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We are currently doing some testing with a stroboscope and "stop motion photography" using a Bergen Intrepid model helicopter. Using a stroboscope capable of over 30,000 flashes per minute (Fpm), we are able to freeze the motions of the helicopter during operation. You can look at the sample videos in this folder: stroboscopic stop motion videos. Illumination of a test article with a strobe light is difficult to capture on film as you will see in the videos. Actually observing the test is much more revealing as human vision is more adept at blending the frames of motion together whereas the camera itself is strobing at a given frame rate and 'confusing' the image created by the stroboscopic flashes.
Helicopters are a happening place when it comes to vibrations. Model helicopters in particular are a difficult case. As contrasted to a full size helicopter such as a Bell 47, Robinson R22, R44 or a Hughes 300, you can't ride along in the seat of a model and feel the movement that results from imbalances in the rotor system or engine.
Typically, the main rotor system will cause vibrations that are equivalent in frequency to the main rotor RPM. Of course the tail-rotor rotates typically around 4 to 5 times as fast as the main rotor so a higher frequency vibration may come from the tail rotor or its drive system. When the tail rotor is the cause of vibration, it will typically cause a buzz in the vertical or horizontal fins which are mounted to the tailboom.
A recent failure of the flybar on one of our helicopters led to a discovery regarding balance of rotating components. The flybar paddles are not mounted so that their chordwise center of gravity is located along the axis of pitch rotation. The result is the same as mounting a small rotating mass at each end of the flybar which cycles according to the inputs from the swashplate.
Another simple way to put it is "put the flybar thru the see-saw, assemble one paddle to the bar, let go, and watch the trailing edge fall, because it is heavy." Even though both paddles are balanced identically on each end of the flybar, they still have a mass that is not at the center of rotation. In the case of the flybar, rotation is really just oscillation between plus and minus ~30 degrees depending on cyclic inputs
Draw a straight line on a piece of paper representing the flybar (see the green line above). Now at each end of that straight line, draw a short line at 90 degrees and put a dot at the end representing the center of mass of the paddle (red markings at an angle to the green line). The lines are on opposite sides of the flybar at their respective ends. Now draw a line from one center of mass to the other (long red line) representing the axis of rotation that the bar will have a natural tendency to rotate upon. It is at a slight angle and passes through the same center (the center of the rotor mast) as the flybar. The flybar's natural tendency would be to rotate around that oblique axis but is forced to rotate inside the bearings.
A wonderful idea would be to place any new helicopter in a dark environment and illuminate it with a strobe light, tuned to specific frequencies of rotational components. In this manner, one could analyze the sources of vibration to determine their root cause.