Subject: NAS2-99090 September monthly report

Date: October 3, 2000

Most of the work effort for September was directed at testing the rotor to determine what was required to produce a smooth stable rotor from startup through 425 RPM (max jump take-off RPM).

The rotor is now smooth and stable at any flight RPM between 75 and 425 RPM.

This was achieved by adding a spar stabilizing bar at the hub whereby weight could be added 24" on either side of the spar spanwise centerline. The weights fit inside the rotor spinner.

A form of cyclic dampening was also required. Hydraulic dampening at the rotor head, not at the control stick end of the push/pull control cable, worked. However, a very reliable and simple solution was to increase the stability margin of the blade, i.e. increase the distance the blade dynamic center of gravity (CG) is in front of the blade aerodynamic center of lift (AC). This was accomplished by adding a trailing edge extension, which moved the AC of the blade rearward. Note it may be that; 1) given enough dampening or 2) a stiff enough cyclic control linkage or 3) the use of hydraulic boost that neither the stabilizing bars or a blade with a large stability margin would be needed.

In order to keep the spindle/stick stable during rotor spin up when 2000 ft-lbs. of torque is accelerating the rotor, an additional spindle stabilizer bar located 90 degrees to the blade teetering axis was needed. This is because any time the drive U-joint is not straight, torque tries to whip the spindle around and feeds back into the cyclic stick. This is only a problem during spin up, as once the aircraft is in the air, the rotor is in autorotation.

Both the spar and spindle stabilizer bars acts like gyroscopes to make the spindle inherently stable without dampeners or a stiff cyclic control. Fortunately it did not take much of a gyroscope to provide the stability needed and any gyroscopic precession forces 90 degrees to the stick movement are not large enough to be noticed.

The aircraft is now ready to fly. We have four new pilots, so we plan to take the testing slow until they gain confidence and experience.

Note at one time when we could not run the rotor over 150 rpm without experiencing an instability, we assumed the problem was because 3 items (mast lateral, blade torsional, and cyclic) had a natural frequency multiple near this rpm. To change the natural frequency of two of the items we added +/- 45 degree carbon to the outside of the blade, which more than doubled its torsional stiffness in places, and operated the rotor with some blade pitch to increase its coning and move its CG closer to the spindle tilt axis. Neither change, collectively or by its self, had any measurable effect.

In addition to the time it took to find a solution to the rotor system stability problem, we had problems with the prerotation drive that had to be resolved. During max rotor overspeed, the constant velocity drive runs at a high RPM which forces all the lubricant to the outside, starving the inner part of the bearings. We had several bearing failures before we realized what was happening. If a small amount of grease leaks out of the boot, the bearing fails.

We also had a gearbox failure. We used a ring gear and pinion out of the salvaged rear end of a small vehicle. The stress analysis on the gears indicates they should handle the load provided the gear did not already have a fatigue crack and that the gears are made from a material with a tough high yield core. Until we replace the gears with new ones and can perform more tests, we plan to limit the torque into the gearbox to 75% of what it was tested. This means we will not be overspinning the rotor but to 375 RPM maximum instead of the design 425. We should still be able to perform zero roll takeoffs and landings, but they will not be as dramatic.

We also took time to show the propeller to a potential licensee, (name withheld). They sent four of their senior management personnel to Wichita Falls in order to review the prop design in more detail and witness the calibration and static thrust tests on the propeller. The static thrust from 275 hp was measured at over 1500 lbs. They were suitably impressed. The CC 4-bladed prop for their turbine engine should not weigh over 60 lbs resulting in a very impressive total package.