Investigation Into the Feasibility of an Augmented Propeller Design With the Use of a Passive Circulation Control System

Circulation control is a high-lift methodology that can be used in a variety of fluid dynamic systems, such as, on the wing of an aircraft. Circulation control increases the near surface velocity of the airflow over a rounded surface of an object, typically a slightly modified airfoil. This is primarily achieved though the addition of a jet of air to a specially designed aircraft wing using a series of blowing slots that eject pressurized high velocity (above the free-stream velocity) jets of air over the trailing edge and/or leading edge. Studies have also been conducted into the addition of circulation control to bodies such ad propellers and rotors. In the early years of circulation control there were three main critical design issues with the addition of circulation control to a rotating body. The first being the exposure of the rotors angles of attack between 0–35° caused by the inflow of air through the propeller plane. Through the study of high angles of attack in wind tunnel testing, it is possible to predict the behavior of the rotor blade at these higher angles of attack. The second obstacle in the prior applications of circulation control to a propeller was the inability to achieve the response times necessary to effectively use circulation control during the rotation of the propeller. A further requirement of circulation control applications to propeller powered aircraft is the power required to supply the airflow. An active circulation control system uses an internal pumping system which can use power from the aircraft or from an additional power source, such as a generator, to pressurize the air plenums in order to use circulation control on the aerodynamic body. With the development of unmanned aerial vehicles (UAVs), propeller performance enhancement is desirable in order to increase the thrust, and/or the overall range of the aircraft. The application of the active circulation control to the propeller, though potentially beneficial, is currently envisioned as creating technical difficulties in the supply of air to the circulation control blowing slot. A passive system in which air can be supplied to a strategically placed circulation control blowing slot can also enhance the performance of a propeller. The proposed passive system will take advantage of the pressure differential upstream and downstream of the propeller plane, forward air velocity, stagnation pressure, and centripetal acceleration to pressurize the internal plenum of the circulation control system and thus not require an additional power source to augment the propeller of the aircraft. Also, because the system will not need to be pressurized from an outside source, no additional weight or requirements will be necessary for the aircraft other than the implementation of an updated propeller. It has been shown that through the addition of a pressure capture device on the front of a propeller, a six to fourteen percent increase in lift coefficient can be achieved simply by allowing the stagnation air ahead of the propeller to pressurize the internal plenums. Although not significant for use in larger propeller driven aircraft, for UAV applications, this can lead to a two to five percent increase in range.