RE: HK90mm viper mw44 conversion she Snaps!
If you have to pull up on the elevator to force the jet in the air you do not have sufficient Angle of Attack. The nose needs to be raised. I had a F-16 that had the nose slightly down and required a yank on the elevator to get her airborne. Because there was not sufficient speed she would yaw from side to side. The F-16 is a forgiving airframe so it rocked for a moment until it obtained suffiecient airspeed. Looks like from the attached email the bungee also did not supply sufficient airspeed for take off. Once the yaw starts to occur the airflow over the flight surfaces is disrupted and proper airspeed is not achieved for stable flight.
Another thing to check is to make sure that the horizontal stab and the elevator are at the proper allignment. Down angle will cause the plane to not fly off the ground smoothly.