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Dynamic Balancing
If the main rotor blades are tracking properly, but the helicopter still vibrates
noticeably, the main rotor may not be properly balanced. To dynamically balance the
main rotor without removing it from the helicopter, stick a small piece of blade
balancing tape to one of the main rotor blades and run the main rotor at flight speed. If
the vibration level decreases, the extra weight of the trim tape is improving the
balance. If the vibration increases then remove the tape, stick it to the opposite blade
and spin the main rotor again. Try different sizes of trim tape until you find one that
minimizes the vibration. Repeat this procedure for the
Arlton Subrotor
stabilizer.
Helicopter Stability
Helicopters by their nature are not positively stable. At best, they are neutrally stable.
The concepts of positive stability and neutral stability can be illustrated by placing
marbles in a bowl and on a table top as shown in Fig. 5-3.
He li cop ter sta bil ity
If you nudge the marble in the bowl with your finger, it will roll back and forth and
finally come to rest where it started in the center of the bowl. This is positive stability;
the marble always ends up where it started. If, on the other hand, you push the marble
on the table, it will continue to roll until you stop it, and will sit still until you push it
again. This is neutral stability; the marble stays put until pushed, and keeps moving
until stopped.
Helicopters act like the marble on the table. When correctly trimmed they tend to
remain in one spot until moved, and tend to keep moving until stopped. Unlike the
marble, however, helicopters vibrate and fly in air that is always swirling and rolling.
As a result they do not stay in one place for very long, and require constant small
control inputs to hover over a spot on the ground. The marble on the table would act
more like a helicopter if you shook the table and tilted it back and forth. You would
have to constantly push the marble from different directions to keep it in one spot.
Op er a tor's Guide
Main Rotor Stability and Control
LITE MA CHINES
5-3
Figure 5-3.