Gyroscopic precession engineering question
 

Ok I have a few questions for the engineer types. Here is what I think I know...the questions comes after:

1-Gyroscopic precession causes the rotor system to react 90 degrees out of phase from the control input (generally speaking, see below). On a 2 bladed teetering system this seems to be the case anyways.
2- The amount of flapping hinge offset can change that 90 degrees to something smaller. The more offset the less phase lag?
3- Pitch link phase angle offset is also a factor. If the pitch link is 45 degrees in front of the blade then that would be 45 degrees removed from the 90 gyroscopic precession. If 45 degrees behind the blade then it would be added to the 90 degrees.

Now here are the questions-
1- Is that everything that would effect the input phase angle? What else am I missing?
2- Is this something that is engineered on paper before they ever make parts? Or does one need to design build and run a rotor system on a whirl tower to see just what the actual phase angle displacement is and then design the flight control system to tilt the swashplate in the right direction? I would guess it is the former not the later but seems like it might be a tough thing to get right on the first shot and difficult to adjust for if you miss the mark.

Thanks
Max

Point 1 is incorrect. A gyroscope is a solid, rotating mass, fixed solidly to the axle. A helicopter rotor system has hinges, albeit in a few different ways. The blades are caused to fly independently to a new position in relation to the rotor axis using aerodynamic forces, rather than gyroscopic precession.

I beg to differ. Any rotating mass is a gyroscope with its accompanying effects. A spinning (fixed wing) airframe can also show the effects of gyroscopic precession. You don't need a fixed axle to identify as a gyroscope, just mass and a rotational speed.

3,2,1, fights on!

Yes but that is not a rotor disc. The spinning fixed wing exhibits precession because external forces are chaning the axis about which the spinning mass is spinning.
With a rotor disc, the disc is not forced to do anything. It flies where it wants to go. The confusion arises from the 90 degree thing because the pitch rods are at a maximum (or minimum) pitch at roughly 90 degrees to the highest point of the blades. But that is because at that (90 degrees "offset") point, the vertical speed of the blades is at a maximum. As the blade approaches its maximum height the pitch reduces until it is flat at the highest point - otherwise the blade would continue to fly up! Ditto when the blade is at its lowest point. So the 90 degrees arises from the difference between blade vertical speed, and blade height. It is nothing to do with gyro precession.

No, "The blades are not not caused to fly independently to a new position in relation to the rotor axis using aerodynamic forces, rather than gyroscopic precession". If that were true, a fixed wing with a positive alpha would always fly up.

A wing, and the airframe to which it may be fixed, will fly in a direction determined by the 4 forces of lift, drag, gravity and thrust.

A rotating wing will do exactly the same, but adds centrifugal and gyroscopic forces into the mix.

Well there is of course no such thing as centrifugal force. Nor is there gyroscopic force where the thing is not being forced to do something it doesn’t naturally want to do.

Anyway lots of people believe the earth is flat. Even more believe in an imaginary friend. So there is no point in trying to convince those who have firm beliefs to change their minds.

Not again. Could everybody please read their Prouty before coming up with that again and again and again? It has been debunked several times here. The Robinsons have an offset of 72°. Not 90°. Can't be gyroscopic precession. Other helicopters have other offsets. Yes, Smarter Every Day got it oh so wrong, too. He isn't that smart. Otherwise he would have talked to somebody who actually knows something about it. And yes, the FAA helicopter handbook gets it wrong, too. It's lies to children. Simplifying things to the point where it is wrong.
In a vacuum you would have that effect, but aerodynamic effects are way stronger and therefore gyroscopic precession is not important on a rotor. It is called phase lag.
And there is no centrifugal force either. Centripetal force is the real thing. Goes in the opposite direction.

I was lucky enough to meet this guy a few times before he passed. Maybe not definitive as he always said but good info.

RW Prouty was a staff engineer-flying qualities at Hughes Helicopters Inc, in Culver City, Calif. He jokingly refers to himself as a "journeyman engineer who has journeyed, over the past 28 years, from Hughes Helicopters to Sikorsky Aircraft to Bell Helicopter to Lockheed's helicopter program and finally back to Hughes" He further observed that "helicopter people tend to go around in circles." As an aerodynamicist, his experiences included preliminary design. performance and flying-qualities analysis, wind-tunnel testing, and flight testing.

As a professional helicopter pilot and military trained instructor since 1979 I probably am rather fixed in my beliefs.

Try replacing the rotor blades with lengths of non aerodynamic, 4 x 2 planking and then see if you can make them precess using the same control system.

Wow....and I thought I was asking an easy question LOL. Good information all around ( from an outsider looking in...not taking sides). By all means keep going. I'll be right back....just gotta go get some popcorn!

The books use "precession" as a way of getting stupid students to sort-of understand why the disc behaves the way it does.

The swash plate and links feed in a pitch angle to the blade. Blade's angle of attack goes up, and generates more lift.
Lift is a force. F=mA
The lifting force starts to accelerate the blade upwards, with a rate depending on the mass of the blade. It takes time from when the max input is fed in, with the max rate of acceleration, until the blade reaches its max position.
While this is happening, the blade is rotating about the mast. By the time the blade reaches its max height, the mast has turned ABOUT 90 degrees. (As stated before the R-22 has turned only 72 degrees, mainly because of the light blade.)
In the meantime, the swash plate has been reducing the pitch angle, so the blade is now being told to flap down, and so it goes on around the full circle. The max acceleration up or down happens ABOUT 90 degrees ahead of where the blade reaches its highest or lowest point.

Some helicopters have the lead angle set 45 degrees ahead, but also have the control rods moved 45 degrees around the mast. Others have the control rods running up the side of the mast, so the about-90 degree lead input is easily done.

And don't get into a tizz about "Flapping to Equality" because it only happens if the cyclic is held fixed, and it results in an eventual crash. People think that in forward flight, the advancing blade must flap up to reduce the extra lift from the forward airflow, and the retreating blade flaps down. All you need to do is look at the disc in forward flight, the advancing blade is flapping down, and retreating blade is going up.
As soon as the cyclic is moved to prevent the flap-back, the blades obey the swash plate. Poke it forward, disc goes forward. Simples...

I don't recall the Rotorcraft Flying Hanbook ever saying that a helicopter's rotor "is" a gyro. I do recall it saying that its "like a gyro", which is not the same thing, but its been a long time since I've looked so,...

Anyway, the FAA only wants us pilots to have a surface level knowledge when it comes to aerodynamics. I know what they want me to know, and it helps me get through my BFRs,...but I'm not going to teach a class of engineering students, or design my own helicopter with it, lol.

Anyway, I thought the R22 was offset around 60°,...?

Gyroscopic precession in a rotor - the gift that keeps on giving. Can we bring Lu Zuckerman back from the dead? Yes, centrifugal force is also not real - it’s a pseudo force.

So....now that I have had a chance to absorb all this, it appears I don't know what I thought I knew. Now that I think I know what I should have known...I will adjust my question accordingly.

If the phase angle displacement is due to aerodynamic forces and the time it takes each blade to react to the inputs as they are provided, then how does one account for this from a design perspective? Is it a matter of calculating the increase in lift due to a control input and how long that force would take to move the blade to its new position? I'm sure there is a way but just seems like there would be a ton of factors at work and hard to engineer correctly right out of the gate.

Thanks for all the input so far. BTW I did read Prouty's book long ago but can't find my copy to reference for this question.

Thanks
Max

On a teeter rotor with no pitch flap coupling and no hub spring rate, the first flap frequency is at 1/rev, i.e resonance with the rotor rpm. A system in this state has a phase lag of 90 degrees. When a hub spring is added (or an offset flap hinge) the first flap frequency is raised and the phase lag becomes less than 90 degrees. A very stiff out of plane rotor can have this phase lag of 35-40 degrees. This isn't the only factor that influences the geometric arrangement of control inputs vs blade position, though. Pitch flap coupling can be intentionally introduced to either damp out flap responses to pitch inputs or, in some cases, act as a "negative spring" and accentuate flap response to pitch inputs. This is relatively uncommon on main rotors, but can serve to drop the first flap mode below 1/rev.

Generally speaking, a rotor design team does not have to guess at the phase lag as calculating the first flap frequency is pretty straightforward these days. That said, there are more complex rotor stability issues, particularly with articulated rotors in some maneuvers, that can be addressed by shifting control inputs, leaning pitch links, etc. Those adjustments are often made during flight test, even by experienced OEMs.

Also, centrifugal force is "imaginary" to physics textbooks (and perhaps people online) but when working and analyzing in a rotating frame, it is the common and even preferred notation, even down to part names.

Digital versions in the link behind his name in post #10.

Hey Robbie, well yes, it says "like a gyro", but with all the explanation following that statement, it is still wrong.
Directly from the Rotorcraft Flying Handbook:
Gyroscopic Precession
The spinning main rotor of a helicopter acts like a gyroscope.
As such, it has the properties of gyroscopic action, one of
which is precession. Gyroscopic precession is the resultant
action or deflection of a spinning object when a force is
applied to this object. This action occurs approximately 90°
in the direction of rotation from the point where the force
is applied (or 90° later in the rotation cycle). (And so on ...)


I am pretty sure it is 72°.

All explained fairly well here:https://en.wikipedia.org/wiki/Reacti...trifugal_force

Fixed that for you.... ;)
I think we have the basics of the issue summarised here, with apologies to the students. There is a lot at work in and around a rotor disc with mechanics and aerodynamics fighting each other on one end and cooperating on the other. In itself, the principle that we call gyroscopic precession is a simplification of the dynamics of a rotating system of masses, but we needed to boil it down to something comprehensible. I still think that it is safe to say that the properties of gyroscopes have a place within the various influences on a rotor system, but what that place is and how it interacts with the rest is something I will happily leave to others to explain.

A lot of our understanding of things like this are based on simplifications, but we then tend to run into the limitations of those simplifications when something doesn't fit into this mould anymore. That doesn't mean that the principle is wrong. We can get into a lovely discussion on how Bernoulli does not fully explain how a wing produces lift and drag. It does not mean that Bernoulli got it wrong. It means that we've got an opportunity to learn.

Exactly. It’s really all about inertia and “forcing” an object to turn rather than go straight on, as it would naturally do it left to its own devices.

Also note that if the rotor system was truly a gyroscope, it would have to follow the first property of a gyroscope - RIGIDITY IN SPACE, i.e. it would be very difficult to disturb it from its axis.

A puff of wind will knock it off, causing flapping and diversion.

Ergo, it is not a gyroscope. But sometimes it behaves LIKE a gyroscope.

I honestly don't care if its wrong. I'm neither an engineer, nor a physicist, but just a pilot. If they wanted me to tell them on my checkride that 2+2=5 in order to get my license, I would have. I'm not designing helicopters. I just want to fly them,...and knowing why the pitch horns are offset (or even that they are offset) isn't going to affect my ability to do that. :hmm:

Oh, Robbie, my excuses, it wasn't my intention to patronise you, not at all. We have met on this forum quite often and I sometimes get the impression, that you try to be at bit too modest. You have crossed swords with quite a few of our most experienced egos lurking here and I must say, your opinion is much appreciated. You do bring sometimes another perspective and that is needed in a discussion. Discussions are made for changing opinions. If we don't think about what others say and even sometimes get things wrong, we don't learn. Exchanging opinions and not thinking about them, is utterly useless.
Common, admit it, you are interested in all things helicopter related. Even if you think it does not make you a better pilot, I think it does. Every little piece of knowledge changes how you look at it. Even if it is a stupid bolt on an airframe that isn't secured properly. It makes you think and you like that. Otherwise you would not hang out here.

Robbiee, you'll like this one.

A while back when this subject was running a physicist got involved, and challenged even the most experienced members on here to justify their reasoning why a rotor is not a gyroscope, or even like a gyroscope. After endless debate, some crusty old naval pilot chipped in with: That was the last post on that thread if I remember. A man after your own heart I guess !

Thanks for all the replies folks. Great info here and lots of new concepts to research. Much appreciated.

Max

There’s not new concepts in this thread… this is basic stuff out of a rotor dynamics textbook.

When you have been here (on Pprune) long enough, it seems like 'groundhog day'! Waiting in the queue, we have HV curves, PC1/PC2/Cat A, and many more topics to bring up

Well, my ass is not a flame thrower, but if I hold a lighter back there and blow a fart, its certainly "like" one. :}

Well they are new concepts for me to look into. If we all knew everything already there wouldn't be anything to talk about !

​​​​​​​It's when the textbook gets it wrong that these topics recur. The FAA handbook is still full of wrong statements and misleading "vector" diagrams.

Robbiee, by that I meant that here's a practical person, speaking to his own experience to which there is no answer. Take a compliment when it's offered !

As amusing as your story is, it points out the major problem that sparks these debates in the first place. The highly complex and often misunderstood concept of a simile.

Anyway, I'm just a bored troll stirring the pot from time to time for his own amusement. I feed on conflict, not compliments. :}

So glad I missed this rehash of an old topic - is there a way of getting PPrune to react to the combination of words 'rotor' plus 'gyroscope' so it automatically leads the poster to all the previous discussions we have had on this topic?

I know there is a search function but nowadays people just like to ask the question and wait.

Pretty perfect illustration:ok:​​​​​​​

Remember that the stick would have been held central, with the swash plate level with the deck, so the rotor couldn't have done much different.

But if the rotor was a gyro it would have maintained position in space - it could have flapped around the hinges quite markedly regardless of the swashplate position. But it didn't because it's not a gyro - as I know you know:ok:

,...but if it were "like a gyro" then it would have something in common with a gyro.

If the pitch horns weren't offset, then when you push the cyclic forward, you wouldn't go forward, you'd go left (i.e. in the direction of the rotor's rotation),...maybe not a full 90° left, but some degree anyway. What other device does something like that? Oh yeah,...a gyro!

:cool:

I thought very carefully about whether to add to this post, but on balance I think there's an important point to be made.

What I think good teachers are trying to do is to give you enough information to do the job, and although it may not be the whole truth, they make sure that nothing they offer is wrong. That way anyone coming in later isn't trying to correct mistakes, they're trying to extend and enrich what you know already. If you teach that rotors are 'like' gyros, you're picking on a single common feature that can't support any extension, and is easily proved to be wrong from the example of the helicopter on the ship.

Robbiee, you've already said that your interest is purely flying, and that these aerodynamic curiosities are just hurdles you have to cross to get to do that. I'd say that's completely fine and I wish you luck and success in your flying career. If you ever have to teach though, I think there's a responsibility to dig a little deeper, and acknowledge the simplifications you might choose to make, and to make those simplifications act as foundations not detours.