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Old Nov 29, 2012, 03:33 PM
um.. yeah, I can make that fly
Carlos230023's Avatar
United Kingdom, Milton Keynes
Joined Jun 2007
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I do like the magnetic attachment for the rotors, that would really save the fuselage and main bar in a crash!

CJ
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Old Nov 29, 2012, 03:36 PM
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KAMAX-T's Avatar
Powell TN
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Quote:
Originally Posted by Freakazoid View Post
Heheh sorry, I should have been a little more clear about that.
Its surprisingly hard to explain with just words, so have a look at this video instead:

http://www.youtube.com/watch?v=xTkaDwI38S0

See how the motor axles point outwards when sweeping forward, and vice versa?
The pivotpoints on which the motorpods sway, are not parallel to the fuselage.
This is apparently called an oblique tilt rotor system, and im trying to figure out the advantages of it.
Yeah freak , this guy is jerry from canada , and has done a lot of work with this project....Also copter ritchie has too.....both are super stable , and are using multi-wii for control..the mass of the prop has as much of the effect ...OAT , as does thrust vectoring ...


Here is one of copter ritchies vids....very solid

Mini-Mollie Equipped with new Motors (1 min 40 sec)


Terry
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Old Nov 29, 2012, 03:45 PM
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Powell TN
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this video here shows the fuction pretty good...This is Jerrys with onboard cam...Great shot....

Terry

post 79 ? second video

http://www.rcgroups.com/forums/showt...1278074&page=6
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Old Nov 29, 2012, 03:53 PM
um.. yeah, I can make that fly
Carlos230023's Avatar
United Kingdom, Milton Keynes
Joined Jun 2007
3,720 Posts
Im still not sold on it, you would only see the effect in forward flight, and he was just hovering in the first vid :P. That and that Bicopter was bobbing around like a buoy in a storm :P. Probably not helped that he was in his back garden but even so id have thought he could keep level flight.

CJ
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Old Nov 29, 2012, 04:08 PM
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try it out carlos .....you might like it ...
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Old Nov 29, 2012, 06:31 PM
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Roosendaal, Netherlands
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Ive spoken with Jerry about bicopters before, back when I was doing my first bicopter experiment (which failed).
His mini molly is a great little bicopter, though I haven't really seen it buzzing around yet.

Still no explanations as to WHY some people make their bicopter "bow legged" though.
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Old Nov 29, 2012, 09:15 PM
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Joined Nov 2011
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This one looks like it flies great:

Twincopter & HK KK2.0 V1.4 Test flight Vol.45 (6 min 24 sec)


Now if I could just figure out how it works!
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Old Nov 29, 2012, 09:29 PM
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this ones better .....http://www.feiyu-tech.com/news-en.php?id=1&nav_num=4
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Old Nov 30, 2012, 04:15 PM
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Look at how that thing is flying with a straight bar.
Im not gonna bother with angling the tilt axis just yet.
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Old Nov 30, 2012, 04:58 PM
um.. yeah, I can make that fly
Carlos230023's Avatar
United Kingdom, Milton Keynes
Joined Jun 2007
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I think the idea with the tilt is (after spending a great deal of time with a calculator and a few scraps of A4) the rotors. If you forget for a minute that they are propellors moving large quantities of air at high speed and you just think of them as flat disks. basically, they are gyros. And the thing with a gyro is it doesn't like being moved off its axis of rotation. The result of this is when you tilt the rotors forward (to move the thrust line so that forward flight is achieved) they try to resist this rotation, this resistance is transferred through the servo to the airframe. So instead of turning the motor, the airframe twists instead. We want to keep the airframe level so that the rotors twist and we get forward flight. Twisting both of them (rotors and airframe) could lead to bobbing, unexpected pitch characteristics etc...

Now, imagine that we turn them in or out (doesn't matter which way) by 45 degrees each, so we have the "handlebar" style frame. Imagine the flat disks again spinning away as two big gyroscopes. they are now at 90 degrees to each other. Now imagine that they both tilt forward. Their gyroscopic effect now isn't in unison. One is tilting forward on one axis (lets call this axis 'x') and one is tilting on another (lets call this the 'y' axis). Where as parallel motors would both add their Gyro effect together, when the x and y axis are offset by 90 degrees, 50% of y gets added to 50% of x. This makes half the gyro force compared with parallel motors (100%+100%).

Basically, so far as I can figure, It stops the body from swinging as much when starting and stopping forward flight (It doesn't make a difference to yaw as the motors axis rotate in opposite directions altogether, so their gyro effect cancels out). But in doing this, you sacrifice flat out speed. If it were me, I would have the rotors arranged so that when they tilt, they push air away from the fuselage, this avoids turbulence during forward flight due to crossing airstreams as the airstreams are forced to the rear and out to the sides of the craft.

It may be something I look at but I think fuselage design and weight distribution is just as important with bicopters. I plan to have a long and thin fuselage to spread out the batteries front and back, weight off the center of rotation makes rotation more difficult due to momentum and leverage. Im also slinging a big fat Lumix GH2 underneath (660g of camera + a gimbal of some sort). This will slow down pitch torque on the airframe with a pendulum effect.

The greater offset of the motors pivot angle the greater this gyro effect is diminished. I may try 30 degrees offset to start with as I'm using quite big hefty props (14x6) with big hefty motors. The greater the mass and the higher the rotation speed, the more pronounced the gyro effect is. so my larger bicopter would be more prone to this behaviour.

Phew, physics lesson over. Time to order parts .

CJ
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Old Nov 30, 2012, 05:10 PM
um.. yeah, I can make that fly
Carlos230023's Avatar
United Kingdom, Milton Keynes
Joined Jun 2007
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Oh yeah, and to add more complication, the gyroscopic torque effect is also amplified by leverage, so when you set up the gimbal to rotate your motors back and fourth, try to set it up so that the axis of the gimbals rotation is lined up across the prop or slightly under it (to compensate for the motor can weight).

CJ
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Old Nov 30, 2012, 05:58 PM
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Roosendaal, Netherlands
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This video demonstrates the effects of the propeller mass:
3-axis Control in Space with Propellers? (/LPs?) (2 min 28 sec)


Quite amazing what kind of maneuverability is achieved with just gyroscopic effect.

So what it boils down to, is slanting the pivot axis of both "disks" into eachother will cancel out some of the torque, allowing for more responsive flight?
Because instead of working against the mass of the central structure, you are making the gyroscopic forces work against eachother?

This is making me reconsider my straight bar plans, Very informative, thanks!
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Old Nov 30, 2012, 10:30 PM
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Taranaki New Zealand
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the oblique axis system is very neat, i have found alot of sucess however buy putting the mass above the pivot point, stops the pendulum effect, and negates teh need for a tail surface to stabilise

its like balancing broomstick in the air

if the body pitches forward, the flight controller tilts the props forward to compensate, the effect of moving that mass helps to actually pitch the body back, and then the off centre thrust now finishes the job of righting it

bottle_copter_V3 (1 min 18 sec)

bottle_copter_finished (1 min 18 sec)
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Old Dec 01, 2012, 06:05 AM
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Solihull, England
Joined Jun 2004
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So I am right that you do not angle the motor axis of rotation by 45degrees but keep it in line with the connecting boom? If my assumption is correct then you are not using the OAT principle. Is that right?

Peter
ps the attached gives the theory behind the OAT

Quote:
Originally Posted by matwelli View Post
the oblique axis system is very neat, i have found alot of sucess however buy putting the mass above the pivot point, stops the pendulum effect, and negates teh need for a tail surface to stabilise

its like balancing broomstick in the air

if the body pitches forward, the flight controller tilts the props forward to compensate, the effect of moving that mass helps to actually pitch the body back, and then the off centre thrust now finishes the job of righting it
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Old Dec 01, 2012, 06:57 AM
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Roosendaal, Netherlands
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The roll/yaw coupling is what already happens in multiwii firmware I believe?

Overhead CoG, slanting motor axis, angling pivot points.
Whoof, somany things to try, I hope my gear will survive all this fumbling.

Any thoughts on how far apart to space the motors?
Same old as other multicopters, further is more stable but slower?
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