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Need Help Related to Rotational Inertia

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Patrick

Sun Feb 17 2013, 04:10AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Ash Small wrote ...

I hope you'll forgive me for posting this in your thread, Patrick, ....enjoy.....

1
very cool, you could also forcibly detain squirels with that machine !

BigBad

Tue Feb 19 2013, 01:13AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

Patrick wrote ...

Rotors counter-rotate at 6000 rpm, using 10x7x3 Master Airscrew props.

Isn't that a really, really, really bad thing for rotational inertia?

As in, to a first approximation, no rotational inertia at all?

BigBad

Tue Feb 19 2013, 01:30AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

How many degrees of control have you got? I only counted 4, don't you need 6 for full control, unless you have aerodynamic stability?

next day edit: no I think that's OK, 4 should be enough

Dr. Slack

Tue Feb 19 2013, 08:04AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

BigBad wrote ...

Patrick wrote ...

Rotors counter-rotate at 6000 rpm, using 10x7x3 Master Airscrew props.

Isn't that a really, really, really bad thing for rotational inertia?

As in, to a first approximation, no rotational inertia at all?

Having them counter rotating means that the airframe itself doesn't have a net gyroscopic reaction. This means that if you try to tilt the frame, there isn't excessive gyro fight-back.

As I understand it, each prop is actively tilted independently. If one tilts one way, the other tilts the other, the two torques add for a net effect on the frame.

Am I right?

<edit>

What I wrote above got me thinking, and now Iâ€™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.

Both gyros and reaction wheels can only apply torque for a limited. Reaction wheels will eventually get up to maximum speed. Satellites use reaction wheels for roll control, but they need to fire roll thruster pairs from time to time to keep the wheel speed limited. Precessing a gyro to generate torque along a specific desired axis will eventually get to a position where the spin axis lines up with the required torque axis, and no more can be had by precession.

This means to me that tilting gyro-behaving props to generate roll torque can only ever be dynamic, and long term static roll trim must be got by one of the more conventional means above. This begs the question that if the long term torque is got by (say) tilting a vane in the downwash, then wouldnâ€™t this be just as fast acting (faster possibly) than precessing a motor+prop for the gyro effect?

Am I missing something? Gyros do peoplesâ€™ heads in at the best of times, so I could be wrong. A vane in the downwash might look ugly and get broken in heavy landings, so maybe itâ€™s a tradeoff to use fast acting tilting props for stability, and a slow acting covert weight shift for long term trim?

<\edit>

Steve Conner

Tue Feb 19 2013, 01:05PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

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.

Ash Small

Tue Feb 19 2013, 01:59PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Gyro's do react in unexpected ways, this can be demonstrated by holding an angle grinder in one hand, and 'tilting' it. There is a 'reaction' that tries to tilt it at 90 degrees to the direction that you are trying to tilt it in.

I generally find when grinding, if I remember correctly, that if I apply a force at 45 degrees, the grinder turns at 90 degrees. It's quite difficult to explain.

Steve Conner

Tue Feb 19 2013, 02:25PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yeah, that's what we refer to as "precession" above. It's why you need to be careful when angle-grinding things up a ladder.

BigBad

Tue Feb 19 2013, 03:03PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

As with I think everyone else, my brain is exploding trying to analyse this vehicle.

But one thing is clear the overwhelming answer to the question: how do I get more rotational inertia is: rotate them the same way, hands down!

Whether that will solve your evident control issues: doubt it, I don't think you've got your head around the effects of the angular momentum of the rotors and explained it to the control system properly. I think you may be able to get the current system to work, with extreme care.

Steve Conner

Tue Feb 19 2013, 03:17PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yeah, but you have to counter-rotate the props so their torque reactions cancel, otherwise you would need a helicopter-style tail rotor to stop the craft from twirling madly around.

BigBad

Tue Feb 19 2013, 09:02PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

Not necessarily, because you can twist the two rotors relative to each other, which can cancel that out.

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