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More Fun with Bellcranks! Jack Sheeks' "Sea Vixon"

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More Fun with Bellcranks!   Jack Sheeks' "Sea Vixon" Empty More Fun with Bellcranks! Jack Sheeks' "Sea Vixon"

Post  Kim on Tue Jul 24, 2012 8:26 pm

Not so much more info on lead-out and bellcrank placement as just an interesting method Jack Sheeks used to evenly actuate a wide elevator on his twin-boom stunt ship without an appearance-spoiling central pushrod .

Still, got to wonder about the springyness of that tall "Z" bend hidden in the rudders!

More Fun with Bellcranks!   Jack Sheeks' "Sea Vixon" Sea_vi10

More Fun with Bellcranks!   Jack Sheeks' "Sea Vixon" Sea_vi11

Some day, I GOT to build a wing with sliced ribs...

More Fun with Bellcranks!   Jack Sheeks' "Sea Vixon" Sea_vi12
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Post  Ken Cook on Tue Jul 24, 2012 8:44 pm

Kim, I agree with your findings. My F-82 twin mustang with the Fox.35's was wrecked early on due to a single pushrod. I was getting flex in the stab itself that would mush out in the bottoms. I switched to a dual pushrod coming from the wing flap. Someone on Stunthangar recently built this plane. Jack is a terrific person and I certainly admire all his designs. The Z as you stated is certainly subject to flexing and I've built planes doing the same thing only to have to rip back into them to redo in a straight fashion. This would certainly be redesigned if I built it probably designing a flap horn with a extended horn on it dropping down to ball link to pick up the pushrod. Ken
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Post  Kim on Tue Jul 24, 2012 9:16 pm

Yeah, the bigger the plane, the fancier the rules for making it work !

Well, I admire the person who'd recreate one of these !!! By my standards, this is an extreme "Builder's Airplane". I'd have a hard time covering that beautiful wood work !
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Post  Ken Cook on Tue Jul 24, 2012 9:39 pm

Nothing like an I-beamer. I've always admired the Jet Stunters of the 60's into the 70's with the I-beam designs.
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Post  Cribbs74 on Tue Jul 24, 2012 11:09 pm

Another fine model Kim!

Must be pretty cool to sit down after a day of flying and read some neat old mags.

Keep em coming!

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Post  Ken Cook on Thu Jul 26, 2012 2:38 pm

More Fun with Bellcranks!   Jack Sheeks' "Sea Vixon" Dscn0728
Kim, I found the plane that was recently built. Certainly an awesome task and I do recall the builder had some serious paint issues requiring an extensive redo. All in all I remember the plane in bones and it was most impressive. Ken
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Post  gcb on Thu Jul 26, 2012 4:07 pm

shawn cook wrote: Kim, I agree with your findings. My F-82 twin mustang with the Fox.35's was wrecked early on due to a single pushrod. I was getting flex in the stab itself that would mush out in the bottoms. I switched to a dual pushrod coming from the wing flap. Someone on Stunthangar recently built this plane. Jack is a terrific person and I certainly admire all his designs. The Z as you stated is certainly subject to flexing and I've built planes doing the same thing only to have to rip back into them to redo in a straight fashion. This would certainly be redesigned if I built it probably designing a flap horn with a extended horn on it dropping down to ball link to pick up the pushrod. Ken

Ken,
I'm not sure what you are saying. Jack's plane has controls in each boom that go from bellcrank to flap horn, then from flap horn to elevator horn. The two bellcranks are ganged (connected) by a 1/8" wire. Perhaps I am seeing it wrong.

George

Edit: If you use an extended horn, remember that you must compensate on both horns and the bellcrank (both sets) to maintain a geometry that will provide proper amounts of control and proper angles less you end up with something that has more control one way than the other. For example, when you move the bellcrank(s) 30 degrees each way from center, the arc must provide equal deflection both ways on the flaps and elevators, at whatever ratio you have them set for...like 20 degrees on the flaps and 30 degrees on the elevator.
Perhaps an easier solution would be to reinforce the pushrod at the "Z" bend with an additional piece of wire. Just a thought.
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Post  Ken Cook on Thu Jul 26, 2012 5:28 pm

George, my statement was in direct regards to the z-bends at the rear of the plane. The plans show what I see to be 3/32 pushrods. It shows 1/8th wire from bellcrank to bellcrank. I feel that the 1/8th wire for as long as it is should also have some type of fairlead as well as that certainly could be subjected to bowing . Although 3/32" wire is quite strong, when placed in that z formation they certainly can flex under load. The fact that there is one on each side obviously takes the load off as both pushrods are sharing the load to the stab. This is more than likely the reasoning that this works. If I was to redo this setup, I would make new flap horns from the stab with an extended horn brazed onto the wire so that I could eliminate those bends. This of course would be on both sides of the stab as the plans currently state. The horn would be internal within the fuse. That's just me though. I've had to redesign control systems before due to flexing that made planes almost unflyable at times. I know it may sometimes sound as if I may be pointing out a flaw. My posting wasn't directed towards that at all. In fact I'm always in admiration of those builders and modelers that have made this hobby the best of the best. Ken
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Post  gcb on Fri Jul 27, 2012 9:12 pm

Ken,

I too would add some fairleads here and there. Does the missing text mention fairleads?

George
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Post  andrew on Fri Jul 27, 2012 11:28 pm

Carbon fiber tubing -- much stiffer and lighter than any wire you can use.
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Post  Ken Cook on Sat Jul 28, 2012 5:54 am

Andrew, I agree with the carbon fiber or arrowshaft rods, it's just that you still have the mechanical disadvantage at the rear with those bends. It is mentioned in the build article about placing the stab low on the booms and making the center portion of trailing edge a moveable flap. I myself prefer the high stab as anyone knows this is a bit of a complicated task. I've seen these situations in planes such as the OV-10 Bronco and the Mig-15 /19. In the case of the OV 10 that I saw, it was scale and wasn't very maneuverable the builder had an additional bellcrank in the boom at the rear. My problem with that is you start developing small amounts of play in each link. I suppose ball links would work.

As I mentioned above, the fact that Jack used 3/32 wire on this build is probably sufficient enough between both pushrods to keep things without flexing. I always find myself anymore trying to get the pushrod in the straightest configuration as anytime a z-bend of a length say greater than 1" always seems to fail me. I can name a dozen designs that use a z-bend as soon as the pushrod exits the sheeting and I've had many fail me. If you receive Model Aviation, Tom Niebuhr is featured in this months control build the "Hobo". Tom is responsible for the build I posted above. I would certainly like to ask him if he made any additional changes to the print. Ken
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Post  andrew on Sat Jul 28, 2012 10:43 am

shawn cook wrote: Andrew, I agree with the carbon fiber or arrowshaft rods, it's just that you still have the mechanical disadvantage at the rear with those bends.
Ken

Yes, there is a significant length where the wire is subject to lateral forces. I don't use Z-bends at all except on throttle linkages -- there is little resistance and it doesn't change with speed.

Like you, my inclination would be to use a bellcrank in the lower boom and come straight up with a pushrod. I know that many of these designs with long elevators suffer from twisting (hence the use of multiple servos on big surfaces in RC), but this design came about before many of the better building materials were available.

I am also a big fan of ball links. They have no slop and are not nearly as subject to binding as the other connectors we use. I have used CF tubing as a leading edge on elevators and ailerons. It adds some work to hinging, but keeps them straight and eliminates almost all twisting.

If I'm not concerned about looks (even my best planes look that way Sad ), I'll just wrap the elevator with clear packing tape.

andrew

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