As Wolverine has said above, you need to adjust your elevator trim. Speaking only for the Spitfire and Hurricane (all marks), expect to trim both elevator and rudder trim frequently. This is simulated accurately in Cliffs of Dover - it was a necessary chore that all the Spit and Hurri pilots had to do with any change in engine settings (including throttle, pitch, and radiator settings), and change in control inputs (climbing, diving, turning, etc). Even adjusting one trim usually changes the flight dynamics of your aircraft enough that the OTHER trim has to be adjusted to maintain your desired flight attitude.
A hassle? You bet! But an accurately-simulated hassle.
Best to assign keyboard keys that are handy to you so you can adjust your trim without needing to take your eyes off the screen. Better yet, if you have a joystick hat available, use THAT to assign your elevator and rudder trims to. (Hey, great excuse to get TrackIR 5 to free up the hatswitch for you!). I actually use a CH Quadrant to use two of its levers to handle trim duties -- very, very precise in its adjustments.
You'll find that the constant trim adjusting quickly makes you a "trim expert" in no time. Not only do you relieve the annoying strain of applying constant pressure on your joystick to maintain a steady course, but trimming your aircraft makes it more aerodynamically streamlined, gaining you a few more precious mph when you are either pursuing or escaping an enemy aircraft.
Hope this helps!
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