The evolution of the modern helicopter is an ongoing process. Each new year brings advances in avionics, flight controls, aerodynamics and structural design. While evolving cockpits often provide the most visibly dramatic upgrades, the less obvious changes made to rotor blades over the years are no less important and have led to major advances in performance.
When helicopters first took to the sky, the earliest rotor blades were made like airplane wings — out of wooden ribs and spars, covered by fabric. Wood was an excellent building material at the time, particularly due to its nearly nonexistent fatigue life. It could endure the bending and twisting aerodynamic loads of rotary-wing flight virtually indefinitely. (In fact, some of the more colorful old-timers with whom I used to fly liked to talk of the “days of old,” when the only things they really worried about were “blade fires and woodpeckers.”) This is not to say that wood was free of problems. Blades had to be manufactured in pairs out of the same lot of wood in order to ensure a balanced set. If one blade was damaged beyond repair, both blades had to be replaced. The wood also tended to absorb moisture, which would dramatically throw things out of balance.
The development of metal blades remedied some of the issues evident in wooden blades, but introduced their own material shortcomings. Much like the continual bending of a paperclip will eventually cause it to break, the constant flexing of metal rotor blades will cause them to weaken over time. For this reason, metal blades have a fatigue life limit in flight hours, after which they must be replaced. Probably one of the biggest issues with metal blades is that any damage that occurs in a critical area can propagate quickly and catastrophically to failure. Nonetheless, metal-skinned blades with metal honeycomb cores provided a significant upgrade in blade strength and performance compared to wood.
Most recently, blades made of modern composite materials, such as fiberglass, carbon fiber or Kevlar skin covering a foam or Nomex core, have brought multiple improvements to rotor-blade design. Composites do have a fatigue life; but unlike metal, composite materials have a failure mode that occurs very slowly. The strength of composites lies in the way they are layered. Criss-crossed layers of fabric resist the propagation of cracks, so damage occurs gradually and noticeably and their life limit is much higher than that of metal. Composites also do not corrode, have a better strength-to-weight ratio and can be made with fewer joints and fewer parts.
Aerodynamically, composites have helped to push the limits of helicopter performance. Complex non-symmetrical airfoil and blade shapes can be achieved by creating custom molds. Composites are anisotropic materials, meaning they exhibit different strength properties depending on the direction they are placed. Therefore, blade stiffness and flex can be controlled by the specific layering direction and number of plies of material. This unique “elastic tailoring” of the blades reduces vibration, improves aeroelastic response and increases fatigue life, helping to match actual blade performance with the original paper design more closely than metal blades.
One of the few disadvantages of composites is their higher upfront cost. They are also more difficult to manufacture, requiring more effort during both fabrication and finishing. But manufacturers believe that the long-term benefits to the customer will outweigh any disadvantage.
In February 2016, composite rotor-blade manufacturer Van Horn Aviation based in Tempe, Arizona, received FAA supplemental type certification of composite main rotor blades that fit the Bell 206B JetRanger. The blades have been approved with an 18,000-hour service life, more than three times that of the original manufactured metal blades. The composite blades can go 2,800 hr between overhauls. Van Horn hopes to have an STC for the Bell 206L in 2017.
As manufacturing challenges continue to be overcome each day, numerous companies are investing more heavily in composite main and tail rotor-blade design. The latest this technology has to offer is sure to breathe new life into existing helicopter models and will most certainly play a major role in new rotorcraft designs to come. R&WI