In some planes, or in some situations, Combat Trim can be quite annoying.
For instance, typically in planes which use a lot of vertical maneuvers, CT tends to fubar low-speed control and increase the risk of the dreaded AH-style, inverted-"fall on your back"-spin.
It's because the combat trim automatically trims your plane to maintain level flight according to speed - the system assumes you want to fly level, and trims it accordingly.
The result is, when going straight vertical, your speed keeps on dropping. Since the CT assumes that you are flying level, it keeps pushing the elevator trim upwards. The result is, by the time you are soaring up, the CT maxed the elvator trim upwards, and your plane keeps want to nose over into a loop.
What's worse, is when you deny going into a loop in that state, and keep your plane zooming straight upwards, by the time you reach about 30~50mph the plane becomes uncontrollable. It falls over to its side diagonally; the speed is so low, that the torque pushes the plane sideways during the stall-reverse phase - the result is, your plane refuses to nose down, and starts to "flutter" straight down either on its back, or on its belly.
In this situation, you have to push the stick forward to send the nose down to recover - and CT comes back with a revenge, denying your stick forward input.
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An opposite effect, is during dives.
When you dive and gain speed quickly, the CT again, assumes that you want to fly level.
The speed of the plane increases - the CT assumes you are flying level - such high speed will urge the plane to pitch up and climb - so, it trims the elevator downwards.
Then you gain more speed, and then CT pushes the elevator trim downwards even more.
Before I got used to manual trimming, the CT caused me a lot of deaths in the 109 - I chase a target on deck alt, push the stick forward to gain a bit of speed, and then suddenly, the nose abruptly pitches downwards and the controls lock! I auger to the ground...!! All because of the CT.
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The good thing about CT, is that you can turn it on/off. When you turn it on, it immediately trims your plane for level flight at current speeds - so, it's possible to sort of use a semi-manual control.
If the speed doesn't change much, you can just trim the plane manually - but when in the heat of the battle, you can just push the CT key twice to turn it on, and then immediately off again. It immediately corrects the trim for level flight at that speed - and you can fine tune it a little bit from there.