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06-21-2018 09:50 PM  15 months ago
garyl

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Joel thanks for the offer! Sandy Jaffey contacted Demonjoe and he said something about an Axon working, I am waiting on his reply to explain??

Jim, between you Corne and demonjoe there was a clear consensus of what really works. Thanks to all who gave their very helpful input.
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06-21-2018 10:14 PM  15 months ago
Jim Ryan

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Let's try this again...

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06-21-2018 10:19 PM  15 months ago
Jim Ryan

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Stay at it, Gary. This is a setback, but you'll get past it. Two or three times I thought my Huskie was at a dead end, but I kept managing to detour around it. Much the same was when I built my little Cheyenne - an endless exercise in problem solving. But hey, that's part of the fun!

Jim
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06-22-2018 11:54 AM  15 months ago
garyl

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I have an idea of how I will be adding a third servo, just need the time to draw it up. It will require a new lower swash ring for 120 degree ccpm, servo mount, machining a slot in the upper frame spacers and new side frames to change the servo locations. Over all it will not be a big change.
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06-23-2018 06:01 PM  14 months ago
Jim Ryan

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Good deal, Gary. Your overall design is elegant, and this isn't a huge setback. Are you going to need an electronic mixer to mix the elevator and rudder outputs to the center cyclic servo?
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06-23-2018 06:24 PM  14 months ago
Jim Ryan

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Skookum Control Setup
Jeff asked if I could post the wiring for my Skookum 720 setup, as he was looking at taking the same approach on his K-Max. I figured I'd reply to the thread in case anyone else is interested.

There's not a lot to it. I'm using standard 3-servo collective with 120 degree spacing. The aileron and pitch outputs (labeled SW-L and SW-R on the Skookum) use compact Y-connectors to connect to their respective servos on both swashplates. The elevator and rudder outputs (SW-C and Tail on the Skookum) are connected to the inputs on a DigiMix-3 programmable mixer, and its outputs are connected to the elevator (center servos). For cyclic and collective control inputs, both swashes move in parallel. For rudder control inputs, they tilt in opposite directions.

One problem is that I think the DigiMix-3 has been discontinued, and I don't know what's a suitable replacement. Cheap V-tail mixers from suppliers like Hobby King aren't designed to handle the high-frequency digital output from the Skookum. Anyone else have any suggestions for high-frequency mixers?

Jim

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06-23-2018 06:29 PM  14 months ago
Jim Ryan

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The schematic I posted dates from when I was first trying to fly the mechanics with a MicroBeast FBL. That didn't work, as the MB couldn't handle the vibration, causing the servos to jitter like crazy. The Skookum is particularly resistant to vibration, and it also has data logging, which can display the frequency, amplitude and axis of all vibration. The frequency points to the source of the vibration, and as you work on reducing it, the amplitude axis will tell you if you're making progress.

As noted earlier, my only issue with the "finished" helicopter is that it's very sensitive in pitch. I really need to get this out and spend some time flying it.

Jim
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06-24-2018 11:24 AM  14 months ago
ozghost1952

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Melbourne Australia

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Linked up a Skookum 720BE and choose Tandem Setup - as follows
This should work AS IS with no additional mixers and stabilisation as required in all three axes

Jeff

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06-24-2018 04:32 PM  14 months ago
Jim Ryan

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As Gary noted, the Tandem option on the Bavarian Demon 3SX didn't work for a his K-Max. With the shafts so close together, controlling roll with DCP (differential collective pitch) causes roll/yaw coupling owing to the asymmetric torque. I would expect this to be an issue for the Skookum as well.
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06-26-2018 04:54 AM  14 months ago
ozghost1952

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I set up a test rig with the Skookum 720 Tandem as per the diagram posted
Left / Right Cyclic - both swash tilt Left and Right = Roll
Back / Forward Cyclic - both swash tilt forward back = Pitch
Rudder - one swash tilts forward / other tilts back = Yaw
Collective - both swash move up and down together (differential set to zero)

Surely this will work? There is no differential collective on roll - which I understood to be causing an issue with the drive system being so close together

Jeff
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06-26-2018 11:34 AM  14 months ago
Jim Ryan

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Jeff, this sounds absolutely correct. I wonder if the Skookum tandem setup functions differently from the Bavarian Demon. It certainly sounds worth giving it a try.

One thing I should note about my mechanics: I designed coning rotor hubs (5 degrees, IIRC). This increased the clearance between the two rotors, but particularly increased ground clearance. With regular straight rotor heads, there's very little ground clearance at the outer edges of the rotor arcs, and a ground strike is a disaster.

Jim
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06-26-2018 12:31 PM  14 months ago
garyl

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Jeff, Thanks for posting that info. Can each rotor head be phased separately?

Jim, Joel is making up a mixer for me to use with the demon.
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06-26-2018 06:27 PM  14 months ago
Notar

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I WOULD NOT THINK THAT A TWO BLADE HEAD NEEDS PHASING. I COULD BE WRONG IN THIS INSTANCE.
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06-26-2018 10:44 PM  14 months ago
Phoenix NOTAR

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I am hoping Gary means “tilted” separately for yaw. Having collective separately would nice as well to aid yaw
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06-26-2018 11:42 PM  14 months ago
ozghost1952

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I agree - phasing on a two blade rotor is unlikely to be required - and if you mean tilted - the answer is yes. It works like a normal CCPM swash with built in mixing to tilt the swash in opposite directions for yaw - just turn it 90 degrees and swap elevator and aileron channels
The Skookum differs from the BD with regard to differential pitch - I have the BD on my Chinook and it is a perfect setup. The Skookum has an option to add differential pitch - not needed in the Kmax - but uses forward tilt of the swash to move forward. The BD relies purely on differential pitch to move forward - and the swash only tilts laterally (roll and yaw)
I have one of Joel's Tandem Mixers and it appears to provide the same mixing as the Skookum but needs external gyros
The Skookum also has two additional "tail" settings
1) Compensation for torque caused by elevator/aileron motion
2) Compensation for torque caused by collective pitch
Jeff
PS If it speeds up your development I can lend you my "Joel" mixer while he makes you one
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06-27-2018 03:35 PM  14 months ago
garyl

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Thanks for the information.

I have finished the design changes for adding the third servo and it keeps everything inside the frames. Now I have to make all the changes to the real parts

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07-02-2018 10:13 PM  14 months ago
garyl

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I have modified the frame spacers and made the new third servo mounts that will attach to the frame spacers. The new side frames arrived from cncmadness in no time, that guy is fast.

Next thing to do is change the lower swash ball positions to 120 degree.
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07-08-2018 11:07 PM  14 months ago
garyl

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The lower swash ring has been modified to three servo 120 degree configuration.

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08-07-2018 10:07 PM  13 months ago
garyl

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Joel and I got together at the club field on Sunday and installed his Helitronix mixer he made. Joel helped me get it all setup, I didn't have all the correct size servo extensions so I made due with what I had for a quick test flight.

No video was taken but it made it into some brief hovers. It hovered for a few seconds before the cyclic would start to oscillate due to high gains. I have to setup some remote gains in my new Futaba 16sz which is anything but intuitive, I like the old 14 channel much better.
Didn't have much time to play so I had to stop for the day at that point, more to come later.
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08-07-2018 10:11 PM  13 months ago
Joel Rosenzweig

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It was a really nice test flight, Gary. I'm glad it got some air under the skids, and was controllable too. You'll get the gains dialed in. Keep up the good work.

Joel
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