltage got cut in half, and the not slowing down part is likely just throttle idle or the I term doing its job, the gyro isn't confused because it thinks it should be going faster, that's not a thing, it's eventually just trying to stabilize the quad while it's on the ground, and if they have say angle mode turned on, with how mismatched the voltage/kv etc is, well they are going to spin at a relatively high throttle value
Also, a bigger frame for bigger props in this case is a terrible idea, it will just make his battery burst into flames

@solid anvil

[livid oracle]

I didn't just ask how fast will 3115 motors go on 3s, i did put in info about prop size

I'm not that stupid

[mighty vessel]

So you asked how fast a 13" tire can go to the predictive text above your keyboard, it's still not something to use while helping someone

[livid oracle]

This is insulting my intelligence at this point, I didn't know at the time what the actual calculations for that were, but the ones that chatgpt gave me check out, and yes, I factored in prop pitch, and motor kv and size.

And either way, unless the numbers are crazy off, the quad is still underpowered.

[mighty vessel]

No need to get cocky, if you don't know it's fine, just don't use chatgpt to answer people's problems

[livid oracle]

Then don't automatically assume that I can't figure out what would be needed in such a calculation

[mighty vessel]

Why did you use chatgpt then ?

[livid oracle]

The precise numbers don't matter anyways, and as I said, take it with a grain of salt

[mighty vessel]

Again, if you don't know it's fine, just don't go using chatgpt to answer people

[livid oracle]

Cause I don't know what numbers go where, but like, I can figure out that thrust won't be affected by what video system they have for example

Correct me if I'm wrong, but for such a calculation you'd need prop size and pitch, motor size and kv, and voltage, yes?

And also

I literally said the numbers may be off

[mighty vessel]

You'd need to know the specific prop, not just the pitch and size, the kv and size of the motor, the quality of the motor, the voltage, the quality of the cell, the air pressure etc, having a decent number requires either extensive CFD simulations or some experience in the subject, chatgpt has neither,
And I am still able to read I know you put "(take it with a grain of salt)", except that the numbers are very clearly wrong, since if it truly was a 2:1 weight ratio he could've had the quad in the air at 50% throttle, which is pretty obviously not the case, so my issue here is that he gave you wrong numbers, which you had no way of checking, and if they were a lot more wrong, you also wouldn't have a way of checking
So this is just circling back to, it's fine to not know but don't use chatgpt to answer people

[livid oracle]

At 50% throttle, with a 2:1 thrust to weight, it'd weigh 0g on a scale, which is neutral, not negative, so not liftoff.

[mighty vessel]

Sure, right above 50% if you want to play semantics

[livid oracle]

specific prop for exact, precise, yes, not for a close number

[livid oracle]

you seem to be doing the same

[mighty vessel]

No I'm giving you the reasons why your numbers are wrong, not just playing semantics

[mighty vessel]

There can be big variations from one prop to the next even if they have the same pitch, blade shape can have a significant impact, and blade count too obviously

[livid oracle]

Again, stop assuming I have an iq of single digits, I put blade count in there

I never claimed to have perfect numbers, only rough ones

Either way, the quad is severely underpowered with those motors, props and battery, yes?

[mighty vessel]

But they aren't even rough, they are just wrong, so stop pretending chatgpt is a worthy way of answering questions

[livid oracle]

At the end of the day, that's the answer

[mighty vessel]

Yes it is underpowered with the current configuration, I didn't put that into question

[livid oracle]

And so, I got the answer, yes, whether I got it through chatgpt or not?

[plush mango]

Those are for 10" or so

On 7" you need like 2807 1300kv for 6s. 3s is so low you'd need much faster motors, like 2600

[mighty vessel]

You did end up with mostly the right solution, but that still wasn't my point, you've ended up with clearly wrong numbers, didn't realize it, so clearly if they were more wrong you probably wouldn't have realized it and could have come to a pretty wrong conclusion, so, again, stop trying to justify the use of chatgpt for answering people

[livid oracle]

Man, if I was gonna use those numbers on something incredibly, incredibly important, I'd do some proper research. As it is, it's already getting late, and I got to the right conclusion.

[mighty vessel]

So giving pretty much financial advice to people isn't important, got it

[livid oracle]

I had an idea that it was gonna be underpowered in the first place, I was checking it

[livid oracle]

I never gave any recommendations on what to buy, I just said its underpowered and they should consider different batteries

[mighty vessel]

Just use prop tip speed if you want to check that, prop tip speed is pretty much a constant for miniquads and x-class quads so long as the purpose is the same, so you can extrapolate a couple inches up and down using it
it's quite accurate and doesn't rely and the generative text above your keyboard

[livid oracle]

And how do I get prop tip speed?

Is that volts times kv for rpm, then circumference of prop times rpm?

[mighty vessel]

You can just do the rpm*size of the prop, no need for constants like π, and if you use that
(900x3x7)/(5x6)=630, so it would be similar to flying 630kv on 6s on a 5"

It is really accurate and Daedalus has a chart somewhere about the ideal prop tip speed for different sizes if you want to help people pick a kv without guesstimating without having much experience with the specific size

[livid oracle]

Aight

[livid oracle]

wait, whered 5x6 come from?

[mighty vessel]

5" * 6s

Again, leaving all the needless constants out

[livid oracle]

Hold up lemme get this straight. 900 is the kv of these motors, 3 is the number of blades? and 7 is the prop diameter. So why divide by 5x6?

[mighty vessel]

No

(900kv x 3s x 7")/(5" x 6s)=630kv

[livid oracle]

ohh lmao

im a little dense occasionally xd

now i get it lmao

prop pitch would change that right?

[mighty vessel]

As an example that it really does hold up
(7×6×1300)÷(5×6)=1820

[livid oracle]

wait

so

[mighty vessel]

Or
(3×4×4600)÷(5×6)=1840

[livid oracle]

7" on 6s 1300kv would be equal to 5" on 6s 1820 kv

ish

[mighty vessel]

Not equal but similar

[livid oracle]

yeah ok

[mighty vessel]

It will still have a crap load more power but the handling will be similar

[livid oracle]

gotta love me using my friends dms as a notebook for stuff like this

[mighty vessel]

Just use the bot's dms

[livid oracle]

doesnt stuff about disc loading come into that? the handling i mean

[mighty vessel]

It doesn't really matter as long as you use reasonable props

[livid oracle]

thats true

so just dont compare a 7040 quadblade to a 4020 biblade and youll be fine

[mighty vessel]

It does, but here we are assuming those are reasonably the same, of course you can't compare a tp3 and a regular 5" with this, but you could compare a tp3 and a 5" toothpick

[livid oracle]

Yeah ok then

[mighty vessel]

It would still work reasonably well, it really just needs for the props to be suited to the quad, while keeping in mind the earlier assumptions

[livid oracle]

oh aight

[solid anvil]

got it, I've got a 4s coming soon so ill test that. I just read 3s-6s on the motors so I went with the cheapest option

[solid anvil]

woah, so I need 6s then?

[mighty vessel]

Yes

[solid anvil]

ive got a 6s comiong too so ill try that aswell

do i need to mind the MAH rating when considering the ESC rating?

the 4 in 1 ESC is rated for about 60 A, so am I going to have to be concerned about the ESC failing if I have a 6s battery with like 10,000 MAH?

[mighty vessel]

The voltage rating of motors is rather meaningless, it depends on the specific quad you will put the motors on, but the motor won't stop working because you are using 2s or 8s so long as you don't pull too many amps through

[solid anvil]

I dont know this stuff about thrust-to-weight ratio and a lot of the specifics yet

[mighty vessel]

Besides how 10,000mah is bonkers high for a 7" (that's more something you would see on 15" lr quads) the esc doesn't really care about the capacity of your battery, the mAh count is simply the "tank size" of the battery, but having a bigger tank won't mean the motor in your car will blow up

[mighty vessel]

If you weren't already experienced, building a 7" was quite a poor idea

[solid anvil]

ah well, I didnt know where to start, so I thought a bigger drone would be easier to fly

if all It takes is a battery replacement, I'm feeling pretty optimistic

[mighty vessel]

It's very much the opposite

[solid anvil]

would an ideal first build be a 3" drone or the smallest one?

[mighty vessel]

What batteries specifically are you planning to get ?

[mighty vessel]

3-3.5" is a pretty nice plateform to learn on, and if you prefer to start flying in your garden/inside, then whoops are a great option too

But going above 3.5" for your first quad is not a good idea

[solid anvil]

this is the 6s one that I'm going to try

[mighty vessel]

That is wayy too big

[solid anvil]

and this is the 4s one

but you said 4s might not be enough

[mighty vessel]

Yeah it will be lethargic on 4s, but I didn't say go buy a battery that would be used on 15" quads

You would use like 3000mah 6s at most on a 7" (assuming freestyle and a lipo, you can go significantly higher with a li-ion for lr)

[solid anvil]

how about a 10" build for that 6s then?

would that be a decent fit?

[mighty vessel]

You are stacking bad ideas at the moment

[solid anvil]

ah okay

so the higher the MAH rating, the heavier the battery is and the harder it will be to fly?

[mighty vessel]

No it doesn't work this way, but your battery is just so dang chonky I'm not sure you will actually end up with much more power at this point

Go get some gnb 3000mah 6s

[solid anvil]

like this?

[mighty vessel]

These

[solid anvil]

does XT60 or XT90 connector matter?

its for higher amperage rating right?

[mighty vessel]

Get whichever is on the lipo

[solid anvil]

ah okay

what percent of the drone cost does the lipo account for in your experience?

[mighty vessel]

Yeah it's because it's a much better battery

[mighty vessel]

There isn't a fixed ideal percentage

[solid anvil]

ah okay

[mighty vessel]

It depends a lot on how much batteries you buy, and which quad specifically you build

Also, just, as a word of advice, if you build another quad in the future, please come seek advice here instead of following, whatever you followed to build this one, cause here your build list is a trainwreck

[solid anvil]

got it

I'm happy it lifted off the ground for a second

if it can stay in the air ill be even happier

can you teach me what parts other than the battery choice make it a trainwreck?

the motors arent a good fit for 7" right?

[mighty vessel]

Way too dang big and low kv (for 6s you would have wanted 1300kv) -> Motors: 3115 900KV 3-6S Brushless Motor
Those are decent Props -> Props: HQPROP 7X4X3 7040 3-Blade PC Propeller for RC FPV Freestyle 7inch Long Range LR7 Cinelifter Drones DIY Parts
Way too small mAh wise, way too low of a voltage, and the cnhl blacks kinda suck in general -> Battery: CNHL black series 2200 mah 3s 30C lipos
I don't even know which brand this is from, if it's a no name stack then that's bad, if it's a speedybee v3 stack then that's also bad -> Flight Controller/ PDB 4 in 1 ESC : F4 V3S 60A ESC
That's a good vtx (assuming it's genuine, they've had issues with fakes in the past on AliExpress) -> Video Transmitter: RUSHFPV RUSH TANK SOLO VTX 5.8G 48CH PitMode Pitmode 25mW 400mW 800mW 1.6W Adjustable 2-6S Microphone
JHEMCU is not a brand you should really shop from, their quality ranges from mediocre to terrible -> Receiver: JHEMCU ExpressLRS RX24T 2.4G ELRS Radio Nano
That frame is a bit crappy and probably a clone which hurts the hobby, and clones are usually made with worse tolerances -> Frame: Mark4 Mark 4 7inch
And that's a decent camera -> Camera: RunCam Phoenix 2

Where did you get that list from btw ?

[solid anvil]

https://www.youtube.com/watch?v=0jOUTYBneVo&ab_channel=HoarderSam
I saw this video and I wanted to make one too

[mighty vessel]

Ok so that's where you goofed

[solid anvil]

is that a bad idea?

[mighty vessel]

Hoarder Sam is uh no go zone

[mighty vessel]

Making a 7" as a beginner is a bad idea, making a hoarder Sam special quad is an extra bad idea since his quads are essentially e-waste

[solid anvil]

is there a reason why smaller drones are better for beginners? the soldering is easier on bigger builds. is it because its cheaper to do smaller builders and mistakes are less costly?

does e-waste mean its not useful after its made?

[mighty vessel]

It means it uses crappy parts

And is just waiting to catch fire mid-air

[solid anvil]

from the ESCs failing??

[mighty vessel]

Smaller quads aren't harder to solder, and they are easier to handle since they have less power, and it's also easier to fly because it requires less space, and it's also less costly to build good micros

[mighty vessel]

From the esc/fc/motor failing

[solid anvil]

ah okay

could you point me to a better small build?

btw thank you for your help @plush mango @livid oracle @mighty vessel , I'm grateful I got any input at all

[mighty vessel]

You could just build up a grinderino, for example

Grinderino
Gnb LiHV 850/650mah 4s (the ones with flower decals)
T-motor pacer 1604 3800kv
Hq 3.5x2.5x3
Airbot fenix
Rushtank mini
Ratel 2
Truerc singularity
Radiomaster xr1

What charger/goggles/radio do you have btw ?

[solid anvil]

I got this HOTA D6 charger, but it doesnt have an wall AC power input so I need to find an adapter

its only got DC input

i got a radiomaster pocket and it feels solid for fpv practice in sims

also cheap analog goggles

[mighty vessel]

It's not an adapter you need, it's a psu, you got the version without an internal psu, hota and toolkitrc both make decent PSUs

[mighty vessel]

You will want to get new ones (like the ev800d/hdzero boxpro) when you will have the budget, crappy goggles are a big bottleneck to the experience

[solid anvil]

Ive never tried better goggles, does the image quality and ergonomics improve?

[mighty vessel]

By a lot yes

[solid anvil]

does Grinderino refer to the frame? or is this a type of drone?

[solid anvil]

I cant find T-motor pacer 1604 3800kv, would these do as a replacement

[solid anvil]

Can I replace Airbot fenix with this? what would be the consequences? or should I go with a speedybee instead of a no name

[mighty vessel]

It's a specific frame

[mighty vessel]

Technically yes but I wouldn't, may as well get 1804 then, with rcinpower/flyfishrc

[mighty vessel]

You should really neither go with speedybee or a no name, going with those will greatly diminish the durability and reliability of the quad

And also that's a 30x30 stack, and t won't fit in the frame

But really avoid at all cost no name stacks, and speedybee, we have skystar/diatone/flywoo etc who all make much better electronics without even being more expensive (stack wise)

[solid anvil]

got it, I'm putting together a build list. where should I post it to get advice?