Craig D. Paxton

High-Performance Airfoil Using Coflow Jet Flow Control

T O ACHIEVE revolutionary aircraft performance, advanced technologies should be pursued to drastically reduce the weight of aircraft and fuel consumption and significantly increase aircraft mission payload and maneuverability. Both the military and commercial aircraft will benefit from the same technology. Flow control is a promising technology to break through the limits of the conventional aerodynamic concepts. Recently, a novel active airfoil flow control concept with zero-net mass flux, the coflow jet (CFJ) airfoil, has been developed by Zha et al. [1–5]. The CFJ airfoil achieves three effects simultaneously in a dramatic fashion: lift augmentation, stall margin increase, and drag reduction. The energy expenditure of the CFJ airfoil is low [1], and the CFJ airfoil concept is straightforward to implement. The CFJ airfoil may create a new concept of an “engineless” airplane, which uses the CFJ to generate both lift and thrust without conventional propulsion systems of propellers or jet engines [6]. A CFJ airfoil [1–5] uses an injection slot near the leading edge (LE) and a suction slot near the trailing edge (TE) on the airfoil suction surface. Similar to tangential blowing, the LE jet is in the same direction of the main flow, but the same amount of mass flow that is injected is removed via suction near the TE, resulting in zeronet mass-flux flow control. A proposed fundamental mechanism [2] is that the severe adverse pressure gradient on the suction surface strongly augments turbulent mixing between the main flow and the jet [7]. The mixing then creates the lateral transport of energy from the jet to the main flow and enables the main flow to overcome the large adverse pressure gradient and remain attached at high angle of attack (AOA). The stall margin is hence significantly increased. At the same time, the high-momentum jet drastically increases the circulation, which significantly augments lift, reduces drag, or even generates thrust (negative drag). The objective of this paper is to demonstrate the high performance of the CFJ airfoil with the windtunnel test results.

Effect of Injection Slot Size on the Performance of Coflow Jet Airfoil

Two coflowjet airfoils with injection slot size differing by a factor of 2 are tested in a wind tunnel to study the effect of injection slot size. At the same angle of attack, the larger injection slot size airfoil passes about twice the jet mass flow rate of the smaller injection slot size airfoil. The smaller injection slot size airfoil is more effective in increasing the stall margin and maximum lift, whereas the larger slot coflow jet airfoil is more effective in reducing drag. To achieve the same lift, the smaller injection slot size airfoil has much less energy expenditure than the larger injection slot airfoil. A coefficient of jet kinetic energy is introduced, which appears to correlate well with the maximum lift and stall margin when coflow jet airfoil geometry varies. Both the jet kinetic energy coefficient and the momentum coefficient correlate well with drag reduction. No optimization of the airfoil configuration is pursued in this research, and the results indicate that there is a great potential for coflow jet airfoil performance improvement.

Jet Effects on Coflow Jet Airfoil Performance

A control volume analysis is presented in this paper to analyze the jet effect on the coflow jet airfoil with injection and suction and on the airfoil with injection only. The formulations to calculate the ducts reactionary forces that must be included for the lift and drag calculation are given. The computational fluid dynamics solutions based on the Reynolds-averaged Navier-Stokes model are used to provide the breakdowns of lift and drag contributions from the airfoil surface force integral and jet ducts reactionary forces. The results are compared with experiment for validation. The duct reactionary forces are also validated with the result of a 3-D computational fluid dynamics calculation of the complete airfoil with jet ducts and wind tunnel walls. The study indicates that the suction occurring on the airfoil suction surface of the coflow jet airfoil is more beneficial than the suction occurring through the engine inlet such as the airfoil with injection only. For the airfoil with injection only, the drag actually acted on the aircraft, or the equivalent drag, is significantly larger than the drag measured by the wind tunnel balance due to the ram drag and captured area drag when the jet is drawn from the freestream. For a coflow jet airfoil, the drag measured by the wind tunnel balance is the actual 2-D drag that the aircraft will experience. A coflow jet airfoil does not have the ram drag and captured area drag. For a coflow jet airfoil, the suction penalty is offset by the significant circulation enhancement The coflow jet airfoil with both injection and suction yields stronger mixing, larger circulation, more filled wake, higher stall angle of attack, less drag, and lower energy expenditure.

A Novel Airfoil Circulation Augment Flow Control Method Using Co-Flow Jet

Anovelsubsonicairfoilcirculationaugmenttechniqueusingco-∞owjet(CFJ)toachievesuperioraerodynamicperformanceforsubsonicaircraftisprovednumericallybyCFDsimulation. Theadvantagesofco-∞owjetairfoilincludehighliftathighangleofattack,ultrahigh Cl=Cdat cruise point, and low penalty to the overall cycle e‐ciency of the airframe-propulsion system. Unlike the conventional circulation control (CC) airfoil which is only suitable for landing and taking ofi, the CFJ airfoil can be used for the whole ∞ying mission. No blunt leading and trailing edge is required so that the pressure drag is small. No moving parts are needed and make it easy to be implemented and weight less. The jet to enhance the circulation will be recirculated. Compared with the CC airfoil, the recirculating CFJ airfoil will signiflcantly save fuel consumption because: 1) the power required to energize the jet is less; 2) no penalty to the jet engine thrust and e‐ciency due to the disposed jet mass ∞ow since the jet mass ∞ow is recirculated. For the NACA2415 airfoil studied, at low AOA with moderate momentum jet coe‐cient, the co∞ow jet airfoil will not only signiflcantly enhance the lift, but also dramatically reduce the drag, or even generate the negative drag (thrust). The mechanism is that the co∞ow jet can control the pressure drag by fllling the wake, and could generate negative pressure drag greaterthanthefrictiondrag. Thismayallowtheaircrafttocruisewithveryhighaerodynamic e‐ciency. AthighAOA,boththeliftandthedragaresigniflcantlyhigherthantheairfoilwith no ∞ow control, which may enhance the performance of taking ofi and landing within short distance.

Numerical Simulation of Co-Flow Jet Airfoil Flows

A CFD calculation strategy is developed to simulate 2D co-flow jet airfoil. The jet ducts reaction forces are added to the surface integral of pressure and shear stress to calculate the total lift and drag. The predicted lift and drag agree well with the experiment at low angle of attack(AoA) and deviate largely at high AoA. The stall AoA of the CFJ airfoil is predicted reasonably well. Details of the flow field results and comparison between the computation and experiment are given in the paper.

HIGH EFFICIENCY FORWARD SWEPT PROPELLERS AT LOW SPEED

Propellers with forward sweep were studied in order to determine whether this type of sweep produces a more efficient propulsion source. The sweep effect is achieved solely by tangentially leaning the leading edge of the blades toward the direction of rotation. Eight 155mm (6.1”) diameter propeller designs were created for testing; one straight blade, used for comparison, and seven swept blades using different sweep angles and configurations. Wind tunnel tests were conducted on all blades and CFD simulations were made for the straight blade as well as the best performing swept designs from the wind tunnel results. While efficiency values from wind tunnel tests and CFD simulations were not the same, the trends created in both were considered to be comparable. The general trend established shows that the swept blades not only have higher efficiencies but also have greater stall margins. Some speculated reasons for these advantages are given.

Velocity field for an airfoil with co-flow jet flow control

*† ‡ § ** The wind tunnel tests reported in this paper demonstrate the ability of the co-flow jet airfoil to dramatically increase lift, stall margin, and drag reduction. Detailed velocity field measurements are reported for a baseline NACA0025 airfoil and two variants of the baseline which implement the co-flow jet. The first variant has an injection slot size of 0.65% of chord while the second has an injection slot size twice that size.