About coax helicopter setup

[_Icarus]

I have a question about coax helicopter design that came up when I started examining how my Esky Lama 4 was put togheter.

I noticed that the flybar is set at a 45 degree angle to the top rotor.
Does any one know why?
Why is it not set at 90 degrees like the single rotor helis?

You can clearly see it on all the coax pictures on the web (lama, BCX, etc...).
So I guess the same thing applies to all coax helis.


Also, the swashplate on the Lama 4 is rotated 45 degrees.
I.E. it tilts on 135 and 215 degree axes, for pitch and roll control, rather than 0 and 90 degrees.
Although that is not so easy to see from pictures, I would guess that it is true for all the coax models as well.

I am really curious to understand why that is.
I am hoping that someone with model design knowledge would be kind enough to explain it.

Thanks

[Brandano]

It is not only a feature of coax helicopters, the Piccoz (Picooz? I never really worked it out, I only own clones) toy helicopter has that same flybar setup, and is stable enough to do away with cyclic input at all. My interpretation of this is that the rules of gyroscopic precession are usually badly worded. What you read from books is something along these lines: A force applied to a spinning body in order to tilt it on an axis perpendicular to that of the spin will manifest itself 90 degrees further on in the direction of the spin (forgive me, I am Italian, I am sure that can be worded better). I think that instead this should be worded as: Part of the force applied to a spinning body in order to tilt it on an axis perpendicular to that of the spin will manifest itself 90 degrees further on in the direction of the spin. therefore you may find empirically an angle where the force on the flybar is better to counteract the gusts, and this angle between 30 and 50 degrees seems to work best, at the expense of control authority. Not an issue where there is no cyclic input, like in the coax or 2 channel helicopters.

[MarkusN]

The law of precession is definitely worded correctly as you usually see it. I guess it's a matter of geometry of the control linkage that makes 45° necessary on coax helis. Googling for "coax, swashplate" first got me a discussion thread right here about converting to a 90° swashplate, so it can be done.

Anyway, the topic seems to have been discussed at length here (http://www.rcgroups.com/forums/showthread.php?t=483542).

[_Icarus]

I tried to google different key words to find this and never succeeded.
I tried "coax, swashplate" like you suggest and there it is.
http://www.rcgroups.com/forums/showthread.php?t=449319

That is exactly what I was looking for.
Now I have to read this and try to wrap my head around it.
Thanks for the pointer.

[_Icarus]

Thanks for your explanation. You don't have to apologize for your wording it's quite correct.

I just have one comment about the following:
It is not only a feature of coax helicopters, the Piccoz (Picooz? I never really worked it out, I only own clones) toy helicopter has that same flybar setup

Actually, I own a Picooz, its flybar is almost aligned with the rotor.
It leads only by a few degrees (clearly less than 5).
So the Picooz and all its clones are probably a different case altogether.

[Brandano]

Marcus, I still think that the "nice and convenient" 90 degrees are an oversimplification of the effect. Gyroscopic precession is again inertia wearing a fake moustache. I believe that all the forces should reproportioned around the path of the spinning disk, but dealing with just the 90 degrees component simplifies things. Why shouldn't the force 90 degrees along manifest itself another 90 degrees along and so on?