Autopilot question

[Tonic Please]

Hello rotorheads.

It just occured to me to ask a perhaps blindingly obvious question...

Do helicopters have autopilot?

I know nothing about nothing about helicopters, but I would like to know the answer nonetheless.

Thanks! :)

[rotorspin]

Welcome to the Professional Pilots Rumour Network

A quick search on google would reveal your answer and probably stop you getting the barrage of abuse that such a dumb question from a professional pilot is about to receive.

:ugh:

[Tonic Please]

Won't be going down that route, thanks.

Guess I'll wait for this to be deleted.

[NickLappos]

If you cant ask a simple question without getting abuse, pprune must be a pretty crummy neighborhood.

Yes, the top of the line helos have autopilots that rival those of airliners, and the SAR helos are even more sophisticated, with auto-hover and precise hover position control.

[Tonic Please]

Thank you kindly.

He got a PM. Not to worry.

[What Limits]

Some helicopters have nothing.

Some helicopters have a Stick Trim

Some helicopters have Stability Augmentation Systems

Some helicopters have Autopilots

Some helicopters have Flight Directors

Some helicopters have Flight Management Systems

Suggest you google these terms to find out a lot more about them. Enjoy!

[Tonic Please]

Thanks, I will.

Regards

[myassisgrass]

Recent heated posts in a Job Advertisement site (see http://www.aeroads.ca/cgi-bin/employment/search.cgi ) seem to indicate that it is much easier for JAR licensed ATPL pilots to convert their license to a Canadian ATPL license and then find work in Canada as opposed to non-JAR licensed pilots (read Canadians and Americans) converting to JAR licenses and finding work in Europe. I agree that a job website is not the place to discuss this and thought PPrune would be. Anybody out there with first hand experience and willing to comment on their experiences?

[rotorspin]

Tonic Please - thanks for your PM calling me a grumpy git...I tend to agree :}

It is infinately boring for me and others when an incredibly stupid question is asked, and yes it makes me grumpy...

I could understand it if you didn't have internet access so couldn't research answers to your questions first but you do otherwise you wouldn't be on pprune.

And now my rantings have made it to the top of the forum which will distract from good topics on rotorheads :eek:

Off to make a strong brew...

[TRC]

It is infinately boring for me and others when an incredibly stupid question is asked.......

It's not as daft a question as "Can a helicopter fly with one blade missing ......"

[O27PMR]

TRC - Good point! That surely tops the dumb question list..?:ugh::ugh::ugh:

Where better to ask a helicopter related question than a helicopter pilot forum?:confused:

If you think the question is dumb, just ignore it and move on. There's no need to give the guy a hard time...

PR

[HeliComparator]

Well I think its a perfectly reasonable question. Its a question any beginner pilot might ask (OK TP is not a pilot but so what) and an area often misunderstood by non-heli pilots, and even by pilots like WhatLimits who quotes several items of equipment that are not autopilots (Stick trim, flight directors, FMSs).

What is often misunderstood is that with suitably equipped helicopters, the autopilot is normally engaged some time before takeoff and not disengaged until after landing or perhaps until shutdown, unlike fixed-wing where
the autopilot is only engaged when the pilot wants to take his hands/feet off the controls in flight.

This is because of the inherent instability of a helicopter versus a plank (fixed-wing to you!). The most basic autopilot has SAS (stability augmentation system) whereby the system makes control inputs to resist rates of pitch, roll or yaw, thereby giving a feeling of artificial stability. It could be argued that even this is not an autopilot because it does not allow the aircraft to be flown hands/feet off - the aircraft will still want to fall over, just more slowly than it would without the SAS. When the pilot is flying hands-on, SAS mode is there to make the aircraft more stable. In large helis, if SAS is not working this would be considered a significant failure especially in cloud.

The next level of autopilot (sometimes called ASE - automatic stability equipment) comes into effect when the pilot lets go of the controls. It has attitude retention, ie the aircraft can be trimmed to a desired attitude (in terms of pitch and roll), which it will try to retain, and usually these have some sort of heading hold on the yaw control when bank angle is near zero and/or feet are off the pedals.

Then the pilot can select what are what are called the "upper modes" - such things as pressure altitude hold, radio altitude hold, IAS hold, selected heading hold, coupling to Area Nav (FMS), ILS, VOR etc, and the cleverer ones have altitude acquire where the pilot preselects an altitude and the aircraft will climb or descend to achieve it, and/or a pseudo ILS function which allows ILS-like vertical and lateral guidance using GPS data to a waypoint in the FMS.

Some aircraft may have SAR (Search and Rescue) modes such as predefined search patterns, automatic guidance from the cruise to the hover over a point defined in the FMS either by previously over-flying it, or by moving a marker over it on the radar screen, automatic hovering, including the ability to input a fixed lateral and longitudinal velocity (eg to allow hovering over a drifting ship), ability for rear crew to modify the hover position (they can see under the heli when the pilots can't), and automatic transitioning up from the hover to cruise flight.

Modern autopilots can also include such things as controlling the rotor rpm in the event of engine failure, reducing collective if the pilot demands excessive power etc, and generally are at the heart of digital aircraft systems, and typically duplexed or quadruplexed (ie there are 2 or 4 separate computers to cater for failures).

So definitey not a stupid question!

HC

[Tonic Please]

Wow.

A great post. I have always wondered it and since I have equal reasons as to why I would have said rotors do and don't have AP, I was completely oblivious.

I don't use google when I can ask professionals.

I understand about stupid questions - I have them in French, Jazz Piano and English teaching... I do have a PPL with 110 hours, (proudly got the higest past mark in the club too)...

Thanks again for the detailed answer. That's all I really wanted in the first place!

But this is proon after all.. :E

[the coyote]

Rotorspin, just a friendly observation:

I would hate to be your copilot if your response and reaction to a question is based purely upon how smart you think it is, rather than the content of it.

When I read this thread, you are the one that actually sounds stupid mate, not him.

Slow down on the coffee man.

[What Limits]

So the misunderstanding regarding autopilots is more widespread than expected.

The basic question was about autopilots. Its my experience that people think that the autopilot does everything including sexual favours. Not quite right.

The autopilot is a mechanical clamp that has two modes, on and off. The position of the autopilot can be trimmed and has several possible inputs.

There is no such thing as the 'upper modes' of an autopilot. There are as many sources of trim information as you like but the typical controller is called a Flight Director. The aircraft can be flown with the FD providing steering information but with the AP disengaged.

Flak jacket on, cover being taken, coffee being drunk.

[rotorspin]

can't believe you are all still posting on this...

new day, off to make another brew..

:ok:

[HeliComparator]

WL
Perhaps you fly a small heli with only basic SAS (hence just an on-off switch) but I'm sorry you lose credibililty when you say ther is no such thing as "upper modes". Unless that is I have been mistaken since I flew the the SA330 in 1981, then AS332, the EC225 all of which have them, as do EC135 onwards, S76 (with the right fit), S92, AW139, EH101 and lots more. (In truth I can't remember if the SA330 did!).

A Flight Director is not an autopilot - its an indication system to tell the pilot how to move the controls (more precisely what attitude is required) to achieve the target of the upper modes engaged. The FD is controlled by the autopilot and as you say, in equipped aircraft the autopilot can be routed to the FD instead of the flight controls - the pilot effectively then becomes part of the autopilot. This is typically to cater for failure modes where the autopilot's ability to move the controls is lost.

HC

[whoateallthepies]

WL
Sorry me old, "upper modes" certainly exist on the EC225. And they fly the damn aircraft a lot better than I can!
http://i.1asphost.com/whoateallthepies/pie.jpg

[Ioan]

In defence of Tonic Please, I've learnt a lot here! Don't forget there's lots of professional heli pilots flying robbos, 206s, etc who would have no reason to know the details and capabilities of advanced autopilots either!
"Modern autopilots can also include such things as controlling the rotor rpm in the event of engine failure, reducing collective if the pilot demands excessive power etc"
That was someone I never realised for starters.
NOW it's time for me to go google the details!

[NickLappos]

The distinctions being made here between FD's and autopilots, upper modes and such are arcane - the definition is not standard, so all comers get a vote on what Autopilot means.

The FBW Comanche had every mode conceivable, including automatic targeting, load factor enhancement, decoupling of rotor modes, control limit clipping for structural envelope purposes and range optimization scheduling of rpm. THERE was an autopilot!

[Overt Auk]

Maybe I'm dreaming.

It would appear that HeliComparitor and Nick Lappos have contributed to the same thread and not only not fallen out but agreed with each other!!

OA

[Fareastdriver]

HC
The 330G autopilot was single channel with big switches, just to jog your memory. The 330C had (probably still has) press buttons like the 332 but still single channel.. The nice thing about it was that it had was a yaw trim, a recessed wheel that would trim out the yaw in the cruise correcting the annoying habit some Pumas have of flying half-a-ball out. If you were really bored you could trim it out in the hover and do spot turns with it.
Then there is the full SRIM 4 axis autopilot, that's something you can play about with for hours.

[212man]

HC, I'd say your description of the FD/AP relationship is reversed: the FD tells the AP what to do, or the pilot, not the AP tells the FD what to do.

ECF do not use FDs so their functionality is incorporated into the AP (in a more appropriate way, I think), however with a/c that have separate FDs, they are independent and can function autonomously from the AFCS system. Take a basic S-76 A model, for instance, with a phase 2 SCAS: no coupling at all, but all the FD info for the pilot.

[kiwi chick]

Well, I have a CPL (H) and I never knew ANY of that ****. :D

So there you go, Tonic Please - it sounds like heaps of us have benefited from your "stupid question".

Keep it up, I could definitely do with learning more! :ok:

(and Yes, I DO google, but with the 1,657,875,998 pages that you get in response, I can certainly see why he came in here and asked.)

[[email protected]]

OK then, for a 'proper' autopilot question - is anyone familiar with the term 'attitude clamp' regarding stab/autopilot systems?

[What Limits]

Yes, Sir. I refer the Honourable Gentleman to my previous post.

Off to make another cup of credibility. (Which aisle is that in Sainsbury's?)

[HeliComparator]

212 my experience of FDs is limited to the AS332L2, which was as I described. No doubt there are other ways to skin the cat and it could be as you describe - it depends on your terminology I suppose. Something has to move the command bars and whether you want to call that bit the FD or the AP probably depends on the manufacturer.

With my L2 heritage I call the FD just the bit the pilot sees on the AI, but it could be that the FD is the clever bit that works out how to position the command bars as well, and the AP is the dumb bit that can couple that info to the flight controls. Personally I would not call that bit an AP because its not doing anything clever, but as you say of course another manufacturer can use different conventions, and in the S76 case you cite it sounds as though the AP does the SAS bit and the FD does the ASE & "upper mode" bit?

And yes WL, it looks like I have to humbly withdraw part of my comment on your post:O

HC

[HeliComparator]

crab - no, but it sounds like the ASE-type thing I was referring to - ie you fly around with the stick trim release pressed, manoeuvre to a pitch and roll attitude, and then let go. The autopilot will then retain that pitch and roll. Is that what you are talking about (in your usual superior military style)?

HC

[[email protected]]

To clarify - on most autopilot systems there is a position transmitter or force sensing link to tell the autopilot not to oppose human pilot input. However, on the Sea King 3A there appears to be an element in the system which is referred to as an attitude clamp that seems to require a specific level of demand before it allows the pilot to override the autopilot.

This is most evident in the hover (not with cyclic modes coupled) where a small cyclic input doesn't actually change the disc attitude - so a larger input is required which, when the autopilot finally concedes control, results in a larger movement than was required. For the most part this is easy to work round but for precision work like deck winching can be offputting, especially when you are working hard - night for example.

This attitude clamp may be normal fare for autopilot systems but our documentation doesn't cover it - any expert knowledge out there?

[HeliComparator]

crab - sounds like a "dead zone" around the trimmed attitude. I have certainly not encountered that and it sounds horrible!

HC

[Tonic Please]

:ooh: :ooh:

Well, I'm glad I have got a debate going. It's all very interesting. Where would I have found this on Google?

Oh, I know.. It would have been a link to PPRUNE!!

:ok:

[NickLappos]

Crab,

I helped develop 2 generations of Sikorsky flight controls, and have intimate knowledge of these things.

The older SAS systems (where your specific comment resides) had no stick canceler system, so that the stability system actually "fought" all disturbances when the trim is engaged and the stick is pushed against trim. The CG trim wheel centers the inner loop actuators to help retain optimum control, and to minimize hardovers.

The S-76 SAS resists your stick motions, too, and it is not at all unpleasant, since the net feel is a soft, but responsive cyclic. In effect, the stick is less sensitive than normal, since the sas desensitizes it. The system was designed to require you to press the trim release button when you move the cyclic, this removes the attitude stability, at least, and reduces the "resistance" to a small rate damping.

One early experimental version of the S76 SAS II used stick movement switches to remove the attitude hold in the given axis when you moved the stick against trim, and when you returned the stick, the system would lock on the new attitude quickly. This command stick steering was so pleasant to fly that we tried to get the FAA to buy it, but the classic (and nearly worthless) stick gradients that the FAR/JAR require stood in the way, since this system had no gradients of any kind. We junked it a few weeks later. Interestingly, this command stick steering concept is precisely what the fly-by-wire sticks now do!

Later systems use stick motion transducers that tell the autopilot that you have commanded a rate, and so the autopilot allows that rate (it "cancels" the resistance to the detected attitude change, thus "stick canceler")

More modern systems have a full model of the helo inside their little heads, and they compute what a given stick motion should do and also what it should not do. The autopilot then gives a pure output that models the ideal helicopter. An example - all helos are cross coupled, so that a pure forward stick always produces a roll. The model following control system knows this, and automatically puts in the correct roll stick when you make a pure forward stick input. Result - you think the helo has pure sweet controls. These "model following" laws are now state of the art.

[Graviman]

Nick, do you know any papers on this written by Sikorsky? I'd like to learn a bit more about the design of these systems. In particular i'm curious how much of the model following system relies on rate gyros to correct either the model or helicopter, and how much hub flapback torque controls stick dihedral position.

[NickLappos]

Grav,

The American Helicopter Society has an annual forum where the various designers strut their stuff, with excellent technical papers and experimental work.

I suggest that you go to your nearest tech library resource and search the Forum publications for the last 15 years or so. The standard texts have a bit on this stuff (Prouty, Padfield) but I don't know that they peel too far back into the details.

The rotor characteristics are part of the "plant model" that is in the model following system, that is, the REAL behavior is cataloged by extensive simulator and flight test ("system identification" in the parlance of the controls engineer). When the model following system crunches out what the controls must do to make the machine obey the pilots request, it must subtract everything that the "plant" does that is wrong, so an accurate understanding of all the characteristics of the helo, main rotor, tail rotor, tail surfaces, engine characteristics are all part of the plant model. With this type of feed forward, the controls don't wait for a wrong motion to occur, they stomp out the improper response before it starts. We do the same thing daily, when we learn to adjust windage on a shot, or when we put a little more on the ball to account for some variable.

wikipedia has a nice set of write-ups, I might add this stuff to them. Note that controls are controls, thermostats have a lot in common with autopilots! I often use thermostats are a way to discuss how feedback controls work.

http://en.wikipedia.org/wiki/Model_predictive_control

Here is a web page by Joe Horn, who is a good controls designer (we worked together a bit on Comanche). He describes a model following control system, but unfortunately, he is a bit too advanced for where this discussion is. You will find most of the papers in the AHS data base similarly esoteric!

http://www.personal.psu.edu/faculty/j/f/jfh19/dmc.html

Here is the whole paper:
http://www.personal.psu.edu/dob104/papers/mfcdmc.pdf

and another. Fig 5 is a bit better for our purposes, the boxes to the right of the "ideal" (which is the model that the controls try to follow) are the subtractive plant model elements:

http://www.personal.psu.edu/users/d/o/dob104/papers/ahsairwakemfc.pdf

[[email protected]]

Nick - the older SAS system is what the Mk3 Sea King has, a simplex system with limited authority requiring pitch and roll trim wheels to keep everything as central as possible. There is a control position transmitter for the cyclic to stop the system opposing pilot inputs.

On the 3A, the ASE computer is allowed acces to the pitch and roll beeper trims on the hyd pack, thus removing the need for trim wheels as it can always beep the cyclic back into authority.

There are FSLs in the control runs to stop the ASE fighting the pilot and the dead zone that these give has been dramatically reduced in the last few years by reducing the tolerances of the microswitches in them.

The problem is that just making the FSLs by moving the cyclic isn't always enough to change the attitude - I believe the 'attitude clamps' are a further element which has to be overcome by sufficient demand (cyclic movement) before the ASE computer gives up control (briefly).

The handling qualities change dramatically when the auto-trim element is bypassed by selecting manual trim - this removes the ASE computer from the equation and reverts to a more Mk 3 mode.

[Graviman]

Thanks for the links, Nick. That info is spot on what i'm after. It's clear helicopter control systems are about as state-of-the-art as FBW gets.

Wikipedia is getting better on engineering articles like helicopter design. The trouble is that like most topics in engineering there is often a great deal to understand before you can document it. Also there are many good texts like Prouty, Newman, and Coyle which invariable become the reference. I'm suprised you don't document your experience - that would be an interesting read.

[NickLappos]

Crab,
That Sea King system is a home-grown British one from back when men were men, and so I don't have much info on it. The same folks who built that also did the tail rotor with an extra blade!

What you mention is a classic issue with older autopilots - either George is flying or you are. The systems I helped work on were always "fly through" autopilots where the pilot can gracefully enter the control loop at any time.

[[email protected]]

Thanks Nick, I was just wondering if the use of these 'attitude clamps' was widespread in autopilots but it seems it is a Westlands special:)