Originally Posted by
B Fraser
I am rather puzzled (it doesn't take much) why manufacturers have not adopted a canard layout. Having an upside down wing at the back seemed daft to me when I first learned about it as a spotty teenager.
The problem with the canard layout is making it intrinsicly safe when stalling whilst maintaining efficiency. With a tail-at-the-back design you can arrange the neutral point so that either the wing or the tailplane stalls first and still get safely close to maximum lift coefficient when needed. If the wing stalls first then the nose drops, reducing AoA and promoting recovery. if the tailplane stalls first then the nose drops, promoting recovery.
With a canard layout it's very different. It's ONLY safe if the foreplane stalls before the mainplane. If the mainplane stalls before the foreplane then the aeroplane pitches nose-up (or pedantically "tail-down") and drops into a deeply stalled condition with no available pitching forces that would pull the nose down. recovery would need to wait for it to drop into a tailslide which (hopefully) would then yaw around into a nose-down condition from which it could recover. This will take a LOT of height. To minimise the risk of this condition a canard must be restricted such that it can never get the mainplane close to Cl(max), and as a result a canard needs to carry more wing for a given loading and landing speed, so the drag is higher and the efficiency is less.
There are other, lesser, issues. The foreplane creates a downwash, and it must not be allowed to impinge on the mainplane because it changes the local AoA and stuffs up the efficiency of what should be the most efficient lift-generating area of the wing (the inner wing area). To avoid this you need vertical seperation, which either needs the foreplane to be about eight mainplane root chords higher than the mainplane, or about two below it. The former isn't achievable because of the effect it would have on forward keel area, and the latter is only achievable if the mainplane is mounted like the tailplane of a C5A or VC10. Aside form the weight penalty of a fuselage structure stiff enough to mount the mainplane up there, the result would place fuel tanks and engines a long way off the ground with consequent issues for refuelling and engine maintenance, never mind the required deep maintenance hangar design.
Finally, it's true that a canard generates lift, but you don't really want to generate much of the overall lift with a smaller surface because compared to the mainplane it either has lower aspect ratio, short chord, or both. And that means that the lift it develops comes at a much higher (drag) price than adding the same area to the mainplane would achieve - the same reason why biplanes fell from fashion as soon as we learned how to make cantilever wings that didn't need the extra moment of inertia available from braced-truss multiwing layouts.
There are other reasons, but those are the main ones.
Also "blended wing" configs are very popular as university student projects, but they're not really suitable for line service. One reason is that there's nowhere to put the fuel, but the main reason is that you can't design a "family" (like the A318/319/320/321) as there is nowhere to easily "stretch" then to adapt them for different mission profiles.
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