Heliport is right, Shawn is the consummate expert!
Having lurked here for a bit, let me weigh in on this pithy subject:
Unlike an airplane, the G's a helo is pulling are only half the story. That is because every helicopter is really two entirely different systems flying in formation - the fuselage, which behaves like an airplane and which we call the "static structure", and the main rotor, which behaves quite strangely under load, and is called the "rotating system".
In turn:
The Static Structure - The G's that the FAR refers to is the load factor experienced by the fuselage - the engine mounts, the tail cone, the feet of the main transmission, and the G meter on the pilot's panel. When the FAR refers to 3.5 g, they refer to the static structure, since the rotor would long prior give up the ghost as the pilot maneuvered that severely.
Frankly, regarding load factor, the FARs were derived from airplane requirements and so they shadow the way airplane behaves. And airplane's structure experiences load factor across the entire aircraft. The wings produce lift and delivered to the whole structure, until the wing stalls or the structure fails. The most extreme maneuver an airplane can successfully perform occurs at the speed where the maximum load factor at stall matches the structural strength of the wings. We call this maneuvering speed and suggest that the pilot slow below maneuvering speed under severe turbulent conditions. Maneuvering speed protects the airplane from structural harm by assuring that the wing will stall before harmful forces are produced. There is no rotorcraft equivalent to maneuvering speed.
The rotating structure – the rotor of a helicopter produces the load factor but virtually no helicopter can produce the load factor required by FAR since virtually no helicopter has a rotor designed for 3 1/2 Gs, except perhaps a very light H-60 or H-64. A rotor designed for 3.5 g at normal weight would consume far too much power in a hover, and require far too much blade chord, again at a cost of considerable payload. The limits to a rotor during high load factor maneuvers are related to the blade stall and the ensuing pitching moment, and the stresses on the pitch links, swashplate and servos. This topic has been much discussed when we've talked about servo transparency, where the blade forces during high load factor maneuvers result in high stresses on the aircraft's controls.
It is safe to say that most helicopters, under limiting rotating system maneuvering, will produce very high blade and control stresses under surprisingly low static system load factors. Because rotor stall is strongly affected by altitude, airspeed, RPM, and collective pitch, it is very hard to predict precisely what load factor would produce excessive rotor stresses. This is why several manufacturers have attempted to employ servo warning limits systems as a means to directly warn the pilot of excess rotating system stresses during maneuvering. The servo limit lights on some EC models and the cruise guide on some Sikorsky models comes to mind.
To give a sense for the kinds of forces the rotor blade can place on the controls and servos here is a thought experiment. Imagine that your helicopter was placed in a hangar and the rotor blade was passed through a hole in the concrete wall and that hole was then bricked up tightly around the blade. Now connect a hydraulic mule to the hydraulic system, bring the system to full pressure and start to move the cyclic around. You can imagine the enormous forces the hydraulic cylinders would impart on the swashplate, pitch links, blade horn, and rotor blades while the rotor blade is trapped in the concrete wall. This condition is precisely what occurs in Jack stall, also called servo transparency except it is the blades that produce the force to drive the hydraulic servos backwards against their maximum force capability.
In a nutshell, load factor is not useful to warn pilots of the damaging maneuvers that the helicopter's rotating system can experience, and in fact a G meter might fool a pilot into thinking his maneuvers were acceptable while the rotor was screaming in protest. Meanwhile, it is very doubtful that the rotor would produce the load factors needed to bother the static system.
As pilots who've experienced Jack stall will tell you, under some conditions, it takes little load factor to create these enormous control system stresses, given some airspeed, altitude and gross weight.
Last edited by NickLappos; 8th Apr 2011 at 07:27.