טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentYana Jessica
SubjectPerformance of Coaxial Rotor System in Hover: Three
Points of View
DepartmentDepartment of Aerospace Engineering
Supervisor Professor Omri Rand
Full Thesis textFull thesis text - English Version


Abstract

A coaxial rotor system is an arrangement with two identical, or nearly identical, counter-rotating rotors along a common spin axis; i.e., coaxially. An important advantage of coaxial rotor configurations over the traditional single main rotor configurations is its improved efficiency and compact design. The efficiency improvement stems from the larger disc area and from the removal of the tail rotor, which consumes power, yet contributes little or nothing to the thrust. The resulting compact design improves maneuver performance, reduces airborne vulnerability, and conserves space in cramped landing areas, such as aircraft carrier flight decks. In addition, significant speed improvements are potentially achievable for coaxial rotorcraft, although at the cost of increased vibration level.


The present effort deals with the problem of the aerodynamic analysis of a coaxial rotor system in hover from three points of view: An analytic point of view; a computational point of view which is based on a rigid wake model; and a computational point of view which is based on a free wake model.


In the analytical model, the upper rotor model takes into account the lower rotor induced velocity as an "equivalent climb speed" and the lower rotor model takes into account the upper rotor induced velocity in a similar, yet more complex way. The proposed analysis also includes a search for aerodynamically optimal coaxial rotor that eventually minimizes the total (induced and parasite) power. This search is founded on a calculus of variations theorem that exploits the Blade Element-Momentum theory.


A relatively simple computational point of view is founded on a rigid (prescribed) wake for each rotor. Wake geometry and its parameters are obtained from experimentally and computationally collected data from the open literature. In this case, the mutual interaction between rotors is accounted for by the rigid wakes induced velocity distribution over both disks.


For the computational point of view with free wakes, both wakes are determined by a time marching numerical scheme. Such modeling offers a higher level of fidelity for the aerodynamic analysis, although at considerably high computational cost. The model is capable of capturing the mutual interference of the wakes which leads to their geometry and strength characteristics and subsequently to the unique coaxial rotor system performance.