TASK 3 - TRACK AND BALANCE MAIN ROTOR IN FLIGHT
Track height should be given only minor consideration during the track and balance process. It is recommended to
keep an eye on track and perhaps use it as a delineating factor when choosing between two otherwise comparably
effective solutions, but to not use track for picking adjustments. If the AVA adjustment set is not manipulated too
erratically, track willremain under sufficient control. This is because minimizing vibration requires only two blades.
The third blade is used to control track with little compromise to vibration. The AVA will always use the third blade to
minimize track spread, as needed. The only mechanism available for having a severely out-of-track condition while
having vibrations in limits is the split tip path plane in which two opposite blades are flying in one plane and the other 2
opposite blades are flying in another plane. It is possible to end up with this configuration if one manipulates or
restricts the adjustables too severely, however, the AVA will itself never drive the rotor to this configuration.
In a perfect world, zero track height would correspond to zero vertical vibration. However, in the real world, each
blade (including lead lag links and pitch cases and pitch links, etc.) is far from identical to the other. Therefore, in the
real world, perfect track usually does not mean zero vibration. The more mismatched the blades, the larger the track
spread may have to be to obtain minimum vertical vibration.
Hub weights are turned off as a default and must be manually turned on (i.e., Edit Adjustables) if needed.
If the ground lateral and hover lateral points on the polar chart need to be brought closer together, hub weights will not
be effective. A hub weight adjustment will essentially move the two points around the polar chart without significantly
changing the distance between them. This is evidenced by the closeness of the sensitivities both in magnitude and
phase angle, for the two conditions. Therefore, if both the ground and hover lateral points need to be brought closer
together on the polar chart, pitch link adjustments are generally required.
The sensitivities of vertical vibration in forward flight to hub weight changes are included in the sensitivity matrix in the
script file. Due to the relative weighting of the vibration measurements, which more heavily weights the total of all
vertical vibrations as compared to the total of the two lateral vibrations, the hub weight solutions will often be rendered
less effective for reducing lateral vibration, since weights will be more effective in reducing vertical vibration. For
these reasons, it is important to get hover and ground lateral under control early in the track and balance process,
when potentially pitch link, weight and tab adjustments are being called for. The AVA will generally not be effective in
coming up with a weight-only solution to minimize lateral.
The AVA may give large pitch link adjustments (2 flats or more) and use a corresponding tab adjustment to counter it.
Usually, these are uncalled for and are not recommended. If the AVA is giving pitch link moves of 2 flats or more, it is
recommended to try other combinations of adjustments (including no pitch links at all) to try to come up with a more
reasonable solution. Rarely would such relatively large pitch link moves be justified as long as ground and hover
track spreads are reasonably small.
Tabs are triggered by blade track height. When the blade relative track height changes 0.5 inches or more from its
hover value at a given speed, a particular tab region is turned on, depending on that speed. The correlation between
speed and tab region is as follows:
4-10 tab: Track exceeds 0.5-inch difference from hover at 60-80 knots
6-10 tab: Track exceeds 0.5-inch difference from hover at 100-120 knots
8-10 tab: Track exceeds 0.5-inch difference from hover at 140 knots
It is recommended to avoid tab bends of 2 degrees or more, if possible. If a 6-10 or 8-10 tab is triggered, but the
adjustment calls for a 2 degree or more bend in that region, turn off the 6-10 or 8-10 tab and turn on the
corresponding 4-10 or 6-10 tab. This distributes a smaller move over a larger span and will serve the same purpose
without requiring excessive tab bending. A 4-10 tab bend of 2 degrees or more cannot be avoided in this manner.
However, a 4-10 bend in excess of 3 degrees is potentially an indication of some other problem. If this happens,
check to see if there is any bend in pockets 0-3. If so, they should be zeroed and a new measurement set taken. If
there is no bend in 0-3 and the large 4-10 bend is required, make a 1/2 degree bend in 0-10 instead, take a new
measurement and go from there. If a large 4-10 is still called for, another 1/2 degree can be put in 0-10 instead, but
do not exceed 1 degree total in pockets 0-3.