TECHNOLOGY AREA(S): Air Platform, Materials/Processes

ACQUISITION PROGRAM: JSF Joint Strike Fighter

OBJECTIVE: Develop a durable and repairable coating or surface treatment for aircraft propulsion turbomachinery inlet components. Demonstrate that it sufficiently reduces the accumulation of ice or enables it to shed off harmlessly before accumulating to a damaging mass or shape, and maintains this capability in the presence of vibration, fouling, erosion by sand or dust, and impact damage by foreign objects.

DESCRIPTION: Ice accretion on aircraft turbomachinery flow surfaces and the resulting detrimental effects on engine performance, operability, and aeromechanics are well recognized as challenges for both legacy and advanced air breathing propulsion systems. The use of hot bleed air from rear compressor stages, which comes at the expense of both weight and cycle performance penalties, has been the traditional approach employed by the propulsion industry. To alleviate some of the cycle performance and weight penalties, electrical ice protection systems with embedded electrical heater mats were introduced. These electrical systems impose significant burden on the aircraft electrical power system while impacts from foreign objects on the heating elements, which necessitate repairs, have posed significant concerns to both operational readiness and affordability of the systems.

Advances in icephobic technology have been made in recent years; however, typically these coatings or surface treatments have been designed either for very low airflow velocities or using coating thicknesses that are unrealistic for use in aircraft propulsion environments. There is a need to develop and demonstrate icephobics for engine realistic air speeds and operational environments to enable consideration for transition by the engine manufacturers and program office.

One of the key requirements is that the coating or surface treatment should not impact engine performance or operability. This includes no negative changes to component surface roughness (varies by application, typically 5-50 micro-inches) or airfoil geometry. The icephobics must also maintain performance in engine operating environments [Ref 4], including exposure to austere media (sand and dust), water, and salt. Another challenge of the aircraft propulsion environment is the ingestion of foreign objects, which can damage engine components. The icephobics need to be durable to impact loading (such as small screws or chunks of concrete) and field repairable in the event of damage. The inlet components are also exposed to vibration and structural loading [Ref 4], therefore fatigue life is a paramount concern. The icephobics must introduce little to no fatigue debit due to added material or the application process. Additionally, the icephobics should be applicable to both carbon composite and metallic (such as titanium) structures. The treatment or coating may be used in conjunction with a legacy anti-ice system; therefore, the icephobics need to retain ice protection properties in an elevated thermal environment.

In order to validate the icephobic technology, it will be necessary to apply it to jet engine fan components (including inlet guides vanes, cases, struts, and nose cones) and test at flight equivalent airspeeds to evaluate its ice accretion characteristics. Additionally, High Cycle Fatigue (HCF) tests need to be conducted to evaluate the fatigue debits or verify that none exist. Other mechanical evaluations such as (1) surface roughness, (2) adhesion characteristics to parent material (particularly titanium, aluminum, steel, and carbon composites), (3) corrosion resistance, (4) erosion resistance, (5) foreign object damage, and (6) local icephobic repair, need to be addressed on a component level to elevate the technology readiness level (TRL) to 5 before an engine or system level test to reach TRL6. Consult MIL-STD-810G for guidance on appropriate environmental test methodologies and baseline exposure levels for the parameters above [Ref 4].

The developed icephobic coatings or surface treatment should be focused on fan and early compression stage components, where temperatures are typically -50 to 140 degrees Celsius, air speed is on the order of 150 m/s, and pressures are typically 0.2 to 2 standard atmospheres. Should early concepts not be proven to meet the full temperature, pressure, or airspeed capability, then a path to achieve these capabilities must be defined and demonstrated during Phase II.

Establishing a working relationship with relevant original engine manufacturer(s) (OEM), while not mandatory, will greatly enhance the probability of successful development and transition.

PHASE I: Develop an icephobic coating or surface treatment application. Demonstrate the efficacy on a turbomachinery representative coupon (or subcomponent) in representative environments (e.g., laboratory-based simulating engine-like operating conditions; an icing environment at elevated airspeeds). Hold a TRL 3 proof-of-concept demonstration of the icephobic coating or surface treatment application at the end of Phase I. Produce plans for prototype product/treatment application to be developed under Phase II. Note: While not necessary to be demonstrated in Phase I, durability and repair should be considered in preparation for Phase II activities.

PHASE II: Build on the results of Phase I to expand development and testing to include more representative geometries and operating conditions as well as durability testing. Perform representative component fatigue testing to verify the impact of fatigue debit of the coating. Surface roughness of the component, both before and after exposure to austere conditions (e.g., erosion, foreign object damage), should be characterized along with the ice reduction performance. Include testing, at a minimum, of further ice accretion testing using relevant components in representative engine temperature, liquid water content, and flow speed conditions. Complete a component level TRL 5 prototype demonstration at the end of the Phase II Base and a system level TRL 6 demonstration at the end of the Phase II Option. Prepare for work beyond Phase II to include planning for engine validation testing to advance to TRL 7 and prepare for transition into a component improvement or acquisition program.

PHASE III DUAL USE APPLICATIONS: Ideally, work with an engine OEM and PMA/PEO to validate the developed technology by completing a system level test in an operational environment, such as an aircraft test in an icing tunnel. This will advance the technology to TRL 7 and allow final consideration by the transition partner and PMA/PEO. Identify and leverage existing test assets and programs if available to reduce cost. Participate in component improvement program and cost effectiveness analysis proposals to support final transition. The technology should progress through final development, validation, and flight clearance in support of an engineering change proposal (ECP) to incorporate the technology into the production design and mature the technology from TRL 7 through to TRL 9. The successful implementation of this technology would have widespread application across the aerospace industry. Commercial aircraft and engines also have ice protection and anti-ice systems, for which this technology could reduce the weight and/or cost of application and maintenance. As a potential secondary application, this technology could reduce or remove the need for existing anti-ice systems in aircraft airframe structures such as wings, tails, and struts.

REFERENCES:

1. Golovin, K. et al. �Designing Durable Icephobic Surfaces.� Science Advances, 2016, Vo. 2, No. 3, e1501496. DOI: 10.1126/sciadv.1501496

2. Laforte, C., Blackburn, C., and Perron, J. "A Review of Icephobic Coating Performances over the Last Decade." SAE Technical Paper 2015-01-2149, 2015, https://doi.org/10.4271/2015-01-2149

3. �Technical Report From the Engine Icing Working Group on Liquid Water Content for Ground Operations in Icing Below -18�Celsius.� DOT/FAA/TC-15/30. http://www.tc.faa.gov/its/worldpac/techrpt/tc15-30.pdf

4. MIL-STD-810G, Department of Defense Test Method Standard, Environmental Engineering Considerations and Laboratory Tests. United States Department of Defense, 31 Oct 2008. http://everyspec.com/MIL-STD/MIL-STD-0800-0899/MIL-STD-810G_12306/

KEYWORDS: Anti-Ice; Icephobic; Turbomachinery; Propulsion; Coating; Advanced Materials