Aileron to rudder mixing can
somewhat reduce the maximum sideslip angles during these maneuvers, but it can
never completely eliminate them. The reason is that the required rudder deflection
is not in general proportional to aileron deflection. In fact, they frequently
must go in the opposite directions(!), as in the case
of a slow tight sustained turn.
Flight path geometry and
Figures 1a and 1b show the
geometry of steady turning flight, with and without rudder deflection. In each
case, the vertical tail tries to yaw the glider so
that the apparent wind at the tail lines up with the tail's zero-lift line. In
other words, the vertical tail seeks its zero-lift position.
In Figure 1a, no rudder deflection
is used, resulting in the tail "sagging" inside the glider's flight
path, and a sideslip is present. In Figure 1b, the rudder is deflected such
that the sideslip is eliminated. The tail now rides outside of the glider's
path, giving the illusion of skidding (negative sideslip), although the wing in
fact sees zero sideslip all along the span. This is likely to be the
lowest-drag flight orientation.
A simple experiment
As a motivation for learning
effective ruddering, first do a simple flight
experiment in calm air, sketched in Figure 2.
Such sideslip angles and
resulting performance losses are typical in aggressive thermalling
maneuvers at slow speeds if the left thumb is asleep. In turbulent thermals it
is not uncommon to occasionally see sideslip angles of 30 degrees or more, with
each such sideslip excursion resulting in considerable altitude loss. Skillful ruddering will prevent these from occurring.
Rolling into a turn
To prevent the appearance of
sideslip in a turn like that shown in Figure 1a, it is necessary to coordinate
the rudder input appropriately with the ailerons. The example here, calculated
with a simulation program, shows the control inputs required for one specific
Discus-Launched Glider, flying at a particular weight and airspeed, and
reaching a specific bank angle. The required control inputs for another set of
conditions or another type of glider will of course be different. The control
input values shown are intended merely to be representative to allow
visualizing what's going on.
Figures 3a and 3b show
turn-entry maneuvers for a DLG in a fairly slow glide at 11 mph, CL = 0.7,
close to the minimum-sink speed. In each case, right aileron is applied,
ramping up from 0 to 10 degrees over a 0.5 second interval, and then
immediately ramped back down to 0 degrees over the next 0.5 seconds. The glider
reaches a 30 degree bank angle over the entire 1.0 second interval. Consider two different rudder actions
during this roll entry maneuver:
a) 1:1 Aileron to rudder mixing, left thumb asleep.
In this case the right rudder
exactly follows the right ailerons in the ramp up and ramp down over the 1.0
second interval. Figure 3a shows the resulting glider motion. Note that a
severe sideslip develops during the ramp-down as the slaved rudder is
(incorrectly) neutralized along with the ailerons. The large sideslip is
sustained throughout the subsequent steady turn.
b) 1:1 Aileron to rudder mixing,
left thumb applies proper rudder deflection.
In this case, substantial right
rudder is independently applied by the left thumb as the intended bank angle is
approached and the ailerons are neutralized, as shown in Figure 3b. This right
rudder is then sustained throughout the subsequent steady turn, producing a
minimal slideslip angle. Note that this rudder action
cannot possibly be achieved with any kind of mixing. A trained left thumb is
Sustained banked turn
a) Left thumb asleep.
The last aircraft position in
Figure 3a shows the 13 degree sideslip which results when no rudder is held
during the turn.
The slight aileron deflection
required to sustain this steady turn depends on the amount of sideslip and the
dihedral angle (zero aileron deflection is shown). With little or no dihedral,
some opposite aileron must be held to maintain the bank angle and prevent
rolling into the turn. With generous dihedral, some rolling moment is already
provided by the sideslip/dihedral combination (just like in a poly glider), and
the required held aileron deflection may be either into or out of the turn.
b) Proper ruddering
into the turn.
The last aircraft position in
Figure 3b shows the control deflections required for a slow, tight, steady
sustained turn without sideslip.
The lack of sideslip means that
some opposite aileron must be held to prevent rolling further into the turn,
regardless of the amount of dihedral. In fact, one way to discern if a
sufficient amount of rudder is being held (in addition to observing the
sideslip) is to note the average aileron stick position which is being held to
maintain the steady bank. Adjust the rudder until the ailerons must be held
slightly opposite. This will produce efficient nearly zero-sideslip circling
Effects of airspeed
How much rudder input is
required depends considerably on the flight speed. In general, the higher the
flight speed, the less rudder action is required. The examples in Figures 3a
and 3b are for a slow thermalling turns. In contrast,
the same glider in a fast upwind glide will require little or no ruddering.
Other types of gliders
DLGs have unusually long tail arms relative
to their turning radius, and hence require considerable rudder input in steady
turns. This in fact makes them excellent left-thumb trainers, because the
sideslip resulting from poor ruddering is relatively
easy to see.
Large TD gliders are likely to
have short tails relative to their turning radius, and also a smaller vertical
tail volume. The main consequences of this are: