AOPA Pilot Magazine - October 2019 - 82

P&E OWNERSHIP
the horizontal tail located above the wing,
the effect of the wing's wake [downwash]
on the tail is minimized, allowing the tail
to be smaller. Sweeping the vertical tail
produces a much longer horizontal tail
arm than would be possible with a fuselage-mounted or cruciform tail." As far
as drawbacks, he says a T-tail makes for
a heavier vertical tail structure and generally moves the horizontal tail out of the
propwash for piston aircraft.
What was the initial motivation for the
V-tail design in the Bonanza?
"A reduction in interference drag at the
vertical-tail/horizontal intersection and at
the vertical-tail/fuselage intersection. A
horizontal-tail/vertical-tail configuration
has three intersections, whereas a V-tail
configuration has two. The drawback is
that the V-tail degrades Dutch-roll damping," he said.
What determines the size of the rudder in a tail? Speed? Weight?
"The rudder and vertical tail are sized
together to produce the right combination of stability and controllability, which
are counter requirements. On a singleengine airplane, crosswind control is a
primary design driver. On a multiengine
airplane, one-engine-inoperative control
is the primary design driver. In both cases,
the rudder must be powerful enough to
meet the design requirements, but if it is
too powerful, it can generate high sideslip angles that can stall the vertical tail,"
Hardin said.
Once the flying is safe, there is art
in tail design. The rounded top to the
Staggerwing tail was a stylistic decision, as was the move from a straight
tail to a swept back version in the
Cessna 150/172/182 models in the 1960s.
According to Hardin, a more angular
design makes the tail more efficient and
thus allows it to be smaller. Yet the sweptback design actually lowered directional
stability by a small amount. In the 1970s,
Cessna tested a T-tail version of the 182,
but, he said, "company pilots disliked the
loss of elevator authority that resulted
from moving the horizontal tail out of the
prop wash."
Perhaps the most recognizable tail
in the sky belongs to the Mooney lineup.
82 | AOPA PILOT October 2019

Instead of swept-back, it appears sweptforward. In truth, though, the leading
edge is vertical and the trailing edge
narrows toward the top. The airplane is
fast and the tail makes it look that way.
According to Kevin Hawley, chief engineer at Mooney International, the reason
for that design was more practical than
exotic. He says, "The main point of the
straight leading edges on the wing, horizontal, and vertical stabilizers was that

Once the flying is
safe, there is art in tail
design. The rounded
top to the Staggerwing
tail was a stylistic
decision, as was the
move from a straight
tail to a swept back
version in the Cessna
150/172/182 models in
the 1960s.

it was easier to wrap the surfaces with
wood (plywood). The M18 and the first
M20s had wood wings and...horizontal
stabilizers. Some people will probably
say it had to do with vertical moment arm
at high angle of attack, but that is an old
wives' tail."
Mooney also sports what it calls an
all-trimming tail. "The all-moving tail for
trim is all about one thing. Al [Mooney's]
favorite topic: performance. It was a
cleaner design and ultimately results in
less trim drag. Angle of attack is more
effective than control surface deflection
in terms of aerodynamic force produced,"
Hawley said.
Of course there are subtleties and
nuances and airflow math involved in tail
design. For example, to counteract things
such as P-factor and spiraling slipstream,
a vertical tail is often slightly offset from

the airplane's centerline. Tabs influence
airflow. And just like propeller shapes are
now taking things such as noise reduction
into their design goals, the future of tail
design involves much more than pitch
and yaw.
At Zenith, when they hold workshops
to introduce people to building their own
aircraft, they begin with the tail. Heintz
said, "We like to start on the rudder tail
section for a number of reasons: First
of all, building the rudder is a relatively
small project that can readily be completed in less than two days from start to
finish, making it a good project for the
workshop, while at the same time the
rudder assembly introduces the builder
to all the elements of the airframe construction. The rudder assembly is similar
to a wing assembly: The assembly is made
up of a main spar, with rear ribs and a
nose rib, and the internal structure is
then 'covered' by the skins-both a nose
skin and a rear skin. Building the entire
rudder assembly is easily accomplished
in a couple of days, but importantly,
introduces the new builder to all the
tools and skills needed to complete the
rest of the airframe, including reading
and understanding the blueprints and
assembly instructions, workshop space,
et cetera."
And there is the imagination of future
design. Again according to Hardin, "One
of the biggest challenges for mechanical control systems is achieving pleasing
flying qualities as airplanes continue
to get bigger and faster, and regulatory
requirements become increasingly more
stringent. The challenges lie in achieving the necessary stability while keeping
control forces sufficiently low and maintaining good control feel throughout the
range of control motion. This requires
appropriately sized tails and control
surfaces with carefully designed and
tailored aerodynamic balances. In the
distant future, fly-by-wire and powered
flight controls may be within reach of the
smaller GA aircraft, and that will change
everything."
AOPA
W. SCOTT OLSON is an aviation writer and

private pilot living in Moorhead, Minnesota.
AOPA Pilot Magazine - October 2019

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