ORIGINAL: john agnew
Tom
Its strange but I have had two twin boomers, an IAD Serpent and my current Jetco which both need quite a lot of downthrust on the turbine to fly on a neutral elevator. David Gladwin reckoned it was the efflux gas from the turbine creating suction on the underside of the elevator making the model pitch up unless substantial down thrust was added to avoid the efflux affecting the elevator.
Bingo! The efflux from the turbine creates acceleration of the air passing over the bottom surface of the stab, and Bernoulli's theory goes to work! You could solve it with downthrust, or by raising the stab. Since the parts are molded.. downthrust is the answer.