Aerodynamic Basics

Before we can get into helicopter aerodynamics, we should know some basic aerodynamic principles.

In order for aircraft that are "heavier than air" to take off from the ground, an upward force must act that is at least as great as the weight of the aircraft. This force is called lift and is generated by the wings.

drive

The wings have a specific shape in cross-section, the profile. There are a variety of different profile types, depending on the flight characteristics an aircraft should achieve.

If a wing now moves forward, the profile divides the airflow into a lower and an upper part (Fig. 1).

suction effect

Since the air is displaced by the curvature around the profile, it has to travel a "longer distance", which increases the flow speed. According to the law of fluid dynamics (Bernoulli equation), the increase in speed leads to a reduction in pressure. A "suction" is created on the surface of the wing (Fig. 2). Since the upper and lower sides of the profile have a different curvature, a different "suction" is also created.

air resistance

With a fully symmetrical profile (a semi-symmetrical one is shown here), 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 an aircraft fly. A wing must be slightly tilted in the airflow, which deflects the air downwards, resulting in excess pressure on the underside of the wing, which increases the overall lift (Fig. 3).

angle of attack / vortex

This angle of attack also causes an increase in the negative pressure on the upper side, as the air has to travel an even further distance and is therefore accelerated more.

However, the angle of the wing also increases air resistance, which must be compensated for with greater power for propulsion.

Basically, it can be said that the faster the aircraft moves forward, the greater the lift. At the same time, however, air resistance also increases. For this reason, aircraft that fly slowly have thick profiles, while very fast aircraft only need slim profiles to generate lift.
However, the angle of attack and the speed cannot be increased arbitrarily because the air flow on the upper side can break off. This means that the flow no longer flows along the profile, but forms vortices (Fig. 4).

First, the vortices form at the trailing edge. If the angle of attack is increased further, more and more vortices form towards the leading edge until the lift is no longer sufficient to keep the aircraft in the air. This flight condition is called a stall and occurs primarily when the aircraft is flying too slowly.

As soon as the airflow flows cleanly along the profile again, the necessary lift is present again and the aircraft flies again.

Airplane

Helicopters are fundamentally different from fixed-wing aircraft. Although aerodynamic forces are also at work in helicopters, they are much more difficult to calculate and explain than in airplanes. This is mainly because the rotating rotor creates additional forces that are not present in fixed-wing aircraft.

In an aircraft with wings, the situation is fairly clear. Propulsion is provided by either a propeller or a jet engine (except in a glider). Lift is generated by the wings and the whole thing is controlled by flaps, rudders and tail units (Fig. 5).

helicopter

The situation is different with a helicopter. The rotating rotor blades generate lift, similar to a wing, and accelerate the air from top to bottom. This happens by simultaneously increasing the pitch angle (angle between the rotor blade chord and the helicopter's longitudinal axis) of all the rotor blades, and thus also the angle of attack. This is known as collective blade adjustment. This "blows" the air downwards, similar to a fan, the overall lift is increased and the helicopter begins to climb. In order for this vehicle to move forward, "only" the rotor plane needs to be tilted forwards so that the air flow is "blown" slightly backwards through the rotor (Fig. 6).

helicopter control

The helicopter is controlled using the same principle. The rotor plane is tilted in the direction the helicopter is supposed to fly. This sounds very simple, but in reality it is a very complex aerodynamic process (we will talk about this later). In fact, a physical law (of Newton) states that an action causes a reaction. This causes the fuselage of the helicopter to want to rotate in the opposite direction to the rotation of the rotor. To prevent this, most helicopters have a vertically rotating rotor, the tail rotor, which compensates for this torque (Fig. 7). With this tail rotor, the helicopter can be controlled around the vertical axis in hovering flight.

In designs with two counter-rotating main rotors, no torque is generated on the fuselage, or rather the torques of the two rotors cancel each other out.

boost

In contrast to fixed-wing aircraft, helicopters can fly forwards, sideways and backwards, but can also remain stationary in the air. This is possible because the main rotor blades are always exposed to air flow due to their rotation, thus providing the necessary lift. A fixed-wing aircraft only generates lift when a sufficiently high forward speed is reached.

For the sake of simplicity, in hovering flight 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 distributed somewhat symmetrically across the entire rotor disk.

In order for a helicopter to stay in the air, its lift must be exactly equal to its weight (Fig. 8).

angle of attack

If the angle of attack of all rotor blades is increased simultaneously using the collective blade adjustment, the air flow from top to bottom through the rotor disk increases, the lift increases and the helicopter begins to climb on the spot (Fig. 9).

If the angle of attack is reduced, the total lift decreases and the helicopter begins to descend (Fig. 10).

rotation of the main rotor

The rotation of the main rotor creates a moment that causes the fuselage to rotate in the opposite direction to the rotation of the main rotor. This unwanted rotation is corrected by the vertical tail rotor (Fig. 11). The greater the power of the main rotor, the greater the torque, and the more the tail rotor has to do to correct the torque.

Since the tail rotor produces a certain horizontal thrust, the helicopter has a tendency to move in the corresponding direction. The direction depends on the direction of rotation of the main rotor.

Stationary hovering by lateral displacement

This lateral displacement must in turn be corrected with the main rotor. The air flow, also called downwash, is directed slightly against the direction of displacement, which keeps the helicopter in a stationary hover.

hovering outside ground effect

In many helicopters, the forces of the main and tail rotors do not act in the same horizontal plane. For this reason, the helicopter may not be horizontal when hovering, but may be slightly banked. Whether the bank is to the left or right depends primarily on the direction of rotation of the main rotor (Fig. 12).

Hovering generally requires more power than forward flight. Air density plays a key role in performance. The denser the air, the less power the engine needs and the more weight the helicopter can carry. As air density decreases with increasing altitude, the weight of the helicopter must be reduced in order to be able to keep it in hovering flight. In general, the higher the outside temperature and the higher the altitude, the lower the performance of the helicopter.

Downwash also has an influence on performance. If the air flow can flow away unhindered, this condition is called hover out of ground effect (OGE) (Fig. 13).

Air cushions - for stationary hovering

If the helicopter hovers close to the ground, this is called hovering in ground effect (IGE). The downwash, which must be diverted to the side, creates a kind of air cushion (Fig. 14). This means that the helicopter needs less power for stationary hovering.

The higher the helicopter hovers above the ground, the smaller the influence of the ground effect. At a hovering height of about 1.5x the rotor diameter, there is no longer any ground effect. The nature of the ground and especially the slope of the terrain also have a major influence on the ground effect. The more the ground is sloped, the better the downwash can flow away and the smaller the ground effect.

forward flight

The greatest advantage of helicopters is that they can both hover and fly forwards. The transition from hovering to forward flight is called the transition and is an extremely complicated process both aerodynamically and mechanically. For the sake of simplicity, we will consider the rotor as a disk and not the aerodynamic conditions on the individual rotor blade.

As already mentioned, the air is accelerated from top to bottom by the rotor during hovering (Fig. 15). In order for the helicopter to enter forward flight, the entire rotor disk must be tilted forward.

inclination

Due to the forward tilt, the air is no longer accelerated vertically downwards, but backwards (Fig. 16). This causes the helicopter to start moving forwards. However, since the lift no longer acts vertically upwards, the pilot must increase the power slightly during the takeoff phase in order to achieve the correct ratio between lift and weight.

flow velocities

The rotation of the rotor creates different airflow speeds on the rotor blades during forward flight. The rotor blade that moves forward in the direction of flight is called the leading blade, and the one that moves backward is called the retreating blade (Fig. 17).

speed of the advancing blade

The approach speed depends on the forward speed, the rotor speed and the rotor diameter. If we assume that the helicopter is moving forward at a speed of 200 km/h and has a blade tip speed of 750 km/h, the following conditions arise at the rotor:

The leading blade reaches an effective speed of 950 km/h (750 + 200) at the blade tip. This speed is already very close to the speed of sound. At the blade root, the flow is still over 200 km/h (Fig. 18).

speed of the returning blade

The receding blade is only approached by airflow at a speed of 550 km/h at the blade tip (750 - 200). The airflow speed decreases the closer you are to the center of rotation. In the area of ​​the blade root, the blade can even be approached from behind and therefore no longer provides lift in this area (Fig. 19).

As is well known, lift depends on the airflow velocity and the angle of attack (as well as the type of profile). In order to achieve a fairly constant lift ratio across the entire rotor disk, the angle of attack must be constantly changed as the blade rotates, since the airflow velocity also changes constantly. This adjustment of the angle of attack is referred to as cyclic blade adjustment.

The limits of forward flight for today's helicopters are around 400 km/h. Above this speed, large parts of the leading blade would be in the supersonic range and a large part of the trailing blade would be in stall. There is no wing profile that could cover such a large speed range.

In order to ensure the airflow to the rotor blade, the rotor must always be driven. But what happens if the drive fails - for whatever reason?

Since the rotor blades have a relatively large angle of attack during forward flight due to the collective blade adjustment and therefore also produce a correspondingly large air resistance, the speed of the rotor drops rapidly without propulsion. This of course also means that the necessary lift is lost and the helicopter crashes within a short time.

Fortunately, that sounds much more dramatic than it actually is. What gliding is for a fixed-wing aircraft, autorotation is for a helicopter. If the helicopter's engine fails during flight, the pilot will immediately reduce the collective blade pitch and the helicopter will begin to descend. At the same time, the air resistance on the rotor blades is significantly reduced due to the smaller angle of attack.

aerodynamic conditions

As we can see in Fig. 20, the air no longer flows through the rotor 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 autorotation, we cannot 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 state in powered forward flight (Fig. 21).

vertical descent

In a flight profile, the lift always acts perpendicular to the incoming flow and the air resistance in the same plane as the incoming flow. Since the incoming flow in a helicopter consists of a horizontal component (rotation of the rotor) and a vertical component (air flow from above or below), we speak of a relative incoming flow in the case of the rotor blade. Since the rotor blade moves at a higher speed towards the outside, but the vertical component remains more or less constant, the relative incoming flow changes constantly over the entire length of the rotor blade. For this reason, Fig. 21 only applies to a small area of ​​the rotor blade. The angle of attack (angle between the profile chord and the relative incoming flow) also changes over the length of the rotor blade, and the angle of attack decreases towards the outside.

In the autoration, the rotor can be divided into three areas. For the sake of simplicity, we first look at the vertical autoration, which means that the helicopter is in vertical descent (Fig. 22).

Vertical autorotation

In vertical autorotation, the areas are distributed symmetrically across the rotor disk. Only the driving area is responsible for the rotation of the rotor. Near the center, the flow velocity is so low that the rotor blades are in a stall.

To explain how these areas come about, we need to examine the aerodynamic conditions on the individual rotor blade in more detail.

In the driving area, the total aerodynamic forces are in front of the rotation axis of the rotor. This results in a force that drives the rotor (Fig. 23).

If the aerodynamic forces lie exactly on the axis of rotation, the rotation speed remains constant (Fig. 24).

In the braking part, the total forces are behind the axis of rotation, which means that the rotor is braked (Fig. 25).

balance

All helicopters are designed to achieve a balance between the propulsion and braking parts. This balance must be maintained from vertical descent up to a certain forward speed. Some helicopters are limited in their maximum forward speed during autorotation. This is because the propulsion part shifts as the speed increases (Fig. 26). This shift always occurs in the direction of the returning blade.

If the forward speed is increased again in this situation, the driving area shifts further to the right, which ultimately leads to the braking part becoming larger than the driving part and thus the rotor speed can no longer be kept constant.

As a rule, 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 raises the nose, which reduces the sink rate and reduces the speed. This braking allows the rotor to absorb even more energy (the speed is increased) and the helicopter can perform an almost normal landing. This sounds very simple, but it is a demanding maneuver for the pilot.