Subject: NAS2-99090 August monthly report

Date: September 5, 2000

The following items were completed or worked on during August.

• Completed new spinner plug and fabricate new spinner mold.
• Added camera on tail boom.
• Modified software and data display per pilots' suggestions.
• Checked out electrical systems.
• Shortened push/pull cables for rudders and horizontal stabilator and installed rudders and stabilator
• Calibrated all data collection sensors
• Tested propeller and controller

The rest of the time was spent testing the rotor and modifying the rotor in an attempt to obtain a smooth stable rotor throughout its operating range (75 - 425 RPM). All 4 of the tests pilots were here at different times to become more familiar with the operation of the aircraft, make suggestions to improve safety, and help with the testing of the rotor.

Some of the modifications made to the rotor include:

1. Adding an additional 10 lbs. of nose weight to each blade in an attempt to move the blade dynamic center of mass ahead of the blade aerodynamic center. Rotor could be forced into a blade weave with a rapid cyclic stick jab above 275 RPM. We were not able to excite the blade weave on the test stand, because the cyclic was locked out and very stiff.
2. After adding the weight to the blades, the rotor developed an instability above 150 RPM. We attempted several things such as preloading the cyclic control system to remove its deadband and adding an inertia bar to provide some gyroscopic stability to the spindle - none of which made much of an improvement. Several helicopter rotor experts reviewed the test data and while they identified some 1,2,3, 4 and some fractional per rev oscillations at the point of instability. They could not provide any conclusive information as to what was causing the problem.
3. Perhaps the rotor C.G. was still behind the rotor AC - hard to believe because the same analysis approach had been accurate on the 33-1/2 ft. diameter rotor. Nevertheless, on two different occasions a trailing edge extension was added to the blades. The last extension was 4 inches long and ran from station 114 to the blade tip at station 260. Neither extension made any improvement.

We noted the mast lateral frequency was at the same RPM as the rotor instability, but since it was heavily damped, we felt something else must be exciting the rotor. We measured the natural frequency of some of the other systems. 1) The natural frequency of the rotor/cyclic control system was measured at various rotor coning angles (various blade CG locations relative to the spindle tilt axis) and found that at zero coning, a no lift prerotating mode, a 1/3 multiple of its frequency occurred near the rotor instability RPM. Note as the coning angle increased (i.e. the rotor CG approached the spindle tilt axis) the frequency increased significantly. We tried running the rotor with increasing collective angles (rotor CG closer to the spindle tilt axis) without much of an improvement. 2) The blade torsional frequency was checked and found it started at 750 CPM (cycles per minute) and peaked at 920 CPM. ¼ of this 750 CPM (188 RPM) was near the rotor instability RPM. We also noticed that when the rotor started to go unstable, we saw a 4 per rev load developing in the cyclic control linkage. We are now adding +/-45 degree carbon cloth to the outside of the blades in select areas to increase the blade's natural frequency.

There is no way we can avoid going through some component's natural frequency as the rotor RPM varies, but to have 3 systems (mast side to side, rotor/cyclic linkage, and blade torsional) with nearly the same frequency is a disaster. Adding weight to the blade to move its CG forward lowered its torsional natural frequency and apparently caused its new natural frequency to coincide with the other frequencies.

The aircraft is ready to fly once we are able to assure ourselves we do not have any known rotor diverging oscillations/instabilities.