Vibration is a natural consequence of flying a helicopter rotor edgewise through the air. High levels of vibration contribute to crew and passenger discomfort, equipment failures and structural problems. So the rule makers have long attempted to write specifications on the amount of vibration that will be allowed. Although these specifications have not always been met, they remain as goals for the ultimate quest of the “jet-smooth ride.”
The human body is not sensitive to all frequencies in the same way. Very low frequencies, such as those of a ship in rough water, cause motion sickness. Increasing frequencies start getting into the range where body parts will shake at their natural frequency — or resonate — causing discomfort. The lowest of these frequencies has been identified as that corresponding to the upper chest organs — the lungs and the heart — bouncing vertically on the diaphragm as a spring. The range of this frequency for most people is from 4 to 7 Hertz (cycles per second).
For the helicopters flying today, this just happens to be the range, which corresponds to a once-per-revolution (IP) oscillating force coming down the shaft caused by an unbalanced rotor or by out-of-track blades. This is why maintaining good track and balance is such an important factor in helicopter operations.
Another possible critical vibration range for humans is at about 12 Hertz. It corresponds to the spine in compression using the pads between the vertebrae as springs. But this is usually attenuated by the characteristics of the seat cushion.
Tests on shake tables have indicated that humans are more sensitive to up-and-down vibration than to fore-and-aft or side-to-side motions.
The literature on helicopter vibration that I’ve consulted also mentions that the hands and feet have their own critical frequencies for maximum discomfort, but no frequency ranges are mentioned.
The designer cannot do much about the IP vibration except to provide means for adjusting the balance and tracking in the field. The vibration frequencies that the designer can possibly minimize are those corresponding to “blade passage” and its multiples.
The discussions in the previous chapter show how only vibrations corresponding to these frequencies can get down into the fuselage. The designer tries to minimize these vibrations by avoiding structural natural frequencies corresponding to the oscillating forces generated by the rotor, and by using various methods of absorbing vibration or of isolating it.
Blade passage frequency depends on the rpm and the number of blades. It is usually referred to as “n/rev” or as “nP”. Since most helicopters have about the same tip speed, the rpm is low for large rotors and high for small ones.
Thus the lowest nP frequency would be for a large two-bladed rotor and the highest for a small multi-bladed rotor. This puts the Bell Helicopter UH-1 at one end and the McDonnell Douglas MD 500 at the other.
Figure 82-1 shows the discomfort level of vibratory acceleration measured in Gs as a function of frequency. These results depict my interpretation of the somewhat confusing results published by a number of researchers. It is meant to represent the maximum vibration level at which the pilot could maintain proficiency. It is a higher level than what a passenger would like while trying to write or drink a cup of coffee.
Since the pilot can tolerate higher vibration for a short time than for a long time, both a one-hour limit and four-hour limit are shown.
For frequencies above about 10 Hertz, the amount of vibration required to cause discomfort rises until, at some high buzzing frequency, you can take relatively high Gs. The report on one flight-test program involving the Bell UH-1H stated that 0.3 Gs at 32 Hertz was “hardly noticeable.” (The first point occurred at low rpm on the ground at the 2P blade-passage frequency and the higher one in flight at 6P.) Also shown on Figure 82-1 are the IP and the nP blade-passage frequencies that define the ranges for several modern helicopters.
The original specifications for both the UTTAS and AAH programs — which subsequently yielded the Sikorsky Black Hawk and the McDonnell Douglas Apache — called for the maximum allowable vibration to be less than 0.05 Gs at frequencies at blade passage and below. Above this range, the vibration limit was raised as shown in Figure 82-2 (between the one- and four-hour limits of Figure 82-1).
None of the four competitors in the two fly-off programs could meet the stringent 0.05 Gs at blade passage. Because of this, the limit was raised to 0.10 Gs at blade passage, but kept at 0.05 below this frequency, for the production Black Hawk and Apache. Thus the designer was relieved of his problem, but the maintenance officer in charge of track and balance was not.
Just because we humans can take high vibration at high frequencies does not mean that equipment and structural components are happy under those conditions.
To be really good, the ultimate goal as stated by some dreamers is to get the maximum vibration level down to less than 0.03 Gs for all frequencies. That would truly be a “jet-smooth ride.”