An aeroplane ESC needs 0% throttle indication at startup, but since a surface radio throttle is set at about 25% by default, it makes the ESC beep as a warning to set the throttle to 0%. For the aeroplane ESC to work, you can either mechanically modify the Tx by setting the default position of the throttle to 0% or if that is not possible, you can do it the software way; the adapter takes the 25% signal coming from the Rx and remaps it to 0%, as an indication of 'no throttle' to the ESC.
I used an ATmega8 for this project just because I had a bunch of those laying around, it's definitely an overkill.
The microcontroller of your choice for this project must have flash memory in upwards of 4 KB and have 16-bit timers. If it's a 3.3v chip, then you'll have to do some level shifting. An ATtiny44 or ATtiny88 would be apt.
1) ATmega8 or equivalent microcontroller- 1 nos
2) 16MHz crystal (one that matches your chip) - 1 nos
3) 22pF ceramic capacitors- 2 nos
4) Servo extender- 1 nos
5) Strip board
Additionally, you'll need an USBasp programmer to burn the code on to the ATmega8.
However, I used an Arduino UNO board for the same.
I did this in an hour with ugly hardwires on a piece of strip board. I urge you to do it on a nice PCB, if you need help designing the PCB for your microcontroller, ask. I'll be glad to help.
Good luck, DIYers!