Improving processes for fabricating aircraft engine inlet ducts

Aircraft engine inlet ducts provide the engine compressor with a constant supply of air to prevent the compressor from stalling. Since the inlet is directly exposed to the impacting airflow, it must create as little drag as possible. The smallest gap in airflow supply can cause major engine problems as well as significant efficiency losses.

The future development of Air Force 

Part of the Air Force 2030 Science and Technology strategy includes the deployment of low cost Unmanned Aerial Systems in mass to assist in future near peer engagements. In order to realize this vision, new manufacturing strategies need to be identified which can support the rapid manufacturing of high quality aerospace components at costs that are lower than what are currently available using legacy manufacturing processes. If the inlet duct is to retain its function of providing sufficient air with minimum turbulence, it must be clean and flawless.

Air Force Research Laboratory’s project

The Air Force Research Laboratory’s Manufacturing and Industrial Technologies Division and the contractor team of Cornerstone Research Group, A&P Technology and Spintech LLC, conducted research to quantify the benefits of replacing legacy manufacturing processes with novel processes for the fabrication of an 11-foot long, S-shaped engine inlet duct.

At present, the legacy fabrication process for the inlet ducts consists of composite material preimpregnated with a synthetic resin, applied by hand, to a multi-piece steel mandrel. The mandrel is packaged and placed in an autoclave for processing. An autoclave is essentially a heated pressure vessel which supplies heat to activate resin curing and pressure to ensure there is minimal absorbency in the fully cured composite part.

The approach replaces the hand applied composite prepreg with an automated overbraid process which applies dry fiber to a mandrel. The very heavy multi-piece steel mandrel was replaced with a light-weight single-piece shape-memory polymer mandrel and the dry braided carbon fiber was processed with a low cost epoxy resin using a vacuum assisted resin transfer molding process.

Evaluating the benefits of the new process

One of the primary goals of this program is to understand part cost and production time benefits from introducing the new tooling and processing solutions.

The team completed element analysis finalization of the overbraid architecture, fabrication of a shape memory polymer forming tool and construction of the SMP mandrel that will serve as the tool during the preform overbraid process.

Because of inlet duct geometrical complexity, multiple iterations were necessary to optimize the overbraid machine settings and thus minimize composite material wrinkling. A total of four inlet ducts will be fabricated and legacy part cost and production time will be compared to the new design.

We believe that the introduction of a reusable shape memory polymer mandrel together with the automated overbraid process and an oven based VARTM composite cure will lead to significant cost and cycle time reductions,” said Mr. Craig Neslen, manufacturing lead for the Low Cost Attritable Aircraft Technology Initiative in the Manufacturing and Industrial Technologies Division. “Quantifying the manufacturing benefits and validating structural integrity will be critical to establishing a positive business case and convincing designers and manufacturers that the new materials and processes should be incorporated into future low cost engine inlet duct designs.”

Test for evaluating strength and stiffness

The final inlet duct will be delivered to the government for further integration into the Aerospace System’s Directorate’s complementary airframe design and manufacturing program. Personnel at the Aerospace Vehicles Division will conduct static ground testing of the integrated braided fuselage and inlet duct structure.

While we have yet to define all of the implications of attrition tolerance on design criteria and the resulting manufacturing materials and processes utilized, we do have a baseline with threshold requirements for strength and stiffness which we will assess via full-scale airframe ground tests,” said Ray Fisher, aerospace engineer in the Aerospace Vehicles Division.

Source: Air Force Research Laboratory

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