My "Ultimate" 5" Freestyle Build (Betaflight 4.1 RPM Filter)

By strikerFPV on Oct 27, 2019

6  792  1

Disclaimer: This is not to be seen as a complete step by step guide. It requires previous knowledge. I highly discourage any beginners to attempt this build following my rough "steps". A lot of information is NOT explained in detail, which are expected to be known or either to be found out by yourself (e.g. Binding Crossfire, setting up modes, etc. in Betaflight, checking failsave, first Flight tests and so on).

Step 1: Assemble the Frame according to it's instruction page

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ImpulseRC made the assembly instructions available in a digital format on their website. You can find it there:

Following the instructions step by step you will end up with a finished frame and attached motors as you can see in the picture to the left.

I decided to use the skids delivered with the frame. Be careful to use the M3x10mm bolts only when using the skids. Otherwise the bolts might hit the motor winding. Additionally I used the rubber battery pad for the perfect hold. You can also use the SMA mount to be able to have a clean antenna mount. You can see this in all my pictures. Later on I decided, that a capacitor is still helping in removing a tiny bit of uneccessary noise from the gyro and replaced the SMA mount with a capacitor because of space problems.

Step 2: Mount the stack and solder motor wires

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  • First of all plug in the FC into your computer and check if it's working. Check the gyro.
  • Solder the XT60 connector onto the ESC.
  • Montage the ESC onto the screws of the frame. Use electrical tape to fix the wire guards delivered with the Apex Frame in place. Be sure to screw the spacer onto the ESC before starting to solder the motor wires. It's hard to install the spacers once the motor wires are soldered onto the ESC. Solder the motor wire onto the ESC. Afterwards mount the FC and plug it into the ESC with the included cable.

Check your soldering so no motor wires touch each other. The space between the pads can be a little narrow. Use a multimeter to check for continuity on the XT60 plug and make sure to use a smoke stopper when plugging in the first time.

Step 3: Flash and Configure Betaflight 4.1 and BLHeli32 32.7

There's a step by step tutorial done by Joshua Bardwell (JB) in the following video. It should walk you smoothly throughout the process:


  • Set gyro rpm filter harmonics number in filter settings to 3. In a later video JB explains that 1 is providing insufficient filtering and the difference between 2 and 3 is negligible.
  • Keep filtering at 1.0. We just wanna test for anything weird first.
  • Configure motor directions in BLHELI32 and check if all four are spinning. Important: Set PWM Frequency to 48k. Otherwise you might notice noise on your yaw gyro (which shouldn't be there like you can see in my maiden flight video)

Step 4: Crossfire Nano Receiver

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You can see a complete wiring diagram below the last step of this build log.

Solder the crossfire nano receiver onto the flight controller. Output1 goes to RX of FC and Output2 goes to TX of FC. Make sure to use the same UART e.g RX4 and TX4 in my case. Bind the Crossfire Nano with your TX and set up your channels according to your liking.

I use the TBS Immortal T antenna and therefore also installed the mount coming with the apex frame. Use the included heatshrink and double sided tape to neatly fix the receiver to the rear of the frame, like seen in the image to the right.

Step 5: Camera and VTX

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You can see a complete wiring diagram below the last step of this build log.

Mount the camera and solder it to the FC. If you are planning to use a buzzer (which I highly suggest) you will want to solder the camera and VTX to the same 5V and ground pads located on the top of the FC. Soldering them to the same ground pad has the additional positive effect, that we won't get noise into the video signal from the ground loop effect. So even if you aren't using a buzzer solder those two to the same ground pad for the optimal video quality.

You can see the suggested VTX mounting in the picture to the right. You might wanna bend the antenna cable a little (e.g. by using a cable binder and letting it sit for a day) so it won't touch the IMU of the FC. This would possibly introduce some noise in the gyro. Solder the camera video to the Video In of the FC. Video in of of VTX goes to video out of FC.

Smart Audio goes to an empty TX slot on your FC. This enables you to use smart audio and the new betaflight 4.1 vtx tables. To configure it you just need to set your VTX Table in the Betaflight Configurator to the one you can download on the Betaflight Github Wiki ( You want to donwload Smart Audio 2.0 for this VTX.

Put everything back together and you should be ready to maiden! Since the build is really tight you might want to "install" your lipo straps before installing the top plate.

Step 6: Maiden

You should do a technical maiden before and check the heat of the motors. Also insert a sd card and enable blackbox logging.
Use Pidtoolbox or Betaflight Blackbox viewer to check your pre filter gyro noise to get ready for the last and final step, the tuning.

Video of my quads maiden flight (default tune with rpm filter, no hypersmooth and no nd filter):

Step 7: Tune

This tune is heavily focused to get a locked in stick feel while maintaining extreme responsiveness.
To achieve this the tune will heavily rely on high Feed Forward and an unusually low I-Term relax for freestyle. The feed forward is really on the edge but isn't resulting in any unpleasant overshoot. Also the tune will aggressively boost Dmin to the D value to counter Propwash and wobbles that may occure due to too high I-Term in sweeping turns. Additionally it really uses a low amount of filtering. High D values can amplify noise in the gyro by 100x or even 1000x resulting in overheating motors. So I suggest to start at lower D Values, D Gain and overall lower PID's and slowly rising it to the desired value. Always check for hot motors.

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Video of the current tune in action (less Dmin gain, Tune above should have much better Propwash handling):

Wiring Diagram

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Part List


ImpulseRC Apex 5" Freestyle Frame

Flight Controller

Holybro Kakute F7 V1.5 Flight Controller + Tekko32 F3 40A 4 In 1 Blheli 32 3-6S Brushless Regler für RC Drone


4 x T Motor F60 Pro III 2700KV grau (1 St.)


Ethix S4 Lemon Lime Props (6 builds)

FPV Camera

Rotor Riot Foxeer Predator V3 2.5mm Linse

FPV Transmitter

TBS Unify Pro 5G8 V3 (SMA) (165 builds)


Lumenier AXII 2 Antennen Set SMA


TBS Crossfire Nano Receiver RX SE (2 builds)

Misc Parts

HellGate FPV Buzzer (2 builds)

Misc Parts

TBS Schrumpfschlauch 38 x 31mm Shrink Tubes 25x

Misc Parts

TBS Schrumpfschlauch 40 x 23mm Shrink Tubes 25x (2 builds)

Misc Parts

MG Chemicals Silikon konforme Beschichtung, 55 ml Liquid Flasche, kristallklar (2 builds)
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Daan   Dec 03, 2019  

Really good flight and that song is like a cherry on top. Overall, it looks like a really good build. Good job.

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