DESIGN:UniCopter ~ Rotor - Disk - 'Absolutely' Rigid Rotor (ARR)
There's no such thing as an 'Absolutely Rigid Rotor' but the phrase 'Rigid Rotor' was already taken.
The expression "Absolutely Rigid Rotor" is used in the context that 'absolute' is the optimal, albeit impossible, objective.
From the report; THE ABC HELICOPTERby M.C. Cheney, Jr., United Aircraft Corporation ~ Feb 1969
"It is evident that an infinitely rigid rotor not burdened with roll trim requirement [i.e. twin rotors] could maintain constant dynamic pressure on the advancing blade by reducing rpm and thus avoiding exceeding the drag divergence Mack number until the rotor was stopped. This, of course, implies potential helicopter rotor forward speed capabilities exceeding 500 knots."
From the report; FORWARD FLIGHT PERFORMANCE OF A COAXIAL RIGID ROTOR by V.M. Paglino, United Aircraft Corporation ~ May 1971
"The rigid retention gives rise to increased control power, increased center of gravity travel and reduction of rotor head complexity through elimination of flapping and lagging hinges."
Rotor Flight Profile:
Posting on rec.aviation.rotorcraft by NL ~ March 29, 2002
To answer the basic question, "why not rigid?":
the stiffness of a "rigid" rotor brings goodies like sweet handling, large CG range and simplicity, but they bring costs, like higher vibration, heavier rotors and masts, more cross coupling (roll control is mixed with longitudinal). It is up to the designer to balance all the needs against the costs. Generally, weight wins and rigid rotors are not favored. For Comanche, the handling and maneuvering quickness was required, so it has a "rigid" rotor. I have banked from 60 degrees left to 60 degrees right bank, and touched 100 degrees per second in the maneuver. It took the blink of an eye to accomplish, because the Comanche is a fighter helicopter.
Posting on rec.aviation.rotorcraft by RC ~ March 29, 2002
Yes and no. A single main rotor helicopter with a rotor so rigid that it won't be able to flap won't be able to fly, except perhaps at hover, because there will not be anything to compensate for the dissymmetry of lift between advancing and retreating side. A co-axial rotor will be able to fly, but the reason for the absence of cross-couplings is not the rotor stiffness, but the presence of the two counterrotating rotors.
A stiffer rotor has *higher* cross-couplings, unless the root section is so flexible in flap that the rotor behaves like an articulated rotor. The reason is that a stiffer rotor has a higher natural frequency in flap (about 1.15/rev for a BO-105, compare with about 1.03/rev for a UH-60, for the Comanche Nick will tell you, but then he will have to kill you), so it will respond to the usual 1/rev excitation in forward flight with a delay different from 90 degrees: that number is valid for a blade hinged on the axis of rotation. For the BO-105 the delay is around 77 degrees, for the UH-60 around 86 degrees. The lower the delay the higher the cross-coupling.
What increases is the amplitude. If the blades are well tracked, the rotor acts like a filter, and only lets vibrations get into the fuselage at multiples of N/rev, where N is the number of blades, regardless of the rotor type. E.g., for a four-bladed helicopter, at 4, 8, 12, 16/rev, etc. IIRC the rotor speed for the UH-60 is 4.3Hz, so the 4/rev (which is usually the strongest) is about 17Hz. If the rotor is not well tracked you'll get all the harmonics.
The following needs converting from SynchroLite to UniCopter
Sikorsky S-69 ABC [XH-59A]
Lockheed Model 286 Rigid rotor helicopter
Article ~ The Second Generation Rigid Rotor in VERTIFLITE Fall/Winter 2000
Outside Web Pages:
An Examination of Selected Problems in Rotor Blade Structural Mechanics and Dynamics
http://rotorcraft.arc.nasa.gov/publications/files/Hopkins_AHS03.pdf Have hard copy.
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