Anti-torque pedals and SAS's

[nickel]

Hi,

I am not a pilot but rather an helicopter enthuisast interested in RC flying and computer simulations of helicopter.

I'd like to know about the contribution of stability augmentation systems in modern helicopters. In MS flight sim 2004 the Bell helicopter was pretty hard to fly because of the need of constant anti-torque correction. In the new version of the flight sim its behaviour is completely different. When I change the collective the helicopter turns just a little but then stops turning without my pedal input as if the control system does the job for me 100%.

I've read many times in internet that the pilot has to play with the pedals any time collective is changed (and this is my experience with RC helicopters even though they have a simple control system that helps).

So the question is how is the real thing? I'd actually think that modern control systems can almost perfectly handle compensating the torque reaction. Is this the case?

Thanks in advance for the answers

[RINKER]

Nickel
I fly models and fullsize helicopters,from experience on R22, R44 ,B206 and SA341G I know these aircraft don't have stability or mixing in relation to the tail rotor.
A properly set up model will climb or descend from the hover without any yaw with no rudder input from the pilot as the gyro in conjunction with the up and down mixing will sort it out for you as I say if it's properly set up.
In the 'real thing' because you are sitting in the machine when you lift the collective or lower it you immediatley feel the yaw and compensate with your feet and in fact with experience you compensate before the yaw even starts as you learn to anticipate it so it becomes automatic.If I change from clockwise rotation to anticlockwise rotation aircraft ie R44 to SA341G I might have to slightly remind myself, or if not the aircraft or my instructor will ! .
So in answer to your question , no the torque is not compensated for automatically the pilot does it.
I can't vouch for larger aircraft but I think for example the S76 has some sort of mechanical mixing and I think the notar machines have some electronic mechanical mixing, maybe someone else knows.
If you haven't done so already I suggest you book yourself a few lessons with an instructor and try the real thing for yourself it will be the best money you ever spend IMHO.
R

[Blind Bob]

I flew the westland lynx in the past, and it had an AFCS which had barometric altitude and heading hold functions. Remove your foot pressure from the pedals and the AFCS would maintain the aircraft heading regardless of the aircrafts direction of travel. Quite handy for making you look good when lifting into the hover! or flying sideways down the runway. There was also a knob on the control panel for minor heading changes (normally less than 10 degrees) but it was interesting for the pax if you decided to more, skidding around the corner in level flight! And then came the landing......not a twitch from the tail......we were professionals.:O

[Graviman]

Good thread! I have often wondered about SAS on tail rotors...

Mart

[Aser]

You can add to the list of "feet-less pilot" friendly helicopters the AW-139,
You can take off, fly, and land, without using pedals. ;)
There is a hat switch in the collective to change heading in hover if you want, anyway I like to have my feet on the pedals when in T/O hover or landing...

[Matthew Parsons]

SAS means "stability augmentation system". They are designed to reduce angular rates (pitch/roll/yaw rate). This makes the helicopter less likely to move quickly from a desired flight condition. SAS doesn't account for control cross coupling such as yaw response due to collective input.

Sometimes you will see control mixing where a collective input will add approximately the right amount of pedal. Pilot input is still required, but smaller inputs.

To create a system that produces no yaw with collective input requires quite a bit more effort and expense. The helicopter needs to maintain a heading, so a reliable heading source is required (gyro=more expense). The inputs to the control system would be larger than a SAS would require, so now you need control actuators that have large authority (actuators=more expense). The logic of the system still needs to know when you do want to change the heading, so you aren't fighting the AFCS whenever you try to fly yourself (computer=more expense). There's more to it, but I think that covers the big points.

Obviously not impossible, and most medium or bigger helicopters built today do this and so much more and do it very well. However, they are expensive. As Rinker said, pilots quickly learn to compensate for this. So all you really need is a good excuse to justify the cost.

[Graviman]

Matthew, i have often wondered if the yaw stability system need be this elaborate. All you really need is something which keeps aircraft yaw rate directly proportional to pedal positions. I have considered this as part of my pet interest in simple mechanical SAS designed into control systems for light helicopters. The idea being that the system is simply a funtion of the control, in all flight conditions, rather than a failsafe add-on.

In the same way that using a gyro can force a cyclic to provide direct pitch/roll rate control, it could force pedals to provide direct yaw rate control. The complication comes from the need to ignore pitch and roll, so this implies two counterrotating gyros. These would be differentially actuated by pedal position above and below gimbles, with gyro axis either longitudinal or lateral. The tail rotor connects to gyro assy to keep the gyro assy in the same orientation inside the helicopter. Any external influence, means gyros don't move so heli gets pulled back. It would mean that Aser's pedalless days would be over, but pedal control would be linear in all flight conditions.

Apologies that this could do with a sketch, but hopefully you will see what i am getting at. The objective is a simple reliable system which gives 90% of what SAS would, but has no wires or electronics - ideal for light heli applications.

Mart

[Matthew Parsons]

Mart,

Interesting idea. Although I'm a fan of what computers have done for control systems, I also try to adhere to the "keep it simple" principle. What you're talking about sounds difficult to explain but quite straightforward in operation. It's amazing how complicated we've been able to make so many things that used to be so simple.

[perfrej]

Simple and effective system that just about eliminates pedal input to compensate for collective changes. I remember when first came across it - actually in a demo video from MD - and the test pilot stabilized a hover, took his feet off the pedals and yanked the collective all the way up (to the red of the tq-meter). No yaw...

I have since flown the MD900N (once) and really liked the system. It works.

[rotorfossil]

The best SAS is a pilot. Adding systems in light helicopters means something else to go wrong. Oddly, hardly any design effort ever seems to go into making what is already there lighter. Every pound that's added is one pound less payload. In the past I've flown helicopters which were loaded with kit (R44 with pop outs, avionics for ever springs to mind), but were severely limited in their ability to do the primary job.
KEEP IT SIMPLE!

[nickel]

Thanks for the answers.

One more question though: Is there a standard term for what we are talking about? I am not sure that it is "heading hold". I was thinking of simpler aids which do not take full control of tail rotor. I was thinking that it is simlply a SAS functionality.

[arismount]

rotofossil you are right on the money, that is something I have been saying for years. Light weight is always better than extra equipment that is "nice to have." Also the more installed equipment, the more chance something will break down.

[Graviman]

KEEP IT SIMPLE!


Good point well made, rotorfossil. The system would probably be reasonably simple in implementation, although complex to explain without sketching (spot on Matthew). Something else to go wrong, definately. The extra weight is something that had worried me, so i can only use the usual remit of small high speed gyros.

As an engineer, it is interesting to try to identify control non-linearities and hence devise solutions. I accept that good pilot training is the key, so the only system justification would be for a fast to train machine. In the end I suspect FBW may actually be more cost effective and reliable, but it is interesting to dream up mechanical solutions.

---

An additional thought: Sikorsky tried reversing the pedals at one time, with good success apparently. The way i think of pedal control is you push the foot you want for TR (or rudder) thrust - pushing the ref point away from that side if you like. If pedals were reversed it would be more like steering a bicycle. Anyone else have any thoughts about reversed pedal direction?

Mart

[Matthew Parsons]

Mart, the pedals on a bicycle are to make it go fast, not to steer. :)

I think what matters most is a standard, rather than finding the most intuitive rigging. Currently, helicopters and fixed wing work the same on most controls, lets keep it that way.

[Blind Bob]

I started my flying in gliders, as I'm sure did alot of other pruners. My first training aid was a broom...........I did sweep hangars too, but I was told to sit down and put my feet on the head of the broom and hold the handle like a joystick. When moving handle to the right, right foot forward.....and left to left of course. Brilliant......KISS.:D

[Graviman]

Bob, I remember raising the question at Enstone a while back (it would have to be :sad: ). Most folks felt that left foot for left stick was natural, indeed as do i now. The odd thing is that when i had my first go in an R22 (not so long ago BTW) i had to reprogram my reflexes when MR torque tried to rotate A/C. Also when thinking about some of the stuff in Prouty i have to think about pedals more carefully than i would expect. It just makes me wonder if convention is not intuitive.

What i really mean is you move stick/cyclic in way which you want A/C to go. You move cyclic or throttle in the way you want aircraft to go. On gliders you even move airbrake in direction you would expect, rearward to cause more drag. It is only pedals where you are pushing the aircraft way from the current ref.

I have the odd feeling that if you put someone in an A/C but didn't explain what the pedals did, they would guess left down pulled the aircraft right rather than pushed aircraft left. I suspect most pilots accept it the way it is because TR or rudder is behind them - rear wheel steering if you like.

Mart

[perfrej]

I beleive Igor Sikorsky was convinced that left pedal meant turn to the right, sort of like you would rotate the floor of the machine right by sliding your left forward and your right foot back, and was persuaded by someone to do it the other way around. He's supposed to at a later time have stated that he wasn't sure it was the right decision...

One of my first tries in a H300 had me doing just that - right pedal felt like a natural way to make the thing turn left.

[Matthew Parsons]

Maybe the initial design came from someone who had differential foot brakes for steering, like on some tractors. Left foot brakes the left wheel, moves the nose left.

[Dave_Jackson]

On fixedwing craft, or at least on some of them, the pedals have toe brakes. Pushing the top of the left pedal applies the brake on the left wheel.

[skadi]

On fixedwing craft, or at least on some of them, the pedals have toe brakes.


The same on most helicopters on wheels.....

[Graviman]

The brake steer comparison is an interesting one. Maybe a little challenge then, to ponder over during next coffee break:

Say you have a skid steer tracked vehicle. The steering is controlled by two levers in front of you, left for left right for right. The question is would you expect to pull or push the levers to apply that steer brake?

The question is not loaded, and there are no right answers. I'm just trying to apply the control system out of context - that way there are no preconceptions. :ok:

Mart

[slowrotor]

Grav,
As kids, we learned to use left foot push for a right turn on the "Flexible Flyer" sled. Carts are set that way sometimes also.
When I moved on to gliders my brain had to relearn, and that is harder than when you are young.
Ultralights often have reversed controls, with the simple cart type nosewheel.
The stick is reversed on weight shift ultralights as well!

[Graviman]

Slowrotor, thanks for your thoughts.

Being a dumb engineer i'm lucky that i can usually picture a simplified model of a system in my head. It's quite a powerful technique for learning new things quickly, and to be honest i would be astonished if any heli pilot didn't use similar tricks - particularly from the detailed level of technical knowledge that appears on this forum.

There are two lines of reasoning when the control gets more complicated, as clearly it does in a helicopter. Either the machine should only be controlled by those able to grasp the (at times) the more subtle details of the dynamics involved, or the machine should be designed so that there is no control complexity. Being a design engineer i frequently find myself having to ask the question what is the driver (or pilot in my interest of choice) expecting from this control system. This applies equally to hydraulics, electronics or mechanical systems.

It is with this in mind that i am interested in thoughts about pedal direction. Truthfully, it is an unfair question to pose on a forum intended for aviation professionals, since the natural reaction will be "i figured it ok". :ok:

Mart

[Shawn Coyle]

There are several things being discussed here.
To keep things straight - a SAS system (like the UK military Gazelle Mk2 and 3) will have a short term heading hold below a certain airspeed (in this case 40 knots). It is pretty simple - when you don't move the pedals, it holds heading reasonably well for a while. When you move the pedals, two small 'switches' open - one for pedal movement, and one for yaw rate. The pedal movement switch stays open as long as the pedals are moving - when the pedals stop, the switch will close a couple of seconds later. When that switch closes, the yaw rate switch waits till the yaw rate drops below 2° per second, and then it closes. The logic is that the pilot isn't moving the pedals, and the yaw rate is pretty low, he must want to hold this heading. It will damp out yaw rates, and try to hold a steady heading, but will enventually drift off heading.
But it won't work above 40 KIAS and it won't hold heading forever with just the simple yaw series actuator.
If you can understand this system, most of the rest are easy to understand.
Above 40 to 60 KIAS, most systems will revert to turn co-ordination (not the Gazelle).
A huge help in controlling a helicopter, and should be considered for every light machine.
I hear that Chelton is coming out with a system for the R-22 and Bell 206 that is pretty nifty.

[Graviman]

Slowrotor, i'll have to reconsider pedal direction at some point in the future. Last post developed into a bit of a rant, but not specifically aimed. :ok:

Shawn, I'll keep an eye out for that Chelton system. Interesting that the Gazelle system does not try to modify pedal input.

Mart

[Shawn Coyle]

Graviman:
When the pedals aren't being moved, the SAS will make small adjustments to try to maintain the heading. Uses a series actuator, which doesn't feed back to the cockpit controls.
Like everything else, you need to know how to use it, and have someone demonstrate how it works.

[212man]

Shawn,
Is it not a little misleading to say that SAS tries to maintain heading? Surely the Yaw SAS (as with SAS in the other axes) is simply correcting short period divergence? This has the consequence of appearing to maintain heading, but is a secondary, rather than a primary, effect. True heading hold (and the reason the Gazelle would not have turn coordination) requires an outer loop AFCS function which requires parallel trim actuators (which do move the controls).

Not teaching Shawn to suck eggs (He's forgotten more than I'll evevr know about AFCS systems!) but trying to keep the discussion pure for those less familiar with SAS etc.

[Shawn Coyle]

212man:
You are correct- all it does is damp out rates, and tries to return to the original heading.
True heading hold classically needs a large authority actuator, but I think if you could look at the Apache AFCS, you'd find it does everything, including hover position hold, with only a series actuator, so an open mind is always necessary.

[212man]

Wow: could make for some interesting actuator runaway conditions!

[sunnywa]

From my experience in a BK117 (with a SAS in Pitch/Roll and Yaw), a SeaHawk and SeaKing (with full AFCS) , a SAS will only dampen the divergence and the yaw will definitely not hold its heading (it could be me but the darn thing always wants to go left). The AFCS aircraft will hold its heading even from takeoff (SeaHawk much better that the older SeaKing) and have a heading hold in flight.
The SAS yaw takes all the twichiness out of the tail in the hover - quite remarkable when we switch it out for training.
On the other thread with regards to which pedal push to turn, didn't the Commanche do some trials with a side controller that, in addition to pitch and roll, it had a twist function to take care of the yaw?

[MSP Aviation]

The B206 in FS9 is a game, not a sim. The B206 in FSX is a joke. If you want a realistic helicopter in FS9, purchase the Dodosim Advanced 206. It is THE only heli for FS that models VRS, rotor droop, accurate autos, LTE, and torque properly (or at all in the case of VRS, rotor droop, and LTE).

If you want some real heli flying in the sim go over to http://hovercontrol.com (http://hovercontrol.com/) and register in the forums. You will find a wealth of knowledge, and a great community.

[Graviman]

On the other thread with regards to which pedal push to turn, didn't the Commanche do some trials with a side controller that, in addition to pitch and roll, it had a twist function to take care of the yaw?


Sunnywa, this is the spirit in which i ask about pedal direction. I accept that most pilots adapt very quickly to "left means left", possibly after those famous words "I have" ;) . What i am really trying to establish is whether Igor Sikorsky was right to reconsider pedal direction from an intuitive ergonomic perspective. Perhaps handle bar yaw control would only be right with a handle bar and foot pedals, then again rear steer is also quite a strong cue.

Shawn, does yaw suffer from a short period oscillation by itself? If the problem was dutch roll wouldn't cyclic SAS series actuator sort this out, as well as hover stability?

Mart

[212man]

Mart,
a SAS will sort out 'dutch roll' using yaw and roll damping. The EC-155 only has a secondary SAS for this very reason. The initial intent was simply to certify it with the AP (which also has a SAS function imbedded, obviously) but the certifying authorities were unhappy that in the event of an AP failure in some phases flight (high power climb) the divergent dutch roll mode would be problematic. There is therefore a secondary SAS, activated with a cyclic buton, for the case of an AP failure. It only operates in roll and yaw: pitch remains undamped, but is naturally so stable it is not a problem.

You should avoid confusing the type of actuators used: a SAS uses series actuators, which are high speed limited authority. Parallel actuators are low speed high authority, and are used for attitude retention and outer loop functions.

[steve_oc]

We did try years ago to adapt a heading hold in the 76A+ (Phase 2, SAS only) by using just the yaw series actuator. We flew with a variable gain box and thought we had it working a couple of times, but each time we froze the gain and put in a fixed gain box it didn't work....We never did get back to the drawing board!

[Graviman]

You should avoid confusing the type of actuators used: a SAS uses series actuators, which are high speed limited authority. Parallel actuators are low speed high authority, and are used for attitude retention and outer loop functions.


Thanks 212man - this forum teaches me a lot!

Mart

[Two's in]

And it's not just an AFCS or SAS function. The Westland Lynx has a Collective/Yaw interlink which is a mechanical coupling in the controls that applies a proportional amount of yaw dependent on how much collective is applied. It's apparent effect to the pilot is to re-centre the yaw datum and counter the torque input. Very elegant and effective design for a bunch of tin-bashers.

[Graviman]

"Two's in", does that system work well in all flight conditions? I would imagine it to be slightly under effective in hover, but over effective in cruise - due to vertical stabiliser effectiveness with airspeed.

Mart