Rotary wing trim?

[alvin-sfc]

While watching a helicopter flying over today,I wondered if they have trim tabs or something similar, and do some have automatic pilot? My flying experience was fixed wing only (obviously) Please feel free to take the p**s if you think this is a daft question. :confused:

[Whirlygig]

Some larger, twin-engined helicopters may have auto-pilot but a small, piston-engined helicopter only has trim controls to the extent that some stick forces can be alleviated; four limbs, four controls and you don't want to lose contact with any one of them for more than a second!

Cheers

Whirls

[muffin]

Auto pilot is quite rare, but many do have a trim control. On the simplest small helicopters the trim is a simple on or off device which just takes the main load off the cyclic control during cruise flight. There is a coarse adjustment but you can only set it on the ground to what you think may be the correct offset, then go fly and try it. My experience is that it is never right, so you always have a control load in the cruise. Makes your arm ache after 30 mins or so, so you try and hold your right forearm between your knees to keep going straight and level. Flight endurance limit is nothing to do with fuel, there is usually plenty of that - either bladder or arm ache gets you first!

[Whirlygig]

so you try and hold your right forearm between your knees to keep going straight and level.

I'm struggling to visualize this :}.

The Schweizer has a little hat on top of the cyclic which can be moved in four directions to remove some of the stick forces through the cyclic in flight. I fiddle with it all the time; not sure that I need to and I sure know when I've got it wrong :ouch: but it does make for a more relaxing flight; therefore, my endurance is inversely proportional to the number of cuppas I've had!

There is a friction control which is supposed to keep the collective where it was left but I find that I still lose height when I'm changing frequency for example. You could take your feet off the pedals in straight and level flight but I wouldn't recommend it :eek:.

Cheers

Whirls

[Whirlygig]

Run that by me again ShyT :confused:


reduce speed to 70 kts


Cheers

Whirls

[davidatter708]

What is the rotor trim??

[Whirlygig]

Not heard of that on a Schweizer, nor any other helicopter but that doesn't mean to say it doesn't existor isn't known by another word!!! Where have you heard that?

Cheers

Whirls

[airborne_artist]

ShyTorque - What would you class the SAS on the RAF and RN Gazelles?

[Fg Off Max Stout]

To go back to the original question, the terms 'trim' and 'autopilot' with reference to helicopters are not directly equivalent to those terms when used on aeroplanes.

On a simple aeroplane, trimming reduces the control force to zero at a required control position, aerodynamically by movement of a trim tab. Control axes that lack a pilot controllable trim tab, often roll and yaw, may have a fixed trim tab which can be adjusted by engineers on the ground but is not controllable in flight.

Helicopter rotor systems often suffer from vibration which can have severely detrimental effects on the crew (the mode of vibration at the resonant frequency of the eyeball is an interesting one) and the airframe. Each rotor blade will have a fixed trim tab and tip weights. These can be individually adjusted to minimize vibration in a fairly complex process known as rotortuning. A helicopter will not have trim tabs like those of an aeroplane though.

Control forces are normally trimmed out mechanically, eg with spring forces, not aerodynamically. A normal, well equipped helicopter will have a coarse trim button or cyclic trim release, which allows the pilot to set the trimmed position of the cyclic, thus trimming out roll and pitch control forces. The yaw pedals will often be trimmed similarly. Cyclic trim will also have a fine adjustment through a 'cooley hat' on the cyclic. Remember that these trims reduce control forces mechanically not aerodynamically. Additionally, because all but the lightest of helos have non-reversible hydraulic flight controls, aerodynamic forces in the rotor system cannot be felt in the flight controls anyway. The collective lever is normally friction locked.

Most helicopters tend to be naturally unstable, and so to reduce the workload required to handle them most will have a system that augments, or modifies the pilot's control inputs to improve handling qualities. These systems vary from type to type but are known variously as the stability augmentation system, SAS, stabs, autopilot, AP etc, etc. More advanced helicopter APs will also include 'aeroplane like' autopilot functions such as heading hold, speed hold, alt hold etc. The most well equipped helicopters will have full autopilot systems roughly comparable to those on airliners, that can follow flight plans and fly instrument approaches.

So in short, helicopters do have tabs and APs, but not in the same sense as GA aeroplanes. Don't start me off on twin rotor peculiarities or horizontal stabilisers on helos or this could go on for a long time.

[Whirlygig]

The yaw pedals will often be trimmed similarly

I wish!!! You should see the size of my thighs :eek:

Cheers

Whirls

[Pilot DAR]

For Alvin's benefit,

One more thing which makes helicopter trim systems different than those of fixed wing aircraft (at least for the helicopter types I have flown) is that the helicopters lack trim position indicators. The result is that before takeoff, you cannot check the trim postitions. During takeoff, the trim may not be centered, and you can surprise yourself. This is particularly the case if you landed the last flight with a lateral C of G off to one side, and the next takeoff is not the same loading.

Pilot DAR

[alvin-sfc]

Thanks guys.:ok:

[davidatter708]

what about rotor trim though

[IFMU]

I wish!!! You should see the size of my thighs
Sounds like we need a picture!

what about rotor trim though

I'm not quite sure what you mean by this. Most trim in helicopters is similar in concept to fixed wing, you trim it until the stick forces are low/gone. Usually it's called cyclic trim. Both big and small helicopters use springs to pull against the stick, usually with an electric trim motor, so you can push against trim ( the spring) to manuver. Then when you are back to S&L flight, or whatever condition you were trimmed for, you are still trimmed.

If you are talking about tracking, then that is a different topic. Blades are first ajusted on the ground with variable length pitch rods, so they are all about the same angle when cyclic is centered. Adjustments are made to both the pitch rods and bendable tabs on the trailing edge of each rotor to make them fly in track, which is to say each blade takes about the same path through the air. Once this is set then it is generally good. Bigger helicopters also use chordwise movable weights to adjust pitching moment.

-- IFMU

[ericferret]

The Dauphin has a rotor speed trim switch. This has a specific purpose and is only used during single engined flight.

It allows the pilot to reduce the rotor speed to the optimum for single engined flight.

It works the same as all other engine trimming systems by reducing engine rpm.

[Akubra]

Interesting thread!
Is this a tracking tab thats adjusted on the ground only then?



http://i119.photobucket.com/albums/o131/acubra/tab.jpg

[ericferret]

That is not a trim tab it is the cyclic control output on a Kaman (Seasprite or similar) I believe.