I saw a similar post dated August 2017 where I found some infos . I suspect the problem may be PITCH2SRV_P value but I’m not sure about it (which could be the “starting” value and also if there’s some other params to check).
So I post here the general specs of the airplane , data-flash log and a short description of what happened.
Wing span : 3000 mm
Engine : Gas twin 110 cc
Takeoff Weight 25 kg
Pixhawk 2.1 Cube – Firmware plane V 4.0.5
When I apply progressive elevator , up to full-up elevator , I see (in the log ) in the servo output I see the pwm limited to not more than 1/3 of the full excursion and a big difference between des-pitch (that goes to the top 25° deg I put as limit) and the effective pitch that does not go beyond about 4° (just enough to leave the ground (not more than 1 meter off the ground ) after a couple of bounch but not enough for a safe climb. So I cut the throttle and put it back on the ground (heavily …. with some damage to the undercarriage).