Then can someone explain to ME why I just shouldn't leave combat trim on the entire time in a fight? I keep hearing about how no one uses it and prefers to trim their own plane, but apparently I'm doing it entirely wrong.
You CAN leave CT on during the whole fight. It does a pretty good job of trimming the plane in the "normal" range of speeds. However, at high or low speeds, "dirty" configurations (flaps or gear deployed, or an off-center DT), or flight attitudes other than level and straight, it has some quirks that could cause it to be seen as a "handicap". CT works very well for basic flying, but not so great for more drastic flight attitudes and maneuvers.
It basically trims the plane (or tries to) so that at whatever direction you point the plane, the trims try to compensate and minimize the required deflection of the stick from neutral.
For example, if you go into a prolonged dive at a given trim setting, without adjusting the trim, as speed builds your nose would have a tendency to come
up. If you didn't dial in some down elevator trim, you'd be required to hold the stick forward to maintain your dive angle. In AH, dialing in the down trim happens automatically with the CT. As your speed builds in the dive, more and more down trim is added until it maxes out at full down trim. This is nice in the dive, because you don't need to hold the stick forward as much, but not so nice when your trying to pull out of the dive. In this case, your trying to pull
up, but the trim is fully
down! See the problem? This is what most people think of as "compression", but really it isn't.
Keep in mind too that "more" isn't necessarily "better" in terms of control surface movement either. Imagine if we could deflect our elevator fully 90 degrees up. How effective would that be? What if we could go beyond that, so the elevator actually pointed forward?
It doesn't take long for people to realize that using manual
up trim makes dive recovery easier. However, they often assume it helps for the wrong reason, and then confusion sets in. The assumption is that it helps because it seems to give additional
up elevator. It doesn't! It gives less down elevator
trim! Visually, it has the same effect, but in reality it's not the same thing.
It's funny you mention dialing in full up elevator for turn fights. That's "funny", because that's basically what the auto CT will do eventually too. You're basically just rushing to handicap yourself in the same manner as the CT would.
As speed builds, CT dials in
down elevator trim to minimize your required stick inputs to maintain the dive. Guess what it does as speed
drops...
In reality, as your speed drops, your nose would feel "heavier", and start to drop. More "back" force on the stick would be required to hold the nose up. In its goal to minimize your work, CT dials in more and more up elevator as you slow down, until it maxes out at full up as you near stall speed. Stall speed and full up elevator is a great way to get in trouble... Also, if you go full down elevator at this point to reverse, dive out, or whatever, CT still has full up trim, and is effectively "fighting" you.
Especially in the last example, the CT could be seen as costing you E. The up elevator trim is a handicap at that point, whether it's from CT, or done by you on purpose. Vertical maneuvers (especially ropes) can benefit from having CT disabled.
Another example- roll inverted, and maintain inverted flight. You'll be forced to use LOTS of down elevator, especially as speed drops. What does CT do to help? It dials in UP elevator, which is exactly the "wrong" input! Ever fly inverted during a fight? Or at least "not level"? Is CT helping you?
How 'bout landing? Notice as you chop the throttle and slow down you need a fair amount of left aileron. Why? Not because the plane wants to roll right... Because the CT is still trimming for the left torque of the engine, which is now gone because you've reduced throttle...
Flap use causes CT to do some odd things too. Normally I'd expect a bit of down elevator trim as I drop a notch, CT puts in some up instead... CT is off in my plane when the flaps get dropped.
Understanding how the CT works is a requirement to understanding how it doesn't always benefit you. It isn't perfect, nor is it terrible. It helps here, and hurts there.
So does Manual Trim. Using either incorrectly is a handicap. Failing to utilize them both is a handicap (IMO). Simply dialing in full up for a turn fight isn't a beneficial, correct use of manual trim, at least IMO.
I use both. I have it disabled, but have it mapped to a thumb button so that I can have it on/off when I want it. Manually trimming turns it off as well. I do best with it OFF by default, and have it accessible when I "ask" for it.
I use CT for ALL of my "normal" flying. Back and forth between bases, and "easy" maneuvering and fighting. I've developed a feel for where I'm better off with it, or without it. When my flying begins to deviate from "normal", I turn it off. When it returns to normal, I turn it back on. A two-minute fight may see it on/off 10 times. That's just how I do it... YMMV
BTW, I don't fly 38's much, so this post is more about CT in general. That said, the 38 is a good, if not great, plane to practice some manual trim with, due to it's low torque.
MtnMan
