Unsteady Aerodynamics of Membrane Airfoils
Abstract
This report results from a contract tasking University of Bath as follows: Background: Membrane wings are used in many applications such as sails and wings of microlights and small unmanned air vehicles [1]. Inviscid [2,3,4,5] and viscous calculations [6,7] were used to simulate the equilibrium shapes in steady conditions. It was found that viscous effects and flow separation need to be included for accurate simulations in steady flow. Unsteady aspects are far more important and critical for micro air vehicle applications. These small vehicles must be able to operate in gusts and unsteady free stream conditions. They may also be subject to the instabilities caused by the fluid-structure interactions of the membrane wings. Unsteady aspects of such flows are neither understood nor simulated to the best of our knowledge. The best known unsteady phenomenon is a flapping instability which is also known as luffing [3]. We have demonstrated these limit-cycle oscillations in our experiments. This instability is usually attributed to the bistable instability, and appears as periodic oscillations between the two modes. The concept of the bistable instability is more suitable at zero incidence when the membrane is just as likely to set itself on one side as the other. However, luffing occurs at nonzero incidences as well. The lift, as a result, exhibits hysteresis [4]. Our experiments indicated that the region over which the luffing and hysteresis are observed strongly depends on the excess length (defined as the difference between the length of the membrane and the chord), which itself depends on the aerodynamic loading for a flexible membrane.
Document Details
- Document Type
- Technical Report
- Publication Date
- Aug 01, 2008
- Accession Number
- ADA521105
Entities
People
- Ismet Gursul
Organizations
- University of Bath