Canard Efficiency Myths Canard owners like to claim their aircraft are more efficient than conventional designs.  Their technical argument is that the forward wing (canard) creates lift rather than a downforce like conventional tails.  The downforce on a conventional tail must be carried by the wing, so for aircraft of equal weight, canards incur less induced drag.  Enthusiasts point to unbroken world records set by the Vari-EZ and Long- EZ to justify their position.  The non-stop, unrefueled, round-the-world flight of Voyager in 1986 is another example of canard efficiency, they say.  They also point out that canard aircraft cruise faster than similar conventional designs with the same engine.  Canard detractors (aka aerodynamicists) claim that canards are not efficient at all.  They point out that most canard aircraft can’t use flaps to lower their stall speed, and the main wing never reaches its maximum lift coefficient because it is not allowed to stall.  For a given payload and landing speed, canard aircraft require larger wings and have higher drag.  They point out that even Burt Rutan moved away from canards, as evidenced by the Boomerang and Global Flyer, which have conventional tails. So who’s right?  Several studies by NASA and AIAA reached the same conclusion: When mission requirements like payload mass, cabin volume, takeoff and landing distance, engine power, construction materials, etc. are equally enforced, conventional aircraft are more efficient than canards.  These studies highlight the aerodynamic disadvantages of canard aircraft, which include: • Angle of attack on the wing must be limited to prevent a main wing stall (which could be unrecoverable).  This means the wing never achieves its maximum lift coefficient. • Wing flaps create a nose-down pitching moment that loads up the canard and causes it to stall at a higher speed, thus defeating the stall-lowering benefit of incorporating flaps. • The inboard wing operates in downwash from the canard and this reduces the wing’s lift over the affected area. • The canard’s small chord operates at a lower Reynolds Number (RN) than the main wing; which is to say that canards create lift less efficiently than a conventional wing operating at higher RN. • Strakes are usually required to store fuel near the flight CG, but strakes are destabilizing and less efficient than wing area. • Swept wings, as implemented in the Long-EZ and derivative designs, promote spanwise flow that reduces wing lift even further. With all the disadvantages, the reader must wonder why anyone designs or builds a canard aircraft.  And how is it possible that some canard aircraft are faster than conventional designs with the same engine?  The answer requires further explanation of the canard configuration.  For example, the following factors mitigate (but do not fully overcome) the disadvantages of canard aircraft: • The canard carries 20% to 25% of the aircraft’s total weight, so the main wing carries just 75% to 80% of the weight that a conventional wing would.  That reduces the need for a larger wing. • The Long-EZ / Cozy / Apollo configuration is very efficient from a packaging standpoint.  There’s no tailcone, so the extra mass and wetted area (drag) of the tailcone are eliminated! • Winglets on Long-EZ type aircraft serve double duty as vertical stabilizers and drag-reducing vortex controllers at the wing tips. • Homebuilt canard aircraft have short fuselages that approach the optimum fineness ratio for minimizing drag. • The pusher configuration probably has a net performance benefit relative to tractor designs. • Lift reduction from canard downwash incurs minimum penalty at cruise speeds because of the following: 1. The flap-less wing is sized for landing speeds and is literally too large at cruise speeds; the oversize wing has no problem creating adequate lift at higher speeds, even with the downwash. 2. When the canard produces lift, downwash on the wing restores some semblance of the desired elliptical lift distribution for the wing and canard combined. These redeeming qualities allow Long-EZ type aircraft to achieve better performance than their inherent disadvantages would imply.  However, the biggest reason for their apparent efficiency is that the Long-EZ, Cozy and E-Racer are point designs optimized for high cruise speeds.  They use supine seating to reduce frontal area and they have retractable nose gear.  They also have higher stall speeds, smaller cabins, and longer runway requirements than the RV (Vans) series of aircraft.  The canards cruise 10 to 15 knots faster and are more fuel efficient than conventional designs only because they give up performance in other areas.  An optimized conventional aircraft designed to equivalent standards could achieve the same or better results. I’m afraid the evidence is pretty clear from an engineering standpoint.  All the studies conclude that canard aircraft are less efficient when mission requirements are held equal.  Fortunately for us, the difference is small and mission requirements for homebuilt aircraft can be unique.  Canard designs can still be optimized for better performance.  Add the inherent safety of a stall and spin resistant design and most people will understand why canards remain the most interesting aircraft on the ramp! Site Map Email the Designer Copyright © 2012 Apollo Canard