Originally Posted by
beardy
Next time you set yourself up in the descent at the same speed as level flight, note which way you trim and think why. (hint: you trim nose up to maintain a greater AoA that is needed to provide more lift perpendicular to the wing chord to provide the same vertical vector that you had before you commenced the descent.)
This is not true. If speed stays the same, AOA will stay the same (actually, a very slight reduction as explained in the next paragraph), and there is no trim due to speed change or anything to do with the vertical motion. There is trim for the thrust-pitch couple, which maybe up or down, or nothing. In my current plane for example, I have to trim down in this scenario.
The missing vertical vector that you (correctly) identify a need for, is provided by a drag component (which is now inclined upward) and not lift. And Just This Once is correct in saying that lift (not the sum of the vertical forces, just lift itself) is now reduced, since its opposing component of weight is reduced. Both are multiplied by the cosine of the descent angle. (They’re both reduced the same way, by the way, for a climb angle.) This reduction is extremely slight (.998 G for a steady 3 degree descent) so is rarely talked about in pilot training materials. Easier to ignore, without significantly affecting anything.