Best Bellcrank system for a competitive stunter

[maverick11359]

 Hi guys, I'm about to complete a new 59'wing and need advice on best setup for bell crank . I currently use 3" bell cranks and up line at the rear of the crank in m other models. My concern using up line on the front is the carbon fiber push rod being on the inside of the crank and interfering with the rear  line connection to the crank..So my question is there much difference and to be gained having up at front .. Another thought, maybe closer leadouts at tip together say 3/4" or 1/2" apart, would this make less difference to having up at front  . I could always make the leadouts  say 1" or 2" different length outside the wing tip, so they don't fowl on each other. Any ideas are appreciated.

[Tim Wescott]

3" bellcrank on anything over 40" wingspan -- no.  Just no.  Use at least a 4" bellcrank.

Up vs. down in back -- with plenty of bellcrank travel (I go for 40 to 45 degrees at least) there will be a crossover point.  Between the ball link that you do use on that bellcrank (right?) and the fact that you're driving the flaps should give more than enough clearance for a good low-friction line attachment to the bellcrank.

If you're really worried about the leadouts fouling the pushrod, hunt down Brett Buck's pictures of the setup he uses (and be careful while building -- you want to use end grain balsa between the aluminum bits, as he designed it; otherwise the thing will fail under load).

[Curare]

Now for the fun question, flap bellcrank in (up line forward) or out (down line forward)??

[Dan McEntee]

   I like the down line in back. It's not a great big deal all by itself, but just one little detail out of a lot of other little details that add up to a well trimmed and good flying model.
     Tons and tons has been written on bell cranks and control systems. Fix yourself a snack and your favorite drink and search.
  Type at you later,
   Dan McEntee

[Dennis Nunes]

Here is a copy of a 4" reverse bellcrank (up line in front). It's basically a modified Brett Buck bellcrank using the same laminated setup. As Tim noted make sure to use "end grain" balsa for the core. I hope this makes sense.

Dennis

[Mike Haverly]

Just made one of my standards pretty much like Dennis' sketch. I use a 3/4" long brass bushing with a .125 dowel pin.  The hole
 is reamed to .1255 for a very close fit. Titanium would be great, I don't have any.  Up line
Is always in front. 

[Dennis Toth]

Mav,
You didn't say what power system you would be using but if electric you could simple set it up for clockwise rotation and use a pusher (lots of APC pushers props), this would allow you to use the conventional bellcrank installation it will act the same as counter clockwise with the back line down.

Best,    DennisT

[Jim Svitko]

There are several bellcrank systems that will work.  I never liked the idea of the stranded leadout cable thru the large teardrop loop of tubing so I went with a slotted end bellcrank with a grooved teardrop fitting for the leadouts.

I now make these from phenolic (Garolite?) since the phenolic is easier to cut and shape.

[Tim Wescott]

I would steer you away from aluminum sandwich bellcrank. I have a big piece of 1/8" garolite LE I got from McMaster Carr I cut any design I need. https://www.mcmaster.com/catalog/127/3947 Just glue little pieces top and bottom to make it 3/8" thick at the pivot so it won't tilt.

Aircraft Spruce has phenolic for much less money.

Yes to everything else -- even the bellcrank.  I was just suggesting a Brett crank because he's worried about interference.  I use the same stuff (probably the same McMaster part number), but I've mostly been making bearings out of aluminum, glued & screwed to the phenolic.

[Mike Griffin]

I always used the control system that Tom Morris made.  I think he sold that part of the business to Okie Air.  I do agree for that size model you would do much better with a 4"  bellcrank.

Mike

[maverick11359]

Here is a copy of a 4" reverse bellcrank (up line in front). It's basically a modified Brett Buck bellcrank using the same laminated setup. As Tim noted make sure to use "end grain" balsa for the core. I hope this makes sense.


Hi , Dennis thanks for the help, I've been out of the flying circle to long since the 70's ,so have not kept up with generations of improvements .I only started building and flying gain in 2020 since covid kept us at home here in Aussie land, so this is my 3rd stunter. Previous kit built chipmunk and then scratch built JVLoo older original modified chipmunk with Sig and metal Brodak bellcranks . So forgive me for maybe asking some probably basic and simple questions. After looking at the diagram of this bellcrank , can i ask is the the 1/8" pivot rod fixed to the bellcrank maybe epoxied,or does the bellcrank pivot on the rod between the wheel collars.Or is the rod and bellcrank all one solid fixed unit and the rod pivots. Do the 5'32 wheel collars clamp onto the rod with grub screws or stay unfixed and can rotate with the washers above and below ...?

[maverick11359]

Hi guys, Thanks for the helpful information.I should have said up front that I'm building another stunter my dad draw up , modifing the JVL chipmunk back in the 70's .Sadly dad passed last year but wanted me to build it because we never got to it back in the 70's, it was going to be the next generation for us to fly. It's the JVL chipmunk 56" wingspan version with 7% increase in fuselage / wing becoming a 59" wing span /46" fuselage with 10%increase in stab and elevator making them also 27" wide .Up front powered by a Super Tiger 46 with uniflow 5.5oz tank. Fuselage design changed with small slim line canopy infront of wing ,it looks something like the stunter Stilletto back in the day. So I've built the tailplane and at about  2/3 of the wing. Bellcrank and controlls stage ..

[Dennis Nunes]

After looking at the diagram of this bellcrank , can i ask is the the 1/8" pivot rod fixed to the bellcrank maybe epoxied,or does the bellcrank pivot on the rod between the wheel collars.Or is the rod and bellcrank all one solid fixed unit and the rod pivots. Do the 5'32 wheel collars clamp onto the rod with grub screws or stay unfixed and can rotate with the washers above and below ...?

Hi Maverick11359,

The wheel collars should be 1/8" not 5/32" as shown in the drawing. My mistake.

The pivot rod is a tight fit through the plywood bellcrank spars but needs to loose enough to slide into place. The wheel collars with the setscrew (grub screw) is tighten to the pivot rod. On the final assembly I use a drop of Loctite on the set screws. The wheel collars act as a retainer to keep the bellcrank from wobbling and from sliding up or down on the pivot rod. Once everything is in place, the pivot rod gets a dab of epoxy on each end to hold it in place. With the wheel collars and the dowel spacer, there should be little to no movement of the bellcrank sliding up or down on the pivot rod or the pivot rod moving up or down.

BTW, this was a bellcrank for a unique situation. Normally on a full built up fuselage I would keep the pushrod from the bellcrank on the centerline of the fuselage and offset the pivot rod as necessary. Ignore the .1875" offset from the centerline of the fuselage to the centerline of the pivot rod.

Attached is an updated drawing. Hope that helps.

Dennis