Aerodynamics of the rotor
Helicopter rotor blade consists of several (typically to from 2 5), connected to the hub.
The sleeve serves to transmit torque from the motor to the blades, which is connected to the drive shaft. Furthermore, the sleeve apparatus allows changing the position in space of the blades.
Changing the position of the rotor blades occurs for two reasons: under the influence of variable forces arising on them in flight, and the pilot's request, which, acting on the blades, the helicopter flight controls.
Where does power come from variables? If hover mode rotor blown air flow and direct all propeller blades are working under the same conditions, in the presence of the translational speed of the screw enters the oblique blasting mode. It turns out that the conditions of the various blades at a given time, or is the same as the working conditions of the blade during one full turn of the screw significantly changed.
Consider this question in more detail. For convenience, taken as a whole rotor to imagine how a single rotation of the plane (which passes through the hub perpendicular to the screw axis). Note that in reality there is not a plane and a cone formed by the rotating blades (sometimes called "tulip" blades).
Considering a plane of rotation of the screw, easily visible attack angles at which the screw moves relative to the oncoming air flow in forward flight.
The angle of attack is the angle of the screw between the direction of the velocity vector of the incoming air flow and a plane perpendicular to the axis of the rotor hub. Represented by the angle of attack by A, in contrast to the angle of attack, which is found under the profile of the blade with the air flow. If the aircraft angle of attack of the wing and the wing profile is the same, the values of various znacheniya- helicopter.
In the case of rotor moves at a negative angle of attack, and in the case b - a positive angle. In both cases, the screw in an oblique working stream.
We expand the rule of the parallelogram vector oncoming flow velocity V on the two velocity components: one - in a plane perpendicular to the axis of rotation of the screw, the other - on the axis of the screw.
Then immediately detected a difference in the operating conditions of the screw in the case.
In the case of air at a rate suited to the screw top. This mode corresponds to the motor rotor forward flight on the horizon, as well as climb or descent with small angles to the horizontal trajectory.
In case air used at a rate suitable for the screw below. This mode of operation corresponds to the rotor motor planning (non-motorized flight to autorotation screw mode) or a steep descent.
Turning to further familiarize with screws, remember of the two circumstances described above, namely:
- The angle of attack of the rotor is not equal to the angle of attack of the blade profile of individuals;
- In the oncoming air flow speed is not a complete operating rotation of the rotor plane V, and the only of its component.
Considering the main rotor on top, in the direction of the arrow, we can see the picture, shown schematically.
In addition, each section of the blade at the radius will be under the influence of two speeds: the circumferential speed and the free-stream velocity V. Putting these geometric rate in each azimuth angle, we see that the blade section (on the dotted circle) for one turn at a speed of air flows about all the time changing in magnitude and direction.
In fact, the azimuth angle (360 °) flow rate is greater (as the diagonal of the rectangle - side) and diverted to the end of the blade. At the azimuth angle = 90 ° equals the sum of the rate of flow velocities. This blade comes to meet the oncoming flow and is therefore called the advancing blade. On the azimuth angle np = 180 ° Flow rate 1 = 180 ° in absolute value is the same as in the azimuth angle 4 = 0 °, but not rejected by the end of the blade and the axis of rotation. At the azimuth angle v = 270 ° Flow rate equal to the difference. This blade as it moves away from the incoming flow, and is therefore called the retreating blade. It is remarkable in that it is a portion which is not flowed round the front and rear edges, whereby this portion does not create lifting forces (reverse flow zone).
So we have established that the working conditions of the blade during the turn change due to changes in flow rates.
In addition, the working conditions of the individual sections of the blades are changed also by changing the angles of attack. " The fact that the total angle of attack for the entire blade no. You can only talk about the angle of attack of a particular section. This angle is always changing. Let section initially set at an angle to the plane of rotation. The angle between the N line of zero lift profile and the plane of rotation is called the mounting angle or the pitch of the blade.
With the start of rotation of any cross-section at a radius flows about the velocity.
The angle of attack is equal to the mounting angle. In the presence of the translational speed occurs in the plane of rotation speed, and perpendicular to it - speed. In addition, the rotational velocity inductive plane appears due to dropping air screw.
The speed decomposed into two components along the blade and perpendicular to it along the section. It is obvious that in the section of the working conditions affect the speed.
Stacked velocity acting in the plane of rotation. Shows a sectional profile and are composed of all the speed acting on it. The angle between the total velocity W and a line of zero lift (aerodynamic chord) is the true angle of attack with the forward motion of the helicopter. This angle (Stork) significantly less incidence.
Furthermore, (due to changes in magnitude as the speed of the fold, then deducted during one rotation), it is clear that the true angle of attack is changing all the time.
Thus, changing all the time in forward flight operating conditions vane in one revolution (cycle). Since the beginning of the next turnover cycle is repeated.
Due to differences in flow velocities and angles of attack sections of cyclically changing the position of the full aerodynamic force generated on the rotor blades
screw. The total aerodynamic force does not pass along the rotation axis of the screw. This is the source of the vibration of the helicopter and one of the reasons of its instability. Due to the difference between the lifting forces in different parts of the surface, the swept rotor, are moments in it, seeking to overturn the helicopter with respect to the longitudinal and the transverse axes; due to the difference lobes have a significant resistance force load in the plane of rotation.
All this makes the designer to introduce a number of design features in a helicopter propeller compared with airplane propeller.
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