Slip Ball or Slip String?
Avoid imitations
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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.
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.
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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.
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.
Last edited by Retro Coupe; 18th Aug 2010 at 22:54.
Avoid imitations
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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!
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!
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"The Final Countdown", there's a brief glimpse of the slip string on an F14 Tomcat which always makes me smile.
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.
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.
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.
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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.
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That joke is pants. 
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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/
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.
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/
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.
