On the bigger heli's even though the phase is less than 90, it's still close to 90, perhaps 80-85. To have 0 phase lag there would have to be 0 inertia, that is, no mass or aerodynamic resistance to flapping. With a flybar there is no damping so the phase lag of the flybar is 90. Most systems have enough flexibility that the lag is close enough to 90.
If you look at the very small helicopters where the blade rigidity is very high you see very small angles and odd swashplate movement. Notice the non 90 degree alignment between the fly/stabilizer bars and the blades on the small helicopters, and the non aligned swashplate movements. By the time you get to 450/500 sized birds the phase angle comes back closer to 90.

So even if we have absolutely rigid rotors in theory , there would never be a 0 phase lag?

Only if they have 0 mass, and 0 aerodynamic forces.
You can't move something immediately without infinite force or 0 mass. The whole phase lag result is because when you push (aerodynamically speaking) on the blade it takes for it to accelerate, move, then come to the maximum amount that it's going to move (flap up or down). For a free flapping rotor with no hinge offset, this turns out to take the same amount of time that it takes for the rotor to turn 90 degrees, hence the 90 phase lag. If you make the rotor stiffer, it gets to the maximum displacement sooner, because there are forces trying to stop if from going so far. Since it reaches max displacement sooner, this is less than 90 degrees of rotation time of the rotor over all, hence the lower phase angle. Its not about gyroscopic precession, it's about resonance.

Originally Posted by mnowell129

The whole phase lag result is because when you push (aerodynamically speaking) on the blade it takes for it to accelerate, move, then come to the maximum amount that it's going to move (flap up or down).

Correct. I agree. The blade takes time from blade pitch change the moment it has flapped fully. but that only aplies if the blades do flap.

Originally Posted by mnowell129

For a free flapping rotor with no hinge offset, this turns out to take the same amount of time that it takes for the rotor to turn 90 degrees, hence the 90 phase lag.

Correct and I agree with you. 90 degrees from the moment you feather till the moment the blade has flapped maximum.

Originally Posted by mnowell129

If you make the rotor stiffer, it gets to the maximum displacement sooner, because there are forces trying to stop if from going so far. Since it reaches max displacement sooner, this is less than 90 degrees of rotation time of the rotor over all, hence the lower phase angle.

Correct, So because the rotor is stiffer "it is going to reach the maximum displacement sooner" i.e. less than 90 degrees hence as you said the lower phase angle. So far both of us understand this the same it way. Now if we make this rotor even stiffer, yeahm, extremely stiff, shouldnt the phase lag decrease considerably or even become 0 degrees, isnt this the logic? Some real helicopters with articulated heads have 57 degrees of phase lag!, like the S-76. The blades still flap though. But what if the blades do not flap anywhere, shouldnt then the maximum displacement be at the same time and place of the feathering itself and hence leading to 0 degrees phase lag?

I truly apreciate and respect your time spending to discus this matter.

Kalle

0 seems unlikely. To be at 0 degrees lag, i'd think you would need the reaction to be instantaneous, i.e. completely rigid so that the bending moment applied to the shaft is immediate. To have 0 bending seems unlikely, but perhaps with a very very rigid system the phase lag gets very low. Say perhaps in a propeller. But even a "rigid" propeller is not infinitely rigid and displaces some. So even a small amount of displacement is going to cause some lag.
This is creeping up on the P-factor discussion with props..... I guess the discussion comes down to how rigid is rigid.

Hmm. it looks like its always gonna be 90 degrees no matter how stiff or how soft the rotorblades are. Helicopter aerodynamic student toled that it will be 90 degrees no matter what kind of rotor head it is due to laws of physics. I must say it starts to make sense now. A physical evidense is my Rc helicopter that is extremely stiff and yet has exactly 90 degrees between maximum feathering and direction of travle. I can buy softer O-rings and softer rotorblades but the heli will fly the same without changing any mixing or controls. So that only can mean that the mechanical gryo precision are the deciding factor. I am not sure but I believe right now that it is 90 degrees for anything and any kind of rotor. It seems different experienced people say divided things about this

I disagree. There is direct evidence to the contrary. Most current models have a solid axle have o-rings. Effectively this is a teetering rotor with some stiffness. Rotor can still tilt. Also the blades are still relatively flexible.

very good lecture. never have enough to learn in this hobby. thanks fellows

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I just want to tell, that I'm reading this topic from beginning. I'm on page 10 atm.
Alot, alot and alot of information I require. I had no idea that rotor aerodynamics is so complicated.
I am plane pilot atm, and I want to try to build autogyro from heli kit. I had the HK450 kit and I'm waiting for tail surfaces, motor mount, servos and props. Want to try 3500k/v brushless motor with 4x4 prop.

mnowell129, I want to thank you for this topic. After all this years it is still needed o)

P.S. Sorry for my english.

yak55x,

I just read this thread for the first time today and noticed your post 283. I can answer the question you had regarding the first video. The camera was attached to the blade by the bolts that attach the control horn to the sleeve looking outboard a couple of inches forward and above the blade leading edge. It went through the same feathering, flapping, and hunting motions as the blade. The only blade motion visible is blade bending. At the time it was taken the tip path plane was nasty. It was a few feet thick. If the camera was mounted on the hub inboard of all hinges it would have looked very different.

duplicate

Mickey, are you still out there answering questions, cause I have a few.
Tom Smith, GyroSchtic driver.

Yep.

OK then Mike, here goes. I scratch built a Gyro Shtick years ago and flew it while living in Florida. Probably have 20 flights on it. Always had an issue with orientation, so I covered it in pink Monokote for visibility. I remember back then, in 2003 or 2005, that I had to put some lateral rotor tilt in it to fly level. It was common practice back then. So, setup like that, in stock configuration, my DC Gyro Shtick flew quite well, as can be seen from the photos, but again, every once in a while I would loose orientation and darn near crash the thing. It was so nerve wracking I just stopped flying her. Back in 2008 I added a Delta head from Flying Balsa that they were designing for their PT25 Gyro. At the same time I added a steerable rudder as well so I could try ROG. Moved north to NC just after doing that and never got to fly the little bugger again. So, now I want to resurrect her and in doing so I installed a DX8 radio in it and a Guardian set for on/off and 2D, for those lost orientation moments.
My problem is, I can't remember if the lateral tilt trim was left or right. Also, since I now have a delta head on her, do I need that tilt trim at all. Once I get her airborne and trimmed I am not so worried, it's just that first toss that concerns me. I read all of your Sticky's and it brought back a lot of the stuff I had learned years ago, but I saw no mention in any of those sticky post about lateral tilt trim. I have a set of 1.5" AeroBalsa blades on it and man do they spin up easy. Sure would appreciate any input from you. Tom