Hello, 

I purchased a Rekon FPV Analog frame with 2 Li-ion cells. Selected suitable motors and FCBs. Below is the part list...

• Rekon 35 FPV Frame Analog 3.5"
  
• HGLRC 1202.5 5600kv BLDC Motor
  
• Gemfan Hurricane 3520 3 bladed propeller - 3.5"
  
• iFlight Succex- D F4 20A V3.2 AIO board (MPU 6000)
  

I did a complete setup on beta flight and flew it with Li-ion cells. However, in flight, it wobbles on cross-arms, like a chair with one leg cap removed. I want easy flight so I am not flying in Acro mode.

Thinking that it may be an issue with the CG balance, I tried to check the CG balancing of the drone. But it was really difficult to do it because of the weird arm position (not in X or + format). Normally, in F450 type quads we do CG balancing with cross arms balancing. and that was not becoming possible here. So, we designed a quad frame in a shape, 3D printed it,  and fitted all the same electronics from Rekon to this new frame. But when we fly, the result is the same. This time too, it is difficult to check CG due to its small-sized arm.

I want to make this as an under 250gram micro quad with a small camera equipped with an Insta 360. Current weight is 180 grams and flight time is 11 minutes, so far with 2S Li-ion 2C 3000mAh.

Questions

1. What can be the probable reason for wobbling?
   
2. How to check CG for odd fpv drones and normal fpv drones
   The drone, when flying from left to right or any other movement, loses altitude. It does not have a baro and magnetometer. Is that the reason? The Drone's up and down movement without controlling the throttle stick is sometimes much.
3. What flight controller would be better to resolve the above 2 issues?
   
4. Any comments on battery part?
   

Thanks for the help in advanced

It shouldn't wobble. The gyro should take care of stability issues.

Things I would do:

1. Check the props, motors and any other screws you have used and make sure they are tightened down.
2. Gyro issues may be caused by ripples in the electrical system. Put a capacitor on the FC at the power leads.
3. If you are using the battery box thing that comes with the Rekon, toss it and just make a battery pack from the Li-ion cells.
- Basic level: carefully solder a wire between one positive and negative and put your XT30/60 on the other two, tape up and if it was me I would throw a small cap (16v 100uf) where your power connector wires are soldered to the cells. Be real careful soldering on to Li-ion, they like to explode.
- Better level: do the same, but use a spot welder.
4. Recalibrate the accelerometer and try and get the quad as flat as you can.

So for instance. I slammed my current build into a rock yesterday and snapped the frame. It still flew perfectly normally with one arm bent. The gyro did it's job.

My guess is it's the gyro / FC. I would get a different FC and test it and see if it makes a difference. Possible choices: DarwinFPV 1-3S 15A AIO would be one, assuming your amp draw is under 15A, which i'm guessing it is.

Also post up a video showing the behaviour. A picture tells a lot more than words.

your issue would match a common power issue well, how is the voltage drop? and voltage recover after flight?

you need the best liion cells available, even these does bring power related downsides compared with lipos.
you need some sony vtc6 or molicel 18650 cells, these are mostly local available in vape shops.
pretty sure the 2c liion battery is your main issue about wobbles

change it from plug in to a decent connector like an xt30 would give better connection, daves battery design doesnt match our needs or standarts in rc use.

a bad center of gravity wont cause wobbles or doesnt need to, a bad cg would mainly give different work to zhe motors and go less efficiency therfor, maybe two hot energy wasting motors.
you can give it a try on a working quad, aslong the motors are able to provide the power and the tune isnt too agressive, a quad with wrong cg will act like on rails wothout wobbles.

i would suggest to leave the fc as aimed source for now and focus power delivery first, high quality "high c-rated" liion cell

A little off topic.

You also have the chance to use 21700 cells if you aren't using the battery box. Yes they weigh a little more, but you get more flight time for your money. So for instance, any of these three cells give at least a 35A discharge rate (45A for the Molicel). 35A should be more than adequate for flying at a set height for many minutes. But as you may already know low discharge rates will make acrobatics much more difficult, so probably best to avoid anything crazy. From videos i've watched, simple tricks are possible (rolls, flips, yaw spins, some loops), but more complicated stuff might equal going into the ground with no chance of pulling out.

Samsung 30T 21700 - 35A 3000mAh - 65g
Samsung 50S 21700 - 35A 5000mAh - 65-70g
Molicel INR21700 P45B - 45A 4500mAh - 72g

Your build would likely stay under 250g and flight time on a 5Ah Li-ion could be a bit more than 11 minutes. Also having a 35-45A discharge rate would be very useful.

But I am not a LR or long flight time aficionado and you would need to do some testing yourself.

Do you by chance have any larger motors to try, keeping close to present kv? 1202.5s are awfully small for pushing 3.5" props...

Have you tried using a smaller/lighter 2S lipo?

I have 2 Rekon35 an analogue one and an custom built Walksnail one

TO Answer your questions:
What can be the probable reason for wobbling?
 -- Possibly a tuning issue
 -- Could be an physical issue like a wire jammed against the flight controller
 -- I Also had this issue when I had bad motor on another quad and it was a brand new motor
How to check CG for odd fpv drones and normal fpv drones.
 -- Rekon35 CG will be off if your using a Insta360 GO -- I would not worry about it, it will still fly O.K.
The drone, when flying from left to right or any other movement, loses altitude. It does not have a baro and magnetometer. Is that the reason? The Drone's up and down movement without controlling the throttle stick is sometimes much.
 -- THis really does sound like a tuning issue, and possible filter issue
What flight controller would be better to resolve the above 2 issues?
  -- Probably the one that you already have if it's working -- The flight controller I used was flywoo F405 1-2S 12A AIO build-in ELRS, I was chasing the extra Uarts as for the GPS
Any comments on battery part?
 -- Good quality 18650 is a must

I did produce a video on the build which you might find helpful 


Cheers

Rekon35 is an unfortunate combination of weak motors with heavy batteries, of which there are only two 18650 cells. Not enough power, not enough voltage and current. I created my own topic here on the forum about the Rekon35 4S, and I have no problems.

(15-Jan-2024, 05:30 PM)Pathfinder075 Wrote: It shouldn't wobble.  The gyro should take care of stability issues.

Things I would do:

1. Check the props, motors and any other screws you have used and make sure they are tightened down.
2. Gyro issues may be caused by ripples in the electrical system.  Put a capacitor on the FC at the power leads.
3. If you are using the battery box thing that comes with the Rekon, toss it and just make a battery pack from the Li-ion cells.
  - Basic level: carefully solder a wire between one positive and negative and put your XT30/60 on the other two, tape up and if it was me I would throw a small cap (16v 100uf) where your power connector wires are soldered to the cells.  Be real careful soldering on to Li-ion, they like to explode.
  - Better level: do the same, but use a spot welder.
4. Recalibrate the accelerometer and try and get the quad as flat as you can.

So for instance.  I slammed my current build into a rock yesterday and snapped the frame.  It still flew perfectly normally with one arm bent.  The gyro did it's job.

My guess is it's the gyro / FC.  I would get a different FC and test it and see if it makes a difference.  Possible choices: DarwinFPV 1-3S 15A AIO would be one, assuming your amp draw is under 15A, which i'm guessing it is.

Also post up a video showing the behaviour.  A picture tells a lot more than words.

Please find the video of the drone here - https://drive.google.com/drive/folders/1...YnOLcGx21E

(16-Jan-2024, 06:53 AM)Cyberess Wrote: I have 2 Rekon35 an analogue one and an custom built Walksnail one

TO Answer your questions:
What can be the probable reason for wobbling?
 -- Possibly a tuning issue
 -- Could be an physical issue like a wire jammed against the flight controller
 -- I Also had this issue when I had bad motor on another quad and it was a brand new motor
How to check CG for odd fpv drones and normal fpv drones.
 -- Rekon35 CG will be off if your using a Insta360 GO -- I would not worry about it, it will still fly O.K.
The drone, when flying from left to right or any other movement, loses altitude. It does not have a baro and magnetometer. Is that the reason? The Drone's up and down movement without controlling the throttle stick is sometimes much.
 -- THis really does sound like a tuning issue, and possible filter issue
What flight controller would be better to resolve the above 2 issues?
  -- Probably the one that you already have if it's working -- The flight controller I used was flywoo F405 1-2S 12A AIO build-in ELRS, I was chasing the extra Uarts as for the GPS
Any comments on battery part?
 -- Good quality 18650 is a must

I did produce a video on the build which you might find helpful 


Cheers

I am not able to understand what's tuning. 
Do you mean, PID tuning? I searched for many blogs and videos and all just said that you should fly your drone as it is. If you will play with tuning, you will mess up things.

To understand the issue better, here is the video link - https://drive.google.com/drive/folders/1...YnOLcGx21E

(16-Jan-2024, 06:24 AM)skywanderer Wrote: Do you by chance have any larger motors to try, keeping close to present kv? 1202.5s are awfully small for pushing 3.5" props...

Have you tried using a smaller/lighter 2S lipo?

Sorry, I do not have a larger motor and I did not try to fly it with a 2S Lipo.

(16-Jan-2024, 05:54 AM)Pathfinder075 Wrote: A little off topic.  

You also have the chance to use 21700 cells if you aren't using the battery box.  Yes they weigh a little more, but you get more flight time for your money.  So for instance, any of these three cells give at least a 35A discharge rate (45A for the Molicel).  35A should be more than adequate for flying at a set height for many minutes. But as you may already know low discharge rates will make acrobatics much more difficult, so probably best to avoid anything crazy.  From videos i've watched, simple tricks are possible (rolls, flips, yaw spins, some loops), but more complicated stuff might equal going into the ground with no chance of pulling out.

Samsung 30T 21700 - 35A 3000mAh - 65g
Samsung 50S 21700 - 35A 5000mAh - 65-70g
Molicel INR21700 P45B - 45A 4500mAh - 72g

Your build would likely stay under 250g and flight time on a 5Ah Li-ion could be a bit more than 11 minutes.  Also having a 35-45A discharge rate would be very useful.  

But I am not a LR or long flight time aficionado and you would need to do some testing yourself.

I have recently bought Samsung 18650 3500mAh 3C cells and today the testing gave me 20 minutes of flight time. 
I also have 1 pair of LG 3000mAh 2C and 1 pair of Molicel 2600mAh 1C. All are 18650 Li-ion.

(16-Jan-2024, 03:53 AM)hugnosed_bat Wrote: your issue would match a common power issue well, how is the voltage drop? and voltage recover after flight?

you need the best liion cells available, even these does bring power related downsides compared with lipos.
you need some sony vtc6 or molicel 18650 cells, these are mostly local available in vape shops.
pretty sure the 2c liion battery is your main issue about wobbles

change it from plug in to a decent connector like an xt30 would give better connection, daves battery design doesnt match our needs or standarts in rc use.

a bad center of gravity wont cause wobbles or doesnt need to, a bad cg would mainly give different work to zhe motors and go less efficiency therfor, maybe two hot energy wasting motors.
you can give it a try on a working quad, aslong the motors are able to provide the power and the tune isnt too agressive, a quad with wrong cg will act like on rails wothout wobbles.

i would suggest to leave the fc as aimed source for now and focus power delivery first, high quality "high c-rated" liion cell

I have recently tested with my new 3500mAh 3C Samsung 18650 cell and we received a flight time of 20 minutes. Are there Li-ion 18650 cells available in the market with a higher C rate than 3C?

sony or molicel 18650 cells

These are links to a UK shop I use for 18650's.  These are pretty much the ones you need if you are concentrating on 18650 based flights.  Obviously if you live somewhere else in the world, find a local supplier that has them.

https://www.nubattery.co.uk/18650-batter...A&count=30

Hi Harry1,

Your Rekon35 looking a bit strange -- looks like your missing the side plates that mounts the camera, and you have legs coming off the arms
Your video looks like your just flying an FPV drone line of sight, just hovering in stabilizer mode? if that is the case, this is how an FPV drone hovers, and you drone just hovering normal for an FPV drone. The Rekon35 is not a camera style of drone that enabled optical stabilization for simple hovering one position, the reckon will drift about like any other FPV when manually hovering.

Give us a close up photo of you drone it looks interesting.

Cheers