Airframe Technology
Advanced Composites & Helicopters
By Greg Mellema
September 2004
In aerospace, we rarely consider a material's strength without regard for its weight. The primary advantage to composite structures is not that they are any stronger than similar metallic structures, but that they are lighter. Helicopter manufacturers have exploited this concept almost from the beginning out of necessity. While weight is an important consideration in fixed-wing design, it has always been paramount in the design of a helicopter largely due to the fact that early helicopter engines were notoriously underpowered.
The Care Flight A-Star helicopter is an example of composites usage, such as the engine cowling being analyzed by mechanic Jeff Vanderwall.
Photo courtesy of Abaris Training, 2004
The evolution of composites
Wood and dope and fabric structures in the 1930s and '40s gave
way to thin-skinned adhesively bonded aluminum structures in the 1950s
and '60s. Many components continued to evolve as fiberglass technology
improved through the '70s and by the mid 1980s fuselage sections
and very large components were being manufactured entirely from carbon,
aramid, or fiberglass composites.
Improving helicopter performance has traditionally been approached in two ways; designing smaller, more powerful engines, and making the airframe lighter. Without question, advancements in engine technology have contributed to the overall performance of helicopters. In fact, the incorporation of the turbine engine is considered the third most important aspect of helicopter development. However, also on the list are unlimited life composite rotor blades and composite fuselage structures.
Advanced Composites 101
How can composite structures be lighter and yet carry the same loads
as their metal counterparts? To answer this we must examine the nature
of the two materials. First, metals are isotropic materials. This
means that they will exhibit the same strength characteristics
no matter which direction we impose a load on them. Any structure
built from such a material would then have inherent strength in
directions it may not need. It would therefore be heavier than
it needed to be to do the job.
In advanced composite structures we rely on continuous fibers embedded in a resin matrix to carry the loads imposed on a part. These fibers are usually in the form of a woven cloth or fabric. A laminate is composed of multiple plies of a fabric and each ply can be oriented in any direction we choose. Since engineers are usually able to predict the type and magnitude of loads a part will be expected to handle, we can engineer strength only in the directions it is needed and eliminate it where it is unnecessary. The end result is a structure no heavier than it needs to be to perform a particular task.
Another significant advantage to advanced composite materials is the variety of properties available to us from the different materials. Fiberglass, aramid, and carbon fiber all have properties that make them particularly well-suited for some tasks and not others. For instance, fiberglass has a fairly high strength-to-weight ratio, good environmental resistance, and is quite flexible (low modulus). This particular set of properties makes it an excellent candidate for making main and tail rotor blades.
Carbon fiber is an extremely versatile material. While it is generally higher modulus, or stiffer than fiberglass, it comes in a wide variety of strength and stiffness combinations making it suitable for many different applications. Because it is stiffer and lighter than fiberglass it is more likely to be used in the fuselage or tail boom of a helicopter, although its stiffness makes it an ideal candidate for the spar in a main rotor blade. In the latter case a helicopter manufacturer may elect to use graphite fiber. A graphite fiber starts its life as a carbon fiber, but goes on to be processed at considerably higher temperatures graphitizing the carbon. This additional processing allows structures to achieve higher strength-to-weight or stiffness-to-weight ratios than with carbon fiber.
Aramid, or Kevlar is an extremely tough, durable fiber featuring a very high tensile strength. It is also the lightest of the advanced composite materials and sees extensive use throughout the helicopter industry. This combination of properties is particularly desirable in the design of landing gear doors, transmission and engine cowlings, and driveshaft covers. Unfortunately, the light weight and durability come at a cost.
Composite repairs
Aramid has a nasty tendency to wick up moisture or any other liquid
it is exposed to such as fuel or oil. The strength of our composite
structures is often compromised by the intrusion of water. This
fact by itself is bad enough, but when we consider that many of
our composite repairs are heated in order to cure the resin system
our problems increase dramatically. Water entrained in a honeycomb sandwich
structure flashes to steam during an elevated temperature cure
and can generate enough pressure to blow the skin off the core
surrounding the repair. Additional steps must be taken to dry the
structure prior to such a repair. Kevlar skins or Nomex honeycomb
core contaminated by engine oil, hydraulic fluid, or fuel can never
be dried or adequately cleaned by flushing with solvents. It must be treated
as damage and removed.
In order to achieve unique combinations of properties engineers sometimes use two or more types of fibers in a laminate. This type of structure is called a hybrid laminate. For instance, let's say that the bulk of a landing gear door structure is carbon fiber. But, since carbon is relatively brittle and more easily damaged we'll lay up one or two plies of Kevlar on the outer surface to resist potential damage from rocks and debris kicked up from the landing zone. The Kevlar plies do carry some of the load, but their primary purpose is to shield the carbon from potential damage. This is just one example of how we can mix and match fiber types to accommodate specific needs.
There's no question that advanced composite materials are here to stay. The dramatic increase of strength-to-weight ratios possible with composites has won them a place in helicopter history and assures that place well into the future.
Greg Mellema is an instructor with Abaris Training in Reno, Nevada. Abaris conducts training in repair, fabrication, and design of advanced composite aircraft structures. Mellema holds both an Airframe & Powerplant certificate as well as an Inspection Authorization (IA).
