A simple (or simplistic?) view of rotor behavior

[Mark E]

A simple (or simplistic?) view of rotor behavior and control. (doesn't really use the terms flapping or gyroscopic precession)

From https://www.rotaryforum.com/forum/showthread.php?t=22760

Ok, forget the different behavior differnces of longer rotors, then. I need to think out some basics:

Reckon I’ve got the left tilt sorted in my head (maybe!) Much is said throughout these threads in one way or another, but it’s often cryptically explained because the knowledge of the instructor is way ahead of the listener (ie, me). (If I’m way off, and someone screams, I’ll delete, but writing this down helps my thinking a bit, and leaves me open to advice)

Consider CBs thrown 3 bladed rotor, or the boomerang. Thrown, it will arc upwards and away to the left. (edit! Wrong! He actually said it would do a normal inside loop!) If it could continue, it would complete a slow leftward roll. (edit, also wrong!)I think a (somehow balanced for independant flight) two blade rotor really wants to do the same. (edit -not any more I don't! I htink it would also do an inside loop)A rotor would like to keep climbing up (and to the left NOPE) , but a control input stops it. (CB has said this many times, but I am not clear on the input, and have not yet found anyone else’s clearly stated opinion on the matter worded to get to my feeble neurons). (Edit - the control input is important, it is just controlling blade pitch in the 3 /9 axis, thereby sustaining straight and level flight)
That control input is the angle at which the pilot (or trim spring) angles the rotor head in the fore/aft axis (and therefore angles the disk of rotation of the teeter bolt) to the oncoming air stream (ie not the left or right tilt stick tilt, but the forward/aft or N/S stick tilt).

Consider a rotor head on a post in a wind tunnel. Spun up by a prerotator to 300 rrpm. The disk of rotation of the teeter bolt and the disk of rotation of the blade tips are completely parallel to eachother. (this tip path is, my terminology here, “the ideal tip path”).

(and I know the tunnel does not work for the ‘up and away’ action, but it does not get that far)

Lock the rearward tilt of the rotor head so the ideal tip path is tilted upward into the airflow the required amount for flight, then instantly (and theoretically speaking!) turn on an 80 mph (130 km/hr) wind (=forward airspeed).

The advancing blade then wants to set off on a climbing course (and the retreating a diving one, I won’t keep stating this opposite reaction) but is restricted by the disk of rotation of the teeter bolt. (ie the airfoil/blade is compelled to remain in a parallel alignment to the disk of rotation of the teeter bolt)

Which is in turn maintained by the control input of the N/S position of the stick.

Due to the angle that this disk of rotation of the teeter bolt is held, the advancing blade cannot increase it’s AOA in response to a 300 mph (450 Km/hr) wind on its nose and an 80 mph (130 km/hr) wind hitting it’s underside at a 2 (edit 12) degree angle, and to then embark on the spiraling upward journey it wants to take, so has no choice but to climb (levitate) across the “ideal tip path” (and yes, thereby reducing it’s AOA* in relation to the airstream – it is rising say 2 meters (edit - by Chris' later comments, the teeter angle is 2* or less, so this rise is only about 20 cm (8 in) for an * m (24 ft) rotor.......?) in one tenth of a second, so there is an airspeed component at right angles downward to the ideal tip path of about 45 mph (edit- only 4.5 on the recalc??), so there develops a downward airflow vector).

The major point at which the advancing (and retreating) blade want (and fail!) to exert their will on the teeter bolt rotational disk, by changing pitch in the direction they want to, is of course at the 3 and 9 O’clock positions, so the advancing blade is forced on a lower path to 12 oclock than the one it intended to take. Ensuring the disk remains level (and a gyro would fly straight). The same but opposite effect is occurring to the retreating blade.

Now the teeter bolt rotation disk and the actual tip path disk are no longer exactly parallel. This is allowed mainly due to flexing of the blades, (edit, and the teeter, the freedom from the control inputs in the 12/6 position)(and must contribute to the 2/rev shake, two different masses rotating on different disk planes, and this difference varies with control inputs**)

At this point the blade tips are levitating across the ideal tip path at about 3 oclock on the advancing side, reaching a peak at the front (near 12 O’clock), and are doing the opposite on the retreating side, reaching a lowest point at the back (ie 6 oclock)

Gyroscopic forces play a huge part in this, there are tonnes of force involved, the rotor disk does not want to move easily, a (theoretical) weightless rotor would be responding dramatically to the changing airflows. (without getting into a gyroscopic precession discussion, the major forces are in fact being expressed in maximum displacement 90 degrees later)

I still think things are not completely in balance, as the argument between the blades and the teeter bolt (more specifically, the plane of rotation of the teeter bolt) (andother 2/rev contribution?) means an upward tilt force is being applied to the teeter bolt disk plane on one side, and the opposite on the other, and these forces are passed through the controls and airframe. (edit , I now think this is relatively minor, and it would just be felt through the stick)

There is also the issue of the frequently mentioned (again by CB) effect of the flexed up blade at the 12 Oclock position encountering a larger force of air on the underside than the other blade in the 6 oclock position (the built in coning, plus the upward blade flex, and the fact the rearmost blade encounters turbulent air, means that the blade at 6 oclock encounters a lesser upward force)

As the N blade begins it’s retreating path, it ends up flying a little higher at 9 O’clock than it had intended).

*I also understand that exactly what an airfoil does in different airstreams is very much dependant on it’s profile, and it’s chord balance, I just find it easier to think of AOA in stages.

** Without going into intricacies of coning and teeter height, and mass above and below the teeter bolt, which seems to be a very interesting subject.

(without mentioning flapping or precession)

I haven’t put any qualifiers in this (such as 'I think', or, 'perhaps') because it is hard enough to follow anyway, but that's what I'm saying all the way through.

.

 

[Resasi]

Thank you Mark for that synopsis of a very interesting thread.

I have always been in slight awe of the, to me, greater complexities of gyro aerodynamics and theory of flight. I do struggle somewhat to more fully comprehend it and am always grateful for the simplest graspable explanations that assume I know nothing.

 

[Passin' Thru]

I have always been in slight awe of the, to me, greater complexities of gyro aerodynamics and theory of flight. I do struggle somewhat to more fully comprehend it and am always grateful for the simplest graspable explanations that assume I know nothing.

Leigh, these "greater complexities of gyro aerodynamics" (and plain old weirdness) are what have kept me absolutely fascinated with these cantankerous contraptions all these years. :der:

I am not a great fan of gyros as flying machines, there much better ways to do that, but the intricacies of autorotation and its interactive dynamics are awesome! -And great exercise for the brain cells.:lol:

I have always appreciated Chuck B.'s ability to break down the most complex concepts and principals into managable elements that even I can understand.

BTW; One doesn't really need to understand all this to be a safe proficient flyer, just listen to your instructor, use common sense and behave yourself!

 

[ckurz7000]

Mark, aka grimwat, I didn't follow your post entirely but I see some discrepancies with actual phyiscs in it. For example, near the beginning you say:

A rotor would like to keep climbing up and to the left...

Actually, a two bladed rigid rotor that's spinning and moving throug the air horizontally (like a rotor on a gyrocopter) wants to tilt backward to equalize lift between its retreating half and advancing half.

There's numerous places in the forum that explain "blowback" angle and "blade flapping". You might want to read one of the recent threads on these topics.

-- Chris.

 

[fiveboy]

I have always thought of it this way. The big thing above me moves in a circle. The little thing in back of me moves in a circle. Somewhere between those two I move up. When the big thing above me wants me to go a way I dont want to, I invite it to go another way with this stick thing that attaches to it by a bunch of little sticks that are connected and it accommodates me by taking me with it. If I make the big one spin faster by adjusting that stick thing and the little extra knob that makes the small round thing behind me spin faster, it makes me go higher.

After that I think its sort of like magic.

 

[Passin' Thru]

I have always thought of it this way. The big thing above me moves in a circle. The little thing in back of me moves in a circle. Somewhere between those two I move up. When the big thing above me wants me to go a way I dont want to, I invite it to go another way with this stick thing that attaches to it by a bunch of little sticks that are connected and it accommodates me by taking me with it. If I make the big one spin faster by adjusting that stick thing and the little extra knob that makes the small round thing behind me spin faster, it makes me go higher.

After that I think its sort of like magic.

You got it! eace:

 

[Resasi]

So Chris, you are stating that a two blade rigid rotor behaves differently to the three blade rotor example that CB gave us?

Ohhh Robert you mean like that magic that happens when I push that switch thingy by the door and I get light.

Pete, just following what your instructor says, I'll say Amen to that. I knew a guy who didn't, and he isn't.

Behaving: well, I try, and succeed most of the time, as for the rest of it, luck, and someone watching over me I guess. But I do like to try to understand the magic as well as enjoy it.

And yes Chuck B is certainly one of the pearls that occasionally appears before us for our delectation, education and enlightenment.

As to flying machines. I guess they occupy a niche all of their own, it is all down to what is asked of them as to whether they are good or not.

 

[Passin' Thru]

As to flying machines. I guess they occupy a niche all of their own, it is all down to what is asked of them as to whether they are good or not.

Absolutely! Inspite of all that flapping, slapping and flailing around in a ridiculous excuse for a flying machine, for shear unadutrated airborne FUN, nothing else even comes close! NOTHING! :first:

 

[Mark E]

I have always been in slight awe of the, to me, greater complexities of gyro aerodynamics and theory of flight. ........ explanations that assume I know nothing.

Leigh, that is exactly what I think. It is very complex, and for me very difficult to conceptulize and understand ........ but the expanation is set out on the basis that I know nothing! I left out all the 'I thinks and maybes' because this stuff is hard enough to read at the best of times, especially with the language difficulties involved. (ie I don't speak gyro or aerodynamics, and only have a smattering of maths and physics... my English is passable)

Leigh, these "greater complexities of gyro aerodynamics" (and plain old weirdness) are what have kept me absolutely fascinated with these cantankerous contraptions all these years. .....

I have always appreciated Chuck B.'s ability to break down the most complex concepts and principals into managable elements that even I can understand.

BTW; One doesn't really need to understand all this to be a safe proficient flyer, just listen to your instructor, use common sense and behave yourself!

Pete, fascinated!? It is is keeping me awake at night!,
and true, CB's contribution here is enormous.
..... and I know I don't have to fully comprehend it, but I greatly enjoy the process of trying.

... discrepancies with actual phyiscs ... (re lift differential advancing vs retreating blade .. ed)

......"blowback" angle and "blade flapping". ....
-- Chris.

Thanks Chris,

Chris, I agree with you there, I have read many of those threads, but have always had trouble with the differential lift issue, I can see how the teeter pivot compensates for it, but in my mind it doesn't entirely. And there is that one major control effort going into the whole process all the time. The set angle of the rotor disk, (or the set of 'blowback angle'.)

I have always thought of it this way. The big thing above me moves in a circle. The little thing in back of me moves in a circle. Somewhere between those two I move up........After that I think its sort of like magic.

Fiveboy. That is all fundamental to the understanding of gyrocopter aerodynamics. ... And this is usually where my thought process leads me!

......stating that a two blade rigid rotor behaves differently to the three blade rotor example that CB gave us?

Leigh ... one of my (many!) issues. ???:suspicious:

 

[Mark E]

that imaginary gyro is flying level! (at last!)

that imaginary gyro is flying level! (at last!)

Chris - I thought hard about this and I think "flapping" and "Blowback angle" are pretty deceptive terms and found I could visualize it all a lot better by banishing those terms from my thinking!

Blowback angle I've replaced by "rotortip disk plane" - it is set at an angle to the approaching air stream and this is determined by the pilot's control inputs, he wants air going through his rotor. (whereas blowback I think implies the disk is tilted back in resistance to airflow....)

Flapping, I've just had to replace with teetering or teeter angle... in my mind it does not occur on the 3 to 9 axis at all (for a gyrocopter flying straight and level) - only cyclic pitch change occurs there. The teeter angle cannot be changing at this point if the rotor tip path is horizontal at the 3 / 9 position, and the rotor tips are on the same plane at this point! (which they need to be for the RTV to be vertical) Any flapping (ooops, I meant teeter angle change) occurs (in S and L flight) as the blades approach and pass the 12 / 6 position. (but even CB sometimes talks about the 3 /9 flapping.... I think?) (late edit - the blades are level at 3 / 9 as they are in the middle of the teetering process, by 12 / 6 they are at the limit of teeter and are reversing teeter direction!)

And the teetering rotor does not, by itself, deal with advancing and retreating blade lift discrepancies, it simply allows the pilot to input simultaneous cyclic pitch changes to both blades at the point he wishes to. In straight and level flight, this is at the 3 and 9 axis.

So, after all this, I found my imaginary gyrocopter was flying straight and level with absolutely no left tilt!

So, only one problem Leigh, what about the left upward inside loop of CBs 3 bladed rotor? Turns out he didn't say that at all. I just went back and read it again, and he said "do a loop"! No mention of left! My mind just read it that way!

:twitch:

 

[RotoPlane]

Mark - And the teetering rotor does not, by itself, deal with advancing and retreating blade lift discrepancies, it simply allows the pilot to input simultaneous cyclic pitch changes to both blades at the point he wishes to. In straight and level flight, this is at the 3 and 9 axis.

So, after all this, I found my imaginary gyrocopter was flying straight and level with absolutely no left tilt!

For quite a while I thought or understood that teetering was responsible for keeping the rotor from pulling the airframe to the left. This notion was causing problems with my rigid rotor control ideas. My understanding now gives me assurance that a rigid rotor will act the same as a teetering one, regarding dissimilar lift.

 

[All_In]

That whole flapping and lesser so blow back angle terminology had me thinking in the wrong direction. But it's finally starting to sink in. I also had a basic problem understanding where you were placing the clock to describe the 12 / 6 position. When you start out knowing less than nothing these term are deceiving to the novice. I still get confused but threads like these really help, thank you!

 

[Mark E]

CB has said all of these things 1000 times, but one thing that still confuses me is that he occasionally talks about lateral flapping in relation to the dissymmetry of lift issue.
I can't see that it occurs that way, but every other thing he has said has sunk in (I think)

 

[Mark E]

For quite a while I thought or understood that teetering was responsible for keeping the rotor from pulling the airframe to the left. This notion was causing problems with my rigid rotor control ideas. My understanding now gives me assurance that a rigid rotor will act the same as a teetering one, regarding dissimilar lift.

Ed, thinking of a rigid rotor in fact was what helped my thinking straighten out my imaginary Gyrocoptor!

I'd got to the point of accepting that the 3 /9 tip planes were on the same level (had to be, for it to fly straight), and the concept that the advancing blade was climbing and retreating blade was diving along the same plane (and thinking 'is this flapping?'). But anyway - there it was, flying level.
So thinking if that is all there is to it, a rigid rotor should fly straight and level if it is at exactly the same tilt, with the same blade pitch etc etc. Thinking the only problem with it would be if you wanted to change speed or direction. Thinking, but with today's computers and superfast servos, even that should be fixable.

But, what is the big difference between those two disks at the exact same angle and speed?

The teetering rotor is getting constant cyclic input on the 3 / 9 axis, changing the pitch of both blades at that point. In straight and level flight that input is reducing pitch on the advancing blade, and increasing it on the retreating blade, so preventing the 12 position from continuing to rise, and the 6 from continuing to fall. The pilot is making it fly straight and level.

So the rigid rotor will always pitch up. You can't make it fly straight and level. (Edit Incorrect statement! My definition of a rigid rotor is incorrect, but even with a "fan blade on a post type rotor" it can be done, it just requires the sufficient (much larger) application of force in the controls, not to mention the aerodynamic stresses involved ... further: see later posts)
:twitch:

 

[Resasi]

Turns out he didn't say that at all. I just went back and read it again, and he said "do a loop"! No mention of left! My mind just read it that way!

Thanks for the correction Mark, I guess I thought that as well.

 

[RotoPlane]

Mark - But, what is the big difference between those two disks at the exact same angle and speed?

The teetering rotor is getting constant cyclic input on the 3 / 9 axis, changing the pitch of both blades at that point. In straight and level flight that input is reducing pitch on the advancing blade, and increasing it on the retreating blade, so preventing the 12 position from continuing to rise, and the 6 from continuing to fall. The pilot is making it fly straight and level.

So the rigid rotor will always pitch up. You can't make it fly straight and level.

You can make a rigid rotor fly straight and level….if you tilt the hub and mast assembly in pitch (move the RTV fwd and aft). This can be done automatically but for other control reasons, I am moving the RTV semi-automatically, with a trim button switch on the cyclic stick. RTV movement in roll is not necessary.

 

[Mark E]

You can make a rigid rotor fly straight and level….if you tilt the hub and mast assembly in pitch (move the RTV fwd and aft). ... RTV movement in roll is not necessary.

Ed, I can't make it work for me (only in my mind, I'm saying!). It will need cyclic pitch control or for sure it runs into the dissymetry of lift issue, so will want to pitch up more with speed. But for a certain set of parameters (set speed, airfoil, rotor disk angle, flexible blades ) with a large stabilizer, you could get it flying, but I'd think it would be draggy, and have a pretty narrow flight envelope. (Edit ............. - pls see posts below!)

But... that's just me, at this stage of my thinking ... I hope I don't have to jump back through that mental hoop again hwell: I'd like to hear more on it.

 

[RotoPlane]

The way I explain things Mark, it would be hard to understand . Let's say you are flying straight and level in trim, and you apply more throttle….the gyro will want to climb. You don't want to climb so you push the cyclic forward. Now because this is a rigid head, the blade tip plane will tip further forward but the head and mast assembly will be held back by the airframe and horizontal stab. In other words the RTV has tilted forward with the tip path. Now to re-trim, you release the mast assembly (with the button switch) and the RTV will pull and tilt the mast forward until it aligns again with the RTV. The mast is snubbed with a hydraulic cylinder and flow orifice….and its movement is controlled by a solenoid valve….which is activated by the button on the cyclic. There….you've just read the best I can do.

 

[birdy]

It will need cyclic pitch control or for sure it runs into the dissymetry of lift issue, so will want to pitch up more with speed.
Far be it for me to explain, coz iv just been educated meself, but ill hava go.

Mark, a gyro and a ridged rotor account for the adv and retreating ASs the same way, only the method is different.
Your gyros allow the difference to balance with the teeter bolt hinge, while the ridged rotors cyclic input comes from the swashplate, which decreases the adv blades pitch and increases the retreating blades pitch.

 

[ckurz7000]

Right on, birdy. -- Chris.