The meaning of "soft in plane"

[calfoote]

Can someone please tell me the meaning of the expression "soft in plane"? I believe it is an engineering term referring to helicopter rotor systems. The question arose in a recent Australian ATPL exam. Also, a reference would be appreciated.
Thank-you.

[EZY AIR]

SOUNDS LIKE SOMETHING YOU GET WITH FAT CABIN CREW:O :yuk:

[moosp]

No, just checked "The Art and Science of..." by our very own SC and found no mention of the term. We can safely assume that it is either an Australianism for the lack of rigidity on the rotor inplane or a made up phrase by the question setter.

These are not as rare as you might think. The exam writer (who may not be a helicopter pilot or engineer) may have taken the phrase "Rigid inplane" (sic - one word) from a POH (Robinson uses the phrase) and developed his own antonym.

As to the aerodynamic effect of an inplane that flexes or is "soft" the subject is way beyond my level, but there are people here who know and may be able to help.

[Shawn Coyle]

Another thing to add for the next edition....

From what I can determine, the best definition for 'Soft-in-plane' refers to a rotor head that has an elastomeric bearing to permit the blade to lead and lag, but not as much as it would if it were a fully articulated rotor. As far as I know, only Bell uses the term (but I'm probably wrong), and they developed the term because the rotor head to which it first referred- the Bell 412, was very different than the 212.
The 212, as a teetering rotor obviously did not permit in-plane motion of the blade as it rotated, whereas the 412 with the elastomeric bearings did permit the blade to lead and lag as it whirled around.

[GLSNightPilot]

It's a Bell term for the 412 head, & it basically means "There is no way to track it smooth, & it will shake your teeth out". I always assumed it just meant "not in plane", since the 412 blades are in different planes, with the two beams stacked on top of one another. Thus, you can never get a perfect track, you just track for an 'acceptable' ride.

[Dave_Jackson]

http://www.unicopter.com/temporary/soft-in-plane.jpg

[Mike Hardy]

Soft-in-plane is a general helicopter stability and control term, not limited to one manufacturer or country.

Soft-in-plane refers to a rotor head system where the lead-lag natural frequency is less than or equal to the shaft rotational frequency. For a fully articulated head (soft-in-plane) the lead-lag frequency is typically 0.2 to 0.3 the shaft rotational frequency (these numbers are from "Principle of Helicopter Aerodynamics", Leishman).

Stiff-in-plane is where the lead-lag frequency is greater than the shaft rotational frequency.

A general discussion of the term "soft-in-plane" can be found in"More Helicopter Aerodynamics", R.W. Prouty.

[B Sousa]

Come on Mike , We all know "Stiff in Plane" is when your flying on Southwest with some of those young lovelys, and " Soft in Plane" is Aeroflot.....