Paving the Way to Quieter Aircraft
Airplanes, one of humanity's greatest inventions, evoke feelings of awe and amazement for most people, unless you live uncomfortably close to a major airport, in which case the feeling may become annoyance and irritation. Constant aircraft noise causes sleepless nights, stress and other health issues, reducing quality of life.
Airports, airlines and aviation authorities around the world have been in an uphill battle to mitigate noise pollution in their neighboring communities. Neither the airports nor the surrounding communities are moving away from each other anytime soon, necessitating the need for urgent solutions. The Advisory Council for Aeronautics Research in Europe (ACARE) Flightpath 2050 document – Europe's vision for aviation, has set a goal of 65 percent noise reduction at airports by 2050. While mitigation strategies include flying restrictions, soundproofing homes and nighttime curfews, the real solution to meeting such goals lies at the root cause of the problem – reducing the noise airplanes generate and radiate to the surrounding community.
The major source of airport noise comes from engine power during takeoff. During descent and approach to landing, the engines are powered down and the dominant noise is often due to the aerodynamic exposure of structures such as control surfaces, landing gear, high-lift devices and speedbrakes. These structures often provide drag needed to maintain a desired trajectory, but may also become the source of excessive noise. One way to mitigate this noise is through “quiet” drag devices that enable quieter landings through steeper, slower and/or aeroacoustically cleaner approaches.
ATA Engineering, in collaboration with NASA GRC, Williams International and the Massachusetts Institute of Technology (MIT) has developed a novel engine air-brake (EAB) concept that will pave the way for quieter aircraft. The EAB is a unique drag device that was conceived at MIT as a ram air-driven nacelle with a stationary set of turning vanes generating a swirling exhaust flow from the engine. With funding from the NASA Small Business Innovation Research (SBIR) program, ATA Engineering has developed and brought this technology to life in the form of a deployable swirl vane exhaust nozzle. STAR-CCM+® and NX™, tools in the Simcenter™ Portfolio from Siemens PLM Software, played a key role in the design of the EAB, accelerating the readiness of the technology by creating and validating a digital twin before building the prototype.
Using NASA's Technology Readiness Level (TRL), a measurement system to assess the maturity level of a particular technology, ATA has now advanced the EAB concept to TRL level 6 – a fully functional prototype. TRL numbers run from 1-9 with level 1 pertaining to basic principles and conception of the idea and level 9 indicating a flight-proven system through successful mission operations.
What is an Engine Air-Brake (EAB)?
ATA's EAB concept was proposed by Shah et al. - as an evolution of the MIT concept. The EAB is a deployable device for drag management in aircraft. Pressure drag is generated through swirling outflow from the engine's propulsion system by reducing thrust. The EAB is stowed during flight but deploys a swirl vane mechanism (Figure 1) during landing, creating a swirling 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.
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 (Figure 2). ATA Engineering partnered with Williams International to demonstrate the EAB on a FJ44-4 engine, a 3,600-pound class, medium bypass, twin spool engine.
Challenging Design Requirements
As with any design, changes somewhere always result in undesired effects elsewhere and this is more pronounced in the aerospace design space. As part of the TRL program, design requirements and technical objectives for the EAB were identified first. The technical objectives were:
Design, fabricate and test a realistic flight-weight EAB on a modern turbo-fan 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 (TSFC) penalty when stowed;
A 15-percent net thrust reduction at “dirty approach” fan speed when 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 3 to 5 seconds, respectively).
Other design requirements included structural and packaging constraints that ensured that 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.
Aerodynamic Design With STAR-CCM+
Parametric solid modeling with NX for Design from Siemens PLM Software was used to create the 3D CAD geometry of the EAB. This allowed rapid generation of designs with varying parameters based on aerodynamic performance. The various design parameters (Figure 3) for the numerical simulation were: vane count (N), swirl angle (S), deployment rotation angle (R), chord length (L) and cutout (area relief) depth (C).
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 (CFD), design optimization and powerful computing hardware, ATA was able to analyze the full aerodynamic design space before identifying the final design that met all the aerodynamic requirements in simulation. The aerodynamic domain is shown in Figure 4.
The domain was discretized with polyhedral cells. Prism layers were used to capture the boundary layer flow. The final designs had a mesh count of 3 to 5 million cells. Total pressure and temperature were specified as boundary conditions at the fan, core and freestream inlet. Steady RANS simulations with ideal gas and k-ω SST turbulence model were carried out. Circumferentially periodic boundary conditions were used, enabling modeling of 1/7th 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.
Contours of dimensionless swirl velocity (normalized to approach flight velocity) and streamline patterns for the stowed and deployed final design are shown in Figure 5. From the swirl velocity, it can be seen that the flow becomes axisymmetric about two nozzle diameters downstream of the vanes. The aerodynamic performance results from STAR-CCM+ were used to update the design parameters and iterate on the NX design. Exactly 150 different designs were evaluated and the best performing design was identified, the parameters of which are shown in Figure 6.
An Analysis-Driven Design With Siemens PLM Software
Aside from the aerodynamic analysis detailed above, ATA Engineering used Siemens PLM Software tools throughout this analysis-driven design process to define the final configuration. The multi-physics capabilities of STAR-CCM+ enabled performance and gap leakage analysis with RANS CFD, thermal analysis with Conjugate Heat Transfer (CHT) modeling, unsteady loads calculation with Large Eddy Simulation (LES) capability and flutter assessment. Thus, a digital twin of the EAB was created, which validated the aerodynamic performance of the final design. Structurally, NX™ Nastran from Siemens PLM Software was used for Finite Element Analysis (FEA), fatigue analysis and prediction of thermal/structural deformation.
Figure 7 shows sample results from various simulations used in creating the complete digital twin. The deployment mechanism was challenging to design due to limited space and the challenges in 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 hand work to bring the nozzle up to specifications.
Technology Maturation to TRL 6 With Ground Testing
Full-scale ground testing of the final EAB design was conducted at Outdoor Test Facility #2 (OTF2) at Williams International's complex in Walled Lake, Michigan. The testing successfully confirmed the performance of the EAB prototype. Results from the test are given below:
Drag and flow/operability targets were successfully met;
Noise was favorable compared to analysis;
Dynamic deployment (<5s) and stow (0.5 s) 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. Figure 8 shows dimensionless flow capacity on the X-axis and drag generated on the Y-axis for stowed and various stages in the deployment cycle. 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.
A Quieter Future in Aviation Beckons
With the initial success, desired next steps are ground testing to test the reliability and durability of the system and an eventual flight test demonstration. ATA Engineering hopes that future aircraft designs will incorporate the EAB, and that the device may also be retrofitted to existing aircraft. There may yet be a day in the future when the general population is lining up to live in close proximity to airports. Sounds crazy? Maybe not. Now that airplanes and airports can be quieter, very few other balcony views can match the sheer splendor of watching our fantastic flying machines take off and land all day.
This article was written by Parthiv N. Shah, Senior Technical Advisor, ATA Engineering, Inc. (Herndon, VA) and Prashanth S. Shankara, Senior Technical Marketing Engineer, Siemens PLM Software (Plano, TX). For more information, Click Here .