2008

Flipperons for Improved Aerodynamic Performance

For a given airfoil design, lift is increased and drag reduced.

Lightweight, piezoelectrically actuated bending flight-control surfaces have shown promise as means of actively controlling airflows to improve the performances of transport airplanes. These bending flight-control surfaces are called “flipperons” because they look somewhat like small ailerons, but, unlike ailerons, are operated in an oscillatory mode reminiscent of the actions of biological flippers.

The underlying concept of using flipperons and other flipperlike actuators to impart desired characteristics to flows is not new. Moreover, elements of flipperon- based active flow-control (AFC) systems for aircraft had been developed previously, but it was not until the development reported here that the elements have been integrated into a complete, controllable prototype AFC system for wind-tunnel testing to enable evaluation of the benefits of AFC for aircraft.

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Figure 1. The Piezoelectric Crystal in a lightweight bending actuator is maintained in compressive preload by a spring-steel substrate.
The piezoelectric actuator materials chosen for use in the flipperons are single-crystal solid solutions of lead zinc niobate and lead titanate, denoted generically by the empirical formula (1 – x)[Pb(Zn1/3Nb2/3)O3]:x [PbTiO3] (where x<1) and popularly denoted by the abbreviation “PZN-PT.” These are relatively newly recognized piezoelectric materials that are capable of strain levels exceeding 1 percent and strain-energy densities 5 times greater than those of pre viously commercially available piezoelectric materials. Despite their high performance levels, (1 – x)[Pb(Zn1/3Nb2/3)O3]:x[PbTiO3] materials have found limited use until now because, relative to previously commercially available piezoelectric materials, they tend to be much more fragile.

What has made it feasible to incorporate (1 –x) [Pb(Zn1/3Nb2/3)O3]:x [PbTiO3] crystals into flipperons is a design and fabrication approach in which the crystals are preloaded and reinforced so as to minimize exposure to tensile stresses, which could break them. The essence of this approach is to place the piezoelectric crystals in each actuator under a compressive preload along the fore-and-aft axis and to bond tapes of uniaxial carbon fibers to the outer surfaces of the crystals to minimize lateral tensile strain in each crystal (see Figure 1). By minimizing tensile strains in the crystals, one minimizes crystal damage, thereby minimizing the probability of actuator failure.