aerodynamic vortex lattice or panel
method program. Assuming that the
canard aircraft’s neutral point location is known, placing the CG ahead
of it for stability will result in a CG
well forward of the 25 percent MAC.
It’s pretty straightforward to determine the load sharing between the
wing and horizontal tail or canard
required to trim an aircraft once the
CG location is known. Figures 1 and
2 show how the load sharing requirements differ between the two configurations. Each figure compares the
percentage of canard or horizontal tail
area (in terms of the combined area
of the wing plus the auxiliary surface)
to the percentage of total lift that the
surface needs to keep the aircraft in
trim. The estimated trends in both figures were generated using a computer
vortex lattice program and include the
following assumptions:
ate a moment that tends to twist
the wing’s leading edge down and
trailing edge up. This nose-down
moment is reacted by the horizontal
tail and reduces the amount of lift it
provides. Most airfoils used on light
aircraft have a CM between 0 and
-0.1 (flaps up), which becomes more
negative with flaps down.
Depending on the horizontal tail
size and wing pitching moment coefficient, Figure 1 indicates that a conventional aircraft can have a slight
lifting load in flight. For those cases,
the CG is behind the wing’s 25 percent MAC, and consequently the
horizontal tail has to provide a lifting
load to trim the aircraft. Moving the
CG forward increases the static margin and has the effect of shifting the
curves down in the figure, and it may
result in a zero or slight downward
load on the horizontal tail.
One of my initial assumptions was
that the wing and either horizontal
or canard have equal aspect ratios.
This is normally not the case for
aft-tail configurations, but they were
kept equal for this study to keep the
number of variables to a minimum.
Using a lower aspect ratio for the
horizontal tail results in the same
general trends, but with the curves
shifted down some.
• CG located 10 percent MAC
ahead of the neutral point.
• Wing and horizontal tail/canard
have an aspect ratio of 7. 5.
• Tail arm equals 2. 5 times the
wing MAC.
• No fuselage or power effects.
Figure 1 shows the trends for
a horizontal tail configuration, and
several observations can be made.
The first is that the percentage of
lift the horizontal tail provides goes
up with increasing tail size. This is
because increasing tail area moves
the neutral point farther aft from the
wing 25 percent MAC. And since I
kept the CG a constant 10 percent
MAC ahead of the neutral point, this
means that the CG moves farther
away, too. The farther the CG is from
the wing’s 25 percent MAC, the greater the load the auxiliary surface needs
to provide. Though the trends are for
a large range in tail area percentages,
most “conventional” aircraft have
horizontal tail areas in the 15 percent
to 20 percent range. Consequently
we will focus on that range for the
other observations.
The second observation is that
the horizontal tail load is affected by
the magnitude of the wing’s pitching moment. Most airfoils have a
negative pitching moment coefficient (CM), meaning that they cre-
Figure 1. Approximate aft horizontal tail lift requirements.
Figure 2. Approximate canard lift requirements.
