Every flight as soon as it starts trim starts to push nose down all the way to the end and plane is uncontrollable.I have even deleted all key commands and joystick commands for trim of elevator and it keeps doing it.Any ideas?

Does this happen with all aircraft or just one in particular?

See if your auto-pilot is On.

See if your auto-pilot is On.

All of them, even Piper Cub with no autopilot

Hi,Does the flight start off this way or does the trim quickly change at the start?

Just press the 'Z' button. If it beeps - you've just fixed it.

Errant key assignment? See f there is something assigned to trim down causing the runaway trim.