Before we can get into the aerodynamics of helicopters, we should know some basic aerodynamic principles: for aircraft heavier than air to be able to lift off the ground, an upward force at least as great as the weight of the aircraft must be generated. This force is called buoyancy and is generated by the wings.
The wings have a certain shape in cross section - the profile. There are a variety of different types of profiles, depending on the flight characteristics of an aircraft. If a wing moves forward, the profile divides the air flow into a lower and an upper part.
Since the air is displaced by the curvature around the profile, it has to travel a "further way", which increases the flow velocity. According to the law of fluid mechanics (Bernoulli equation), the increase in speed leads to a reduction in pressure. A "suction" arises on the surface of the wing. Since the top and bottom of the profile have a different curvature, a different "suction" is also generated. For a fully symmetrical profile (here a semi-symmetrical one is shown), the negative pressure on the upper side of the wing is exactly the same as on the lower side.
For a fully symmetrical profile (here a semi-symmetrical one is shown), the negative pressure on the upper side of the wing is exactly the same as on the lower side. These purely aerodynamic forces are not enough to make a plane fly. A wing needs to be turned slightly in the airflow, deflecting the air downwards, resulting in overpressure on the wing bottom, increasing overall lift.
This angle of attack also causes an increase in the negative pressure on the top, since the air has to travel an even further distance and thus accelerated more. Through the employment of the wing the air resistance is increased, which must be compensated with a larger power for propulsion. Basically, it can be said that the lift increases the faster the aircraft moves forward. At the same time the air resistance is increased. For this reason, aircraft that fly only slowly have thick profiles, with very fast aircraft, slim profiles are sufficient for generating the lift.
However, the angle of attack and the speed can not be increased arbitrarily because the air flow on the top can tear off. That is, the flow no longer flows along the profile, but forms vortex. First, the vortices arise at the trailing edge. If the angle of attack is further increased, more and more swirls form towards the leading edge until the lift is no longer sufficient to keep the aircraft in the air. This flight condition is referred to as “stall” and occurs especially when the aircraft flies too slow.
As soon as the current flows cleanly along the profile, the necessary buoyancy is restored and the plane flies again.
differences helicopter / plane
The helicopters differ fundamentally from the surface aircraft. Although helicopters also act with aerodynamic forces, these are much more difficult to calculate and explain than with an airplane. This is mainly because the rotating rotor creates additional forces that are not present in a surface aircraft.
For a plane with wings, the situation is fairly clear. The propulsion is delivered either by a propeller or a jet engine (except for a glider). The buoyancy is generated by the wings and the whole thing is controlled by flaps, rudders and tail units.
The situation is different with the helicopter. The rotating rotor blades produce, similar to a wing, the buoyancy and accelerate the air from top to bottom. This is done by simultaneously increasing the setting angle (angle between the rotor blade chord and Heli longitudinal axis) and thereby also the angle of attack on all rotor blades. This is called collective pitch. As a result, the air is "blown down", similar to a fan, the overall lift is increased and the helicopter begins to rise. In order for this vehicle to move forward, "only" the rotor plane must be tilted forward so that the airflow through the rotor is "blown" slightly backwards. The helicopter is controlled according to the same principle. The rotor plane is tilted in the direction the helicopter should fly. This sounds very simple, but in reality, is a very complex aerodynamic process (we'll talk about that later).
Commonly, the law of physics (by Newton) states that an action causes a reaction. This causes the fuselage of the helicopter to rotate counter to the direction of rotation of the rotor. To prevent this, most helicopters are equipped with a vertically rotating rotor, the tail rotor, which compensates for this torque. With this tail rotor, the helicopter can be controlled in hover around the vertical axis. In constructions with two counter-rotating main rotors no torque is generated on the fuselage, resp. the torques of the two rotors cancel each other out.
Opposite the surface aircraft, the helicopters can remain in the air. This is possible because the main rotor blades are always flowing through the air because of the rotation and thus provide the necessary lift. A surface aircraft only generates the lift when a sufficiently high forward speed is reached. In hover, for the sake of simplicity, we will consider the rotor as a disk and not examine the conditions on the individual rotor blade. This is possible because the aerodynamic forces are fairly symmetrically distributed over the entire rotor disk. In order for a helicopter to hold itself in the air, the buoyancy must be exactly the same as its weight.
If the pitch is increased at the same time on all rotor blades with the collective blade adjustment, the air flow from top to bottom through the rotor disk increases, the buoyancy increases and the helicopter begins to rise in place.
If one reduces the angle of attack, the overall lift becomes smaller and the helicopter begins to sink in a similar manner.
Because of the rotation of the main rotor, a moment arises which causes the hull to rotate counter to the direction of rotation of the main rotor. This unwanted rotation is corrected by the vertical tail rotor. The larger the power of the main rotor, the greater the torque, and the tail rotor has to perform correspondingly more to correct the torque. Since the tail rotor produces a certain horizontal thrust, the helicopter tends to shift in the appropriate direction. The direction depends on the direction of rotation of the main rotor.
This lateral displacement must again be corrected with the main rotor. The air flow, also called Downwash, is easily directed against the direction of displacement, which keeps the helicopter in stationary hovering flight. The forces of the main and tail rotor do not work in many helicopters in the same horizontal plane. For this reason, it may be that the helicopter is hovering, not horizontally, but with a slight transverse position. Whether the transverse position is left or right depends primarily on the direction of rotation of the main rotor.
Hover usually requires more power than forward flight. Above all, air density plays an essential role for the performance. The denser the air, the less the propulsion has to do and the more weight the helicopter can carry. Since the air density decreases with increasing altitude, the weight of the helicopter must be reduced in order to keep it in hover. In principle, it can be said that the higher the outside temperature and the higher the altitude, the lower the performance of the helicopter.
Another influence on performance is the downwash. If the air flow can flow away unhindered, this condition is called hovering out of ground effect (HOGE).
If the helicopter is hovering near the ground, hovering is called hovering in the ground effect (HIGE). The downwash, which must be diverted to the side, creates a kind of air cushion. As a result, the helicopter requires less power for stationary hovering.
The higher the helicopter hovers above the ground, the smaller the impact of the ground effect. At a flying height of approx. 1.5x the rotor diameter, there is no soil effect left. The soil quality and, above all, the slope of the terrain have a major influence on the soil effect. The more the soil is inclined, the better the downwash can drain and the lower the soil effect.
Probably the biggest advantage of helicopters is that they can both float and fly forwards. The process from hover to forward flight is called a “transition” and is an extremely complicated process, aerodynamically as well as mechanically.
For the sake of simplicity, we will consider the rotor as a disk and not the aerodynamic conditions on the single rotor blade. As already mentioned, the air in the hover flight is accelerated from top to bottom by the rotor. In order for the helicopter to go into forward flight, the entire rotor disk must be tilted forward.
Due to the inclination forward, the air is no longer vertically downwards, but accelerated to the rear. This will cause the helicopter to move forward. But since the lift is no longer acting vertically upward, the performance must be slightly increased by the pilot in the starting phase in order to achieve the right balance between buoyancy and weight.
Due to the rotation of the rotor, different forward velocities occur on the rotor blades during forward flight. The rotor blade, which is seen moving in the forward direction is moved as a leading blade, the one which moves backwards as a returning blade.
The flow velocity depends on the forward speed, the rotor speed and the rotor diameter. Assuming that the helicopter moves forward at a speed of 200 km / h and has a blade tip speed of 750 km / h, the following conditions arise on the rotor:
The leading blade achieves effective blade tip speed of 950 km / h (750 + 200). This speed is already very close to the speed of sound. At the blade root, an influx of over 200 km / h is still reached.
The returning leaf is only flown at a speed of 550 km / h at the tip of the blade (750 - 200). The flow velocity decreases, the closer you are to the center of rotation. In the area of the blade root, the blade can even be flown from behind and therefore provides no more buoyancy in this area.
The buoyancy is known to depend on the flow velocity and the angle of attack (in addition to the type of profile). In order to achieve reasonably constant buoyancy ratios over the entire rotor disk, the angle of attack during the rotation of the blade must be constantly changed, since the flow velocity indeed changes constantly. This adjustment of the angle of attack is referred to as cyclic pitch adjustment. The limits in forward flight are, in today's helicopter, at about 400 km / h. Above this speed, large parts of the leading blade would be in the supersonic range and a large area of the returning blade would be stalled. There is no wing profile that could cover such a large speed range.
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