the confusion arises IMO, as previously clearly stated, as a result of the reference system used to describe flapping.
Most people (UK CAA for example) would use the definition of Flapping being the Angular displacement of the longditudinal axis of a blade
referenced to the HUB AXIS (or hub plane) - this has
some merit since this is the Flapping which a flapping hinge actually has to accommodate. however for forward flight the Tip Path Plane can be inclined forward more than the Hub Plane - leading to the odd consequence that Flapping, defined in that way, would be UP on the retreating side (!) - NOT WHAT MOST PEOPLE WOULD EXPECT. It is fairly 'meaningless' for understanding helicopter flight except in as far as it does show clearly how a blade's flapping hinge will physically move. It would result in a different value for Flapping if the CoG were changed in a nil wind hover for instance - it is not what people mean when they use the term flapping.
Simply (by this definition)
it is the difference between Tip Path Plane and Hub Plane (accommodated by Flapping Hinges, Teetering Hinges or blade (root) flexing depending on the Rotor Head).
More commonly / usefully Flapping can be described with the
reference being the Tip Path Plane - if a blade is rotating in the TPP and encounters a change in cyclic pitch or cyclic airspeed change (advancing/retreating sides) then the blade
would Flap (at a rate) into a new plane of rotation - this is the definition most commonly used for describing the Flapping to Equality phenomenon - the TPP
would change resulting in FlapBack. In this case it is clear that to
maintain a constant accelerative
attitude the cyclic stick (Control Plane) will have to be progressively displaced forward (in general) sufficiently to effect a Pitch (and AoA) dissymmetry
sufficient to negate the need for any Flapping -
as a result no Flapping occurs.
There is another plane to which 'Flapping'
could be
referenced - this is the
Control Plane - it was most favoured in the Dark Ages before the breakthrough into the understanding of helicopters which was marked by Cierva allowing his blades to
freely flap - this marked the beginning of the viable helicopter. This plane is a little arbitrary - if the Blades are Referenced to this Plane then then
Pitch does not vary around the cycle at all but no part of the disk physically rotates in this plane ( the merit to using this plane is that it
can be the plane of the swash plate - which is marginally interesting ) although largely useless as a Reference in a modern helicopter it does have the interesting feature that Referenced to that (arbitrary) plane
the Pitch does not vary cyclically but instead the blades ONLY FLAP. Does anyone use the Control Plane as a reference for Flapping? - An Analysis of Flapping With Respect to the Control Plane would be fairly obscure and unhelpful. It is also noteworthy that a (truly) rigid rotor with NO FLAPPING HINGES and NO BENDING would
still be said to 'Flap'
relative to the Control Axis
For Crab to
begin to be right he would have to be using this Reference Plane (Control Plane), and he should he have said so (since it is NOT the Default Plane of Reference) , he should also have agreed when I pointed out that the likely cause of the (his) confusion was the Reference Plane he was using. NOT only that but he would also have had to be consistent in the use of that plane - which he was not - frequently using descriptions relevant to other reference planes.
So in short an argument could be made for Crab's correctness on this issue - but it would be a pretty weak argument IMHO - and
he clearly was not talking about Flapping referenced to the Control Plane - (was he SASless?)
The friendliest 'judgement' would surely be that both were correct - and to say:
Referenced to TPP; the Pitch varies Cyclically (as required to make the appropriate AoA such that 'lift' is constant around the disk) and the Blades Do Not Flap - except to change the plane of rotation.
or
Referenced to the Control Axis; Pitch is constant around the disk
and the blades Flapping is Huge (and extremely difficult to analyse any element of this definition of Flapping attributed to (apparent) Cyclic Speed Variation!)
sorry a bit rushed.
If anyone wants to read someone else saying it 'in print' then this reference says it (approximately) quite nicely (reading from 4.3.3 on) click the link:
Art of the Helicopter - John Watkinson - Google Books
please vote (SASless) - should Crab continue to do my spell-checking?