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> Rudder vs. Aileron Shallow Turns
[Courtesy of Dan deVries soarhead "at" iserv.net
and the Michigan Soaring League Newsletter, May 1999]
As a full scale sailplane pilot as well as R/C, I can tell you that anything less than
"coordinated" aileron/rudder flight is terribly inefficient! Yawing the fuseage
(which has an absurdly poor L/D) causes enormous drag compared to ANY bank angle. Sink
rate indicated on the cockpit vario almost doubles with skidding or slipping flight. Think
about it -- yawing (or
slipping) is the way you substancially increase your sink rate for landing approaches in
"real" planes without flaps like Cubs or Champs.
The extra wing loading due to banking isn't nearly as great as you suspect.
The sink rate increases:
or for 15 degrees: 1/COS 15 = 1/.966;
or 1.035 x level flight sink rate ( only a 3-1/2% increase!)
or for 30 degrees: 1/COS 30 = 1/.866;
or 1.155 x level flight sink rate ( only a 15% increase!)
or for 45 degrees: 1/COS 45 = 1/.707;
or 1.414 x level flight sink rate ( only a 41% increase!)
The ability to stay within the strong lift near the center of the thermal offsets the
slight sink rate increase with TRENMENDOUS relative rate of climb. The important part --
MOST IMPORTANT -- is to fly CENTERED in the
rising column of air.
The need for "top" aileron or rudder in steep turns does,as you suggest, relate
to the glider passing thru effectively a circular or conical flow. This effect is most
noticeable at bank angles over 45 degrees and
otherwise has little significance on sink rate.
Keeping the plane aligned with the airflow is of most importance in minimizing sink rate.
Others may want to add or critique this commentary. Feel free!
DEVRIES DESIGN, Inc.
Product Design --+-- Patent/Litigation Graphics
soarhead "at" iserv.net
>Which is a more "efficient" method for somewhat shallow turns, a normal
>aileron/rudder/elevator input, or using a bit more rudder, a bit of
>ailerons to reduce the bank, and a bit less elevator (since the wings are
>more level)? It seems to me that the trade offs are: "normal" turn:
>elevator drag, and induced drag from the extra wing loading during a
>banked turn; "cross-controlled" turn: drag from the ailerons,
and drag from
>the fuselage since there's a "skid" condition.
>In tighter turns, such as trying to stay in a thermal, I've seen some F3J
>gliders use a pretty heavy bank angle (about 20 degrees), and still
>climb up fairly effeciently.
>I've also read that in tight turns, dynamics come into play; a
glider in a
>turn is effectively "rotating" about it's CG, in effect swinging the tail
>(so that the air flow direction at the tail is a bit sideways compared to
>the direction at the nose/wing), so you need to feed in some rudder, and
>then a bit of opposite ailerons to make such a tight turn coordinated
>(to reduce the excess bank caused by rudder and the slower moving