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Old Dec 10, 2014, 07:36 PM
Master of the 1 point landing
ckimmey's Avatar
United States, IN, New Palestine
Joined Aug 2011
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Bellcrank alternative

I was just looking at my Lysander wing and getting ready to work on the bellcranks for flap controls. To my dismay, I found that I don't have any in stock! The space between ribs is only 1", so I would have to cut down the bellcrank arms to begin with or make relief cuts in the ribs to accommodate movement.

Then, I looked at a little box just full of unused servo horns. A standard round Spektrum or Futaba servo horn (of which I have many) would fit nicely in the bays and would have the exact movement of the flap servo (if I mount a round horn on it).

So, has anyone ever used a servo horn in place of a bellcrank? With the proper washers and a nice lock nut it would be stable.

Any thoughts?
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Old Dec 10, 2014, 07:58 PM
KE your cub.
Curare's Avatar
in the gutter, again....
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I have.

Provided the loads aren't so great, it works OK. The big problem I hit was the head of the bolt snagging on the splines, (i ran it upside down) but a little bit of work removing the splines made it smooth enough. It lasted the lifetime of the model , a .25 sized Cessna 152, which was about 3 years.

These days I would probably aim to hide a servo in the wing, rather than use bellcranks. I really got irritated by the amount of slop produced by having one servo drive the ailerons through a linkage train. Personally I prefer no slop setups to scale fidelity but to each his own.


...Oh and for what it's worth I wouldn't even dream of using gold'n rods in a wing ever again. Temperature changes would change the length of the rods so much that the ailerons would kick up when cold, and then droop on a hot summers day, effectively changing my incidence! ICK!
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Old Dec 10, 2014, 08:56 PM
Master of the 1 point landing
ckimmey's Avatar
United States, IN, New Palestine
Joined Aug 2011
453 Posts
These will be for flaps only, so the only load on them will be in a pulling motion. I have a plan for minimizing the slop and for avoiding the teeth in the mounting ring. I will post it once I am done (the glue is drying right now).

And, the ailerons will be on dual servos.

Thanks for the gold'n'rod advice. I will be using them on elevator and rudder, but the run to the flaps is too short to mess with the rods.
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Old Dec 11, 2014, 01:14 AM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
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If you use the right size wire and use an "L" bend in the ends instead of a "Z" you can achieve a very nice slop free fit. So the only source of possible play becomes the center bearing. But there again if you can drill out and sleeve the output wheel with a snug fitting brass tubing bushing then use the next size down as a sleeve for the hold down screw you can have a low friction bearing with very little play.

The "L" bends on the end of the wire can be secured very nicely by using a Dremel and cutoff wheel to grind a hollow on one side. Then fit on a collar of suitable size brass tubing and crimp the tube into the groove to lock the collar in place.

In the end bell cranks can be a source of unending slop. But if done with some attention to the fits we can attain very low slop. The devil is in the details though.
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Old Dec 11, 2014, 07:59 PM
Master of the 1 point landing
ckimmey's Avatar
United States, IN, New Palestine
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So, here's what I came up with:

Take 1 round servo horn. Decide what size screw/bolt you will use to mount the bellcrank (#4 screw is just right). Find a bit of tube that allows free rotation of the screw/bolt ( I had 1/8" ID aluminum tube on hand). Drill the center hub to allow tight fit of tube. Slide in the tube and allow 1/32" to stand above the highest point of the hub (the lower side, with the mounting collar). Drip some for of glue in between the collar and tube (I used thinned wood glue) and allow to dry. Cut and file the opposite end of the tube. Place washers on both ends to allow smooth rotation. I will be using a lock nut to be safe and 1/8" ply base inside the wing. I am trying to remember that I will hopefully never see this bellcrank again once the plane is finished. I think it will do the job nicely.
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Old Dec 11, 2014, 08:33 PM
KE your cub.
Curare's Avatar
in the gutter, again....
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I wasn't using the wheel servo arms, I was using the cross style with two legs cut off to make an L style bellcrank.

My only concern with that setup is the distance from the pivot to the hole isn't a lot, so your mechanical advantage isn't great. Ideally you'd want a large bellcrank that moves a little, rather than a small one that has to move a lot for the same travel. Your linkage geometry will suffer as a result.
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Old Dec 11, 2014, 08:46 PM
Master of the 1 point landing
ckimmey's Avatar
United States, IN, New Palestine
Joined Aug 2011
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Quote:
Originally Posted by Curare View Post
I wasn't using the wheel servo arms, I was using the cross style with two legs cut off to make an L style bellcrank.

My only concern with that setup is the distance from the pivot to the hole isn't a lot, so your mechanical advantage isn't great. Ideally you'd want a large bellcrank that moves a little, rather than a small one that has to move a lot for the same travel. Your linkage geometry will suffer as a result.
The rib bays are only 1" wide, so the wheel fits best. Also, this is a .25 sized plane with fairly thin flaps. I am not looking for a lot of throw in the movement of the crank, so I am betting on it being adequate for my needs. If I need to make it L shaped, I will cross that bridge when I get to it. And, the way I see it, the bellcranks will turn the exact amount as the servo since I am planning to use the wheel as the servo horn also. Everything will be a 1:1:1 ratio with no gain or loss of mechanical advantage.
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Old Dec 12, 2014, 01:07 AM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
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Quote:
Originally Posted by Curare View Post
I wasn't using the wheel servo arms, I was using the cross style with two legs cut off to make an L style bellcrank.

My only concern with that setup is the distance from the pivot to the hole isn't a lot, so your mechanical advantage isn't great. Ideally you'd want a large bellcrank that moves a little, rather than a small one that has to move a lot for the same travel. Your linkage geometry will suffer as a result.
Actually he'll be OK as long as the pushrods both meet the holes at a 90 angle to the line through the pivot hole. That way the differential action will cancel out. If it wasn't for this things would get ugly in a hurry.

Ckimmey, even with that you need to ensure that the your wheel style bellcrank does not need to move to an angle greater than around 50. If you try to use a bigger angle then more and more of the push and pull at the extremes will go into trying to move the bellcrank mount inside the wing instead of to the control surface. So realistically you are limited to using the same size output at the servo as you're using for the bellcrank.

If this last fact can't be done then you MUST go to a larger bellcrank.

Although I do agree that it's nice when the bellcrank can be made bigger. But it seems that ckimmey has built himself into a corner by the rib spacing and apparently the rib thickness at the location of the bellcrank. Otherwise Curare's worries will bite you where you sit.
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Old Dec 17, 2014, 05:37 PM
I'm a pilot... 100 yrs to late
Thermalin's Avatar
USA, FL, Palm Harbor
Joined Jan 2005
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Why not semi flex nyrods - one piece, no slop and can easily be supported in multiple places along the way so no flexing.
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Old Dec 17, 2014, 07:26 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
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Quote:
Originally Posted by Thermalin View Post
Why not semi flex nyrods - one piece, no slop and can easily be supported in multiple places along the way so no flexing.
Two reasons. One is that there's inevitably some slop between the inner and outer tubes. And in a high angle bend like a 90 as needed to run flaps or ailerons that slop really shows up as dead band in the movement of the surface and play at the surface which can aid in supporting flutter.

Second is that Nyrods are highly temperature sensitive. The two times I've used the darn things on gliders where the rudder throw angle matches the servo throw angle the rudder trim at the trailing edge of a 2 inch wide rudder would move almost 3/8 of an inch. I needed almost the entire rudder trim lever throw to compensate for the temperature change from coolness of the morning to a hot afternoon.

I still use Nyrods. But now I only use them for throttle controls on engine powered stuff.
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Old Dec 18, 2014, 12:31 PM
I'm a pilot... 100 yrs to late
Thermalin's Avatar
USA, FL, Palm Harbor
Joined Jan 2005
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hmmmm I have not experienced that with the inner star shaped cross sections.. only 5 pts of contact or so and the contact is constant on all sides throughout the application, but I d understnad what you're saying with with the round in the round type. as well as the expansion \ contraction due to temps across all types. why not just stick a servo out there in each panel
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Old Dec 18, 2014, 12:50 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
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The ones that gave me the worst issue was the old white versions. But the Dubro red and yellow are not immune by a long shot.

Form the Dubro ones around in a big loop and then with one finger resting lightly on the inner where it exits on one end move the other in and out to get a feel for the slop. I think you'll be surprised at how much there is even with this better option.

In nearly straight line applications it's not as big a problem. But one of the more often used roles for the stuff is a central aileron servo and 90 degree bends out in the wings.

But in the end I tend to agree that in this age of cheap and yet still decent performing servos the obvious option is small minis or micros out in the wing with a direct wire link.
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Old Dec 18, 2014, 03:47 PM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
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Bellcrank alternative ?

Pull-pull cables, you run them round a pulley wheel to change direction, e.g. 90deg.
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Old Dec 19, 2014, 09:46 AM
I'm a pilot... 100 yrs to late
Thermalin's Avatar
USA, FL, Palm Harbor
Joined Jan 2005
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?? what you talkin bout Willis... (different strokes ?)

I understand the turn-around and the pulley but not sure how its applied here...
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Old Dec 19, 2014, 11:03 AM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
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Quote:
Originally Posted by Thermalin View Post
?? what you talkin bout Willis... (different strokes ?)

I understand the turn-around and the pulley but not sure how its applied here...
It's an alternative to a bellcrank and pushrods.

For ailerons you could use a single servo in the middle, pull-pull wires out to a top and bottom horn, with a pulley wheel used to change the direction through 90deg.

For flaps, (never tried it on flaps), you could possibly use a single pull cable with a spring return to neutral.
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