Characteristic features of a coaxial helicopter aerodynamics

Characteristic features of a coaxial helicopter aerodynamics

Three major helicopter schemes are used in the modern global helicopter industry: single-rotor, coaxial and longitudinal, with that the vast majority of rotorcraft are built as single-rotors. The pioneers of helicopter engineering were well aware of the fundamental advantages of a coaxial aircraft design. However, foreign designers managed to bring to mass production and widespread use only single-rotor helicopters with a tail rotor. This scheme is now referred to as a classical. In Russia, in addition to single-rotor helicopters, coaxial helicopters are also widely used.

The applications of coaxial helicopters were determined by their characteristic features – small size, high thrust-to-weight ratio and maneuverability, aerodynamic symmetry. These features provided them with convenient basing on small-sized runways of various special purposes ships. In the conditions of takeoff and landing on a swinging deck and flight over the sea, the unique qualities of coaxial helicopters were clearly manifested. Ka-25 and Ka-27 helicopters are used in the Navy. In civil aviation, Ka-26 and Ka-32 are operated. These helicopters are also appreciated abroad for the high efficiency of their work. In light of this, it is especially important to conduct an objective comparative analysis of the features of coaxial and classic single-rotor helicopters.

Coaxial Torque Compensation

The features of coaxial helicopters are associated with the implementation of a fundamentally new method of compensating the reactive torque of rotors in comparison with single-rotor helicopters. The reactive moments of the coaxial helicopter propellers are mutually balanced directly on the axis of their rotation. In a single-rotor helicopter, in order to compensate for the reactive torque of the rotor, it is necessary to create lateral force of the tail rotor applied to the fuselage. Coaxial helicopter designers, in essence, created a new type of carrier system without reactive moment. Propeller torques are automatically compensated throughout the flight without any intervention by the pilot. Due to this, a change in power on the rotors of the coaxial helicopter does not lead to unbalancing of the helicopter in terms of travel. In a steady flight, the top and bottom rotors of the coaxial helicopter have zero total torque. When moving the pedals, there is a difference in reactive moments, due to which the helicopter is controlled on the course.

The method of reactive torque compensation used on a single-rotor helicopter requires the pilot to maintain unceasing attention in flight and to regulate the tail rotor thrust in order to balance the helicopter.

Fig. 1. Aerodynamic excellence of coaxial and single-rotor helicopters on the hover

Energy opportunities

From an energy point of view, the optimal solutions for an aircraft are those in which the power plant’s capacity is primarily provided for useful needs. For a helicopter, this is the creation of the necessary lifting and propulsive forces at a given flight mode.

On a single-rotor helicopter, part of the power is spent on the tail rotor drive, which creates the traction force required to compensate the rotor torque. These costs amount to 10-12% of the power supplied to the rotor shaft and are net losses.

On a coaxial helicopter, all the free power of the propulsion system is used to drive rotors, that is, to generate lifting force. In this case, the reactive moments are mutually balanced. Therefore, there is no direct power cost to compensate for reactive moments. In addition, in hovering mode, the coaxial rotors produce a positive effect on each other, which also leads to power savings. This circumstance is illustrated in Fig. 1, which shows a diagram of an air stream coming from the upper and lower rotors of a helicopter in hovering mode. Since the airblast from the upper rotor narrows in the plane of the lower rotor by 15-20%, the lower rotor has the ability to carry out additional air intake. This broadly increases the cross section of the airblast and reduces the cost of power to create lift. In addition, due to the opposite direction of the rotors rotation on the coaxial support system, the energy costs for swirling the airblast are significantly reduced, which also leads to a reduction in unproductive power losses.

The results of flight tests and other experimental materials indicate that the efficiency of coaxial rotors is on average 1.06-1.1 times (6-10%) higher than this of single rotors, as can be seen in Fig. 1. Considering the saving of power used to compensate for reactive torque (10-12%), we find that in general, the efficiency of coaxial helicopters is 16-22% higher than the efficiency of single rotors. The listed energy features provide the coaxial design with significant advantages in the hovering ceiling and vertical climb rate.

At first glance, it seems that due to the presence of a double-rotor column, coaxial helicopters should have greater drag than single-rotor aircraft. However, this advantage of single-rotor helicopters in the required power was not proved by test flights which can be explained by the following factors:

– favorable mutual influence of coaxial rotors in translational motion;

– additional power expenditure for single tail rotor drive;

– additional resistance of the single tail rotor helicopter, especially taking into account the interference of the tail rotor and tail boom of a helicopter;

– additional harmful drag of a single-rotor helicopter fuselage in flight with gliding, since it is preferable to pilot a helicopter without a roll;

Dimensional and mass characteristics

The coaxial design reduces the dimensions and weight of the helicopter, which gives it a number of advantages.

For a comparative assessment of the overall dimensions and mass characteristics of coaxial and single-rotor helicopters with tail rotor, it is advisable to consider two cases. The first case with coaxial and single-rotor helicopters having the same flight mass and the same available propulsion system power, and the second withcoaxial and single-rotor helicopters having the same diameters of rotors.

In the first case, the use of a coaxial load-bearing system makes it possible to reduce the overall dimensions of the helicopter by 35-40% compared to a single-rotor one. In the second case, lower aerodynamic quality and additional power loss on the tail rotor drive of a single-rotor helicopter derive a lower value of the flight mass. Due to the presence of a tail rotor, the overall dimensions of a single-rotor helicopter are 20% larger than the coaxial one.

The compactness of the coaxial helicopter airframe and the concentration of heavy aggregates near the center of mass lead to a noticeable decrease in the moments of inertia with respect to the vertical and transverse axes (Fig. 2). This plays an important role in ensuring high handling and maneuverability characteristics.

Figure 2. Moments of inertia of coaxial and single-rotor helicopters

Stability and controllability

The most important feature of a coaxial helicopter, which significantly improves the stability and controllability characteristics, is its aerodynamic symmetry. In the process of development and formation of the aircraft industry, designers have repeatedly turned to aerodynamically symmetric schemes. Aerodynamic symmetry of the aircraft provides a number of important aerobatic properties, and most importantly, ease of control. A very obvious example is the development of aircraft construction: they designed and built solely symmetrical aircrafts.

A single-rotor helicopter is an aerodynamically asymmetric aircraft with a number of unique characteristic features. In helicopter engineering, constructors reconciled to this asymmetry as an inevitable payment for the simplicity of a technical solution. However, the history of helicopter industry development has shown that this simplicity is only an illusion. The creation of a workable tail rotor and transmission having acceptable resources and characteristics is an extremely complex problem, which still remains unsolved.

The aerodynamic symmetry of the coaxial helicopter scheme is ensured by the absence of reactive moment on its body, the relative proximity of the upper and lower rotors and their favorable influence on each other, which leads to a small difference in their thrusts in a balanced position. The lateral forces of the rotors oriented in opposite directions balance each other, and the transverse moment arising due to a low spacing between the rotors is small. Due to the absence of a tail rotor, a coaxial helicopter is exempt from permanent variable lateral force. The design of coaxial helicopters provides a harmonious combination of control efficiency and aerodynamic damping, which provides reliable controllability characteristics.

Due to aerodynamic symmetry on a coaxial helicopter, there are practically no connections between longitudinal and lateral movement, which ensure independence of control channels and ease of piloting. Operation of such a helicopter is available to average skilled pilots.

Aerodynamic symmetry significantly changes the “face” of the helicopter. The absence of a variable (in flight modes) yaw moment and lateral force acting on the fuselage improves stability and controllability characteristics, increases flight safety and facilitates problems solving in extreme conditions. On a coaxial helicopter, there are no links between changes in engine power (total pitch of the propellers) and directional and lateral control. On a single-rotor helicopter, all maneuvers during which the operating mode of the engines changes (acceleration and braking, changes in flight altitude, turns, etc.) are accompanied by direction rebalancing and the need to counter the arising lateral forces with roll and slip. Due to the lack of symmetry and to the constant change in the relationship between movement in the vertical and horizontal planes, piloting a single-rotor helicopter becomes complicated, which requires more thorough training.

Coaxial helicopters in control simplicity are comparable to initial training aircrafts. At the same time, in terms of flight characteristics of stability, controllability and maneuverability, they leave single-rotor helicopters behind.

“Flying in a coaxial helicopter is not difficult and, in essence, is performed as reflexively as walking, freeing up all human resources for solving a tactical task,” wrote Hero of the Soviet Union Honored Test Pilot N.P. Bezdetnov. It is hard to say anything better about the controllability of a helicopter coaxial.

Maneuverability

Due to its design features, the coaxial helicopter has unlimited possibilities for implementing a “flat” turn, outmatching the capabilities of single-rotor helicopters. Helicopter construction envisages concentration of all the most important functions on the coaxial rotor: the creation of lifting and propulsive (driving) forces, longitudinal, transverse and directional controls and common pitch control.

The directional control of a helicopter with a coaxial rotor is carried out using the difference in torque on the upper and lower rotors. This leads to the fact that the control system as a whole becomes almost independent of the slip angle. It is this circumstance, as well as the absence of a tail rotor that allows the coaxial helicopter to perform a “flat” turn with large slip angles.

For a single-rotor helicopter, a “flat” turn is fundamentally impossible. On a single-rotor helicopter, permissible slip angles are significantly limited by the presence of a tail rotor. The fact is that a change in the angle of slip leads to a change in the angle of attack of the tail rotor, the conditions of its operation and the swinging motion of its blades, especially at high flight speeds. An increase in the amplitude of the flywheel movement of the tail rotor blades beyond the permissible level is a direct threat to flight safety. This is due to the fact that there are no swashplates on the tail rotors, and the prevention of an excessive increase in the amplitude of the flywheel movement is provided only by the swing regulator, quite limited in its capabilities. Therefore, with an increase in the amplitude of the flywheel movement beyond the permissible, the tail rotor blades might strike against the beam. In addition, with an increase in the amplitude of the flywheel movement, the loads in the construction elements of the tail rotor increase, which also imposes restrictions on the slip angles.

Coaxial helicopters allow significant slip angles. As we have already said, this is due to the lack of a tail rotor and the independence of the directional control system from the slip angle. The tail unit of the coaxial helicopter does not impose any restrictions on the magnitude of the slip angle, since it is designed to change the slip angle in the range up to 180 °.

On a single-rotor helicopter, the effectiveness of directional control is excessive. This is due to the need to provide directional balancing in the entire range of power plant capacity changes.  However, this efficiency cannot be fully realized in flight. The limitation of the angular rotational speeds on these helicopters is caused by the need to prevent the tail rotor from entering the vortex ring mode, as well as by the strength conditions of the tail boom, tail rotor and transmission.

On coaxial helicopters, the directional control is harmonious and balanced, while ensuring the optimal degree of directional control efficiency. The rudder increases the efficiency of directional control in proportion to the increasing aerodynamic moments of the airframe with increasing flight speed. Piloting a coaxial helicopter, pilots quickly get used to the new conditions and find out that it is possible to perform maneuvers that are not available to a single-rotor helicopter.

The absence of a tail rotor on a coaxial helicopter gives the pilot the ability to control the course by deviating the pedals to the stop at the maximum possible pace, which ensures the shortest turn time at a given angle. A coaxial helicopter has a great advantage over a single-rotor one in terms of rate of rise and maximum angular rotation speed, as well as a large reserve of directional control on hovering, including on a static ceiling, regardless of the barometric height. This advantage translates into significant tactical superiority.

On a single-rotor helicopter, with an increase in flight altitude or with an increase in the temperature of the outside air, due to a decrease in the excess capacity of the power plant and an increase in the pitch of the tail rotor, the available directional travel and, consequently, the turning efficiency are significantly reduced: on the hovering ceiling, where all available power is used, single-rotor helicopter can not make turns without loss of height.

Overload maneuvers

The efficiency and power of the longitudinal control of a coaxial helicopter is significantly higher than that of a single-rotor helicopter. This high efficiency is ensured by lower moments of inertia (see Fig. 2) and large available control moments, explained by the large value of the arm of force applied to the bushings of the upper and lower rotors relative to the center of mass of the apparatus. On a single-rotor helicopter, due to the fact that the process of launching the overload is delayed, there is a noticeable drop in speed and, therefore, a lower level of maximum overload is achieved. Thus, a coaxial helicopter, having greater efficiency and power of the longitudinal control, has significantly larger available overloads.

Pulling a coaxial helicopter down into a dive is more efficient and safer than a single-rotor. Pulling a helicopter down, you need to give the handle away from you, while the vertical overload gets significantly reduced, the trajectory curves accordingly and the angular velocity of the fuselage dive increases. In the process of quenching this angular velocity, the pilot pulls the handle on to go into a steady dive.

In this case, the flywheel motion of the blades develops faster than the angular velocity of the fuselage changes. If the change in the angular velocity of the fuselage is insufficient due to the low efficiency of the longitudinal control (as, for example, for a single-rotor helicopter), then due to the oncoming relative motion of the tail boom and the blades, their dangerous approach and even collision are possible. On coaxial helicopters, such phenomena are impossible. Thus, performing maneuvers with reduced vertical overload on a coaxial helicopter is more efficient and safe.

When maneuvering in the horizontal plane, a greater excess of power due to the lack of a tail rotor and a higher aerodynamic quality of the coaxial rotors compared to a single rotor allow the coaxial aircraft to accelerate from hovering mode with maximum acceleration and build up to a predetermined speed much faster.

Maximum permissible side and back speeds also characterize maneuverability. The speed of movement of a coaxial helicopter in any direction from the hovering mode is limited only by the maximum available moves in the propeller control system. On a single-rotor helicopter, the presence of a tail rotor imposes a significant limitation on the speed of movement sideways from the hovering mode due to the possibility of the tail rotor getting into the swirl ring mode.

Coaxial helicopters have advantages when performing all spatial maneuvers, especially when performing maneuvers such as turning on a “hill”, when it is necessary to develop high angular speeds and use deep glides.

In addition to the above maneuvers, coaxial helicopters successfully perform aerobatics such as a skewed loop, spin, ascending roll, etc. When performed on coaxial machines, pitch angles reach 90°, rolls get to 130-140°.

Windmilling

Analysis of statistical materials obtained on the basis of flight tests shows that, with the same load per square meter of area swept by the main rotor, the minimum vertical speed of decrease in autorotation mode for coaxial helicopters is slightly lower than for single-rotor helicopters. This is due to the presence of a biplane effect on the coaxial carrier system, which reduces inductive power losses. In addition, in autorotation mode, despite low thrust, a single-rotor tail rotor consumes a certain amount of power, which also leads to an increase in the vertical speed of descent in single-rotor helicopters.

– there is no significant unbalance of the machine in space when switching from motor flight to autorotation mode due to the aerodynamic symmetry of a coaxial helicopter and to the fact that such helicopters do not have ‘collective pitch – pedals’ type cross connections in their control channels;

– landing speeds of coaxial helicopters in autorotation mode are approximately 15 km/h lower than that of single-rotors. This is explained by lower (by 20-30 m) energetic alignment of machines with large (up to 10°) pitch angles, which is ensured by higher longitudinal control power and smaller glider dimensions. Lower landing speeds increase landing safety, especially on rough terrain.

Directional controllability of coaxial helicopters in autorotation mode is ensured by a developed vertical tail unit and the difference in torque on the rotors. The Flight Operations Manual contains recommendations for reducing the rotor speed by 3-4% for gliding in autorotation mode and landing at low pitch speeds. While maintaining the gliding speed, this leads to a 2-3 m/s decrease  in the vertical speed of descent. The difference in the antitorque moments that occurs simultaneously on the rotors leads to an increase in the efficiency of directional control and an improvement in landing characteristics.

Flight Safety

Human factor is crucial in ensuring flight safety. Coaxial helicopters are safer than single-rotor helicopters, as they are simpler to operate, have better handling and maneuverability, and high aerodynamic quality.

A coaxial helicopter with smaller dimensions compared to a single-rotor helicopter of the same class is safer when maneuvering near obstacles and at low altitudes. Due to the fact that the dimensions of a coaxial machine are determined by the diameters of the rotors, during the flight near obstacles damage to the tail unit of the coaxial helicopter is practically impossible. However, even damage or loss of the tail unit, for example, during a heavy landing, does not significantly affect the safe completion of the flight, since the directional control is provided by coaxial rotors. On a single-rotor helicopter, in case of damage and loss of the tail rotor the situation is close to catastrophic.

When comparing the flight safety of coaxial and single-rotor helicopters, opponents often pay attention to the danger of the blades’ collision on coaxial helicopters.

It should be noted that the problem of rapprochement of the blades with the structural elements is equally relevant for coaxial and single-rotor helicopters and the methods to solve it are well known. It should be noted that on single-rotor aircraft, cases of impact of the rotor blades with the tail boom, the cockpit, as well as with the end beam have also been recorded.

The rotors of coaxial helicopters are designed to ensure the required flight safety. In addition, during the design process of the helicopter, structural reserves are provided between the lower propeller blades and the helicopter design elements. The distance between the upper and lower propeller blades and between the lower propeller blades and the helicopter structural elements is meticulously and accurately measured during flight tests over the entire range of operational flight modes, including all the maneuvers. These measurements are carried out using special equipment. On the basis of measurements and generalization of the test results of coaxial helicopters at all operating conditions, including when performing aerobatics, constructive measures have been developed to prevent the dangerous approach of the upper and lower propeller blades, as well as the lower propeller blades with glider design elements.