Wichita Falls, Texas: Thursday, December 23, 1999

THE CARTERCOPTER SUFFERS DAMAGE

On Thursday, December 16, during flight testing at Olney, Texas, the CarterCopter suffered substantial damage during an emergency, forced landing. The aircraft landed at a relatively high forward and vertical speed. The landing impact caused the nose gear to compress too far, allowing the nose boom to dig into the soft asphaltat the edge of the runway. The dipping nose and other factors caused the rotor to hit both vertical tails, the horizontal stabilator, and the prop. The aircraft spun sideways around the implanted nose boom but the right wing prevented a roll. The right wing was broken in the process. The rotor hit the ground after the aircraft spun around, breaking several feet off each end.

FIRST FLIGHT

The accident occurred during the first flight of the day -- after 2½ weeks of down time. The plan was for the test pilots, Don Farrington and Jay Shapley, to make a few conservative flights down the runway to get the feel of the aircraft again before making a traffic pattern and starting the flight test program. Don, as PIC, was to pre-rotate the rotor to only 450-RPM instead of the usual max of 550 RPM. The rotor collective stop had been blocked for the first set of tests to have 5 degrees of pitch with the collective full down. As soon as forward airspeed reached 30-MPH (approximately 2 lengths of the aircraft) Don was to quickly pull full collective (10 ½ degrees) and hold it until the rotor dropped to 350 RPM. At this point he would reduce collective to 5 degrees for the remainder of the flight. He would then continue with full throttle and climb at 75-MPH. As the end of the 5,000-foot runway approached, Don would reduce throttle and land.

Don had previously flown the aircraft in steady, smooth, climb outs at up to 7.5 degrees collective. The strategy for this first flight was to quickly use the excess energy stored in the rotor and thus get the rotor RPM slowed down to where it would operate most efficiently (less drag). What we did not expect was how dramatically this tactic would improve the aircraft's performance.

CAUSE OF ACCIDENT

Following the pre-flight briefing and before the flight, there were three separate delays that caused distractions for the task at hand. This was not a concern at the time since everyone was expecting a short flight down the runway similar in most respects to those done numerous times before. Neither the pilots nor ground control noticed how fast the aircraft accelerated. Don did notice he was having to hold a higher forward stick force than normal, but was not concerned at the time, and let go of the collective to support his other hand on the stick.

The collective has a new air boost cylinder, which automatically drops out at 350-RPM and lowers the collective approximately 2-1/2 degrees. When the pilot's hand is on the collective, he can feel when the boost drops out and knows it is time to reduce the collective without having to look at the instruments.

This time Don did not have his hand on the collective and was distracted by the increasing stick force and stick shake. Due to the lower rotor RPM and subsequent much lower drag the aircraft accelerated much faster than usual to a speed faster than the rotor would auto-rotate at the given collective. Now without auto-rotation to drive it, the rotor continued to slow down. The centrifugal force on the rotor continued to decrease with the decrease in RPM, which resulted in more rotor coning, which, in turn, caused the center of rotor drag to move up and away from the spindle pivot. This increasing moment tried to tilt the rotor spindle back, which Don had to resist with more stick force.

Don now realized his rotor RPM was low, but because of his concentration at just flying the aircraft never realized his RPM had dropped as low as 230 RPM or his airspeed had climbed to nearly 100 MPH (he learned this later only after reviewing the recorded flight data). Don also did not realize the excessive blade flapping and stick-force visual alarms had been activated in the cockpit. Jay Shapley, riding as co-pilot, had been temporarily distracted by cockpit management duties and was thus unprepared to react to the sudden and totally unexpected turn of events. Had the planned dual flight control system been installed, Jay would have been able to provide some stick force help.

Don managed to take his hand off the stick long enough to reduce the throttle and collective, but then increased the throttle again to keep from hitting the ground at a pitched-up attitude of approximately 30 degrees. At this attitude and 74 MPH, the wings were in a deep stall, which, in turn, reversed aileron control. The rotor had slowed to 230 RPM - yet the aircraft continued to fly. (In the post flight review of the data, everyone was surprised that the aircraft continue to fly in this situation, which was entirely outside the theoretical flight envelope. The aircraft proved once again that it is very forgiving.)

Not knowing for sure what had happened, Don landed the aircraft as soon as he got the aircraft leveled out. As the landing gear compressed to absorb the vertical impact, the nose pitched over and Don instinctively pulled back on the stick. The rotor acts like a gyroscope and will not change its plane of rotation as fast as the aircraft can pitch over. Therefore, when the aircraft pitched over, the rotor was pulled down toward the rudders. This combination of the aircraft pitching over, the rotor being tilted back more than 9 degrees due to the flapping caused by low RPM, and Don's pulling back on the stick when the nose dropped - all contributed to the rotor hitting the vertical tails, horizontal stabilator and prop. Almost simultaneously with the rotor striking the rear of the aircraft, the nose boom dug into the soft asphalt at the edge of the runway -- causing a hard left yaw that almost rolled the aircraft to the right. The right wing stopped the roll but was broken in the process.

Click on pictures to view larger versions

LESSONS LEARNED AND PLANS FOR AVOIDING A RECURRENCE

Had the nose gear not compressed as far or had the rotor not been flapping as much (due to low rotor RPM) or had Don pulled collective rather than tilt the rotor back after landing - it is likely no damage would have occurred.

A higher rotor RPM would have prevented the rotor from flapping as much. Three changes will be made to help insure sufficient rotor RPM at this stage of future flights. First, a large (additional) rotor RPM display will be added to the top of the instrument panel. Next, a headset voice alarm will be triggered automatically by the flapping sensor. The urgent warning will state "Flapping over 5 degrees, reduce collective." As a failsafe, the onboard computer that monitors 60-plus channels of data will provide redundancy. The computer will trigger the voice alarm when it calculates that rotor flapping is over 5 degrees based on collective pitch, rotor RPM and forward speed.

Had the nose not dipped so dramatically on landing, Don would not have reacted instinctively by pulling the stick back instead of pulling collective. Also, the nose boom could not have dug into the ground at the edge of the runway causing the aircraft to spin sideways and try to roll over. Two changes will be made to assure the aircraft remains in a more level attitude should a similar situation occur in the future. First, the nose gear will be extended an additional six inches. It will also be redesign to insure that the gear does not compress as much for a given load.

EXTENT OF DAMAGE

PARTS THAT APPEAR TO BE "OK": Fuselage and everything inside -- including the cabin, the engine and the drive train. The rotor head and the left wing are also OK. The landing gear appears to be undamaged even though it absorbed a severe impact and skidded sideways down the runway.

PARTS THAT ARE DAMAGED OR DESTOYED (see pictures): Rotor, prop tips, right wing, both vertical tails & rudders, and the horizontal stabilator.

The rotor could be repaired since the carbon fiber spar appears to be OK. However - it will be replaced since work had already begun on a new generation, larger diameter rotor (see below).

Two inches of each prop tip are damaged, but otherwise the prop looks fine. However - it will be replaced since work had already begun on a new generation, slightly larger diameter prop (see below).

The right wing spar broke where it slides into the center section. It will be repaired.

ADDITIONAL IMPROVEMENTS TO BE MADE

All of the improvements listed below were planned and in the process of being implemented before the accident. They will be included in the rebuild process.

1. Finish installing dual controls
2. Install our next generation rotor.The rotor diameter is increased by 10-feet, from 33½ to 43½-feet. The new spar design requires less force to change pitch. John Roncz designed the new airfoils.
3. Change the drive ratio that will be used to pre-rotate the new, larger diameter, slower turning rotor.
4. Install our next generation prop. The prop diameter is increased by 2-inches, from 94 to 96-inches. The new prop design makes it easy to build a four-bladed prop, which will be needed for future turbo-prop engines. The new design will also incorporate a closer-fitting cuff to a larger diameter spinner, which will reduce drag.
5. Add more moveable rudder area.
6. Add more horizontal stabilator area. 3-foot extensions will be added outside of the tail booms and rudders. Studies show the additional surface area may be needed at airspeeds above 200-MPH. This will also move the aircraft aerodynamic center of lift rearward, allowing us to remove some weight from the nose boom. The extensions will be removable so the aircraft can continue to be transported by trailer.
7. Eliminate the air-scoop on the bottom of the aircraft. The air will be brought in from the top over the engine - then through the radiator, the turbo inter-cooler & after-cooler and oil cooler before exiting through a variable opening flap at the bottom rear of the aircraft. This new design will work for both piston and turbo-prop engines.

OUR TWO WORSE FEARS

Our worst fear has always been that during flight testing someone would get hurt (or worse) for any reason; whether it be a design deficiency, structural failure, or human error. In this accident there were no structural failures leading up to the accident and, in hindsight, there were very human mistakes made by the pilots and the ground crew. Even though both pilots feel that they were never in mortal danger, everyone involved still feels fortunate no one was hurt. For this we are very thankful.

Our next worst fear was that the CarterCopter would be destroyed before we had a chance to prove its concept. This would give the skeptics a chance to say, "See, I told you they would never achieve their claims." In this instance, damage to the aircraft could have been much worse. For this we are also thankful. We also learned a lot about the aircraft. Under the circumstances, it behaved better than expected. It seems to want to fly, regardless of our mistakes. More than ever, everyone associated with the project believes "we have a winner". The rebuilt CarterCopter will be much improved. Estimated time required to be flying again is 4 to 6 months.

Although the planned flights for the stockholder meeting held on Sunday December 19th did not occur, the unanimous decision among the stockholders was that additional funding would be immediately made available and that maximum effort be expended to expedite repairs and implementation of the numerous improvements.

Image Gallery of Flight Incident

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