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Aero-Tips 12.11.06
An airplane that is in rig is one which, in cruising flight,
presents the straightest alignment between relative wind and the
airplane's longitudinal axis. That is to say, a well-rigged
airplane will "fly straight" without unusual control inputs.
What is rig?
Airplane rig is the sum total of the way it was put
together-whether the wings and tail were mounted precisely to
specifications, flaps and primary control surfaces were hung
correctly, and the landing gear struts are properly attached and
inflated (this last item is, of course, negated in most flight
regimes in retractable-gear airplanes).
Why is rig important?
Airplane rig is important for a couple of reasons:
- Reduced need for compensating trim increases airplane
performance
- Lack of need for active pilot input reduces pilot workload and
fatigue
Let's say you're flying an airplane that's a little out rig-for
instance, one wing was mounted a millimeter off square. In cruise
flight you'll need to compensate for this out-of-rig condition with
trim. Compensating trim corrected for an out-of-rig condition, but
it does so by deflecting a trim mechanism, and therefore a control
surface, into the slipstream. Now the airplane "trims up" properly
(i.e., it flies closer to "hands off"), but extra aerodynamic drag
created by the deflected control reduces airspeed.
If the pilot does not (or cannot) trim off the pressures
resulting from an out-of-rig condition, he/she will have to
manually overcome the condition with control inputs. This will
still result in reduced performance from control deflection, but
additionally robs part of the pilot's attention (increasing
workload and reducing situational awareness) and adds to fatigue if
this constant input is necessary throughout the duration of a
flight.
Other rig factors
Other factors besides airplane construction can affect rig. Have
ground-adjustable trim tabs (those flat pieces of metal on the
trailing edge of many control surfaces) been bent out of position?
If the airplane asymmetrically loaded (more fuel in one wing than
the other, or a lot of the passenger or baggage load on one side of
the cabin)? These factors will affect rig as well. When assigning
passenger seats or loading cargo, consider the left-right loading
as well as weight-and-balance loading along the longitudinal
axis.
Aero-tip of the day: Know and account for the
performance, workload and fatigue effects of an out-of-rig
airplane.