Slip Ball or Slip String?

[Shawn Coyle]

I wrote an article for Vertical about the Slip Ball which has generated some discussion, so I thought this might be a suitable subject for a separate thread.
One of the questions asked was to the effect of - 'When you have both slip string and slip ball, which should be used when'

My answer would be (like most of my answers) - It depends.
If you're flying at night or IFR, the slip ball is the only device that's lit and on the instrument panel.
If you're flying during the day and want to minimize drag, zero the slip string. Especially if you're trying to climb at any speed below 60 knots.

Any other thoughts?

[birrddog]

When the string is curled and twisted around itself, then use the slip ball :ok:

[Three Blades]

Perhaps..
If you are flying a machine with the tail rotor significantly below the main disc and wish to navigate by DR (typical ppl training in Robbo or S300) then perhaps you should fly on the string so that the machine is flying in the same direction that the compass is pointing. If you fly with the ball then will there not be a greater sideslip element that will not have been taken into account with the DR calcs ?
A small difference, and negligible if you are navigating from one big visible town to another.
I would guess that this difference is less pronounced with types with a raised tail rotor (never flow one).

[idle stop]

'a suitable subject for a separate thread.'

Ho-ho!

[[email protected]]

Shawn, I only realised the existence of inherent sideslip when I read your Cyclic and Collective book 15 years or so ago but it seems to me you have a choice:

fly wings level ball in the middle and accept you are sliding sideways gently through the sky and the heading will change or

fly the string in the middle with a little AoB and maintain a heading

In the Gazelle, as I am sure you know, we used to teach the string below 90 kts and the ball above.

[soggyboxers]

"On regarde la ficelle monsieur, la ficelle", as my old Aerospatiale instructor used to say. It's just as crab says, the string below 80 knots, the ball above :ok:

[helisdw]

Shawn - thank you for starting the new thread and also for addressing my question. The points about the sting's limitations are duly noted!

From Crab's information about the Gazelle, I assume that the higher cruise speed will negate the heading deviation and thus ball trumps string?

Thanks again for the input - clarifying these fundamental concepts is most helpful.

Simon

[Fun Police]

aside from when it is a twisted up mess, as noted above, i think the string is a work of genius. most of my time is in the astar and their airspeed indication is somewhat lacking at low speeds, so the string is ever useful when working at high density altitudes. the 407 has a pretty good pitot-static system, but i find the string much easier to use when on short final to cliff side pads.
just me, tho'
regards,
fp

[topendtorque]

Had an amusing episode once many moons ago when flying with the senior dept dude from Adelaide when Adelaide (south Australia) was first set up by our glorious aviation dept, I think it may have been called DofT then, as separate from Canberra to administer a large area of our continent.

Barry Lodge was his name an exceptional fixed wing driver and a high respect I had for him because of a couple instances where we were out in the boonies and his very able assistance came in large doses just when it was needed and where we were both bending the rules. however I digress;

He hadn't flown a helicopter before and we are out demonstrating the mustering disciplines etc.

'Here have a go Barry', I says, we are in a '47. I couldn't figure out why he was trying to fly the dashed thing nearly sideways. Then I realized that he was trying to centralize the ball, not the string, I says, as I covers the ball, 'just fly straight for a bit', then I twisted the ball to be level and says; 'there you go, don't let it be said that you can't fly a helicopter straight'.

as anyone who has flown a '47 solo will know that the ball sits at a different angle in balanced flight than when it has a balanced load. this particluar ball was only mounted to the panel on one end.

it was also only there because of some stupid new rule which said we had to have them.

[Bravo73]

Can I also suggest that the 'seat of your pants' will tell you what the slip ball is telling you, before the slip ball tells you?

Pressure on one side of the seat, push the corresponding pedal. Easy.

[rick1128]

Bravo, while the seat of your pants works quite well visually, myself like most instrument pilots has had that tendency beat out of us by our instructors during instrument training. It can get confusing at times. I flew a 300 setup for instrument training and I had the string showing one thing, the ball another and my butt saying they were both wrong. For instrument flying I would use the ball, for no other reason than to keep from moving my head too much due to the potential of spacial disorientation. It is bad enough that most instrument training ship have some of the worst panel setups for instrument flying.

[Bravo73]

Well, Rick, I think that you might be confusing your inner ear with your butt! ;)

When flying under instruments (an offshore departure in an EC225, for example), I still find that my butt will tell me that the aircraft is out of balance slightly before the ball does.

[spinwing]

Mmmm ....

Interestingly .... if you have an issue with the ball not playing games correctly (pls excuse the pun) you may find that if the aircraft is placed in the 'rigging' position that they may in fact have been installed incorrectly ... also the balls may indicate differently across a wide cockpit ... I have had issues with both 412s and 212 with this .. especially after rebuilds if the panels have had major work done on them.

Also ... the airframe has to be 'straight' for the balls to show correct trim.


:ooh:

[handbag]

or if flying with the doors off and you have wind rush on your mike, then get your foot in !:uhoh:

[Darkhorse30]

When I flew Cobras in RVN I found that when firing rockets in a dive that if you held the slip ball 1/2 ball width out of trim to the left the rockets would fly parallel and true on the sight. Obviously, this was because I was correcting for the inherent side slip.
I have used the string on a couple of occasions and during tests with a boom and side slip vane out front several feet I found that most of the time it didn't agree at least in Engineering terms. The string on the wind screen is subject to all sorts of conflicting airflow due to unsteady rotor wash, particularly near the best rate of climb speed and lower. So.... I'm convinced that the string is better than nothing, but not necessarily the best indicator for side slip. Maybe if the string was calibrated with windscreen marks against a yaw vane, it might be more valid.
Just my two cents worth.

[Shawn Coyle]

Darkhorse:
You're right, slip string operation does depend a lot on the airframe geometry around the nose. The length of the support for the slip string plays a part - perhaps that's why the R-22 / 44 slip string is very close to the fuselage?

[Darkhorse30]

I flew an Mi-8 for a number of years and for whatever reason there was no turn and slip indicator on the panel for the first year. I put a string on the wind screen and used my proprioceptive feel ( my butt) to sort of estimate where the zero side force was using anti-torque pedals in forward flight. We had a slip indicator installed later and the agreement between the string and the slip indicator was just sort of okay. Not very scientific but better than nothing, I guess.

[ShyTorque]

The slip ball on the RAF Gazelles read differently to the Army ones in my time instructing on the type; the Army ones were more accurate. Strange but true and there was a simple reason why it was so! :ok:

[Shawn Coyle]

shytorque:
And that reason would be????
(anxious in Philadelphia!)

[sunnywa]

From my time on AS350's, VFR was the string (more efficient, less drag), IFR the ball (and accept the non streamlined flight).

The most important question is the one was why the 6"piece of red wool (string) cost $500 from Eurocopter (the AeroSpat). Was it made with special gold (non metallic) thread so it wouldn't effect the DG - or some super secret wool from a European Apline wilderbeest that had to be fed special grain for breakfast and plucked at midnight by a blind Corsican monk (no offence to any blind Corsican monks that may read this post)?

Always tickled my fancy that one.......:)

[ShyTorque]

Shawn, the design of the RAF instrument panel was different; it had a larger extension to the right hand side which made it sit heavier on the right hand flexible rubber "Barry" mount. The panel on many aircraft therefore hung slightly right hand low so the ball sat out to the right in balanced flight. We used to notice it when flying on instruments; if you put the ball "in the middle" the aircraft turned gently right as it was actually yawing.

The simple cure? We always carried an En-route supplement book (containing ATC info, airfield details, frequencies etc), normally in our flight suit lower leg pocket. Suitably jammed under the right lower edge of the panel, it brought the instruments to a true level and put the ball back where it should be. ;)

[Shawn Coyle]

shytorque:
Thanks! Amazingly observant!

[Lord Mount]

As my instructor used to say "lets share the breeze, shall we?"

LM

[Retro Coupe]

One of the things I noticed the most when changing from AS355 to EC135 for police work was the absence of the string. It made hovering into wind at night a doddle.
If you ever get a chance to watch "The Final Countdown", there's a brief glimpse of the slip string on an F14 Tomcat which always makes me smile. :)

[ShyTorque]

Shawn,

Even more amazing for me was that this discussion brought that snippet of information from some dusty corner of my brain; it's been more than 25 years since I instructed on the Gazelle!

[MartinCh]

"The Final Countdown", there's a brief glimpse of the slip string on an F14 Tomcat which always makes me smile.
Let's not imagine what'd the aerodynamic forces do on jet fighter if flying badly uncoordinated.

I recall (from my meagre experience, PPLH training not so long ago) bashing circuits in R22 in SVFR weather with some drizzle - less RT chatter from FW VFR training, that I found it bit hard to take off, speeding up and climbing, while the wet strings stuck to window slowly bothered to work about 50-60kts. Unfortunately, no ball to look at. Flying gliders, I prefer string, keeping eyes out (although there isn't any ball usually anyway).

Shawn's book is nice. Have to start reading it up for part-66 mod 12 which I'd like to pass over next year or two.

[Jack Carson]

Shawn, Great Topic of discussion.
It has been my experience that one has to understand the dynamics of the machine to decide which, if either is of more value. Typically, the Yaw Sting is well under the rotor thus influenced by rotor flow as well as the basic air stream. Similarly, the ball installed in the airframe has a curvature of its race designed to provide some level of accurate measure of side force for each specific machine. Others may want to comment on some of the specific machines they have flown and how the slip string or ball react to aircraft lateral trim. Two aircraft that come to mind are the SK-64 Skycrane and the H-53 Sea/Super Stallions. Both have similar main and tail rotor systems. The cranes main gear box is shimmed 3.5º left wing down to accommodate level flight in a hover. It also has a very limited fuselage structure, thus limiting much side force buildup in out of trim flight. It is almost impossible to determine if the aircraft is in lateral trim in cruise. By contrast the H-53 with its large cabin displayed very strong indications of out of trim flight on the ball as well as the seat of your pants. The Wessex 5 was another aircraft that I personally had difficulty keeping in trim. The AV-8 Harrier has a Yaw string with a cone attached to provide required cross wind indications during slow flight and hover.

[Hilico]

The R22s I did a few lessons on had not one string, but two, one either side of the central pillar in the canopy. In 'balanced' translational flight (say 40-80kt), they formed an arrow pointing straight down. Get out of balance and the 'arrow' would helpfully point the way you should be pressing pedal.

[krypton_john]

If your g string slips, a ball can fall right out.

Just another safety issue to be watch out for.

[ShyTorque]

That joke is pants. :p

[helisdw]

Following on from the useful advice already offered, I came across an article from Helicopters Magazine's Fall 2007 issue that had a flight test of the MD902 Explorer.

The pilot/author wrote the following:
"Still, one must be careful to keep the ball centered as any deflection causes an increase in fan pitch thereby using engine power. For instance, at gross weight, a slip with the ball touching the right vertical line will reduce the rate of climb by 200 fpm while slipping in cruise can increase torque significantly. Flying without yaw in level cruising flight is quite important as slight displacement of a pedal will cause the helicopter to sashay somewhat."
Ken Armstrong, Helicopters Magazine, http://www.helicoptersmagazine.com/content/view/465/132/ (http://www.helicoptersmagazine.com/content/view/465/132/)

Further proof, should it be needed (!), of the importance of maintaining balanced flight. However, as the Explorers I am familiar with don't have slip strings, it doesn't add much to the string vs. ball discussion!

Simon

PS I hope the author of the article doesn't object to the direct quotation - I couldn't find contact details to seek permission.