Flow structures near the models of helicopter configurations
Aerodynamics and heat-exchange processes in flying vehicles
Аuthors*, *, *
Central Aerohydrodynamic Institute named after N.E. Zhukovsky, TsAGI, 1, Zhukovsky str., Zhukovsky, Moscow Region, 140180, Russia
Flow patterns obtained in water tunnel are presented for helicopter models and their components. Flow visualization was performed using the method of dyed jets. A detailed description is given for the models in which the jet blowing was realized.
Visualization of Ka-50 glider model has shown that with decreasing the angle of attack the rearrangement of limit streamlines occurs in nose fuselage so that at α = −10о they are drawn in the underwing area. At α = −10о the vortex cores are generated initiating from the nose section then following under fuselage and along it to the area of horizontal tail. During slip these vortex cores become greatly distorted that may have a significant influence on horizontal tail operation.
Investigations of flow structure in the vicinity of Ka-50 glider model with backswept wing has shown that trailing-edge vortices running from tip to root wing sections and further along the aft fuselage to stabilizer area are clearly visualized on this model. In the presence of slip the said vortex structures of the model become visually distorted that may have a significant influence on its longitudinal static stability.
Investigations on Mi-26 helicopter model with main rotor simulation has shown that even in the absence of slip (β=0) the flow over the right and the left side of fuselage is essentially asymmetric due to the impact of rotating rotor. On the right side behind the pylon the area of disturbed flow generated by the main rotor disturbances is formed. This is confirmed by comparison with the flow pattern in the right-side view with inoperative rotor. Then this area drifts downstream towards horizontal tail. The flow over the left side of fuselage is much smoother. Only some of pylon disturbances reach the area of tail boom and loading ramp joining. Flow patterns obtained at negative and positive slip of this model are also presented in the paper.
The paper presents the results of the investigation of the cylinder model with controlled circulation using a specially contoured slotted nozzle. Visualization was performed in the water tunnel and weight tests were carried out in the wind tunnel. The azimuthal position y of the slotted nozzle determined in reference to the direction of the incoming flow velocity vector and the jet pulse coefficient сμ varied over a wide range. Tests showed that in the region of 100° ≤ Ψ ≤ 135° in lift coefficient and drag increment dependencies on ψ there appeared discontinuities and ambiguity characteristic of hysteresis effects indicating the presence of two possible flow regimes. At small values of cμ separation occurs above the slotted nozzle, the lifting force increment is caused by air suction from separation zone near the cylinder and by reactive force of the jet. With increasing cμ the boundary layer is attached by the shock, the lift increases drastically, the flow over the upper part of the cylinder is without separation.
Investigations of Ka-60 helicopter model in water tunnel with simulation of jet blowing from the slotted and jet nozzles, as well as the air intakes operation have shown the following. The air intakes take water from behind the rotor hub allowing for elimination of separation area behind it that causes the essential part of parasitic drag of helicopter body. The unseparated flow over the tail boom in the vicinity of the slotted nozzle is clearly seen that induces a side force compensating a considerable part of rotor reaction torque.
Flow patterns near Yak-24 tandem-rotor helicopter model are also presented in the paper. Visualization of tip vortices shedding from the aft rotor is especially clear. At fixed rotation frequency of rotors the spacing between these cycloid vortices increases substantially with increasing the incoming flow velocity.
It is noted that flow visualization in low-speed water tunnel is a highly effective and low-cost technique to reveal characteristic features of the flow over aircraft models in addition to wind-tunnel investigations.
Keywords:method of dyed jets, vortex cores, main rotor, slotted nozzles
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