Seems we're running in circles; still new replies appearing as to why tail downforce is required for stability, despite multiple explanations and references as to why that is not at all necessary.
I guess we can at least agree that
most aircraft have a tail downforce in
most parts of their flight envelope. How many that actually have a tail upforce in some remote corner of their flight envelope is still an open question (but if the C172 does, as demonstrated, then I'd imagine it can't be all that uncommon!).
But just to pick up on this line then:
On a canard the reverse is true. Always an upforce when in stable flight . This gives them the advantage of no trim drag.
There will be trim drag. When lift is produced, whether it is upwards or downwards, induced drag will always be generated.
But isn't the drag from the canard offset by the reduced lift requirement from the wing? No it's not! Because the wing is far more effective in generating lift than the canard is, you get an overall reduction in lift to drag ratio as more lift is generated by the canard rather than the wing.
It is true that a canard always has to produce an upforce in stable flight. The positive decalage requirement (the thing in the front needs a higher angle of attack than the thing in the back), together with the obvious fact that the wing always has to have a positive angle of attack [*], means the canard has to fly at a rather high angle of attack, always. The possibility for very low downforce, or even a slight upforce in some remote corner of the flight envelope, that exists for conventional aircraft, is not possible for a canard. Therefore, a canard always produces significant lift and significant trim drag.
To try to get rid of the canard trim drag while still maintaining positive decalage throughout the envelope, people are using canard flaps, variable incidence canards, variable canard sweep, or even retractable canards. Such devices are also needed to keep the stall speeds down and get reasonable takeoff and landing performance (and meet the 61 kt stall speed requirement for singles, if certification is desired). It adds complexity, and it adds weight.
So I guess that is why we see so few successful canard designs.
Now, can anyone sort out which surface produces what forces on the Piaggio Avanti?
Canard, wing
and tailplane...
[*]: ... with zero angle of attack defined as the angle of attack giving zero lift, as can be done without loss of generality.