Full size stunt ship. Well, the plane flys like the lines are sticking together or there are springs in the flying lines. Just sluggish, jumpy and uncontrollable. No way I can fly any kind of good pattern with it. Been trimming it up and down every parameter no joy.

I figure I got some bad advise about bellcrank design and I went ahead and built the plane with it in there. My fault, I did it as an experiment that failed. Now I want to change the bellcrank to something I know works. 

It's a full fuselage plane with a sheeted foam wing. No great finish on it so cutting into it is no problem. I'm worried about strength in the center section and wing folding after I'm done. Do you think it would be a problem?

I plan to cut the top off the fuselage, dremmel the ply plate that holds in the axle then cut the push rod exit big enough to get the bellcrank out.

If I keep my destruction inside the fuselage do you think it will be ok after I patch it up?

Thanks,
MM

I had to do that. Doing what I thought of as a "good job" was much harder with the foam wing. The picture shows the job partway done, and before any structure was added back in. I had to have enough space to replace the bellcrank, the pushrod and the leadouts. I added back in the equivalent of planking and there was some glass added as well. I'm still trying to get the thing painted, so I can't prove my repair was adequate--but I think it will be ok.

Good luck with yours....

 

Full size stunt ship. Well, the plane flys like the lines are sticking together or there are springs in the flying lines. Just sluggish, jumpy and uncontrollable. No way I can fly any kind of good pattern with it. Been trimming it up and down every parameter no joy.

I figure I got some bad advise about bellcrank design and I went ahead and built the plane with it in there. My fault, I did it as an experiment that failed. Now I want to change the bellcrank to something I know works. 

It's a full fuselage plane with a sheeted foam wing. No great finish on it so cutting into it is no problem. I'm worried about strength in the center section and wing folding after I'm done. Do you think it would be a problem?

I plan to cut the top off the fuselage, dremmel the ply plate that holds in the axle then cut the push rod exit big enough to get the bellcrank out.

If I keep my destruction inside the fuselage do you think it will be ok after I patch it up?

Thanks,
MM

(I like the plan of going in through the top. Most people ignore that a fix like this has to address flex in both directions.  I would remove the top all the way back to well aft of the flaps. I would then cut away as little of the center section planking & foam as cleanly as you can and leave at least a 1/4" lip on the sides.  With that much room to work you should be able to get the old BC out.  Hopefully you did not glue in the wire post.  Don't forget to attach something to the leadouts  to pull the new ones through.  Spectra works good for that.  Your problem will be spreading of the fuselage top in outside turns.  The only way to prevent that is with exceptionally strong covering of the opening and a couple of new formers.  Assuming you are planked with 1/20 or 1/32 you haven't much to glue to so try this.  Get some thin hardwood construction sticks from a craft store.  Plywood will flex too much.  The ones I use are 1.5mm x 10mm.  Cut them long enough to slide under the planking about 1/2" on each side pushing the foam a bit.  Sharpening them like wedges helps. Work them in so that you can glue them in under the existing planking.  One on each end and ever inch or so through the opening.  They will serve as the floor for the center planking. Epoxy them in then carefully drill small holes through the fuselage sides through the strips and run a toothpick through.  Soak the toothpick with CA.  This both attaches the wing planking for the pull of an outside and stiffens for the crush of an inside. Plank the hole and glass the crap out of it.  And add a former or two while you are at it.
Finish and fly as usual.  Shoot us some pictures, I want to know what the bellcrank that didn't work looked like.  If this doesn't work, please delete this thread!  If this repair will make the plane lighter all I can say is 

Good luck.  This message will self destruct in 5 seconds

Ken

Before you go full Ben Carson on it can you get a camera in there and see what you actually have.  4 1/4 isn't the problem, the offsets could be but what if you do all of that and the BC was just a big BC with ball bearings.  I got my hands on a plane like that a while back and came to find out that the leadouts were reversed in the wing, rubbing and sawing away at the ribs.  Not likely with foam but....  Now if it is one of those crazy "Y" shaped ones, Call Ben.

Ken