so anyway this has been happening for a few weeks and i cant figure it out. running a 4" longrange build, 4s, props just randomly come loose during flight. not every flight but often enough that ive had two pretty rough crashes from it. im using standard self tightening props with the correct rotation direction (checked this like five times) and theyre tightened down properly before each flight.

someone at my local field mentioned it might be a motor shaft issue but the shafts all look fine to me visually. could this be a vibration thing causing them to back off over time? the tune isnt perfect admittedly, theres still some propwash on quick direction changes but nothing crazy. im running HQ 4x4x3 props if that matters

vibration absolutely can cause this yeah. if your tune is loose enough that youre getting propwash youre also probably getting enough mid-frequency vibration to work those nuts loose over time, especially on a 4" where the resonance frequencies are already kind of annoying to deal with. id look at your RPM filter settings and make sure dynamic idle isnt set too low. also check if the prop nuts themselves are worn, those little aluminum ones strip out and lose their grip way before you can tell visually

had this exact thing happen to me on a 5" build ages ago. in my case it was actually that one of my motor bell faces had a tiny burr on it that was keeping the prop from seating flush, so even though the nut was tight the prop was rocking slightly. took me embarasingly long to spot it. might be worth pulling each prop and spinning them by hand on the shaft to feel for any wobble before assuming its a tune issue