Notre Dame Micro Aerial Vehicle Development Group

University of Notre Dame

Micro Aerial Vehicle Research

MAVs, Reynolds Number, and Low Aspect Ratio Wings
Micro aerial vehicles are by definition small aircraft which fly at relatively low speeds (usually less than 30 mph). Because of this combination of flight characteristics, MAV flight is said to be associated with low Reynolds numbers (less than 200,000). The Reynolds number is a ratio of inertial forces to viscous forces and varies from a few thousands for insects to several millions for commercial aircraft (see figure below).

The design of vehicles which operate at Reynolds numbers below 200,000 is difficult because many of the simplifications of the governing equations which are valid for large Reynolds numbers are not valid for MAV flight regimes.
Another complication of MAV design is that MAVs are required to be as compact as possible, leading to wings of low aspect ratio (the chord is roughly equal to the wingspan). The aerodynamics of low aspect ratio wings (LAR wings) are drastically different than their higher aspect ratio counterparts. Furthermore, LAR wings at low Reynolds numbers have essentially not been studied until very recently.

MAV-Focused Research at Notre Dame
The Department of Aerospace and Mechanical Engineering at the University of Notre Dame has been involved in the study of low Reynolds number aerodynamics since the 1980's, with an emphasis on MAV-focused research starting in 1996. Some references related to two-dimensional airfoil studies and three-dimensional, rectangular-wing studies at low Reynolds numbers are given below (research conducted between 1996 and 1998):

Pelletier, A. and Mueller, T. J., Low Reynolds Number Aerodynamics of Low Aspect-Ratio, Thin, Flat and Cambered-Plate Wings, to be published by the AIAA Journal of Aircraft.
Mueller, T. J., Aerodynamic Measurements at Low Reynolds Numbers for Fixed Wing Micro-Air Vehicles, presented at the "Development and Operation of UAVs for Military and Civil Applications" course held at the von Karman Insitute for Fluid Dynamics, Belgium, September 13-17, 1999.
This paper is available online in Adobe Acrobat PDF format (1.5 MB).
Pelletier, A. and Mueller, T. J., Aerodynamic Force/Moment Measurements at Very Low Reynolds Numbers, Proceedings from the 46th Annual Conference of the Canadian Aeronautics and Space Insitute, Montreal, Quebec, Canada, May 3-5, 1999, pp. 59-68.
In 1998, a more detailed study of LAR wings at low Reynolds numbers was launched. This program consisted of extensive wind tunnel experiments of wings with aspect ratios between 0.5 and 2.0 at Reynolds numbers between 50,000 and 150,000. The aerodynamic performance of wings of different planform shapes was determined experimentally and used to generate relationships applicable to practical MAV design. In fact, this experimental data was partly used to design Notre Dame's entry for the 2000 MAV Competition. The papers listed below discuss the research performed in the 1998-2000 period at Notre Dame:

Torres, G. and Mueller, T. J., Aerodynamic Characteristics of Low Aspect Ratio Wings at Low Reynolds Numbers, presented at the Conference on Fixed, Flapping, and Rotary Wing Vehicles at Very Low Reynolds Numbers, Notre Dame, IN, June 5-7, 2000. Proceedings to be published as AIAA book in 2000.
Torres, G. and Mueller, T. J., Micro Aerial Vehicle Development: Design, Components, Fabrication, and Flight Testing, presented at the AUVSI Unmanned Systems 2000 Symposium and Exhibition, Orlando, FL, July 11-13, 2000.
This paper is available online in three formats:

Paper in HTML format.
Paper in Adobe Acrobat PDF format (268 KB).
Presentation at the AUVSI 2000 meeting in Adobe Acrobat PDF format (887 KB).

Flow Visualization of Low Aspect Ratio Wings
Flow visualization experiments have also been conducted on various planform shapes to study the structure of the tip vortices and the existence of separation bubbles. In addition, flow visualization experiments have also been conducted on full-scale MAV prototypes to study the aerodynamic performance of test-flight vehicles. Below are a selection of the visualization photographs taken using a variety of flow visualization techniques.

Multiple-exposure photograph of the tip vortex on a rectangular wing of aspect ratio 1 (visualized with kerosene-smoke and a laser sheet)

Smoke-wire technique applied on same wing, showing the breakdown of the flow structure over the upper surface of the wing and the development of the tip vortex

Multiple-exposure photograph of the tip vortex on a inverse Zimmerman wing of aspect ratio 1 (visualized with kerosene-smoke and a laser sheet)

Fluorescent surface fluid technique applied on same wing, showing the shape of the vortex structures; a separation bubble near the leading edge is also visible as an area of collected fluid

Kerosene-smoke flow visualization used on a full-scale MAV prototype to determine tip vortex structures and identify regions of separated flow

Work Currently in Progress
Current work in the low Reynolds number aerodynamics project include the wind tunnel testing of low aspect ratio wings with varying camber configurations, including reflex airfoil shapes that neutralize the pitching moment in tailless aircraft. Also under development at the moment is a vortex-lattice-method-based computer code to be used to predict performance characterstics of low aspect ratio wings.

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