I love the sausage-making of conceptual design, the way configurations evolve, and the paper presented at SciTech, authored by TUM researchers Michael Iwanizki, Niclas Randt and Sky Sartorius (who gets my Best Name for an Aircraft Designer award), shows how they started with a market analysis and ended up with a four-turboprop, double-deck, 420-passenger short/medium-range airliner.
They picked 3,000km (1,620nm) as the design range after a route analysis showed 90% of flights (in June 2008) were 3,000km or shorter. The majority were flown by Airbus A320s and Boeing 737s, so they started with a 300-seat design capacity, which would allow one aircraft to replace two A320s or 737s.
They chose a turboprop for its short take-off capability and fuel efficiency -- its lower speed compared with turbofans was less of an issue on shorter routes. Span was constrained to within 52m (170ft) and length to within 48.5m to allow the aircraft to use shorter runways (a field length of 1.8km) than conventional widebody aircraft.
The researchers expected to need at least 10,000kW (13,400shp) of take-off power from each engine -- only Russian turboprops get that big, so they looked at the Tu-95's Kuznetsov NK-12 and the Antonov An-70's D-27 "propfan".
TUM started out with a high-wing concept resembling the An-70 airlifter, but the design evolved into a low-wing configuration with a double-bubble, double-deck fuselage. That way the wing structure passes through the lower cargo compartment, rather than one of the passenger decks. Nine-abreast seating on the main deck kept the fuselage as short and light as possible.
The researchers looked at various engine locations, pusher and puller, on the wing, fuselage and tail before deciding to go with conventional tractor turboprops mounted on the wing. This creates some drag from immersing the wing in the propwash, but improves high-lift performance. The inner nacelles were moved up in line with the wing to provide prop clearance with a shorter, lighter landing gear.
The result of all this conceptual design work is a 420-seat aircraft with a 163-tonne (390,350lb) maximum takeoff weight and a 380kt/Mach 0.64 cruise speed powered by four 12,700shp D-27s. Wingspan is 51.65m and fuselage length is 47.7m. Fuel burn is 3.4liters (0.9gal) per 100 passenger-kilometers at 80% load factor.
The TUM researchers acknowledge the higher noise of turboprops versus turbofans is a critical challenge to the concept, but conclude that growth of global air traffic volume in the face of rising energy costs "is likely to make high-capacity turboprop aircraft ... more attractive."
Well, attactiveness might be in the eye of the beholder but I rather like the Tu-95...