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HomeAircraftHelicopterHelicopter Main Discussion › Is There An Elliptical Circle In A DFC Head Rotation?
09-23-2013 01:08 AM  5 years agoPost 21
AWittleWabbit

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O.C., CA

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What am I missing?
In regards to icanfrys post, nothing.

Never mind, I wrote a little blurb but TMoore said it best above.

This all reminds me of trying to shut the pantry door with misaligned hinges.

I think we get away with it at model sizes due to really stiff damping and being overbuilt for the stresses.

At the 450 size, I've never had a problem or premature wear.

Heli-itis sufferer.

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09-23-2013 01:10 AM  5 years agoPost 22
Craigdieslemac

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Valdosta, Ga USA

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The return forces that would normally be dampened by softer dampeners, which DFC cannot run. My logic may not be clearly conveyed, but it is sound. The more rigid you make the head, the less reliable it will be, the weakest point, in this case, is the link, or link bolt,is where your failure is going to occur. It's the price paid for performance, and it is paid by everything mechanical that we push to the limits. All rotating masses cause vibrations that need to be countered or dampened. If neither is done, failures wil occur, it's science.. You can google that mess. DFC heads are cool.. And they are precise, but as I already said, the price for that precision is reduced life expectancy. I need sleep..thanks fellas for this post.. Hope there will be more good reading when I get up.
Jeff

Aut viam inveniam aut faciam

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09-23-2013 01:22 AM  5 years agoPost 23
TMoore

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Cookeville, TN

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The so called dampeners are not dampening anything, the term dampener is actually a misnomer as it applies to model helicopters. Technically they are teeter rubbers or teeter springs. The feathering spindle contains the blade grips and that spindle is contained in the hub and held in place with the rubbers. It's free to move around but on the old Schluter heads we used a Heim ball to centralize the shaft and on Hirobo heads they use a set of ball bearings supporting a pivot shaft that the spindle is allowed to rotate on. Newton's law states: "For every action there is an equal and opposite reaction." That is what is occurring here. Return forces will appear in the control system and won't be dampened out by the teeter rubbers.

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09-23-2013 01:53 AM  5 years agoPost 24
icanfly

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ontario

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well
Rigid heads force the designer to make the blades flex so that they can flap which replaces the teeter action of the feathering spindle.
that says quite a bit in a nutshell (icanfry heri pitty goot)

I fly with Titanium blade clamp bolts. As you know titanium is a flex metal, an "elastometal" if you wish. A metal that flexes and does not fatigue and break off. First line of defense against dfc link failure might be to replace the hardened steel bolts with ti, and if given the chance to increase the diameter a millimeter or two on that bolt at the blade clamp.

A small difference in grip hole placement on a pair of my 450 blades has netted a magnificently smooth disc while the blades are cf and stiff, I forgot to mention the main shaft is slightly bent. They have more span wise area at the grip bolt than normal. This has resulted in less twisting near the bolt but even up and down flex near the clamp. If you took a 12x1x1/8 strip of brass and held it between your fingers while tapping it with a small mallet it would ring. That's because it has harmonics and is vibrating at frequencies within it's length, like a xylophone bar. Your blades are doing the same thing. If those harmonics can be isolated and not passed onto the clamps and links I think it would be an ideal solution. Yes harmonics screw with a fbl controller, saw it happen a few days ago, a wild flutter. Your blade design grip extension may be the cause of a problem regarding flutter and harmonic vibration of the blades passed down to the links.

Test that theory out by tapping along the length of your blade with a thin rod, you'll hear it ring. Most metal doesn't like to be flexed back and forth a lot, it tears.

One other solution is to have the clamp attach to the dfc link at both ends of the link at the clamp, the swash end would remain normal. Biggest problem in all of this is that it's a ridged design but the nature of a blade rotation is one that vibrates in ff, one side is in greater pitch and the blade is flapping during it's 360 degree rotation, equals? vibration.

OK, I just got back from testing the blade harmonic problem. Seems your root extension where the grip bolt is is a node and lowers the harmonic frequency of the blade a bit. You can hear it when first putting your fingers about where the blade is span wise the same (a centimeter in from the bolt hole) while tapping it with a small diameter rod on the blade's span, and then backing your fingers off to the bolt hole. I think that lower harmonic is adding to the failure of the dfc linkage. The dampers/feathering shaft are taking some of the harmonic vibration normally but the link is ridged and does not flex. This would result in stress. When your blades hum they are oscillating in tiny waves to adjust to aerodynamic forces upon them such as drag.

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09-23-2013 02:41 AM  5 years agoPost 25
rcflyerheli

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Granbury, TX USA

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Maybe someone can clarify this for me. Why are the DFC style heads more precise?

The main purpose of the DFC configuration was, I thought, to let the DFC links serve as the driver for the swash to allow a shorter head block and main shaft to get the CG of the heli closer to the blade axis. I can't picture anything about the DFC mechanical configuration that would make it a more accurate control system.

The main reason for the super stiff dampers is due to the decreased distance between the boom and the blades, which started leading to boom strikes after the introduction of the DFC head configuration. I have no disagreement on the effects of blade flapping on the links and bolts. I also have a belief that in particular on the T700 V3 DFC, that the problem with tail hum and vibration is (at least partly) caused by the vibrations induced by the blade flapping forces being transferred through the grip links to the swash to the main shaft to the frame to the boom to the tail.

This isn't any intent to hijack this thread over to the tail problems on the 700 DFC, and I have put forth this theory over on that thread, but I do believe there is really no way with a DFC head to eliminate this type of harmonic vibration in the head.

Logo 700, Specter 700, Goblin 700, Trex 700DFC, Gaui X7, Logo 690SX, Logo 600SX; Trex 470 Trex 500
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09-23-2013 02:44 AM  5 years agoPost 26
icanfly

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ontario

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ahh but I've discovered there is. Re read the last part of my reply one up.

When you hold a blade by your fingers let it hang. Tap only the very tip of the blade, or equally at the other end from where you are holding it (a little in from the bolt holes) and you'll see what I'm talking about.

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09-23-2013 04:08 AM  5 years agoPost 27
Eco8gator

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Palm Beach, FL

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Watch at YouTube

Bottom line...regardless of how stiff those dampers are the spindle will teeter some. That stress goes straight to the bolts that hold the links to the grips. When that screw breaks you are DOOMED!

And the assumption that you need the DFC design to make a low profile head is rubbish...just look at the Minicopter Diabolo for reference...as low as it gets and it still uses a proper swash driver. There is just enough flex in the plastic link to allow the swash phase to advance and retard slightly when the grips teeter.

C

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09-23-2013 08:30 AM  5 years agoPost 28
Craigdieslemac

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Valdosta, Ga USA

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I feel like we are all re-wording the same phrase. In the interest if good form, I agree with all. I don't fly DFC because it is too rigid.. And will cause more problems than it is worth.

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09-23-2013 08:41 AM  5 years agoPost 29
icanfly

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ontario

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DFC would work provided there was dampening on the link. dfc was a smart way of eliminating parts thus decreasing weight.

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09-23-2013 12:12 PM  5 years agoPost 30
Eco8gator

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Palm Beach, FL

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Compass has a more flexable link...

But I still prefer the swash driver...I don't think the parts reduction was worth the price in this case.

C

Xera Motors
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Minicopter
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09-24-2013 02:20 PM  5 years agoPost 31
Mike Fortin

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USA

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What about all the people flying "DFC" style heads without any issues?

Have Rotors, Will Fly!

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09-24-2013 03:51 PM  5 years agoPost 32
Peter Wales

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Orlando Fl

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They dont count

Peter Wales
http://scalehelicopters.org

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09-24-2013 04:24 PM  5 years agoPost 33
icanfly

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ontario

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I don't think the parts reduction was worth the price in this case.

C
if only someone told everyone sooner.

re MF's reply that some people have no problem, sounds like hard dampers and a good fbl gyro are working well. JR's 3 blade head is rigid. I think we'll be re-visiting rigid 2 blade set ups soon.

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09-24-2013 05:12 PM  5 years agoPost 34
Mike Fortin

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USA

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On my helicopters using both a rigid and a soft damper we haven't seen any problems.

Hundreds and hundreds of flights on my own machines and it's still as solid as it was on day one with zero maintenance.

There are far more people not having problems with "DFC" style heads then pilots that are.

Have Rotors, Will Fly!

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09-24-2013 05:19 PM  5 years agoPost 35
icanfly

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ontario

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but Mike, that sets up a false sense of security.

Are any of these dfc components x-ray'd for defects in the aluminum? I've heard of grip bolts failing for no reason.

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09-24-2013 07:47 PM  5 years agoPost 36
Mike Fortin

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USA

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Anything can fail for no reason, doesn't have to be on the head.

I've had flybars snap, tail belts break, tail grips fly off, main shaft break, blade failures, booms sheer off and look at how Align had spindle bolts breaking for a while.

Anything can break at anytime.

Have Rotors, Will Fly!

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09-24-2013 10:46 PM  5 years agoPost 37
icanfly

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ontario

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Mike, I'm questioning "where" they break as much as the fact they repeatedly did break on different manufacturers helis. If a part fails in one place on several helis of one manufacturer it is often a manufacturer specific design flaw. If it is a part that fails over several different manufacturers then it is a general design flaw.

My guess is no x-ray'd parts from any brand. I over heard a story of a hundred grand aluminum part chucked because a little void was inside the part. I think one guy said it took two weeks 24 hrs a day to make.

Hey'all, I got to thinking
Titanium links and bolts on a dfc
would be the kitty's meeyew, I'll make'em myself if you don't first steal my idea (I'know you're gonna pay for it later. Make em before anyone gets hurt Avant, Goblin, Align, whomever, give me some sets free, LOL)

regarding the heli stuff breaking, qc in eastern countries where a lot of helis are made isn't always the best is it? Niether are guys who beat and or crash their heli and fail to replace vital bolts/screws/and nuts.

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09-24-2013 10:49 PM  5 years agoPost 38
Mike Fortin

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USA

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The EPIC MD7 already utilizes titanium, perhaps that's why we haven't had any issues.

Have Rotors, Will Fly!

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09-24-2013 11:00 PM  5 years agoPost 39
McKrackin

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Lucasville,Ohio

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The video makes it clear as a bell now.

Thanks.

They need dampers in the the grip bolts.
I'll just stick with my swash drivers.lol...

I literally never use the word literally right.

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09-25-2013 01:41 AM  5 years agoPost 40
icanfly

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ontario

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next I'll show you why your favorite blade counter weight is both DANGEROUS and totally unnecessary, or you can tell me how dangerous it is. (nuther topic) The point about my next topic might demonstrate what vibes are transmitted from the blades.

DFC works, eliminates the driver, needs some deflection absorbing bolt/link to the swash (balls at 90 degrees to the main shaft), but I think the elliptical circle the swash balls take is different than that of the grip linkage, therefore adds to the amount of off axis torsion being transmitted to the swash through said links. Looks like some manufacturers have caught on to the fix early, some have not, take a lesson.

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