Frise ailerons on sailplanes

The use of spoilers as roll control on a sailplane has two major drawbacks
compared to ailerons with adverse yaw. These comments are based on flight
test of a roll spoiler equipped flying wing glider.

First, the relationship of drag to reduction in lift produced by the roll
spoiler changes dramatically with airspeed. There is only one airspeed
where the drag and roll produce a perfectly coordinated turn entry.
(Remember that drag increases with the square of airspeed.)

At airspeeds above the coordinated airspeed, the spoiler will produce too
much drag and the turn entry will skid unless outside rudder is used. This
is called proverse yaw.

At airspeeds below the coordinated airspeed, the roll spoiler will not
produce enough drag and the turn entry will be a slip unless into-the-turn
rudder is applied. (Adverse yaw)

The second major problem with roll spoilers is during a steady turn where
one would normally use top aileron to oppose overbanking. If the pilot
attempts to oppose overbanking with roll spoilers, the drag of the spoiler
will cause the glider to yaw away from the turn. The pilot will then use
into-the-turn rudder to center the yaw string which will cause the
overbanking to resume unless still more top spoiler is used. This will
quickly progress to the point where full top spoiler and full into-the-turn
rudder is applied. This is hardly conducive to a low sink rate.

The lession here is that adverse yaw is actually needed for steady state
turns. In a turn the airspeed and angle of attack varies across the full
span. This results in the outside wing having more drag and lift than the
inside wing causing overbanking. Top aileron restores spanwise symmetry of
lift and drag neatly taming the overbanking.

Adverse yaw is also useful for crosswind landings. Into the wind aileron
produces a down wind yaw that helps align the fuselage with the direction of
flight.

Another way to look at it is that differential spoilers produce a control
response that is not unlike the rudder. If the glider is equipped with both
rudder and roll spoilers it has, in effect, two rudder systems and therefore
lacks true 3-axis controls.

That said, differential spoilers actuated through the rudder control curcuit
can significantly augment a weak rudder.

Bill Daniels