yaw trim release switch on pedal or button on grip
 

Which one is more preferable on helicopters, a thumb-operated yaw trim release button on collective/cyclic grip or feet-operated yaw trim release switch directly on pedals ?

A109e has trim release on cyclic which also does trim release on pedals. This is a diabolical arrangement in my opinion. It should of course be on the pedal but there needs to be considerable finesse in how its done. Take the EH101 system for example. In this case it has 2 microswitches, one centrally mounted under each pedal face. To move the trim point you have to make the micro switches. However if your feet aren't central you are not moving the pedals..if you apply a lot of force (cos the pedals are locked in trim) then shift your foot slightly to actually make the microswitch the pedal moves quickly under that force. Also if your idle feet (great Autopilot) touch the pedals when the plane is flying itself suddenly you have control. Some aircraft by Airbus helicopters have pressure sensitive trim which I find much more seamless provided the chosen breakout force selected by the manufacturer is not too much and not too little.

A109E and A109S have the yaw trim release button on the cyclic grip. Not at all natural until you get used to it. The most unnatural thing is that when the AP "ALT and "HDG (or NAV)" upper modes are engaged, the aircraft still isn't really flying itself because the pilot still has to use the cyclic trim release button and the yaw pedals to keep the aircraft in balance.

Micro-switches on yaw pedals, such as on the S-76, wins hands down.

The Chinook has a pedal trim release button on the cyclic.....it's part of the function of the cyclic trim release button. So when you trim the cyclic you trim the pedals at the same time (unless you use the vernier trim on the cyclic - when the cyclic will move if you trim in the longitudinal direction, but not the lateral!). It sounds complex but in practice it's very straightforward.

All AW139's 4 axis equipped I actually fly and the only 4 axis Bell 412Ep I flew had FTR on the pedals.

kintyred - but since the Chinook uses cyclic control to produce yaw, that makes much more sense than on a conventional helicopter.

As Maeroda says, however on the 139 you have to apply equal pressure on the pedals for them to release properly and the microswitches can get a tad sticky sometimes.

Remembering that the aircraft has yaw trim above 41kts accelerating and kicks out below 45kts decelerating.:ok:

Not sure that this is any more true than of any other helicopter. The cyclic produces roll when displaced laterally and the secondary effect of roll is yaw.