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Old Dec 01, 2012, 08:42 AM
um.. yeah, I can make that fly
Carlos230023's Avatar
United Kingdom, Milton Keynes
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Exactly. Imagine using the furthest holes on a control horn compared to the closest ones to the surface. The further in you get. The more responsive it is. The further out you get, it's a slower reaction but easier to make corrections. So more stable. This is also true of your weight placement. Batteries on the COG will be easier to move. Batteries further from the COG have a greater momentum when it comes to moving about the COG. So in my case. As I want a stable camera platform. The batteries ideally will be as far from the COG as I can get.

CJ
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Old Dec 01, 2012, 09:38 AM
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So the harder a center mass is to move, the easier it will be to fly it..
Heres a thought, would the greater momentum due to dispersed weight also work adverse in some situations?
Because while it is true its harder to move the central body when weight is further from the center,
it will also be harder to stop once it gets going, right?

I had a design for a while that looked like a "T", that I doodled out on paper one day.
It was meant as a miniature FPV platform, and I even intended to fly it "sideways", but the idea is the same.



Would building a "gauntry" like this scaled up to say 2 foot, with the battery swung way low under it as a base, work well?
I would imagine flying it sideways like that would make forward flight a tad easier to manage? Or would roll stability suffer tremendously?
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Old Dec 01, 2012, 11:06 AM
um.. yeah, I can make that fly
Carlos230023's Avatar
United Kingdom, Milton Keynes
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I guess you'd have to try and find out. im going for a more horizontal approach. Id imagine It would be quite difficult to land something like that. heres what ive roughted out.

Motors swept forward by 30 degrees each (so as not to loose all of my forward thrust :P) tilted outward by 7 degrees (to help prevent lateral buffeting) and a long fuselage with batteries mounted on top, the FC bang in the middle, and a big fat camera swung underneath on a stabilised mount. Oh, and it'll be tailless, I really can't see a point to a tail unless your planning on transitioning into full on forward flight

Obviously this is a pig ugly rough design, but more work still be done to make it pretty :P

CJ
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Old Dec 01, 2012, 11:50 AM
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The main reason im thinking of putting the battery down low, is this way gravity will assist in keeping it upright a lot.
I'm guessing this way it would pretty much behave like one of those coaxial helicopters, dangling from its lift point.
Having all the weight too low down might cause a pendulum effect though.
What im thinking of doing is making a compact unit with big ol' magnets,
and then just trying to spread balance in various spots to see what it does.

I'm not gonna think about the tail too much yet, that's just for sustained forward flight.

I am curious about flying a bi-copter "tandem" style though, like a chinook helicopter.
That way pitch, and thus forward flight, will be achieved solely on motor speed,
possibly avoiding some of the gyroscopic twisting of the center frame.

I have already attempted this once with a smaller design which failed miserably.
I'm blaming the small size or possibly the boards upright orientation (although corrected in firmware).
So I'm hoping another attempt about 5 times bigger might yield more favorable results.
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Old Dec 01, 2012, 12:44 PM
1400 watt RW Zephyr
matwelli's Avatar
Taranaki New Zealand
Joined Sep 2003
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Quote:
Originally Posted by Freakazoid View Post
The main reason im thinking of putting the battery down low, is this way gravity will assist in keeping it upright a lot.
I'm guessing this way it would pretty much behave like one of those coaxial helicopters, dangling from its lift point.
Having all the weight too low down might cause a pendulum effect though.
What im thinking of doing is making a compact unit with big ol' magnets,
and then just trying to spread balance in various spots to see what it does.

I'm not gonna think about the tail too much yet, that's just for sustained forward flight.

I am curious about flying a bi-copter "tandem" style though, like a chinook helicopter.
That way pitch, and thus forward flight, will be achieved solely on motor speed,
possibly avoiding some of the gyroscopic twisting of the center frame.

I have already attempted this once with a smaller design which failed miserably.
I'm blaming the small size or possibly the boards upright orientation (although corrected in firmware).
So I'm hoping another attempt about 5 times bigger might yield more favorable results.
can i sugest you have a look at putting your mass above the props, not below.....conventional wisdom dosent always make it right
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Old Dec 01, 2012, 12:46 PM
1400 watt RW Zephyr
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Taranaki New Zealand
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Originally Posted by Peter Seddon View Post
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
correct - not using OAT, using momentum change for intial correction and thrust vectoring

weight needs to be above the props, not below
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Old Dec 01, 2012, 02:57 PM
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In your experiments, be sure to note that prop rotation must be correct for angled foreword or angled rearward axis - wrong rotations will cause instability rather than stability. For inline axis, rotations don't matter. So further thinking is that axes at 90 degrees, maximum OAT effect. Using lesser (fore-aft) angles lesser OAT effect.

Roll and yaw are also somewhat coupled to gyroscopic pitch due to precession.

In forward flight, rotor/prop aerodynamics tends to pitch the nose up, hence the fixed 'down elevator' seen the the Gress unit to compensate.

As for amidship angles, I've been happy with the improvement using 4 degree tilt inward (although 6 may be better), creating a slight self-righting effect similar to dihedral in wings. Used in a Gaui 330X quad. Not tried with bicopter, but additional complication due to axis coupling need consideration.

BTW, I'm working on getting an actual nVader 600 V1 tuned.

Also BTW, even Goddard was mislead by the 'pendulum fallacy' on his first free flight rocket !
Jim
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Old Dec 01, 2012, 03:08 PM
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Quote:
Originally Posted by matwelli View Post
can i sugest you have a look at putting your mass above the props, not below.....conventional wisdom dosent always make it right
Putting all weight above the props, it actually sounds a bit terrifying to be honest.
But the flying bottle tells no lies, it darts off quite adequately.
Is there a trick to getting it going, some special firmware or something?
Because im envisioning the props just tilting forwards when you give it the forward flight command,
making all of its mass pretty much fall over backwards?

If its easy to do though... flying Christmas tree anyone?
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Old Dec 01, 2012, 05:51 PM
1400 watt RW Zephyr
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Joined Sep 2003
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dont know about special firmware, i use openpilot copter control, its a std 2 servo, 2 motor mix

servos do yaw and pitch, motor speed does roll

remember if the body tilts forward, set the servos up so they tilt the motors forward to compensate
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Old Dec 01, 2012, 05:59 PM
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Yes, but apparently there's some magic voodoo going on that im wondering about.
Imagine your bottlecopter, hovering in place, props and body straight up.
Now imagine you initiate forward flight, youd say the props pivot forward,
applying a force on the body which then in theory starts to tip backwards, right?
So now you have the body falling backwards, the props trying to pull it forward.
How is it possible for it to pull itself forward while stopping the main body from falling over backwards?
Does it "pop" backwards briefly to let the body fall forward and initiate forward flight,
letting the props chase it and eventually catch up once you wish to stop?

Or do the motors actually just swing backwards when you take it into forward flight,
their gyroscopic force being stronger then the bodyweight countering it,
basically flying forward and backward by letting the body fall in that direction?

Right now it seems forces are being applied that don't make a lick of sence to me.
Id imagine its pretty much a flying segway, that bottle of yours, and we know how segways work.
So spill the magic if you can, cause im completely lost on the logic behind this thing.
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Old Dec 01, 2012, 06:10 PM
1400 watt RW Zephyr
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Taranaki New Zealand
Joined Sep 2003
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ok. hovering vertically. push the pitch stick forward , the props move slightly back , the body pitches forward , the flight controller now adjusts the servos to keep the desired rate of change/angle that you are commanding with the pitch control , if it starts to pitch to far forward it will tilt the motors forward to compensate.

no different to how a quad flies , to fly forward the controller briefly increases the rear thrust , then reduces it to stop the forward pitch and keep the desired angle
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Old Dec 01, 2012, 06:13 PM
um.. yeah, I can make that fly
Carlos230023's Avatar
United Kingdom, Milton Keynes
Joined Jun 2007
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Its all about balance, all flight is, no matter if its a Monoplane, biplane, helicopter, missile...Its ALL about balance. The whole time your flying, your basically balancing a ball bearing on one thing or another, on a stable aircraft (easy star or a basic trainer, something to this tune), imagine your balancing the ball in a shallow dish, its pretty easy.

A sport plane, your balancing the ball on a plank, a flat surface, but should you fall off the edge things will go wrong, and it won't self center.

A Quad, or Heli, is like balancing a ball on top of another ball. You need to be incredibly precise, as its very easy to get is wrong. Gyro's and Accelerometers help you go from the last scenario to the second or first depending on how well you controller works and how well you have it tuned in.

Weather you have a high or low COG just adds to the difficulty of the flying problem presented to your control board. But 9 times out of 10 they'll make it work

CJ
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Old Dec 01, 2012, 06:18 PM
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Ok, so with the weight up top the servos run "backwards" too?
And the way you describe it responds to your stick, I take it you always fly these things in ACC mode?
Because on gyro only a copter will just keep tipping more and more into the direction you are inputting.
Ive always griped about the rigid behavior of copters in ACC mode, but I wont be doing the crazy stuff I do with multicopters normally,
so I think ill have less of a fight with it then I usually do.

Good to know which way the servos are supposed to go though,
its not the motor vectoring doing the forward/backward flight,
its actually just shifting the main body balance to achieve it.
Id think this would limit its speed before you can nolonger recover its balance,
but the bottle seems to scoot up and down the road at a fair pace regardless.

The balancing story though, one would say dangling a broomstick on a string would be easier to manage then balancing the broomstick on the tip of your finger.
Thats what is so amazing about this bottlecopter, it seems to do a great job doing it "the hard way".
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Old Dec 01, 2012, 06:25 PM
um.. yeah, I can make that fly
Carlos230023's Avatar
United Kingdom, Milton Keynes
Joined Jun 2007
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Quote:
Originally Posted by Freakazoid View Post
Ok, so with the weight up top the servos run "backwards" too?
And the way you describe it responds to your stick, I take it you always fly these things in ACC mode?
Because on gyro only a copter will just keep tipping more and more into the direction you are inputting.
Ive always griped about the rigid behavior of copters in ACC mode, but I wont be doing the crazy stuff I do with multicopters normally,
so I think ill have less of a fight with it then I usually do.

Good to know which way the servos are supposed to go though,
its not the motor vectoring doing the forward/backward flight,
its actually just shifting the main body balance to achieve it.
Id think this would limit its speed before you can nolonger "recover" its balance,
but the bottle seems to scoot up and down the road at a fair pace regardless.
Its the same as a helicopter if you think about it. A helicopter doesn't just tilt the rotor forward (before anyone explodes, I know this isn't exactly how it works, more differential pitch from front to back) it also tips the fuselage back. Lets not forget our good friend newton and his laws. You can tilt a helicopter too far forward so that it falls over, in the same way you can a quad, or that bottle bicopter. You just need to control it so that event doesn't happen.

Though if your asking me, its easier to keep a stick upright when help from the top, not the bottom :P. So far as self stabilising modes using accelerometers. A handy feature no doubt, but I tend to use it more for emergency recovery or when I'm busy lining up a shot with the camera mount, as opposed to using it all the time while flying. Ive got to use some sort of skill to fly my aircraft or it feels like I'm cheating :P.

CJ
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Old Dec 01, 2012, 07:03 PM
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If you want to compare it to a helicopter,
I suppose you should think of the bottle copter as an upside down helicopter, right?
Since all of the main body weight is above Its lift point.
So then, when an upside down helicopter wants to go forward,
it would turn its thrust vector towards its tail, right?
Or would it push the body weight forward to make it "fall" to that direction?
Because the way i see it, its thrust vectors changing to push in a direction,
and balance shifting to the opposite direction as a result.
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