Why aren’t there any lifting-canard airliners?

Question

A lifting-canard aircraft, such as the Long-EZ, is an aircraft with the main wing at the back end of the fuselage and a pair of small, highly-loaded canards attached to the forward fuselage; the canards fly at a higher angle of attack than the main wing, and, consequently, provide a significant amount of lift in addition to pitch control.¹

During a sharp pitchup in a lifting-canard aircraft, the canards, being at a higher angle of attack than the main wing, stall first, causing the aircraft to automatically pitch down before the wing ever stalls. As the wing, with the ailerons on it, remains unstalled throughout, roll control is preserved, without any of the violent rolling inherent in stalled control-tail aircraft.² As the canards are stalled, pitch control is lost until they unstall, but, in this case, you don’t need pitch control, because the aircraft pitches down and recovers all on its own, without the need for any manual control inputs;³ this actually has the benefit of making lifting-canard aircraft practically unstallable (for the main wing, anyways) barring the separation of large parts of the airframe. In addition, as the surface providing most of the aircraft’s lift never stalls, the overall loss of lift when the canards stall is fairly minor, and the aircraft’s handling characteristics remain benign throughout.

In contrast, while a control-tail aircraft will usually also pitch down when the main wings stall, this is dependent on the stabiliser trim setting, and, if the center of mass is near the forward limit (necessitating considerable nose-up stabiliser trim), the aircraft may actually pitch up in a stall. Also, if the aircraft does pitch down, it does so quite violently, as the majority of the aircraft’s lift is suddenly lost, and with large, violent, largely-uncontrollable roll oscillations (as the ailerons are located on the main wing, which is stalled). And, because the surfaces used for pitch control are not yet stalled, pitch control is preserved, allowing the pilots to hold the aircraft in the stall.

Given the considerable safety advantages of the lifting-canard configuration, why don’t we see any lifting-canard airliners?

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¹: As opposed to a control-canard aircraft, such as the Flyer, where the canards fly at a nominally-zero angle of attack, are used only for pitch control, and make these aircraft utter beasts to fly without a computer making constant control inputs.

²: Aircraft with a tail-mounted horizontal stabiliser and elevator which (nominally) fly at a lower angle of attack than the main wings. Examples include most aircraft ever.

³: In contrast, control-canard aircraft pitch violently up as they stall (because the main wing, being at a higher angle of attack than the canards, stalls first), and, therefore, require aggressive nose-down control inputs to recover before the canards stall as well and pitch control is effectively lost; lifting-tail aircraft (with a tail-mounted horizontal stabiliser and elevator that fly at a higher angle of attack than the main wings) also pitch up, for the same reason, except that this happens well before the main wings stall, and is coupled with a simultaneous, near-total loss of pitch control as the horizontal tail stalls (making stalls completely unrecoverable for lifting-tail aircraft using only the usual aerodynamic surfaces for control).

Answer 1

Sean I have no idea where you got the idea that the horizontal tail on a conventional aircraft lifts up, like a canard surface but at the opposite end, and can therefore stall and let the nose rise. It's exactly backwards.

The tail lifts down. When the airplane stalls, the center of lift of the main wing shifts sharply aft creating a strong nose down pitching moment that the horizontal tail lacks authority to counteract until the plane speeds up, and the nose drops. If the down lifting tail surface itself stalls, that is a disastrous situation and is abnormal for any aircraft.

Canards generally suck because they have a host of limitations that negate most of the theoretical benefits and that's why they have been commercial failures. Otherwise, you'd see lots of them.

Answer 2

One good reason, is that airliners have main wings with flaps galore dropping down to increase the coefficient of lift at slow speeds. This also greatly increases the nose-down pitch moment, which must now be countered by the already highly loaded canard. Canard stall then becomes the limiting factor on approach speed. You either need to have a less loaded canard (there goes your natural stability) or flaps on it, or some other aerodynamic kludge.

Answer 3

None other than the great Clarance Kelly Johnson had similar thoughts in the 1930s, before testing validated the advantages of rear mounted stabilizers.

The Achilles heel of canard designs was that once relative wind shifted to beneath the aircraft at high AoA, the lower surface of the canard acts as a lever to push the nose up even further. The US Army Air Force Ascender required additional area to be added behind the CG to counter act this tendency. Secondly, a loaded canard essentially makes the airplane a less efficient biplane.

Let's also clarify one very important basic aspect of forward set CG and downforce on tail design. As the plane loses speed, the forward CG pitches the nose DOWN. As the plane gains speed (increasing aerodynamic forces) the elevator trim pitches the nose UP.

Notice with a properly designed rear mounted horizontal stabilizer (also validated by around 150 million years of bird evolution) the forward wing also stalls first. It would serve us well to review the design of the horizontal stabilizer/elevator/trim system.

Firstly, a review of tail force created at various AOA. Yes, at lower AOA the tail creates DOWN force to balance main wing lift force and CG. What happens at a higher AOA (even without change of trim or elevator input) ?. The tail force of a properly designed horizontal stabilizer of ADEQUATE AREA should begin to generate UP force BEFORE the plane stalls, to compensate for the forward shift of the wing center of lift and help push the nose down. Of course control inputs help this, excessive ones should be unnecessary.

Notice the canard design is nothing more than a tiny wing and a giant tail. This is why deltas (lots of rear area, stall at higher AOA) go so well at the back end with canards up front, which also makes a successful supersonic design.

But in the fuel efficient high subsonic realm, planes will probably look "the same" for a while, just like birds. But it may be good to keep working on that tail.