If you're going to design a canard (or a three surface plane), what are the pros and cons of having the canard a) lower and b) higher than the main wing?

From : Don Stackhouse

First of all, for the record, I am not a big fan of canards. From what I've seen, they're a lot like pusher props, they appear to have advantages when you look at them in generalities, but in the vast majority of cases those advantages disappear or become disadvantages when you get into the details of a specific design. And yes, I do have a pretty fair amount of experience with canards and with pushers to back up this statement. In fact, among other things I have the beginnings of a full-scale Rutan VariEze sitting out in the barn, collecting dust. It will continue to sit out in the barn and collect more dust for at least the foreseeable future.

The big problem with canards, aerodynamically speaking, is downwash. A wing makes lift by grabbing chunks of air and shoving them downwards. Whenever you have a flying surface making lift, the air behind it is moving downwards. We call this "downwash". If the flying surface is making negative lift, such as most cases of conventional tails in positive "G" flight, then it is accelerating the air upwards.

The big argument trotted out in favor of canards is that both surfaces are lifting upwards, so the canard isn't opposing the wing, and therefore the system is supposedly more efficient than a conventional horizontal tail that is lifting downwards and supposedly fighting the wing.

In actual practice it doesn't really work this way.

It's possible to design a positively stable conventional tail arrangement that requires very little or no lift at all from the aft-mounted tail. Since the tail is making very little downwards lift (in some cases it could even make a small amount of upwards lift, although that really isn't very efficient either), it isn't opposing the wing significantly. In addition, the downwash from the wing actually helps the horizontal tail do its job more efficiently. Yes, its downward lift does oppose the wing, but the overall net effect on the airplane's total induced drag is relatively small.

The opposite is true for a positively-stable canard. To have positive static stability in pitch, the "wing loading" of the canard generally has to be somewhat higher than that of the wing; in other words, the canard needs to be doing a little more than its fair share of supporting the total weight of the airplane. Unfortunately, the canard is smaller than the wing (if it wasn't, it would not be a canard), and therefore it has either a smaller span or a smaller chord (and therefore lower Reynolds numbers) than the wing, or both. Because of this it is less efficient at producing lift than the wing. Any work you take away from the more efficient wing to give to the less efficient canard makes the airplane's overall efficiency less, and there's quite a bit that you must give to the canard to make the airplane statically stable.

Worse yet, the canard can only make that lift by causing downwash. This means that the wing is now flying through a continuous downdraft created by the canard.

The net result is that with regard to the total induced drag, for a given amount of static pitch stability a canard layout is generally at best about equal to an equally well designed conventional tail.

With regard to the effects of mounting the canard higher or lower than the wing, the downwash field from the canard extends quite some distance (about half the canard's span both above and below is a good approximation) above and below the canard, so it's not practical to get the wing completely out of the canard's downwash. However, a low-mounted canard will be better in this regard than a high-mounted one. It's sort of an extreme case of a tailless negative stagger biplane where the vertical separation between the two wings isn't unusual, but the amount of stagger is huge. The end result is that the worst of the low-mounted canard's downwash passes below the wing, while that of the high-mounted canard runs directly into the wing. Sketch the downwash of the two flying surfaces on a side view of your airplane and you'll see what I mean.

There are some stability effects involved with vertical positions of the wing and canard, but they tend to be very dependent on the details of each individual design in question. It's dangerous to try to make any sweeping generalizations about that particular issue.

However, from a structural durability standpoint, canards tend to be relatively fragile compared to the wing, and they're way out in front where they will be the first thing on the airplane to reach the scene of an accident. On models, where we tend to see a lot of dings, etc. from rocks, clumps of grass, etc. in the landing area, a high-mounted canard in most cases probably does a better job of avoiding these obstructions. However, it's still not likely to be as good in this regard as an aft-mounted tail, especially an upright V-tail or a T-tail.

There's a lot more discussion of these and related issues in the "Ask Joe and Don" section of our website.

Don Stackhouse
DJ Aerotech