Phase Angle - What is it??

[gwelo shamwari]

Listening in on a ground lesson where a instructor was talking about phase angle. He said that it the rotor system on a helicopter was not a true gyro because it did not spin quickly enough, hence it did not precess at 90 degrees. He said that in the S300 phase angle was approximately 78 degrees.

Is there a certain "RPM" that a spinning object becomes a true gyro?

I read somewhere online that phase angle had to do with flapping, I looked at my books but can't find anything. When I brought this up he look puzzled and said that he would get back to me, but still has not.

Could someone please explain this to me simply and then in detail, as I am only starting out and would like to get the basic's now then more detail later.

thanx
Zulu

[Heli-Ice]

TGZ

Thank you for the info... I didn't know I had a phase angle in my H300 :rolleyes:

Basic helicopter theory= It'll cost you loadza money... the heli pilot license that is...

[The Rotordog]

Oh, life used to be so simple. In the good old days, we had this thing called "gyroscopic precession" and we used that term to describe how you had to put the pitch-change in back *here* to get the rotorblades to react up *there*. We said that the rotor's largest displacement happened "90 degrees" beyond the point at which we made the control input. We had all played with gyroscopes as children, so this simple analogy sufficed to explain how a helicopter rotor worked. It still does.

But then some egghead engineers came along and said, in effect, "Whoa, wait a minute."

In a "real" gyroscope, they say, the rotor is fixed solidly to the axle. And in a "real" gyroscope, the reaction actually is 90 degrees beyond where we put in the force. You can measure it, supposedly.

But helicopter blades are not fixed solidly to the mast. There are these things called flapping hinges. And even if there are no hinges per se, the blades are quite limber and act as a hinge. But remember, we're not just applying a force to the edge of the disk to cause it to tilt. We're changing the pitch of the individual blades themselves, causing them to fly up or down to a new position as they go 'round. Only they don't react instantaneously. This is where the term "phase angle" comes in. It's a term engineers invented to confuse pilots.

And so, okay, the precession is "not quite" 90 degrees, not exactly. It is something less (but it could be more). Nevermind, it does not matter. Those wacky engineers have figured out just how much "phase angle" they're dealing with, and they design and rig the cyclic controls so that when *you* push forward on the cyclic, the disk tilts down in front. Clever, eh? As a pilot, that's all we need to know.

[[email protected]]

And since precession on a gyro is always 90 degrees later than where the force is applied, the fact that different helicopter rotor systems have phase lag angles of less than and more than 90 degrees rather puts the whole 'precession in a rotor system' argument to bed, once and for all. The precession argument is for people who don't understand flapping.

TGZ - in a still air hover, all the blades experience the same speed of airflow ie that of the rotation of the blades. Now add 20 kts from the front. The blades over the nose and the tail will have the same speed of airflow since they are crosswind but the blade at 3 o'clock will have an extra 20 kts and the blade at 9 o'clock will have 20 kts less.

Now start with the blade at the tail and move it towards the nose - it starts to see an increase in speed as soon as it moves and begins to flap up, slowly to start with but increasing as the speed increases (lift increases as the square of the speed) until, at the 3 o'clock position, it has maximum speed and therefore maximum flapping up. Even though the speed reduces between the 3 and 12 o'clock, the blade doesn't flap down, it reduces its rate of flapping up until it reaches its high position at 12 o'clock.

Now from 12 to 6 o'clock on the retreating side, the process is reversed.

This is basic flapping to equality and highlights that 90 degrees after the maximum effect (adding 20 kts in the 3 o'clock), the blade reaches its high point. So if we want to make the blade high or low, we must rig the controls so that we input the required pitch change 90 degrees before hand. This is achieved by putting the hydraulic jacks 90 degrees out ie fore and aft jack in the 3 or 9 o'clock position - or by having the pitch change rod on the blade mounted ahead of the blades feathering axis - or more usually by a combination of the two.

Now all rotor systems are not equal and are not gyros so, because of the rotor head design and the weight and inertia of the blades, phase lag on some helicopters is not 90 degrees (your instructor has quoted 78 for the 300) and the rigging of the controls and pitch change arms will reflect this so that any desired blade position is initiated by changing the pitch 78 degrees earlier. The reason for doing this is so that forward cyclic in the cockpit results in forward disc tilt (apparently Igor Sikorsky's early model didn't have this yet he still managed to fly it).

Phase lag changes with density altitude as the Lock number changes (blade inertia v aerodynamic damping) which also doesn't happen on a gyro for those doubting thomases.

Hope this helps TGZ, if not - do a search on PPrune as this has been debated extensively before.

[soupisgoodfood]

And since precession on a gyro is always 90 degrees later than where the force is applied, the fact that different helicopter rotor systems have phase lag angles of less than and more than 90 degrees rather puts the whole 'precession in a rotor system' argument to bed, once and for all. The precession argument is for people who don't understand flapping.

Not quite. If you play with a real gyro, its RPM determins the amount of force that gyroscopic precession will have, therefore the actual angle of tilt will be somewhere in between the orginal, and 90 degrees after.

Of course, as has been gone over again and again, the this force is so small on most helis that it's much closer to 0 than 90.

I know that a few people here are going to scream in horror if this topic goes on any longer, but I'd like to put across a perspective I haven't seen before:

If the 90 degree phase lag really was mainly because of gyroscopic precession, then surely the downdraft when applying the cyclic would preceed by 90 degrees. E.G. if your rotors spin clockwise, then appling forward cyclic should create the strongest downdraft on the right hand side. Seems like a fairly easy way to debunk the myth. How they conduct these experiments is their own problem. But playing with my RC heli (which probably feels gyroscopic forces more than most large helis) clearly shows the strongest downdraft at the back.

[Ascend Charlie]

Wouldn't Lu Zuckerman love to get into this post?:ok:

[ShyTorque]

>>If the 90 degree phase lag really was mainly because of gyroscopic precession, then surely the downdraft when applying the cyclic would preceed by 90 degrees. E.G. if your rotors spin clockwise, then appling forward cyclic should create the strongest downdraft on the right hand side. Seems like a fairly easy way to debunk the myth. How they conduct these experiments is their own problem. But playing with my RC heli (which probably feels gyroscopic forces more than most large helis) clearly shows the strongest downdraft at the back.<<

The downdraught change on the upward side would only be a transient effect, as the blades were accelerated upwards. Once they are flapping up it becomes a steady state, i.e. no acceleration so no further increase in downdraught. The place where the upward acceleration takes place is at the rear of the disc ;)

[soupisgoodfood]

I assume that just sarcasim, ShyTorque? It can be hard to tell when it comes to this subject. :}

[Greaney]

If you would like to read a nice derivation of the dynamic
equations of blade motion about the flapping and the
lead/lag hinges, check out Ch 4, Rotating Blade Motion
in "Principles of Helicopter Aerodynamics" by Leishman.
Section 4.7 ends with an expression for phase angle for
a blade with an offset flapping hinge (as they are on
articulated heads). In the text he states that the value
of hinge offset only varies from 4 to 6% for an articulated
blade, so the natural frequency is only slightly greater
than Omega (1/rev). The forcing frequency from the
swashplate is, of course, at Omega, so the phase lag
will be slightly less than 90 deg, as it would with any
resonant system.

If you really want to understand the physics behind the
phase lag I think you are going to have to refer to a text
similar to Leishman. I don't believe you will get the level
of detail necessary in a few posts.

Shaun