Item 0893
OTHER:
Aerodynamics - Vibration - Rotor Induced - Analysis of Coaxial
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The dotted line is with the blades intersecting at the sides (azimuths of 90º and 270º). |
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Copilot Station |
Pilot Station |
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Vertical |
0.85 |
-0.25 |
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Longitudinal |
0.1 |
0.0 |
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Lateral |
0.9 |
0.9 |
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Cockpit 3P vibration at 200 kts. |
Note:-Sikorsky reduced the vibration by indexing the rotors so that the blades' intersection is at azimuths of 0º and 180º. |
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The upper rotor turns CCW and the lower rotor turns CW.
On the starboard side, over the pilot station, the upper blade is the advancing one, with the high lift.
On the port side, over copilot station, the lower blade is the advancing one, with the high lift.
Auxiliary Propulsion:
Rotor induced vibration increases as the rotor thrust increases.
"...the aircraft will have significantly reduced vibration when flying with auxiliary propulsion, when the rotor is relieved of its propulsive force requirements." ~ from Advancing Blade Concept (ABC) Dynamics, May 1997
Moments, Forces and Rotor Crossing Azimuths:
"If the rotors are installed to cross each other at the 90º-azimuth position, the predominant rotor excitation was the rolling moment. Secondary excitations were the yawing moment and the lateral force. The pitching moment, vertical force and longitudinal force cancel between the two rotors and are not transmitted to the fuselage.
If the rotors are installed to cross each other at the 0º-azimuth, the predominant excitation is the pitching moment; secondary excitations are the vertical and longitudinal force; and the lateral force yaw moment and roll moment cancel."
~ from An ABC Status Report
My Thoughts re the Sikorsky ABC:
View of Reverse Velocity Region at Desired Top Speed.
Re the difference in vertical vibration between the copilot and pilot stations.
On the starboard side, the lower blade may neutralize much of the upper blade when they pass because the lower blade is in the downwash of the upper blade.
On the port side, the upper blade may neutralize less of the lower blade when they pass because the upper blade is in the downdraft of the lower blade and therefore more intermittent 3P lift is coming from the port side.
Re structural resonance and two RRPMs.
The S-69 operates at rotor speeds of 650 ft/sec in the helicopter mode and 450 ft/sec. in the compound mode. It appears to me that it would have been very difficult to reduce structural resonances at both frequencies. Perhaps they would have done it for the compound mode (high-speed flight) only.
Kamov:
The 3/rev vertical vibration was reduced by arranging the blades to pass 15º off the longitudinal axis, with this decrease being most apparent at higher speeds. ~ Source
http://ntrs.nasa.gov/archive/nasa/atrs.arc.nasa.gov/975555_coleman/975555_coleman.pdf, end of page 11
UniCopter:
The UniCopter blades must intersect at the sides. A disadvantage that the UniCopter has is that the distance between the advancing blade's center of thrust/drag and the centerline of the craft is greater. This is because of the lateral offset of the rotor hubs and the rotors turning outside forward. This may result in more rolling and yawing moments.
Rotor Crossing Azimuths:
Re: Intermeshing Configuration:
This might be relevant;
DESIGN: UniCopter ~ Rotor - Disk - Lateral Dissymmetry of Lift and Drag? - (3-blades)
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Last Revised: March 8, 2007
