Movimiento de las palas del autogiro

Tipos de autogiros y helicópteros, modelos, características, etc.

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NTWNTW
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Registrado: Sab Feb 03, 2018 20:28

Movimiento de las palas del autogiro

#1 Mensaje por NTWNTW » Dom Sep 02, 2018 11:43

Comprendo que sea de interés solo para los interesados en los autogiros, y dentro de estos, solo para aquellos que gusten del tema técnico, pero es un tema fascinante. Una de los grandes atractivos del autogiro son las peculiaridades técnicas de su funcionamiento, que no son tan conocidas. A continuación, copio y pego un 'post' publicado, hace ya años, por un importante experto en la materia (Chuck Beaty) en el 'Rotary Wing Forum'. Es muy notable que el movimiento en batimiento (flapping) de las palas de un autogiro sea solo cosa de punto de vista, literalmente...


Rotorblade motion
08-25-2012, 12:01 AM
Rotorblade Motion and Control

Does it flap?

What is the axis of rotation of a rock being twirled on a string? The twirler’s forearm? His wrist? His thumb and forefinger?

Actually, none of the above. The rock clearly rotates about the axis of the circular path it travels.

So it is with freely hinged rotor blades.

The early fixed rotorhead Autogiros that were controlled by tilting the complete airframe via ailerons and elevator certainly presented the appearance of flapping rotorblades to the view of a stationary observer. The advancing blade ascended and the retreating blade descended relative to the rotorhead.

However, viewed from the real axis of rotation, the tip plane axis, the blades don’t flap, don’t speed up or slow down and the only thing that might seem strange to a casual observer is a cyclical variation of pitch.

Axis of view

Gyroplanes controlled by tilting the rotorhead have two axes from which the rotor may be viewed; the tip plane axis and the rotorhead axis. Swashplate controlled helicopters have three possible axes of view; tip plane axis, powered shaft axis and swash plate axis.

Any view except along the tip plane axis is quite complicated and requires some fancy mathematics for analysis.

Viewed from the rotorhead axis, the blades appear to flap and with a coned rotor, the CG of the upward flapping blade moves nearer to the center of rotation and must speed up to conform to conservation of energy law. The retreating blade must slow down since its CG moves away from the center of rotation. The same law that governs pirouetting ice skaters as they tuck in or spread out.

Cierva’s rotor analyses always used the rotorhead frame of reference, perhaps to bedazzle and befuddle his competitors. When the rotorhead reference frame is used, to make the math work, Coriolius theory must be applied to explain the need for drag hinges on rotors with three or more blades.

The NACA as well as textbook authors picked up Cierva’s analysis and ran with it. Gessow and Meyers (“Aerodynamics of the Helicopter”), to their credit, point out that Coriolius forces are in the eye of the beholder.

Drag Hinges

Viewed from the tip plane axis, the blades don’t speed up or slow down and drag hinges are a kinematic rather than a dynamic necessity.

Flap and drag hinges serve the same purpose as the universal joint of a socket wrench as illustrated in figure 1. With only a flap hinge, tilting the rotorhead of a coned rotor would require swinging the rotorblades through an arc, rendering control by human muscle power impossible.

With both flap and drag hinges, tilting the rotorhead can only rotate the blades about their feathering axes. Flap/drag hinges also permit the rotorblades to rotate at uniform angular velocity about an axis that differs from the rotorhead axis, like the rock on a string.

The Autogiros of Cierva employed a rotorhead configuration similar to that depicted in figure 1 with the flap hinges crowded as close together as possible; but nonetheless, very high stick force was required for cyclic control. With several tons of centrifugal force acting on each blade, there is high resistance to rotorhead tilt; the “T” bar effect.

The Sikorsky S-51 helicopter rotorhead employed centrally located flap hinges as shown in figure 2. Any modern tilt head 3-blade gyro should utilize a similar scheme.

Teetering Rotors

Cierva experimented extensively with 2-blade rotors but experienced little success. The vibration problem appeared to be insurmountable. There is a 2/rev aerodynamic drag variation that can’t readily be solved through the use of drag hinges that must be located far enough outboard from the center of rotation for centrifugal force to keep them in alignment. Then the “T” bar effect comes into play and centrifugal force doesn’t permit sufficient drag hinge motion to accommodate aerodynamic drag variation. In the case of 3-blade rotors, the periodic aerodynamic drag force of each rotor blade, when added together, becomes a steady force.

Arthur Young, the designer of the Bell-47 helicopter, was the first to solve the vibration problems of 2-blade rotors; at least partially.

Young’s solution was to undersling the rotor so as to locate the teeter bolt at the CG of the coned rotor and to allow the periodic drag variation to be accommodated by providing the softest possible mounting of the rotorhead. With soft mounting, the rotor is free to move fore and aft relative to the airframe. The rotor doesn’t actually move fore and aft relative to the airstream; it merely has a 2/rev speed variation.

Another requirement is for the mass of the rotorhead and stuff mounted thereupon to be as small as possible. Mounting electric starter motors, batteries and the like at the rotorhead may provide a ride with less shake of the airframe but increase the periodic in-plane flexing of the blades and hub. Some attempts at utilizing crisscrossed seesaw rotors have resulting in cracks developing in the rotorblades.

Teetering rotors combined with a rigid rotor pylon can’t provide a smooth ride and can even be dangerous due to the increased stresses imposed on the blades and hub.

Cyclic pitch control

The traditional explanation for equalization of lift between advancing and retreating sides of the rotor disc is that the advancing blade flaps upward and the retreating blade flaps downward; the vector sums of rotational velocity, forward velocity and flapping velocity reduce the angle of attack of the advancing blade and increase the angle of attack of the retreating blade. True, but needlessly convoluted.

Since a hinged rotor system permits the rotor plane to have an axis that need not be aligned with the rotorhead axis, the concept of lift equalization is simple and straightforward when the rotor is viewed relative to the rotor plane.

In forward flight, the axis of the rotor plane tips rearward with respect to the rotorhead axis; the amount dependant upon airspeed.

Referring to figure 3, blade pitch is fixed relative to the teeter bolt. The blade on the near side (advancing side) has less pitch than the blade on the retreating side. Lift is thus equalized without jumping through vectorial hoops. It is important to understand the equality of cyclic “flapping” and cyclic pitch.

The rotorhead of a tilt rotor system is an exact equivalent of a swashplate controlled feathering rotor system minus the collective pitch capability. A tilt head gyro in no way is, as some have speculated, a kind of weight shifter like a trike. There is no way the rotor could be tilted against its own inertia without cyclic pitch control.

NTWNTW
ASESOR TÉCNICO
Mensajes: 80
Registrado: Sab Feb 03, 2018 20:28

Re: Movimiento de las palas del autogiro

#2 Mensaje por NTWNTW » Mar Sep 04, 2018 12:58

Como parece que el asunto no atrae mucha atención, quizá porque la explicación sea algo oscura, adjunto dos ilustraciones. Se trata de un hecho no muy conocido entre los autogiristas, en cuanto a que el batimiento de las palas produce una rotación del eje de éstas, lo que varía cíclicamente su ángulo de ataque. Todo ello, claro, visto desde el eje del plano generado por las puntas de las palas al girar, que determina el eje real de rotación...

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