Suspended Bellcrank Questions

[Motorman]

I'm making a 40 sized profile plane and I'm considering my bellcrank placement options.

Of course there is plywood top and bottom but, is it ok to put the (1/8") pivot rod through the (1/4") center rib? The way the push rod lines up with everything it's a real good place and easy to space top and bottom but I worry about the rib breaking. 

Do you really need to have the rear line pull for down because that puts the control junk on the inside where I can see it while flying. If it's no big deal I'd rather keep it clean looking on the inboard side. Lead outs are about 3/4" apart.

Thanks,
MM
   

[Perry Rose]

As long as the post is captured top and bottom it should be fine. There is no noticeable advantage having the up line in front. There is a down side though. If the line snag a weed on the take off run the plane will jump into the air pivot around the snag until the line comes free and by then you are the target.

[Brett Buck]

Do you really need to have the rear line pull for down because that puts the control junk on the inside where I can see it while flying. If it's no big deal I'd rather keep it clean looking on the inboard side. Lead outs are about 3/4" apart.

   No, you don't need that. I am surprised people still do it, that was a marginal concept from 40 years ago.

     Brett

[Tim Wescott]

If you really want the up line in back then don't worry, just put it there.  The advantage, if any, of putting the up line in front is minor.  If you're really worried, put the bellcrank a couple of inches into the outboard wing, or just cross the leadouts inside the wing.

I would seat the pivot in plywood.





Here's an alternative, from a Brodak Legacy kit -- the pivot for the bellcrank only goes into balsa in the wing, but after it's all put into the fuselage, there will be a plywood plate on top and bottom that ties it into the fuse structure.  This doesn't help on a profile -- but on a profile you could put the pivot just the right distance outboard of the fuselage center and extending out of the wing by a bit, then install it flush against the outboard fuselage doubler.

[Gerald Arana]

What I do in this case is to run a piece of (hard) balsa, 1/4" sq. between the ribs at the front and back, top and bottom, of your ply mount to attach it to the sheeting. I've yet to have a strength problem.......or a BC pull out!

Good luck, Jerry

[Air Ministry .]

Yea , you tend to put one or two braces in there fairly securely , so you can pull test it up to ' line stretch ' level , on solids .

Tried to get sense on a Twin Boom job , wether it afected ' things ' putting the Bellcrank at the Inner or Outer BOOM .
My view is the Outer Securing point would be preferable .

Which leads us TO , if Your Bellcranks Reversed , The Center Rib is on the centerline , the Pushrod arranged to be adjacent to outer fuse side - likely Less Drag outboard

The PIN / Pivot would end up say 1 1/2 in outboard from fuse centerline . All my profile twins ive done like that .

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The Big aces shifting adjustable leadouts for optimum performance  , one assumes they leave down aft .
If at the bellcranks Up Aft the Leadouts would cross ( in the wing ) with the Lineadout Guides adjusted to optimum . ??  ONE ASSUMES ! .

[Motorman]

If you cross the lead outs don't they rub?

MM

[Avaiojet]

If you cross the lead outs don't they rub?

MM

You flip the bellcrank 180 degrees.

Mike Griffin did a Thread on the floating bellcrank. Great photos and text. It may have been a video that was moved to the "Video Place," Where Robert's construction videos are.

Worth looking at.

Charles

[Brett Buck]

If you cross the lead outs don't they rub?

   Don't cross the leadouts, it's unnecessary, just have them come straight out the guide.

      Brett

[Tim Wescott]

If you cross the lead outs don't they rub?

MM

Yes.  I don't think it's worth that much to put the up line in front, but if you really want it...

[Steve Helmick]

If you tilted the BC (up at the rear is the usual way), you might be able get away with crossing the leadouts...but I still wouldn't do it. Bad Tim, BAD!  Steve

[Tim Wescott]

If you tilted the BC (up at the rear is the usual way), you might be able get away with crossing the leadouts...but I still wouldn't do it. Bad Tim, BAD!  Steve

Some local pilot told me to try it on one of my planes.  What was his name?  Sam Strider?  Pete Strider?  Oh wait -- Walker, that was it, Paul Walker.  If anyone else had suggested it, it wouldn't be in my repertoire.

(I wonder how often he suggests really oddball stuff because he knows how much weight his name carries.  To date he hasn't made a direct suggestion to me that has turned out badly, and putting a 46LA into a 64-ounce, 700 square inch Atlantis turned out to be a Really Good Idea).

[Igor Burger]

That was already several times analyzed - rear line up and tractor prop is the way :- )))

And regarding crossed lines - I had it, no problem on 49 strand 1mm cable in wing, and yes BC tilted down (front down, back up - just to make pushrod to flaps paralel to BC)

[RandySmith]

There is another  problem no one is talking about with crossed lines, I have seen a leadout guide worn out  quickly  from doing that
and if not very careful the  leadout cable will  rub each other at the  leadout guide

Randy

[Geoff Goodworth]

First, putting the controls on the inboard side of a profile model keeps exhaust crud out of the system—a plus if you use ball joints.

Second, on profile models, I like to have two ribs aligned with the edges of the fuselage plank so that any bending loads between the wing and fuselage result in compression through the ribs rather than bending in the centre sheeting.

With that in mind, I mount the bellcrank between the ribs with a tiny piece of engine bearer under the spar leaving just enough space for the bellcrank bearing.

I drill through the spars, spar doublers and the little blocks. I fit the bellcrank with the wire flush on the top and bottom of the wing and then drop Cya on the holes.

As per the pic, there is about 5/8" of timber support for the pivot rod both top and bottom. That pivot rod is never going to worry its way through the fuselage plank for the bellcrank to come loose.

[Motorman]

tractor prop is the way :- )))

I have yet to see this flying tractor.


MM

[Tim Wescott]

I have yet to see this flying tractor.


MM

[Mark Scarborough]

I rebuilt these beasts,, they are indeed tractors...

[Mark Scarborough]

big bad beautifull beasts

[Howard Rush]

That was already several times analyzed - rear line up and tractor prop is the way :- )))

1. Assuming "tractor" prop has + rotation about the x axis, wouldn't both the yawing moment from pitching the rotating machinery and the yawing moment from pulling harder on one line be in the same direction? 

2. Has anybody ever quantified these moments?

[Howard Rush]

 

And regarding crossed lines - I had it, no problem on 49 strand 1mm cable in wing, and yes BC tilted down (front down, back up - just to make pushrod to flaps paralel to BC)

49-strand cable?  What material was the cable?  Were the strands soldered together?  I would like to use more flexible cable than our traditional .031" 7-strand stainless steel cable, but I fear that individual strands would break, causing bad things to happen. 

[Brett Buck]

1. Assuming "tractor" prop has + rotation about the x axis, wouldn't both the yawing moment from pitching the rotating machinery and the yawing moment from pulling harder on one line be in the same direction? 

2. Has anybody ever quantified these moments?

  Yes, but I would have to dig around for it. The precession in the corner was MUCH larger than the static yaw torque for the spacing and line tension I used.

     Brett

[Igor Burger]

I have yet to see this flying tractor.


MM

that? : -P

... and no, it is not PSed :- ))

[Reptoid]

1. Assuming "tractor" prop has + rotation about the x axis, wouldn't both the yawing moment from pitching the rotating machinery and the yawing moment from pulling harder on one line be in the same direction? 

2. Has anybody ever quantified these moments?

   I think you got it Howard
Even though the reaction to front line up is small on a relatively large stunter I don't see how you others can think rear line up is better for a tractor propeller. That would mean that Rabes movable rudder was moving in the wrong direction?? In addition, when I was flying a lot of combat I installed the horn on the bottom of the stabilator on a couple (giving rear line=up) and they definitely did not fly as even inside vs. outside as the normal front line up set-up. Enlighten me please

[Igor Burger]

2. Has anybody ever quantified these moments?

Yes I have it in my old spread sheet, (I took it from old program do by Lou Crane, just improved enumeration of moment inertia for BE carbon props we used that time - 10 or 15 years ago)

Result was clear, modern models with large bellcranks and limited feedback from controlls need very large LO distance to make something usefull so it supports what Brett says.
 

[Igor Burger]

1. Assuming "tractor" prop has + rotation about the x axis, wouldn't both the yawing moment from pitching the rotating machinery and the yawing moment from pulling harder on one line be in the same direction? 

... so (after previouse commet) the answer is clear, does not matter.

The point of up line back is in influence of precession to controlls. If model pitches UP because of something, the precession will yaw nose out, line spacing will make input to lines which will pitch model DOWN. That makes negative feedback which will make some kind of damping and gives better controllability.

If you put up line front, it will, as any other positive feedback in controlls, make positive feedback what will make hunting worse and also more difficult fly offs from corners.

However if you use Rabe rudder, it can change sometimes, I wrote it somewhere in past - when I trim model, I always try both ways, from all my models I had only one which flew better with front line up, that one had crossed lines inside, because I build always up line back.

that is also answer for Reptoid.

[Igor Burger]

49-strand cable?  What material was the cable?  Were the strands soldered together?  I would like to use more flexible cable than our traditional .031" 7-strand stainless steel cable, but I fear that individual strands would break, causing bad things to happen. 

It is Robbe Stainless Steel cable, very flexible, wires are not soldered. No problems with that so far. BUT I had some other cable in aproximately 2008 model which did just that what you write in LO only after one seasone, so I needed replace it. It was brain stroming how to do it, but after all I found way with only one 8mm hole in wing later covered by sticker :-P

Report from opartion will be also somewhere here in history.

Now I ordered 19strand cable, may be my new model will use that, I will see when it arrives. But it is not so flexible.