Background:
In aircraft, the ram-air intake's airbox is located within the larger inlet of the airplane (Fig. 1). This intake is a critical component that leverages the aircraft’s forward motion to compress incoming air within the airbox (Fig. 2), converting its kinetic energy into higher static pressure via the Venturi effect. This increased static pressure enhances engine performance by improving fuel-air mixing and cooling, which are essential for efficient combustion and thrust generation. As part of my broader capstone project focused on optimizing inlet design for a sponsor's Long-EZ airplane, I developed this side project to specifically analyze the airbox of the ram air intake to identify design improvements for future teams that could further enhance pressure recovery and overall aerodynamic efficiency.
Objective:
Identify and quantify the static pressure recovery efficiency (η = P₂/P₁) of the current ram-air intake airbox under varying flight speeds, and determine potential design modifications to the airbox—that could enhance airflow (increased incoming air pressure, reduced drag) and improve engine performance.
Methodology:
Using a dimensioned CAD model of the airbox, I adapted the geometry in OnShape to create a single, enclosed flow volume. The model was then exported as an STL file and imported into SimScale for CFD simulations. Flight conditions were defined using sensor data at a typical cruising altitude of 762 m, with simulations conducted at airspeeds of 60, 100, 140, and 180 knots. Air properties (density, dynamic viscosity, ambient pressure) were obtained from Wolfram Alpha, and the k‑ω SST turbulence model was applied due to the turbulent regime (Re > 4,000) of the intake flow. A pressure probe was placed near the outlet of the airbox to measure static pressure, and the efficiency η was computed by dividing this value by ambient pressure. This data was plotted to generate the intake's efficiency curve, then compared to the ideal efficiency curve for the specified flow conditions (Fig. 4). Additionally, I analyzed streamline patterns, velocity magnitude, and turbulent kinetic energy (TKE) solution fields to identify flow separation, spillage at the inlet lips, and eddy formation within the airbox (Fig. 3).
Key Results:
CFD simulations revealed that the static pressure recovery efficiency of the airplane's ram-air intake increased modestly with airspeed, yielding efficiencies of 0.8627, 0.8664, 0.8722, and 0.8798 at 60, 100, 140, and 180 knots, respectively. The results closely followed the predicted ideal ram-air intake efficiency curve (Fig. 4), indicating that the current airbox design is near-optimal under the tested conditions. However, qualitative analysis of the flow fields identified localized regions of high turbulence and inlet spillage, suggesting that minor design modifications—such as rounding and thickening the intake lips or lengthening the diverging duct—could further improve pressure recovery.
Conclusion:
My CFD analysis confirmed that the current ram-air intake airbox recovers a high percentage of ambient static pressure, validating its near-optimal design for the sponsor’s flight conditions. I quantified pressure recovery efficiency and identified potential areas for improvement (enhancing fuel-air mixing and engine performance), offering direction for future teams working on the sponsor's airplane. These suggested geometric changes (lip thickness, duct length) can be varied and quantitatively compared through further CFD simulations with the methodology I used for this airplane's air inlet.