Despite extended opposition here (and in my case sometimes via private messages) Lou Zuckerman probably went to his grave convinced that the rotor system was a gyroscope and control inputs were causing precession of said gyroscope, rather than blade pitch control inputs causing blades to fly to a new position.

When I asked him to explain how and why he thought aircraft like those made by Kaman could be controlled by “servo tabs” mounted on the main rotor blades, he never answered. Same with an explanation of why a helicopter was dynamically unstable, rather than possessing the very stable characteristics of a gyroscope.

Phase lag occurs because a blade cannot instantly attain a new position, it has to be made to fly there under aerodynamic forces. It’s similar to gyroscopic precession, but definitely not the same phenomenon.

When we compare rotor system designs....particularly the BO-105/117 series with the rigid rotor system which has been quite successful, against say a fully articulated rotor system.....would the rigid rotor come closer to reacting like a gyroscope than the articulated head?

Is the Rigid Rotor more stable aerodynamically?

My experience in the "Vomit Comet" (un-sas'ed BK) I have to wonder.

I don't know who this Lou guy is, but are you sure he truly believed that a spinning rotor "was" a gyroscope, or that it just acted "like" a gyroscope?

I agree. Many of the diagrams in my class notes, that then found their way into AP3456, had their inspiration there.

You can look at a copy at https://archive.org/details/sikorsky...-and-mechanics

Very interesting discussion, and I appreciate everyone's input (and patience!) as I attempt to make sense of all of this. It's surprising how little of this information is in the current helicopter training manuals.

From this discussion, I would expect that the effects of inflow roll would be much more pronounced on takeoff versus flapback, but I've found the opposite to be true in flying and instructing. In theory, while in a no wind hover, we are experiencing no flapback or inflow roll. We move the cyclic forward to tilt the rotor disc and start our transition to forward flight. We need a little left cyclic pressure and some footwork to keep the nose straight, but somewhere around 20-ish knots, we need a distinct forward push on the cyclic to keep everything where it was going just a moment before. What is it that makes this one moment seem so different than any other phase of flight? Or is it just in my head?

Thank you for the recommendation, I just managed to grab a decent '64 version off Amazon.

ISTR he thought it was a gyroscope.

He was a very clever guy but sometimes simple things escaped him - I had a long argument about pitch change rods with him - he couldn't grasp that, much like a piston in an engine, they didn't have constant rate of vertical movement as they followed the swashplate and actually stopped moving vertically at the top and bottom of each stroke before they started moving again in the opposite direction.

The phase lag on a Lynx rotor head was quite a way from 90 degrees so No, I don't believe so.

Another factor is the relationship of blade inertia to aerodynamic damping (Locke Number if memory serves) where a heavier blade will flap further than a light one given the same aero forces or the same blade will flap more when aero forces are reduced (high DA for example).

So called “rigid” rotors still work by flying the rotor blades to their required positions, rather than by gyroscopic precession.

The “rigid” description is in that there are no specific hinges on the head, ie there are no leading/lagging, flapping or pitch change hinges. The blade support system is all one piece, which can twist/bend to allow relative blade movement and pitch changes. The Lynx type of head was/is semi rigid in that it does have conventional pitch change hinges.

Both the BO 105 and the Lynx exhibit some unusual pitch/roll coupling as a result of the phase lag not being 90 degrees.

It might be worth emphasising that all the forces that the rotor exerts on the helicopter are transferred through those hinges/supports. For a fully-articulated rotor head that force is just the blade tension [1]. All that aerodynamic forces on the blades are doing is altering the angle at the hinge. I suspect this is largely true for rigid types.

[1] For those interested, this is discussed on p13-14 of that Sikorski manual.

Lu was always asking the question "Where are the missing 18 degrees?" for the R-22, which had a phase lag of 72 degrees.

The BK had feathering hinges but no flap or drag hinges. Without SAS, it could be uncomfortable to fly, with the slightest puff of wind or teensy cyclic movement passed straight into the cabin. Our Chief Pilot was known as Chuck, because most of his crewmen did.

That's actually kinda funny. :}

Oh no, we'll be getting into 'wee-wah' next!

beats WIWOL!

Could it be due to the relatively instantaneous change of the airflow from static hover with its specific airflows through the disc to 'normal flight' airflow which happens when going through ETL? The Rotorblades 'suddenly' getting clean air from the front. This could potentially lead to some 'overshooting' of the blade track, somewhat similar to a gust hitting the disk.

It certainly could, but I would expect inflow roll to have at least as big of change, since the front of the rotor system is first and the rear is last to realize the airflow changes. However, the roll seems minor compared to the amount of forward cyclic needed to keep the nose where it was.

The roll IS minor, and gets less with increasing airspeed, whereas the flapback increases with airspeed or collective increase.

What AC said :ok: remember the roll is due to a change in inflow angle, not a massive amount but enough - the pitch is due to velocity differences between advancing and retreating sides of the disc and is V squared so much bigger

It is also accompanied by vibration, because the inflow angle only changes at the front of the rotor disc and the effect is assymetric until the airflow regime for forward flight is established. In a normal transition from the hover to forward flight the vibration may not be noticed.

However, if you have spent endless hours in the hover in a SeaKing over the Atlantic with a strong breeze (25+ knots) blowing you cannot ignore it. That is firmly in the vibration bracket and it manifests itself by shaking the body so that the fleshy tip of the nose wobbles and tickles almost unbearably. ;-)

Same with doing cliff winching in an updraft in the Sea King Syd - no chance of reading any instruments at all.

However the inflow angle changes at the rear as well with the front seeing a bigger change than the rear, hence the dissymmetry.

Normally the vibration is associated with translational lift rather than inflow roll.

In a still air transition the inflow roll happens first, then the flapback and then, when you overcome both, the vibration of ETL.

The effect of which I speak is quite subtle. Seeing the instruments was not a problem.

I suspect that your vibration is likely to be caused by interaction with complex vortices, as is the vibration of translational lift.

I should have said that it 'progressses from the front to the rear'.

Yup. ;-)

Syd I realised after posting that you were referring to 'Sea King Nose' - well known to crews.

Agreed

Got it, thank you!

I thought the vibration was due to the uneven forces across the disc, when inflow roll was at its worst. That period in slow acceleration when only part of the disc has moved out the induced flow. What would be causing a vibration at ETL?

One bit that puzzles me is the "dastardly" warning that comes from hovering next to a hangar or a cliff. The warning is that the air is recirculated next to the vertical obstruction, and there is thus increased induced flow next to the hazard, less lift means the helicopter will get sucked over to the cliff or hangar and nasty things happen.

Well, if the downwash increases in that sector, the effect should be felt around 90 degrees later, which would pull the aircraft along parallel to the hazard??

And having done quite a bit of hovering next to vertical obstacle walls, I have never felt any problem with it. Any contributions?

the rotor has to pass through the tip vortices which, in still air IGE, are spread outwards from the rotor - it is passing through those vortices that creates the vibration of ETL.

If you do an IGE transition over longish grass (an inch or two) you can see the pattern of the outflow of the rotor on the grass and as you catch up with the forward edge of that, you will experience the vibration and the onset of ETL.

AC - I'm with you on that one - heard all sort of theories about the aircraft being sucked towards the obstacle or requiring more forward or aft cyclic to prevent the aircraft moving forward or back - never felt a problem either on ops or in training moving in from a stable OGE hover to close proximity to cliffs/hangars/buildings. if there is an effect it is negligible.

Full recirculation is a different matter - had a colleague make a very firm landing in a wriggly tin fort in South Armagh - we 'fell out of the sky' as we got to the hover and used a lot of unanticipated power to cushion the landing.

The dreaded “settling without enough power”……..;)

I think we did with the lag <90...

When the controls positions are recorded, the S shape that occurs is pretty to break down, the lateral change is initially from inflow roll, the hover longitudinal to ETL to high speed is from the flap back initially and then from the balance of forces with the thrust/drag couples around the beast.

Alternatively.... for a 2 blader.... the hub reaction forces are "simple"... Per Uncle Wayne :}

https://cimg7.ibsrv.net/gimg/pprune....10a02a8e1a.png
19.8 Rotorcraft Aeromechanics, Johnson (2013)

Now you are using maths - so unfair:)

Helicopter Principles of Flight was explained by the venerable Lofty Marshall as 'an explanation of something we know happens, not mathematical proof of why it happens' :ok:

Everybody knows that a helicopter is "a triumph of science and technology over common sense." Also known as "White man magic".

Mast Bumper - I have found a copy of Lu's book on a dusty shelf in the office.....

DM sent.

May I just say that I've read every post in this thread but said nothing because I could neither add to the discussion nor explain things as clearly as some of you guys. The level of knowledge about this subject here is truly impressive. And...not to single anyone out, but Ascend Charlie should be teaching this stuff if he isn't already. His explanations are incredible in their simplicity and clarity. A lot of us old-timers know this stuff but have a hard time putting it into words and terms that a newbie might understand. If any pilot were to ever wonder why the rotor does certain things, I would point them to this thread. Nice work, men!

Thank you Mr FH1100.

Yes I used to teach this stuff from 1976 onwards, but retiring from flying and instruction leaves me with these sites. I think Crab, fdr and Shy are in a similar boat.

But if you want REAL knowledge, you have to ask Nick Lappos, Sikorsky's chief test pilot, who cruised these sites in the past. Sadly Shawn Coyle has departed the helipad, but his book Cyclic and Collective is around, with knowledge and wit combined.

And pay attention whenever John Dixson visit the site, his knowledge is the supplement to Nick's, two gentlemen who were in on some of the best testing ever.

That was me a month ago, on being pointed to the AP3456 online and discovering its use of my own class notes from 1977. This stimulated me into doing something that has been in the back of my mind for ages, and I have digitised them. The only copy I have is rather poor and yellowed, but with the wonders of a modern phone it has been relatively straightforward.

I have posted these as a PDF and there is an online HTML version. Remember, these are 45 years old and completely unchanged, so do not expect too much. I welcome comments, but be gentle with an old man.:)

Shawn's "little book of Autorotations" & "40 years afore the mast" are good reads. Last time we spoke he was putting together the notes on the Ray Prouty Lecture series, I read the first chapter, and it was well done, perhaps Patricia would consider letting the whole set be published. The last 10 years of his health was a tough road that he travelled with his positive attitude.

On the topic of vibrations and ETL - can anyone explain WHY certain rotor systems, generally with 4 blades or more, seem to exhibit these bone shaking vibrations right around that 15-30kt airspeed range? In an Agusta it makes your vision blurry, in bigger machines like the Skycrane, it can apparently cause premature tail boom cracks.

lelebebbel - my guess would be the tip vortices interfering with the following blade.

When they created the BERP blade for Lynx and subsequently EH101, the first iterations had problems with what was referred to as 'cobblestoning' in the hover - this was mostly alleviated by adding an anhedral tip to help shed the tip vortices downwards.

The more blades you have the more vortices you are producing.

You could also factor in the blade passing frequency over the cockpit and tail boom as a source of pressure pulses that could affect structural integrity if they match the resonant frequency of the airframe - I believe this is a problem on the S92 creating higher noise levels in the cockpit.

The "Sikorsky shuffle."