Wichita Falls, Texas: Monday, May 17, 1999.

THREE UNEXPECTED LAST MINUTE DELAYS:

Three times in less than a month, the CarterCopter appeared ready for the next flight test series. When the aircraft is ready to fly, several members of the flight test team must schedule their arrival from distant locations. Three times these members were notified the aircraft was ready, and three times the unexpected happened.

The first delay occurred during the week of April 19th. On this occasion, the AV-10 unit in the CarterCopter failed during the final ground tests just prior to moving the aircraft to the Olney airport. The failure required that the AV-10 unit be disconnected from its numerous sensors and returned to the manufacturer to fix a non-linearity in a number of parameters. Once returned, all aircraft sensors had to be recalibrated

The second delay occurred on Wednesday, May 5th. The CarterCopter had been moved to Olney in preparation for flight testing that afternoon. During the final inspection the ground crew noticed an unexplained powder on the prop hub root block. Vigorous manual movement of the prop's blades produced a slight movement between the prop hub root block and the spar caps where no movement should be. This prop has been used for the jump takeoff endurance tests conducted on the test stand, for all engine calibrations, and for all flight tests to date. The prop's root block is mechanically locked (encapsulated) and does not depend upon a bond to transmit the torque drive loads. However, the bond failure between the root block and spar caps does result in a very slight movement between the two that eventually would produce excessive wear. The prop spar is without any joints and so was in no danger of failure. The problem was fixed by (1) reworking the prop hub root block so that a thicker bond line between the root block and spar caps was obtained to allow for stretch discontinuities and (2) by using a tough (high strength - yet flexible) adhesive to bond it to the spar caps. The new method produces a much stronger attachment joint than before.

COMPONENT FAILS ONE HOUR BEFORE FLIGHT TESTS:

Once again everything was ready at Olney on Wednesday morning, May 12th, for the resumption of flight tests. The chief test pilot, Don Farrington, was flying in from Kentucky and was scheduled to arrive in another hour. The other test pilot, Jay Shapley, was already at Olney and was assisting in some pre-flight tests.

The CarterCopter was going through final ground tests with maximum performance run-ups on the engine and pre-rotation system. The purpose of these pre-flight tests was to recheck the aircraft's systems with emphasis on any changes that were made since the last flight tests. Any failure that occurs on the ground causes only minor delays (relatively speaking) and is wonderfully educational.

Once again the unexpected happened. The steel prop-shaft broke on the opposite end from the prop - between the second and third bearings. At the time, everyone was puzzled, because the break occurred in a part of the shaft that has a very low stress.

THE FAILURE IS DIAGNOSED:

The prop-shaft failed as a result of improvements made to cure problems uncovered in the last flight test series (see April 5 press release). The engine-to-prop pulley ratio had been changed from 2.38 to 2.75 - allowing an increase in max engine RPM from 5500 to 6350 while maintaining the same prop RPM. The resulting increase in engine RPM together with the optimization of the spark advance and fuel/air ratios produced a static HP increase from 250 HP to 325 HP and an increase in static thrust to 1400 LBS.

When the new pulleys were installed, the drive-belt tensioner was removed. The purpose of the belt tensioner was to apply pressure to the slack side of the belt so the belt could be run with more initial slack. The two 3-inch diameter wheels on the tensioner were not large enough for the relatively stiff drive belt and were causing the belt to delaminate due to the tight bend radius of the wheels. The reason for the drive-belt slack, and thus the tensioner, was to compensate for the expansion of aluminum components as they heated up. The pulleys, engine, and brackets are all made of aluminum. As they expand, the drive-belt gets tighter. With the previous re-design of the exhaust system (see March 3 press release), engine compartment temperatures had been greatly reduced - so it was thought that less initial slack in the drive-belt could be used. With less slack, the tensioner was not needed - or so it was reasoned.

The drive-belt is rated at 600 HP for 2000 hours and has very little stretch. Once the aluminum had heated and expanded to the point where the belt was tight, any additional temperature rise increased the belt load way beyond the normal load the belt would see even at 600 HP. During the pre-flight testing at Olney, the engine compartment temperature rose to a much higher value than normal. When testing the rotor-drive at full HP, testing was stopped only when the engine water-temperature reached 225 degrees. High engine temperature coupled with no aircraft movement and little airflow through the engine compartment - all added to high engine compartment temperatures. The resulting drive-belt load put a bending load on the prop-shaft sufficient to break it. The shaft broke near the edge of the weld that connects the prop-shaft to the forward bearing support.

CHANGES NEEDED BEFORE FLIGHT TESTS RESUME:

If drive-belt loads are only those loads associated with the HP transmitted, then the bending loads on the prop-shaft are very low. Therefore, the prop-shaft does not need to be strengthened. The problem can be fixed by adding another belt tensioner - this time with a 5-inch diameter "roller" that extends across the full width of the drive-belt. The drive-belt can then be given enough initial belt slack to handle any engine compartment temperature. Since the prop-shaft has to be rebuilt anyway, it will be made with 4130 steel and heat treated. The shaft as originally built had a yield safety factor greater than 10 - based on belt loads associated with 300 HP. The new heat-treated shaft will be twice as strong.

The repairs should be completed in another week. The long awaited 8th flight test series will be flown soon thereafter.

ADDITIONAL CHANGES SINCE THE PREVIOUS FLIGHT TESTS:

A complete engine optimization of the fuel/air and spark advance from 1000 to 6250 RPM was performed as a result of earlier changes made to the engine's compression ratio (see April 5 press release). These optimization test-runs added 6 additional hours to the engine, prop, and drive-train.

The solenoids for the landing gear were reworked to make the orifices larger. The landing gear now extends in less than nine seconds, down from 56 seconds previously. The oil in the landing gear was changed to synthetic 5W50, which makes it operate smoother.

Both fuel pumps were fixed so that if one looses prime it can re-prime itself against the pressure from the other fuel pump.

The rotor clutch release solenoids and hydraulic return lines were reworked so that the clutch will release in 1-½ seconds, down from 4 seconds previously.

The prop controller was fixed so that it does not "over-hunt" for the best RPM. The controller now keeps the prop at flat pitch for four seconds after the clutch is disengaged - or until the engine reaches 6100 RPM. It then changes the pictch to hold the engine at 6100 RPM for the balance of the four seconds and then adjusts the pitch for best efficiency.