DESIGN: UniCopter ~ Rotor - Disk - Enhanced Precone

 Overview:

• To give the rotor a precone that is greater than conventional calculations dictate.
• This is particularly relevant to the Intermeshing configuration; both the UniCopter and the intermeshing Electrotors.

 Advantages:

• Static Longitudinal Stability: In forward flight at speeds greater than 40 knots (+/-) the coning angle will generate a nose up moment that will offset the nose down moment resulting from the parasitic drag of the fuselage, and the landing gear if not retractable. At lower speeds, transverse flow effect must provide the static stability.
• Static Lateral Stability: See UniCopter_Stability.html
• Reduced Drag and Negative Lift: A reduction in the size of the horizontal stabilizer and perhaps the vertical stabilizer. This reduction will reduce drag from the tail and reduce the negative lift of the horizontal stabilizer,
• Greater Ground and Personnel Clearance: With 3º of pre-cone, the obliquity will be 9º. This results in the blade at 90º azimuth having only a 6º downward angle from the horizontal. The pre-cone will result in the blades at the front and the back having an upward angle of 3º from the horizontal.
• Thrust is Closer to Vertical: The advancing blade at 90º azimuth provides most of the thrust because of the inclusion of Advancing Blade Concept. The advancing blade is now closer to being horizontal. The blade at 270º azimuth will still have a 12º upward angle from the horizontal. In other words, there is higher tip on the outer (advancing) blade and no change to the angle of the retreating blade.
• Greater Rotor - Rotor Clearance: The pre-cone improves the blade to blade clearance at the front and the back, which are the critical intersection locations.
• Reduced Bending Moment on Blades: A small precone angle will be closer to the mean coning angle therefor the out-of-plane forces will be reduced. A large precone will increase the bending moments on the rotor.
• Reduced Craft Pitch Moments from Rotor Torque: The reduced angle between the masts, due to the inclusion of precone, will mean a reduced rotor torque in the X-Z plane.
• Increased Propeller Diameter: The diameter of the propeller can be slightly greater because of the increased rotor blade elevation above the propeller.

 Notes:

• Because this is an absolutely rigid rotor the cone will not move the rotor's center of mass off the centerline of the mast.
• Initially consider a precone angle of 2 to 3º. The Sikorsky ABC had 3º of precone; in addition, the rotor was not absolutely rigid.
• A greater precone, such as 4º or 5º, may be better, for a number of the reasons mentioned above.

 Various Precone Angles:

 Related Ideas:

• Intermeshing Rotors w/ Precone/Decone.
• Intermeshing Rotors w/ Curved Anhedral

 Outside Information that may be Relevant:

• http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770019124_1977019124.pdf

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