Flap Rotors

There have been several attempts to harness unsteady mechanisms similar to those found in nature, such as flapping and pitching, to enhance the performance of conventional lift production mechanisms. For example, in a conventional heli-copter rotor in hover, an airfoil at any spanwise location on the rotor blade experiences a steady aerodynamic environment. At higher levels of rotor thrust, as the airfoils operate close to their static stall angle, they experience a loss of lift and an increase in drag, resulting in a decrease in the hover efficiency of the rotor. It may be possible to improve this efficiency by creating an unsteady aerodynamic environment at the airfoil. The unsteady motion can be created mechanically in different ways. Due to the large forces involved in creating such motion, this approach is only feasible at the microscale.

Bohorquez and Pines [61] developed an active flapping and pitching mechanism for a 20 cm diameter, two-bladed helicopter rotor. The mechanism enabled the rotor blades to be actively pitched and flapped in an oscillatory fashion at a frequency independent of the rotor speed. The goal of the oscillatory pitching was to induce dynamic stall on the rotor blade, resulting in a large increase in lift coefficient.

The goal of the flapping motion was to generate a radial flow along the rotor blade, which was expected to stabilize the leading-edge vortex created during the dynamic stall event. An appropriate combination of flapping and pitch-ing amplitudes as well as frequencies was deter-mined by experiments.

FIGURE 5.13 Mechanical samara microflyer, total mass 9.5 g, maximum dimension 75 mm [59]. Credits: E.R. Ulrich, D.J. Pines, J.S. Humbert, From Falling To Flying: The Path To Powered Flight Of A Robotic Samara Nano Air Vehicle, Bioinspiration & Biomimetics, 5, 045009, 2010.

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A schematic of the rotor hub with articulation for flapping and pitching is shown in Figure 5.14. A scotch-yoke mechanism converts rotary input from a small electric motor into linear motion that is conveyed through the hollow rotor shaft into a lever mechanism on the rotor hub. These levers actuate the rotor blade in flap-ping and pitching about their respective axes.

Large-amplitude flapping (total angle of 46°) and pitching (±20°) motion is possible, and the frequency of the motion depends on the rota-tional speed of the motor. A picture of the rotor

with the blades at a large flap angle is shown in Figure 5.15.

The mechanism was tested by spinning the rotor at 2,000 rpm, keeping the flap motion fixed and prescribing a pitching motion of amplitude 6° at frequencies varying from 0.25 to 2 per revo-lution of the rotor. It was observed that while operating at mean pitch angles close to the static stall angle of the airfoil, the oscillatory pitching resulted in around 50% improvement in hover efficiency. As expected, there was negligible effect at lower angles of attack, where the

Rotor blade Rotation

Flapping axis

Pitching axis

Actuation through rotor shaft

FIGURE 5.14 Schematic of flapping rotor operation. Adapted from Ref. 61. F. Bohorquez and D.J. Pines, Design and development of a biomimetic device for micro air vehicles, Volume 4701, 503–517, SPIE 2002.

Rotation Rotor blade

Flap angle

FIGURE 5.15 Rotor blades actuated to a high flap angle. Adapted from Ref. 61. F. Bohorquez and D.J. Pines, Design and development of a biomimetic device for micro air vehicles, Volume 4701, 503–517, SPIE 2002.

oscillatory pitching does not cause dynamic stall to occur. In addition, the largest improvements were observed at low pitching frequencies (0.25–0.5 per revolution).

A further extension of this concept is to use oscillatory flapping of the rotor blades to reduce or eliminate the torque required to rotate the rotor. This idea is based on the Knoller–Betz effect:

When an airfoil undergoes plunging motion in an incident freestream velocity, it can produce thrust, i.e., a force opposite the direction of the freestream velocity. Flyers with flapping wings utilize this effect to generate a propulsive force in flight. The effect is summarized in Figure 5.16. The airfoil is shown plunging in an incident freestream of velocity V. The plunging displace ment of the airfoil is h, and the apparent velocity of the air is Vh= dh/dt. The resultant velocity incident on the airfoil is Vres at an angle of attack ∝. The lift L and drag D on the airfoil are perpendicular and parallel, respectively, to the resultant inci-dent velocity. The thrust or propulsive force Fp

is given by the summation of horizontal compo-nents of L and D, i.e.,

Therefore, based on a specific range of values of freestream velocity and plunging velocity, it is possible to create a positive propulsive force. This effect forms the basis of a unique microflyer devel-oped by Jones and Platzer [62] in which the lift and propulsive force are generated by a pair of (5.6) F_p=L sinα−D cosα.

straight biplane-like wings located at the rear of the vehicle, flapping in opposition to each other.

This gives the two wings an oscillatory pitching and plunging motion with respect to each other that results in both a lift force and a thrust force.

Heiligers et al. [63] developed a single-rotor helicopter, called the Ornicopter, with a mecha-nism that actively flapped the blades. The flap-ping resulted in the production of a propulsive force on the blades that created the torque required to spin the rotor. As a result, there was no reaction torque on the helicopter fuselage. A radio-controlled model helicopter was modified to accommodate the required flapping mecha-nisms. A series of experiments was performed to evaluate the yaw control authority and the optimum settings of rotational speed and flap-ping amplitude [64].

The rotor diameter was 1.5 m and the flap-ping was phased such that opposing pairs of blades on the four-bladed rotor flapped with the same phase and were out of phase with their neighboring blades. In this way, oscillatory iner-tial forces along the rotor shaft were eliminated.

The prototype was tested at a rotational speed of 500 rpm, over a range of collective pitch settings and flapping amplitudes. Torque measurements indicated a range of settings over which thrust was produced at zero rotor torque. For example, at 4° collective pitch and a flapping angle of 8.3°, the rotor produced 8 N of thrust at zero torque.

Yaw control was achieved by varying the

h V

V_h

V_res

D L

α

F_p

FIGURE 5.16 Schematic of thrust production by a plunging airfoil in a freestream.

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flapping amplitude at a given collective pitch setting. Note that rotor thrust was insensitive to flapping amplitude at a constant collective pitch.

Future work should focus on increasing the thrust produced by the rotor by operating at higher rotational speed.

Fitchett and Chopra [65] developed a micro-scale rotor, called the Flotor, that was powered by blade flapping (Figure 5.17). A prototype was constructed and tested in three modes: pure flap-ping, pure rotation, and combined flapping/rota-tion. The geometry of the blades of the Flotor, as well as the rotational speed, was determined based on the wings of bats of similar size and their reduced frequency in cruise flight. The pro-totype rotor had two blades that were flapped in phase, a rotor radius of 80 mm, and a blade aspect ratio of 6.5. To ensure that pure flapping initiated rotor rotation in the correct direction, the blades were constructed with a main spar at the leading edge. This resulted in sufficient elastic twist in the blades to generate the appropriate propulsive forces at low rotational speed. The blades were constructed out of 0.25 mm thick mylar sheet and a carbon fiber framework.

In the pure flapping mode (passive rotation), a maximum disk loading of 10 N/m² was meas-ured, which is low compared to conventional

shaft-driven rotors. For the pure rotation and combined flapping/rotation tests, rigid blades with a circular arc profile were tested. Com-pared to the pure rotation cases, an increase in thrust of up to 20% and a decrease in torque of up to 30% were measured during combined flapping/rotation. Several recommendations were made for future research leading to a flight-capable prototype.

Although the concept of active blade flapping has been shown to enhance the performance of a conventional rotor, the main challenges to this approach are the mechanical complexity of the rotor hub, the inertial forces due to active blade-flapping, and the additional power required by the blade-flapping mechanism. These challenges must be addressed appropriately to enable flight testing of such a configuration.