Future Falcon business jets may use composite materials for their wings, but the fuselages will likely continue to be constructed from aluminum alloys. Speaking at a conference organized by the French Air and Space Academy in Paris in late November, Philippe Vautey, a Dassault senior expert in aerostructure technologies, outlined the company’s vision for Falcons that might enter service “four to eight years” from now. He would not clarify whether this includes the still-under-wraps Falcon SMS.
A 200-kilogram (440 pound) weight increase equates to a 2-percent gain in lift-to-drag ratio at the aerodynamic level and a 2-percent increase in specific fuel consumption for the engines, Vautey explained. Design engineers looking to save weight must consider three basic questions when choosing between composites and metal, according to Vautey: How much weight are we saving, if any? What are the maintenance costs? What are the production costs?
The questions may be the same, regardless of the aerospace company you work for. But the answers can be different if you work for Airbus or Dassault.
“For a business jet, the purchase price is a stronger driver for purchase decision than it is for airliners,” Vautey emphasized. For airliners, structural maintenance costs and overall direct operating costs are more important. Therefore, on a business jet the higher cost of composites over aluminum is more difficult to justify. While composite construction offers benefits related to fatigue and corrosion, business jets fly significantly fewer hours.
Moreover, the first major overhaul on a Falcon takes place at 12,000 flight hours. At an average utilization of 500 hours per year, the first buyer will have sold the aircraft before it reaches that requirement, Vautey said.
Vautey tells the story of a wealthy man at the delivery ceremony of his new Falcon. “I’m so happy about my new Falcon and its full-carbon wing,” the customer said, tapping on the airplane’s metal wing. The customer was likely influenced by the fact that his sports car had large parts made of carbon-fiber reinforced plastics (CFRP), and he had also probably heard of the major adoption of CFRP in Airbus and Boeing aircraft construction. At the time, Vautey had to explain to the new owner that Dassault had made the best choice–metal–for his new Falcon’s wing.
But that was yesterday, and this may change. Dassault’s research has shown that CFRP wing boxes could be 15 to 20 percent lighter than their metal counterpart, saving a few hundred kilograms per aircraft. Based on materials and processes that are at technology readiness level 5 today (TRL 5, technology demonstration), these weight savings might be achievable with no increase in production costs. Vautey cautioned, however, that questions still need to be resolved about design optimization, process automation and durability.
The fuselage is a different story, though, and metal is likely to remain the material of choice. Due to the relatively low level of stress imposed on it, aluminum alloy sheets used for its skin can be as thin as one millimeter. Trying to save weight by moving to CFRP could further slim these parts but makes no sense, according to Vautey, due to other constraints such as surface quality, lightning-strike protection and electrical grounding for systems.
In-development alloys, such as aluminum-magnesium-lithium, promise lower densities that could shave 5 to 8 percent off the fuselage’s structural weight–a few dozen kilograms. The higher material cost could be offset by replacing riveting with friction-stir welding, which Dassault is considering.
In-production metal Falcon wings could also benefit from upgrades with newer but mature alloys such as aluminum-copper-lithium, which offer a small gain in density along with higher structural properties, Vautey said.
Dassault will continue to use its installed aluminum machining facilities for a long time. One reason is the recent €12 million ($15.6 million) investment in expanding its factory in Seclin, northern France.
Another way to save weight could be to trade maintenance costs for kilos. Dassault is considering reducing the 12,000-flight-hour number for first major overhaul. Some material choices could be pushed in the direction of “less durable but lighter,” and “we could save dozens of kilos,” Vautey concluded.