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The descent and landing of the helicopter
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The descent and landing of the helicopter

The descent and landing of the helicopter.

 

 

Shutter or scheduling is performed in such a mode of operation of the rotor, when the air passes through the rotor surface swept upward. The angle of attack of the screw A can vary within wide limits, from slight angle to + 90 °. When k = + 9SR air passes through the rotor surface swept from bottom to top in the axial direction, and the helicopter descends vertically.

In other cases there is an oblique blasting propeller and the helicopter descends along an inclined path. Consider the descent along an inclined path.

Furthermore, there should be balanced torques acting around the three axes.

The helicopter can be planned at any angle to the horizon. In the case of a steep planning rotor is set at a positive angle to the oncoming air flow stream passes obliquely through the bottom surface of the rotor swept-up). In cases when a shallow planning rotor is set at a negative angle to the flow, the flow passes through the oblique surface of the rotor swept from top to bottom) as it took place at level flight or a climb along an inclined path. The value is determined by the installation of the lift rotor collective pitch and engine speed.

With a large step (high gas) helicopter will climb. If you decrease the value of the pitch and gas helicopter will fly horizontally. For a small step (idling) - plan.

Steep descent is made on A = + 90 °, the flow will blow the rotor in the axial direction, upwards. Screw down swinging mass of air will create a lifting force which balances the weight of the helicopter and providing descend at any speed up to hang at any height below the ceiling static.

With the increase in shutter speed rotor absorbs less and less engine power, as the incident on the screw at the bottom air stream hyping it as a windmill.

At sufficiently high rates of descent mode comes autorotation (autorotation) of the rotor when the motor operation can be generally unnecessary, since all the work rotation of the rotor performs the bottom of the incoming stream.

When approaching the ground at a distance equal to the diameter of the rotor, and less thrust it increases significantly, and thereby decreases the rate of decline.

This ground effects are called "earth pillow."

Helicopter landing can be accomplished in two ways: "in a helicopter", ie. E. Vertically and without a run, Shea "an aircraft", ie. E. With forward speed and then run.

When an engine failure a landing can perform under the conditions of the rotor autorotation (autorotation). As we already know, in normal flight lift helicopter rotor creates (rotor) is driven by a motor.

To make stops in autorotation mode - it means to plan due to the lifting force generated by the screw, which is rotated by the action of the incident and swept them passing through the surface airflow.

The school makes such an experience. Take two vessels: one air pumped, and from no other. If we place the top of each vessel a piece of cotton wool and simultaneously to omit them, in a vessel where the exhausted air, cotton reaches the bottom of the vessel before the vessel with air.

This is because air has a falling body resistance. The force of air resistance directed upward against the force of weight.

The strength of the resistance directed against the forces of weight, and there is lift.

In the incident in the air body lift (resistance force) with the growth rate falling increases all the time, until it becomes equal to the force of weight, and then the fall has been going at a constant speed. For example, when a falling skydiver skydiving.

The strength of the weight applied to the center of gravity of the body. The strength of the resistance - in the center of pressure of the body.

If the center of gravity of the accident will coincide with the center of pressure, or will be directly below the center of pressure, the body will fall safely. If the center of gravity will be in the side of the center of pressure, then under the action of forces falling body will rotate around the center of gravity. When the body rotates, it changes its position the center of pressure, wherein the rotation may begin at the other side and to continue until the center of gravity will be directly below the center of pressure. So, swaying in the autumn falls on a windless day, fallen from a tree leaf.

But the seeds of the maple fall differently. They do not sway, and rotate about a vertical axis.

Created with the lift slows down the fall and the wind carries away the seeds. Maple seeds fly on the "landing" on the regime of autorotation.

They rotate at the same time under the influence of air flow, creating a lifting force. If the lift is not directed vertically and is slightly inclined, then the component of the lift in the report as a thrust body forward speed.

Let's now some up. "

To produce lift and thrust necessary that rotor rotates.

Typically, the rotation of the engine is created, and if the engine is not (has failed, is switched off), it is necessary to act on the propeller airflow.

In order for rotor air flow, it is necessary that the screw moves relative to the air - or lowered or moved progressively made these movements simultaneously.

These conditions are always there at the helicopter, whose engine failed.

If the helicopter is flying horizontally at a speed of, for example, 150 km / h and the engine failed, then, of course, the speed of the helicopter did not immediately fall. As well as the inertia for some time and will rotate rotor.

The helicopter will begin to gradually decline, while gradually be quenched his forward speed and increase the vertical speed. If the first screw is rotated by the engine, and then when the engine is switched off - by inertia, now, he will continue to rotate under the influence of the incoming air flow to it by the forward speed and reduce the helicopter.

However, this requires that the angle of the blades (angle of attack, the pitch of the screw) to be completely defined, as will be discussed further.

To determine the condition of self-rotation of the rotor blades analyze the work item.

Forces on the blade element depends on the magnitude and direction (angle of attack) the total velocity W, with which the profile meets the air. When the angle of attack of the screw is negative, the rate of W is the solution to the profile as shown.

Under the influence of air flow on the blade element arises aerodynamic force. We expand this force in two directions: downstream and perpendicular to it.

Force directed perpendicular to the flow, and force directed downstream determine how a lift force generated by the blade member, and rotational resistance. The resistance to the rotation of the blade is determined by the sum of the projections of the forces. and both projections directed back against rotation. In this case, of course, about any self-rotation of the rotor can not be considered. On the contrary, to turn the screw, it is necessary to overcome the drag torque, telling the screw rotation of the motor.

The case, which is typical for a positive angle of attack of the rotor and small setting angles of the blades, it turns out a different direction forces. Now, if the projection onto the plane of rotation forces directed backwards and prevents the rotation of the blade, the projection of forces onto the plane of rotation is forward and contributes to rotation.

In the case where both projections are equal, stable autorotation mode occurs rotor r. E. Rotation at a constant speed without the cost of power from the engine, since there is no force preventing rotation seeking stop screw. Rotation, once started, continues with constant speed. Rotor cooled by air flow from the bottom-up, works like a windmill.

Consequently, the regime of autorotation begins with a positive angle of attack of all the screw when the blades are mounted on a small installation angle (small step).

This screw will create a lifting force equal to the amount of projection of forces on the axis of rotation (t. E. Equal force. On mode autorotation a helicopter, of course, can not climb. He can not also continue to horizontal flight. In mode, autorotation, the helicopter can only decrease . behalf reduction is the source of airflow, providing self-rotation screw.

To create conditions for autorotation rotor pilot when an engine failure must first reduce the installation angle of the blades (to reduce the pitch of the screw). This must be done as quickly as possible so that more, but had greatly reduced speed and to reduce the speed of the screw rotating now only by inertia. To reduce the pitch of the main rotor helicopter can install "automatic reset of pitch", which comes into play immediately after stopping the engine.

The transition to autorotation mode when there is no automatic reset of pitch is complicated, because there are many factors distracting the pilot: decreases the number of revolutions changes the velocity field of the screw, change points on the fuselage (a tendency to nose down), change efforts on the control handle.

The transition to autorotation mode is possible at various speeds helicopter flight. These speeds are usually less than 100 km / h. Possible transition to autorotation and is also hovering. However, in the latter case, the transition from hover to autorotation requires a relatively long time during which the helicopter loses height. Therefore it recommended to hover at altitudes of at least 150-200 m. Otherwise, you can

do not have time to translate the screw on the regime of autorotation in case of engine failure.

You can also hang at a height of less than 10 m. If the engine fails, then before the meeting with the land the helicopter did not have time to dial a large rate of descent.

The speed reduction on the autorotation mode during vertical descent can reach 10 meters or more per second, depending on the specific load on each square meter of rotor swept area. The vertical rate of descent is proportional to the square root of the specific load, t. E. High loads lead to a rapid decline.

The decline in the autorotation mode in the presence of forward speed is much safer. So, during the descent along an inclined path with the most favorable rate of vertical descent rate of 0-8 m / sec, which is less than the rate of decrease in the parachuter.

To reduce the speed of a meeting with the ground is recommended at the approach to land on 15-30 m produce "undermining" the helicopter, t. E. Continuously increasing collective pitch. As a result, short-term, yet by inertia continues the rapid rotation of the screw increases lift vultures, thus mitigating the impact on the ground.

Self-rotation of the rotor can be used for towing the helicopter, or even a whole "train" of helicopters after the aircraft. In this case, the source of forward speed of the helicopter through the air for autorotation rotor will be the thrust of the aircraft.

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