HELICOPTER AERODYNAMICS WORKBOOK
cyclic control available for rotor tilt. These limits are reached sooner with a downslope wind
condition. Extreme lateral CG loading on the upslope side of the aircraft will further restrict the
amount of controllability.
The final phenomenon we will discuss deals with helicopter vibrations. Vibrations of low
magnitude are inherent in helicopters. It is important one have the ability to identify the type of
vibration should it become excessive. It is important to note sources of vibrations can only be
from rotating or moving parts. Other parts may vibrate sympathetically with these rotating or
moving parts, but may not be a source.
Helicopter vibrations are classified into three categories: low, medium, and high frequencies.
Low frequency vibrations are the most common and originate from the main rotor. The
frequency beat can be either one or two frequency beats per revolution. "One per" revolution
vibrations can be classified as vertical or lateral. The source of one per vertical vibrations is the
main rotor in an out-of-track condition. This occurs when one blade develops more lift than the
other blade at the same point of rotation in the rotor disk. These vibrations are felt through the
airframe as a vertical bounce and can be corrected by maintenance personnel.
Lateral one per vibrations are also caused by the main rotor system due to an imbalance in
the main rotor from either a difference of weight between the blades (spanwise imbalance) or a
misalignment of the blades (chordwise imbalance).
Rigidly controlled manufacturing processes nearly eliminate differences between the blades.
These minor differences do affect the vibration level and are correctable by adjusting the trim
tabs, blade pitch settings or small balance adjustments. Imbalances can also occur in the rotor
Two-to-one vibrations are inherent in two-bladed rotor systems. A slight two-to-one
vibration will be felt in the TH-57 during normal flight operations. A noticeable increase in
vibration is an indication of a worn rotating control part.
Medium frequency vibrations have a frequency of 4 to 6 beats per revolution and are also
inherent in helicopters. An increase in normal medium frequency vibrations can be caused by a
change in the aircraft's ability to absorb normal vibrations, or by a loose aircraft component
vibrating sympathetically with the rotor system. A rattling in the aircraft structure indicates
High frequency vibrations are characterized by a frequency too fast to count and are felt as
a "buzz". High frequency vibrations are always present and sometimes difficult to determine
when they become abnormal.
Sources of high frequency vibrations can be anything rotating or vibrating at a speed equal to
or greater than that of the tail rotor. Common sources are the tail rotor, engine, drive shaft, and
5-12 FLIGHT PHENOMENA