Voyager: unrefueled flight around the world. Image courtesy B. Rutan.
The same stability and trim equations may be satisfied with negative values
of the tail length. Such designs are called "canard" designs.
(Canard is the French word for duck. Some of the early canard designs bore
a strange resemblance to their namesakes. Oddly enough, canard in English
means a gross exaggeration or hyperbole.)
These notes deal with some of the basic analyses required in the design
of canard aircraft. These analyses differ in several ways from those methods
which are usually used in the initial design of aft-tail configurations.
Although several differences are apparent we do not address in detail here
the general question of the relative merits of canard and aft-tail designs.
(See Refs. 1 and 2 for this.)
The large variety of canard designs illustrates that, unlike the case of
aft-tail designs, a consensus on optimal canard size has not yet emerged.
Some of Burt Rutan's canard designs.
One possible explanation for this lies in the fact that aircraft performance
is much more sensitive to canard size than to tail size. The proper canard
size depends strongly on the most critical design condition (e.g. climb
unimportant, high speed cruise critical vs. a long endurance design with
high speeds unnecessary.) Here we consider simple methods of estimating
key performance parameters for a given design. For a particular project,
several canard sizes may be tried to achieve the best compromise.