This may be a stupid question, but I'm trying to understand the physics behind my Blade CX2's flight. What is the purpose of the flybar? What would happen if the flybar was removed and the top blades were fixed from rocking? Why do RC helicopters have flybars, but full size ones don't?

There are many more experienced pilots than myself around here who would be better qualified to answer this question, but I'll do my best.

In the case of the CX/CX2, the flybar acts as a means to stabilize the heli. The weights on the flybar create a gyroscopic effect that serves to keep the flybar in place while the heli moves around. The link from the flybar to the upper blades causes them to pitch in the opposite direction from which the heli is tilting, resulting in self-leveling and a very stable hover. The trade off, of course, is mushy flight characteristics.

So, the more you reduce the gyroscopic effect of the flybar, the more responsive and less stable the heli gets. At a certain point, the heli becomes impossible to fly... by a human at least. Your example of removing the flybar and locking the upper blades would likely result in some very short "flights".

Shortening the flybar and/or removing the weights is a good overall compromise between stability and responsiveness. It makes for a heli that is not well suited to beginners, but good for someone looking to try something other than hovering.

That's a good explanation. Do full-sized helis not require a flybar because their rotor blades have enough mass by themselves to stabilize the heli? Also, the CX flybar is not parallel to or perpendicular to the upper blades, it is at an angle. What effect would changing the angle of the flybar mount have on the flight characteristics?

Oops! Sorry, I just realized that I forgot to answer this one the first time. Well, I read the answer to this question somewhere, but now I don't remember what it is. I believe the main purpose of the flybar is to stabilize the relatively small size of model helicopter rotors. Many scale RC helicopters that do not use flybars - they use "scale heads" that better match the appearance of a full size heli. I have played around with a couple of scale head helis in RealFlight G3.5, and they are basically just a little less stable. I still do not know why full size helis do not use flybars... Who knows, maybe some do, but I've never seen one. Anyone?



Another good question. The short answer: I dunno. I'd be interested to find out myself. I'm sure it is related in some way to the fact that the swash plate on the CX/CX2 is actuated 45 degrees from the actual direction you wish to travel. Ever notice that? When you press forward on the cyclic, the swash tilts diagonally forward and to the left. I have been told this characteristic is intrinsic to a coaxial rotor setup. What I'm not sure about is whether the 45 degree flybar causes this effect or is a result of it.

From what I understand the 45 degree inclination of the flybar is to counteract the fact that the swashplate tilts forward/left in 45 degrees when the cyclic input is "straight forward".
Apparently, the 45 offset of the flybar will counteract this and produce the resulting force that drives the chopper approximately straight forward.

One advantage with this setup is it's simplicity. Only two servos required. Cheaper to produce and consume.


Biker-74

it would not fly stable and would be very hard to fly.

Hey. I'm a helicopter mechanic, and may be able to shed some light on this. You're confusing the balance bar (CX, CX2) with a flybar (CP, CPP). They perfrom 2 different tasks.

Helicopters have a natural tendency to rotate opposite the direction the main rotor is turning. Most helicopters use a tail rotor to counteract this torque. In the case of a coaxial heli like the CX & CX2, it uses an additional main rotor turning in the opposite direction to cancel the torque. Even though you have two main rotors, you still need one to perform the actions of a tail rotor and one to perform the actions of a main rotor, or the aircraft would be completely uncontrollable. That is why only the lower blades are linked to the swashplate. They change pitch to move the aircraft in flight. The balance bar keeps the upper blades level relative to the orientation of the aircraft. This provides a constant source of lift and stability regardless of what inputs may be affecting the lower blades. The CX/CX2 then uses variable motor speeds to facilitate what we call pedal turns (we call them that because the foot pedal controls in a helicopter control the tail rotor and cause the aircraft to pivot around the main rotor shaft).

The purpose of a flybar is a little of the same concept, and a little different. First there's some facts you have to take into consideration. I'll use the UH-60 Blackhawk helicopter as an example. The blackhawk in flight configuration weighs about 14,000 lbs. The main rotor turns at about 250 RPM and has 4 blades. In comparison, The Blade CP weighs about 2 lbs with the main rotor turning about 2000-3000 RPM with 2 blades. The sheer inertia alone in trying to move a 14000 lb aircraft creates some stability in addition to force dampers and other systems. Hence the flybar on the CP. The flybar is heavier and will maintain a more stable rotational plane during in flight forces than the lightweight blades would. That is why the swashplate is linked to the Flybar , and the flybar linked to the blades. The flybar basically forces the blades to align themselves in the same plane as the flybar. A direct linkage at this weight at that rotational speed would make the aircraft too responsive to control. The Bell-Hiller head for the CP/CP Pro has a direct swashplate to blade linkage in addition to the above configuration, and even though it still has the flybar, you can really see the difference in responsiveness. A buddy of mine flew like a champ with the CP. When he upgraded to the CPP, he crashed and almost destroyed it 1st time out. It's extremely quick.

Anyway, hope I helped a bit.

Thanks for the answers. I like understanding how things work.