Tricky Woo,
There are some aspects of canards that that have already been mentioned by Prof2MDA, but I have to disagree with Beagle with his comments.
There are several attractive features of a canard surface. The canard, if properly positioned, is free from interference from the wing or the engines (unlike a conventional tail plane). Canard control is more attractive for trimming the large nose-down moment produced by high-lift devices. To counteract the nose-down pitching moment, the canard must produce lift which will add to the lift being developed by the wing, a conventional tail plane produces a down load to counteract the pitching moment, or as many aircraft do they pump fuel to the tail plane to change the CG location to minimize the amount of down load required.
They must be designed so they stall before the wing. This is normally done by the selection of aerofoil section (this effects Clmax), and the angle of incidence. When the canard stalls it produces a nose down pitching moment, and the downwash at the wing also disappears so the lift on the wing increases, the wing center of pressure moves forward and this also gives a nose down pitching moment. The hard bit is making sure this occurs at all airspeeds, control deflections, cg locations, and flap configurations.
Normally the flow over the canard has to be made turbulent as it has a short chord a laminar flow canard would uncontrollably go turbulent when it encountered ice or rain, so it is better do design it to be turbulent from the scratch.
To quote the AW&ST article again
<font face="Verdana, Arial, Helvetica" size="2"> A classic difficulty with canards is that the wing fuel is so far aft, causing a large center-of-gravity (CG) shift as it burns off. This is at least partly compensated by placing fuel farther forward in the glove. There is no fuel in the canard or fuselage, Bair said. "It requires different CG management than a conventional airplane but computers are wonderful," he said. As the flight progresses, the canard will carry a greater fraction of a lesser load, causing handling shifts that should be manageable with a fly-by-wire system.</font>
it would seem Boeing are going for the variable incidence canard.
If Boeing decides to employ a variable incidence canard with elevator on it, the stall (i.e. exceeding the critical angle of attack) will be a function of the trim (cg location) which is undesirable but controllable with a fly by wire system. The variable incidence canard used primarily for longitudinal control and the aircrafts flight control system is able to predict the impending stall scenario and prevent the pilot from entering this regime. This may require Boeing to come up with a new flight control law called “canard alpha protection”, if I was in the aircraft when one of the flight control computers or hydraulic systems controlling the canard developed the fault I would be very worried (if it employed variable incidence canard). There is no doubt that this aircraft would need a full fly by wire system with little chance any direct mechanical backup as found on previous Boeings.
The disadvantages of a canard is that it produces a destabilizing contribution to the aircrafts static stability. The canard also interferes with the span wise lift distribution on the main wing, which has its most dramatic effect on wing stall, which can induce wing tip stall in an extreme case.
The canard downwash and trailing tip vortices cause in increase in induced drag as the it causes a deviation from the elliptical span wise loading of the lift distribution. For those who remember Cd=Cdo+k*Cl^2, where k=1/(pi*e*AR), where AR is the wing aspect ratio, and e is Oswald’s efficiency factor (also know as span efficiency factor), the efficiency of the lift distribution compared to the theoretically efficient elliptical span wise loading, and also effects the efficiency of the wing structure to get a gradual and constant increase in bending moment.
The Beech Starship overcame these issues with inefficiencies caused by the downward of the canard by adding and removing camber to the wing. Camber was added where the trailing vortex had a negative effect on the wing lift (this was done for a small distance parallel to the tip of the canard on the wing towards the wing root), and camber was removed from the wing for a small distance from distance of the tip of the canard outboard along the wing (to visualize this looking from behind the aircraft, the vortices off the canard are in a clockwise direction on the port side, and anti-clockwise direction on the starboard side).
As you can imagine this lead to a number of inefficiencies in the structural design of the wing, how Boeing is going to overcome these problems as well as the transonic drag rise is unknown.
Prof2MDA I would have thought it be prudent to reduce the fuselage cross section at the canard to minimize drag using the area rule ?
