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I am designing a UAV and have settled on a canard design. However, I’m curious whether it would be helpful to have a typical horizontal stabilizer behind the wing as well as a canard wing in front of the main wing to increase lift towards the front of the plane and for better stablity. Is this a bad idea? I’m looking to carry cargo ranging from 0-1000g, and initially fly for around 30 minutes. It is also using a special VTOL system I’ve designed.

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    $\begingroup$ Hey Awesome, could you maybe add some info on the mission profile of your UAV? $\endgroup$ – rul30 Dec 10 '17 at 17:46
  • $\begingroup$ Are you carrying weights/cargo so the center of gravity would shift during the mission? $\endgroup$ – rul30 Dec 10 '17 at 17:46
  • $\begingroup$ I made some info changes! $\endgroup$ – Tristan Semmelhack Dec 10 '17 at 18:19
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    $\begingroup$ If you optimize for extreme efficiency you should avoid generating any lift, positive or negative, except the main wing, because only the main wing has the highest lift to drag ratio. The reason canard has the advantage of lower drag under some conditions is because at super sonic speed, center of lift shifts backwards so much that the leverage of aft elevator is too short which calls for a very large negative lift on the elevator. Your plane won't fly supersonic so it won't take advantage of that. $\endgroup$ – user3528438 Dec 10 '17 at 19:36
  • $\begingroup$ Oh! Thank you. The canards are necessary because I need to be able to put things, most notably the vtol system, in front of the wing. Canards will allow the cg to go out much further. But I Definitly see your point. $\endgroup$ – Tristan Semmelhack Dec 11 '17 at 4:35
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Actually, for most tasks it would be most helpful to add the horizontal tail and to remove the canard. There have been only a few canard designs and even fewer three-wing designs in the past. Off-hand I remember just two:

Voisin-Farman biplane in the Musée d'l Air in Paris

Voisin-Farman biplane in the Musée d'l Air in Paris (picture source)

Gabriel Voisin was the founder of the first commercial aircraft factory and designed a biplane with forward elevator and aft stabilizer in 1907 for Léon Delagrange and Henry Farman. He had chosen a similar layout for his very first glider design in 1905 already, most likely because the question where the elevator goes on an airplane was a matter of hot disputes at those times.

Piaggio Avanti in flight

The second one is of course the excellent Piaggio P.180 Avanti (picture source), where the highly loaded canard allows to move the wing behind the pressurized cabin and the lightly loaded elevator provides ample control authority at all flap settings and center of gravity locations.

If you don't expect to have a wide center of gravity range in your UAV, use fewer wings. One big one for the lift and a second one for stability and control should be all you need.

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    $\begingroup$ There are many variants of the SU-27/30/35 family which have both canards and tail planes. The SU-30MKI is an excellent example. $\endgroup$ – Moo Dec 11 '17 at 1:18
  • $\begingroup$ Interesting! The reason I need the canards is I need to balance out the vtol rotor system and payload both of which are forward of the wing. The vtol config won’t work if it’s behind the wing for various reasons, so I need the canards to help allow me to move the cg forward. $\endgroup$ – Tristan Semmelhack Dec 11 '17 at 4:34
  • $\begingroup$ @Moo: Yes, good addition - I was probably too focused on civilian planes. $\endgroup$ – Peter Kämpf Dec 11 '17 at 8:19
  • $\begingroup$ @PeterKämpf sorry I didn’t tag you on my comment ^^ $\endgroup$ – Tristan Semmelhack Dec 11 '17 at 12:32
  • $\begingroup$ @TristanSemmelhack: No problem, if you answer the author of a question in a comment, he/she gets an automatic notification, even without tagging. $\endgroup$ – Peter Kämpf Dec 11 '17 at 16:45

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