Vertical Takeoff-and-Landing (VTOL) Micro Air Vehicles (MAVs) provide a versatile operational platform which combines the capabilities of fixed wing and rotary wing MAVs. In order to improve performance of these vehicles, a better understanding of the rapid transition between horizontal and vertical flight is required. This study examines the flow structures around the Mini-Vertigo VTOL MAV using flow visualization techniques. This will gives an understanding of the flow structures which dominate the flight dynamics of rapid pitching maneuvers. This study consists of three objectives: develop an experimental facility, use flow visualization to investigate the flow around the experimental subject during pitching, and analyze the results. The model used for testing features a low aspect ratio (AR), low Reynolds number (Re) Zimmerman planform wing with two contra-rotating propellers in a tractor configuration. The experimental facility, located at the Department of Aerospace and Mechanical Engineering at The University of Arizona, consists of: a closed loop open test section wind tunnel capable of airspeeds up to 15m/s and controlled with a variable frequency drive (VFD); a power source and wire to generate vapor from a mixture of turbine oil, petroleum jelly, and iron powder, which is placed across the wind tunnel nozzle outlet; a five axis robotic arm mounted below the test section capable of controlling the experimental subject for pitching maneuvers; and, a pair of video cameras capable of recording the flow visualization at 600 frames per second. The flow within the wind tunnel was carefully examined in order to insure that the experimental subject was placed within a region of flow unaffected by boundary effects and that there were no significant disturbances or oscillations within the flow. The flow around the experimental subject was studied in both static and dynamic testing. For the static tests, the angle of attack (AOA) of the experimental subject was varied across a range of AOA from 15 to 70 degrees. For each range of AOA, the Re was varied to 10700, 22600, and 35500, and advance ratio (J) was varied from undefined, 0.60, to 0.47. Several conclusions can be drawn from the static testing. The flow is dominated by the propeller slipstream effects. The slipstream drastically delayed leading edge (LE) separation and vortex shedding. It also causes flow to be either deflected downward into the slipstream or to deflect outward towards the wing tip before passing over the LE. The slipstream strength also increases the turbulence in the slipstream and relative velocity of the flow at the wing surface compared to freestream. The Re affects the LE (visible only without slipstream) and trailing edge (TE) vortex shedding frequencies, increased Re increases the frequency. Additionally, it appears that the non-dimensional LE and TE vortex shedding frequencies are constant at a value of 0.216, irrespective of both Re and advance ratio. This is important because it means that these observations are likely valid across a broad range of flight conditions. Dynamic testing also varied the advance ratio and the Re. It also varied the reduced frequency. Both positive and negative pitching was examined. Many of the conclusions drawn were the same as those from static testing. Increasing the Re increased the vortex shedding frequency. The slipstream delayed LE separation and caused significant deflection downward and towards the wingtip, as well as increasing turbulence and relative flow speed at the top surface prior to separation. Dynamic testing also found that in the presence of the slipstream, increased Re decreases the AOA of LE separation, while without the slipstream, increased Re increases the AOA of LE separation. In addition, the pitching rate has several effects on the flow. For positive pitching, increasing the pitch rate decreases the AOA of separation and for negative pitching; increasing the pitch rate has no apparent effect on the AOA of separation. This is contrary to expectations. Previous study1 has shown that increasing the pitching rate delays stall and nose down pitching hastens stall. Additionally, greater positive pitching rate slightly increases the TE vortex shedding frequency. In the absence of a slipstream, LE and TE vortex-shedding frequency are generally the same. Some interesting phenomena were found at the LE. In the presence of a pulsating slipstream from the propellers, the LE separation bubble oscillates in both height and length. It does so at the same frequency as the propeller rotation and is due to variation in the flow speed at the LE. During pitch down maneuvers, the flow reattaches at the LE first and then the region of attached flow moves aft, opposite of the characteristics of pitch up. With only minimal variation, the non-dimensional TE vortex shedding frequency remains constant at an average value of 0.229. However, it appears that increasing the pitching rate increases this value slightly. Re and advance ratio have no appreciable effect on this data. It is therefore possible to extend this result to a large range of flight conditions. A comparison of the static and dynamic testing resulted in several findings that correlated very well with previous research on this model. During positive, nose-up, pitching, the increase in lift found previously was due to the increased downward deflection of the flow and the delay of stall was due to the delay in LE separation. The opposite effects were found in negative, nose-down, pitching. There was disagreement in the findings based on the size of the turbulent separation wake and the increase and decrease in drag. Positive pitching was found to increase the drag on the model however positive pitching reduces the size of the turbulent separation wake which should decrease drag. The increase in downward flow deflection caused by pitching rate was significantly less than that due to the slipstream. Therefore the increase in lift due to the slipstream is greater than that due to pitching. The flow around the Mini-Vertigo VTOL MAV is dominated by the slipstream from its propellers. The slipstream delays LE separation and causes drastic deflection in the flow. While the frequency of the vortices shed from the LE and TE varies with flow speed, the non-dimensional frequency does not. It does, however, vary slightly with the pitching rate. These results are applicable across a wide range of flight conditions.
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