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Old Nov 17, 2012, 10:35 PM
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BanditBushman's Avatar
United States, OH, Groveport
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Small Tandem Flybarless?

I was wanting to know if i could build a tandem flybarless chinook on the cheap? Small for me would be that the helicopter is about 1.5-2 feet long. So are there any parts out there? Please point me in the right direction, i tried the search function but couldn't find anything...

-Nathan
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Old Nov 17, 2012, 10:44 PM
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Balr14's Avatar
Germantown, WI.
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You need to describe what you call cheap. I think you could do it for under $400, but it wouldn't be easy. There is no kit you could buy.
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Old Nov 17, 2012, 11:04 PM
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hk450's, hxt900 servos, hk401b gyro, cheap motors and esc, and a couple of zyx gyros... ...then hk shipping. you may be able to do it.
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Old Nov 17, 2012, 11:42 PM
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Thank you!
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Old Nov 17, 2012, 11:47 PM
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I might just pick up a Gaui chinook. Its about $400 and 1/32 scale. Or build one around its fiberglass body...
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Old Nov 18, 2012, 01:18 AM
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HK450s would be too big, 250s would be better. You need a multi-rotor flight control board, too. I've been playing with tandem designs for several years and haven't come up with something I like. One design point I found that needs to be used always is the rear mast must be several inches higher than the front mast and angled away front the front mast. It's probably better to splay the front rotor to the right and rear rotor to the left to cancel torque from both heads, but I'm not convinced that's worthwhile.

The Gaui chinook was discontinued two years ago due to severe problems in the control system. Even if you found one you would have a hell of a time finding parts.
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Old Nov 18, 2012, 10:08 AM
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United States, SC, Irmo
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There was a twin rex a years back. Would love to see one again. I wonder if you could frame 2 450 frames together with aluminum stock and use multirotor control board. But who is going to write the software.
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Old Nov 18, 2012, 07:03 PM
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United States, PA, Pittsburgh
Joined Sep 2012
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I would like to build a 450(ish) electric V22 Osprey. They have a cheapie flybarred kit out there, but I would like to build a real one.
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Old Nov 19, 2012, 09:10 PM
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Canada, SK, Regina
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Quote:
Originally Posted by Balr14 View Post
One design point I found that needs to be used always is the rear mast must be several inches higher than the front mast and angled away front the front mast.
If both rotors ran off the same shaft (synchronized rotation), then you could position the heads so the blades inter-mesh and you wouldn't have to worry about one rotor smashing into the other. That would take care of any mast shenanigans?

Let's say, hypothetically, from off the shelf parts...

You combined an blade nano (a micro tandem - oh ya baby!) with a second set of nano mechanics (frame, motor, servos, everything but the board). According to another mcpx thread I was following, it's possible to simply reverse the direction of the main motor and flip the blades to get a counter clockwise main rotor. I know, it sounds crazy, and I'm still not sure I believe it (figured the AS3X wouldn't go for it), but it's true (supposedly).

Using a trick from the brushless guys, you could take something like a 'steve' converter and a micro brushed motor ESC to power the second nano motor off the main motor signal of the first. Some bevel gears (130x parts) and a carbon fibre drive shaft would keep the two rotors in sync (each motor would share the same load). So now you've got counter rotating rotors under the same throttle control, synchronized for blade mesh.

The signal wires from the first nano servos can be split to power the second nano servos. So far everything 'should' be more or less in full control.

The trick is to take the tail motor output (again the 'Steve') and mix it with the aileron servos. HK sells a 'dual Steve' that would work perfectly. The trick is to take a single servo signal and supply just the right amount reversed left/right mix between the two rotors to control yaw. This is the tricky part I'm not so sure about. You'd probably need a custom micro-chip to pull this off.

I don't know how a single AS3X would react with two rotors. I don't know if forward/back control (elevator) is as simple as feeding forward/back cyclic to each rotor simultaneously. Maybe elevator control might need to be mixed to produce uneven pitch in each rotor (drop the lead rotor, raise the tail rotor), maybe not.
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Old Nov 20, 2012, 01:38 AM
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Been there, done that. Driving both masts with one motor is mechanically complex and heavy. It limits you to controlling all functions entirely by pitch changes. Two motors with blades short enough that they don't interfere with each other gives a lot more flexibility. Canting the masts eliminates a lot of unwanted torque, movement and cyclonic effects. A MultiWii flight control board ought to provide most of the control; I'm not even sure if programming would be required.

I have already built several versions using EXI 250 kits with flybars. But, I didn't have a good flight control board and didn't understand them very well. A lot of what I was doing with the electronics was real "Mickey Mouse" and not very satisfactory.
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Old Nov 20, 2012, 06:34 AM
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United States, SC, Irmo
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Twin Rexx Flight (2 min 13 sec)


I think with modern advances in electronics, this one could be brought back. Someone could reprogram an existing KK board to control this think probably. Imagine 2 450's? Scary.
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Old Nov 20, 2012, 09:22 AM
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The cost of the Twin Rexx was $1200 - $1600, depending on the components you chose. They stopped making it at least two years ago. The only other viable tandem kits were from Hirobo. They run from $3000 - $8000 and are 700 size and above. You can probably still get one, though. Tandems simply magnify the flaws of single rotor heli design and add a few more flaws in the process. They don't handle nearly as well. Essentially, they are an expensive novelty.

It may seem I'm really down on tandems, but I'm a huge fan and have been trying to come up with an economical and reliable model for several years. You have to be realistic about their shortcomings, though. I do not want to repeat other failed attempts. I want something under $400 that won't break if you look at it too hard.
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Old Nov 20, 2012, 12:29 PM
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That video is really cool. If you run them off of the same motor how would you get the heli to yaw?
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Old Nov 20, 2012, 12:42 PM
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I wouldn't go near a twin rexx. Setup must be a nightmare.
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Old Nov 20, 2012, 12:44 PM
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Canada, SK, Regina
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Quote:
Originally Posted by RFDelie View Post
That video is really cool. If you run them off of the same motor how would you get the heli to yaw?
Pitch. Front rotor pitches right, back rotor pitches left and between the two you end up with a yaw to the right.
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