Flyer & G-Arms similarity functions

[Charles King]

A great over-view of both the flyer & G-Arms.

"Adjustment on the Flyer is identical to the "Lift" adjusment on the G-arms. And this same adjustment is used on the other Iso-elastic arms too, the Master/Ultra and the Provid/SK arm.

How springy a spring is depends on how tight it is. A loose spring will be bouncy, and a tighter spring is less bouncy - more iso-elastic. So changing the tension of a spring changes not only how hard it's puling (lifting) but also how iso-elastic the arm is.
Most low end arms, in fact all non-Steadicam arms, are adjusted by changing the tension of the spring. This is is also how older Steadicam (3A/EFP..) arms are adjusted. With an arm like this, the tighter the springs are, the more iso-elastic it is. A 3A arm is actually prety iso-elastic when holding it's maximum weight.
All the arms that use a turnbuckle on the end of the spring to adjust do the same thing.

There is however another way to change the lifting force of an arm: Instead of changing the physics of the spring itself, you can change how much mechanical advantage it has. The higher up the end block a spring is connected, the stronger it can lift. - And this is the idea behind the vertical adjustment of "lift" force on Iso-elastic arms.
Now changing this height and mechanical advantage does also slightly effect the physics of the spring. This is why the adjustment isn't exactly vertical, but rather at an angle. As you lift the end of the sping to help it lift more on the end of the arm, you also pull it *slightly* tighter, helping keep the spring tuned in it's "sweet spot" of iso-elasticy.
A directly vertical adjustment of the end of the spring will work, but it can only be iso in one point.

The end of the spring moves up and down a threaded rod that is rotated by a knob at the top, this is no secret. But the reason for the grooves/slots in the side is structural. The spring is pulling *very* hard on the threaded rod from the side, possibly enough to bend or break it, resulting in arm faliure. The grooves on the Flyer create a strong 'track' for the end of the spring (in it's end mount) to move along, holding it in place horizontally so it can't break the threaded rod, who's job then becomes to just move the end of the spring vertically to adjust left angle.

By attatching the threaded rod at it's bottom end (the Flyer's isn't) too will certainly add strength to it, and with a strong enough rod (and a light enough load) can work just fine. I don't know how thick rod nor what maximum load Tom is going for, but this desing can work fine. - As it does on Robert's arm in the thread linked by Cedric a few posts ago. - Worth dooing some math on first though.

The G-arms take a 3rd approach to this:
The Flyer arm has no "ride" control, only "lift". To control "ride" you would have to somehow adjust the tension of the spring. One way would be to move the distant end of the spring along the bone of the arm (similar to how a Glidecam V-16 type arm is adjusted) but this is complicated.
The way that the G- arms do the "ride" adjustment is by allowing the user to change the angle of the "lift" adjustment rod. Instead of beeing fixed at a pre selected angle to maintain iso, you can move the top of the rod in and out (via the 'Geo' cam-linkage), therfore changing that magic angle and the spring's tightness, and therefore iso-elasticity.
Now you obvisouly can't change the angle of a groove cut into an end block, and the G-arms don't have that grove. Instead they use a 2nd, very thick (and strong) solid rod running parrallel to the threaded lift rod that acts as a 'track' and takes the lateral strain of the spring and transfers it to attachment points on each end. - In a similar fasion to Tom's plan, but using 2 rods (one smooth, one threaded), not just one.

There's some arm theory for all."

- Mikko

[trung dau]

What amazing article! What a wonderful theory! Thanks a lot dude! This forum is really great. Finally You r the man who answer the question which stick to me since firstly of my life touched the G-arm.

Please put to picture for demonstration of the article. English is not my second language, same as many people i this amazing forum.


 "In a similar fasion to Tom's plan, but using 2 rods (one smooth, one threaded), not just one." R they 2 rods from 2 side of the spring? You mean to nob control "lift" and "ride" will be 2 side of per section of the arm.

[Tom Frisch]

I think there is another point about the G series arms that is missed in this discussion- that is that the GEO linkage dynamically changes the spring tension at the highest and lowest angles of arm extension.  This should keep the arm from locking up at the top, as the lifting force is decreased gradually at the top of the arm motion.  (I still don't understand the purpose of decreasing spring force at the bottom of the arm motion).  This feature allows a greater boom range without risk of 'bumping' at the end of arm travel, and ruining an otherwise smooth shot.

While the 'ride' adjust is physically connected to the Geo-cam, it doesn't require the cam to function, it just moves the top pivot of the slide that hold the spring in and out to change the angle (and spring tension).

See my cad drawings for a better explanation. Please correct me if I'm misunderstanding the theory, I'm all talk until I actually build something.

-tom

[MikkoWilson]

Yes, Tom (A different Tom than my original post was refering to) is correct regarding the "Geo-cam".

The adjustments operate as I originally described, but that magic "cam" style linkage (It's not actually a cam, but a series of linkages that operate in mutch the same way [sort of how a piston connects to a crankshaft]) dynamically fine-tunes the tension of the spring - therfor effectly re-tuning the arm's iso-elasticisty & "sweet spot" as you boom. This keeps you in the sweet spot thought the entire boom range. (As you lift the sled, it gets higher, therefore has more potential enegery, therfore the springs need to lift just *slightly* harder to keep it in place.) The linkage's limits are just within the limits of the arm's boom range. Which, as described by Tom, causes the arm to become more springy just as you get to the end of the arm's boom range. As a result, as you boom up, just before you hit the "hard stop" at the end of the boom range, the arm starts to pull back down. Effectivly it turns the arm's lift-springs into (really good) "bumper springs" for the end of the travel.

Earlier (non-Geo) Steadicam arms have seperate little springs to act as bumpers just before the arm "locks up" at the end of the boom-range.
You can see the little bumper springs on the upper bones in this picture:

Source: http://www.steadicam.ca/mainfile/article/arm/suparm.html
With the Geo arms, those bumper springs arn't needed as they are built in via the linkage.

To answer Trung's question..

In this photo:

Source: http://www.homebuiltstabilizers.com/TheSteadicamFlyerinDetail/photos/photo23.html
You can see the threaded rod (that is turned by the "lift" knob) that moves the end of the spring up and down the end block. Notice the long hole in the side of the end block that acts as a "track" for the end of the spring to move up and down. Without the track, the pull of the spring would bend the threaded adjustment rod.

Because the angle of that threaded rod is adjustable in a Geo arm, they don't have that hole in the end block as the track, but rather they have a thicker (smooth) rod next to the threded rod that acts as the track.
See this picture:


Notice there are also both a "track" and a threaded adjuster for the "ride" adjustment too.

 - Mikko

[Charles King]

Great work Mikko. Thanks.