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Author Topic: Proper Line Tension  (Read 1362 times)

Offline Dick Fowler

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Proper Line Tension
« on: September 29, 2006, 01:18:09 PM »
Bill used this term in the thread on Side force generators and it's a good one. Worthy of further discussion.

The term gets tossed around without much elaboration as to what they really are talking about. There's the handle pull you feel in level flight and then there's the tension you feel when performing a maneuver... probably should be referenced using some other term...stick or handle load has been used and seems like a good one.

What factors affect line tension? Some I can think of are...

Model speed and weight - Centripetal force

Aerodynamic Forces – Can increase or decrease line tension.

Vectored Engine Thrust – Engine Offset.

Position in the hemisphere – Gravity

Wind

Handle Loads -  Function of aerodynamic forces and control system mechanics

When I measure lap times of most flyers, 5 seconds seems to be a nominal number. This translates into something like 2.8 to 3 G’s of Centripetal force with typical line lengths. So if no other contributions are made to tension then a 50oz. Model pulls at around 150oz.. That’s a bit more than 9 lbs.

 Is that proper line tension? Don’t know but lots of folks fly in that ballpark. My personal preference is a fair amount of tension. I don’t feel that I have much control when the plane is pulling around 2 G’s… has happened when I get a really  bad engine run and it feels like a “potato chip on a string”.

What do the rest of you think is “Proper” line tension.
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Offline phil c

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Re: Proper Line Tension
« Reply #1 on: September 30, 2006, 07:21:50 AM »
dick, for any plane, the harder it pulls(whether because of speed or weight or sideforce) the straighter the lines are and the less they affect how the plane maneuvers.  The "magic number" is the leadout rake needed.  Looks to me to be around 1.5 deg(.7 in. of leadout rake on a 27 in. panel) for a stunter.  If much more rake than that is needed the lines have enough curve in them that they will start to whip the plane around in sharp corners and interesections.
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Re: Proper Line Tension
« Reply #2 on: October 01, 2006, 11:48:21 PM »
Dick and Phil C...

A lot of good thinking in these two threads!

If I understand it right, the Centripetal Acceleration requires a force, which, since Isaac Newton, we've called Centrifugal Force, to cause it to happen. An object will travel in a straight line unless acted on by an external force. The rate of change from a straight line defines the Centripetal (toward the center of the curved path) Acceleration. Per Newton, F = mA (F= Force; m = mass; A = acceleration.)

The required force wasn't given a name by Newton. Calling it Centrifugal Force is as valid as calling it Nova Scotia Lox, but we never did that!

BTW, the line "aft sag" is pretty irrelevant to airspeed, whether we measure at the model, or in terms of lap time. Both are affected as the square of the forward,  -OR-  the rotational, velocity. As Phil says, though, the total forces increase with velocity, so we don't collide with what Dick correctly reminds us is the Netzeband Wall.

The control surface deflection required to make a certain rate of change in pitch is also tied in with forward, or rotational, velocities. For smaller forces, we need smaller deflections - IF we think of the counter-forces on the flap and elevator surfaces. At half the airspeed (which is what affects the airloads on the controlling surfaces) we need 1/4 the force to move the flap/elevator surfaces to the required point.

The line tension has decayed by the same factor!

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Offline phil c

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Re: Proper Line Tension
« Reply #3 on: October 04, 2006, 09:31:25 AM »
I've seen several flyers approach trimming a plane out from the long lines, heavy lines, flying too slow school of thought.  It never works.  Trying to fly a 42 oz. Vector 40 on 65 ft. 018 lines at 5.8 sec. laps.  Just doesn't work well. When you bring the plane to the field most of the trimming is done.  You have a certain weight to work with.  The engine will put out a certain amount of power comfortably.  You want to fly at a certain speed for ease of control.  Put those together and the line lenth and airspeed are pretty much pre-determined.  that is where having some rules of thumb really help out.

50-55 mph  use that to calculate a lap time
Mid forties for weight on moderate size 40 powered planes on 015 lines
60 oz. plus or minus for 60 size planes on 018 lines.
Adjust the line length for best flight.  My own preference is to keep the lines as short as consistent with good looking maneuvers.  I find that the longer the lines are the harder it is to trim a plane to fly well.
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Offline Dennis Moritz

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Re: Proper Line Tension
« Reply #4 on: October 04, 2006, 10:07:17 PM »
Often stunt planes boogie. They don't look slow to me. There's a sense of reserve power and authority. I've seen many a plane come out to the field fly too slow and get put in doing something overhead, even a wingover. I think the ideal is wrong, looking for planes that fly real slow. Too slow and you're done for. Five second laps on 60 foot lines, or let's say 4.8 second laps, is moving. Not slow combat speeds, but on the fast side, nonetheless. I've seen lots of planes go splat trying for a slow fly "ideal" stunt speed. Even my Twister, when I first took it out, fell victim to my misunderstanding of what a truly useful stunt speed is. I had to put the wing back together twice before I realized the obvious, I was trying to fly too slow. When I let the FP40 sing everything started to come together. The plane flew, did the maneuvers and stayed out on the lines.
« Last Edit: October 05, 2006, 06:11:29 AM by Dennis Moritz »

Offline phil c

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Re: Proper Line Tension
« Reply #5 on: October 05, 2006, 09:35:57 AM »
Flying slower is good, if the plane/engine can handle it.  Slower lets you fly more precisely, at least as long as it isn't too windy.

5 sec on 60 ft lines is 51.5 mph, a good speed for most flyers.  It is on the verge of being too fast, but most folks have reflexes that can handle that speed.  It's also fast enough that the plane can handle 5-10 mph of wind without too much trouble.  Trying to go much slower than 50 without doing something else to increase line tension is asking for trouble, as you pointed out.
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Re: Proper Line Tension
« Reply #6 on: October 05, 2006, 10:22:38 AM »
Taking it to the most basic answer...

Proper line tension depends on what is comfortable to the flier, as long as it is at least enough to move the control surfaces as far into the airstream (i.e., away from neutral) as it takes to do the figures.

The crucial 'feel' is through the longer overhead maneuver paths. I guess it's a mixed blessing to have ample, dependable pull up top... It is bound to be noticeably more down lower.

Then, the change to to the lighter- but still 'adequate' - pull up high feels like a loss of pull, by comparison. This is another area where a fully free-moving linkage system is good. The flier has confidence that, if there is any reduction in pull, he most likely still can get the message to the control surfaces.

Speed is something I feel by laptime, more than by actual model airspeed. The same laptime on 70' lines is 116% the airspeed as it would be on 60' lines. In Phil's example of 50 MPH at 5 sec on 60's,  5 sec on 70's is 58 MPH.

Lift goes by the square of velocity, so, theoretically, potential lift at 58 MPH goes to 135% of its value at 50 MPH.  "Centrifugal" Force pull - and lift-related drag - go up by the same factor, too.

But the laptime is still 5 sec.
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Offline Dick Fowler

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Re: Proper Line Tension
« Reply #7 on: October 06, 2006, 09:35:56 AM »
I r not an ingineer, so that being said, to me if I can do four consecutive over head eights with no line slack, then I have  enough line tension. Does than make sense?

Ya, that's about right... except you aren't going to get pattern points!  <=
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Offline Randy Powell

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Re: Proper Line Tension
« Reply #8 on: October 06, 2006, 10:57:14 AM »
I think I'd look more at "consistent" line tension. My current plane pulls stongly as such things go (still trimming it), but there is very little change in the tension wherever I'm at in the pattern. One of things I worry about is the plane that pulls much harder in places like the pullout at level flight on a square corner. I've had planes that would almost pull you off your feet.

So I look for consisent line tension that doesn't change much in maneuvers and isn't too heavy. I'm flying a 625 square inch plane on 65 foot lines with about 5.2-5.3 second lap times.

The other thing (my own quirk) is I try to trim so that the plane is as tangential to the circle as possible. Square to the pilot with no or little yaw if that makes sense.
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Offline Dick Fowler

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Re: Proper Line Tension
« Reply #9 on: October 06, 2006, 12:16:24 PM »
Randy..."I think I'd look more at "consistent" line tension. My current plane pulls stongly as such things go (still trimming it), but there is very little change in the tension wherever I'm at in the pattern. One of things I worry about is the plane that pulls much harder in places like the pullout at level flight on a square corner. I've had planes that would almost pull you off your feet."

That's an interesting comment.  Do you think it's a manifestation  of the control surfaces load against the handle?  As Lou pointed out, that would make a good case for keeping airspeeds down.
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Offline phil c

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Re: Proper Line Tension
« Reply #10 on: October 06, 2006, 05:13:55 PM »
Lou, the longer lines also have more drag, so the leadout sweep to keep the plane tangent to the circle is about 40% more.  I figure .9 in, vs. .66 in for 60 ft lines.  Even though the plane is flying faster on the longer lines with more line tension, the extra bow in the lines is going to have significant effects on how it handles.  Longer, heavier lines whipping around more will make the plane feel much less in solid control.

Higher airspeed is not good for precision.  Most flyers seem to have conniptions when their Nobler winds out to a 4.8 sec lap(53.5 mph).  Trying to fly  a decent RWO at 58 mph would get very tricky for most flyers.  5.3 sec/lap is a much more common target on 65 ft lines, but with a much heavier, more powerful plane than a Nobler.
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Re: Proper Line Tension
« Reply #11 on: October 06, 2006, 06:13:03 PM »
You know you have enough line tension when you can do the maneuvers up wind.
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Re: Proper Line Tension
« Reply #12 on: October 06, 2006, 08:45:36 PM »
Well my T-Bolt would probably do it in 10 MPH It pulls that hard!
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Re: Proper Line Tension
« Reply #13 on: October 06, 2006, 11:35:21 PM »
Phil, to your comment on longer lines -

Of course! The comparison I tried to make is for the same length, diameter and number of lines. In that case, airspeed, or laptime (another way of seeing it) is irrelevant. Change line length, and you have a different case demanding its own solution.

Either here or in another site, I mentioned the way to place the leadout guides is so the direction of pull force, as it reaches the guides, aims at the CG when the fuselage is at the desired angle to the imaginary line from flier to CG. And, that the best compromise is when the fuselage is perpendicular to that imaginary line.

Since the angle at which the lines reach the guides depends on length, number and diameter, ideallly it should be determined for whatever the best combination of THOSE terms turns out to be.

Soule's LineII is great for doing that, and I find the earlier Aeromodeller Annual equations at least as useful.
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Offline Dick Fowler

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Re: Proper Line Tension
« Reply #14 on: October 07, 2006, 07:25:42 AM »
If you like to be in the 3G's range then I can see where there would be an advantage to longer lines... especially with the diminishing reaction time of some of us more senior types.

Below are the "Numbers" and they both create about 3 G's of line tension assuming no aerodynamic influences (my rock on a string thing again)

                                                      60 ft lines         70 ft lines

Laptime (sec.)                                    4.9                    5.3

Speed (MPH)                                      52                       56
 
Angular Velocity (deg/sec)                   72.8                    67.2   


The angular velocity is down about 8% so the big thing is that you slow down the maneuver times by about 8%.  The arc lengths are bigger in a 70 foot hemisphere than a 60ft. Even though the model is flying faster it still takes longer to fly an arc of  45 deg .

Although the longest lines I fly on are 65ft, I have flown models with speeds up in that 56MPH range. It doesn't seem to me  that those speeds would make  trimming too difficult. 

Edited - This begs the question...What are the problems associated with using longer lines?  
« Last Edit: October 08, 2006, 06:26:36 AM by Dick Fowler »
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Offline phil c

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Re: Proper Line Tension
« Reply #15 on: October 08, 2006, 06:53:19 AM »
I think guys that we are discussing several different aspects of the problem and maybe not keeping them clear of each other.

One aspect is the bow in the lines and how it affects the plane during maneuvers.  Lou, you post seemed to be saying that using longer lines and higher airspeed would result in more line pull and hence better results.  To me the longer, heavier lines and greater rake can quickly run into problems if the plane isn't big and heavy enough for the lines.

Second aspect is the infamous lap time.  It is happenstance that many stunters end up in the range of 5 sec plus for lap time, it's not a requirement.  Lap time is just a quick, fairly accurate way to evaluate engine runs.  If the plane always flys 5-5.1 sec/lap then you know to look for a problem or maybe wave off the flight if it takes off and turns 5.5 sec.  This is not a requirement that all planes fly 5.0 laps.  It just tells you when your well-trimmed plane that has routinely been flying 5 sec laps suddenly turns in a 5.5 lap that something has changed.

Third aspect is the various speeds involved.  We've got airspeed, that supports the plane.  We've got the ground-closing velocity the pilot sees when trying to position and fly maneuvers.  We've got the angular velocity of the plane around the circle.  To my mind the most important one is airspeed.  It flies the plane and generates everything else, so control that and everything else falls in line.  I think everybody agrees that a steady airspeed makes the plane easier to fly precisely.  It just gets hard to fly when the plane is accellerating and slowing down, with greatly changing line pull and control loads.  Slower is better, as long as the pilot can maintain good control.  As the airspeed mounts up over 50 mph and change the "window" to perform each maneuver gets tighter and tighter.  At an airspeed of 40 mph you can fly down and clip the dandelions.  At 100 mph that gets pretty iffy.

Angular velocity is tricky.  People aren't built to work with it.  Just try and guess how fast the prop is turning by looking at it, or how fast you are turning around with your eyes closed.  We use all sorts of visual cues to deal with angular motion, but we don't sense 1/4 of a circle by feel with our inner ears..  I the case of CL flying, the angular motion comes directly from the planes airspeed.

The result of all this is that a new plane will have a certain line length, line diameter, and airspeed that will make it fly is well as possible.  Our job is to find it, and hopefully, the speed turns out to be slow enough that we can fly the plane in PA.

Another thing to keep in mind- you can trim a plane to fly with zero line rake by adjusting thrustline, rudder offset, etc.  That doesn't mean the curve in the lines goes away.  The curve and drag of the lines is what causes the problem.  That can only be changed by airspeed, plane weight, or some kind of side force generator.
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Re: Proper Line Tension
« Reply #16 on: October 08, 2006, 09:53:01 AM »
  That can only be changed by airspeed, plane weight, or some kind of side force generator.

Or changing to a smaller line size! This is why I would rather fly a .40 size plane.
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Offline Bill Little

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Re: Proper Line Tension
« Reply #17 on: October 08, 2006, 10:06:34 PM »
I really have no clue as to the "lbs" that I like to have for line tension, but it seems that I like a *little* less than many........  this coming from a guy that used to power lift competitively in the 242 and up weight classes.  I just do not like to have a plane really pull on the lines.  Seems to me to be harder to try and fly it precisely and have any *feel*.

I have tended to fall around 64'-65' eye to eye lines and 5.3-5.4 lap times.  I am comfortable there.

Can anyone tell me how much line tension is created (in lbs.) by a 54 oz. stunt plane (USA-1, ST .51) flying on 64 1/2' lines (eye to eye) at ~5.4?

Thanks!
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Offline Dick Fowler

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Re: Proper Line Tension
« Reply #18 on: October 09, 2006, 06:28:33 AM »
Hi Bill, assuming no tension contributions from engine offset or aerodynamic stuff (the rock on string thing). A rough cut is:

Speed ~ 55MPH

Tension ~ 2.9 G's so a 54oz. stunter "pulls" around 156oz or 9.7lbs.
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Offline Ted Fancher

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Re: Proper Line Tension
« Reply #19 on: October 09, 2006, 08:53:20 AM »
This has been an excellent discussion.  Most everything that I've read rings pretty much true.

I'd like to merely emphasize what I have felt for years was one of the biggest "institutionalized roadblocks" to effective stunt flying there is; the belief that there is inherent value in flying at a particular lap time ... most generally, a lap time that is way too slow to use effectively.

I think the germ of this concept was formed clear back with the original Nobler article that was titled something like "Stunting can be slow" or something like that.  Up to the Nobler era stunters tended to fly pretty quickly with large engines, skinny wings and no flaps.  The Stuntwagon actually bragged about stunting at around 100 MPH.

George Aldrich's revelation that you could fly effectively somewhat slower was an eye opener and his success in doing so resulted in nothing less than a revolution in the approach to stunt flying.  All of a sudden it was not only OK not to fly "zippy" patterns, it was "the" thing to do.

Unfortunately, like many such revolutions, the myths tended to overwhelm the facts and talk such as six second laps on a Fox .35 powered Nobler started to be taken as gospel.  As Phil and Lou and others have pointed out here, successful flight of a Fox .35 powered, 40 oz or less airplane on 60' or less of line at such lap times is pretty much the stuff of cartoons.  The physics just don't work out.

The residue of such legend has, nonetheless, instilled itself into the collective stunt consciousness with the implicit belief that slower is better and that he that flies the slowest lap times should receive some sort of award for doing so.

Number one, I've yet to see a plaque or plastic trophy handed out for the slowest lap times and, number two, it is seldom true that airplanes fly better at a slower speed than they do at a higher speed.  We need the lift thing for our tricks and lift goes up as the square of airspeed.  To suggest that we are better off the slower we go flies in the face of the physics.

Like everything else in stunt, optimum lap times are a compromise between many factors, including but not limited to: the lift necessary based on the wing loading, the power available to overcome the drag produced by the creation of the lift, the "Netzeband Wall" (a good example of which I judged last weekend with Larry Wong's OTS entry at a local Vintage Meet in Napa), and ... most important of all ... the pilot's ability to fly effectively and precisely to garner the most possible points from his/her particular assortment of compromises.

The search for ever slower lap times can often be counterproductive.  Unless a ship "locks in" to the necessary pitch rates and can be trimmed to maintain constant radius loops and corners with little pilot input, errors and corrections to them will be both more numerous and more evident.  The slower you go the more  the time available for the judges to notice errors and degrade the resulting score.  Thus, the pilot who flies slower lap times simply because he can complete a "recognizable" pattern at those speeds may in fact be harming the bottom line (his score on the scoreboard) by doing so.

No matter what anyone says, flying slower makes the ship more susceptible to inclement conditions.  Turbulence and winds will effect the flight path more: in turbulence because of reduced inertia to overcome the bouncing; in winds because whatever the level of winds they are a greater percentage of slower speeds than of higher thus making the disparity in ground speeds (lap times -- what the pilot sees and feels) greater.

Here again, my old saw about the importance of airplane trim becomes a major factor.  An airplane in good trim can be flown effectively at higher speeds because its response is predictable and uniform. The pilot needn't make physical adjustments to the "probable" response of a poorly trimmed ship.  Thus flying faster is much less of an issue.  This is also why a well trimmed ship can handle even the worst of winds that keep a lot of guys with more questionable ships on the ground.  The speed variation can be dealt with, again, because of the linearity and predictability of response.

Probably the ideal is to fly a well trimmed stunter no slower than what is comfortable for the pilot's personal physical limitations to fly comfortably to the best of his ability.  Artificial attempts to fly slower for the sake of flying slower are unlikely to prove beneficial in the long run and the trophies for doing are not only rare but not very pretty. ::)

Ted

Offline Bill Little

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Re: Proper Line Tension
« Reply #20 on: October 09, 2006, 09:43:34 AM »
No argument from me, Ted, about flying too slow!  It seems I must be comfortable in that "around 55 mph airspeed" and I attempt to make the plane fly at that speed, if not at least subconciously.  I do know that the feel of the airplane is most important to me.  If the plane feels like it is flying too slow, I know it is.  Did that make sense??  I experiment with airspeed after I get the plane decent in trim.  Flying slower and faster to see the effects on that particular plane.  When the feel starts to get right for me, I take a lap time.  Then go from there.  Unfortunately I can't get to fly very much!

Thanks for your input.  y1

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« Last Edit: October 09, 2006, 02:05:33 PM by Bill Little »
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Offline Dick Fowler

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Re: Proper Line Tension
« Reply #21 on: October 09, 2006, 01:24:40 PM »
OK... how do you guys like your handle loads? It is a distinctly separate yet integral part of "the feel".  I can tell you from personal experience it took me some time to get use to the 4" bellcrank!
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Offline Ted Fancher

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Re: Proper Line Tension
« Reply #22 on: October 09, 2006, 07:18:09 PM »
OK... how do you guys like your handle loads? It is a distinctly separate yet integral part of "the feel".  I can tell you from personal experience it took me some time to get use to the 4" bellcrank!

Dick,

You're certainly right that control loads are part and parcel of the same dynamics but a separate issue for the flyer.

There are a couple of givens regarding control loads that must exist before you can even start to talk about how it feels to the flyer.  Quite simply, whatever the physical relationships of all the various levers in the system (handle line spacing, bellcrank leadout arm length, bellcrank pushrod arm length, horn length, control surface size, etc. the full range of control deflection built into the airplane should be obtainable from the handle by the pilot with an amount of handle, wrist and arm motion with which he is comfortable.

In addition, we must also remember that the total load on the handle will can only equal the line tension available at the time of control input.  All we are doing is changing the ratio of how much of the existing line tension is supported by which of the two lines, up or down.

Thus, the control "load" is really a matter how much variation from one line to the other one experiences when giving up or down controls.  The sum total of both remains the same.

there will, of course, be some difference in feel to the pilot when he loads one line or the other more than the other because the individual lines are displace from the point of rotation (probably the wrist joint) and you've thus got a "load times an arm" situation.

Generally speaking, the greater the amount of wrist rotation necessary to achieve a given amount of control deflection, the less will be the variation in load.  Again, the limiting factor is that you must have sufficient mechanical advantage to deflect the control surfaces with the amount of line tension available (the Netzeband wall discussion again).

Ted


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