Hush! One company’s journey toward quieter skies

ATA Engineering needed a prototype of its Engine Air Brake

At the heart of the airport noise problem lies the conundrum of aircraft coming in for landing. Unlike engine-dominated takeoff noise, the noise during landing results mostly from the air disturbed by aircraft structures, such as edges, control surfaces, landing gear, high-lift devices and speedbrakes. While these provide the drag needed to safely maintain the desired trajectory and slow the aircraft, they also generate turbulence that is responsible for much of the excessive noise around airports.

An alternative would be to incorporate quiet drag devices that permit steeper, slower and/or aeroacoustically cleaner approaches. One such device is an Engine Air Brake, or EAB, prototype developed by ATA Engineering, headquartered in San Diego. It consists of vanes embedded in the exhaust nozzle of an engine, which could deploy during landings to induce swirl in the exhaust flow as a source of quiet drag. The EAB prototype was developed under NASA sponsorship, with Williams International of Michigan and Utah as an engine partner, and with subject matter expertise from the Massachusetts Institute of Technology.

On NASA’s Technology Readiness Level scale, ATA has now advanced the EAB concept to TRL 6, defined as having a fully functional prototype. TRL 9, the highest level, indicates a system flight-proven through successful mission operations.

Parthiv N. Shah of ATA Engineering explains:

Conceiving the EAB

The pursuit of quiet drag received a big boost from the “Silent Aircraft Initiative,”a research project carried out in the mid-2000s by the University of Cambridge in England and MIT with a host of aerospace industry partners. I became involved in this initiative in 2004 while working toward my Ph.D. at MIT. The aim was to develop the conceptual design of an aircraft whose noise would be effectively imperceptible outside the perimeter of an average daytime urban airport. This design was unveiled to the public in late 2006 and was a hybrid wing-body aircraft with highly integrated, boundary-layer ingesting engines. Because the aircraft had a high lift-to-drag ratio, it would be “slippery” on approach, meaning it would be difficult to control without intentionally induced drag.

One idea considered by the team was to induce quiet drag with the propulsion system. An early concept switched the engine fan during landing from pumping mode, in which compression work is done on the air by the engine, to extraction mode, in which energy is extracted from high speed air spinning the blades, making it behave like a turbine.

I began to work with MIT professor Zolti Spakovszky at the school’s Gas Turbine Laboratory to extend the concept to the introduction of swirl in the propulsion system exhaust. The goal was to generate pressure drag equivalent to that of an equal diameter bluff body, an aerodynamic shape whose airflow has separated over much of its surface. Together, we discovered that a stable, swirling exhaust flow could not only create drag comparable to a bluff body (without the associated separated airflow), but also maintain flow capacity and a low-noise signature. In fact, the swirling flow maintained capacity better than a work extraction device. In essence, introducing swirl into the exhaust broke the high-drag, high-noise relationship of other conventional drag devices. This work was sponsored by NASA’s Langley Research Center in Virginia, which afforded the opportunity to validate the noise predictions in tests at the LaRC Quiet Flow Facility using a ram-air driven nacelle with stationary swirl vanes, a so-called swirl tube. Aerodynamic performance was validated at the MIT Wright Brothers Wind Tunnel.

While promising, the concept still presented practical implementation challenges. Since engines at approach idle power are not ram-air driven, Spakovszky and I recognized that propulsion system integration, including deployment of swirl vanes, would be a challenge in today’s conventional ducted turbofan environment. Additionally, the ram-air driven nacelle’s swirling wake flow would need to be replaced with a fan-driven swirling jet flow, with an as-yet unknown noise signature.

When I joined ATA Engineering in 2007, upon completing my Ph.D., I had the opportunity to address these concerns and take the swirling exhaust flow idea from a promising concept toward a possible commercial product. As part of a Small Business Innovation Research project sponsored by NASA Glenn Research Center in Ohio, whose first phase ran between 2008 and 2011, ATA examined the noise and performance of swirling exhaust flows in high bypass ratio nozzles. In these nozzles the fraction of air passing through the bypass duct is significantly higher than the fraction of air passing through the gas generator core. The work included computational fluid dynamics analysis plus aerodynamic and aeroacoustic evaluation at Glenn’s Aero-Acoustics Propulsion Laboratory. The work also examined integration of swirl vanes into a two-stream nozzle with a variable geometry pylon.

As a result of this effort, ATA invented a mechanism that could stow and deploy swirl vanes in an exhaust nozzle while appropriately regulating flow capacity. The EAB would be stowed during flight but deploy a swirl vane mechanism during landing, creating a streamwise vortex from the jet engine exhaust flow. The constant flow of swirling air creates additional drag by reducing thrust and is sustained by the radial pressure gradient from the swirl vanes. The system enables a slower, steeper and acoustically cleaner approach/descent when engine thrust cannot be further reduced. At a system level this can be significantly quieter than a landing where additional drag is induced by flight control surfaces such as flaps and speedbrakes.

Design considerations

In the second phase of the SBIR project, which ran between 2012 and 2015, ATA partnered with Williams International to demonstrate a prototype deployable nozzle on the FJ44-4 engine, a 3,600-pound class, medium bypass, twin spool engine. As with any design, changes somewhere usually result in undesired consequences elsewhere. This becomes more challenging when multiple design considerations are present, such as aerodynamics, thermal and structural integrity, noise, and real estate. As part of the technology maturation program, the team first identified design requirements and technical objectives for the EAB. The technical objectives were:

• Design, fabricate and test a realistic flight-weight EAB on a modern turbofan propulsion system
• Quantify the equivalent drag, effect on operability, noise, cost and weight of the system
• Perform system-level analysis of the proposed impact in terms of steep approach for noise reduction 

For the aerodynamic design of the EAB, the following requirements were identified:

• No measurable thrust or thrust-specific fuel consumption penalty when stowed
• A 15 percent net thrust reduction at fan speeds for “dirty approach” (high-powered approach throttle setting) when the EAB is deployed, measured as a percentage of the stowed nozzle’s gross thrust at same condition
• No measurable fuel consumption penalty or flow reduction when fully deployed
• Adequate surge margin during all operation, including dynamic deployment and stowing
• Meet stow/deploy time requirements (0.5 seconds and three to five seconds, respectively)

Other design requirements included structural and packaging constraints that ensured the EAB could be integrated into a typical aircraft installation, such as the Cessna CJ4, without performance penalties while providing the noise reduction benefits. The design activity involved performance assessment of various systems, including aerodynamic, mechanical, acoustics and structures. To model the performance before constructing the EAB prototype, we created a digital twin of the EAB with the STAR-CCM+ and NX, tools in the Simcenter Portfolio from Siemens PLM Software. This accelerated the readiness of the technology.

Parametric solid modeling with the NX for Design tool from Siemens PLM Software created the 3-D computer-aided-design geometry of the EAB. Researchers could then rapidly generate designs with varying parameters based on aerodynamic performance. The various design parameters for the numerical simulation included vane count, swirl angle, deployment rotation angle, chord length and cutout (area relief) depth.

One of the foundations for the maturation of the EAB technology is the analysis-driven design effort using STAR-CCM+ to quantify flow performance and operability and to predict the thermal operating environments of the design. With the power of computational fluid dynamics, design optimization and computing hardware, ATA was able to analyze the full aerodynamic design space before identifying the final design that was estimated to meet all the aerodynamic requirements in simulation. The fluid domain was discretized with polyhedral finite volume cells. Prism layers captured the boundary layer flow. The final designs had a mesh count of 3 million to 5 million cells. Total pressure and temperature were specified as boundary conditions at the fan, core and freestream inlet. Steady RANS (Reynolds-averaged Navier-Stokes) simulations with ideal gas and a k-omega Shear Stress Transport turbulence model were carried out. Circumferentially periodic boundary conditions were used, enabling modeling of one-seventh of the upstream region. A mixing plane interface was used to cope with the non-uniformity of the flow emanating from the 14-lobe mixer. The reduction of the computational domain in this way enabled faster exploration of the design space. Full-annulus simulations were carried out on final designs to verify consistency and the performance prediction.

ATA Engineering applied the Siemens tools throughout its analysis-driven design process to define the final configuration. The multiphysics capabilities of STAR-CCM+ enabled performance and gap leakage analysis with RANS CFD, thermal analysis with conjugate heat transfer modeling, unsteady loads calculation with large eddy simulation capability and flutter assessment. Thus, a digital twin of the EAB was created that validated the aerodynamic performance of the final design. Structurally, NX Nastran from Siemens PLM Software was used for finite element analysis, fatigue analysis and prediction of thermal/structural deformation. The deployment mechanism was challenging to design due to limited space and syncing the operation of the 12 vanes. Solid modeling in NX with assembly constraints allowed for visualization of the deployment and checking for interference between parts. Manufacturing of the physical EAB prototype was done through a combination of a 5 axis mill and handwork to bring the nozzle up to specifications.

Maturation to TRL 6 with ground testing

ATA performed the analysis, design and fabrication of the nozzle prototype to be integrated with the WI FJ44-4 test engine. The EAB ground demonstrator consisted of a spool piece, an aluminum nozzle, 12 high-temperature aluminum vanes, 12 stainless steel shafts, 12 dogleg lever arms and adjustable linkages, three hydraulic rams, three extension springs, a stainless steel actuation ring and a string potentiometer.

Full-scale ground testing of the final EAB design was conducted at Outdoor Test Facility Number 2 at Williams International’s complex in Walled Lake, Michigan, in October 2015. The testing confirmed the predicted performance of the EAB prototype:

• Drag and flow/operability targets were met Noise was favorable compared to analysis
• Dynamic deployment (less than five seconds) and stow (less than 0.5 seconds-faster than required engine spool up time during a go-around maneuver) were demonstrated
• Fuel burn on deployment was reduced
• Mechanism fits within a notional cowl
• Thermal performance matched prediction and no structural dynamic concerns were found
• Quiet steep approach glideslope potential was demonstrated in a system simulation
• Testing confirmed the performance of the EAB as a function of the vane rotation angle, which had been predicted with the digital twin. STAR-CCM+ predictions agreed well with test results for all configurations, reinforcing the use of STAR-CCM+ as a valuable design tool to bring this new technology to life. A steep approach flyover analysis predicts a 1 to 3 dB reduction in noise on the ground, confirming the performance of the EAB as a system noise-reducing device.

A quieter future beckons

With the initial success, desired next steps are ground testing the reliability and durability of the system and an eventual flight test demonstration.

With EAB at Technology Readiness Level 6, getting to TRLs 7, 8 and 9 will require light-weighting of the assembly (the prototype was developed for a ground demonstration), additional ground testing for reliability, and participating in a flight demonstration program to show the potential for quiet steep approaches. Our team is actively looking for a well-suited flight demonstration opportunity where we could partner with an airframer, engine manufacturer, and/or nacelle supplier to bring the technology to the skies. An estimate for time to introduction to market would be five to 10 years, if the steps detailed here are completed.

ATA Engineering hopes that future aircraft designs will incorporate the EAB, and that the device may also be retrofitted to existing aircraft. With innovative devices like these helping to reduce noise pollution, there may yet be a day when the general population is lining up to live in close proximity to airports.

Prashanth S. Shankara of Siemens PLM Software contributed to this article.