Originally Posted by whizz101
I have now upgraded to quad flying on 6 cells and 14" props. <snip> With my old hexa, i always took off in manual mode 1 very easily, with the new quad take off in mode one is very difficult, it's very jittery and always wants to tip over. <snip> however what really happened is i gained a bit too much altitude, then switched to mode 1 then dropped the throttle( right down to 7 in playback) to stop the assent.all fine so far, however once i put the power back on , it flipped and fell to the ground. <snip> Shake and vibration seem quite erratic on this frame. on the old hexa it was always between 3 and 8. on the quad its between 3 and 14 generally. putting on 11" props helped reduce the vibrations and made take off in mode 1 less jittery.
I think you have two problems, vibrations and motor timing issues. Both stemming from using larger propellers.
Larger propellers generate MUCH more vibration if they are unbalanced. With 14" and larger it is crucial to get them perfect. Sadly there is no sure way of getting there. Read forums and experiment with different propellers, and take your time. A very good/pro setup should not have vibration values above 3-4 during flight, and it can be a lot of work getting there.
Larger propellers makes it harder for a motor to quickly change speed. Especially if you apply throttle aggressively. Worst case if the ESC timing and motor is to mismatched with the propeller, a quick throttle punch can make the motor lock up and stop completely (sounds like a screaming pig).
Problems getting a stable takeoff can be a combination of both issues. Instead of just applying throttle until you get lift-off, try (in manual mode) slowly applying throttle until you feel the copter is just about to lift. Then take off normally from that point. If this helps you have motor timing issues, if not then most likely vibration problems (assuming you stab settings are correct).