Future Flight: Composite Changes

Part 7 of 12

An easy-to-understand composite material would be an adobe brick, which is wet mud and straw that are mixed together and then dried. The result is a material that is stronger than either mud or straw. If thermosetting of epoxy resin and carbon, graphite, or glass fibers is substituted for the dried mud and straw in the adobe brick, the basics of composite structures can be understood.

While many people think of composite airplanes as those little ones that people build in their garages, in reality anyone who has ridden in a Boeing 777 or an Airbus A320 has ridden in an airplane with a largely composite empennage.

If viewed only from an aesthetic point of view, the smooth, sensuous shapes of composite airplanes are prettier and more pleasing to the eye than the aluminum airplanes designed a few decades back. This can be verified at any airshow—a cluster of people always gather around the composite airplanes. It doesn't matter whether the airplane is a Glastar homebuilt, one of the new production composite airframes, or a Beech Starship; composite airframes are exciting.

History

In the late 1950s, composite sailplanes were certified in Germany. Since 1972, when pioneers such as Ken Rand of KR–1 and KR–2 design fame started producing flying composite airplanes, the utilization of composites in airplane construction has grown—from an inexpensive, easy-to-use material for a few free-thinkers who wanted small, easy-to-build flying machines into the airframe building material-of-choice for companies such as Raytheon, Cirrus, AASI, Diamond, and Lancair.

The fact that the first two general aviation airplanes to garner new type certificates in the past several years utilize all-composite airframes shows that the technology necessary to use this material for four-place certified, mass-production airframes is presently available. While these successes are encouraging, both Lear Aircraft and Beechcraft spent tremendous amounts of money in the late 1980s attempting to build and market all-composite-construction business class aircraft. While these early efforts at all-composite construction are noteworthy, they didn't result in _any airplanes that were competitive in the market.

Business-class composite airplanes

Bill Lear's Lear Fan 2100 was built with an all-composite airframe featuring graphite/epoxy and Kevlar/epoxy materials. The weight savings over conventional structural construction techniques was 40 percent. The airplane was never certified because of financial concerns.

The Beechcraft Starship, a twin-turboprop pusher design, was the first all-composite pressurized airframe ever certified. The wing structure used graphite/epoxy skins with Nomex honeycomb cores. Beechcraft built 53 Starships in the late 1980s and early 1990s before ceasing production.

Today there are a number of manufacturers that promise delivery of a cabin-class all-composite business airplane with great performance numbers and reasonable prices, but none is certified at this time. The poor track record of larger all-composite designs; senior design engineers who are comfortable and familiar with metal construction; and improvements in metal construction materials, machinery, and techniques are all reasons why a number of today's airplane manufacturers are opting to produce a hybrid airplane instead of an all-composite design.

Hybrids combine aluminum and composite construction. The Safire S–26, Raytheon Premier I, Hawker Horizon, and AASI Jetcruzer 500 are all recent designs that fit the hybrid construction label because of their aluminum airfoils and composite fuselages.

The new Eclipse 500 will be manufactured of conventional aluminum construction, apparently because of the preferences of the engineering team hired to head the production effort.

Composite construction techniques

First-generation methods were simple but required a lot of hand-finish labor to achieve satisfactory results. This technique used blocks of polystyrene or urethane closed-cell foam that were cut to shape by drawing an electrically heated wire over forms.

After the foam was shaped, the skin thickness was built up by applying layers of fiberglass cloth and resin. This method is known as foam-core construction, or the Rutan method.

Since the resin was semifluid and flowed under the influence of gravity, there was a lot of handwork necessary to smooth the surface and remove sags and resin buildup, but the system was workable. Setup costs were minimal, and as long as cleanliness and the temperature restraints of working with glues and fiberglass layup materials were rigidly adhered to, strong light structures could be manufactured locally with inexpensive materials.

Molds

While foam-core construction served as a workable system for basic wing and empennage structures, it wasn't too practical for fuselage construction—nor was it suitable for large-quantity production runs. Enter the mold. In the mold system either a male or female mold is constructed and then cloth and resins are applied. After the resins cure, the finished part is removed from the mold and any imperfections are cleaned up. Development of the mold construction technique was necessary for manufacturers who needed to be able to produce numbers of like parts.

Cold molding and hot molding

When the resins are applied at the optimum 70 degrees Fahrenheit and cured by the addition of a catalyst, the process is called cold molding. Soon it was learned that fiberglass cloth that was preimpregnated with resin, compressed into a mold by atmospheric pressure, and heated to approximately 250 degrees F would yield a homogeneous part with better resin control and a tighter sandwich, if you will.

By simply covering the prepreg cloth laid in the mold with a piece of plastic that was sealed airtight at the edges, and then evacuating all the air under the sealed plastic bag with a vacuum pump, the pressure of the atmosphere could be brought to bear on the laid-up prepreg. Sliding the bagged part into an oven that could achieve and maintain 250 degrees F was sufficient to cause the preimpregnated resins to flow out.

This hot-mold technique was a step forward from the hand-layup method, mainly because an even overall pressure could be applied to uniformly force the layers together. Also, the amount of resin in the prepreg cloth could be tightly controlled. This led to more consistent control of the weight and quality of the finished part. Vacuum-bag construction is referred to as the low pressure cure (LPC) method.

Autoclaves

LPC is fine for smaller airframes, but when many composite layers are required to adequately handle the higher stresses of larger airplanes, a method of controlling and applying higher pressures while heating the part was developed. An autoclave is, in essence, a composite-part pressure cooker that can apply heat and higher pressures than the vacuum-bag method. Typical pressures in an autoclave are 100 psi, although some are capable of 300 psi. This method is called the high-pressure cure (HPC) technique and requires a much greater financial investment than the LPC construction. The autoclave that Beechcraft used for its Starship is reputed to cost $100 million. The same autoclave is presently being used for composite fuselage construction of the Premier I.

Production for the future

Fuselages, being tubular in shape, can be built by winding filaments or tapes of fiberglass, carbon, Kevlar, or other reinforcements around a full-size mold. The tape or filaments are usually preimpregnated with resin so that after winding all the layers over the mold, the whole assembly is slid into an autoclave for curing. This technique is being used on the Premier I and the Horizon. The wound-tape or filament technique eliminates some of the bonding steps now necessary with two-half construction, and the possibility of joint delamination and pressurization leaks. The drawback of this technique is the requirement for complicated and expensive tooling.

Bonding

Bonding or gluing composite parts together to form a complete structure is common. For instance, top wing skins and bottom wing skins produced by the LPC method are bonded together to form a complete wing structure. Military and commercial users often back up the bonded joints with hardware fasteners as a fail-safe backup system. In general aviation, hardware fasteners are generally only used during the bonding process to apply pressure.

One of the FAA's requirements for certification of composite airplanes is a test of the manufacturer's bonded joint techniques. As part of the production certification testing, each company is required to assemble a complete airframe using mishandled bonding materials, poorly joined seams, and below-standard quality control. This worst-case airframe is then subjected to an extensive series of stress tests at normal and elevated temperatures.

Lancair put its worst-case Columbia 300 airframe through 170,000 two-second-long stress cycles in blocks of 5,000 to 15,000 cycles with no failures. Included was a test cycle done at 180 degrees F. The FAA is very concerned about bonded joints. The Lancair airframe, with stress sensors installed throughout the structure to detect stress levels and failures, passed the testing required for certification with no problems.

One-cure construction

Burt Rutan and his group of engineers at Scaled Composites Inc. in Mojave, California, recently completed a flying proof-of-concept airplane called the Adam M–309. One of the landmarks achieved during this airplane's design and construction was that a number of composite airframe parts that ordinarily would require bonding for completion were fabricated using a newly developed technique that did away with the bonding requirement.

This advanced technique was used on the wing outer panels, the horizontal stabilizer, the flaperons, and the rudders of the Adam M–309. Rutan referred to this technique as single-cure construction. Engineers at Scaled Composites said that this construction method gives the same strength as bonded construction with 25 percent less weight.

The execution of this concept is an example of how the leaders in the composite industry are developing methods that eliminate some of the labor-intensive steps previously required for construction and certification of composite structures. In addition to the impressive weight savings, the new techniques will lower costs because less labor is required and parts count is reduced.

"Fast-build" kits for homebuilt composite airplanes are common today. Thanks to improved manufacturing materials and procedures, the build times for some of Lancair's kits have been reduced from 6,000-plus hours to as few as 1,500 hours.

Possibilities for the future

Based on the progress that's being made in composite construction techniques, it's a sure bet that the light airplane of tomorrow will be manufactured out of composite materials, at least in part. Because of advances in future powerplants, it will be easy to pressurize the cabins. New technology is available that may result in full cabin active noise reduction and next-generation airframe ice-shedding equipment.

Tomorrow's general aviation powerplants will be based in diesel technology and pocket-size, high-bypass turbofan engines. Designing the turbocharging/ supercharging systems of the diesels and the compressor sections of the turbofan engines for enough capacity to pressurize the cabin of tomorrow's airplanes should be considered since pressurization, even at lower altitudes, reduces fatigue and increases safety.

A cabin active noise-reduction system developed and STCed by Quiet Flight LLC, of Dallas, Texas, combined with the installation of passive sound reflection and absorption materials, has reduced cabin noise levels into the mid-80 dBA levels in Cessna 210 and 310 models (see " Pilot Products: Quiet Flight Cabin Noise-Canceling System," October 1999 Pilot). This technology reduces pilot fatigue, can be fine-tuned to each airplane, and doesn't require any increase in pilot work load.

In 1995 NASA licensed an airframe ice-shedding system that promises to be an improvement over current inflatable rubber boots or alcohol applications. The Electro-Expulsive Separation System (EESS) has been licensed to Ice Management Systems Inc. of Temecula, California. This system sheds ice by shooting a microsecond-long burst of high-current electricity through a thin back-folded conductor that is installed on the airframe. Magnetic fields are created that repel each other, and the surface of the system moves instantaneously, shattering any ice into table salt-size particles. An ice-detection system is under development that will automatically trigger the system when ice accumulates.

All-composite airframes are a reality today, especially in smaller airplanes. Components and entire sections of business aircraft and Transport category airplanes increasingly will be made of composites. The reasons are obvious. Every composite engineer repeatedly says that composites don't fatigue like metal; composites don't corrode like aluminum; and composites can be molded and formed into shapes that are difficult and expensive to achieve using today's metal technology. Repairs of minor damage often can be accomplished in the field without a large outlay of money, using readily available materials.

The future of composites

There are still some hurdles to overcome before composites will replace aluminum completely, especially when it comes to larger airplanes. Improved manufacturing techniques, further education of the approving government agencies, and a history of successes by composite airplane manufacturers will change airplane manufacturing techniques in the twenty-first century. Right now, it appears as if manufacturers are being conservative by combining improved aluminum and metal construction techniques, where they work best, with composites—where they work best. Engineers will tell you that their hybrid designs have been created with all-composite construction in mind, but first they have to sell airplanes.

Next month's "Future Flight" will explore developments in engine technology. E-mail the author at [email protected] .