Rotorhead Dynamics 101

Chapter One: what is the rotor head

The rotor head in most modern helicopters is a complex mechanism that turns swashplate movements into pitch changes in the main rotor blades and flybar paddles. Collective pitch change - changing the pitch of both blades together to make the helicopter climb and fall - is relatively straightforward. Cyclic pitch - changing the pitch of both blades is different directions at a specific rotor head orientation to make the helicopter pitch and roll - is much more complex.

Notice the path of the linkages in the Futura SE rotor head pictured above. Start from the blade grip and working downward. The blade grip ends with a "mixing lever" from which two links extend downward. The link on the left side of the mixing lever (attached to the back of the mixing lever and thus partially obscured by it) extends down to the swashplate; this link is commonly referred to as the "direct link" since it provides the pilot with direct control over the blade pitch. On flybarless helicopters, this is the only link between the swashplate and the rotor blade grip. The link on the right side of the mixing lever attaches to the flybar. We'll refer to this as the "flybar link" for the purpose of this discussion. The flybar's pitch is also controlled by another set of links (sometimes called Hiller links), which extend downward from L-shaped arms.

The Hiller links on the Futura SE are connected to the swashplate via a mechanism known as a "washout" or a "collective pitch compensator," depending who you ask. I'm fond of the latter myself, since I'm not sure what "washout" means and I think "collective pitch compensator" describes the mechanism perfectly. Its function is to translate swashplate tilt into flybar pitch, even as the swashplate slides up and down on the main shaft. In other words, it compensates for the swashplate's collective pitch movements.

Both the direct link and the flybar link influence the pitch of the main rotor blade. The mixing lever "mixes" the influence of each link and determines the resultant rotor blade pitch. The Futura SE rotor head above has a 1:1 mixing ratio and is not adjustable. Some helicopters, including the XCell series, Bergen Intrepid, and Concept 30s with Zeal upgrades, have adjustable mixing levers. This allows you to set the relative influence of the flybar links (flybar movements) and the direct links (swashplate movements).

Note that the design described above is not the only way to do things - while the Futura SE (and the XCell and Raptor and many other helicopters) use a sliding swashplate to control collective pitch, the Kyosho Concept (and some Kalt helicopters) uses a fixed swashplate that can only tilt. On these helicopters, the collective pitch is controlled by linkages that run parallel to, and spin with, the main shaft. Though this changes the configuration of everything else between the swashplate and the blade grips as well, the resulting functionality is basically the same. Perhaps I'll take some pictures of my Concept's rotor head and go into more detail here later.

Chapter Two: why is the flybar

1. turning servo motion into main blade cyclic pitch changes
2. stabilizing the rotor disk against pitching and yawing motions

The flybar is free to move in two directions.

First, it can twist along its length, changing the pitch of the paddles mounted at the ends of the flybar. Flybar pitch is controlled by the cyclic servos, via the swashplate. When the one paddle's pitch is increased, the pitch of the opposite paddle increases. The swashplate "cycles" the flybar pitch - the pitch varies as the rotor head turns. For example, if you hold forward cyclic, the flybar paddle pitch will be decreased on the left side of the helicopter, increased on the right side of the helicopter, and neutral when the flybar is aligned front-to-back (if you're looking at a counter-clockwise rotation helicopter, exchange the words "increase" and "decrease"). As a result, this cyclic flybar pitch causes the flybar to deviate from its plane of rotation, just as main rotor cyclic pitch causes the main rotor to change its orientation.

Second the flybar can tilt, moving the flybar paddles up and down. This tilting action is controlled by the gyroscopic effect of the flybar paddles, and by the aerodynamic effect of the flybar paddles, when they have pitch applied as described above. When the helicopter is at rest, both of these forces go away, and the flybar is free to flop around.

The aerodynamic behaviors of the flybar and the main rotor blade grips, and the geometry of the linkages between the flybar and the main rotor blade grips, are designed to keep the main rotor and the flybar rotating in the same plane. If the flybar's plane of rotation changes, the flybar link and mixing lever cause corresponding changes in the pitch of the main blades as the rotor head turn. These pitch changes are proportional to the difference between the planes of rotation of the flybar and the main rotor grips, so they act to bring the main rotor blades into the same plane of rotation as the flybar, and vice versa.

In practice, this behavior has at least three obvious results:

1. When the pilot applies a tilt to the swashplate, the flybar's cyclic pitch causes it to lean in the direction commandad by the pilot. The main rotor's cyclic pitch causes the main rotor to lean in the same direction. Watching my Futura SE's rotor as I give it quick stabs to cyclic, it's quite clear that the flybar's orientation "leads" the main rotor's orientation. The flybar's orientation changes first, and the main rotor quickly catches up.

2. When external forces conspire to change the helicopter's orientation against the pilot's wishes, the flybar acts to damp these forces.

   This can be demonstrated (dangerously) by holding onto an LMH while the rotors are spinning at flying speeds, and rolling the helicopter by hand with the cyclic centered. You should never actually attempt this of course, as it's very dangerous and you will put your eye out.

   This can also be demonstrated (less dangerously) by comparing the fast forward flight characteristics of your helicopter with heavy and lighweight paddles fitted.

   When the helicopter is in fast forward flight, the advancing blade generates more lift than the retreating blade. Due to gyroscopic precession, this manifests itself 90 degrees later as a tendency to "pitch up." On most helicopters (the Lite Machines LMH series being the sole exception that I'm aware of), the flybar does not generate lift and is not so affected by the advancing/retreating differential.

   Helicopters set up for novices or scale flight typically use heavy, gyroscopically stable flybars and/or mixing levers that favor the flybar link, and the "pitch up" tendency is negligible. On helicopters set up for aerobatic flight, lighter flybars and mixing levers that favor the direct link make the flybar less effective at countering the pitch-up tendency.

   It has been suggested that the advancing/retreating lift differential that causes the main rotor blades to pitch up may in fact have the opposite effect on the flybar. The advancing flybar paddle is pushed downward by the oncoming air, and this further counters the tendency to pitch up. I can only guess as to how effective this is in practice. Logically, one should be able to adjust flybar paddle size to create a helicopter that pitches down in forward flight, but I have yet to hear of anyone doing this.

Chapter Three: how do the variables

Much of the rotor system's complexity centers around the flybar.

The holy grail, I suppose, would be a system that gives maximum cyclic reponse (or at east, as high a roll rate as the pilot can handle) yet maximum stability as well, (one that stops rolling or pitching as soon as the swashplate is leveled).

The most obvious benefit of a highly stable system is that the helicopter should hover easily, with no tendency to drift or to change attitude without pilot input. A less obvious benefit (less, obvious, until it goes away!) is that the helicopter exhibits less pitching up during high-speed flight. When I tweaked my setup for more roll rate and less stability, I was somewhat suprised at the tendency for the helicopter to want to slow down and climb after I built up significant speed.

What follows is a work in progress that reflects my understanding of what's going on. I welcome your corrections, additions, and suggestions.

flybar paddle mass

This is probably the most commonly adjusted variable. Generally speaking, lighter paddles give strong cyclic response faster roll rates, and less stable hovering.

flybar paddle area

A larger paddle area will make the flybar more responsive for a given flybar pitch change, and thus increase the cyclic response.

flybar paddle airfoil

This is one variable that I don't understand very well, as I haven't experimented with it.

flybar length

A longer flybar should make the flybar deflect more with a given flybar pitch change, due to increased airspeed of paddles. Increased flybar length leads to increased flybar moment, like adding heavier paddles, but the end result is an increase in flybar effectiveness. I have experimented with an extremely long flybar setup (710mm JR flybar and Schluter Futura SE paddles) and found that flybar weights were necessary to tame the cyclic response.

flybar link placement

This is the link that connects the flybar seesaw to the mixing levers, and the variable that inspired this web page.

Many of us tend to assume that the flybar enhances the pilots authority over the rotor disk, but this is not necessarily the case. In fact the flybar reduces the pilot's authority. Reducing the flybar's authority on the main blades will increase the roll rate and make the helicopter more sensitive in a hover.

Hiller lever length

A shorter Hiller lever should yeild increased flybar pitch changes for a given swashplate angle. This should mean more cyclic response, with no change in stability. However, one must wonder when the flybar paddles begin to stall, causing more drag than useful leverage.

rotor speed

Higher head speeds give more cyclic reponse due to increase air flow over the flybar paddles and main blades as well as more stabilization, due to increased gyroscopic effect of both the flybar and main rotor. This is desirable for aerobatic flight. However, an excessively high head speed can contribute to rotor head failure, so don't get too carried away.

main blade moment

I would call this 'main blade mass,' but the distribution of the mass along the blade is critical. Greater moment (more mass, or center of mass closer to blade tip) yields increased stability (and better autorotations, but that's beside the point).