While the tiny tailwheel pant/rudder of the Quickie in its original form was perfectly adequate in flight, it resulted in demanding
ground handling characteristics. A taller vertical fin and a conventional rudder surface made the Quickie easier to fly and taxi.
The last observation from Figure
1 is that the percentage of lift (either
up or down) on the horizontal tail is
quite a bit less than its percentage of
total lifting surface area. The implication is that the horizontal tail’s lift
coefficient requirements are lower
and historically have been met by
using a symmetrical airfoil equipped
with an elevator.
Figure 2 shows the estimated
load-sharing trends for a canard
configuration. The major difference
from Figure 1 is that the canard
always has a positive lifting load.
As I mentioned earlier, this is because
the canard aircraft’s
neutral point (and
resulting CG location for stability) is
always ahead of the
wing’s 25 percent
MAC. Focusing on
the same 15 percent
to 20 percent area
range we did earlier,
you can see that the
canard would need
to provide roughly 20 percent to
30 percent of the total lift to keep
the aircraft in trim. Moving the CG
farther forward would result in shifting the curves up some, indicating an increase in canard loading.
The canard lift requirements are fur-
ther increased if the wing’s pitching
moment coefficient becomes more
negative due to deploying flaps. This
is one of the reasons flaps are rarely
used on canard designs. The Beech
Starship was one of the few that did
use wing flaps, and it incorporated a
forward-swinging canard to increase
distance from the wing to the canard.
Other designs have used speed brakes
located on the belly of the fuselage to
allow steeper approaches for landing.
The final observation is that unlike
the aft horizontal configuration,
the percentage of canard loading is
always higher than its percentage of
total lifting surface
area.
The canard’s
higher lift requirements mean that
careful attention
must be paid to
the design or selection of the airfoil.
Designers often use
a high aspect ratio
planform for the
canard, because
it stalls at a lower
angle of attack than a comparable
lower-aspect-ratio surface, and you
want the canard surface to stall at a
lower angle of attack than the wing.
The consequence is that the canard’s
chord length tends to be short compared to the average wing chord.
An airfoil’s characteristics are subject
The canard’s higher lift
requirements mean that
careful attention must be
paid to the
design or
selection of
the airfoil.
to scale effects, and aerodynamicists
found that they can quantify those
effects by examining an airfoil at different Reynolds numbers. Reynolds
number is the ratio of the inertia
forces of the air flowing around an
airfoil to the viscous forces of the
air. The inertia forces depend on the
air’s density and speed, whereas the
viscous forces are a measure of the
air’s “stickiness.” At sea level, stan-dard-day conditions, the following
formula can be used to estimate the
Reynolds number of an airfoil:
Reynolds number ≈ 9360 x airspeed x chord
…where the airspeed is in miles
per hour and the wing chord is in
feet. For example, the Reynolds number for a wing on a light aircraft can
range from 2 million at stall speed
to 7 million or more at top speed.
However, a canard’s Reynolds number on a homebuilt can be around
500,000 or so. Normally an airfoil’s
maximum lift coefficient goes down
with decreasing Reynolds numbers.
Rutan initially used the same airfoil
(NASA’s GAW- 1) for both the wing
and canard on the prototype VariEze.
Initial flight testing showed that the
aircraft had a high stall speed, so he
installed a new canard with an airfoil
specifically designed to operate at
low Reynolds numbers. This airfoil
had a rather unwieldy designation of
