Well when you gain in speed, you need to trim in accordance. I dont use manual in Aces High, but fly a real plane, and you'll know.
As speed gets higher, your nose often pitches upwards and your nose yaws in the direction of the prop's spinning direction. As extra power is applied very quickly, the kick of wind often will yaw the nose AGAINST the prop's spin. This is called the slipstream effect.
As a result, you will never be able to trim your aircraft PERFECTLY since an increase in speed will cause the pilot to add in more trim input. At top speed, you'll be able to trim the aircraft more easily as it can't really change its speed unless you slow down. More often than not, any adjustment of the throttle will require an adjustment in trim to get the plane stable. In real life, it's more complicated than that. Wind, air pressure, blalala etc.
hope that helps!