by John Salt - last updated January 2024
The Easy Explanation...
Due to gyroscopic precession, you'll likely notice something very strange when you move your cyclic RC helicopter controls.
The pitch change on the rotor blade doesn’t occur where it should. It seems to happen 90 degrees out of phase before it should…
“I must have built something
wrong or programmed the wrong
mode into my radio” you say to yourself.
Nope! All is good.
What you are witnessing are the swashplate linkages up to the blade grips compensating for the forces of Gyroscopic-Precession. They will be imparting the force application up to the rotor blades 90 degrees before the actual movement takes effect in the rotor disc.
Why? Well, funny, non-intuitive things happen with large, fast spinning masses; and as you can appreciate, our RC helicopter rotor blades become a large spinning mass (the rotor disc) once they are spooled up.
Here we go again...
Say we are looking down at our helicopter from above. The nose is pointed forward (12 o-clock position) and the tail is pointed back at our feet (6 o-clock position). In this example we will assume the rotor rotation is clockwise (the most common direction for most RC helicopters).
Now we give a forward cyclic command. The natural assumption is the rotor blade will have more positive pitch at 6 o-clock than at 12 o-clock so the rotor lifts at the back more than the front and pitches the helicopter forward.
However because of gyroscopic forces acting on our spinning rotor disc, the lift forces of the rotor actually occur 90 degrees later, or "in front" of where the input force to was applied to our rotor disc.
This phenomenon is called Gyroscopic Precession or Torque Induced Precession.
In the above example with a clockwise rotating rotor and where the rear blade at 6 o-clock has more lift than the front blade at 12 o-clock - our helicopter would actually roll to the right, not pitch forward. Why?
Remember, the gyroscopic-precession will cause the force applied to the rotor to occur approximately 90 degrees later in the revolution. So the rotor force actually lifts more a 9 o-clock than at 3 o-clock.
The rotor disc wants to tilt to the right and our helicopter follows along and rolls right. In essence, we have just given a right cyclic helicopter control command.
This is pretty hard to visualize, but this video will clear it all up!
Most RC helicopter or setup instructions don’t include this very useful tidbit of information and believe me, it has had more than a few of us scratching our heads when we first start into the hobby.
I told you this was fun... or maybe just funny :-)
The harder & CORRECT explanation...
Everything I just told you about gyroscopic precession in an RC helicopter was a lie!
Well, not exactly... Everything about the 90 degree phase delay you will see/experience on your RC helicopter holds true, but is gyroscopic precession the primary reason for it?
Turns out it's not!
Just as I explained in my RC Helicopter Lift article, regarding the two theories of lift; there are also two for explaining this 90 degree phase shift. The incorrect and commonly accepted way (precession), or the less common but fundamentally accurate way (phase lag or phase delay).
As an RC helicopter instructor, like most instructors, I try to make this RC heli learning curve as easy & quick as possible for beginners. When it comes to explaining the reason that 90 degree phase delay or lag occurs, gyroscopic precession is simply the easiest for most beginners & newbies to grasp in a short amount of time. It's also by far the most popular reason you will read about in aviation articles, even in the full size world.
PRIMARILY however, the precession theory is what most if not all RC helicopter examinations if your club or instructor has a formal exam, will teach and want for a correct answer. Again, I'm coming at this from an instructor's point of view, not a rotor system dynamists'.
Yes, I actually want you to pass your RC helicopter exam which will almost always want the gyroscopic precession answer.
However for those out there that want the the correct phase lag explanation - I'll try to sum it up in a very generalized and broad scope brush.
It's much too complicated to cover in detail and I'm most certainly not qualified to cover it.
Yes, rotor systems, even on our little RC helis are extremely complex, complicated & dynamic.
The misconception is that a helicopter rotor behaves like a gyroscope. Not really.
Rotor systems actually behave more like a spinning pendulum. The blades oscillate up and down through each rotation with cyclic inputs, and like a pendulum, the blades are in dynamic flap resonance that is out of phase from where the cyclic force was applied.
The video below shows this pretty well. Best to turn the playback speed down to 0.25 in the settings icon so you can see how the blade doesn't reach peak amplitude / peak flap (using the horizon as a gauge) until about a quarter rotation after achieving maximum pitch angle. This is blade flap resonance phase lag.
Notice the cyclic input where the blade pitch angle reaches maximum is on the left side of the heli, but the blade flap doesn't reach maximum amplitude (force) until it's pointing toward the tail rotor. This helicopter is obviously flying forward (maximum cyclic lift on the back half of the rotor disc, minimum on the front half).
Phase lag or flap lag, is simply the distance (usually measured in degrees) of arc travel, it takes the rotor blade to respond and reach its peak amplitude (and thus peak resultant cyclic force) from the time it's first given the command to do so.
Another way to think of this is it takes time for the rotor blade to respond to the initial change in cyclic pitch to the time it fully reacts; it's not an instantons reaction. This is what makes the most intuitive sense to me.
Coincidently, on our RC helicopters and some full size ones, that duration of time to react (lag) occurs about 90 degrees later in rotation from where the input command was given. This is why the gyroscopic precession theory is so convenient to explain it. However, if was truly gyroscopic precession, it would be exactly 90 degrees on every helicopter out there, that is not the case.
There are several factors that affect exactly how much phase lag occurs due to blade pitch response speed and flap resonance.
The further out from the center of rotation on the mast the flapping occurs (called flap hinge offset), the smaller that phase lag angle will get. Some full size helicopters down into the 70 degree range for example but most full size helicopter have a phase lag of about 85 degrees.
Helicopters with teetering rotor heads such as the Bell 206 essentially have zero flap hinge offset (teeter pivot point at rotor head center) so they on the other hand are very close to 90 degrees of phase lag.
Since most of our RC helicopters use a dampened feathering shaft (head axle) that also teeters in the center of the rotor head, our flap pivot, just like the 206, is more or less centered (zero flap offset) and thus the phase lag is almost 90 degrees.
There are other minor influencers to the phase lag angle as well such as head dampening stiffness, blade stiffness, blade lead & lag and dissymmetry of lift but they are minimal. Again, this is the ten-thousand foot, generalized explanation.
Call it what you want, Precession or Phase Lag; all you need to know when building or setting up an RC heli, is that on our RC helicopters, it occurs pretty darn close to 90 degrees later in rotation. And if you are taking an RC heli exam, most clubs/instructors will want precession theory for the answer. I'll accept both, but encourage the correct theory of phase lag :-)
If you ever get good enough to feel or experience a slightly different phase lag angle from 90 degrees on your RC heli (ie. heli crabs or corkscrews slightly); many higher end FBL systems have a phase adjustment setting. I've personally never found a need to mess with this adjustment however as the systems themselves are really good at correction for phase lag variation these days.