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Author Topic: Three wire bellcrank  (Read 2372 times)
Fred Shattuck
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« on: March 23, 2008, 09:11:54 PM »

I am modifying my Chipmunk. I am going to a larger R/C type engine. I would love to try a three wire configuration, but don't have a clue how to do it. Does the tension on the carb wire interfere with the line tension on the others? We fly with 70ft cables and a little faster than most, with bigger engines. Does the engine rev more with or without the line being pulled? Do I need a little more right rudder or outside wing weight?  I appreciate any advice I can get on this. Thanks, Y'all   Fred Shattuck
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Steve Helmick
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« Reply #1 on: March 23, 2008, 09:32:10 PM »

With the J.Roberts system, the three lines share the line tension, and will require more tipweight and probably more leadout rake. At least some of the leadout position calculator programs ask if you'll use one, two, or three lines, how large and long they'll be, how fast the plane is expected to go, etc. That'd be a good place to start, if you insist.

Beyond that...I've not heard of much good coming from winding up three lines instead of the usual two. No doubt, some have done it, but then some folks have also flown monoline stunt. Probably not an easy path for most, plus the handles are incredibly clunky.

Remember that bit about sharing the line tension with three, not two, lines. Most our crashes are because we run out of line tension....that what powers our control system.   Cry Steve
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« Reply #2 on: March 24, 2008, 05:08:35 AM »

In theory, and according to carrier rules, it's supposed to be rigged with equal tension, 1/3 on each line.

However, on a sport stunter, you could set it up so the "up & down" bellcrank "bottoms out" inboard, thereby letting the throttle line get a bit limp and having more force to work the flaps and flippers. 

The in-an-out motion of the bellcrank will make rigging the flap horn a bit of a challenge.

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Paul Smith
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« Reply #3 on: March 24, 2008, 07:54:33 AM »

As you haven't done it before it is probably best to stick to a known workable system such as the J. Roberts based Brodak 3 line bellcrank and handle. It is designed to compensate for the elavator line slack when the throttle line is pulled.   Don't plan in it being much of a stunter though.  Simple loops and wingovers will work but overall that third line will be a detriment to flying the full pattern.  It is fun but does not offer any advantages in the stunt event.  If your interest lies in throttled flight, I suggest think more along the lines of one of the Scale or Navy Carrier events, or just use a throttled stunter as a sport and demo plane.
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« Reply #4 on: March 24, 2008, 08:43:37 AM »

A couple of years ago when I got to attend a contest in Fargo, North Dakota there were several people flying a three line system in there respective stunt class.  They did a very credible job.  They would throttle back between maneuvers.  DOC Holliday
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« Reply #5 on: March 24, 2008, 09:52:19 AM »

Just a caution on the three-line systems.  The older, J-Roberts and newer, Brodak have different ranges of motion of the the third line, I'm not sure which has more movement.  It is possible to get ALL the airplane weight on the throttle line if components from the two brands are mixed.  Make sure to stay with the same brand.
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Fred Shattuck
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« Reply #6 on: March 24, 2008, 02:45:09 PM »

Ok Y'all, I'm starting the reconstruct this evening. I am thankful for the info and cautionary comments, and admit it seems a little less the good idea it originally did, but you know how it is when you are stubborn.   I have noticed on the posts that at least one person uses electronic additions to his flying. Without going crazy on this thing, does anyone use electric throttle control, and if so, how?  I have never seen these three wire bellcranks, and I have a burning desire to build it myself. I envision the third wire curving around a separate pulley (above the bellcrank) to the carb with a light tensioned spring for recoil. The throttle would be wide open with the auxillary being pulled via a loop on the end for my finger to hang onto,  I hope to trim it on the ground to get all the lengths correct. The third wire will exit in between the original two and I'm going to try make a lighter wire work to my advantage. This is not really intended for any serious competition, just for the fun and challange. The responses I get mean a lot, so thanks again Y'all      Fred Shattuck
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« Reply #7 on: March 24, 2008, 03:12:39 PM »

The J-Roberts/Brodak system is the norm in Carrier because they make it so simple.  There's no reason to not do it your way, just requires two hands to do it.  In by-gone years some carrier flyers used a third wire attached to the aft end of the fuselage to yaw the airplane out for slow speed.  NOT useful for PA though.
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« Reply #8 on: March 25, 2008, 04:25:12 AM »

I have never seen these three wire bellcranks, and I have a burning desire to build it myself.    Fred Shattuck

Here is the installation instructions for the J Roberts unit.


* Roberts044 resize.jpg (66.92 KB, 1024x802 - viewed 145 times.)
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« Reply #9 on: March 25, 2008, 07:04:14 PM »

Fred
I have been building and flying C/L models with electronic throttle for over 25 years.  See my info in the Vendors Corner under U/Tronics or send me an Email and I will try to answer your questions.

Clancy
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Clancy Arnold
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bill marvel
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« Reply #10 on: March 26, 2008, 09:56:32 AM »

I am modifying my Chipmunk. I am going to a larger R/C type engine. I would love to try a three wire configuration, but don't have a clue how to do it. Does the tension on the carb wire interfere with the line tension on the others? We fly with 70ft cables and a little faster than most, with bigger engines. Does the engine rev more with or without the line being pulled? Do I need a little more right rudder or outside wing weight?  I appreciate any advice I can get on this. Thanks, Y'all   Fred Shattuck

Fred

Throttle control on a stunt plane is a lot of fun.  I flew a Shark 15 kit-bashed into a P-39 with a Webra RC 3.5 (.21) for 8-10 years in the 1970's.  The trike gear of the P-39 made touch and go's very easy.  What was the most fun was to do a half loop to inverted and throttle back.  It was so slow and so quiet.  I used a J. Roberts handle and bellcrank.  It used to be referred to as a "balanced" system because the tension of all three lines stays exactly the same regardless of throttle position (high, low and in between).  I experimented with the third line similar to what you are describing.  It worked but the Roberts system was loads easier and far more precise.  Jim Walker, one of the pioneers of control line flying, used a similar system to the one you are describing.  He could fly three airplanes at once so it worked for him.

The three lines do make for a lot of drag!  I found the airplane easiest to fly in a dead calm.  But, she would fly loops much better with some wind to hold her out on the lines.  I wish I had known more about trimming in those days.  I was the best of my flying friends and I wasn't very good!!!  I would have tried moving my lines as close together as possible and have an adjustable leadout guide.  Despite an offset engine, an adjustable rudder and different tip weights, the P-39 was incredibly soft on the lines above about 60 degrees.

Joe Just (Big Country Joe) has an ad in the Classified Section of Stunt Hanger for instructions for making your own 3-line bellcrank out of two standard metal bellcranks.  It is a system I have used.  It works very well and Joe's instructions are well worth the modest cost.  I loved the suggestion of using an inexpensive "toy" RC car receiver and transmitter with a servo to actuate the throttle.  That would eliminate the problems caused by the extra line.  In the Vendor's Section of Stunt Hanger, Clancy Arnold advertises a remote system that gets high praise from those who have seen it.  I think his system works similar to a TV remote.

Keep experimenting!  Having fun is the name of the game.

regards,
bill marvel

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Bill Marvel, AMA 793835
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Clancy Arnold
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« Reply #11 on: March 26, 2008, 07:38:57 PM »

Bill
Actually in all of my designs the control is through the two lines in full compliance with AMA regulations.  The single channel unit only weighs 1/4 oz. and the Multi Channel Decoder weights 1/2 oz.

Clancy
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Clancy Arnold
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bill marvel
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« Reply #12 on: March 27, 2008, 09:22:12 AM »

Bill
Actually in all of my designs the control is through the two lines in full compliance with AMA regulations.  The single channel unit only weighs 1/4 oz. and the Multi Channel Decoder weights 1/2 oz.

Clancy

Thanks for the correction and additional information, Clancy.  I did know that but simply forgot.  Mea culpa!!!

regards,
bill marvel
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