A question on flybars if I may?

[waveydavey]

There was a superb article in MHW which explains very well the flybar and then the mystery around FBL systems Jim Davie I think. I've read it a half a dozen times as it's so good. Theres history of the flybar both bell, hiller and the hybrid in there and then an explanation of the electronic side of it, superb.

[pchristy]

One of the reasons the early Kavan JetRanger had control issues was due to the absence of damping on the flybar. Its very difficult to make hydraulic dampers (as used on the full-size) at our kind of scale, but a similar effect can be achieved by using a paddle shaped weight instead of just a round lump of iron! This provides aerodynamic damping.

Unfortunately it also makes the flybar susceptible to being disturbed by gusts - something a pure Bell flybar avoids. And once you have the paddles, you may as well control them - it only takes an extra pushrod.........

Helicopter design is all about compromise.......

[scallybert]

Yeah, but the Bell flybar dampers aren't there for damping oscillation.

They're to make the flybar follow the heli rolls, etc.

[I know you don't make the mistake, but it could be read as such.]

[tomatwalden]

There was a superb article in MHW which explains very well the flybar and then the mystery around FBL systems Jim Davie I think. I've read it a half a dozen times as it's so good. Theres history of the flybar both bell, hiller and the hybrid in there and then an explanation of the electronic side of it, superb.

Don't suppose you know which edition of MHW?

[waveydavey]

Don't suppose you know which edition of MHW?

I do Tom, Ill post it later. I did ask permission to post it which I got. I'll see if I can get a copy

[gazmk2]

Is MHW still going? I haven't seen it for ages.

[tomatwalden]

I buy the ipad edition of MHW and rotorworld


Sent from my iPad using Tapatalk

[waveydavey]

It was The June edition this year tom, I've been a subscriber for years. Great little room reading material

[pchristy]

Yeah, but the Bell flybar dampers aren't there for damping oscillation.

They're to make the flybar follow the heli rolls, etc.

[I know you don't make the mistake, but it could be read as such.]

Yes, thanks - I hadn't read it that way, but I see what you mean!

But they do need oscillation damping as well - a Bell flybar wobbles quite badly under some conditions without it!

You can get away without damping if all you are doing is hovering. That's why contra-rotators get away with it. And indeed, it didn't show up early in the life of the Kavan JetRanger, as everyone was struggling to teach themselves basic hovering! Those who could afford such an expensive machine weren't going to risk smashing it trying to fly it around until they were reasonably confident! However, once people started to fly them slightly more aggressively (fast, and high speed turns), the deficiencies of an un-damped Bell flybar soon became apparent. A lot of crashes were blamed on unexplained radio failure, until it became apparent it was actually a fundamental design flaw. The JetRanger had other issues as well, most noticeably inadequate engine cooling, but it was by far and away the prettiest of the "original three" helicopter kits, which helped maintain its popularity despite these early issues.

Eventually most of the bugs got sorted, and today, a pristine JetRanger is highly sought after and can command good money!

A lot of people took the flybar off, and flew them as flybar-less machines (with suitably weighted blades) as per the full size. Yes, back in the 70's, with no electronic stabilisation and only 60 size engines for power! There truly is nothing new under the sun!

[scallybert]

[...]
But they do need oscillation damping as well - a Bell flybar wobbles quite badly under some conditions without it!
[...]

Ok, but, there's the paradoxical question of whether those viscous dampers actually provide a damping effect.

This sounds like nonsense, as they obviously provide a torque opposing the flybar's seesawing angular velocity (that's what dampers do.)

Except the flybar behaves like a gyroscope; so that torque makes the flybar disc precess about an axis perpendicular to the torque and the axis of rotation. (ie 90 degrees out of phase).

To illustrate this, the dampers' torque is max when the seesawing angular velocity is max; which isn't at the points when the flybar is most inclined; but when it's perpendicular to the main shaft. At the points when it's most inclined, the torque is zero.

So the dampers' torque makes the flybar disc precess to minimize the inclination between flybar disc axis and the mainshaft. [Cool - what we want.]

So, in PID terms, the dampers' torque is acting like the proportional 'P' or spring; rather than like derivative 'D' or damping.

So, if the dampers are actually damping the flybar, it's due to a more subtle effect than my hand-waving above is describing.

What you can say is that since the dampers will be taking energy out of the flybar system, it's likely they're calming things down - but you can never be sure with active systems [it's generally the damping gain that makes tails get exciting.]

Dunno if I mentioned, but The Art of the Helicopter is quite good on Bell flybars and Hiller flybars. [But not Bell-Hiller, as the book is fullscale.] It's also more intelligible than my psychobabble...