Originally Posted by Dozy
To my mind the advantage of a trimmable stabiliser over tabs in a technical sense has to do with command authority - a trimmable flight surface can potentially have a far greater degree of pitch authority in the event of an elevator system failure or jam than a trim tab on the elevator itself. I don't think any airliner has used tabs over surfaces for trim since the days of the B707 and DC-8.
It's got nothing to do with pitch authority. One cannot "fly" an aeroplane, in the normal sense of the word, using the horizontal stabiliser alone. It doesn't react fast enough (unless it's an all-flying stab like a fighter). That's why there is an elevator surface hanging off the back of it, connected to the stick.
Originally Posted by Dozy
You'd have to ask an aerodynamicist to be sure, but I'm confident that the difference would be negligible - to say the very least!
Negligible? How about massive? Good aerodynamics equals good economics, Dozy. When I take off, my stab trim is around 5°. In the cruise it is zero. If I had a fixed stab, the drag would be huge. The compromise, setting a fixed stab at 2.5°, would be crazily draggy in the cruise and would require a massive elevator for takeoff and landing.