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HomeAircraftHelicopterHelicopter Main Discussion › Is There An Elliptical Circle In A DFC Head Rotation?
09-22-2013 02:16 PM  5 years agoPost 1
icanfly

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ontario

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This has come to me because there was a unique type of anti rotation mechanism I am looking at and have to overcome the 3 axis of rotation to perfect it.

The number of failures by dfc links people report recently has caused me to examine the dfc head more closely. I was at the local hs holding an Align 700E DFC when I thought about putting a substantial deposit on it. The dfc gives me a little problem if guys are reporting failures. I see the links form a cone.

In a Elliptical circle two sides are larger radius than the other "ends". This means the dfc links are being stressed to the sides off axis from the main shaft. In a perfect world the links would be perfectly parallel to the main shaft at all times. The problem exists because the swash ball link circle is smaller in its elliptical rotation than the blade clamp link circle.

I've bought a DFC head for my 450 and know they are over built in scale compared to larger heli dfc machinery so I have comfort in that fact. I may mess around with a modded dfc head.

Here's what you can do to examine this for your self. On the bench or dinning room table without blades tilt your dfc swash to it's extreme angles and turn the head by hand. If you notice any tightness in the 360 degrees of rotation then you are looking at binding aka twisting. There will be a vibration present when the swash is tilted past 90 degrees to one link axis when it's flying and under load.

Correct me if I'm wrong.

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09-22-2013 06:03 PM  5 years agoPost 2
McKrackin

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Lucasville,Ohio

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In a perfect world the links would be perfectly parallel to the main shaft at all times.
Not possible because the blade grip ball turns in an arc.
Closer to the shaft at full negative degrees.
Furthest away at zero degrees.
Closer to the shaft at full positive degrees.

The only way to do what you want is some sort of gear driven blade grip and that is just extra weight that does nothing for the heli.

If you have binding,you have too much pitch.
More flexible links are the best answer.

I literally never use the word literally right.

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09-22-2013 06:07 PM  5 years agoPost 3
McKrackin

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Lucasville,Ohio

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And the stress I think you're talking about doesn't truly exist.
As the swash turns,the link will adjust the pitch of the blade to relieve stress from itself.

All FBL heads pitch links are the same angle.
All of them go from the blade grip to the swash.
All swash plates inner links are on a smaller circle than the outer links.

The extra stress is because the links are driving the swash plate and that bends them.
But it bends both at the same time and at the same angle so there's no effect on their up and down angle.

I literally never use the word literally right.

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09-22-2013 06:21 PM  5 years agoPost 4
TMoore

rrMaster

Cookeville, TN

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If you lock the swash at extremes of travel the bumping can happen on almost any head if there is interference during rotation of the inner ring of the swash. If you teeter a DFC head as will happen in flight the inner ring of the swash will go out of phase relative to where it should have been if the head couldn't teeter. This constant in/out of phase condition plays havoc with FBL controllers and can cause vibrations through the system. It is interesting to note that the designer of the T700, Jason Krause was at the NE Phenom sporting the old style head pictured here:

While Jamie Robertson was sporting the DFC offering:

I'm not a big fan of the DFC head design. Even one of the early proponents of this style of head as basically quit offering this design.

TM

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09-22-2013 06:24 PM  5 years agoPost 5
don s

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Chesapeake, VA

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I think me means links parallel to the shaft when viewed from the side, with the blades fore and aft.

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09-22-2013 06:52 PM  5 years agoPost 6
McKrackin

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Lucasville,Ohio

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I think me means links parallel to the shaft when viewed from the side, with the blades fore and aft.
I think he means the other way.
The swash balls are closer together than the balls on the grips.

That's the same on non-dfc heads.

They will stay parallel from the side.
Well...not exactly parallel but perfectly in line.

No head will ever keep the links parallel...Just not possible without some sort of gear train or a ball that moves in the grip.

I literally never use the word literally right.

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09-22-2013 06:56 PM  5 years agoPost 7
icanfly

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ontario

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This constant in/out of phase condition plays havoc with FBL controllers and can cause vibrations through the system.
and the two elliptical circles joined by the links are ridged however the longer stems on the clamps form a cone when looked at down the length of the blades.

Here's what I think happened in the progression toward dfc. First it was fbl with extended link stems on the clamps to compensate for servo torque or lack of. Then it went dfc but nothing else changed except the removal of the swash driver and lowering of the head.

If you were to examine the mechanism I think you might see some lateral deflection due to the mismatched diameters of swash and clamp stem geometry. (Where's a computer simulation analysis when you need one?)

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09-22-2013 06:59 PM  5 years agoPost 8
McKrackin

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Lucasville,Ohio

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The angle of the links on a dfc head and a traditional head are identical.

They ALL have links direct from the blade grip to the inner swash.

I literally never use the word literally right.

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09-22-2013 07:01 PM  5 years agoPost 9
TMoore

rrMaster

Cookeville, TN

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Y'all are missing what's happening entirely. The head teeters in flight due to aerodynamic forces. That means that when the feathering spindle moves in an arc relative to the mainshaft centerline the rods that are rigidly attached to the blade pitch arms now describe an arc and the swash inner moves in the rotational plane to reflect the arc that the DFC links are being driven through. All you have to do is grab the rotor head and teeter it and you can see the swash inner rotate.

TM

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09-22-2013 07:02 PM  5 years agoPost 10
TMoore

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Cookeville, TN

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The angle of the links on a dfc head and a traditional head are identical.
They ALL have links direct from the blade grip to the inner swash.
They may start out that way on a DFC head but they don't stay that way in flight and that's the difference. On a standard head the swash puller makes the swash inner stay put and the teeter that the blade holders naturally do is not going to affect the inner swash location.

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09-22-2013 07:04 PM  5 years agoPost 11
McKrackin

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Lucasville,Ohio

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Just used to my rigid heads I guess.

I literally never use the word literally right.

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09-22-2013 07:06 PM  5 years agoPost 12
TMoore

rrMaster

Cookeville, TN

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Your rigid heads aren't rigid either. They teeter too. If the blade flaps and they do, the displacement takes place at the blade holder and if you had DFC links to the inner swash, the same thing would happen.

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09-22-2013 10:39 PM  5 years agoPost 13
icanfly

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ontario

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therefore the dfc link is transferring all the teetering down to the swash events, and probably shaking its self to obliteration when the stress is too much from hard swash angles.

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09-22-2013 10:56 PM  5 years agoPost 14
McKrackin

rrProfessor

Lucasville,Ohio

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If the head teeters down,it won't push down on the swash.
The blade grip is a hinge so when the head teeters down,the swash stays put and the blade pitch changes.

What am I missing?

I literally never use the word literally right.

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09-23-2013 12:29 AM  5 years agoPost 15
TMoore

rrMaster

Cookeville, TN

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What you're missing is that when the feathering spindle teeters the pivot bolts sweep through an arc which csuses totation of the inner swash.

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09-23-2013 12:33 AM  5 years agoPost 16
McKrackin

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Lucasville,Ohio

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I'm not sure how different that is from traditional heads.

When I move my cyclic stick around in circles with the heli on the bench...it will spin the head...pretty fast.
All my helis have.

There is some interaction no matter what.

I'm still not seeing the point...Not arguing...I just don't understand it.

I literally never use the word literally right.

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09-23-2013 12:43 AM  5 years agoPost 17
TMoore

rrMaster

Cookeville, TN

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That's why it is always best to have a frame of reference during discussions like this. A DFC head is different in that it has no swash inner puller like a traditional head would.

Since the DFC Link is attached to a pivot bolt and the DFC Link is rather stiff when the head teeters the pivot bolt is swept through a small arc which causes rotation of the swash inner. On a traditional head that uses a puller it doesn't matter if the ball pivot sweeps through an arc because there is a spherical ball compensating for the pivot arc.

TM

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09-23-2013 12:45 AM  5 years agoPost 18
Craigdieslemac

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Valdosta, Ga USA

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DFC links are failing quite simply because the nature of the rigid dampeners. There is very little, if any, give to allow normal lead, lag, flap, and droop that all blades experience in flight. Dampeners on a traditional style head, allow a blade to do all of these things without transferring all of the motion or vibrations to the links and down to the rotating swash. Precise and rigid go hand in hand, the trade off for the performance is shorter life expectancy of parts. This holds true for anything high performance.. Race cars, dirt bikes, Dfc heads. Look at Funny car engines.. Absolute perfection.. And they run beautifully until they detonate. Same will happen with DFC heads.. Sharp and precise.. Handles like its on rails.. Until it fails.

Aut viam inveniam aut faciam

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09-23-2013 12:47 AM  5 years agoPost 19
McKrackin

rrProfessor

Lucasville,Ohio

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Actually...I think the FIX for DFC heads has always been more rigid dampers.

Prevent the teetering or limit it.

I literally never use the word literally right.

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09-23-2013 12:50 AM  5 years agoPost 20
TMoore

rrMaster

Cookeville, TN

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DFC links are failing quite simply because the nature of the rigid dampeners. There is very little, if any, give to allow normal lead, lag, flap, and droop that all blades experience in flight.
Not altogether true. They fail because the pivot bolts are not capable of withstanding return flight forces from the head teeter action that sweeps the pivots bolts through those tiny arcs. The design would actually work better if the head were completely rigid but that is not a panacea either as that situation causes its own problems. Rigid heads force the designer to make the blades flex so that they can flap which replaces the teeter action of the feathering spindle. We've run rigid and near rigid heads in the past and they have rpm related vibration issues not connected to a DFC design.

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HomeAircraftHelicopterHelicopter Main Discussion › Is There An Elliptical Circle In A DFC Head Rotation?
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