Vibrations

CHAPTER 5

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.

VIBRATIONS

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

hub.

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

these vibrations.

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