I've never heard of pendular action despite nearly 3,000 hours in the 234; however we did experience one interesting characteristic.
Over the North Sea it is common to experience severe frontal weather especially around occlusions; no sane pilot flies straight into it and if you can't get round it you must duck under it.
Strong temperature differences lead to severe windshear; the Chinooks ADC's would become seriously confused as the front of the aircraft pitched up or down and the LCT's tried to fight the uncommanded divergence. At 500 feet over a Force 10 the PF is now trying to control the attitude; he ends up in a divergent fugoid which only ceases as you pass through the shear. 44 less happy punters in the back.
Cure was to take pre-emptive action: slightly before encountering the squall line, make a sharp forward cyclic input and release it immediately. The 234 would then sail through with it's arse in butter. No coffee spilled.
It occurred to us that the military (and Boeing Vertol) must have known about this charming characteristic but didn't feel it necessary to tell us.
Please can some kind American enlighten me (you guys invented the phrase I think)- was this "pendular action"? Because it was not like the effects in this thread caused by underslung loads, either with short or long line.