Variable Area Fan Nozzle Concept for UHBPR Engines

1st September 2016

The ENOVAL project sets the objective to develop the next generation of the turbofan engines in civil aviation industry. The key to achieve this objective is to ensure that the technologies are developed taking into account the system requirements all the way from aircraft and propulsion system level to specific module level to allow optimum technology solutions to be achieved. The target of ENOVAL is to support the reduction in CO2 and noise emissions by introducing Ultra High By-Pass Ratio (UHBPR) engines which can have By-Pass Ratios (BPR) of up to 20. These engines require a significant step forward in the low pressure system technologies. Such a new low pressure technology can be achieved through an innovative fan design with a low fan pressure ratio and a radically increased fan diameter compared to the current conventional turbofan engines. The aerodynamic stability of such large fans is, most likely, very sensitive against the backpressure variations. The back pressure regulation of such a large fan, especially during the take-off operation, can be achieved through the introduction of a Variable Area Fan Nozzle (VAFN). The VAFN will be needed to overcome the fan stability issues associated with the reduced fan pressure ratio allowing it an optimum stability during the desired flight conditions. With the introduction of large fans in UHBPR engines with low pressure system technologies, the fan tip Mach numbers will decrease. This will result in lower noise from the fan side. A VAFN could play an important role in reducing the overall noise signature. During take off, a VAFN with an increased nozzle exit cross-section, will reduce the jet exit velocity and therefore the jet noise which depends on the nozzle exit velocity [* Willy J. G. Bräunling, Flugzeugtriebwerke, Springer 2009]. The Brandenburg University of Technology at Cottbus-Senftenberg (BTU) is involved in the ENOVAL program to develop the concepts of such VAFN systems for UHBPR turbofan engines.

An UHBPR engine offers two potential design spaces for a variable fan nozzle, one in the by-pass duct nacelle and other under the inner fairing structure of the by-pass ducht nacelle. The concept at the outer nacelle space uses three large moving flaps at each side of the by-pass duct (see figure 1). Their kinematic is realized on curved rail structures at each side of the flap which results into an over area- (larger nozzle throat area) and under area- nozzle positions. The thin winglets are designed between the flaps to avoid leakages in longitudinal directions. The kinematics is realized through one linear actuator per flap, connected at the centre of the flap’s leading side. The locking mechanisms are used at each side of the flap to stabilize the flap’s positions.

 Figure 1: Large moving flaps as VAFN concept with variable nozzle positions and illustrated kinematic

Large moving flaps as VAFN concept with variable nozzle positions and illustrated kinematic.


The second design space is selected at the inner fairing structure which, potentially for the UHBPR envelop, provides sufficient clearance for the different kinematic possibilities (see figure 2). The original inner fairing structure is divided into smaller segments, which are fixed rotationally at their outer edges. The segments are folded inwards (towards the engine axis), resulting into an increment of by-pass duct throat area (over area nozzle). The movement away from engine axis results in an under-area nozzle. The actuation concept designed for this VAFN concept comprises of two large half-ring segments at each side of the inner fairing structure partition. The moving inner fairing structure segments are connected to the rings through drag-links. Both the rings are actuated linearly through 3 actuators at each duct sides.

Figure 2: Moving fairing segments as VAFN concept with variable nozzle positions and illustrated kinematic

Moving fairing segments as VAFN concept with variable nozzle positions and illustrated kinematic