AIN Blog: Component Improvements Drive Aircraft Efficiency Gains

 - December 28, 2015, 7:55 AM
GE turboprop schematic
GE Aviation entered the GA turboprop segment with a series of engine announced in late 2015.

Breakthroughs in aircraft design are few and far between; what we are seeing now are incremental improvements that nevertheless aim for ever greater efficiency and performance. Many of these improvements are under the skin of modern aircraft, such as new avionics, improved systems and, most important, the engines that make flying possible.

While there have been major breakthroughs in electric motor design, it’s unlikely that electric-powered aircraft will be carrying significant loads–more than a handful of passengers–anytime soon. Battery technology, while gaining ground, just isn’t up to the task and might never match the energy density of carbon-based fuels.

That said, we do expect to see widespread adoption of lithium-ion for main-ship battery applications. Manufacturers such as True Blue Power know how to make these batteries with safe lithium-ion technology and, compared with nicad versions, these batteries are far more reliable, last longer, deliver more power for low-temperature starts and operate much longer in a battery-only electrical emergency.

Turbine engine designers have steadily improved efficiency by an average of about one percent per year during recent decades, and this points to reliably improving performance. Unfortunately, turbine engine performance comes at a high cost, and although there are some promising small engines in the works, there is little opportunity to dramatically lower the cost of turbine engines and spark a resurgence in general aviation powered by turbines replacing pistons.

Nevertheless, GE Aviation’s entry into the 850- to 1,650-shp market offers some long-awaited competition, and this will surely stimulate new turboprop designs powered by both GE’s new engine and Pratt & Whitney Canada’s PT6. The first to emerge will be Textron Aviation’s GE-powered single-engine turboprop, but given the popularity of this segment–driven by Daher’s TBM series and Pilatus’s PC-12–we expect to see further developments.

The slow but steady pace of engine improvements doesn’t mean there aren’t technological changes that could speed up engine development. Ceramic matrix composites are headed toward production engine programs. And engine manufacturers are pouring many millions of dollars into 3-D printing of complex components, which should ultimately lower costs and accelerate prototype development.

Fly-by-Wire Technology

The trend on the business jet side is toward increasing complexity that makes pilots’ jobs easier. Most clean-sheet new business jet designs have fly-by-wire flight control systems, witness Embraer’s Legacy 450/500, Gulfstream’s G500/G600 and Bombardier’s Global 7000/8000. For those who say that fly-by-wire is suited only to larger aircraft, the Bell 525, although a relatively heavy helicopter, shows that modern flight controls make sense in smaller packages.

Thales, which is developing the fly-by-wire rudder control system for Textron Aviation’s Longitude, doesn’t see any reason why fly-by-wire won’t be beneficial for smaller business jets, too. While the weight savings aren’t as significant as for larger aircraft, they are a factor. But more important are the safety benefits of fly-by-wire. Flight envelope protections built into fly-by-wire systems can help get pilots out of trouble and could eventually eliminate the need for some upset recovery training, although pilots will always need training in the fundamentals of flying, especially how to recover a fly-by-wire aircraft in a mode where protections have been removed. Recent crashes involving fly-by-wire airliners show that further training is an urgent necessity. As more OEMs gain experience in fly-by-wire and as initial development costs are covered by new aircraft, they will gradually see the benefits of moving the technology into smaller aircraft.

Another benefit of fly-by-wire is that it enables designers to extract even more performance from future airframes. One way to do this is by eliminating traditional flight control surfaces and replacing them with flexible portions of wings and horizontal and vertical stabilizers. Winglet manufacturer Aviation Partners has teamed with FlexSys in a joint venture to market the FlexFoil variable-geometry flight control surface technology. FlexSys has been testing it on a GIII, in which the flaps were replaced with FlexFoil seamless continuous surface units that can morph from -9 degrees to 40 degrees. The surfaces could eventually be used for mission-adaptive profiling, where operators can adapt the control surfaces for the shape best suited to the phase of flight.

For the near future, the pinnacle of aerodynamic design will likely be a supersonic business jet, and the Aerion-Airbus team has steadily built engineering and financial capabilities that could lead to the first Aerion AS2 entering service in 2023. Fractional-share operator Flexjet is slated to become one of the first operators to place the AS2 into service, having signed a firm order for 20 of the SSBJs.

One development that seems fully established is flat-floor cabins for midsize and larger business jets. And while ultra-long-range jets have commanded significant market share, Textron Aviation’s recently announced Citation Hemisphere is going to test the market with relatively long range (4,500 nm) and a 102-inch-wide cabin (the same width as the Falcon 5X, but less range than the 5X’s 5,200 nm). Time will tell how this new market niche develops