No abstract

Circulation control is a high-lift methodology that can be used on the wing of an aircraft. This technology has been in the research and development phase for over sixty years primarily for fixed wing aircraft when the early models were referred to as "blown flaps." Circulation control works by increasing the near surface velocity of the airflow over the leading edge and/or trailing edge of a specially designed aircraft wing using a series of blowing slots that eject high velocity jets of air. The wing has a rounded trailing edge, and ejects the air tangentially, through these slots inducing the Coandă effect. This phenomenon keeps the boundary layer jet attached to the wing surface longer than a conventional wing and thus increases the lift generated on the wing surface due to the relaxation of the Kutta condition for the rounded trailing edge. The circulation control airflow adds energy to the lift force through conventional airfoil lift production and by altering the circulation of stream lines around the airfoil.

No abstract

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.

The purpose of circulation control for fixed wing aircrafts is to increase the lifting force when large lifting forces and/or slow speeds are required, such as at takeoff and landing. Wing flaps and slats are used on almost all fixed-wing aircraft. While effective in increasing lift, they do so with penalty of increasing drag, weight, and control complexity. The goal of this research was to find an alternative way of pumping pressurized air to the trailing edge slot on a UAV propeller. This design called for rerouting stagnation pressure from the frontal propeller area through the inside of the propeller blades to ejection slots on the trailing edge. This allows for the forward velocity of the aircraft to drive the pressurization of the circulation control plenum passively, without additional hardware. For this study, a Clark-Y airfoil section propeller with an overall diameter of 0.609 meters was designed and tested. The comparison of the augmented to unaugmented propeller showed a 5.12 percent increase in efficiency, which is shown to act over the entire range of flight envelopes of the aircraft and is shown to be particularly beneficial at advance ratios above 0.30, normal operating conditions of propeller-driven UAVs.