DFC vs non-DFC Head

[scallybert]

Can anyone describe the difference in terms of feel ?

[Especially in respect of 'tidy' hovering manoeuvres.]

[KevDavies]

Ive found my DFC Goblin very very aggressive. Really fast cyclic response and not that neat in the hover tbh. My Raptor has a traditional FBL head and has fast cyclics but is what I can describe as accurate. And hovers really well.

I like both but for hovering I prefer the Raptor over the Goblin

[SGD]

Here's a good read on the subject I found quite insightful.

DFC Rotor Heads Demystified

[Ade_Law]

Comparing like with like...

fbl dfc and standard link fbl

the difference is not really in the stronger links but more about the requirement of DFC to have much stiffer damping. They require stiffer damping in order to stop it effecting the swash phasing. Compass had prototypes of this floating around when I was in the process of quitting the sport.

Stiffer damping is great for 3d, it means the body of the model follows the rotor disk much better, stops the tail bounce you get when you stop a fast flip suddenly and is just generally better.

the downside is that in pushes up the resonant frequency of the rotor disc. This resonant frequency is the wobble you see models do when the head speed gets too low. I always found the trade off between nice autos and hard damping for 3d a hard compromise to make as autos require softer damping and 3d requires hard.

The other disadvantage of harder damping is that it can (on a fly bar) make the model feel nervous on the cyclics on a flybarless heli the electronics do all the hard work in this respect.

I have never played with a flybarless rig only flew some. I know it is possible for a flybarless heli to be positively stable (like a contra) with the right setup as I have flown some that are. If you want a model to hover nicely then softer damping is defo a good place to start followed by correct setup of the flybar system (heavier paddles, more flybar input, slower headspeed) or setup of the fbl electronics.

hope this helps!

Ade

[trillian]

At the end of the day there's no magic in the DFC concept, nothing that another type of FBL head type cannot duplicate. One difference that is often part of the equation when comparing Align helis with DFC vs non DFC is when the DFC head has also been used with a lower main shaft which places the rotor disc lower (or effectively raises the C of G). That's really the main justification for the DFC design, because it no longer needs a swash driver taking up space, the headblock can be closer to the swash. So that and the fact that it will have very hard dampers, as Ade mentioned, are the two biggest factors, not really the constuction of the links themselves.

--just to add, regarding tidy hovering. I have flown a Trex 550 DFC with a 3GX that was excellent in a hover. I will say, it was a very calm day so it might be different in windy conditions but I was really impressed. Headspeed was fairly high but certainly not crazy. The 500s I have flown with DFC heads were not as nice but that was more than likely due to the FBL setup (my 500 with RJX head and BeastX is a pig to hover but I don't mind).

[Gixxer]

At the end of the day there's no magic in the DFC concept, nothing that another type of FBL head type cannot duplicate. One difference that is often part of the equation when comparing Align helis with DFC vs non DFC is when the DFC head has also been used with a lower main shaft which places the rotor disc lower (or effectively raises the C of G). That's really the main justification for the DFC design, because it no longer needs a swash driver taking up space, the headblock can be closer to the swash. So that and the fact that it will have very hard dampers, as Ade mentioned, are the two biggest factors, not really the constuction of the links themselves.

--just to add, regarding tidy hovering. I have flown a Trex 550 DFC with a 3GX that was excellent in a hover. I will say, it was a very calm day so it might be different in windy conditions but I was really impressed. Headspeed was fairly high but certainly not crazy. The 500s I have flown with DFC heads were not as nice but that was more than likely due to the FBL setup (my 500 with RJX head and BeastX is a pig to hover but I don't mind).

This ^^^

The DFC head also looks nicer but to offset that, it's the cause of a lot more rebuilds.

It's a ridiculously stupid design. It's as if the design brief was to make a head that would inevitably fail.

[Varelco]

The whole concept of lowering the head with the DFC design isn't really an advantage any more. Swash drivers are now integrated into head blocks lowering the head anyway.

[Ade_Law]

The whole concept of lowering the head with the DFC design isn't really an advantage any more. Swash drivers are now integrated into head blocks lowering the head anyway.

And in my experience the limiting factor isnt the space there any more. Its the blade boom gap which is another reason they need hard damping as this gap is smaller. Im actually interested to see how this differs with flybarless. Due to the control system and algorithms in place it is possible that just by being flybarless means that the system can be setup to prevent boom strikes.

[tomatwalden]

The whole concept of lowering the head with the DFC design isn't really an advantage any more. Swash drivers are now integrated into head blocks lowering the head anyway.

Arguably, lowering the head too far is overly increasing the risk of boom strikes too. It's fair to say, I'm not fan of the DFC concept either. Interesting to note that the new Compass swash-driver head uses exactly the same main shaft, so doesn't change the height of the heli over the standard (DFC-style) head. Proof, if proof was needed that you don't need to lose the swash drivers to lower the CoG.

[gazmk2]

Pardon my ignorance, but what is meant by "damping"?

[tomatwalden]

Dampeners are the inserts in the head that allow the feathering shaft to move axially (teeter). This movement of the feathering shaft is what allows our small helis to have blade flapping - necessary for stable fast forward flight ... see : http://www.dynamicflight.com/aerodynamics/flapping/

[gazmk2]

So it kind of allows the leading blade to lift up in forward flight then? effectively reducing the amount of lift it generates?

[jwatts007]

I put one on my 700n when they came out...it was spot on very solid or crisp on the sticks with the V bar...took it off though as saw the damage it caused in a crash...stuff got bent I had never seen bent in a crash before...I also like too fly FIA blades sometimes so use a V 2 head now.....

[tomatwalden]

Good video here (The Helicopter Speed Limit - Helicopter Physics Series - #7 - Smarter Every Day 51 - YouTube) on the dysymmetry of lift problem. Destin vaguely mentions at 4:03 that the retreating blade is given more pitch but doesn't mention how. It's teetering that does this - and the dampers in the heads of our small helis are what hold the feathering shaft in such a way as it can teeter. Softer dampers = more teetering. Hard dampers = less teetering.

[tomatwalden]

So it kind of allows the leading blade to lift up in forward flight then? effectively reducing the amount of lift it generates?

spot on!