## 6 Experimental Methods for Evaluating HAPS Propeller Performance
To validate the aerodynamic predictions of the 1.2 m diameter propeller, optimized for high efficiency under stratospheric conditions using BEMT with minimum induced loss and low-Reynolds CFD airfoil data, a wind tunnel campaign was conducted at the Korea Aerospace Research Institute, using a closed-circuit low-speed tunnel (4 m \(\times\) 3 m test section, max speed 120 m/s, turbulence intensity 0.07% at 88 m/s). The test apparatus featured a custom measurement rig integrating an industrial AC motor (Mitsubishi HG-JR 153, max 6000 rpm), a torque sensor (HBM T-22), and a load cell (DacellUMI, 20 kgf), ensuring precise force and moment measurements. Propeller tests were performed at fixed rotational speeds (700-1050 rpm) while varying freestream velocity (up to 10 m/s) to explore a range of advance ratios. Performance coefficients (\(C_{T}\), \(C_{P}\), and \(\eta\)) were computed from directly measured thrust and torque. A Glauert correction was applied to account for wall blockage effects, shifting \(J\) by approximately 0.005-0.01. The wind tunnel experiments were conducted under standard atmospheric conditions (near sea level) without simulating the low-density environment of high-altitude flight. However, by adjusting rotational speeds and freestream velocities, the tests reproduced equivalent advance ratios and Reynolds numbers consistent with stratospheric operation, enabling a meaningful aerodynamic assessment of the propeller performance in realistic flight regimes. The experimental results show good agreement with CFD predictions obtained using the transition SST model in Fluent, with efficiency differences limited to 1-1.5 percentage points across the tested conditions. The residual discrepancies, especially at low advance ratios, were primarily attributed to wake contraction and blockage effects, which were subsequently confirmed through additional CFD simulations that included the physical test apparatus. Propellers designed for high-altitude applications typically have large diameters, which often exceed the dimensions of standard wind tunnel test sections. To overcome this limitation, experiments are commonly conducted on scaled models, applying similitude principles to ensure geometric, kinematic, and dynamic similarity with the full-scale propeller. Key non-dimensional parameters, such as advance ratio, Reynolds number, and Mach number, are matched, allowing the scaled models, with appropriately adjusted diameters, rotational speeds, and inflow velocities, to accurately reproduce the aerodynamic behavior of the full-scale propeller [151]. However, since it is often impossible to fully satisfy all similarity requirements during model testing, attention is generally focused on matching the two parameters most relevant to the problem. This is the case of [45], in which, to verify the results of CFD simulations with wind tunnel tests, only the advance ratio and Reynolds number were ensured to match those of the prototypes. Since the Mach number at the propeller tip was below 0.4, compressibility effects were considered negligible and thus disregarded. The goal was to verify that a Gurney flap would enhance the performance of a propeller (with a diameter of 2 m) working in a low Reynolds number environment, as previously predicted by CFD simulations The experiments were conducted in a closed, single return-flow wind tunnel. The test section measured 12 m in length and featured a rounded rectangular cross-section of \(3\,\mathrm{m}\times 3\,\mathrm{m}\). It featured a low turbulence intensity, between 0.1% and 0.13%, and allowed for continuous wind speed control ranging from 3 to 100 m/s. The scaled propellers tested had a diameter equal to 0.7 times that of the full-scale propellers. Two tests were performed: one at a free-stream velocity of 4.2 m/s, corresponding to a Reynolds number of approximately \(4\times 10^{4}\), which represents the design point of the propeller; and the other at a free-stream velocity of 16 m/s, corresponding to a Reynolds number of about \(1.6\times 10^{5}\).