Advantages of pull-pull via pulley

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A better way to control the rudder via cable
Many modelers reach for a tiller (double-horn servo arm) when installing pull-pull cables. Only thing is, there's a better way.

- Common setup is a tiller like this ProModeler PDRS40D-25T on a DS210CLHV servo in a 84" TBM


Problem with this setup is the downwind cable, the one *not* doing the pulling, goes a little bit slack. Obvious downside is during certain maneuvers, like knife edge, turbulent air may cause the model to fishtail slightly because it lacks positive stability due to cable being a bit slack. No, not a huge amount, just a little. Point being, those seeking to refine their control setup adopt a different approach.

And it's a superior solution for dealing with cables. Moreover, it's the exact same one developed by sailors centuries ago for use with ropes, and is familiar to every mechanic in the form of a block and tackle. We're talking about incorporating a pulley instead of tillers into the control loop because this keeps both cables taunt.

Best part is, despite a modest diameter you get plenty of throw. Here's how it works.

- The guts of this 91" EF Extra 300 V2 with ProModeler DS630BLHV and PDRS50PP-25T for rudder

Step 1, align the rudder with the vertical fin. Best/easiest way is two bits of balsa with rubber bands. Select balsa wood of spruce stiff enough, but flexible enough to conform to airfoil shape. Rubber bands in front of the leading edge, and rubber bands again behind the trailing edge. As they clamp together the rudder and vertical fin become perfectly aligned. These are going to work as a 3rd hand to help you when making your crimps.

Note, alternatively, as in this photo, just use tape!

Reason for this is we're going to be working with a continuous loop of steel leader cable instead of two individual pieces. This means only two ball linkage assemblies instead of four.

- Two pieces of wood align the vertical fin with rudder to make fastening and crimping easier.

Step 2, is simple enough, crimp your hardware as usual. 30-50# leader is plenty. I'm sourcing a really nice 7x7 cable out of China, stainless, coated. More flexible than the normal 1x7 I find in tackle stores, but either work fine. Anyway, I am still working on sourcing the remaining bits and bobs so as to offer everything you need in one package. Or buy your own, I don't really care because this is not the kind of thing you make money on (meaning these are commodity items available at any fishing an tackle store in America). They're more about convenience for the customer.

So the trick, if there's one continuous wire lead from one rudder control to the one on the other side. Begin by threading the turnbuckle into the plastic link. Run it all the way in to where you want the taunt cable inside the plastic ball link, and then before you crimp it, back it out 5-8 turns. This is enough let you tauten the cable and remove slack.

This is very easy, just don't go painting yourself into a corner where you're unable to remove the slack and then have to snip the cable and crimp it again.

- Note the savvy use of anti-twist bearings, also available in tackle shops

Step 3, is easy enough, you loop around the pulley's fixed post, making a figure-8 beneath the clamping bolt (M3 Allen Head), and route the cable around and back out to the other side's rudder pickup point.

So this is how it looks heading aft . . .

- Extreme Flight's larger models have carbon tube through which you route the servo extensions

. . . and this is where the magic happens.

- A continuous loop makes a figure-8 around the fixed post and under the head of the M3 Allen

And the benefit of the pulley for pull-pull control is unlike a conventional tiller arm where the non-pulling side goes a bit slack, the principle of the pulley keeps both legs taunt. The more perfectly you build your rudder pick up points (the control horns), the better. The goal, just as with a tiller, is for a straight edge between the two pick up points to be perfectly in line with the hinge line.

Since a pulley works rather nicely for cables, as shipwrights discovered centuries ago, why models reach for a tiller for this job is a bit of mystery. And yes, we offer tillers but their principal purpose is linking two mechanisms together, e.g. harnessing two or more servos together into a gang. But for cables? Pulleys are far superior for this job.

We offer three, the 43mm in 25T and the 50mm in both 25T and 15T;

. . . the 34mm suited for models to 78" and the two 50mm being great for 85" through 40%.

The 25T being suited for servos folks are familiar with featuring the nominal 6mm diameter 25T as popularized by Futaba. And the 15T being for gnarly output shaft as used in our BLS2 servos (DS635/845/1155) and Quarter-class servos.

How do they compare, This photo shows two servos, the 25T ISO (nominal 6mm diameter as used by us, Futaba, Savox, MKS, et al) side-by-side with our 15T (8mm nominal diameter).

- Gnarly 15T output spline (8mm nominal) dwarfs the ordinary 25T output spline shaft

Reason for this is some of our servos are getting to be so fast and powerful they can accelerate loads quickly enough to destroy servo arms and damage gear teeth. So the fix was easy, we delved into the parts bin and adopted the larger spline from our Quarter-class servos (ranging from 1505oz-in to 2685oz-in).

Another thing we did is increased the shaft diameter, and added two more (so the BLS2 are a 5-shaft design instead of a 3-shaft design). Larger diameter and more shafts lowered the pressure loading on the components enough to allow them to live (e.g. stop rapid self-destruction under load).

- The observant will note we reinforce the 6061-T6 case with bronze inserts - hardpoints

The other thing I did was go to a physically larger gear tooth, when the profiles are longer (or taller, however you want to reference it), this put more gear face to work and once again, served to lower the pressure loading on the gear teeth. Survival of gears is all about managing the loads and these steps sorted the issues associated with increasingly powerful servos altogether rather nicely. This is best explained with a photo showing a servo familiar to many, the high quality and very nicely made Savox SB-2290SG Monster.

- A larger bearing due to the larger spline shaft means BLS2 servos handle higher loads

Finally, since we use 303 stainless for the bull gears and 18/4 for the pinions gears, I don't know of a finer assembly in the business. The bronze inserts serve as hardpoints within the 6061-T6 aircraft aluminum which we use to make cases. These, are fitted expressly where the steel gear shafts fit the bores for the shafts.

Honestly, without this critical step, then damaging the gears usually means also replacing a case. So once you add together the price of gears *and* a replacement case then you're hosed because the sum is usually uneconomical. This is the economic downside of buying cheaply built servos, e.g. folks don't discover until later what they own is a one-and-done.

Of course, words are cheap but our finer quality competitors (e.g. MKS and Futaba) reinforce their cases similar to what we do, also. As for the many others we have bought to take apart and examine? Well, the phrase caveat emptor has been a saying since Roman times. Or more succinctly, as the bard himself once put it . . . all that glitters isn't gold.

Last thing, the tremendous benefit of investing in better quality servos equipped with hardpoints within the case is so obvious you don't even need to be an engineer to understand it, you just need a set of eyeballs Mark II. To see what I mean, when a customer had a hard arrival with his 40% model and damaged two servos quite badly, he wrote asking if I thought they could be repaired, or if he should toss them and buy new replacements.

I suggested nothing ventured, nothing gained. So he returned them and we took photos of the repair process.

Thanks for allowing me to explain how the pulley-system works compared to a conventional tiller set up. If you have further questions, or if anything isn't quite clear, reach out to us by phone at 407-302-3361, or via email: info@promodeler.com

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