With the SGA DC-9/MD-80/717 series (or most T-Tailed aircraft in general), you MUST set trim. Your flying experience will be difficult and dangerous if you don't. In addition, the SGA flight dynamics of the pre-MD80 Douglas narrowbodies are programmed such that the engine spooling takes a much longer time to spool up (which is actually realistic on the real older DC-9 aircraft).
When taking off a high-altitude destination such as DEN
, make sure you spool up before releasing brakes on the older DC-9 aircraft. Don't forget to set trim! (in case I haven't mentioned it already)