ASME-DE Section Outstanding Senior Design Final Presentation Award
Overview: This project improved the design of the air inlet of a sponsor’s Long‑EZ homebuilt airplane to increase airspeed and engine efficiency. I redesigned the inlet geometry to achieve better airflow (higher pressure) for engine performance while meeting engine safety constraints. I performed CFD and experimental analyses with 3D-printed prototypes.
Background:
Air enters through an airplane's inlet and both feeds the engine/ram air intake and cools the outside of the engine (Fig. 1). Air pressure and velocity varies from external flow as it passes through the inlet geometry—higher pressures generate better propulsion and cooling performance. As a flow-facing orifice, the inlet generates significant unavoidable drag. The sponsor suspected the inlet of his homebuilt airplane was oversized and limited his top speed in competitive timed events.
Objective:
Increase the aircraft’s top speed and engine efficiency by redesigning the inlet geometry to increase incoming air pressure, minimize drag, while maintaining engine temperature requirements.
Methodology:
I led the concept development, CFD analysis, and experimental data analysis.
Benchmarking & Concept Generation: After extensive background research into airplane inlet design and performing 2D CFD analysis of the entire airplane myself (Fig. 3), I developed multiple inlet concepts (Fig. 4), varying in shape (existing, elliptical, "splitter"), size (24, 28, and 32 in²),and lip angles (0°, 2°, and 5°).
Modeling: My teammate modeled my inlet designs in CAD/Blender. I performed a photogrammetry scan of the current inlet (Fig. 17) and we worked together to develop an effective interface to mount our prototypes onto the existing inlet for easier testing (as opposed to a complete inlet swap). Comparative assessment of the prototype concepts was more important to the sponsor than the final product for this project. A teammate conducted FEA to ensure structural integrity of the mounting interface.
CFD Analysis: I performed CFD comparisons of all inlet concepts to determine the most viable designs to manufacture and test.
Since a complete model of the airplane was too complex to model efficiently, I developed a CAD model with tunable geometry parameters to represent the rest of the plane (Fig. 7). By tuning these parameters aft of our inlet models, I could calibrate this CFD model to output the same pressure and flow data as in-flight data collected from previous years, circumventing the need to model every complex crevice of the internal/engine components typically found in this section of the airplane. The CFD data was within 8% of measured results (0.09 in Hg), validating its accuracy for the scope of this project.
Using flight flow conditions (airspeed, atmospheric conditions, altitude), I ran simulations of hundreds concept variations to determine the most promising parameters to explore, then the most viable of these parameters to fabricate for in-flight testing. I explored multiple computational parameters to identify optimal simulation conditions (e.g., mesh geometry). I compared pressure and drag measurements throughout each model to identify the best inlet designs (24, 28, 32 in² splitter, 28 in² elliptical).
Fabrication & Experimental Validation: Our rapid prototyping process involved 3D-printing, wrapping in fiberglass, and mounting each chosen inlet design to the airplane (Fig. 10). Using 4 engine thermocouples (cylinder head temperature), a tachometer (RPM), a pitot tube (airspeed), and manometer (pressure), we collected data for each inlet design by the millisecond for the same flight path (Fig. 13).
Key Results:
The CFD analysis identified that the elliptical and splitter shapes outperformed the current inlet shape in increasing cowl pressure and decreasing drag (Fig. 9). A greater lip slant angle and size correlated to greater pressure recovery as well. The 32 in² splitter inlet offered a ~5% reduction in drag, which hand calculations determined equated to a max speed gain of ~1.3 knots.
Experimental data, after extensive processing and statistical analysis, revealed (1) cowl/manifold pressure increased with inlet size, (2) engine temperature slightly decreased with inlet size, but all tested sizes still kept temperatures within safe limits (Fig. 15). These results held for cruise and climb conditions (when the engine is more active). My analysis quantified that the optimized 32 in² splitter inlet offered 0.29" Hg increase in engine manifold pressure, while keeping engine temperatures safely within a 3–10°F increase from baseline values. Since manifold pressure correlates to engine power output, this enabled us to determine speed gains without needing to repeatedly push the engine (which the sponsor was against).
Our team also completed the project 37% under budget due to CFD analysis reducing experimental overhead.
Conclusion:
These quantitative improvements directly translate to better engine performance and fuel efficiency for the sponsor's airplane, with the 32 in² splitter design outperforming the rest. Completely replacing the current inlet with these new designs (instead of simply mounting the new inlets on), would result in further improvements in drag reduction and pressure recovery due to far smoother external surfaces. Future teams can fabricate inlets with the prototype geometries identified, using complete surface finishes. They can also explore a slightly larger inlet (36 in²) that is still smaller than the existing inlet (38 in²), as my data found a trend in pressure with inlet size.
Critical Reflection:
We could not experimentally determine gains in maximum airspeed because the airplane owner did not want to repeatedly stress the engine during testing. To work around this, we compared manifold and cowl pressures at the same airspeed and engine RPM across inlet designs. A higher pressure reading under identical conditions indicated improved engine efficiency and correlated to a gain in maximum airspeed, validating the 32 in² splitter design.
Due to time constraints, we mounted prototypes onto the existing inlet rather than fully replacing it, avoiding permanent modification. While this introduced extra drag from the mounting interface, identical mounts for every trial ensured that any drag penalty was consistent across designs—including a control with the original inlet geometry. This method isolated the impact of inlet geometry, our primary focus. We advised future teams to replace the existing inlet entirely, with seamless integration, to realize further performance gains.