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a closer look at Gyroscopic Precession


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According to standard PPL-Theory a gyroscopic precession is usually mentioned with a 90 degree lag. However on a helicopter rotor system supplementary effects such as Wee-Wa or cross-coupling take place.

 

My basic understanding of the cross-coupling effect is that the rotor disc when tilting no matter in which direction creates a force that will also lag 90 degrees later.

 

So if you want to go forwards (anti-clockwise turning rotor) you will have to tilt the rear of the disc upwards and front downwards. During the tilting process the downward movement of the rotor blades over the nose create lift which again will tilt the rotor slightly to the right. Therefore left cyclic is required to maintain a level flight. Again this is only one example of many because tilting can occur in any direction.

 

So the effective lag is a sum of the primary input and cross-coupling. Both together however are not 90 Degrees. Manufacturers take this into account and rig their helicopters with swash-plate offsets or other means:

 

{Al Hammer says ...} The Linx, and the Squirrel have 72 and 78 degree offsets. It is to eliminate the cross coupling. Depending on altitude and temperature,aft cyclic will get you a right roll or a left roll component or none at all. The R22 design eliminates the cross coupling effects quite well, but at higher speeds more and more right stick pressure has to be held, and there is a cruise trim knob for that.

 

A very interesting discussion about this subject can be found under Unicopter (Wee-Wa, Delta3, cross coupling) and FRobinson_Rigging.rtf.

 

Any other suggestions or comments to this topic are welcome.

Edited by HumblePilot
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  • 11 months later...
This is the answer I got ...

 

 

Gyroscopic precession is a force when applied to the rotor system takes effect 90 degrees later. Just like a gyroscope spinning, when a force is applied you notice that the effects take place 90 degrees later. In a helicopter rotor system there are a few things happening as you transition from a hover or from the ground into forward flight. There is Dissymmetry of Lift, which is the advancing blade having more lift than the retreating blade. This occurs around 10 - 20 knots. Because the advancing blade has more lift its force is applied 90 degrees later which is at the nose of the helicopter. Thus, forcing the nose of the aircraft upwards. This also occurs around Effective Translational Lift. Which occurs between 16 - 24 knots. The two, while very different are associated with one another most often because they happen around the same time. Then you have Transverse Flow Effect. Which is as the helicopter is moving forward there is more lift in the front half of the rotor disk than there is in the rear half of the rotor disk. This is because the front half of the disk is in "cleaner" air than the rear half and provides more lift. The force is applied 90 degrees later and it causes the helicopter to roll to the right. As the helicopter starts to outrun its "dirty" air it becomes more efficient. So, as you transition from a hover or the ground the helicopter will have a tendency to pitch its nose up and roll to the right. This is taken care of by rigging and pilot inputs.

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  • 4 weeks later...
According to standard PPL-Theory a gyroscopic precession is usually mentioned with a 90 degree lag. However on a helicopter rotor system supplementary effects such as Wee-Wa or cross-coupling take place.

 

My basic understanding of the cross-coupling effect is that the rotor disc when tilting no matter in which direction creates a force that will also lag 90 degrees later.

 

So if you want to go forwards (anti-clockwise turning rotor) you will have to tilt the rear of the disc upwards and front downwards. During the tilting process the downward movement of the rotor blades over the nose create lift which again will tilt the rotor slightly to the right. Therefore left cyclic is required to maintain a level flight. Again this is only one example of many because tilting can occur in any direction.

 

So the effective lag is a sum of the primary input and cross-coupling. Both together however are not 90 Degrees. Manufacturers take this into account and rig their helicopters with swash-plate offsets or other means:

 

 

 

A very interesting discussion about this subject can be found under Unicopter (Wee-Wa, Delta3, cross coupling) and FRobinson_Rigging.rtf.

 

Any other suggestions or comments to this topic are welcome.

That is an interesting and complicated discussion, especially considering Frank's input in the link. Unfortunately I didn't find it clarified everything, may have made for more confusion given his mention of the forward blade increasing angle of attack and climbing and the aft blade reducing angle of attack and diving. I say this in respect to the universally taught notion that to counteract dissyemmetry of lift the advancing (aft) blade flaps up and reduces angle of attack while the retreating blade flaps down increasing a of a. Also I am a little fuzzy about the relationship he speaks of between the 18 degree delta three angle and the 90 degree pitch link.

Not disputing what Frank says, obviously we have to apply the left lateral cyclic. Just curious for others take on this.

 

blave

Edited by blave!
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  • 9 months later...

Some time in the past, it was decided to draw an analogy between a rotor and a gyroscope. Unfortunately, this analogy is incorrect and has caused confusion ever since. A rotor is not a gyroscope, due to the articulation, real or virtual, of its blades. A rotor is a system of whirling pendulums, and its reaction to displacement inputs is governed by the laws of vibration. In accordance with those laws, the only time a rotor blade responds with a 90 degree phase lag to a cyclic control input is when that blade is hinged at the axis of rotation and does not include any pitch-flap coupling. As the flapping hinge is displaced away from the axis of rotation, or as pitch-flap coupling is applied, the phase lag steadily reduces below 90 degrees due to the increase in natural frequency of the blade. In a given helicopter rotor, the phase lag also changes with the Lock number, i.e. increased aerodynamic damping reduces the blade natural frequency and thus increases the phase lag, such as would occur if flying one day in freezing conditions in Death Valley, and another day in hot weather at the top of Pike's Peak.

 

In summary, a rotor is a dynamic system in resonance, i.e. an oscillating system responding to periodic disturbances. All else follows.

 

The phase lag may be referred to as "precession", but this is a less than rigorous - and confusing - use of this term.

Edited by JPHarrison
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There is Dissymmetry of Lift, which is the advancing blade having more lift than the retreating blade. This occurs around 10 - 20 knots. Because the advancing blade has more lift its force is applied 90 degrees later which is at the nose of the helicopter. Thus, forcing the nose of the aircraft upwards. This also occurs around Effective Translational Lift. Which occurs between 16 - 24 knots. The two, while very different are associated with one another most often because they happen around the same time. Then you have Transverse Flow Effect. Which is as the helicopter is moving forward there is more lift in the front half of the rotor disk than there is in the rear half of the rotor disk. This is because the front half of the disk is in "cleaner" air than the rear half and provides more lift. The force is applied 90 degrees later and it causes the helicopter to roll to the right. As the helicopter starts to outrun its "dirty" air it becomes more efficient. So, as you transition from a hover or the ground the helicopter will have a tendency to pitch its nose up and roll to the right. This is taken care of by rigging and pilot inputs.

 

Just to clear this up. Dissymmetry Of Lift will occur at 1kt and is present with any amount of airspeed. As long as there is airspeed there is a difference of lift between the advancing and retreating halves. Transverse Flow Effect on the other hand will occur generally between 10-20 kts for the reasons stated already. I would say that TFE and ETL both tend to get confused with each other as one passes through them because of the similar A/S. The "rattle" used to determine ETL on approach is really TFE and DOL both trying to flap in 2 different directions. DOL starts to flap up on the advancing while TFE is at max down flap at the same time and visa versa on the retreating. Really you could feel the "rattle" between 10-15 kts and be technically out of ETL. Usually one doesn't feel this on T/O because we speed through TFE so quick on T/O. I also want to make clear that none of this requires any movement of the helicopter. It's all based on A/S alone so if one is to hover with a wind whether it's a headwind, x-wind, or tailwind, the retreating and advancing halves will act accordingly.

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  • 11 months later...
In summary, a rotor is a dynamic system in resonance, i.e. an oscillating system responding to periodic disturbances. All else follows.

 

The phase lag may be referred to as "precession", but this is a less than rigorous - and confusing - use of this term.

 

You are correct. Unfortunately, the gyroscope analogy persists, probably because it's easier to understand. Equations of motion of the rotor blades, interestingly, do not include gyroscopic terms (angular momentum, torque, precession, nutation).

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  • 7 years later...

Some time in the past, it was decided to draw an analogy between a rotor and a gyroscope. Unfortunately, this analogy is incorrect and has caused confusion ever since. A rotor is not a gyroscope, due to the articulation, real or virtual, of its blades. A rotor is a system of whirling pendulums, and its reaction to displacement inputs is governed by the laws of vibration. In accordance with those laws, the only time a rotor blade responds with a 90 degree phase lag to a cyclic control input is when that blade is hinged at the axis of rotation and does not include any pitch-flap coupling. As the flapping hinge is displaced away from the axis of rotation, or as pitch-flap coupling is applied, the phase lag steadily reduces below 90 degrees due to the increase in natural frequency of the blade. In a given helicopter rotor, the phase lag also changes with the Lock number, i.e. increased aerodynamic damping reduces the blade natural frequency and thus increases the phase lag, such as would occur if flying one day in freezing conditions in Death Valley, and another day in hot weather at the top of Pike's Peak.

 

In summary, a rotor is a dynamic system in resonance, i.e. an oscillating system responding to periodic disturbances. All else follows.

 

The phase lag may be referred to as "precession", but this is a less than rigorous - and confusing - use of this term.

I am working on a workbook for my school. We are trying to make training materials that coincide with the FAA's Helicopter Flying Handbook. The HFH describes the cyclic in terms of gyroscopic precession but my boss and I dislike that. I am trying to find a clear way of describing to early students how the cyclic works and why the control inputs are off-set. I really enjoy what you said about the whirling pendulums and the oscillating system. I think it is much better to think of the blades as blades. The duality of form is something photons do, not helicopter blades.

 

However, I am a bit unclear about the difference between phase lag and what has been taught to me as gyroscopic precession. I can understand that the term precession has been widely misused and that a phase lag is truly occurring, so what then is precession? And does it affect our rotors at all? Or is all of it a resonance thing that has been explained through the more simplistic answer of precession?

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Gyroscopic procession. An inherent quality of rotating bodies, which causes an applied force to be manifested 90° in the direction of rotation from the point where the force is applied.

To compensate for this the pitch linkage is offset 90° so when you move the cyclic forward the rotor disk actually tilts to the right. Then gyroscopic procession applies that force 90° in the direction of rotation, thus the helicopter moves forward.

 

The simplest explanations are always the best!

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I am trying to find a clear way of describing to early students how the cyclic works and why the control inputs are off-set.

 

I'd say the odds of getting a response to a post that is over eight years old is well,...not good, so here's what I'd say;

 

Today kids we're gonna talk about how a helicopter's controls work.

 

There's this principle in physics called gyroscopic procession, which says that when a force is applied to a spinning object the effect happens 90 degrees later in the direction of rotation. :huh:

 

Since a helicopter's blades are spinning this principle applies to them. So when you move the cyclic forward the rotor disk tilts forward, but because of gyroscopic procession its effect is experienced 90 degree later and the helicopter moves to the left. :o

 

You can't see it, but I'm using my hands to help demonstrate this, partly because guys are visual, but also because I'm Italian. ;)

 

Now to make flying a helicopter much easier, they offset the pitch horns 90 degrees to the right. So now when you move the cyclic forward the rotor disk actually tilts to the right, then gyroscopic procession brings the effect 90 degrees later and the helicopter moves forward. :D

 

There's really no need to delve any deeper into gyroscopic procession. After all we're pilots not physicists!

 

By the way, I'm not a CFI,...but I did stay at a Holiday Inn last night!

Edited by r22butters
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However, I am a bit unclear about the difference between phase lag and what has been taught to me as gyroscopic precession. I can understand that the term precession has been widely misused and that a phase lag is truly occurring, so what then is precession? And does it affect our rotors at all? Or is all of it a resonance thing that has been explained through the more simplistic answer of precession?

 

Gyroscopic procession as referred to in most standard textbooks, is a simplified way of explaining the motion of a helicopters rotor as it turns. As a quote from Aristotle:

 

“It is the mark of an instructed mind to rest satisfied with that degree of precision which the nature of the subject admits, and not to seek exactness where only an approximation of the truth is possible.”

 

If we seek exactness, the rotor is not a gyro and the rotor flapping is not a pure precession as in “Gyroscopic Precession.” However, the rotor system exhibits “similar properties” since its part of the same family of mechanics. When a control input is applied that increases the blade pitch at a given point the blade will have its maximum flapping amplitude sometime later in the direction of rotation. We had a post on this subject back in 2010 at link: Gyroscopic Precession, Nov. 2010

 

However, we can wrap this up quickly if we look at some of the basic forces.

 

When we look at the forces acting on the rotor:

 

1. Aerodynamic forces

 

2. Weight forces (mass)

 

3. Inertia forces

 

4. Centrifugal forces (Centripetal force if you prefer)

 

vs.

 

When we say "Gyroscopic Forces" lets list some:

 

1. Weight (mass)

 

2. Inertia forces

 

3. Centrifugal Forces (Centripetal force if you prefer)

 

If the forces aren't equal the mechanics of motion aren't equal. Our rotor has one additional force that sets It apart from a Gyro and the Gyro lacks this force. Aerodynamic forces are what sets the rotor apart from the Gyro. If you took away the aerodynamic forces, essentially placing a rotor in a vacuum, the rotor would then behave like a gyroscope.

 

“The phenomenon of precession occurs in rotating bodies that manifest an applied force 90 degrees after application in the direction of rotation. Although precession is not a dominant force in rotary-wing aerodynamics, aviators and designers must consider it, as turning rotor systems exhibit some of the characteristics of a gyro.” - Raymond W. Prouty; Helicopter Performance, Stability, and Control

 

“The addition of hinge offset changes the characteristics of the rotor from being a system in resonance to one whose natural frequency is higher than the rotational frequency, thus the phase lag is less than 90 degrees.” - Raymond W. Prouty; Helicopter Performance, Stability, and Control

 

Forces%20Acting%20on%20a%20Blade%20Eleme

Edited by iChris
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so what then is precession? And does it affect our rotors at all? Or is all of it a resonance thing that has been explained through the more simplistic answer of precession?

 

Let's define terms; “Phase lag” is the delay between cause and effect.

 

The Effect:

 

We have a rotor system that responds to our input a time later (delay) after we’ve made them and in a different position ahead of our initial input.

 

The Causes:

 

Aerodynamic Forces, Centrifugal Forces, Weight (mass), and Inertial Forces.

 

Gyroscopic Forces are in order, to simplify the explanation, because any rotating mass exhibits similar properties to that of a gyro. The rotor disk as a rotating body exhibits an ability to maintain its angular momentum and maintain its direction in space, however weak. In doing so, aerodynamic forces, centrifugal forces, weight, and inertial force are the primary causes of flapping action.

 

Also, hingeless rotors or rigid rotors may have no single point at which flapping occurs, but an “Effective Hinge Offset” can be determined that will give the same characteristics as a blade with an actual mechanical hinge at that point. Remember, bending and flexing along defined points along the blade accomplishes flapping on a rigid rotor system.

 

Blade flapping moments without hinge offset

 

i) an inertial force opposing the flap motion, with moment arm r about the flap hinge

ii) a centrifugal force directed radially outward, with moment arm z = rβ

iii) an aerodynamic force normal to the blade, with moment arm r

 

Blade flapping moments with hinge offset

 

i) an inertial force opposing the flap motion, with moment arm (r − e) about the flap hinge

ii) a centrifugal force directed radially outward, with moment arm z = ηβ

iii) an aerodynamic force normal to the blade, with moment arm (r − e)

 

The moment generating capability of the helicopter is increased greatly when ν>1 (flap frequency higher than rotors rotational frequency). An articulated rotor normally obtains about half its moment from hinge offset and half from the thrust tilt. For a hingeless rotor the direct hub moment can be two to four times the thrust tilt term. Moreover, the direct hub moment term is independent of the helicopter load factor.

 

Consider next an articulated rotor with the flap-hinge offset from the center of rotation by a distance e. Such an arrangement is usually mechanically simpler than one with no offset and in addition has a favorable influence on the helicopter handling qualities, because the offset produces a flap frequency above 1/rev. Articulated rotors typically have an offset of e = 0.03 to 0.05.

 

Excerpt From: Wayne Johnson. “Rotorcraft Aeromechanics. - Wayne Johnson worked at the U.S. Army Aeromechanics Laboratory from 1970 to 1981, at the NASA Ames Research Center. He was with NASA from 1981 to 1986, including several years as Assistant Branch Chief.

 

Without%20Offset_zpskcepnt6n.png

 

With%20Offset_zpsgbennh33.png

Edited by iChris
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Guest pokey

it sure would be nice if helicopter flight wasn't rocket science. But for most of us laymen & wanna be pilots, it does suffice. Now? speaking of rocket science, it is not only about getting off the ground, & then accelerating to escape velocity, how about tossing in planetary orbit?...............some things are just better left to the experts. in case you space cadets don't get my point (there is a time & a time not to fly)

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iChris - Thank you. So far your post most clearly defines the lines behind what all the talk about gyroscopic vs no gyroscopic precession is all about. I highly appreciate the quotes you put in by Raymond Prouty, that is the exact explanation I needed.

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  • 2 weeks later...

Mr Newton gets a look in here, too - you apply a force to the blade (Pitch increases, angle of attack causes lift to incease) and the blade has mass, so then F=mA steps up and the blade starts to accelerate upwards.

 

That's the whole key - it STARTS to move upwards - it takes time to get up to where you want it to be, and taking the rotation into account, the blade has turned a bit since the force was applied.

 

By the time the blade gets up to where we want it, the swash plate has already started to feed in the opposite input, so the upwards acceleration force has stopped and a downwards acceleration is starting. The difference in position between where maximum accelerative force is applied and the maximum position is reached, either up or down, is your phase lag. About 90 degrees.

 

But as explained above, your average student has an IQ that varies with the room temperature, so on cold days they need a simple explanation, such as : "A rotor is LIKE a gyroscope".

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