AERODYNAMIC FEATURES OF COAXIAL CONFIGURATION HELICOPTER
Nowadays, the world helicopter industry employs two configurations - single-rotor and coaxial-rotor helicopters with most helicopters featuring the single-rotor configuration.
The pioneers of helicopter-building were aware full well of the fundamental advantages presented by the coaxial configuration. Plenty of projects and attempts of building coaxial helicopters at various times are known. However, it was only single-rotor helicopter featuring the tail rotor that western designers managed to put in extensive operation. Single-rotor helicopters were developed and widely used in the Soviet Union and Russia too. In developing the national helicopter industry, considerable funds and efforts were employed by this country into the further development of the single-rotor configuration, which, however, failed to deal with certain fundamental flaws inherent to this configuration.
In 1947, Soviet designer N.I. Kamov commenced his work on actual development of coaxial-rotor helicopters. Over the 50 years, the 'Kamov company has developed and put into series production many a coaxial helicopter: Ka-10, Ka-15, Ka-18, Ka-25, Ka-26, Ka-27, Ka-29 as well as world-renowned Ka-32 and Ka-50 Black Shark.
Due to their small dimension, high thrust-to-weight ratio, superb manoeuvrability and aerodynamic symmetry, coaxial helicopters were widely used as shipborne rotary-wing aircraft operated by the Soviet Navy. The civil aviation began extensive operation of coaxial Ka-26s and Ka-32s.
By the late '70s - early '80s, all necessary prerequisites had appeared for the development of a combat coaxial-rotor helicopter later designated Ka-50. The stiff and above-the-board competition between the Ka-50 and Mil Mi-28, with the latter featuring the classic single-rotor configuration, resulted in the Ka-50's impressive victory and was fielded with the Russian Army.
The coaxial configuration helicopter is so special due to the fact that it embodies a principle of the reactive moment compensation fundamentally different from that of the single-rotor configuration. To compensate for the reactive moment of the single-rotor helicopter's main rotor, there should be developed the tail rotor's side force applied to the airframe, while the coaxial-rotor helicopter has its rotors' reactive moments compensating each other directly in their axis of rotation. This removes the need for any additional forces. Rotors' reactive moments are compensated automatically throughout the flight, thus requiring no interference on the part of the pilot.
Apeculiarity of the coaxial rotor featuring zero reactive moment in the balanced flight is the fact that the pilot's operating the pedals creates disparity between the upper and lower engines' reactive moments with the resulting summary reactive moment being used as the direction control capability.
The reactive moment compensation method employed in the single-rotor helicopter requires the pilot's constant attention to adjusting the tail rotor's side force to maintain the helicopter's balance throughout the flight, thus putting the helicopter to certain disadvantage.
As far as power is concerned, the coaxial helicopter has a considerable edge over its single-rotor counterpart, since all free power is transferred to the rotor drive, i.e. used for developing the lift, while the single-rotor helicopter's tail rotor power consumption accounts for 10-12% of total power.
Another important feature of the coaxial configuration is revealed when the helicopter is hovering. The upper rotor race grows considerably more narrow in the lower rotor plane, which allows the lower rotor to suck in additional air thus increasing the rotor race cross-section and reduces the power used for developing the lift.
The contra-rotation of coaxial rotors leads to significant reduction in power, which is required for swirling the jet. Flight-testing as well as other experimental data shows the coaxial rotors to be 6-10% more efficient as compared to the single-rotor helicopter.
Coaxial rotors and single rotor aerodynamic quality in the hover
Given the coaxial-rotor helicopter does not have to use power for compensating the reactive moment, coaxial helicopters appear to be 16-22% more effective than single-rotor aircraft. The above power considerations provide the coaxial configuration with substantial advantage in the hover ceiling (by 500-1,000 m) and vertical rate of climb (by 4-5 m/sec).
Despite the fact that the twin-rotor system mast creates greater drag for the coaxial helicopter as compared to its single-rotor opposite number, the flight testing of coaxial-rotor and single-rotor helicopters of the same type displayed no obvious increase in drag, which is owing to the following reasons:
beneficial mutual effect of coaxial rotors in forward flight; it appears as the biplane cell effect and ensures substantial reduction in power that is developed by the engine for creating the lift;
lack of the tail rotor and need for powering it;
lack of the tail rotor drag and negative interference of the tail rotor and the tail boom;
measures taken in designing the coaxial-rotor helicopter (Ka-50) with lesser drag (i.e. by retracting the landing gear in flight).
The coaxial configuration allows the helicopter, while being smaller and lighter than the single-rotor one, to feature important tactical advantage.
To assess the changes in the dimension and weight of single-rotor and coaxial-rotor helicopters, it makes sense to compare the following cases:
coaxial-rotor and single-rotor helicopters have the same airborne weight and available power developed by their engines (Ka-50 and Mi-28);
coaxial-rotor and single-rotor helicopters have the rotor blades of the same diameter (Ka-50 and AH-64).
Coaxial-rotor and single-rotor helicopters movement of inertia.
The coaxial-rotor helicopter's reduction in size and different weight distribution along the airframe results in considerable reduction in longitudinal and directional moments of inertia. This is fundamental for providing the required controllability of the helicopter.
Aerodynamic symmetry is the most important feature of the coaxial helicopter. It enhances its controllability and stability substantially.
With the progress in helicopter-making, designers have repeatedly turned to symmetric aerodynamic configurations, understanding full well the importance of aerodynamic symmetry for achieving the ease of controlling the helicopter.
The fixed-wing aircraft manufactory experience is exemplary in that respect. Only symmetric planes are built. It is hard to imagine a plane with two engines set at various points of their respective wings and developing different thrust whose disparity would change depending on the flight mode.
However, helicopter developers put up with asymmetric single-rotor configuration taking it as an avoidable evil - retribution for the perceived simplicity of that technological solution. At the same time, in practice, developing an efficient tail rotor and its transmission proved to be a tall order.
Aerodynamic symmetry of the coaxial configuration is provided by the lack of reactive moment on the airframe, relatively close upper and lower rotors and their beneficial mutual effect, which results in little difference in their thrusts when balanced. Rotors' side forces directed in different directions balance each other with their lateral moment, which emerges due to their separation, being insignificant.
Thanks to the lack of the tail rotor, the coaxial-rotor helicopter is not subject to the constant effect of the alternate side force. The coaxial design ensures a smooth combination of efficient control and aerodynamic damping, which provides good controllability.
Helicopter controllability levels (hover and frequency flight/banking)
For example, the Ka-50's lateral controllability characteristics have been evaluated under the ADS-33C standard (Manual Control Requirements to Military Helicopters) of the US Army Aviation Department. The picture shows the results of the evaluation for hovering and low-speed flying. It is obvious the Ka-50 controllability characteristics match Level 1 (excellent controllability) of the ADS-33C standard with the Ka-50 having significant edge in lag value and frequency as compared with the other helicopters.
Owing to aerodynamic symmetry, the coaxial-rotor helicopter has literally no relation between the longitudinal and lateral movement. However, it has independent control, ease of flying it and is easy to master by any pilot irrespective of his flying skills. Aerodynamic symmetry changes the helicopter dramatically. The lack of flight mode variables, yaw moment and side force on the airframe, as well as the lack of a relation between the change in power (collective pitch) and directional and lateral control, improves the coaxial helicopter's stability and controllability. Due to this, flight safety enhances and flying in extreme condition gets easier, which is especially true as far as low-altitude flying, small landing pads, broken terrain, high barometric altitude and systems' failure are concerned. Controlling the coaxial-rotor helicopters is as simple as flying initial training aircraft. At the same time, as far as their stability, controllability and manoeuvrability are concerned, they could give their single-rotor rivals a run for their money.
Fast-changing environment of modern combat and the need in gaining tactical advantage put high on the agenda the necessity in expanding speeds and modes suitable for the 'flat' manoeuvre, i.e. the manoeuvre intended to change the direction of flight without the use of regular g-load.
Unrestricted efficiency of the coaxial helicopter in performing the flat manoeuvre is rooted in its design. The coaxial configuration concentrates all important functions in the coaxial rotor: development of the lift, propulsive force, longitudinal and directional control as well as collective pitch control.
Concentration of the whole control system in the coaxial rotor and the availability of the coaxial rotor directional control capability through the disparity in moments provides coaxial-rotor helicopters with another important feature - the control system becomes nearly independent of the angle of slide. It is this and the lack of the tail rotor that provides limitless opportunities for performing flat manoeuvres at high angles of slide.
The coaxial-rotor helicopter's empennage places no restrictions on the value of the angle of slide since it is expected to deal with the changing of the angle of slide within 180 degrees.
Aradically new manoeuvre - 'flat' turn - has been tested through the use of the Ka-50 and accepted for use. At speed of up to 90-100 km/h this manoeuvre could be performed at 180 degrees both left and right in the horizontal plane while at a speed of up to 230 km/h it is performed within 90 degrees in both directions with the banking being close to zero.
The flat turn is a purely combat manoeuvre ensuring the directing of the static weapon towards the target in the shortest time possible. This makes the bulky ring mount unnecessary while gaining valuable time in turning at high angles to boot. The lack of the tail rotor enables the coaxial helicopter to use all the advantages of its directional control and develop high yaw-ing rates with no restrictions while manoeuvring. Though single-rotor helicopters boast greater available directional control moment, that moment cannot be employed in full, which is especially true for the sharp step control input. This is due to the restrictions on the yaw rate caused by tail rotor and transmission strength considerations, insufficient strength of the tail boom and the considerations given to maintaining controllability should the tail rotor get into the vortex ring. On the assumption of the above, the lack of the tail rotor allows the helicopter to be controlled in the horizontal plane by hitting the pedals fast, which results in faster turning at the required angle. Due to the invariability of the directional control margin coupled with variations in the hovering altitude up to the hover ceiling, this capability turns up to be a significant tactical advantage vital to winning the duel.
Yaw angular speed in the hover
Employing the flat manoeuvre by coaxial-rotor helicopters ensures taking off and landing irrespective of the force and direction of the wind.
When landing on small pads or when obstacles are present, this method of taking off and landing grants most important operational and tactical advantages.
Let us examine certain peculiarities of coaxial-rotor and single-rotor helicopters' manoeuvring in the horizontal plane. Such manoeuvring witnesses a sharp change in flight speed, which influences the helicopter manoeuvrability.
Achieving the required vertical g-load is performed mainly by increasing the pitch angle and the rotor's angle of attack with the g-load rate depending on the pitch angle value and rate, i.e. depending on the longitudinal control system's capabilities - its efficiency and power. The more effective the longitudinal control, the faster the pitch angle and g-load change with the g-load growth rate has no time to diminish, which makes the manoeuvre more efficient. Should the manoeuvre happen not to be efficient enough, the speed drops faster than the g-load grows, which could result in problems with achieving required g-loading.
Coaxial-rotor helicopters feature far better effectiveness and power of longitudinal control than single-rotor ones do.
This is provided by lesser moments of inertia and greater available control moments due to great values of the arms of force applied to the hubs of the upper and lower rotors due to their separation. The above is confirmed by statistic dependencies of maximal available acceleration and longitudinal acceleration of coaxial-rotor and single-rotor helicopters.
Boasting greater control efficiency and power, the coaxial-rotor helicopter enters the dive with better efficiency and greater safety. The point is when entering the dive, the controls are pushed forward with the resulting drop in vertical g-load, curving of the trajectory and increase in the airframe's angular speed in diving. When negating this angular speed by pulling the controls to go into the steady dive, the rotor blades' flapping motion increases faster than the air-frame angular speed changes. If this is accompanied by insufficient change of the angular speed due to inefficiency of the longitudinal control (like that of single-rotor helicopters), the collision of the tail boom and rotor blades is possible as a result of their conflicting movement. Thus, the efficiency and power of the coaxial helicopter's longitudinal control ensures more efficient and safe manoeuvring accompanied by a decrease in vertical g-loading.
Coaxial helicopters have a substantial advantage in low-speed horizontal manoeuvring, which enhances both their combat efficiency and survivability.
These advantages are produced by the redundant power thanks to the lack of the tail rotor and better fineness of the coaxial rotor as compared to the single rotor at a low speed. Hence, the coaxial-rotor helicopter has greater hover stripping acceleration in comparison with its single-rotor counterpart and, hence, lesser acceleration time to achieve the required speed.
The presence of the tail rotor places stringent limitations on th
e hover stripping speed due to the threat of the tail rotor getting into the vortex ring. Coaxial-rotor helicopter aerodynamic characteristics provide moving from the hover in any direction at a speed of up to the maximum capacity of the control system. Low-speed manoeuvring is much safer when you fly a coaxial-rotor helicopter.
When the helicopter, accelerated rearwards, accidentally accelerates to the speed at which the control stick hits the limit, the only thing the pilot has to do is to deflect the pedal and turn the coaxial helicopter at 180 degrees at a great angular speed.
While examining horizontal manoeuvres, one should note two main manoeuvres - the correct (coordinated) turn that is performed nearly identically by both single-rotor and coaxial-rotor helicopters, and a radically new manoeuvre called 'funnel'. The funnel's tactical purpose is that the helicopter, while performing the funnel, can keep ground targets in the crosshairs and engage them for a long time despite the negative pitch angle. Otherwise, maintaining the negative pitches angle leads to acceleration, loss of the target and need for multiple passes, which is accompanied by breaks in the fire. This degrades the hit probability and increases the helicopter's vulnerability. The funnel manoeuvre giving a vital advantage in combat can be performed only by coaxial-rotor helicopters.
The funnel is performed at a speed of 100-180 km/h at a negative pitch angle of up to 30-35 degrees and is, in fact, a side turn, in performing which the pitch and banking angles trade places. When the funnel is performed, the rotor thrust's constituent parallel to the horizontal plane is directed to the centre of the notional cone and balanced by forces of inertia, which emerge while the helicopter rotates along the near-circular trajectory at a slip angle of 90%. Thus, the funnels based on the ability of the coaxial rotor to perform deep sideslipping and lateral movement at a high speed.
The accelerated turn is a combat manoeuvre too and is employed to quickly alter the direction of flight. It could be effective in attacking ground targets and in aerial combat at the head-on course. The peculiarity of accelerated turns made by coaxial-rotor helicopters is their use of deep sideslipping, which considerably increases the manoeuvre's efficiency.
This is due to the lack of the restrictions on angular speed of rotation and ability to perform accelerated turns with deep (60 deg.) sideslipping, which increases the efficiency of the turn. The coaxial-rotor helicopter has this capability owing to the lack of the tail rotor.
Coaxial-rotor helicopters have advantages in performing other manoeuvres. These advantages become really awesome when the helicopter makes manoeuvres like a turn while performing a zoom, during which great angular speed is a prerequisite just as well as deep sideslipping is.
To cap it all, coaxial-rotor helicopters can perform aerobatics: 'slant loop', ascending roll, etc. While performing aerobatics, the helicopter develops pitch angles of up to 90 degrees with banking reaching 130-140 degrees.
Aerodynamic symmetry, good stability, controllability and manoeuvrability provide development of the coaxial-rotor helicopter automatic stabilisation and control system capable of automatising many flight modes, including complex enough - terrain-hugging flying, etc.
Coaxial-rotor helicopters' operation in extreme modes are worth examining too. Their minimum vertical descent rate in autorotation are 1 sq. m less as compared to that of single-rotor helicopters with the same loading.
This is due to the biplane cell effect of the coaxial rotor system reducing the induced loss of power as described above. Besides, despite low thrust in autorotation, the tail rotor of the single-rotor helicopter takes certain power, thus adding to the increase in the vertical descent rate of single-rotor helicopters.
The minimum vertical rate of the combat coaxial helicopter with the loading of 57.3 kg per sq.m as compared to that of the single-rotor helicopter of the same class with the loading of 43.4 sq.m is 8-10% more. The difference has no impact on the landing owing to the following:
aerodynamic symmetry of the coaxial configuration, control simplicity, lack of cross-coupling (e.g. 'collective pitch - pedals') and efficient longitudinal control provide the coaxial with easy transition to autorotation;
autorotation landing speed of coaxial-rotor helicopters is approximately 15 km/h less than that of single-rotor helicopters due to the lower (by 20-30 m) levelling with a greater (by 10degrees) pitch angle, which is possible thanks to powerful longitudinal control and lesser size of the coaxial-rotor helicopter. Lesser landing speeds enhance the landing safety, especially when flying over the broken terrain.
The problem of coaxial-rotor helicopters' directional stability in autorotation has been solved in full. Besides, autorotation landing methods have been developed and adopted, which employ the rotor rotation frequency that is 3-4% less than normally. This reduces the vertical descent rate substantially (by 2-3 m/sec), enhances directional control efficiency and landing characteristics.
In cooperation with research institutes of the helicopter-making industry and the Ministry of Defence, Kamov has undertaken an extensive flight-test and mock-up research programme dedicated to the vortex ring in support of the coaxial-rotor helicopter development. The results confirm the following:
the vortex ring's top boundary for both the upper and lower rotors is the same with the right and lower boundaries of the vortex ring (where the characteristics of this mode are minimal) are somewhat more extensive in coaxial-rotor helicopters;
for the coaxial-rotor helicopter the entering into the vortex ring mode and exiting it is safe if there is enough altitude margin to leave the vortex ring (same is true for the single-rotor helicopter).
The human factor is fundamental for flight safety. Coaxial-rotor helicopters being easier to fly boast better controllability and manoeuvrability as well as better fineness and are, thus, safer in comparison with their single-rotor competitors.
The helicopter's dimension is key to its flight safety. The coaxial-rotor helicopter's lesser size enhances its flight safety in the vicinity of obstacles and at a low altitude, which is vital for any combat helicopter. Since the coaxial-rotor helicopter's dimension is literally matching the diameter of its rotor, there is no chance for it to have its empennage damages while flying close to some obstacle. However, should the empennage be damaged or lost altogether (e.g. during the rough autorotation landing), this is irrelevant for the flight safety.
Comparing the flight safety of coaxial-rotor and single-rotor helicopters, opponents often consider the issue of rotor-blade overlapping in coaxial-rotor helicopters. It should be mentioned the issue of blades getting too close to the airframe is topical for rotary-wing aircraft of all types.
Based on lab tests, experimental research and flight testing data analysis, it has been proved coaxial-rotor helicopters provide flight safety in all flight modes (including in aerobatics) as far as rotor-blade minimum distance is concerned.
Coaxial-rotor helicopters have no restrictions on deflecting pedals to the right in either direction to their utmost as well as on performing both right and left turns. The impossibility of using the pedal to their full capacity is typical of single-rotor helicopters and is caused by flight safety requirements to the tail rotor operation.
Thus, the coaxial-rotor helicopter is far safer to fly than the single-rotor is.