Localized fluid flow measurements with an He-Ne laser spectrometer

While much research has been carried out on propellers for full-scale aircraft, not much data exists on propellers applicable to the ever growing number of UAVs. Many of these UAVs use propellers that must operate in the low Reynolds number range of 50,000 to 100,000 based on the propeller chord at the 75% propeller-blade station. Tests were performed at the University of Illinois at Urbana-Champaign (UIUC) to quantify the propeller efficiency at these conditions. In total, 79 propellers were tested and the majority fit in the 9- to 11-in diameter range. During the tests, the propeller speed (RPM) was fixed while changing the wind-tunnel speed to sweep over a range of advance ratios until reaching the windmill state (zero thrust). To examine Reynolds number effects, typically four RPM’s were tested in the range 1,500 to 7,500 RPM depending on the propeller diameter. Propeller efficiencies varied greatly from a peak near 0.65 (for an efficient pr opeller) to near 0.28 (for an exceptionally poor propeller). Thus, these results indicate that proper propeller selection for UAVs can have a dramatic effect on aircraft performance.

Propeller Performance Data at Low Reynolds Numbers

*† ‡ Vortex control concepts employed for slender and nonslender delta wings were reviewed. Important aspects of flow control include flow separation, vortex formation, flow reattachment, vortex breakdown, and vortex instabilities. The occurrence and relative importance of these phenomena strongly depend on the wing sweep angle. Various flow control methods were discussed: multiple vortices, control surfaces, blowing and suction, low-frequency and high-frequency excitation, feedback control, passive control with wing flexibility, and plasma actuators. For slender delta wings, control of vortex breakdown is achieved by modifications to swirl level and external pressure gradient acting on the vortex core. Effects of flow control methods on these two parameters were discussed, and their effectiveness was compared whenever possible. With the high-frequency excitation of the separated shear layer, reattachment and lift enhancement in the post-stall region is observed, which is orders of magnitude more effective than steady blowing. This effect is more pronounced for nonslender wings. Re-formation of vortices is possible with sufficient amplitude of forcing at the optimum frequency. Passive lift enhancement on flexible wings is due to the self-excited wing vibrations, which occur when the frequency of wing vibrations is close to the frequency of the shear layer instabilities, and promote flow reattachment.

Review of flow control mechanisms of leading-edge vortices

A quadcopter, DJI Phantom II, was tested in the Virginia Tech Anechoic Chamber to study its aeroacoustics performance. Noise and thrust measured by a single microphone and a load cell were acquired for 4 different rotor configurations, two plastic and two carbon fiber rotors. To study the effects of multi-rotor interaction, the quadcopter was also set to operate with 1, 2 and 4 rotors. Results of 4-rotor operation show that tones at the blade passing frequency, shaft rate and their harmonics dominate the quadcopter acoustic spectrum up to 6000 Hz without much deterioration. Also significant is a broadband hump present in the mid frequency range which increases over 10 dB above the broadband level at low frequencies. Motor noise is also noticeable in the mid frequency range. For a smallscaled rotor, thrust performance is greatly influenced by rotor configuration whereas its acoustic signature is only altered near mid and high frequencies resulting in 1-2 dB change in the OASPL for the same thrust setting. Having 1, 2 or 4 rotors operating does not affect the acoustic signature but a significant increase was found in broadband noise when switching from 2 non-adjacent rotors to 4 rotors.

Experimental Study of Quadcopter Acoustics and Performance at Static Thrust Conditions

https://purehost.bath.ac.uk/ws/files/417226/Progress%2520in%2520Aerospace%2520Sciences%2520review%2520article%2520flow%2520control.pdf

The recent boom in Unmanned Aerial Vehicles (UAV) and Micro Air Vehicles (MAV) aircraft development creates a strong demand for accurate small-diameter propeller performance data. Small-diameter propellers as defined in this paper (diameter 6 inches to 22 inches), operate at low Reynolds numbers (typically between 30,000 and 300,000), rendering performance scaling from larger counterparts inaccurate. An Integrated Propulsion Test System (IPTS) has been designed, developed and validated at Wichita State University (WSU). A large number of propellers have been tested and a reliable database of performance data has been created. This paper discusses the salient features of this measurement system and propeller test data for a few propellers.

Propeller Performance Measurement for Low Reynolds Number UAV Applications

The meandering nature of a single vortex filament is examined. The flowfield is typical of that in the vicinity of an aircraft wing tip vortex. Self-induced motion of the vortex filament is examined within the confines of potential flow theory and is compared with experimentally obtained results. A single-sensor constant temperature anemometer is used to measure the amplitude and rate of growth of vortex meandering experimentally. Data are presented that show the variation of the core radius with distance downstream of the point of origin as a function of the vortex strength. Also, experimental data are presented to show the variation in the amplitude of wandering downstream of the point of origin. Using the velocity fluctuations in the close proximity of a vortex filament, it is shown that 1) this quantity grows with downstream distance from the point of origin and 2) the degree of wandering depends on the vortex strength

Exploratory study of aircraft wake vortex filaments in a water tunnel

The goal of this research was to examine the relative safety and stability of stationary motor vehicles exposed to severe winds. The focus was on private passenger vehicles. 1) The behavior of two instrumented storm-chase vehicles that were exposed to severe winds, 2) the behavior of 291 vehicles exposed to a tornado, and 3) the wind speed required to upset a sedan and a minivan exposed to winds in a wind tunnel were studied. A wind as strong as 47 m s−1 (105 mph) has been measured by a storm-chase pickup truck and 44 m s−1 (98 mph) by a storm chase sedan. The vehicles were not adversely affected by the wind. Also studied were 291 vehicles parked outdoors at homes struck by tornadoes, and the behavior of the vehicles was compared to the F-scale damage to the house. At sites with F1 or F2 damage, 72% of the vehicles were not moved by the wind and 96% were not tipped over. At sites with F3 or F4 damage, 50% were not moved by the wind and 82% were not tipped over. Wind tunnel tests on a sedan and minivan sho...

Unsafe at Any (Wind) Speed?: Testing the Stability of Motor Vehicles in Severe Winds

The effect of a canard on delta wing vortices was investigated in the 2 3 3 ft water tunnel at Wichita State University. It is well known that the leading-edge vortices generated by a delta-shaped wing greatly enhance a vehicle’ s performance at high angles of attack. In this experiment, different canards were placed in front of a 70-deg swept main delta wing. Dye e ow visualization was used to observe the vortex breakdown location during dynamic pitch-up and pitch-down motion with varying pitch rates. Compared to the no-canard cone guration, results showed that there was a delay in vortex breakdown because of the presence of the canard and the dynamic pitch motion. The most favorable delay was obtained when the canard was located closest to the main delta wing and the model was pitched up at a fast rate or pitched down at a slow rate. Complete vortex breakdown on the main delta wing (i.e., full stall ) occurred at 53 deg for the static case without canard. In comparison, complete vortex breakdown occurred past 90 deg when a canard cone gured delta wing was pitched up at the fastest rate tested (i.e., k = 0.2).

L. Scott Miller

Papers

Propeller Performance Measurement for Low Reynolds Number UAV Applications

Exploratory study of aircraft wake vortex filaments in a water tunnel

Unsafe at Any (Wind) Speed?: Testing the Stability of Motor Vehicles in Severe Winds

Effect of Canards on Delta Wing Vortex Breakdown During Dynamic Pitching

Evaluation of a basic doppler global velocimetry system