NASA’s project Sceptor (Scalable Convergent Electric Propulsion Technology and Operations Research) is a test aircraft, to be built to study the use of distributed electric propulsion (DEP). It involves replacing the wings on a a twin-engined Italian-built Tecnam P2006T (a conventional four-seater light aircraft) with DEP wings each containing electrically driven propellers.
Test flights are planned to commence in 2017.
Tecnam P2006T specs
- Crew: 1 (pilot)
- Capacity: 3 passengers or 618 pounds (280 kg) payload with full fuel
- Length: 8.69 m (28 ft 6 in)
- Wingspan: 11.40 m (37 ft 5 in)
- Height: 2.84 m (9 ft 4 in)
- Wing area: 14.75 m2 (158.8 sq ft)
- Aspect ratio: 8.80
- Empty weight: 760 kg (1,675 lb)
- Max takeoff weight: 1,180 kg (2,601 lb)
- Fuel capacity: 200 litres (44 imp gal; 53 US gal)
Distributed electric propulsion (DEP) involves increasing the number and decreasing the size of airplane engines. Electric motors are substantially smaller and lighter than jet engines of equivalent power. This allows them to be placed in different, more favorable locations. In this case, the engines are to be mounted above and distributed along the wings rather than suspended below them.
NASA 18 engine test wing
The propellers are mounted above the wing. They will increase the air flow over the wing at lower speeds, increasing its lift. The increased lift allows it to take operate on shorter runways. Such a wing could be only a third of the width of the wing it replaces, saving weight and fuel costs. Typical light aircraft wings are relatively large to prevent the craft from stalling (which happens at low airspeeds, when the wing cannot provide sufficient lift). Large wings are inefficient at cruising speed because they create excess drag. Sceptor’s wings will be optimised for cruise, with the engines protecting it from low-speed stalls (achieving the small aircraft standard of 61-kt.)
The speed of each propeller can be controlled independently, offering the ability to change the over-wing airflow pattern to cope with flying conditions, such as wind gusts. When cruising, the propellers closer to the fuselage could be folded back to further reduce drag, leaving those towards the wing tips to move the plane.
Such aircraft would have no in-flight emissions, operate with less noise and reduce operating costs by an estimated 30%. Cruising efficiency was expected to increase 3.5-5-fold.
The latest design is for a 31.6-ft.-span wing with an aspect ratio of 15, compared to 37.4 ft. and 8.8, respectively, for the stock P2006T wing. The slender wing’s chord is 2.48 ft. at the wing root and 1.74 ft. at the tip
The wing features 12 cruise propellers are 1.89-ft. diameter that each require 14.4 kW of motor power at 55 kt and turn at 4,548 rpm. The five-blade props fold in cruise to reduce drag. Each wingtip hosts two 3-blade 5-ft. diameter cruise props that each require 48.1 kW at 150 kt and turn at 2,250 rpm. The wingtip location offers favorable interaction with the wingtip vortices is expected to provide a 5% drag saving.