To ensure the flow on the rotor blade, the rotor must always be driven. But what happens if the drive - for whatever reason - fails?
Since the rotor blades in the forward flight by the collective pitch adjustment have a relatively large angle of attack and thereby also produce a correspondingly large air resistance, the speed of the rotor without a drive drops rapidly. As a result, of course, the necessary buoyancy is lost and the helicopter crashes within a short time.
Luckily, that sounds much more dramatic than it really is. What is gliding flight in a surface aircraft is the autorotation of a helicopter. If the helicopter fails during flight, the pilot will immediately reduce the collective blade pitch and the helicopter will start to drop. At the same time, due to the smaller angle of attack, the air resistance at the rotor blades is substantially reduced.
As we can see on the left, the rotor is now no longer flowing through from top to bottom, but from bottom to top. Due to the aerodynamic conditions, which we will look at in more detail, the rotor speed can be kept constant in this state.
In order to explain the processes in the autorotation, we must not consider the rotor as a disk as before, but must examine the conditions on the individual rotor blade. And to do this, we first look at the condition in powered forward flight. In the case of a flight profile, the buoyancy always acts perpendicular to the flow and the air resistance in the same plane as the flow.
Since the flow in a helicopter consists of a horizontal (rotation of the rotor) and a vertical component (air flow from above or below), we refer to the rotor blade of a relative flow. Since the rotor blade moves outwards at a greater speed, but the vertical component remains more or less constant, the relative flow changes constantly over the entire length of the rotor blade. For this reason, the sketch applies only to a small area on the rotor blade. Also, the angle of attack (angle between the chord and the relative flow) changes over the length of the rotor blade and that decreases the angle of attack to the outside.
In the autorotation, the rotor can be divided into three areas. For the sake of simplicity, we first look at the vertical autorotation, that is, the helicopter is in vertical descent.
In the vertical autorotation, the areas are distributed symmetrically over the rotor disk. Only the driving area is responsible for the rotation of the rotor. In the vicinity of the center, the flow velocity is so small that the rotor blades are in stall. In order to explain how these areas come about, we need to investigate the aerodynamic conditions on the individual rotor blade a little more closely.
In the driving range, the total aerodynamic forces are in front of the axis of rotation of the rotor. This results in a force which drives the rotor.
If the aerodynamic forces lie exactly on the axis of rotation, the rotational speed remains constant.
In the braking part, the total forces are behind the axis of rotation, which means that the rotor is braked.
All helicopters are designed to balance the driving and braking parts. This balance must be ensured from vertical descent to a certain forward speed. Some helicopters are restricted to maximum forward speed during autorotation. This is because the driving part shifts with increasing speed (see below). This shift always takes place in the direction of the returning blade.
If the forward speed is increased again in this situation, the driving range shifts further to the right, which ultimately causes the braking part to be greater than the driving and thus the rotor speed can not be kept constant. In general, an autorotation is always flown at a certain forward speed. To ensure a safe landing, this speed must be reduced as much as possible. This is achieved with the so-called flare. Just above the ground, the pilot picks up the nose, which reduces sinking and reduces speed. This deceleration allows the rotor to absorb even more energy (speed is increased) and the helicopter can make an almost normal landing. This sounds very simple, but is a demanding maneuver for the pilot.
During stable hovering, as already described, the air is accelerated from top to bottom by the rotor. The air flow is not constant over the length of the rotor blade.
The vortex or vortex ring condition arises in a helicopter when it is in hover or slow forward flight, with a high rate of descent. The rate of descent must be in the range of more than 500 ft / min (2.5 m / sec) and the forward speed below the so-called transition zone. In addition, the rotor must be driven by the drive. These conditions are given especially during a steep landing approach with tailwind.
The sinking speed creates a flow of air which counteracts the downwash.
As a result, the air is accelerated from top to bottom in the inner region of the rotor plane, but by the air flow from below immediately transported back up.
The air is sucked in again from above and thus creates a self-contained system, the vortex or vortex ring state. In this condition, the helicopter begins to sink even more, even if the power is increased. Although the helicopter is still controllable, but there are some strong vibrations.
The Vortex can basically be terminated in two ways: As the first possibility, one goes into the forward flight, since the Downwash is derived in the forward flight to the rear, thereby the rotor can be supplied with "new air" from above. This variant must be used when the vortex occurs near the ground, for example during a landing approach. As a second possibility, you can go into the Autorotation, whereby the air flow is missing from top to bottom and the rotor is only flowed through from bottom to top (as in a normal Autorotation). In any case, the pilot must react very quickly when the vortex occurs, because the enormous descent speed may mean that the altitude is too low to stop the vortex.
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