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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Dr. Slack wrote ...
What I wrote above got me thinking, and now I’m concerned. While gyro reaction sounds really neat and sexy, AFAICS, neither gyros nor their close cousins reaction wheels can apply continuous torque to an airframe, in either roll or pitch. If an airframe is subject to a loading imbalance for instance, which has moved the centre of mass out from under the centre of lift, then it will apply a continuous couple to the airframe, which must be resisted. Such a trim is conventionally got from unbalanced fan thrust, or a vane deflecting an airflow, or a weight being shifted about under the lift centre.
Steve Conner wrote ...
Seems about right Dr. Slack, that might explain the tail stalls. The gyro effect would give plenty of transient torque to damp pitch oscillations, a similar effect to turning up the "D" in the PID controller. But in the steady state, there will only be as much pitch control authority as the vertical distance between centre of lift and centre of gravity allows. Once that authority runs out it will flip over.
I just had a radical idea, if you added another two props you would have full control authority in both axes.
Crap on a stick!, i realize that the torque only exists while the servo tilts the prop, then there is none at all. But when mine flip over it doesnt seem to be at its maximum angle.
however, this CL / CG comment worries me, CopterRitchie (rimshotCopter) is experimenting with "active mass shifting" on his Bi's, but he wont tell me why... maybe he trying to prevent the "satelite saturation reaction wheel problem" in relation to forward translation?
EDIT: should i have the machine nose heavy? (CG closer to nose than the CL), to help keep the nose down? right now i have them right on top of each other.
EDIT: ive flown with half the brass missing i now definately see the difference, it is more quick to stall, and each stall is worse, also the prop missing the brass, was being controlled more to keep the pitch level, i think i should add more brass to each prop, and see if the stall is diminished even more.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
I know what's going wrong.
You have put the cross-couplings in.
When you move both rotors to the right or left, that introduces a torque that tries to flip up one end or other of the vehicle because when the rotors are no longer in the same plane, they have angular momentum.
The fix is to cross couple the speeds of the rotors so that as you steer on one axis it applies an aerodynamic couple that cancels that out.
There's going to be several cross-couplings that you need, that's just one of them.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
BigBad wrote ...
I know what's going wrong.
You have put the cross-couplings in.
When you move both rotors to the right or left, that introduces a torque that tries to flip up one end or other of the vehicle because when the rotors are no longer in the same plane, they have angular momentum.
The fix is to cross couple the speeds of the rotors so that as you steer on one axis it applies an aerodynamic couple that cancels that out.
There's going to be several cross-couplings that you need, that's just one of them.
I was thinking the same thing, it's just that my brain started spinning and I couldn't put it into words.
It's to do with torque reactions, that's the reason for the tail stalls. I was re-running the vids and pausing them to try to see exactly what was happening, but without knowing exactly what manoeuvre you were trying to achieve it's difficult to understand exactly what is happening.
It is to do with torque reactions that happen at 90 degrees, though. It has to be.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
The thing is here when the two rotors aren't parallel, say they both lean right and inward, if you were to squint past both rotors, then the tips are now going around in little loops, both in the same direction, hence the vehicle now has angular momentum around the axis you're squinting down.
In order to get that, there must have been a torque at 90 degrees, or if there wasn't, the vehicle is now twisting along that axis at 90 degrees, hence the pitch-up issues.
Chinook helicopters have the same problem, when you do one thing with the controls, the other axes also have to respond, there's cross coupling built in.
edit: of course for this to happen the two rotors must be spinning at different speeds, due to the cancellation so as to generate the angular momentum to cause it to spin at 90 degrees.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Ash, let me make better vids and well slo mo them and ill text narrate what i command vs. what happens.
BigBad... when level the rotor are both at the same speed, in fact the onboard flight logger says the props are at 6000ish rpm, and the machine is level through oout the tail-stall crash cycle...
lets watch some more vids, and keep thinking, ill ask greg gress to since he has a PhD in physics and developed this idea.
BigBad, can you further explain the Chinook example?
EDIT: i just doubled the gyro mass to 600 micro grams per tip, and found the nose-rise, tail-stall phenominomna slower to set in, and less severe when it did. there is a definate relationship
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
Not only, for example when they pull collective on one end to pitch that end of the Chinook up, that causes increased drag on the rotor, which tends to yaw the aircraft, so they pull cyclic oppositely at each end to pitch the rotors a bit to cancel that out. The pilot doesn't have to do this, it's all cross-coupled for them.
Without this, the aircraft would be a total nightmare to fly
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
"Cyclic pitch changes are accomplished by tilting the swashplate. Collective pitch changes are accomplished by vertical movement of the swashplate. Combined collective and cyclic pitch changes result from combined control inputs by the pilot."
The Chinook steers by tilting the swashplate, thus giving more lift on one side than on the other.
The rorors , I think, from watching them (they fly over, and sometimes past the windows, of my house all the time), and due to the fact that both rotors can be driven by each engine, always spin at the same speed, relative to each other, thus cancelling out any torque reaction. Tilting fore and aft is accomplished by vertical movement of the swashplates. Foreward motion is achieved by increasing the angle of the rear rotors, thereby tilting the whole craft foreward, as far as I can tell.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
im thinking i may have an explaatio for my drone problems... though i dont like where it leads. Perhaps, i was putting 5 seconds of forward acceleration into the drone, which it does faithfully. then as the human pilot i cammand a pitch -back to decelerate in 2 seconds. hence a proportionately greater rise in the nose, which starves the props of pitch authority?
i wonder if i should limit the forward speed? and/or the rate of deceleration?
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