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Old Mar 01, 2013, 08:13 AM
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Bicopter stability and control stuff

Anyone interested in a general discussion about bicopter stability and control techniques and issues?

There is one aspect I'm particularly interested in at this time and it has to do with gyroscopic precession.

Check out the bicopter in the video below. It tilts the fans in and out to control pitch. The control seems to be pretty good. This is directly using gyroscopic precession for pitch control

Gyroprop Bicopter: tailless trial (0 min 53 sec)


In the next two videos you'll see bicopters with canted fans. I'm wondering if the combination of canted fans and tilt control provides a similar gyroscopic precession effect.

New Concept RC Heli maiden flight (3 min 29 sec)


RCvertts' Osprino

RC Osprey Osprino FullConversion A (2 min 13 sec)
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Old Mar 01, 2013, 09:01 AM
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I've been thinking about this for a while. Recently RCvertt's success with his Osprino and especially his ability to hover when very tail heavy got me thinking. If a bicopter is really tail heavy, there has to be some compensating torque or force to keep it stable. Assuming there is no airspeed over the tail to lift the tail there has to be some other force. The only thing I can figure is that the canted fans provide a torque due to conservation of angular momentum. If this is the effect and compensates for a tail heavy set up then it must be a very strong effect which makes it a very useful tool for controlling pitch of a bicopter.

If this is the effect, then I think you could also cant the fans out if you reversed the direction of rotation of the two fans to get the correct pitch response.

Check out the second half of this video for a demonstration of conservation of angular momentum.

Conservation of Angular Momentum (2 min 14 sec)
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Old Mar 01, 2013, 09:15 AM
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What? No Mini-Mollie?
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Old Mar 01, 2013, 09:26 AM
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Originally Posted by rimshotcopter View Post
What? No Mini-Mollie?
Mini-Mollie thread http://www.rcgroups.com/forums/showthread.php?t=1558500

Thanks for the heads up; I hadn't seen this before. Nice work. Looks like another creative way of using "canting fans" to enhance pitch control.

Just for clarity I use the term "tilt" for forward and back tilting and the term "cant" for side to side tilting.


http://m.youtube.com/#/watch?v=Sa6lw...%3DSa6lwc-VN4U
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Old Mar 01, 2013, 09:44 AM
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Originally Posted by leadfeather View Post
Mini-Mollie thread http://www.rcgroups.com/forums/showthread.php?t=1558500

Thanks for the heads up; I hadn't seen this before. Nice work. Looks like another creative way of using "canting fans" to enhance pitch control.

Just for clarity I use the term "tilt" for forward and back tilting and the term "cant" for side to side tilting.


http://m.youtube.com/#/watch?v=Sa6lw...%3DSa6lwc-VN4U
Yes lead.,
verry usefull..Allways i wonder my self,how i can have more distante from pivot point to cg...
Lattelly i have learned something and promise,i will be a good student...
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Old Mar 01, 2013, 10:30 AM
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You presumably know of the work of Gary Gress? His website is but a shadow of its former self but I have the attached paper.

Peter
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Old Mar 01, 2013, 12:00 PM
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Interesting discussion.

My question/thought is

Gyroscopic procession requires a torque to apply a torque. That means, correct me if I'm wrong, but in the presence of a constant disturbance, the rotors can only tilt so far before you can no longer create a torque. Doesn't this mean that using only gryoscopic forces can't fly an aircraft indefinitely?
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Old Mar 01, 2013, 12:17 PM
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Originally Posted by ilektron View Post
Interesting discussion.

My question/thought is

Gyroscopic procession requires a torque to apply a torque. That means, correct me if I'm wrong, but in the presence of a constant disturbance, the rotors can only tilt so far before you can no longer create a torque. Doesn't this mean that using only gryoscopic forces can't fly an aircraft indefinitely?
I'm not sure??

In the video in post #2 the girl will keep on rotating as long as the flywheel has been tilted so that angular momentum is conserved. It takes an outside force on the system (such as penulum effect, force from tail etc. in the case of the bicopter) to create a new steady state.

I think it gets kind of complex in real life (lots of forces acting on the bicopter) but the effect is strong enough and lasts long enough for control corrections... I think??
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Old Mar 01, 2013, 01:46 PM
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The problem that I foresee is drift. It is conceivable that the system will drift far enough that the props won't be creating enough vertical thrust for a hover. This is the same problem with integrating gyros. Eventually, you need an additional source of error correction. It may work for short flights, but flying in wind it could be a problem.

I think the 'canted' angles give the best of both worlds, by using gyroscopic procession and aerodynamic forces.

I think, in addition, the air friction on the wheel when turned will provide some torque on the girl, causing her to continue to rotate. Much like quadrotor yaw.
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Old Mar 01, 2013, 01:56 PM
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I am ONLY Stating my opinion based upon my observations and personal experiences. With the Props that I've used on my builds, the gyroscopic effect is minimum and do not overcome the other flight dynamics. In a total or near vacuum, then gyroscopic effect is supreme. The mass and the speed of spinning object is very important but to add enough mass or RPMs is a very hard challenges. I have a number of videos to support my claims but I digress.
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Old Mar 01, 2013, 02:01 PM
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Good stuff .. thanks for sharing lead
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Old Mar 01, 2013, 02:11 PM
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Also, I should state, this does not means to discard the gyroscopic effect of a spinning prop but to complement it and building upon the effect.
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Old Mar 01, 2013, 06:50 PM
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Also, I should state, this does not means to discard the gyroscopic effect of a spinning prop but to complement it and building upon the effect.
+1

Yep, precession/angular momentum is another possible tool in the bicopter controls tool box.

In the first video of post #1 the bicopter appears to be using angular momentum exclusively for pitch control. The fans cant outward to tilt the copter forward and cant inward to tilt the copter backward with the fan rotation he has chosen.
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Old Mar 02, 2013, 08:27 AM
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Vertical CG stuff

Here is one issue that does not seem intuitive at first. Copters with higher vertical cg seem more stable. Check out the video of the Hiller flying platform. The vertical cg is well above the thrust yet it looks amazingly stable. So what is the magic???

When a propeller tilts forward and begins to attain some forward air speed, the center of lift from the propeller disk shift forward. This creates a torque which tries to level the prop out...a good thing actually. However it is often too much of a good thing and leads to pitch oscillations.

Now if the cg is low, the pendulum effect is also trying to restore the craft to level. The combined effect of prop self leveling effect and low cg results in the over correction of pitch. What you see is an initially small swing in pitch of the craft that keeps growing in amplitude until the copter crashes.

If the cg is high; as example the Hiller flying platform; the high cg is destabilizing but, since the prop self level effect is over-stabilizing... the net result of the two effects is a stable craft. At some point, if you keep raising the vertical cg, the net effect will be an unstable craft.

Hiller Flying Platform (2 min 40 sec)
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Old Mar 02, 2013, 08:48 AM
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Pitch control

There are just a few options for pitch control of a hovering bicopter. In forward flight , the flight surfaces can augment and enhance the pitch control but in stationary hover here are the choices:

1) Cyclic control, helicopter swash plate, blades change angle of incidence and can shift the center of lift of the rotor disk to creat a pitch controlling torque.

2) Prop tilt/weight shift. This method creates a control torque by changing the the relative position of the thrust line and the cg. If the cg is above the prop tilt point control will be reversed. If the cg is at the pivot point, no torque is created by tilt.

3) Control vanes in the prop wash. The vanes create a pitch controlling torque by deflecting the prop blast. The greater the distance the vane is below the prop and the bigger the vane the stronger the pitch control effect will be.

4) Conservation of angular momentum. Discussed in the first few posts.

Of course you can have combinations of the above such as tilting and canting props etc.


For some reason I can only upload one attachment at this time from my ipad??? I have a few more diagrams I'd like to add below.
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