It is recommended you use a Capacitor with your RaceFlight build, this can have many benefits including better noise handling and protecting your components from voltage spikes.
Here is a video of what difference capacitors can have –
It is recommended to use low ESR capacitors, a minimum 25v is recommended but if you want to run 6s, it is recommended to use 35v.
Capacitors come in many uF (microfarad) sizes, the larger uF value capacitor you can use, the better.
For a 5” build running 4s batteries, a recommended size would be a 25v 820uF Low ESR capacitor or two 25v 470uF Low ESR capacitors. If you can fit larger uF capacitors in your build, it is better.
Now there are many brands of capacitors out there, the most recommended would be fm series Panasonic Capacitors, you can get these at many retail outlets or at https://shop.raceflight.com/product/low-esr-capacitors/
Capacitors should be installed on the lipo power chain of your quadcopter, if you are using the RaceFlight Bolt esc, there are pads at the front of the esc to accommodate two capacitors. If using a PDB, find an appropriate spot on the PDB with direct lipo power.
You can also install capacitors on each individual ESC.
For best results and easier installation, solder wire on to the capacitor pins –