XV-15 litho - NASA's History Office

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Following the completion of controllability flight evaluations at Ames with modified
SCAS components installed in N703NA, efforts began to prepare the ATB
for flight tests. XV-15/ATB ground runs on the ramp and on the tiedown stand
were conducted between September and early November of 1987 and the first
hover flight with the new blades was performed on Friday, November 13, 1987.
From the first operations with the ATB there were problems. The initial difficulties
surfaced during the runs required to obtain a satisfactory proprotor track and
balance. Balance of the two interconnected proprotors presented problems on the
XV-15 since a change on one proprotor provided an excitation that resulted in a
change in the dynamic behavior of the other proprotor. Obtaining a proper balance
with the ATB presented a special problem which stemmed from the frequent
addition or removal of small weights from a fiberglass weight block located
at the tip of each blade within a removable tip cover. The frequent removal of
the tip covers to alter the weights resulted in the failure of the metal screw-retention
inserts installed in the fiberglass weight blocks. Other problems included the
deformation of the skin material under the retention screws at the fiberglass tip
requiring the installation of metal washers, the failure of the bonds within the
tip-weight assembly, and the delamination (unbonding) of the blade skins from
the underlying nomex honeycomb material. Many of these material issues continued
to cause problems during operations with the ATB.
When the expansion of the flight envelope in the helicopter mode with the ATB
began in June 1989, higher than expected oscillatory blade control loads were
measured at airspeeds as low as 40 knots. These loads increased with airspeed
and reached the allowable limit at about 65 knots, too low to allow a safe envelope
for initiating conversion. At that point, efforts were intensified to analyze
test results and initiate analytical studies in order to determine the cause of the
high loads. In addition, the loads investigation, headed by John Madden from
Ames, included a series of tests on the XV-15 control system to determine stiffness
characteristics as a function of the rotational (azimuthal) position of the proprotor.
The results of this evaluation revealed that a major mechanical rotor control
component, called the swashplate inner ring, did not provide uniform stiffness
at all azimuthal positions. The lower than expected stiffness, coupled with
the increased blade mass and inertia of the ATB (due to the larger solidity than
the metal blades) resulted in lowering the natural frequency of the control system
to the 3/rev (3 vibrations per proprotor revolution). When the three-bladed proprotor
was flown in forward helicopter mode flight, the 3/rev aerodynamic excitation
coupled with the system’s natural frequency to produce high structural
loads.
A temporary remedy was proposed by John Madden and was subsequently
implemented. A set of shims was installed between the inner ring and the transmission
housing which locked out the lateral cyclic input to the rotor (used for
flapping reduction in helicopter mode flight) and provided the required increase
in the control system stiffness. A permanent modification to change the inner