Chapter 16 Coaxial • • • • • •

Most pioneering designers did not chose a configuration with a main rotor and a . They almost always solved the unbalanced torque problem by using two or more main rotors. Some located the rotors in the same plane as tandems, side-by-sides, or quad-rotors, but many chose to use two rotors on the same shaft—the coaxial arrangement. In the beginning The first designs of Igor Sikorsky and Stanley Hiller were coaxials and even Arthur Young, developer of the Bell helicopter, toyed with a coaxial. In this they joined pioneer designers Pateras Pescara in Spain, Corridino D'Ascanio in Italy, Louis Breguet in France, and Peter Papadakos with his Helicopters in America. Today (1996), only the design bureau in Russia is actively designing coaxials; one example is the Ka-32 (Figure 16-1). And even Kamov seems to be having second thoughts; having recently brought forth the Ka-62, a design with one main rotor and a fenestron.

Figure 16-1 Ka-32

The good news Having no tail rotor, the coaxial design has superior hovering performance when compared to the single-rotor helicopter. And it is insensitive to wind direction during low-speed flight. Its compactness makes it possible to operate from smaller areas on ships and platforms. The bad news However, the primary Achilles Heel that has hampered the acceptance of the configuration is the difficulty of obtaining good directional stability and control. Another drawback is that the configuration—while compact in the top view—is tall in the front view. This is bad from a drag standpoint and also may require a hangar with high ceilings.

57 Coaxial Helicopters 16 Hover performance The coaxial helicopter has superior hover performance, even without considering the lack of a tail rotor. The two sets of blades, being separated, do not interfere with each other as much as if they were in the same plane. Figure 16-2a shows a coaxial helicopter whose rotors are separated so far that they operate independently of each other, with low disc loadings and correspondingly low power requirements.

Figure 16-2 Coaxial Rotor Inflow Affected by separation

A second orientation is shown in Figure 16-2b. Here the rotors are closer together, but it is assumed that the lower does not significantly increase the induced velocity through the upper rotor. Another assumption is that only the lower rotor's inner portion out to 70% of the radius is in the fully- developed wake of the upper rotor. These assumptions are probably true at a separation distance of about half a rotor radius. With these assumptions, the upper rotor again acts independently, as in the first example. The lower operates as a rotor whose inner portion is in a vertical climb condition but whose outer portion is free from the interference of the upper. To maintain yaw trim, the two rotors must be absorbing the same power. My calculations show the corresponding split of thrust. Figure 16-2c shows a single rotor helicopter. Because all the thrust is generated by this one rotor, its disc loading is higher than in either of the previous examples. This rotor will require about 10% more power than Figure 16-2b's coaxial rotor. About 1% of this difference is because some power is associated with swirl in the wake of a single rotor. The contra-rotating rotors of the coaxial system produce a wake with no swirl and thus less energy loss. Of course, moving the rotors closer together, as in a more practical design, increases their mutual interference. The lower rotor is now sucking air down through the upper rotor, and the induced velocity from the upper rotor is covering more of the lower rotor, but with a lower velocity. Tests made by Kamov show that when the separation distance is as little as 20% of the rotor radius, there is still a 7% power benefit. Kamov aerodynamicists have developed a sophisticated vortex- analysis program accounting for the complicated flow situation which correlates with the test results.

58 Coaxial Helicopters 16 The directional control problem On coaxial helicopters, there obviously must be a means to control the two closely-spaced rotors. The mechanism to do this appears complicated—but it works.Figure 16-3 shows the system installed on the Kamov Ka-26.

Figure 16-3 Ka-32 Rotor Head

In practice, the control of the cyclic and collective functions is the same as on a single-rotor helicopter. But the lack of a tail rotor requires a different system for directional control. How it's done On most coaxial helicopters this is done with differential collective pitch. This produces an unbalance of torque between the two rotors. On the system used on the Kamov Ka-32 (Figure 16-4), the collective pitch is handled with coaxial upper and lower "slider rods," which are located inside the hollow upper rotor shaft. At the rotors, they protrude from slots in the shaft to attach to the blade feathering mechanisms. At the bottom, they are encased in a barrel with right and left threads (sort of like an inside-out turnbuckle). The barrel can be rotated by pedal action from the cockpit, thus lengthening one rod and shortening the other for differential collective pitch. The barrel is raised and lowered as a unit for ordinary collective pitch. How it works Differential collective pitch works well in powered flight. Increasing the pitch of one rotor increases its torque and the opposite effect governs the other rotor. The unbalance in torque will turn the helicopter opposite to the direction of rotation of the rotor producing the most torque—just as it would on a single- rotor helicopter without a tail rotor. But in , however, differential collective pitch has the opposite effect. Figure 16-5 shows why this is true. In powered flight, the lift vectors are tilted backward, and so an increase in lift produces increased torque opposite to the direction of rotation. In autorotation, the lift vectors are pointed forward to compensate for the drag of the blades. Now an increase in lift will produce torque in the same direction as rotation

59 Coaxial Helicopters 16 .

Figure 16-4 Ka-32 Control System Schematic

Stanley Hiller's first helicopter was a coaxial. When he made his first autorotation test, he found himself spinning around even though he thought he was doing the right thing with his feet. As he approached the ground, he decided he had nothing to lose by reversing control—and it worked.

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Figure 16-5 Co-axial rotor in Powered flight and autorotation

On synchropters too The same characteristic is found on synchropter helicopters such as the Kaman designs. Kaman has solved the problem by including a reversing linkage in the directional-control system that is actuated when the collective stick is lowered below a certain point. Of course, right at that point there is no directional control from the rotors at all. I have been told that this can produce a problem when taxiing with low rotor thrust. To prevent this from being a problem in flight, descents are always made in forward flight where the stability contribution of the vertical stabilizer can be used to maintain a straight flight path. The Kamov solution The Kamov designs do not include such a reversing device. The pilot's manual for the Ka-32 specifies a minimum speed in autorotation of 100 km/h (54 knots). At this speed the large vertical stabilizers with their rudders controlled by the pedals provides satisfactory control. There is some evidence that this situation was not always true. The slat (in Figure 16-6) is undoubtedly an "engineering fix" to solve problems in the early flight-test program.

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Figure 16-6 KA-32 Rudder slats

An alternate system There is another solution to the coaxial directional control problem. This was used on the Gyrodyne designs and consisted of movable drag vanes at the blade tips controlled by the rudder pedals (Figure 16-7).

Figure 16-7 Yaw control with Tip Vanes

When a turn was desired, the vanes on one rotor were deployed to increase its torque, while the vanes on the other rotor were left in their streamline position. This system works well in all flight conditions, but has the drawback that the drag of the vanes hurts performance even in their undeflected positions.

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