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