Wichita Falls, Texas: Monday, November 20, 2000

Edited by Rod Anderson

Fig. 1 - View from wireless camera on top of rotor in
spinup during rotor testing

ONE NASA GOAL COMPLETED - FOUR TO GO

The four days from November 10th through the13th were our most successful test period since the flight-test program began in September 1998. We performed our first zero roll takeoff. This was the 1st of 5 goals set for us by NASA under their SBIR Phase III grant program. We feel confident that the CC rotor control problem with the new rotor head is solved and that no major barriers remain to prevent us from completing the remaining four NASA goals.

SOLVING THE ROTOR CONTROL PROBLEM

On Thursday, November 9th, we returned to the pit to test the most recent control system modifications. They worked so well that we were able to (1) eliminate the spindle stabilizer bar, (2) reduce the weight on the spar stabilizer bar by 80% (may be able to eliminate completely) and (3) disconnect the cyclic dampening. We limited the rpm to a maximum of 375 to keep from over-loading the pre-rotator gearbox any more than necessary. At 375 rpm the rotor proved very stable even when the cyclic stick was given a hard jab. The modifications also reduced cyclic stick forces by more than one-half. The results of these tests indicated that the CC control system would now respond similar to other rotorcraft the CC test pilots had flown.

Friday morning we returned to Olney. By early afternoon we were making test runs. Test pilots Rusty Nance, Dick DeGraw, and George Mitchell were present. They confirmed almost immediately that eliminating the spindle stabilizer bar had also eliminated any tendency for the spindle not to follow the cyclic stick due to gyroscopic precession. With the cyclic dampening disconnected, there was just enough stick shake to take a majority of the friction out of the controls. This gave the pilots a better feel of the flight controls. The pilots all agreed the stick forces had been greatly improved.

Saturday morning was spent making a few minor changes. By early afternoon the pilots were flying the full length of the runway for the first time since we completed the CC rebuild program. There was still some yaw instability (see below), but it was manageable. The pilots felt confident in their ability to control the aircraft and began pulling collective to get airborne at (approx) 30 mph -- resulting in several short takeoff runs.

SOLVING THE ENGINE COOLING PROBLEMS

Sunday morning was spent making a few changes to the air pressure system and removing the cowl flap. We determined that a permanent solution to the cooling would require the following changes:

1. Create a vertical air-vent slot where the left and right sides of the fuselage come together just below the prop. The new air-vent will be 18 inches long and contain a clam shell cowl flap controlled by a thermal switch to vary its width as needed from 4 inches to 10 inches.
2. Remove the current air-exhaust fixture from the bottom of the fuselage and fill in the hole with fiberglass.
3. Extend the edges of the air inlet (on the front of the rotor mast) forward by 8 inches. It will look similar to the air scoop found on the bottom of a P-51 Mustang - only inverted.

SOLVING THE YAW INSTABILITY PROBLEM

By early Sunday afternoon we had determined the cause of the yaw instability problem. The rudders were not aerodynamically stable about their pivot axis. They were trying to align themselves at an angle of (approx) 15 degrees to the air stream - which caused the aircraft to yaw in the opposite direction. When the pilots corrected for the yaw by applying opposite rudder pedal, the rudders would snap to the 15-degree angle in the opposite direction as soon as they crossed over the center. The back and forth over-control caused a slow oscillation in an otherwise straight flight down the runway.

The temporary fix was to adjust the rudder crossover control cable so both rudders had their trailing edges pointed inward toward the aircraft center line (approx) 15 degrees. This change put more of the rudder surface area into the prop slipstream -- providing the additional benefit of improving yaw control at low forward speeds. The permanent fix will be to move the aerodynamic center of the rudders behind the pivot axis by adding 8 inches to each rudder's trailing edge -and then rebalancing the rudders about their pivot axis.

AT LONG LAST - THE FIRST ZERO-ROLL TAKEOFF

Following the temporary rudder fix, Rusty and Dick began progressively increasing the throttle after releasing the rotor clutch and prior to releasing the brakes - resulting in shorter rolling takeoffs. When they finally reached full throttle before releasing the brakes, they accelerated to 30 mph very quickly. Next, they began pulling collective at progressively lower air speeds until they suddenly found themselves making the first zero-roll takeoff for the CarterCopter. The digital video camera mounted on the top of the right vertical stabilizer clearly recorded this historic event along with the radio conversation.

Observers unfamiliar with the CC program would probably not be impressed with this first success. Initially, the aircraft lifted vertically only a few feet off the runway and never climbed higher than 10 -12 feet. To everyone involved, however, the takeoff indicated the potential for some very spectacular takeoffs in the near future. This time we were concerned about overloading the pre-rotator gearbox and causing it to fail again. We therefore over-speeded the rotor (prior to takeoff) to only 365 rpm instead of the 425 rpm the rotor is designed for. Everything else was also conservative. The pilots pulled just enough collective (6.8 degrees) to clear the ground. They then reduced the collective as the aircraft accelerated. This kept the rotor rpm and the stored energy (much less than would be available with normal rotor over-speed of 425 rpm) as high as possible until the pilots were sure they had reached 50 mph. They remained close to the ground on purpose. Insufficient rotor energy (for any reason) would simply have caused the CC to settle gently back to the runway. As it happened, the rotor rpm dropped to only 285 rpm by the time they reached 50 mph. Rotor rpm at 5 degrees collective and 60 mph is 210, approx one-half of the design takeoff rpm. Following this flight, the pilot's jump takeoff confidence went up dramatically. The late hour caused this to be the final flight of the day.

Fig. 1 - View from chase vehicle after zero roll takeoff

PRE-ROTATOR GEARBOX FAILS FOR THE FIFTH TIME

Monday morning we were able to start flying earlier than usual. The only preflight change was an adjustment to reduce the rotor clutch disengagement time. Since the first flight of the day had in the past always given us the most problems, we decided to make at least 3 progressively shorter rolling takeoffs before we started optimizing the jump takeoff procedure. This was to make sure everyone was comfortable and back in the test groove.

For the 4th flight and the 1st jump takeoff of the day, we planned to duplicate the previous day's success. After that the pilots would progressively pull more collective on each successive takeoff and hold it longer -- causing the rotor rpm to drop to a more efficient rpm quicker. Recall that rotor drag to the aircraft is a function of the rpm ratio cubed -- so slowing the rpm quickly does 2 good things; (1) the rotor energy is much more efficiently converted to altitude -- which is desirable, and (2) the drag is greatly reduced so the aircraft can accelerate to its minimum HP condition much faster.

During the over-speed of the rotor prior to the start of the 4th flight, the rotor had reached 340 of the planned 365 rpm when our pre-rotator gearbox failed for the 5th time. It is now obvious that the gearbox is overloaded, even at the reduced torque required for the lower pre-rotate rpm's. At the time of this failure, everyone felt confident we were going to perform several very impressive jump takeoffs before the end of the day.

Fig. 2, 3 - View from rear after short takeoff on last day of testing
 

PREPARATIONS WELL UNDERWAY FOR THE NEXT FLIGHT-TESTS

The 8-inch rudder extensions necessary to solve the yaw instability problem are almost complete. The engine air-inlet and air-exhaust alterations necessary to prevent future ground cooling problems have been designed and will be tackled next.

The preliminary pre-rotator gearbox design is completed and the required helical bevel gears have been delivered. Based on specs received from the gear manufacturer, we estimate these new gears will last 100 times longer when using the same loads we used previously -- but their life expectancy will decrease dramatically with increased loads. To compensate, we will do a rotor confirmation test in the pit at 450 rpm (rotor tip speed of mach .92) and then limit the number of max performance takeoffs at the max takeoff rpm of 425 (for show). Most of the jump takeoffs will be made at a lower rpm and pre-rotate torque in order to extend the gear life. There was insufficient room in the present engine compartment configuration to make the gearbox larger. We will not have this space problem when maximizing the gearbox design for use on future CC prototypes.

There is a 50-50 chance of being ready to fly again by Friday, December 8th. The following weekend will be a better bet. Everything depends on the weather and other factors that are beyond our control. We are anxious to begin flight-testing again as soon as possible. The next NASA goal we hope to achieve will be a zero-roll landing. Our pilots believe this will be easier than the zero-roll takeoff, as they now feel very comfortable with the aircraft's flying characteristics and the amount of energy that can be stored in the rotor. Your interest in our R&D program is appreciated. Please forward this press release to your friends.

Fig. 4 - Short takeoff from side view