Kaman's servo-flap intermeshing rotor system is unique. Flying it is also a unique, but rewarding, task.
Peter Gray/BLOOMFIELD, CONNECTICUT
A HELICOPTER TAIL ROTOR requires power to drive it, sometimes as much as 30% of that available from the engine(s). The greater the angle of pitch on the tail-rotor blades, the more power is required to overcome the increased drag. In an extreme case, for example, hovering in a crosswind with a heavy load at high altitude or high outside air temperature, or even just a strong crosswind at lower levels and temperatures, full pedal may be required to maintain heading.
If insufficient power is available, handling difficulties and other problems can occur, particularly if the helicopter is close to the ground. A tail rotor requires a shaft to transmit power from the main transmission and a gearbox at the end of the tail boom to change the angle of drive through 90° and reduce the speed. There is often an intermediate gearbox half way along the shaft. Furthermore, a system for changing tail-rotor pitch is necessary. All this requires lubrication and maintenance. When working in confined areas, there is the additional problem of the tail rotor hitting something or someone and of preventing people from walking into the rotor - it is often invisible at 100% RPM.
IMPROVING PERFORMANCE
In the early days of Kaman Helicopters (the company was formed in 1945), its aerodynamic servo-flap system was designed to reduce the forces fed back to the controls, the very high control vibration and overall rotor instability. The company also sought a configuration to improve performance and eliminate the disadvantages of the tail rotor. The intermeshing-rotor design was the solution. By eliminating the tail rotor, space was made available to fit longer blades and so reduce disc loading. The large area of the four blades, gives greater lifting efficiency (ie, they can lift more weight per unit power), low rates of descent in autorotation, good engine off landing flare characteristics and increased inertia which resists acceleration and, more importantly, deceleration (reassuring in the event of an engine failure) - all good safety features.
The venerable Kaman H-43 Husky and, now, the single-seat K-Max heavy-lift helicopters, both have intermeshing-rotor systems. Flight International's introduction to the system came with the Husky.
The rotor system consists of a pair of two-bladed rotors, mounted side by side on separate shafts driven by a common gearbox and phased 90° to one another. As viewed from above, the rotors are counter rotating, with the right rotor turning clockwise and the left rotor turning counter-clockwise - like a swimmer doing the breast stroke. No tail rotor is required because the counter-rotating rotors nullify torque reaction. Furthermore, because the rotors are teetering, there are no flapping hinges, dampers or pitch-change bearings to maintain.
In this unique system, raising the collective-pitch lever causes the servo flap on the trailing edge of each blade to move up, just like the elevator on a fixed-wing aircraft. This twists each blade by the same amount, raising the leading edge and lowering the trailing edge, as in any other helicopter, thus collectively increasing the pitch and so the lift. The lever is mechanically linked to the moving elevators on the tail boom to help minimise any attitude changes with lever movement.
The cyclic-pitch stick is also connected to the servo flaps. Moving the stick forward and aft tilts both discs forward and aft equally.
Applying lateral cyclic, however, does not tilt both rotors sideways, only one. Thus, left stick causes only the left rotor (and the fuselage) to tilt. This system means that there is lot of rotor power available to the pilot in the fore- and -aft plane, and half this in the lateral plane.
With no tail rotor, what is the function of the pedals, and how do we keep the helicopter in balanced flight and do spot turns?
Movement of the pedals has three effects - for example, application of left pedal causes:
the collective pitch of the left rotor to decrease and that of the right rotor to increase, turning the helicopter to the left;
the left rotor to tilt aft and the right rotor to tilt forwards, also helping to turn the aircraft left. If the helicopter is hovering, the result is a spot turn. (As pedal is applied, however, the helicopter will also roll to the left, unlike other helicopters, so opposite stick has to be applied to prevent the aircraft flying sideways as well as turning);
the mechanical connection to move the rudder. In forward flight, this increases directional stability.
FLYING THE HUSKY
With all of this in mind, I was ready for my first flight in the Husky, Kaman's dual-seat trainer powered by a single Lycoming T53, with Greg Barnes, experimental test pilot. The 1959 engine is remarkably modern in that it has an automatic start - you just press the starter button, release it immediately and sit back to watch the engine slowly and coolly burst into life.
After a short demonstration of the differences in technique required, Barnes handed over control to me.
During take-off, the nose wants to pitch up as the lever is raised, so a little forward nudge on the stick is required, followed immediately, by an equal rearwards nudge, as the aircraft breaks ground. A landing requires opposite procedures. Since there is no torque reaction, both pedals are kept level throughout, a weird procedure initially. Because of the difference in fore- and -aft and lateral rotor power mentioned, combined with a small lateral "dead band" in the stick movement, one's first hover is unstable, with over-controlling and a failure to anticipate the aircraft's reaction to cyclic inputs.
To complicate matters further, any pedal movement will cause the aircraft to roll. The box-type fuselage and rotor system tend to hover into wind, so the trick is not to move anything, relax and let the aircraft do its stuff. Too low a hover at 3ft (1m) and below causes down-wash turbulence beneath the helicopter and a very unstable hover.
SPOT TURN
Now for my first spot turn - the first time I moved the pedals from their neutral position. Barnes had previously demonstrated some extremely fast turns in both directions, with no ill effects, but I tried a more moderate rate of turn. Remembering to apply opposite stick during the entry and recovery, it worked out well. A session of take-offs, hovering, spot turns and landings helped reprogramme my brain after 37 years of flying conventional helicopters.
Sideways flight was straightforward. Because of the box shape of the Husky fuselage, however, either the nose or the tail wanted to turn into the direction of movement. I went to about 30kt (55km/h), which required full pedal - pretty fast for a 1957 aircraft. Recovery, from fast backwards flight, needs care to prevent an extreme attitude developing. I was now ready to launch into forward flight.
Remembering not to move my feet as I moved the lever, we accelerated normally. There was none of the in-flow roll (that unannounced drop of the advancing blades) which other rotors produce. Translational lift, that sudden boost, came in at about 10kt. The aircraft wanted to yaw slightly into the crosswind and Barnes advised me to let it.
Straight-and-level flight was "normal". Turns required leading with the pedals, keeping an eye on the slip ball, and appeared to require, more pedal than other helicopters. Although this was not a test flight - indeed, I was the one being tested - I went to the never-exceed speed (Vne) of 100kt. Intermeshers are built for lifting capacity, not speed.
A speed of 100kt, however, was quite respectable in 1957, when the Husky was first flown. Vibration levels were quite acceptable. Kaman says that the aircraft will go comfortably faster, but it is hard on the components, so it restricts the Vne - a common decision, even with modern helicopters.
We slowed down to normal cruise, and Barnes invited me to pump the lever up and down (no pedal is required to keep in balanced flight). The nose-up pitch as the lever is raised and the nose-down pitch when lowered is severe. I made a note to anticipate this when flying the circuit.
I now joined the circuit and slowed to 50kt, noting a flock of migrating geese pass beneath us at 55kt, heading south to supposedly warmer climes.
All helicopter pilots will be pleased to learn that intermeshers do not suffer from vortex-ring condition/settling with power.
Remembering the lessons I had learned so far, I did nice balanced turns and settled on final approach with the pedals level. Again, the aircraft wanted to yaw into the crosswind and, again, I let it, keeping my speed up (good single-engine technique) and kicking it gently straight along the runway as we came to the hover. This can avoid getting into a crossed-controls condition. Because of the high fore- and -aft rotor power, care has to be taken not to overcook the flare and stop too suddenly.
By now, I was getting a good feel for the aircraft and did not have to refer to the ball so often. Good balanced turns were coming naturally and I kept the aircraft level when required.
We did some running landings as a lead-up to auto-rotation and engine-off landings. Again, I let the nose stray into wind and kicked the aircraft straight just before touchdown.
When flying an intermesher it is much easier to get in to a crossed-controls, out-of-balance, condition (ie, too much pedal and opposite stick), particularly on final approach, so I deliberately tried this while Barnes was on board. Provided you can feel the out-of-balance condition or keep an eye on the ball, you cannot get into trouble. My deliberate side-slip, however, was severe. The solution, as nearly always in an intermesher - level both pedals. ENGINE-OFF LANDINGS
The entry into autorotation is one of the most benign of the many helicopters I have flown. In the event of an engine failure, the servo flaps respond automatically to the change of airflow direction through the discs, from above to below, and the decreasing rotor RPM by reducing the angle of attack (and therefore drag), giving the pilot much more time to react. So, the throttle was closed first, then the lever lowered gently, no hurry. No pedal is required. The nose wants to go down, so a co-ordinated stick movement is necessary. The Husky does most things best at 50kt - climb, auto-rotate, etc - so this was selected. Rate of descent in auto-rotation is 1,200ft/min (6m/s). It is also one of the most benign. The glide angle is quite flat. It can be easily and effectively adjusted by reducing/increasing airspeed and/or reducing rotor RPM. Remembering again the power of the rotor in the fore and aft plane, the flare is effective and prolonged. Lever is used while leveling the fuselage to hold the height, then however much more it takes to land. No pedal is required for the manoeuvre.
I next tried engine failures in the hover. As the throttle is chopped and the first bite of the lever is taken, the nose wants to go down, so just a small amount of corrective aft cyclic is required. Remembering not to move your feet, sufficient lever is used to cushion the landing. There is plenty of time. If one did nothing, the aircraft would land itself safely with some forward movement.
CONCLUSIONS
Converting to intermeshers should present no problems for any helicopter pilot - just a little brainwashing, particularly on pedal management. It was a delight to revert to seat-of-the-pants-type flying again, although all the necessary instrumentation is there should you need it. The dual-controlled Husky obviously provides a perfect introduction to intermeshing for pilots converting to the single-seater K-MAX, which is why Kaman has brought the aircraft out of retirement.
Source: Flight International
