Rotor physics and gyros

[tbourner]

Do CP R/C helis act exactly the same way as full size helis? Predominantly in the way the swash plate actuates the rotor blades, ie: the swash acts as you want the rotor disc to act but it actually pushes on the connecting rods to actuate the blade 90 degrees earlier in the rotation.

I wondered because I've seen some R/C helis where it looks like it acts at 45 degrees, and I was confused, is it a scale thing? I can understand fixed pitch heli swash plates acting directly, as it's forcing the disc to move rather than allowing the blades to apply a force, so what's the deal with CP? Someone teach me physics please.

[myxiplx]

They follow the same basic rules, but with helicopters every design reacts slightly differently, and the smaller scale of our models will have an effect too.

They're not really acting like gyroscopes though, the effect is similar but the root cause is actually the aerodynamic forces on the blades. So unlike a gyroscope where the effect is always felt 90 degrees around from the force, with helicopters that angle (called phase lag) can vary. There are some full size helicopters where the phase lag is 72 degrees, and it's affected by a ton of factors.

[tbourner]

Ooh OK will have to go look up phase lag then.

[tomatwalden]

...

I wondered because I've seen some R/C helis where it looks like it acts at 45 degrees, and I was confused, is it a scale thing? ..

I've never seen one that tilts the swash at 45deg phase lag. There are fixed-pitch helis that have 45-deg flybars if that's what you're thinking?

As myxiplx says, RC helis are subject to the same force/physics as full size helis - but it is phase lag, and not gyroscopic precession.

[tbourner]

I can't find the video now, it was something on youtube showing how collective pitch (R/C specific) works - it was maybe the connecting rods to the spinny bit that was at 45 degrees to the body, I didn't think to look at where the actual pitch change was happening to the rotors. You can see in this vid how the rods aren't at the same angle as the pitch change occurs https://www.youtube.com/watch?v=HxtXXuMhKKc. I'm just confusing myself I think.

[scallybert]

AIUI (which isn't *too* badly), the big feature of rotor discs is that they aren't rigid discs; whereas a theoretical gyroscope is; and also that the blades flex (in conjunction with other factors) to allow the rotor disc to tilt relative to the mainshaft.

If you look at how a flybar disc (ie the rotating flybar with paddles), responds to the swash; you can do a simple analysis that expects the phase angle to be 90 deg, and the disc to follow the swash.

The main blades are more complicated, but similar...

[myxiplx]

If you're interested in this stuff, there are some lively discussions from time to time on Helifreak about this in the Physics and Aerodynamics section. There are a couple of guys on there who really know what they're talking about. Probably more interesting reading a thread than a textbook :-)

I certainly learnt a lot by listening in to the threads on there over the last few years :-)

[tomatwalden]

I can't find the video now, it was something on youtube showing how collective pitch (R/C specific) works - it was maybe the connecting rods to the spinny bit that was at 45 degrees to the body, I didn't think to look at where the actual pitch change was happening to the rotors. You can see in this vid how the rods aren't at the same angle as the pitch change occurs https://www.youtube.com/watch?v=HxtXXuMhKKc. I'm just confusing myself I think.

Check out this series : https://www.youtube.com/playlist?lis...ECC2E56B68A2C3

Destin incorrectly attributes the phase lag to gyroscopic precession ... but it's a brilliant series nonetheless in my humble opinion!

[waveydavey]

Scallybert above hides his light under the proverbial bushel, he knows a thing or 2 about the physics of flight

[dillwhacker]

Seems like simple mechanics to me....
You want to tilt the disc forward.
So you start pushing it 180 degrees before, at 12 o clock, over the nose..
You apply maximum push half way, at 3 o clock.
You stop pushing when it is there, at 6 o clock, as high as it goes, cos you have to reverse the input now to get the front of the disc down.
Just like kicking a hedgehog over a series of hills really, not rocket science.
BAAAA

[scallybert]

Seems like simple mechanics to me....
You want to tilt the disc forward.
So you start pushing it 180 degrees before, at 12 o clock, over the nose..
You apply maximum push half way, at 3 o clock.
You stop pushing when it is there, at 6 o clock, as high as it goes, cos you have to reverse the input now to get the front of the disc down.

That's pretty much how I look at the behaviour of the flybar rotor-disc.

You look at the pitch of the paddles relative to the rotor disc [this is quite an important concept] to determine whether the lift is +ve, -ve or zero at any point; do some hand-waving calculus [the vertical speed will be max when the vertical acceleration is zero]; then deduce that the front of the disk will be going down fastest, and the back going up fastest; ie rotating about a 3 o'clock to 9 o'clock axis; ie following the swash.

The disk stops this rotation, when the disk orientation is such that the flybar paddles lie flat to the flybar disc [where the concept above comes in - it also applies to flapping, flap-back, etc on the main rotors].

So, in this case, you'd expect a 90 degree phase difference.

The big differences between the flybar and the main rotor is that the former has a fixed zero collective pitch, is free to teeter and the flybar is relatively stiff ; the latter has collective pitch, various or no articulation, and the blades flex.

It gets very complicated...

[waveydavey]

You're not kidding me, thank God for easy to setup FBL units