In addition to its comprehensive use of ‘conventional’ aerodynamic advantages rarely seen, Synergy uses experimentally proven techniques for active drag reduction. A small amount of energy is used to control the behavior of air close to the skin, resulting in a tremendous reduction in overall power requirements. On this aircraft, boundary layer control minimizes drag by creating ‘pressure thrust’ and up to 100% laminar flow.
To explain what was happening as experiments began violating convention, in the 1980’s NASA began describing this equation-changing field of “powered” drag reduction by the term “open thermodynamics,” because we are bringing energy into an otherwise closed system. It’s not really about how we use heat, although that is still relevant.
The assumption of “closed” thermodynamic calculations (the ones always used to figure up a new aircraft) is that the drag when gliding unpowered is always equal to the drag at the same speed even when under power, which isn’t necessarily true when one is using that power to reduce drag. Synergy uses power first to dramatically reduce drag, then to make thrust.
Active drag reduction takes general aviation far beyond mere streamlining to explore new technologies for fast, efficient, comfortable flight in the 21st century.
A small amount of engine power can be used to reduce the boundary layer thickness of the aft fuselage. This “powered pressure recovery” can be used to create a larger area of high pressure in the back of a body than in front of it, creating a slight forward push called pressure thrust.
However, attempts to use the phenomenon for propulsion are misguided and inefficient. Done properly, pressure thrust is actually just major drag reduction for the price of a little suction.
Zero or even negative drag has been easily reached in experiment, but it quickly becomes inefficient to go beyond the goal of drag cancellation.
Boundary Layer Control
On Synergy, power can be used to create less drag on the airframe by sucking turbulent air off the wings. This not only stabilizes the boundary layer for 100% laminar flow, but with correct geometry, creates pressure thrust for extremely low drag.
Although the physics are slightly different, boundary layer control is like the difference between playing air hockey with the table turned off… or playing with it turned on.
Done well, this is neither complicated nor a safety issue. Synergy provides the ideal form for utilizing several proprietary BLC technologies developed and refined for laminar bodies by John McGinnis since the late 1970s.
Wake Immersed Propulsion
Hydrodynamicists have likewise known for decades that wake-immersed propulsion greatly reduces drag: because of the high drag environment of water, it was obvious. Synergy uses a specially designed quiet wake impeller, (not shown) which reduces turbulence ahead of the engine. Cooling drag is also minimized by pressure recovery flow design and integral exhaust scavenging.
Having the prop behind the body is stabilizing and greatly preferable over having it in front, as long as the inflow is not disturbed by turbulence as in the vast majority of prior designs. As in most areas of aeronautics, opinionated myths persist in this area due to the perception that prior work was adequate to establish a superiority of the tractor configuration. In fact, the primary reason for tractor props becoming dominant was the influence of the air-cooled engine in conjunction with very poor aerodynamics for propulsion, including among most good-looking pushers.
For more information on the historical development of advanced drag reduction technologies, or to read NASA studies on their impact when applied, visit the CAFE Foundation technical library here.