so I helped it can be flown (gently) on low power (20%) but you really need to understand the basics before you try to build a high speed jet. click the menu button and check COM/COL/Thrust option, the one rule you really can't break is red should be in front of blue always. you can add nose weight to do this (fuel in this case behind the nose cone) or move the wings backwards which is good but in extreme cases makes the aircraft hard to control without excessive speed. then understand wing shape, I've altered your tail so that it provides lift to make your plane fly in a nearly straight line. then you need to be careful about control surfaces, big ones make a fast jet rip apart, small ones make slow speed landing approaches rather tricky. it's about finding a balance or a work around such as swing wings for example (swept forwards for good low speed, back making the aircraft less responsive to certain stick inputs).