5 inch sub 250g possible?

published : 2018-12-04 changed: 2018-12-04

category: global --> drones

Thanks to the not so new drone laws and the rules that were introduced, it is in most countries only allowed to fly drones freely without regulations if the drone is lighter than 250g - especially if you want to do FPV. (in Germany, if the drone is heavier than 250g you need a Spotter, which keeps an eye on your drone and shouts out "Danger" if something happens or whatever).

Regarding to ensurance a sub 250g drone is considered a toy and is also better.

But real racing quads do have 5" props, sub 250g is only possible with 3" - or is it?

Druckbär Ultralight

Have a look at Druckbär. The frame Ultralight 3, only weights 30,7g in 5 Inch!

Using a 20x20 Flightcontroller-Stack it is possible to keep the weight unter 250g including a Runcam Split mini. Then you can do full-HD recordings AND fly FPV.

Parts used:

  • Druckbär Ultralight 3
  • HGLRC XJB F440 Stack inkl. VTX
  • Runcam Split mini
  • XNOVA 1804 Motoren - 15g jeder!

All put together the mini racer weight with props about 160g. With a small Lipo (650 or 850 mAh minimum) you end up with about 250g.

Hints for the build

The Stack is simple to build, but we need to keep an eye on some specialities. I wanted to achieve some things with that build:

1.lighter than 250g (that’s the point, right?) 2. I wanted to control the Runcam Split Mini via the FC and the Taranis (switching on and off would be enough for a start) 3. I want SmartPort Telemetry on the Taranis. The OSD would warn about end empty lipo, but the audible warning is a lot better. 4. I wanted also to be able to control the VTX settings using the OSD or Taranis.

Spoiler-Alert: I got everything to work, except for number 4.

How did I do that?

The FC does have 2 "free" UARTs (serial ports that can be used for some periperals). One is used for ESC-Telemetry (the FC can read out temperature, RPMs etc) the other should be used for controlling the VTX.

All that is a bit of a problem, as we need 4 UARTs: ESC-Telemetry, RuncamSplit Mini, VTX and SmartPort.

FrSky SmartPort Telemetry

The simplest thing first: Smart port is possible using the LED-UART, which I did not use for this build. So jus solder the SmartPort Telemetry from the FrSky-Receiver to the LED OUT port

in Betaflight you need to remap the corresponding resources, this was in my case like this:

resource LED_STRIP 1 NONE
resource SERIAL_TX 11 A01

For that to work, the checkbox for "softserial" needs to be checked (Configuration Tab). After running those commands, you can see the virtual UART in the Serial Port Settings - set this to SmartPort then.

That was it for me. If it does not work for you, you should try to change the settings for tlm_inverted (on or off). In any case tlm_halfduplex should be set to on

set tlm_halfduplex = ON
set tlm_inverted = OFF

(Hint: the smartport of FrSky is inverted. But the FC is expecting an inverted signal. So if you set inverted to on, it will invert its expections! Crazy thing, but this acutally meens inverted from expectation.)

Controlling Runcam Split Mini

from the hardware this was quite simple. I used UART6 for the Runcam. You solder RX to TX, meaning RX6 from the FC is connected to TX from the Runcam, and TX6 from the FC to the RX pad from the runcam. Ground needs to be added also - voila!

Do not vorget to solder the video out of the runcam to the CAM-Port of the FC.

Well, this should work. But it did not... at least not really. It seems there are some problems with the firmware and runcam, so it works from time to time but not reliably. With Betaflight 3.4 new settings were added, which should fix those problem:

set rcdevice_init_dev_attempts = 8
set rcdevice_init_dev_attempt_interval = 2000

It seems, the problem was that the Runcam needed to long for initializing. That means, the FC was up, the runcam not. And so the FC did not find it reliably. So I increased the timeout with those settings. Theoretically the Runcam may now need 16secs to init.

Remark: there seems to be a bug in Betaflight, i could not store those settings! Did not work as expected

ESC Telemetry

This is also quite simple. ESC telemetry should be configured for UART3 and you are done. Well... almost ESC telemetry is read only, means, you solder the RX-Pad of the UART (here UART3) to the ESC-Telemetry port of the FC.

VTX Steuerung

This is not really working yet, I do not know why. The Idea is, that the vtx control is connected to a sending only pad of the FC. So I soldered it to the TX-Pad of UART3. Now you only need to create a new virtual UART that is using this pad:

resource SERIAL_TX 3 NONE
resource SERIAL_TX 12 B10


Now I could set the VTX-Control in the port settings. The Setting was possible, but it is not working. I also tried to configure it the other way round - creating a virtual RX-Pad. Did also not help.. (then nothing works)

I will put an update here if I know more

created Stephan Bösebeck (stephan)