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Old Oct 03, 2012, 04:28 AM
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Originally Posted by jnmardister View Post
so im guessing your original endeavor was to make a tandem rotor helicopter aka a chinook? if so that would be a job... the mixing involved in those birds is ridiculous. but in that case any multi-rotor bird is going to have alot of mixing. to answer your question, i dont see why you cant build any design you would like, as long as you get the mixing and balance correct. which is the hard part. hope that helped.
Yes something like the chinook.
So its possible to do like a skateboard shape and have two quadcopter type of motors to fly it through a course?
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Old Oct 03, 2012, 08:15 AM
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There have been many attempts at making a decent tandem over the years, none lasted very long. There are too many issues with this configuration. But, if you can come up with a successful design, lots of people would buy one. Here is the last commercial attempt, which is no longer available: http://www.flying-hobby.com/shop/chi...ck-p-4694.html. Personally, I think it could be done with two motors and two flybarless CP heads. Trying to cancel torque while retaining lift with FP heads doesn't work. You also need to drop the nose for forward flight, which is harder than it sounds, because the natural tendency is to lift. I think I've spent about 400 hours playing with tandem designs. I may try again someday, if I can come up with a better design.
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Old Oct 05, 2012, 01:23 AM
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So with that saying two motor on a skateboard design is gonna be very hard ? If using tiltrotor will it be able to help in anyway?
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Old Oct 05, 2012, 08:47 AM
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I recall several failed attempts at two motor configurations using tilt-rotors, mostly for VTOL configurations. None worked worth a damn. The most practical way to fly a "skateboard" is with a 4 motors.

However, the design I mentioned using two motors with CP heads should work. All of the commercial tandems have attempted to do this with one motor. They got it to work, but creating the drive for one motor to handle two rotors was always complicated. Both rotors have to turn at the same speed and all of your functions have to be handled simply by blade pitch changes, which is not that easy, nor is it very effective.

However, two independent motors can control function by speed variations and pitch changes. There are already MultiWii boards out that will handle this type of mixing; it would just take some programming. I think it could be done for less than $400.

Here are some design points about tandems you should be aware of:

1. The rear rotor should be higher than the front rotor and the blades should have some overlap.

2. The rotors should not be on the same vertical plane. Splaying the front one forward and rear one backward works better. It may work better with left/right splaying as well, I never tried it.

3. Fixing one rotor so it does not have either aileron or elevator function seems to have promising results. I don't know if this works better in the front or rear.
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Old Oct 05, 2012, 11:06 AM
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Balr14, the "mCX Chinook" works like you say - it has 4 motors, two coaxial drives.

The reason the "commercial" kits didn't do it that way is because they were mostly trying to build a scale Chinook, and one of the cool things about it is the interfering rotors, which requires them to be geared together, even if you use two engines like the real one.

For this project I think a standard quad copter in "rectangular" configuration might be interesting, or a coaxial design like the little Chinook...
http://www.e-fliterc.com/Products/De...rodID=EFLH2580
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Old Oct 05, 2012, 12:20 PM
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Those design parameters are my own, from many hours of experimentation. I wasn't trying to create a scale heli. The blade overlap is only by an inch or two and since the rear rotor is elevated by 2" - 3" and canted rearward, blade strikes are not an issue. It may sound strange but this configuration has much better movement authority and manages yaw better. I forgot to mention the rotors turn in opposite directions.

H design quads are not that uncommon. There are a number of them designed after airborne carriers copied from movies, as well as steam punk inspired designs. I was seriously thinking of designing my own steam punk quad. I have many years of model ship building experience. Here's one that's currently being worked on that also shows the carrier and other design possibilities: http://www.rcgroups.com/forums/showthread.php?t=1682868. I think they are stressing the ship aspect too much, but otherwise it looks pretty cool. Why would you have lifeboats for an airship?
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Old Oct 05, 2012, 12:58 PM
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Hehe, that's kinda what Steam Punk is all about - there's always levers and knobs and things that aren't necessary.
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Old Oct 10, 2012, 04:54 AM
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But with two motors only how is it able to move around ?
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Old Oct 10, 2012, 08:03 AM
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Originally Posted by zappygon View Post
But with two motors only how is it able to move around ?
The rotor blades rotate downward into the direction of movement and upward when they reach the opposite direction. This is in addition to pitch changes, which control lift. The difference is for pitch, the change is consistent throughout the rotation, regardless of the direction of movement. For example, to move forward the rotor blades may rotate - 8, when they reach the forward position in their rotation, while the backward side rotates +8. At the same time, you could be giving pitch changes to climb, which causes the blades to rotate upward +5; the net would be -3 forward and +13 rearward. Of course, these numbers are just for illustration. Actual values will vary a lot.

This is how it's done with a conventional single rotor or tandem, collective pitch heli. With the design I was working on, I was allowing only the front rotor to supply directional and pitch changes. The rear could supply compensating pitch changes, but that is all. In theory, this aids the ability to control movement and stability, as the rear rotor acts as the anchor or pivot. Having both rotors doing exactly the same thing introduces variables that are hard to control, IMO.
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Old Oct 10, 2012, 11:55 AM
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LOL, you eliminated precession to make the explanation easier
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Old Oct 10, 2012, 01:15 PM
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LOL, you eliminated precession to make the explanation easier
Yes I did! I really didn't know how to word it and sometimes I just can't explain stuff well. That's why we have you.
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Old Oct 10, 2012, 02:11 PM
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No I like that idea... bringing in the precession makes it way harder to explain.

To the OP - we're talking about the fact that Balr14's explanation isn't 100% correct about how cyclic pitch works. It's actually 90 degrees out of phase - but it would take a paragraph to explain what that means. One of these days I'll make a video about it, but I've spent about 5 hours this week trying to finish my "HK vs. Align" video... and it's no where near done. Yeah, I have 5 hours worth of material on that issue
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Old Oct 10, 2012, 02:28 PM
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Notice I also made it a point to avoid mention of flybars.
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Old Oct 10, 2012, 04:33 PM
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Originally Posted by Balr14 View Post
Notice I also made it a point to avoid mention of flybars.
Stupid flybars... I've about had enough of those!
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Old Oct 11, 2012, 01:43 AM
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wow thanks for the explaination guys ! anyway i was reading through kkmulticopter. I have this kkmulticopter redboard atmega48. Just wanna ask whats power tower ? it means two motors huh

btw the kkmulticopter has the power tower firmware thing.
this is pretty similar to what i wanted.

The flying Power Tower! (2 min 20 sec)


So with this configuration on the FC is it still hard to fly ?
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