|Quoting Jetlagged (Reply 18):|
You never operate near max AOA, so not quite reaching it is hardly an efficiency penalty. Wing efficiency is primarily measured by L/D ratio not CLmax, as Mir said. The canard wing is just as efficient at normal AOA.
Yes and No.
The issue with canards is not max efficiency, and in fact, canards climb just as well as conventional aircraft, (which you are correct, is determined by L/D ratio).
However, saying that aircraft do not operate near max AOA is patently false, that is the fundamental definition of a conventional aircraft's stall speed, and herein lies the rub with canards.
As a canard's CG moves aft, more lift is carried by the main wing and less by the canard (by definition), therefore in trim, the canard flies at a lower AOA. Conversely, move the CG forward, and the canard flies at higher relative AOA to the main Wing for any given airspeed to maintain trim.
Now, at the aft most CG limit in a canard, with the canard flying 'flatter' and hense at the same AOA as the wing, then yes, conventional aircraft and canards are roughly equivolent (only roughly, because the canard still must stall at a lower AOA than the wing in all cases, including control margins). Start moving the CG forward, however, and the canard must fly at a higher AOA simply to compensate for the forward weight. More AOA to carry this load, means less AOA available for lift at the same low airspeed, means a higher airspeed necessary to keep the canard flying.
Thus your limiting factor for minumum stall speed is no longer the critAOA of the wing, but instead the critAOA of the canard, which must by definition be lower than that of the main wing, meaning your not getting as much lift from the main wing. This means you either need a bigger main wing, or a main wing with a higher Lift coefficient to carry the same weight at the same speed. Either case means more drag.
Thus, while a canard will climb just as effeciently as a conventional aircraft, which as you correctly state has nothing to do with critAOA, it will NOT be able to utilize its entire wing planform for higher AOA operations as a conventional aircraft can, so while Vy will be the same (and in fact, the Canard will probably climb better due to more lift going into climb rather than fighting tail downforce), Vs will always be higher for the Canard. Thus if you want the same Vs for a canard and a conventional aircraft, you will need a larger/higher lift (either way, more draggy) wing for the canard, regardless of Vy or L/D max.
And as others have noted, the Avanti (which I have a bit of a fetish for) is _NOT_ a canard aircraft, and in fact, 3LS is a very elegant way of getting (almost) the best of both worlds... but that is a rant for another time.