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TM 1-6625-724-13&P
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Trim Tabs:
The trim tabs are the primary adjustment for vertical 1/REV in forward flight. Coarse adjustments are made on the
ground to reduce changes in track from idle to 100% rpm and fine adjustments are made to reduce the vibration in
forward flight. The AVA specifies trim tab adjustments as a bending up or bending down of the tab in ½ degree
increments. Bending the tab up causes the blade to climb with rpm or airspeed while bending a tab down causes it to
dive. The current limit for bending tabs is +/-15 degrees. The proper method of bending a tab is to space the bender
away from the blade trailing edge approximately by 1/8 to 3/16 inches. When bending tabs, bend the tabs to a setting
approximately 2 degrees past the desired setting and then bend it back to the desired setting. This over bending
minimizes creeping of the tab over time. Note: Stiff tabs are the most likely to creep over time as the adhesive has a
memory. If a tab creeps back do not bend the tab further, make the opposite move on the opposite blade to minimize
the vibration.
Proper Methodology Used to Smooth the OH-58D
As stated, the recommended method for smoothing the OH-58D is conducted only with the MMS installed. The
technique of smoothing the rotor without the MMS and then installing the MMS is obsolete and has been found to
result in high airframe and MMS 1/REV vibration at idle. Despite some teachings, the only way to balance an OH-
58D with the MMS is at idle. If the idle is rough, the rotor is out-of-balance.
The OH-58D is smoothed in two steps: a ground or initial tracking and an in-flight final smoothing of the rotor.
Properly conducting the ground or initial step greatly reduces the amount of effort required to smooth the rotor in flight
and yields a superior ride.
The recommended ground/initial method to smooth the OH-58D uses these basic steps:
a.
Use GRNTRK to adjust pitch links to track the rotor at 65% rotor speed. This corrects for basic lift variations
between blades. Getting the rpm correct is critical, it has been found that using the NP to set the rpm correctly is
the easiest method because of its fast response.
b.
Use INIT-SG with weights turned off and adjust trim tabs at 100% rpm and 35% mast torque as per the MFD to
correct for changes in track from idle to 100%rpm. This corrects for pitching differences between blades.
c.
Once the track is close for INIT-SG turn hub weights on and use hub balance weights to reduce fore-and-aft
1/REV at idle. This is the last step as pitch link and tab adjustments affect the balance of the rotor.
In the AVA the GRNDTRK flight plan only adjusts the 65% track while the INIT-SG conducts all three operations
simultaneously. When initially tracking the rotor using INIT-SG it is best to turn off the hub balance weights until the rotor is
in close track. Once the rotor is close to proper track, the hub weights can be left on to finish balancing and tracking the
rotor. It should take approximately three initial runs to get the rotor in track. If after three runs the rotor track is not within
the 0.1-0.2 inch range, review the adjustment procedures to make sure the correct adjustments are being made and re-
orient the aircraft to make sure that the tracker is not looking at the sun or that the sun is at the left front of the aircraft. As
long as the 65% and 100% track are in the 0.1-inch range (2-3mm) it is acceptable to discontinue the initial mode and
press into flight if problems are being encountered getting the 65% balance acceptable.
NOTE
The better the ground track is the less vibration will be
encountered in the first flight and the rotor will be closer
to finished levels.
Once the ground tracking or initial phase has been conducted and the aircraft is taken to forward flight, only the tabs
and hub weights are used to smooth the rotor. When smoothing the rotor in flight the 65% rpm condition and at
least one forward flight condition must be taken on every run to ensure that the program specifies correct
adjustments. History has shown the idle balance to vary from run to run, this is a characteristic of the aircraft. While
the balance may vary, the tabs for reducing vertical in flight are very accurate. If the aircraft is not responding
properly to tabs make sure that the over bending method described above is used. If a stiff tab is found use the “Edit
Adjustables” to turn off that tab and recalculate the adjustments. When the vertical is close and you do not have
confidence in making small tab moves, use the editor to turn off the tabs and turn on the pitch links for the final moves
(no more than 1-2 flats). The only criteria for successfully completing the in flight smoothing of the rotor is the
vibration levels, the final track does not matter. If all that is out is the track, it is acceptable to stop and consider the
aircraft finished. Once the vertical is acceptable and if the fore-and-aft at 65% rpm is still too high, then it is
acceptable to conduct only 65% rpm balance runs to get the rotor smoother using the FLT-SG flight plan (edit out trim
tabs). Balance moves do not change the vertical 1/REV in flight significantly.
