Not enough power for take off

[Total_noob354]

Hi
2 main problems for today!

1. The motors do not seem to have enough power to take off properly, it just hovers about a centimeter off the ground. It worked fine before, I removed the camera for a bit and changed the fc and now doesn’t take off even with full throttle. Checked and double checked motor and prop directions. Maybe need to change some settings on betaflight?

2. When throttle goes over about 30%, motors start spinning at full speed and no longer respond to throttle. Only way to stop them is to disarm the quad. Confusingly, it seems to respond to pitch, roll and yaw changes (as in, left motors spins less if right stick goes left for example). I originally thought this was because the drone realised it wasn’t going up so just applied more power, but doesn’t seem to be the problem?!?

thanks for all the help now and in the past!

 

[wafflejock]

Double check your receiver tab values in betaflight make sure 1000 for min 1500 for mid and 2000 for max, maybe reset pids to defaults that come with the firmware as well or if the frame mfg supplies pids to use on a specific version of bf then can do that to eliminate any broad tuning issues.

 

[Total_noob354]

All the receiver tabs look fine, I genuinely don’t know what to do. Haven’t changed any pid settings. The only other reasons I can think of is that it is too heavy, or that the fc is restricting the current to the motors somehow? Can I increase the threshold in beta flight or something? (Btw it’s a brushed drone)

 

[wafflejock]

All the receiver tabs look fine, I genuinely don’t know what to do. Haven’t changed any pid settings. The only other reasons I can think of is that it is too heavy, or that the fc is restricting the current to the motors somehow? Can I increase the threshold in beta flight or something? (Btw it’s a brushed drone)

Might seem obvious but make sure using a freshly charged battery. Betaflight doesn't limit current in any way really it will just give low voltage warning, but doesn't really do anything else to restrict power from the batteries.

 

[wafflejock]

If weight or props didn't change significantly then the power to weight should still be about the same, generally speaking 1 or 2S quads will only fly for a few minutes before needing a recharge though. Motor rpm is only other thing that effects thrust and motor rpm is dependent on the voltage applied (the torque or turning force for accelerating a prop is related to the current since that is what makes the magnetic force)

 

[Total_noob354]

How could I add more thrust then? Better props, higher voltage battery?

 

[wafflejock]

Yup either could be an option, if the components support higher voltage then could try higher cell count battery, alternative can get a lower mAh capacity therefore lighter battery to reduce weight. A higher pitched prop or a larger surface prop will both get more thrust per rotation (or one with more blades) but it takes more torque from motors to keep things up to speed or rather changing speed quickly enough and larger props or higher pitch props put more "load" on the motor and whole system cause need more current to change speed of a bigger/heavier prop so it is a bit of balancing act.

Ecalc.ch site is good resource to plug in numbers and see how they effect the theoretical flight time

 

[wafflejock]

Another variable I didn't mention yet is motor kv because I assume sticking with the same motors but a motor kv is effectively how many rpm it gets per volt applied so a 300kv motor does 300rpm at 1V and 600rpm at 2V but if you push too much current through a motor in a given time you'll end up cooking the wires in it (or in case of brushed motor possibly damaging the brushes and making them more resistant). Another thing to note the brushed motors have a limited lifespan and will die after about 6 hrs of runtime (hence why I fly primarily brushless stuff even on the small scale). Brushed quads are fun for indoor or tight spaces but the limited lifespan of motors and batteries means I usually end up finding a field to rip the 5" or wait for good weather

 

[Total_noob354]

Update:
Spent a while trying to optimise weight by getting rid of some of the random plastic casings on batteries etc. Gained another centimeter of Max height!
While I was doing this i realised that the remote controller would start beeping as I flew and I found out that this was because the voltage going into the receiver was going below 3.5 volts during fly time which caused an alarm to go off. The voltage when it isn’t flying is well over 4.5 volts though, can’t think of why the fc would suddenly stop supplying voltage?

 

[wafflejock]

With 1S it should only really ever be charged to 4.2V anything higher is gonna be fire hazard for the cell, but my guess is the voltage you're seeing reported is just off because of analog to digital converter that is munging the actual values. If don't have one suggest get multimeter and check the DC voltage with that you'll get far more accurate readings.

Regarding voltage "dip" during flight basically the more current a battery has to deliver the more the voltage will drop from whatever its "resting voltage" is. A small drop is normal like dipping from 4.2V down to say 3.8V pretty fast when using some throttle but a full volt dropped under load is an indication the cells discharge rate is too low for what is being drawn. The "c-rating" on cells is meant to tell you basically how much max current you can draw given the capacity of a cell, so a 10C battery that is 1Ah (or 1000mAh) capacity can discharge 10C * 1Ah = 10A at any given moment, a 20C 1Ah would be 20A "continuous discharge". It might be the case you just need a higher c rated or higher discharge current capable cell because the motors are "over drawing" current from the cells and making the voltage drop which in turn drops the prop rpm and keeps ya barely hovering.

 

[wafflejock]

For 5" quads I typically fly 1500mAh 4S batteries with 75-100C rating from the mfg and tend to work well but this is a bit of a lottery with which mfg you get cells from and if they can actually deliver the current advertised.

 

[Total_noob354]

Looking to buy better propellors, worked out I need about 40 g thrust per motor. Which variables do I need to look for in props to get maximum thrust? Higher diameter, higher pitch? What values for these are good or bad?
Thanks for all your help!

 

[wafflejock]

Looking to buy better propellors, worked out I need about 40 g thrust per motor. Which variables do I need to look for in props to get maximum thrust? Higher diameter, higher pitch? What values for these are good or bad?
Thanks for all your help!

Oof just noticed my c value examples were off by factor of 10 but I fixed hopefully that isn't terribly confusing... Anyway regarding prop changes suggest go to ecalc.ch pick out the quad calculator and adjust the pitch or prop sizes in there to see what kind of effect that has on everything. Generally speaking if you have double the diameter disk (prop diameter) then per sweep you are going to be pushing about twice as much air, so more load on motors and all but more thrust per rotation (double if talking double the prop size) because area of the disc is proportional to the radius. Since prop size mostly dictated by frame size and what motors can push you general want as big a prop as will fit and will help to increase overall flight time (again assuming motor has enough power to push bigger prop). Similarly pitch will effect how much air you push per rotation but is more "slapping the wind", think easier to think about this in terms of sails on a sail boat, if you have a very deep sail (high pitch) it can catch more air up to some limit for quads this equates to more thrust/punchiness but drains more power so basically less efficient (less flight time) but more manuverable and able to correct for wind etc. Also people who fly high altitudes (Denver for example) have thinner air and so a higher pitch prop is just needed to stay in the air.

 

[Total_noob354]

New props did nothing. Realised that, even though the remote controller says the receiver has no power, the indicator light on the receiver says it is on the whole time. They seem to re-connect after the props stop spinning. Could they be interfering with the signal maybe?
This is my last chance before I give up for a while. Thanks for ur help over the last few weeks!