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icanfly

Elite Veteran

ontario

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my little 300, used to be a Dynam 450 until I chopped the boom, put a 450 belt in. cut out most of the frame undercarriage, relocated the tail servo below and behind the main gear, put mostly Align tail and dfc head parts, use 1300mah and 1800mah 3s lipos, shortened 450 tail blades to 55mm and main blades to 270 maximum (sometimes 250mm) to give you the short stop on the heli that's now sporting my first non dual dfc arm head assembly. For inexplicable reasons the heli was crashing all the time and has been a cursed bird for the most part with even one flight this morning ending up in yet another crash due to a connection with a tree branch. On this heli I've been forced to improvise.

So a few flights ago one dfc arm got badly bent at the swash link end and a few servos got taken out another flight before that. I don't know what in gods name has this heli so prone to being crashed without making a full lipo's worth of flight without impact. Yesterday the tail connected with a fence and damaged all the tail components, then as I was reassembling the tail a vital screw just completely disappears, same with a shim washer WTF I have to say.

Ok, this morning take a minute to test the rebuilt heli, with the single side dfc arm swash driver, and first discover the tail crank loose on an arm that isn't tighten'd all the way. This was due to using a too long screw to replace the one that disappeared last night. Size the screw properly and prepare for another flight out back. Take her up, little vibe problem but flyable so I take it around the yard for a minute then loose it into a small central tree and can't believe it. Two main blades in half each and one link missing, lipo and lipo checker tossed the other direction as the heli was going and heli lands upside down in the grass below the tree, I cannot freakin believe it as the heli has been nothing but crash after crash seems every time I try to fly it for the past two weeks.

Ok, so I look around for the missing swash/arm link but didn't put the complimentary neon orange tape on it before it was lost so little to contrast it from the grass. Take the heli in and examine the feathering shaft, size up two more blades from some 325's down to 270mm, get another link sized and installed, and make sure nothing else is out of order. Got the heli spooled up with a wobble, put some blade balance tape on to more closely balance the two, wobble is smoothed out and go for a few circuits a minute before the lipo checker starts to beep.

Hey, the dfc didn't wreck any servos this time, the dfc driver arm popped off the swash and the other link flew off the heli somewhere after/before the impact (I'll go take another look at the location the heli landed to see if I find it there. But miracle of miracles all the servos are ok, cool. Wouldn't you want to have a dfc driver but not wreck servos all the time? Single side dfc seems to be the answer as one grip/arm/blade will pop off the swash while the dfc arm does it's own thing without being bound into a unified assembly, it works and works pretty good I might add. What does it really take to turn the upper swash around anyway? very little no doubt.

DFC single side, regular link the other side, not looking back, it works well and is crash friendly on the heli compared to dual dfc arm, no kidding.

One other thing, a true test of how much tail blade pitch is being applied to keep the heli straight can be observed if you spool up beside or in grass, not tall grass, just place a piece of plywood large enough to old the helis skids and hold the tail off the grass and watch the grass on the side the tail rotor is blowing. I should add all my helis are raked forward to help the tail fan clear the ground better.

08-20-2016 01:14 AM
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Dr.Ben

rrMaster

Richmond, VA, USA

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At least one company does that:

Ben Minor

Team Synergy Team Futaba Team Kontronik USA
Progressive RC

08-20-2016 02:38 AM
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Ace Dude

rrProfessor

USA

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minicopter!

  

08-20-2016 03:30 AM
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Aaron29

rrProfessor

USA

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Wall of text. I'm genuinely curious but not that curious. Can I get the Cliff's notes version?

08-20-2016 04:05 AM
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icanfly

Elite Veteran

ontario

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hehe, one reason the two side dc arm driver might be a problem with vibes, and someone did testing in this regard, is that essentially the grip arm pathway in pitch events takes on an ellipse. The upper swash also takes on an elliptical path and here's where things go poorly with the two pathways, the grip link and swash ellipses are different and what you have is a cone. Why this is important to point out is that the elliptical path of the lower pathway becomes more angular and the two pathways distance wise travel different to each other. It's easy enough to go to your dfc rig and while bladeless and detached from the servos rotate the assembly while manually tilting the swash to one side or the other, you will notice a node or bump where the grips have a little resistance. Now take one dfc arm off and try again, node goes away. You have to tilt the swash plate to extremes to really see it in action. If the links were purely parallel to each other in all pitch events the node would not exist.

Discovering single arm dfc first hand out of necessity even though someone else like minicopter has done it for awhile now, it is not as hard on servos as two dfc arms because one side allows the other link to float, and pop off in an accident.

I never knew it could be so good, and, the link end on the dfc arm did not bend at all where it usually did in a crash while it was a two side dfc assembly. We're not talking serious crashes just inconvenient mid air shut-offs and low level impacts with back yard objects, obstacle course style mistakes, it's fun.

Now I feel like converting all my dfc heli heads to this program, 700e, etc, in future.

08-20-2016 12:55 PM
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icanfly

Elite Veteran

ontario

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one thing to absolutely remember, you have to balance both link arms as well. Before I did mine the heli head had an annoying wobble, so I weighed the link and dfc arm and got 1.7g on the dfc arm and .6g on the ball link, mind you the ball on the grip and the screw for the dfc to grip should be equal. After making a sleeve counter weight of ss metal tube and twisting the two ends of the ball link together with the tube over them I spooled up the heli and wobble was gone.

08-20-2016 11:14 PM
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Aaron29

rrProfessor

USA

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Interesting. I understand you mass balance these opposing assemblies, but wouldn't there be a difference in effective delta between the two blades? I mean, one is able to pivot around the ball and one isn't. Wouldn't this matter?

Is there an advantage to this over a traditional swash driver assembly?

08-21-2016 01:23 AM
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don s

Key Veteran

Chesapeake, VA

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Rotor head asymmetry doesn't seem natural.

E820, Raptor G4N, X50F/E, E620, Forza 450, and some planks.

08-21-2016 01:36 AM
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icanfly

Elite Veteran

ontario

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Is there an advantage to this over a traditional swash driver assembly?
and
Rotor head asymmetry
both very good questions/considerations I wish I had concise answers for.

First because the single dfc arm is not restricted by duplicating exactly what the other arm is doing it would be free to allow the grip to "play" as it were and not depend on rigid dampers to sustain equal actions for both dfc grips. Secondly, the link only grip is essentially going to perform the same as a standard follower type head in rotation so there's nothing more to see in that, except.

Except that, the aspect of asymmetry does enter here more so in phasing as the blade in pitch will want to lift the grip slightly therefore retarding phase angle by the same amount, I didn't experience any difficulty with it at all. You can imagine the larger the heli the less of delta3 is introduced due to soft dampers and loose bearings. I don't know how many of rc heli fans have actually tightened a head button screw down onto the feathering shaft and locked it, the heli will fly better, more true to stick actions, more direct.

One thing I have to note is that I tried my 300 with phase lag mechanically set to something like 68° by turning the dfc arm around 180° and flew it but it required a different feel on the pitch stick. I returned the phase mechanically to where it was (not original 90° but more like 82° where it is now because I shortend'd the grip link arm slightly before I did the experiment and) only setting it far back of 90° to test some ideas in all regards of this nature like will it fly, it did.

On my heli the links of ball and dfc types are symmetrically positioned so both sides are equal but I see how the phasing would be offset by how much out of true vertical the dfc arm may swing back and forth because of pitching/flapping moments.

Regular swash drivers are basically doing what a dfc arm is doing and not prone to rigidity of the grips which are independent of the fixed pathways of regular followers.

In the test results submitted by the member who tested all variations of follower/driver/dfc, he found that one dfc arm with two regular drivers resulted in the lowest vibe numbers in his testing. I don't think he posted a single dfc arm driver result however though I did ask him to test all variations of drivers like one driver two links, two drivers, two dfc, two dfc two drivers, etc. He didn't get to it all but posted some good info of what he had tested.

I'd like to test all variations with vibe logging if I could, just not set up to explore that topic fully as yet.

If it's easier on servos, well.

It just dawned on me now, in a two arm dfc pitch event the blades may take on opposite directions on the hub say like in a roll. With dfc using both grips you would have opposite bending of the grips and feathering shaft and would push the arms on the swash against each other, like pushing both swash link ends to one side of the main shaft at the same time. Rigid dampers or locked spindle lessening the effect however the metal spindle will still flex and return though not much. This is probably the flaw and source of vibes of full dfc and perhaps short inflexible dfc arms.

08-21-2016 03:48 AM
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