Helicopter Mechanics

I'm still amazed, even after almost a century of powered flight and a decade of doing it myself, that airplane wings can lift us through such a tenuous medium as the air we breathe. I find helicopters even more fascinating. Indeed, those whose interests in life don't take them anywhere near flying machines often wonder just how a helicopter can hover there, as if by magic. As a kid, I certainly did. Even more peculiar is that even among those who fly, only a few seem to regard without uneasiness or suspicion those aircraft that fly by means of wings that generate their own velocity. I've met more than a few fixed-wing pilots who half-expect them to go into auto-flail mode and fling themselves asunder. But a little information can brighten a dark hangar, so let's flip the switch and find out: What does hold helicopters together?

Though they take many forms, of course, they all have rotating blades, a transmission, a fuselage, and a powerplant. (How the blades generate lift is another subject, but let's make one thing clear: Helicopters are not so ugly that gravity repels them, and they don't just beat the air into submission.) However, since how they fly has much to do with how they're put together, certain general explanations of their unique controls are necessary in order to provide a proper backdrop for how the subsystems are connected.

The helicopter has four primary controls. The collective is one. Resembling a lever pivoting at the aft end much like the emergency brake in a car, but to the left of the pilot's seat (which in most helicopters is on the right and not the left side), it uniformly increases the pitch of the rotor blades throughout their span. Unlike airplanes with constant-speed propellers, movement of this control increases manifold pressure (or torque in turbine helicopters), rather than rpm.

On the end of this lever in many helicopters is a second control, the twist-grip throttle, which is the primary control for rpm and not manifold pressure. (Not all have throttles on the collective, especially among turbine helicopters.) This operates much like the throttle on a motorcycle, though the closed position is toward the thumb, which is opposite to that of the motorcycle. There is usually a correlator that in a crude way adds throttle to meet the increased load on the engine when the collective is raised. To keep rotor rpm constant, a sensing device called a governor provides the "fine tuning."

The third control is the cyclic, positioned in front of the pilot and held in the right hand, which looks like a longer version of the control stick found in some airplanes. It selectively varies the pitch at a desired point in the blades' rotation and thus provides the "tilt" of the rotor blades, often referred to as the rotor disc or tip path plane, moving the helicopter in the desired direction. For example, if pushed forward, pitch is only increased as blades pass over the tail. This causes more lift over the tail, the rotor tilts forward, and the fuselage follows.

The fourth control is at the pilot's feet and resembles rudder pedals, but these control the pitch of the tail rotor or anti-torque rotor, which counteracts the tendency of the fuselage to spin in a direction opposite the main rotor.

A helicopter's rotor blades are its airfoils. Generally rotor blades vary in number, from two to six. There are three basic layouts: single rotor, twin rotor, or two rotors one above the other. Their construction usually features a strong solid leading edge and thin covering over a less-dense inner structure, such as a metal honeycomb. Some manufacturers use composite materials. They also have some slight "twist" in them to account for the differing tangential velocities along their length. Also, being so much longer than propellers, they have a visible "droop" when at rest, which could give one the mistaken impression of fragility. However, when rotating, the approximately 20,000 pounds of centrifugal force of even small rotor systems overcomes this. In fact, when the collective is raised, the resultant of lift and centrifugal force causes an upward bending, which is called coning. Unlike airplanes, the entire wing (blade) is rotated about its longitudinal (also called a feathering) axis to vary lift, either cyclically or collectively, via the pitch change links mentioned above.

The main function of the transmission is to translate the much higher engine rpm to a lower rotor rpm (as well as change the engine's horizontal axis of rotation to the rotor's vertical one). Even a small training helicopter like a Robinson R22, having blades with a rotating arm of 12.5 feet and rotor blades spinning around almost nine times a second, has a tip speed that's about 0.6 Mach - and that's just in a hover! Gears are used along with various bearings and shafts to transmit and change the direction and/or the speed of applied power. They are often splash lubricated, such as in the spiral bevel gears in the main rotor and tail rotor gearboxes of the R22. A second drive shaft passes through the long tail boom to the tail rotor gearbox. There are usually intermediate hanger bearings to keep the rotating shaft from bowing out in the middle (like a jump rope).

A key element in the transmission is the clutch; it's important on the way up and critical on the way down: Since the rotor's inertia compared to the engine's power is so much greater than that of an airplane's propeller compared to its engine, reciprocating-engine helicopters use a clutch to allow the main rotor to gradually "catch up" to its operating speed. (The rotor's inertia is too great to do otherwise.) This is done either by a centrifugal clutch, or a simple means of tightening a drive belt by moving one pulley in relation to another. (Free turbine engines don't need a clutch here, as this function is inherent in their design.) However, if the engine stops in flight - this is the "down" part - a one-way "sprag" clutch or other freewheeling unit, found on all helicopters, is essential. One form consists of inclined surfaces inside an outer drum which disengages the engine from the main rotor, allowing the helicopter to enter autorotation where the air now passing upward through the rotor keeps the rpm at a high enough speed to allow a soft landing by trading rotational energy for lift (accomplished by "pulling collective") when nearing touchdown. In the R22 this is inside the hub of the upper drive belt pulley.

The most intriguing part of a helicopter is the hub, or rotor head, where the blades attach to the drive shaft. Curiously, the cyclic and collective controls don't actually connect directly with anything that rotates. They are linked to a lower swashplate - though a more proper name might be "non-rotating star" - which stands still. It can be raised, lowered, or tilted by means of bell cranks and pushrods actuated by those cyclic and collective controls. The upper swashplate, which does rotate, rides on bearings over the lower swashplate, and its motion directly follows that of the lower one. The rotor shaft is also connected to the near end of all rotor blades at about mid-chord, but there are also pitch control rods that connect the upper swashplate to the front edge of the rotor blades. (This is how the blades' pitch is varied.)

Along the connection between the rotor shaft and the rotor blades in many larger helicopters are flapping hinges, articulating up and down, which allow the blades to flap up and down when at different positions as they rotate. This is what prevents a helicopter from rolling over when moving forward, due to the dissymmetry of lift between the advancing and retreating halves of the rotor disc. A side effect of upward flapping is the slight inward movement of that blade's center of gravity and resultant increased rotation speed of the upward blade, akin to what happens when a figure skater retracts her arms while spinning. This effect, misnamed the Coriolis effect, necessitates another hinge at the blade root allowing movement about the chordwise axis so the slightly faster upward blade can advance (or the downward moving blade can move slightly aft).

Many helicopter fuselages combine monocoque and tubular truss, with increasing use of composite and bonded structures. The primary structure of the R22, for example, is welded steel tubing and riveted aluminum, with the tail cone employing a load-bearing monocoque structure. Unlike fixed-wing aircraft, which take thrust and lift loads at separate points, the helicopter fuselage carries them at the same point: the center area of the rotor hub. Landing loads are also usually absorbed only in the vertical. Vibration levels are much higher because of the many rotating components. Although the slipstream and vertical fin alleviate some, there are still significant side loads that the tail boom must handle, especially in a hover.

There's one important fact as far as the powerplant goes - piston engines in helicopters are often derated, meaning that they aren't really breaking a sweat in normal operation. In the case of the R22, the maximum five-minute rating is for 131 of its 160 horsepower, after which it loafs along at a max continuous 124 hp.

So yes, there are many interconnected moving parts. Unlike fixed-wing aircraft, retirement times must be tracked for many rotorcraft components. For the R22, all critical moving parts are replaced during a mandatory overhaul, regardless of appearance or condition, after 2,200 hours or 12 years, whichever comes first.

Another key point is that hardware safetying is used much more on helicopters, due to the higher vibration levels inherent in moving and rotating assemblies. This can be done with wire, cotter pins, jam nuts, castellated nuts having self-locking inserts, one-time locking tangs, and other devices. There are various means available to fulfill the common requirement of keeping one part attached to another, whether through the use of nonremovable fasteners such as rivets, or by the more prosaic means of nuts and bolts. To the casual observer, hardware selection may seem arbitrary, but it isn't. In reality it requires careful consideration of loads, temperature, corrosion, vibration, fatigue, and many other factors.

Vibration is not merely incidental, being the primary cause of deterioration, and is closely monitored. This is because wear rate follows a geometric, and not a linear, path. For example, a bearing may wear one thousandth of an inch in the first 500 hours, but the next thousandth might occur within the next 50 hours. Naturally, inspection frequencies and replacement tolerances account for such expected wear rates, and with added margins of safety. These tolerances are quite specific. In the R22, for example, the maximum values for looseness or "play" in the tail rotor teeter hinge and blade root feathering bearings are 0.010 inch and 0.005 inch for axial and radial play, respectively.

All rotating components have a natural or resonant frequency, and designers are usually able to fabricate components so that their operational envelope does not include these frequencies. When it can't be avoided, however, a transit range is established and marked on the tachometer by a red arc. Low-frequency vibrations of 300 to 500 rpm are usually associated with the main rotor. These are periodically reduced by blade tracking to remove vertical vibrations (such as adjusting the length of the pitch change rods on individual blades), and span-wise balancing, which addresses lateral vibrations (just as on an automobile tire, although tape is used instead of lead weights). Medium-frequency vibrations are those in the 500- to 2,000-rpm range, and high-frequency vibrations above 2,000 rpm are usually associated with the tail rotor, engine, and drive train.

Present in great number are push-pull tubes, such as in the cyclic and collective systems, and for transmitting pitch change inputs to the main rotor and tail rotor. Because of higher vibration levels and the numbers of consecutive tubes in some systems, the "play" in rod ends that would be acceptable on fixed-wing aircraft is not on helicopters.

Another type of articulation hardware item in helicopter flight control systems is the torque tube. Usually mounted perpendicular to the aircraft centerline, in a saddle and attached at each end by a bearing, the shaft partially rotates within it - converting a rotary motion to a linear one, as in the use of the collective. Bell cranks, often used with push-pull tubes, are used to change direction of travel, as well as to vary the mechanical advantage. In addition to these there are often added components such as counterweights, bungee springs, friction adjustment controls, and hydraulic boosts - especially on larger aircraft - to aid in control and flying qualities.

All pilots must have a basic mechanical understanding of the aircraft that they fly. By now it should be obvious that students of the helicopter have a bit more to learn in this regard than their fixed-wing brethren. However, helicopter pilots consider knowledge of these more complex systems and controls to be a small price to pay for the vertical freedom that these fascinating aircraft provide.