Trim Release - What does it do?

[Two_Squirrels]

Hopefully, a simple question. What does the trim release on helicopters actually do?

[oldbeefer]

Uhh, releases the trim! Trim holds the cyclic in the position at which you release the trim button, Stick can be moved against springs to manoeuvre, then returns to trimmed position when pressure relaxed. To move stick to new datum, button pressed, stick moved button released - hey presto!

[ShyTorque]

In my aircraft, if you press it, the door panels and roof lining fall off... :E

[gulliBell]

Hopefully when you press the trim release button the floats don't inflate or the cargo hook doesn't release :{

[Teefor Gage]

Whenever I press it, my trouser belt slackens off causing my beer belly to push the cyclic forward. Only useful if you are intending to accelerate quickly or descend providing you can get control of your belt back when required.........

[Two_Squirrels]

What does the trim release do?

--------------------------------------------------------------------------------
Whenever I press it, my trouser belt slackens off causing my beer belly to push the cyclic forward. Only useful if you are intending to accelerate quickly or descend providing you can get control of your belt back when required.........

Yes, OK, but what else does it do, i.e. does it disconnect the Autopilot? or the stabilisation?

[Um... lifting...]

In many machines it involves a set of magnetic clutches which hold the cyclic in a given position, but can be moved against springs as one of the other posters pointed out. Press the button, the clutches release, release the button, they reengage to whatever position you've moved the cyclic while you were holding the button down. A beeper trim (or coolie hat or chinese hat) will slowly reposition the clutches in whichever direction you move the hat.
Many folks believe for most smooth flying that best practice is to press the trim, move the stick, release the trim, repeat. If you move the stick then press the button, the stick will tend to jump. There are many arguments on this in different flight regimes and once you wade into that swamp you're never coming out. There are also differences between systems on different machines, with the more complex machines being well... more complex.

[VeeAny]

On the 109 it doesn't disconnect the autopilot, it releases the magnetic brakes (cyclic and pedals only) but while pressed you are flying the aircraft, when released the autopilot will keep trying to do what you asked it to do previously.

Once again on the 109 pushing the trim release will recenter the SAS actuators, which can give the impression that the SAS has been turned off sometimes, particularly if you manoeuvre (spellling ?) hard. But it doesn't turn off the SAS.

Can't comment on any other types as I don't know enough about them.

GS

[the coyote]

It will also reset any coupled flight director datums, to those occuring at the time of re-engaging the trim. For example, if you have altitude hold selected, press the trim release and fly the aircraft to a new altitude and then release, the new altitude will then be held.

[Um... lifting...]

press the trim release and fly the aircraft to a new altitude and then release, the new altitude will then be held.

Not necessarily. In some types, yes, with some autopilot or flight director systems. The original question was general in nature. In other types, one can fly-through autopilot or flight director, but upon releasing, the aircraft will return to whatever the original settings were. In several types of multi-axis autopilot helicopters I've flown (and a couple with force trim but NO autopilot), each had different operating characteristics.
It depends upon the aircraft.

[Shawn Coyle]

A detailed knowledge of the AFCS is necessary in order to answer the question.
In most helicopters with stabilisation, the trim system is a necessary part of the system. It anchors the cockpit end of the flight control system and lets the SAS (or series) actuators do their thing at providing short term, limited authority 'stability' - mostly taking out small rates. With the trim turned off, the pilot has to hold the stick in position for the actuators to only move the flight control 'surfaces' - the rotor blades.
Without the SAS portion of the AFCS (in most systems) the rest of the AFCS won't really work. If you have attitude hold engaged, but either turn off the force trim, or hold the force trim button down, the system can't maintain a 'datum'.
If you have attitude hold engaged, the system will always be trying to maintain the datums of pitch and roll attitude and slip ball position. (assuming forward flight). If you press the trim release button down, the system will remove the 'datum' and wait till the button is released. When the button is released the attitude hold is effectively told - 'hold this pitch and roll attitude and slip ball position). This is one of the reasons you don't want to use the trim release in forward flight - it resets all the datums when you may have only wanted to change one of them.
If you just move the stick and don't touch the trim release with only attitude hold engaged, the system should take you back to the original pitch and roll attitude and slip ball position when you let go of the stick.
With an autopilot in the system, you really need to know the logic of the autopilot (which should be well explained in the factory training you've taken... - you have taken factory training, haven't you???). It's doubtful that the trim release will reset the altitude hold or heading hold.
My experience has been that if you have a trim release system on a helicopter, learn to fly with it on - takes about 5 hours to get comfortable with it. Persevere with it, and once you understand it, you'll never turn it off again.
And there are lots of helicotpers without trim releases that have trim systems (FH1100, MD500 series to mention two).
If you want more info, please pm me.

[Bladecrack]

Interesting thread, however going off at a slight tangent, can any experienced 355 pilots tell me what they expect from their autotrim? I previously flew an EC135 and it appears to me that the 355 doesn't actually 'autotrim'.
To clarify, when the pilot moves the cylcic in the 135 the autotrim follows through, (quite a wide range of movement) holds the new stick position and the actuators return to neutral, however in the 355 when the pilot moves the cyclic the amount of autotrim seems to be very small and the acuators reach their limit of travel very quickly, and the pilot then has to push the trim relase to re-centre the actuators. Is it perhaps just the fact that the 135 is more advanced and has a better system?

BC

[fkelly]

Err..bladecrack that's because the AS355 doesn't have an auto trim, it has a conventional trim/stab system as previously described, ie/ you set an attitude. No amount of holding the cyclic against the trim will alter the datum. BTW the 135 autotrim only works fully at low speeds ~40kt and below cos it's dead clever, otherwise known as the logic of the autopilot.

[Bladecrack]

The manual for the SFIM AFCS 85T31 states:

Whenever the series actuator nears saturation, the parallel trim actuator with full authority moves the stick at a slow rate so as to displace the anchoring point of the cyclic stick and maintain the series actuator around its mid point, thus restoring full authority

It also states:

When illuminated, the amber TRIM warning annunciator indicates:
1. A trim actuator runaway.
2. A trim computer failure.
3. A "Trim Demand" This is triggered whenever a P or R series actuator is out of centre by more than 2/3 of the actuator's stroke. This might happen in the case of an inoperative Auto-Trim

Note: When I converted onto the 355 F & N the helicopters I trained on had no autopilot fitted (VFR only), hence A/P theory and operation was not covered during the groundschool. The one I now fly has a full A/P system.

[oldbeefer]

'Whenever the series actuator nears saturation, the parallel trim actuator with full authority moves the stick at a slow rate so as to displace the anchoring point of the cyclic stick and maintain the series actuator around its mid point, thus restoring full authority '

Yes, but that's when 'hands off', and the AP is trying to maintain the attitude. The series actuators do most of the work most of the time, but if the displacement is too great for them to cope with, the parallel actuator moves (and the therefore the stick as well). The series actuator will be moved (by the computer) to its null position. If the pilot wishes to change the attitude, then he presses the trim button, moves the stick then releases it - this sets the new datum for the AP to work to. On most aircraft there is also a beeper trim to make small changes of attitude.

[bast0n]

The Whirlwind did not have a stick trim. The only way to trim your stick was to use a penknife. But remember not to let go of the stick whilst whittling or you die.................................:O

[HeliComparator]

The EC225 has an interesting variation - below 40kts there is follow-up trim, ie if you move the stick against the spring trim, the trim actuators motor the trim position so you are back in trim again. So you get some spring trim feel to discourage over-controlling, whilst always being in attitude-hold trim. you can manoeuvre in the hover, then let go of the cyclic to scratch your bits, and the heli's current attitude is held.

HC

[theavionicsbloke]

Hi guys.

Don't worry, many AS355 pilots who bring problems to me are just as confused with the good old 'Trim Release' functions.

You ask a popular question, one that I get asked and have to demonstrate several time a year from operators, newbies, CPL's & ATPL (Yes.... including TRI & TRE's!!! :ugh:).

So OK, lets try and sort this one out, here goes,

The AS355 autopilot system is the SFIM 85T31
The Flight Director is a factory option and for the F2 & N's the Failure Monitoring Unit is also an option to satisfy UK & EASA (JAR OPS 3) IFR certification requirements.

Firstly this sytem DOES have automatic trim. It is operational all of the time and it's purpose is to sychronise (Keep Centre) the Primary AFCS (Automatic Flight Control System) servos. The position of which is displayed on the 'Galvinometers' on the instrument panel. The Galvinometers should be centred at all times! and indicate the position of the relevant servo.

If they do not centre then the AFCS is Un Serviceable. This is normally due to a failed trim system (Fuse is a good one because the airframes guy has put the wrong size fuse in :ugh:). However it is usually due to flight crew not knowing what the trim release switch does and that they have inadvertantly selected it to OFF :O. It could also be due to a stuck servo (Nothing that a tap with a hammer by your friendly local airframes engineer won't sort out. Well, at least for a few flying hours or so....)

Warning !! if you see the Galvinometers permanently out of centre with the AFCS ON and particularily with the AFCS OFF, then DO NOT FLY...:= ground it! This is because in a nut shell, you will not have the correct primary flight control range of movement required. Particularily look out for a failure in pitch as you may not have sufficient primary flight control movement available to achieve a full flare needed for Auto's or a fast run on landing / Engine Off flare. Thats what the auto trim looks after for you and thats why the Galvonomiters are there for. By the time you learn this, it may have already of spoiled your day!:(

The trim release effectively switches the autotrim OFF (By removing the power to the trim Motor). This switch is there for emergencies only!,
ie AFCS Trim Runaway. In this situation you push the trim release switch and this will stop an AFCS Trim Runaway. Unfotunately, It is on the preflight check list to check that the Trim Release switch is selected to ON. The poblem is that many pilots do not know whether ON is with the switch In or Out. And some...just don't check it! You would logically think that it it is IN. But, the good old French and Logic, well they kinda do not go together. It should be, would you believe, Out ie. flush with the others and hence the confusion :confused:.

May be, one day AS355 flight crew will consist of a pilot and his dog.
The dog will be there to bite the pilot for touching the 'Trim Release' Switch' :E.

OK so may be we want to put in a large and fast longterm attitude change. The auto trim will not cope with this. So for this we can temporarily switch the trim off (Known to the veterans as 'Floppy Stick') via the momentry push 'Cyclic Trim Release' button. Set your new attitude, release the 'Cyclic Trim Release' button and Bingo,.... Servos are sychronised and the aircraft now maintains it's new attutude :ok:.

Now for the 'Fly Through'. OK, so we have the AFCS engaged and we want to take evasive action around the microlight (the one without a transponder that didn't ping the TCAS) that we have just got a little bit too close for comfort to :eek: because we were too busy heads in messing around with the portable satnav aerial connector that just vibrated out due to the tracking and balance issue that we have been carrying since the annual..... well guys at this point, the force gradient spings and sensor in the trim motor knows you are fighting it ...so guess what...yep, it automatically switches off the autotrim until you are ready to carry on sorting out the Garmin! Clever stuff ayyy :D

And finally...Turn & Slip... (Back to basics on the old theory of flight here...:sad:) The SFIM 85T31 takes care of this just the same way as you should ....through the pedals...ie Yaw not attitude. It has a Yaw channel. This should be engaged. Again the preflight check list covers this, you engane Pitch, Roll & Yaw. If your Green Yaw Light will not illuminate when you press it, ... then It's broke! :{

So in summary for the type training guys.....
The Avionics Blokes AS355 AFCS Type Tips
1) The pupose of the Trim Release switch is for Emergency Disengagement of the trim motors.
If you have a suspected AFCS trim motor runaway. Push the Trim Release Button on the LHS Ohd switch panel.

2) Don't fly around with 'Floppy Stick'
(Trim Disengaged ie'TRIM RELEASE' Depressed :=

3) Don't fly around with Galvonomiters out of centre :=

4) If you operate the cyclic Trim Release or 'Fly Through' then
MONITOR the GALVOS. you need to look after the servo
synchronisation during these modes of operation.

5) But seriously, Always follow the Check List, SOPS and operate iaw with
the Flight Manual. and you shouldn't go wrong....

And finally, well, pllease always refer to your TRI for a demonstration and confirmation of the above. They do take the trouble to go the extra mile and find out this stuff and are constantly asking me awkward questions.

Hope this sorts out the confusion in some earlier posts.

Love and best wishes
...... your friendly local Avionics Bloke:8

[ShyTorque]

The 'N' I flew only had a floppy stick.

[Bladecrack]

Firstly this sytem DOES have automatic trim. It is operational all of the time and it's purpose is to sychronise (Keep Centre) the Primary AFCS (Automatic Flight Control System) servos. The position of which is displayed on the 'Galvinometers' on the instrument panel. The Galvinometers should be centred at all times! and indicate the position of the relevant servo.

If they do not centre then the AFCS is Un Serviceable. This is normally due to a failed trim system (Fuse is a good one because the airframes guy has put the wrong size fuse in ). However it is usually due to flight crew not knowing what the trim release switch does and that they have inadvertantly selected it to OFF . It could also be due to a stuck servo (Nothing that a tap with a hammer by your friendly local airframes engineer won't sort out. Well, at least for a few flying hours or so....)

avionicsbloke,

This is exactly my point, because if what you are saying is correct, and the autotrim should keep the galvinometers centred all the time, then I shouldn't have to re-centre them manually all the time by pressing the cyclic trim release. I will have a word about this with our avionics bloke to see if there is a fault in the system. (Which wouldn't surprise me as there was a very serious fault with the A/P when the helicopter was delivered, which took ages, and a new FDC to solve eventualy, and after I had got a few more grey hairs trying to recover from UA's while IMC!) I understand about switching the trim release off on the OHP of in the event of a runaway etc.

BC

[theavionicsbloke]

Shy
Your 'N' had a faulty trim release system then. This would have been due to trim release power defect, trim release switch / wiring defect etc.

Blade
Yes, you were and are spot on. The only exeptionto the rule is if you carry out a rapid turn, climb or decent eg: 60 deg bank angle. Again (As expected) the trim system will not cope and here you may have to momentarily press the trim release switch on the cyclic to effectively resychronise the system.

However, normal flying (Rate 1 turns etc) you should be able to engage higher modes (HDG, ALT HOLD etc) and fly complete hands off without any problem at all.

[ShyTorque]

Shy
Your 'N' had a faulty trim release system then. This would have been due to trim release power defect, trim release switch / wiring defect etc.


No, it was de-modded by removing all the stab equipment to reduce the weight for police role equipment. We flew it for four years as a floppy stick-er. Our "loaner" aircraft had stick trim as per the original specification. It was interesting to fly, especially at night or in poor visibility.

[oldbeefer]

Having checked the manual, my comments refer to thre SFIM AP85 T31 as fitted to the 350s.

[Avtrician]

In older choppers such as the Hughey, the trim switch disengages the mag clutches of the force trim. This (as previously said) allows repositioning of the stick to a new "Neutral" position. The force trim really provided an artificial feel due to large springs, as with out the feel it was easily possible to over control the aircraft due to the powerful hydraulics.

With other airframes I dont know, as I havent worked on them.