Okay, I have to preface this question by admitting a very guilty pleasure. I’ve recently been reliving my childhood and bought the first three seasons of Airwolf on DVD (the fourth season includes a cast change, network change, and the loss of the Bell 222, so all flight scenes used recycled footage - season four is godawful.)
Anyway, the show has Airwolf equipped with “turbos” that are essentially dual jet engines mounted on the wing stubs to allow it to go supersonic. Whenever the pilot wants to engage them, it shows a closeup of his hand on the cyclic control, and he presses a little red button. It looks like they used the stock Bell stick, without re-labeling anything, because the “turbo” button actually says “Force Trim.”
I’ve googled the term and found some references to the Huey and other Bell whirlybirds, but nothing that explains it in terms that I can grasp. One website makes it sound as if force trim simply adds a little (magnetic) resistance to the cyclic, which is usually very ‘loose’ (to the point of just flopping over if one takes his hand off it.)
The other, which sounds more plausible to me as a fixed-wing pilot, is that force trim allows the pilot to “trim” the helicopter by essentially zeroing the cyclic in a certain place - for example, in forward flight, he could push the “force trim” button and then bring the cyclic back to a neutral position, but the bird would be “set” as if he was continually pushing the stick forward. Seems that would be more comfortable, but I’m not a rotary wing pilot, so I have no idea.
So, dopers, anybody care to clear up the mystery of force trim?
Answer deleted because it is too basic now that I see you are a pilot. I assume that it has to be related to something like a trim system that we know on airplanes or maybe they just made it up.
I’ve only flown Robinson R-22s and Schweizer 300CBs, so I’ve never encountered ‘force trim’ buttons. The Robbo has ‘right trim’. This is a little knob that is pulled up to, as the name implies, add right trim to the cyclic control. Basically it’s a bungee. It’s used in cruise flight to reduct the tendency of the cyclic to pull to the left. The pre-landing checklist is ‘right trim - off’. Really high-tech! (Incidentally, the right trim is just to the left of the mixture control. The mixture control has another piece of high-tech engineering: a small plastic cylinder that fits over it so you don’t pull the mixture to idle cut-off when you mean to pull on the right trim!) The Schweizer has a proper cyclic stick as opposed to the R-22’s T-bar. On the top of the grip is a ‘coolie hat’. This activates electric trim, which is much superior to the Robbo’s two-position bungee. Both helicopters have friction knobs to increase or decrease the amount of force needed to move the controls.
I found this site that has this to say about force trim:
It sounds as if force trim sets a new ‘neutral position’ to the controls. I can see this being helpful on a slope. Let’s say you’ve got one skid on a slope. You want to keep a little pressure on the skid so that you don’t fly away from the slope, but you also don’t want to input too much uphill cyclic or else you may get into dynamic rollover (basically getting the helicopter over-center and causing it to roll over). So you’re holding pressure on the cyclic. ISTM that if you’ve got the cyclic where you want it you can turn on force trim so that you’re not feeling the pressure you’re holding.
But as I said, I’ve never encountered it. Piston-engine helicopters tend not to have a lot of the goodies you get with jets.
IANA helo pilot but I am a fixed wing pilot & have a few hours at the controls of OH-58s & UH-1s. A mis-spent youth if ever there was one.
At any rate, force trim is exactly as Johnny L.A. says. It performs the same function that ordinary trim does in fixed wing aircraft; neutralize the forces on the controls at a particular point in their travel.
On large jets (& pre fly-by-wire jet fighters) the rudder & aileron trim systems operate using the same basic mechanism, applying adjustable spring tension against the control linkages at some point. Pre fly-by-wire jet fighters used a similar system in all 3 axes, while large jets use a movable horizontal stabilizer for pitch trim.