Trimmed Turns

[Bladestrike]

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?

[IHL]

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?

[rotordk]

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

[IHL]

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

2)Auto pilots always fly in trim.

[rotordk]

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.

[Bladestrike]

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.......

[[email protected]]

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.

[Bladestrike]

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)

[GLSNightPilot]

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.

[Nick Lappos]

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.

[Xnr]

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

[Winnie]

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

[Shawn Coyle]

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.

[212man]

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.

[Nick Lappos]

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.

[Xnr]

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

[Nick Lappos]

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.

[heedm]

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?

[Bladestrike]

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.

[GLSNightPilot]

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.

[Xnr]

Bladestrike

I told you that you've been doing it all wrong.....you old sea dog.....thanx for the info on the H/V charts and it still is a ball of wax.....Just for your info i asked an old friend at Transport about the 704 H/V curve and he said "don't ask"

Nick

If the S76 is u/s IFR with out a cyclic stick trim......what about guys who fly it IFR with there thumb on the FTR button or the cyclic force trim turned off? They will not get any force feel from the cyclic in either case?

PS I sent you an email on our Cat A problem.

[Nick Lappos]

Xnr,
The way you fly is not governed by the design FAR 29, have a ball with the trim button! You can use the techniques that suit you best when you fly (unless otherwise restricted by operational FAR), but when we certify to the requirements for the design, we must show that a means to provide force vs airspeed exists. On an S-76, I myself would not turn the trim off with the panel switch in IFR, because I then disable the Attitude stability and leave only SAS, which is less suitable.

GLSNightpilot points out a problem with flying against trim that I failed to mention previously. In most aircraft when you push the stick against trim, the inner loop actuators fight you with some of their authority (since the stability system sees this maneuver as an outside disturbence). In the S76 we limit the attitude authority of the actuators to half their travel, thus saving the other half for rate damping. Biased actuators in a staedy maneuver then reduces stability authority and makes it somewhat more prone to running out of authority in turbulence. In other words, if you ride against trim you can get closer to the point that turbulence can upset the aircraft. This probability is much more real than the possibility of disorientation causing you to need to let go and hope the aircraft rights itself and saves you against yourself.

Regarding the beeper hat authority, the S76 beeper is timed out so that the failure of that switch does not cause the AFCS to run away in a hard over. The upset at high speed would be too large, and the loss of the attitude hold would be guaranteed, so we put the beeper thru a timer to limit the runaway.

heedm points out ANOTHER reason why trusting the trim to save you when you give up and let go of the stick is not a swell idea. If the trim has a failure that causes it to run away (this can be a switch failure in the cyclic) then the stick center is wrong. If the pilot gets disoriented (as heedm did ) and then trusts the trim to get himout, his goose is cooked. I am not sure why heedm deduced from his experience with this failure that letting go of the stick was a good idea.

Nick

[Skaz]

hi all you chopper jocks, from an admiring 'seized-wing' pilot,

Obviously I dont have the in depth knowledge that you guys do, but after reading this thread I'd like to give my 2-cents worth

it seems the 'safest' way to go is trim the helo for straight and level flight, with the pilot feeling stick force in a turn, as a tactile feedback cue from the a/c systems re the rate etc of turn. Same with the speed issue, more speed = more force felt thru stick....uh, cyclic:} all the while keeping a scan going of the instruments to check the tactile feed back you get thru the stick corresponds to what you think the a/c is doing and this is confirmed thru the instruments...:cool:

or am I missing something;)

[GLSNightPilot]

Skaz, yes you're missing something. Reread the thread carefully. It depends on the design of the AFCS. Some, such as on the Bell 412, are designed as 'hands-off' systems, & any attempt by the pilot to move the cyclic other than through the trim switch or depressing the interrupt button, will confuse the system, & can result in unusual attitudes. Most S76 pilots I know fly the same way, not pushing against the force trim, but using the 'coolie hat' or interrupt button if necessary. I don't claim that's the only way or even the best way, but that's the way we do it down here.

[Shawn Coyle]

It really does depend on the AFCS design. French designed AFCS systems have a completely different method of operation in this regard than most N. American systems.
The problem is that none of them are particularly well described in any of the manuals.
So, most people don't understand them, and lots of old wives tales (hows that for both age-ist and sex-ist?) develop.

[heedm]

Nick, when you find yourself in a UA, you know something went wrong. What were you doing prior to the UA entry? Might be that whatever you were using was what got you into trouble. Then again, maybe it wasn't. In the recovery you have to use everything at your disposal, but you might not be able to trust any of it.

That's what I did. I trusted the stick force. Why? During the numerous UA recoveries I did prior to that the stick force worked. Without fully realizing it, I learned to trust the stick force to help find a stable flight regime. Had I not trusted it, I still would have made reference to it but been more diligent in observing other clues during the recovery.

The situation I was in was fairly compounded. Partial panel so I already was without some attitude indication, runaway trim, so I had that to deal with, then a simulated UA that led to the real thing. Oh yeah, I had less than 100hours total time and Terminal was trying to contact us on guard because they vectored us into the Snowbird's training area, then cancelled our IF clearance and switched us to a traffic frequency. A little bit busy.


I wasn't advocating letting go of the stick. I say fly her to the ground. What I do advocate is, if able, leaving a trimmed position that you can feel while hands on flying. Try it...it works.

[Nick Lappos]

heedm,
Thanks for the explanation, I understand.

I have always taught to roll wings level, then pull up, using purely visual cues. One must learn to fight feelings, which in the slight cases we call the "leans" and in major cases can feel like you are housed inside a tornado. The trimmed stick recovery idea relies on the fact that you would use a perceived stick force to help you, but ignore all those other false forces from your body and inner ear. Neat trick, might work, but I put that into the same basket as the school that says every helicopter has LTE, and any hover descent can get you VRS. The "rules of thumb" that we get when we train are comforting and helpful, but somewhere along the line we learn when they can apply, and when to doubt them.

If the AFCS gives you good responses when you push against the stick, and you want to rely on that cue, have at it! Speaking for the S-76 design team (I helped design the AFCS and did much of the development flight test on both the SAS II, SAS III and Honeywell systems) we had no intention to use them as a wings leveler system, and would make no claim that they will help you. In fact, if formally asked, we probably would say that one should never rely on the AFCS as a UA escape tool, and that pushing against the trim will slightly degrade AFCS performance because it biases the actuators away from center and therefore slightly reduces stability peformance.

That being said, it is certainly not terrible to push against trim. There are many correct techniques in our world, have at it!

[Lu Zuckerman]

The troll speaks.

When Sikorsky first developed a stability control system it was originally installed in HSS-1 and eventually into the H-34. The automatic stabilization system was comprised of an autopilot originally installed in an F-86 fighter. There were some minor modifications necessary. The aux servo was equipped with electric motors that received signals from rotary variable differential transformers, which were attached to the flight controls (Collective stick, the cyclic stick and the rudder pedals. If any of these controls were used the RVDT would send an input to the auto pilot which in turn would command one or more electrical motors in order to displace the pilot valve(s) on the aux servo which would modify the disc attitude. The cyclic controls would be locked in place by magnetic brakes, which were in turn connected to the controls by force gradient springs. The pilot could make minor adjustments to the cyclic by moving the stick and compressing the springs. When the pilot removed his input force the cyclic would return to the original position.

If the pilot wanted to change the attitude or direction he would press a disconnect switch which cut out the autopilot and energize the magnetic brakes which allowed unfettered movement of the cyclic. By releasing the cutout the auto pilot would reengage and the new course would be maintained. There were similar cutouts on the rudder pedals. When the pilot wanted to set up a new direction (heading) he would move the pedals and in the process cut out the auto pilot allowing the directional gyro to assume and hold the new heading. By removing his feet from the pedals the course would be set.

If the autopilot detected a deviation from the established heading or the attitude of the helicopter it would send a signal to the aux servo returning the helicopter to the preset conditions. When the autopilot made a correction the flight controls would not move.

A similar situation existed in the collective as it had a barometric hold provision. If the pilot wanted to rise or lower he would disconnect the bar alt hold move to the new altitude and reengage the bar alt sensor.

Granted the system was by today’s standards antiquated. It had a lot of problems mostly human induced but the design philosophy was good. Why then have there been so many deviations from this design philosophy. The apache flight control system is similar but with more modern hardware and the EH-101 is a step backward in their selection of the means to maintain stability through the autopilot.

I anyone finds fault with my description it must be understood that I went to Sikorsky ASE school 48 years ago.

:confused:

[cpt]

Beeing rather new on the "76" I realize I cannot have a better opportunity than this forum, to ask some questions about its "standart" AP. My "shadow areas" (amongst others) are:
1/ When testing the coolie hat and the trims after start, we have to observe the deflections of galvanometers needles on the 2 lanes. That works well in roll, a little less in yaw but why do we have to apply "short and rapid movments" on the cyclic pitch to test pitch actuators ? (I don't have the flight manual with me and the terms are maybe not the exact ones)
I have never seen a deflection in doing this. Delections rather occur when maneuvring the coolie hat in pitch.
Is it something I have misunderstood in the flight manual? (english is not our mother tongue here, as you probably have already guessed !!!! :O )

2/ Recently our yaw damper went unserviceable, therefore with the "phase 3" capability, but we barely noticed any difference in yaw even with the "phase 3" selector switched on ( maybe must be switched "off" to operate the yaw lane "open loop"with less authority on actuators ? )


I personnaly make my turns with the coolie hat in night/IMC on the "76" but I rather do that against the trims on S365's. Once again thanks for your highly valuable explanations!

[Xnr]

When you are testing the pitch you are not testing cyclic movement....the AFCS gets its info from the ADI ....if the ADI does not change in pitch or roll axis then the AFCS channel will not move.

[Bladestrike]

What suprised me was the fact that alot of companies dictate that you do not trim your turns. Luckily I have the choice.

Xnr, call next time you're on the coast, I'll buy.