imabell,
The term "soft inplane" is a term that helps differentiate between various "rigid rotors" where the blade is attached to the mast by solid (usually composite) structure, but where the structure is tuned by its inherent softness and sometimes a small damper to have some "spring" and allow a high degree of lead-lag motion. Thus, you can have various degrees of rigidity in a rigid rotor with modern materials, so we need to describe them differently.
The Bell 680 rotor head is an example, where it is bearingless and appears to be rigid, but in fact is relatively compliant in lag and also in flapping, making it behave more like an articulated rotor and much less like a "rigid" rotor. By being soft inplane, the rotor delivers less stress to the rotor head and mast, and less vibration. It also delivers less cyclic control power, and so feels softer to the pilot. These rotors were developed for retrofit onto teetering rotor helicopters, so the stresses they deliver to the airframe must be very low, to match those the airframe was originally designed for.
A true "rigid" rotor like that on the Bo-105 is much stiffer, and delivers much higher stress to the mast, and feels much crisper to the pilot. These rigid rotors have almost no lag motion.
We judge the stiffness of the rotor by measuring the first lag frequency of the blade relative to the blade's passing frequency, where the exact term "soft inplane" refers to a rotor where the lag frequency is less that 1 per revolution.
see this paper for the first few paragraphs that discuss it, before the paper takes you into some fun engineering math:
http://www.enae.umd.edu/AGRC/faculty...SMSVol5No5.pdf
The flapping response of many soft in-plane rotors is also softened, so the cyclic feel is much less crisp than a classic rigid rotor. This also reduces the stresses on the airframe, of course. I'd be glad to discuss the effects this has on handling.
here is an illustration:
