I was hoping if someone could tell me exactly why it is that some single-rotor helicopters (such as the Blade mcx) are "self-stabilising" - in other words, they automatically revert to a stable hover when you release the controls (like all coaxials seem to do), and why it is that other single-rotor helicopters don't.

I'd be happy just with a good answer to the above , but I was wondering about some other related things :

1. Is this "self-stabilising" ability more common to fixed-pitch or collective-pitch helis in general? If yes, which of the two, and why?

2. Are all flybarless helis self-stabilising? If no, can they be made to be?

3. Are multicopters (tricopters, quadcopters, hexa-, octa- etc.) self-stabilising without any special controller boards (i.e. the really basic multicopters with just four or five standalone heli gyros and some mixing)*?

4. There seems to be a lot of mention in the thread that the flybar phase angle (45 or 90 degrees) affects the handling of the helicopter. Could someone please explain whether this is just a rule-of-thumb, or whether flybar angle does actually make the helicopter behave differently (and if so, how?)

*I know this question probably belongs in the multicopter section, but since it's related and such a short question I figured it wouldn't hurt to ask it here

Do a search on "gyroscopic precession" and "phase-lag" as it relates to helicopters, then come back. The explanation will make a lot more sense once you understand those two topics.

Originally Posted by norcalheli

Do a search on "gyroscopic precession" and "phase-lag" as it relates to helicopters, then come back. The explanation will make a lot more sense once you understand those two topics.

I already did before posting the question . It seems the world is undecided on whether gyroscopic precession or aerodynamic precession is the true cause of the lag. And I also found some great articles about how flybars work (here), but they don't answer my questions .

To be specific, what I want to know is how you can make a single rotor helicopter hover hands-off, and why most of them don't seem to have been given the ability to do it.

Balr14 has a great explanation of the physics involved in the self-stabilizing helis, as well as the difference between a 45* offset flybar (Nine Eagles, etc.) and a 45* offset head(Blade mSR, 120 SR). You might try searching for that. To specifically answer your first question, the self stabilizing helis are engineered that way to make them easier to learn than a 90* flybar heli. The compromise in that is that they aren't very responsive. It's harder to get them into forward flight, and to keep them going. The offset head is better for this than the offset flybar. With a regular 90* flybar heli there is an exponential increase in sensitivity and response. A regular heli will go where you point it, and keep going there until you tell it to stop. The downside of all that responsiveness is a total lack of stability. You have to learn how to fly one of these beasts, and you have to fly it every second it is in the air. Will they hover hands off? Yeah, for no more than five seconds or so. But they will do things that you would never expect a helicopter to do. 3D would be impossible if the heli were built to self stabilize.

Hope this answers some of your questions. Maybe some of the others will check in and provide some more answers.

1. The term self-stabilizing is often misused to include self-leveling. In any case, FP helis are much more likely to exhibit both of theses characteristics. But, it's strictly because 95% of CP helis currently made are intended to fly 3D, where these traits would be undesirable. There's no reason why a CP couldn't be self-stabilizing and self-leveling. To a large degree, they already possess superior stability due to their higher and more constant headspeed. Some use electronic stabilizations systems that can be dialed down or turned off, for the best of both worlds.

2. Flybarless helis are not self-stabilizing, by themselves. But, they are with a 3 axis gyro or flybarless control system.

3. Quads and other multi head designs use 3 axis gyros and mixing systems. They also use accelerometers. These are what gives them self-leveling properites.

4. Gyroscopic precession dictates that it takes 90° of rotation, from the point where a signal is given, to be acted upon. Which is the reason why flybars are typically 90° from the point where the swashplate linkage connects. 45° offset heads create a leading flybar, 45° offset flybars create a trailing flybar. Both create an out of phase condition that can used to mix with movement hat occurs at the regular 90° interval to affect and damp head movement. Mixing in-phase and out-of-phase conditions is a recurring effect, that has echos. So, the damping effect is multiplied, giving your self-stabilizing effect.

An offset head with an out-of-phase leading flybar will not impede movement; if anything it will enhance. But, may have some rebounding when you attempt to stop. An offset trailing flybar is purely for damping movement.

However, neither of these will make a heli return to level. That requires accelerometers. Self-stabilizing FP helis tend to self level because they have a long vertical CoG (due to the extra linkage and connection points required for self-stabilizing) and a low battery mount right under the main shaft.

I have never got any single rotor heli to hover hands off, nor have I ever attempted to do so. I don't know what point that would serve.

Thanks for that Balr14. And norcalheli too. So if I understood correctly, to make a helicopter self-levelling, all it takes is for the flybar to be displaced 45* from its usual 90*-to-the-blades position, in a direction opposite the direction the rotor turns?

Originally Posted by Balr14

I have never got any single rotor heli to hover hands off, nor have I ever attempted to do so. I don't know what point that would serve.

I used to think that was the case too, but I saw a video (attached) of the mSR hovering hands off, and that's what prompted me to ask the question.

It got me thinking, that a largish single rotor collective pitch self-levelling helicopter would be the best of all possible combinations - the beginners won't have a hard time because it practically flies itself, intermediate users will be able to fly outdoors even when it's very windy, the most basic of 3D manoevers (like flying inverted) will still be possible, and it could replace multrotors in aerial photography - all with the same machine. Plus with everything powered by one motor upgrades and replacements will be cheaper.

Blade mSR Hover Stability (0 min 48 sec)

No, a 45° flybar will not make a heli self-leveling, that will make it self-stabilize. When it stabilizes, it may be at a level position, but doesn't have to be. The heli has no idea where level is, that is entirely up to gravity. Try fastening a 20' rope to a bucket and swinging it around your head at a 30° angle. Now, try to change it to swing level. It doesn't want to change. That's stabilization. So, self-leveling helis simply utilize gravity (long vertical CoG and low battery position) to help them level. Copilot II or Flymentor systems utilize accelerometers or photocells, in conjunction with 3 axis gyros, to stabilize and level. Flybarless helis use similar Align or BeastX stabilization systems.

Conventional helis still can not reach the same level of stability a good large quad can achieve. A quad has a much wider base.

My MSR never came close to hovering like the one in the video.

I don't really understand how it works, but the self-leveling helis seem to work by limiting their airspeed - once the heli is moving, the head tries to slow it down.

If it's tipped away from level, it'll pick up speed in that direction and the head will react to that speed by tipping the heli the other way, effectively leveling it.

The problem with this is that the head can't tell the difference between a moving heli and a gust of wind, which is why mechanically stabilised helis aren't much use in the wind.

OTOH advanced co-pilot systems have a horizon sensor, and don't react to airspeed.