DFC / Driverless Rotor Heads Demystified

So what exactly is a DFC (direct flight control) or Driverless rotor head and why is there so much hype & debate going on about them?

Trex 800E DFC HeadTrex 800E DFC Rotor Head

There also seems to be a fair amount of misunderstanding, at least going by some of the email questions I've been getting lately but that's not the reason I'm doing a web page explaining direct flight control / driverless heads.

The main reason is while on our monthly city supply run last week, I snuck off to a new hobby shop I had never visited before while the wife was stocking up at Costco.

Anyways, the sales person was a nice enough fellow but then started into the all too familiar sales pitch that DFC is the new thing and is so much better than flybarless.

The funny thing is DFC is just a somewhat simplified flybarless/Bell rotor head that does away with the washout (also known as the swash driver). This is why driverless head is a more accurate description; but DFC is the most commonly known and used term due to its vastly more popular and somewhat generic usage.

In other words, all DFC heads are flybarless, but not all flybarless heads are DFC / Driverless. Moreover, all DFC heads are driverless, but not all driverless heads are DFC. Clear as mud right?

Not being the confrontational sort, I just let the matter slide and saw it as an opportunity to write about this newer form of rotor head and hopefully explain exactly what qualifies as a direct flight control rotor head and the pros & cons (if any), over a conventional flybarless head that uses a standard washout.

Let's start by looking at a conventional flybarless rotor head with a washout and that way it will be easy to spot the difference between these two types of flybarless rotor head designs.

The photo to the right is showing a conventional Flybarless (FBL for short) rotor head with the washout base, washout arms, and black radius arms all circled in green.

The sole purpose of the washout assembly is to rotate the top half of the swashplate with the head so it remains properly phased with the rotor head while still allowing the full range of up/down & tilting movement/articulation of the swashplate.

The washout base is clamped to the mast (or it can be integrated into the bottom of the head) so as the mast or head turns, the rotational loads are transmitted to the washout arms and then to the radius arms, which are then connected to two balls on the upper swash. This is why this part is also called the swashplate driver or swashplate follower as it drives and allows the swashplate to "follow" and keep in phase with the rotor head.

Please take note however that the main rotor grip pushrods still do attach directly from the upper swash balls to the blade grip control arm balls by way of a ball link on each end. They provide a direct linkage between the swashplate and the blade grips, no more or no less so than a DFC linkage.

In other words, D-F-C is somewhat misleading in what it really does, but the majority of the industry has adopted/copied the acronym. Even "driverless head" is fundamentally incorrect because the swashplate is still being driven, just not by the washout arms. Washout-less is a more accurate description, but certainly not as catchy for the marketing gurus.

DFC Head

At any rate, now let's look at a direct flight control rotor head. Notice there is no washout/swash driver on this rotor mast. The blade grip pushrods have now been replaced with a much beefier control linkage that is integrated directly into the blade grip arm.

This design now allows this stronger linkage to transmit the rotational/lateral loads down to the upper half of the swashplate to keep it phased with the rotor head the same way the washout would. The obvious advantage is less parts count and there is no question it looks great! I will go over other advantages and disadvantages further along in the write-up.

DFC Head Linkage

I have circled (in red) the main design feature that all driverless/direct flight control head designs share and that is the integrated linkage that drives the lateral/rotational loads from the head/blade grips down the linkage to the swashplate.

This is the main component difference over a simple ball link. This linkage can rotate on the blade grip arm axle still allowing the blade grips to change pitch angle, but it can't pivot like a ball link could and that is what allows it to transmit lateral force down to the swashplate to "drive it".

In yellow I have circled the other end of the linkage which still uses a conventional ball link as there has to still be some sort of fully articulated pivot point. You will notice the swashplate (in this particular example) has ears extending at a 90 degree angle to the swash where the ball link attaches to the ball.

The proposed advantage to the "swash ear" design is that it's very unlikely a link will pop off under combined prying and centripetal loading. This leads us to direct flight control drawbacks and potential issues; it all comes down to avoiding head axle feathering movement at all costs (in other words, little to no head dampening) .

DFC - The Potential Issues

Before going into this, have a look at the video below to get a better understanding of the mechanics involved with a direct flight control head if super stiff dampening is not used or if any play develops in the head axle shaft supports and is allowed to feather up and down.

As you can see, any feathering movement of the head axle will tilt the integrated control linkage and that will put large amounts of strain on the linkage itself, the linkage bearings/bushings, the bolt or pin where it attaches to the grip arm, the ball links down at the swashplate, the swashplate, and even down to the servos. In effect, the blade grip control linkage has now become a very formidable pry-bar. This "prying" action also throws the swash phasing out slightly depending on how much the axle shaft is allowed to pivot.

These excess lateral / "prying" forces on the linkage arm in combination with centripetal forces pulling on larger, higher mass linkages is what can lead to ball link separation down at the swashplate. That is the main reason Align and some others are now using those updated swashplates with the swash ears with right angle ball placement to reduce the possibility of centripetal separation.

At the end of the day, the main thing to keep in mind is head axle movement is the DFC KILLER  and must be avoided at all costs.

Most direct flight control heads will therefore use very stiff head dampeners (basically not far off an un-dampened/rigid setup) to keep axle feathering movement at the bare minimum, but still allow some vibration absorption qualities; not to mention any built in flexing within the blade grips.

Even with super stiff dampening, play can develop within the stiff dampeners as they "ovalize" over time from the high amounts of axle loads or even within the head recesses where the dampeners sit allowing excessive axle movement. I'm sure as time goes by, direct flight control heads will continue to evolve and improvements/solutions will be introduced to help minimize/eliminate these issues.

Non Rigid Driverless Heads

To alleviate most of these rigid DFC issues, some manufacturers incorporate a flexible control linkage allowing for normal head dampening to be used.

When the head axle feathers, the prying force is no longer transmitted down the control linkage, it is absorbed within the "flexible" linkage. This not only reduces the stress loads in the linkages and attachment points, it allows lower head speeds to be run just like with any other "conventional" dampened head.

Still, the vast majority of driverless head designs right now continue to use rigid DFC type linkages and if that is the type of driverless head design you have (DFC), an important pre-flight check should always be checking for any head axle play by pulling up and down on your blade grips. I check for it constantly on my Trex 800 DFC and so far with over 300 flights logged on that big rascal, all is well! In other words, I've notice no accelerated wear whatsoever.

The one other reported issue I have heard with rigid direct flight control head design is with all this super stiff and hard linkage attachment from the grip arms to the swashplate; blade strikes (even minor ones) are likely to transmit more forces down through the blade grips to the swashplate and into the servos.

I experienced this myself on a Trex 600E Pro DFC when a huge gust of side on wind tipped it over just after I landed as the rotors were still spooled. I did manage to level it out, but not before the rotor tips had just kissed the ground. Very little damage was even evident on the rotor tips, the head axle was not bent, nor was the rotor shaft.

I basically brushed it off as a non event and no change of underwear was required, until I started moving the swashplate servos around. One of them was very noisy and I found some teeth of the metal gear set torn out which takes a good deal of force. Perhaps it was just a weird coincidence of the right set of conditions existing all at once to strip out a strong metal servo gear set, but I have had much worse impacts on 600's that never once stripped out a servo and as I said, I've heard of this before from people who contacted me with direct flight control stories.

With a traditional pushrod setup, chances are the plastic ball links would have popped or broken off, or the pushrod's would flex or bend to absorb some of the impact energy from reaching the servo (the good old sacrificial mechanical fuse device in other words). Not so with a much stronger integrated linkage.

In short, if you have a blade strike (even a very minor one) with a rigid direct flight control head, make sure you closely examine every linkage component right down to the servo arms and gears to make sure nothing is bent, cracked, or stripped (nothing new you shouldn't be doing anyways with any head type after a mishap, but damage is more likely to be hidden on DFC).

DFC Scale Warning

Lastly, if a scale fuselage is perhaps a future project on your direct flight control equipped machine; better think twice on that... Because the rotors are sitting so much lower on most (not all) driverless heads, it may not be possible to fit a scale fuse or even if it is, it's very possible the blades will contact the scale boom which are almost always larger in diameter and may have horizontal fins with vertical side fins that will get chopped off with the slightest amount of blade flex.

Scale birds almost always run lower head speeds (not only for realism but to prevent high frequency fuselage resonance issues) and once again, running super low head speeds is generally not possible with DFC, unless you have one of the dampened driverless head designs.

Before getting any comments from the peanut gallery, I'm talking about general "average Joe" scale builds here - you know, throwing a generic fuselage over a typical off the shelf DFC equipped pod & boom heli kit. I'm not talking about super scale mechanics that are purpose built for these applications as some will use full scale multi blade driverless heads with scale grip flapping or grip flex built in.

Super Scale DFC Rotor HeadSuper Scale DFC Tri-Blade Rotor Head

Shown above for example is a 700 size Roban Compactor Super Scale Tri-Blade DFC rotor head for an AS350B I'm currently building. Both the blade grips and scale non-symmetrical blades have a fair amount of built in flex. 

Moreover as shown below, the boom is mounted very low in the mechanics compared to most pod & booms giving more blade to boom distance (green arrow), helping avoid boom strikes.

Scale DFC Head With Lowered Tail BoomScale DFC Head With Lowered Tail Boom

DFC - The Good

With these potential direct flight control issues, is there anything good about it?

You bet! I already pointed out two items earlier and that is the lower parts count and how good it looks. In addition to that, head setup is a bit easier because you don't have to worry about washout base height etc. to get the washout arms level while setting head geometry (if you have a non integrated washout). In the case of that super scale DFC head, no swashplate phasing was needed since the DFC control links are purpose designed to correctly phase the head to the swash; but what about flight performance improvements if any? 

With super stiff dampening used on most direct flight control heads, there is no question cyclic response is very crisp and immediate (provided your flybarless stabilization system is setup for that type of immediate control response). That said, you can stiffen the dampening on a conventional washout type head and basically achieve the same results so we can't say that is something unique to a direct flight control head.

What is unique however is the minimized helicopter body's center of mass to rotor disc distance. Due to the head design itself and the elimination of the washout base, the mast & head can be shortened which brings the rotor blades (rotor disc) down closer to the helicopter's center of mass. This further improves cyclic roll and flip rates and gives a more symmetrical feel between right-side up and inverted maneuvers. Again, all that is for conventional pod & boom RC helicopters, not purpose designed scale mechanics.

DFC Boom Strikes?

Scale fuselages aside, having the rotors sitting lower on the heli brings us to the highly debated DFC boom strike topic. As you can guess, with the rotors being closer to the tail boom, there is a greater chance & potential the rotors will contact the boom if the right set of conditions are present. With some heli's this seems to be a bigger issue than others, but it can certainly happen with any direct flight control equipped RC helicopter just as it can with any conventional flybarless head or even a flybared head if you don't pay attention to some basic boom strike rules & physics.

First off, head speeds have to be kept up at manufacturer's minimum recommend settings for the type of flying you are doing. This is pretty much a given anyways with stiff head dampening which necessitates higher head RPM's to prevent blade flutter & cyclic pitch wobbles, but it's even more critical to follow and be aware of with direct flight control, especially when performing aerobatics.

With high rotor RPM's, the blades will not flex as much under high disc loading or high negative G maneuvers because they are under higher centripetal force preventing them from bending as close to the tail boom.

Likewise, the increase in the pseudo centrifugal force will also keep the blades from leading or lagging excessively which under high collective or cyclic pitch angles means they will not be tilted down as close to the boom further minimizing catastrophic blade to boom contact.

Blade bolt torque also plays a roll here and should be kept fairly tight (depending on the size of helicopter of course) but if you find after you land and rapidly spool down, one or both of the blades is loose enough in the grips that it visually leads or lags in the blade grip, it is much more likely for that blade/s to hit the boom if the lead/lag condition is present at the same time as a large positive or negative collective pitch angle (blade/s will be angled down dangerously close to the boom).

Head dampener stiffness & condition are of course considerations when talking about boom strike prevention since a soft or worn out dampener will allow the head axle to pivot/feather up and down excessively causing the rotor blades to do the same and contact the boom if the movement is sever enough. Since I already covered the importance of dampener condition and axle play with rigid direct flight control, there is no point to go over that again but it remains something to keep a close eye on and is worth repeating. 

Lastly there is the condition/quality of the rotor blades. Depending on how you fly no matter what type of rotor head design you use (flybar, flybarless, DFC/driverless), rotor blades can weaken over time and start to flex more depending on how you fly. If you fly nothing but gentle type scale, a good set of blades can last for thousands of flights. However, if you are a hard core 3D'er and really stress the blades flight after flight; most blade manufacturers warn that blades are a consumable part that have to be replaced occasionally even if they don't have a single scratch on them and look like new (just like in the full size helicopter world).

So, boom strike causes & avoidance are more or less exactly the same as they are with a conventional flybar or flybarless head; it's just a little more critical with most DFC/driverless heads that the causes of boom strikes are understood so you know how to best avoid them.

Micro Heli DFC?

Here's a common email question I keep getting: "Do micro collective pitch helicopters like the Blade mCPx or 130X have DFC heads"?

Well, when you look at a micro CP head close up, at first glance it may seem that it is using a direct flight control head since there is no washout to drive the upper half of the swashplate.

The two main pushrods from the blade grips are indeed supplying the lateral force to turn the swash. What is missing however to consider this a true DFC head is the rigid integrated linkage that connects to the blade grip arm that allows the lateral forces to be transmitted down the linkage to the swashplate.

Instead, most CP micros use a set of forks or prongs on the head (circled in yellow) that the pushrods slide between and it's those forks that provide the lateral swash driving force to the pushrods.

So one half (the bottom half) of a micro setup does qualify as DFC / driverless since the main pushrods are in-fact driving the swashplate, but the upper half is being driven by those forks, not the blade grips themselves so it's not a true direct flight control head.

Perhaps it would be appropriate to call it a DFC hybrid head?  DFCHH - hehe...

Who Really Benefits From DFC?

This is a hard question to address and I'm staying well away from the direct flight control / driverless debate but here's my lousy 2 cents if it's even worth that...

From a purely aesthetics point of view, direct flight control heads for the most part look beautiful. I'm a big believer in keeping things as mechanically simple as possible on complex CP RC helicopters so a lower parts count is right up my alley. This is where I really like direct flight control type rotor heads and why I'm somewhat sold on the idea. Nope, it's not perfect yet and keeping your heli (at least the head side of things) in 100% tip top condition is more critical with a rigid DFC design than any other head type.

From a performance point of view, it's likely only the advanced and hard core 3D pilot has the skill and experience to fully appreciate the improved center of mass/rotor placement factor in my opinion. I certainly know I don't have the skill to feel or appreciate it on my 600 or 800, but did I mention how good it looks - LOL.

I actually warn in my Setup & Tips ebook that beginners stay away from rigid DFC for many of the reasons I have mentioned on this page. This is somewhat helicopter dependent (as pointed out in my review of the Trex 450 Plus DFC); but in general and again specifically with rigid DFC head types, performance and component awareness does not match or suite beginner skill sets.

To end off here, there is one thing missing in all this and that is a recommendation. Nope, no DFC yea or nay here, just the facts about what it is, how and why it works, and the pros & cons. Hopefully enough information to help you decide if a direct flight control head design is right for you should you be faced with the nonsensical "DFC is better than flybarless" pitch...

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