This experimental study addresses late transition stages over a conventional airfoil at Reynolds numbers of several ten thousands. The study is based on extensive volumetric flow measurements using constant temperature anemometry. This technique provided both the time-averaged description of the flow field in the vicinity of the wing and the high fidelity spectral analysis inside the separated boundary layer. Large cellular flow structures were observed above the wing. They resemble the stall cells at Reynolds numbers of a few hundred thousands and separation cells at Reynolds numbers of a few hundred. It is shown that most of the energy of the velocity fluctuations within the boundary layer over the forward half of the wing is concentrated in the low-frequency range of the spectrum, where the chord-based Strouhal number is small as compared with unity.

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The presented aircraft is capable of alternating between two singular working points by folding the exterior surfaces of the wing underneath the interior surfaces. This allows for a significant change in wingspan, lift surfaces, aspect ratio and airfoil (camber and thickness). The motivation for this type of morphing is twofold: The increase in wingspan due to unfolding, results in an increased endurance of the aircraft, while the opposite process, which eliminates the camber of the airfoil and reduces the moment of inertia, is translated into improved manoeuvre capabilities. An analysis was performed to assess the additional endurance gained by the morphing capabilities, factoring in a spectrum of aircraft geometries and flight missions. It was concluded that this morphing concept can, in theory, improve the endurance up to 50% compared to the standard counterparts. The penalty due to the additional weight of the morphing mechanism was factored in, which had an adverse effect on the endurance improvement. The concept also calls for unique airfoil selection process. Selecting a proper airfoil for either working point, results in irregular airfoil geometry upon morphing. The two possibilities were subjected to analysis and wind tunnel testing.