Trimmed Turns
 

I've always flown my turns with the trim hat, but I've recently been told this is a no-no. Apparently the CAF wants their aircraft trimmed for level flight and turns are made holding the cyclic against the force trim springs. Is this a standard I've been unaware of?

I quess if your in the CAF then you do it the CAF way; but if your not in the CAF and what you have been doing works fine, then really who cares?

Yup, idea is to prevent inadvertent loss of control. Vertigo hits you, you release pressure, and vupti, it's back into straight and
level mode. Works nice for night rigs app. And makes your
co-joe happier :-)

BTW Thanks for the drinks in Stavanger

Two points:
1)What are the chances of getting ( vertigo) srewed up in a 2 pilot cockpit ?

2)Auto pilots always fly in trim.

I said the idea behind it .........( wasn't that the question ? ).

1) I agree. But easier for the other guy ( if both know that
the A/C usually is trimmed for straight + level ). On a day
to day basis it works fine.

2).....night rig take off is a more probable scenario
until reaching Vtoss. Part of the no no regarding trimmed config.

No worries re; the brews, and thanks for the replies

-No autopilots on this bird,

-No, the question wasn't the idea behind it, but how common is the practice. Are there many out there that trim your turns?

-I appreciate the argument, but if I'm trimmed into a rate one turn and get vertigo, I'm still trimmed for that rate one turn. There's not much chance of losing control.

-On a night rig departure, I doubt there'd be much turning prior to Vtoss, but I do control pitch with the hat as well and I could see the benefit of having the nose come back up to level if something were to go wrong.

hmmmmm.......

Bladestrike, the British Military have always taught not to trim into turns for the exact reason that rotordk mentioned. I take your point about 2 pilot ops but we operated a lot of single pilot helos in the past and I guess thats where the reasoning came from.

I am curious as to the intent of the design of the system. Does it come down to personal preference or was the system designed to be utilized in a certain way?

If you are turning against the force trim and not retrimming with the FTR or the hat, aren't you saturating the roll channel and cancelling out any stabilization in that channel for the duration of the turn?

(Unless you're in an AS355, where I believe the AFCS recenters around the cyclic)

Which aircraft? What configuration? IMO it makes a difference.

In the B412 & S76, I use the hat, or the interrupt button. Flying the 412 especially, against the AFCS, is counterproductive, at least in ATT mode. That will get you into all sorts of trouble, ESPECIALLY if you get vertigo. You completely confuse the AFCS, & if both of you are confused you're in a heap of trouble. Bell says it has to be in ATT for IFR, & every night takeoff in my aircraft is in ATT, because it's IMC as soon as you clear the platform helipad, or the ship. I simply consider night as IMC all the time.

OTOH, if you're in something without AFCS, then the CAF technique might work, but I still prefer to keep the aircraft trimmed all the time, whatever the attitude. If you have vertigo, a little pressure from the cyclic is not going to put you straight & level, no way, nohow.

Some thoughts:

Autopilots have two general philosophies that serve this issue. The type that Sikorsky generally builds uses a "fly through" concept, where the pilot can move the stick against trim, and some degree of smooth acceptance by the autopilot keeps things stable. This requires attention in design and development, usually by turning off the computational integrators in pitch and roll. Integrators look for an unsatisfied command and increase that command over time to build up a larger correction. This forces the aircraft back to trim over time. In a fly-through system, stick sensors detect your stick movement and shift the stability system integrators off, so the system behavior is acceptable.

On non-fly-through autopilots, the integrators are never turned off, so if you move the stick, the autopilot sees you as another form of outside disturbence, and works up an ever-increasing correction to wipe your input out. In such a system, if you push the stick and the system works up large corrections, when you let go you can be in for an awful experience as the big correction drives you wildly into an opposite maneuver, at least until the integrated correction gradually washes out.

Some specific examples of each: Fly-through is an S-76 cyclic, any version, not coupled. Non-fly-through is a coupled S-76 with the Honeywell autopilot.

The idea that you have a "wings leveler" system to help you out if you are disoriented is probably one of those bright ideas that looks great in the snack bar, but serves little use in the cockpit. If you are truly disoriented, the last thing you will do is let go of the stick! In fact, the first thing you will do is distrust the machine, and search for the fault - the classic cockpit voice recording is "My gyros have tumbled, all of them!"

That being said, the typical problem in disorientation is seldom an attitude problem, it is a sneaky descent or geographic disorientation that cause CFIT (controlled flight into terrain). The emphasis on "Controlled" bears discussion. It is the cumulus granite cloud that gets us, the ground that's not supposed to be there.

Want to survive disorientation? Trim the cyclic any way you want, then buy an EGPWS.

Nick

A quick question....

If you are to use the cyclic trim "anyway you want " and not as a "wing leveller" in IMC why is an S76A not certified for IFR flight if the cyclic trim is U/S?

I use it as a wing leveller and fly against it in uncoupled IMC flight.

This is probabaly the only time I agree with the CAF.

Cheers

Xnr

For IHL:
You were asking about vertigo in two pilot crews
well I think there has been a lot of instances where just that happened, not to me yet, since I only fly single engine, single pilot VFR for the time being. Look at the statistics on the crashes that do occur in bad weather where CFIT is the cause, couldn't Vertico/loss of Situational Awareness be one of the issues leading to it in the first place?

Just for some thoughts anyway...
:D

It's more subtle than that.
I believe the concept arose from the UK and specifically the Royal Navy.
The RN helicopters that were used for over-the-water, night dipping had an AFCS that would let you roll into a turn using just lateral cyclic, but with the requirement to hold lateral cyclic into the turn to maintain the bank angle. The only way to get the AFCS to maintain the bank angle without lateral displacement and pressure was to trim into the turn - something the pilots were told not to do.
The rationale was as early stated, that if you got vertigo, just let go of the stick, pull power and you would stop going into the drink.
The problem comes in the configuration of the AFCS. Most AFCS will let you roll to a bank angle with only lateral cyclic, but then maintain the bank angle with the cyclic laterally centered, just like a helicopter without an AFCS. In this type of machine, you don't need to hold any displacement or force to maintain a turn, and you don't need to trim into the turn.
Having done a small experiment with some unsuspecting operational pilots on two different types of helicopters - RN Sea King with the hold-cyclic-into-the-turn, and the Chinook with the cyclic is centered when stabilized in the turn - (and having RN pilots fly the Chinook and RAF pilots fly the Sea King), neither one noticed a particular difference in the way the helicopters handled when doing simulated IMC.
The lesson?
Know the AFCS, and don't blindly follow what others give you as advice and ways to do things.
Personally, if I was doing a lot of IFR / IMC, I'd prefer the AFCS that requires lateral displacement / force for a turn. You're not doing continuous turns (typically only 180 degrees of heading change is required), and it is very easy to get the leans in most helicopters.

On APs with Yaw coordination, using the roll trim may cause problems too. Generally they allow small roll displacements using stick force (say up to 4 degrees), but maintain heading, and larger roll angles result in a coordinated turn. If you simply use roll trim it will still try to maintain heading and you'll end up out of balance, which is a good start to getting the 'leans', and will require additional input (pedals) which rather defeats the object.

If you use the trim release button you will also have cancelled the pitch trim datum too. I personally was taught not to trim into turns. I was also taught (at the FW stage) the importance of having the a/c accurately trimmed and have found these maxims have stood me in good stead for the IFR stuff I done. The whole philosophy of flying IF should be to reduce your flying workload as far as possible.

Xnr,

This thread sounds like the coffee talk around the Sikorsky Pilot's office, where the Army pilots laugh at the Navy pilots about this subject. Since I was Army, I am duty bound to say "Pshaw!" But really, there is nothing very wrong with fighting trim when turning, if you don't mind fighting the aircraft whenever you maneuver. In the Army, we were taught that you have the most fun when you make your partner WANT to maneuver, if you get my drift! ;)

Regarding the S-76 trim and IFR limits, the reason the trim is needed is that the IFR requirements demand that there be a stick force vs speed cue to the pilot, so that any increase in speed causes a rearward stick force to be felt by the pilot. This force comes from the stick trim, so no stick trim, no IFR.

Nick


You wrote:
"Regarding the S-76 trim and IFR limits, the reason the trim is needed is that the IFR requirements demand that there be a stick force vs speed cue to the pilot, so that any increase in speed causes a rearward stick force to be felt by the pilot. This force comes from the stick trim, so no stick trim, no IFR."

Is this not a function of the PBA?

Cheers

Xnr

Xnr,
The PBA helps the stick move forward with airspeed increase, but that movement only produces a force if the trim is on, and the FAA asks specifically for force for IFR flight.

I was doing partial panel IF Unusual Attitude recoveries when my instructor gave me a runaway trim. The training UA turned into a real UA rather quickly. Ended upside down, vectored to the ground approaching 300kts (it was a jet not a helo and we were VMC).

The reason I got so screwed up is because I was used to recovering an aircraft that was trimmed for a safe flight regime. I wasn't letting go of the stick, but I was feeling where the stick was neutral.

With that in mind, I think leaving the cyclic trimmed to a level attitude position is helpful after disorientation (unless your AFCS can't handle it).

__________

GLS, Agree what you say about the 412 in ATT mode. I'm used to flying against the trim springs and from what I've read I should be able to do this in SAS mode on the 412. Every time I try it I get a caution light (not sure anymore if it's an Autotrim Light or an AP Light). Any idea why?


_______
Also, what is CAF?

CAF, Canadian Armed Forces, we have alot of ex-mil guys at our base.

Thanks for all the replies guys, and the aircraft I've flown trimming my turns are the 222, 76 and 61.

Yes, in the 412 you should theoretically be able to push against the trim while in SAS mode. You get the light because the AFCS reaches its limits rather quickly when you do. I tend to fly the 412 in ATT all the time, because it's required to be in ATT for IMC, & at night it's all IMC to me. I don't push against the trim in either SAS or ATT. I use the interrupt button most of the time, because the coolie hat is so slow in ATT mode. You just can't keep up using the coolie hat alone, at least I can't, unless the air is very smooth.

In the S76, the hat quits working much too quickly. It's very quick, & easy to use, but you have to use the interrupt button fairly often, especially in the roll channel, because you reach the limits of the servos & pushing the hat has no effect. Most S76 pilots I know keep the indicator in the roll position, so they can see when the effectiveness is about to be lost, or more often, verify that that's already happened.