Question on Retreating Blade Stall

[cburg]

Cross-Posted from another forum same subject:

 

bryancobb

--------------------------------------------------------------------------------

GrantR,

 

On a forum like this, you will get opinions from dozens of ill-informed/confused posters who tangle you up in confusion. I may be one of these.

Nevertheless, I'm going to regurgitate what I have been taught and understand. I will limit my post to a 2-blade, tetering, under-slung, pre-coned rotor system.

* Any airfoil has a "set-in-stone" angle of attack, beyond which it will stall. Somewhere around 18 degrees, I think.
* Anything that puts the rotor blades beyond that will cause it to stall in that portion of the disk where it exceeds that angle.
* Whether or not a rotor is being driven or autorotating is irrelevant. It only matters that the critical angle of attack is exceeded.
* Comparing AOA's of the retreating half of helicopter vs. gyro rotors, there is a huge difference. Most helicopters have <->twist. Most gyros have (+)twist.
* In level flight, the retreating half and advancing half of the disk must make exactly equal amounts of lift.
* To make that happen, designers attach the rotor so it can see-saw. While see-sawing, if a blade goes downward, its incidence increases and if going upward, it decreases.
* That's purely mechanical. It's commonly called Delta-3. The same also happens aerodynamically since relative wind direction changes as a blade ascends or descends.

Now let's look at things that can cause blade on the retreating half of the rotor disk to exceed its critical angle of attack AT THE TIP. The first few are things we all agree on.
* Thin air (High Density Altitude)
* Low Rotor RPM
* High Airspeed
* High Gross Weight (debatable on gyros because I have been told that more weight = more RPM)
* Turbulence and/or bumpy air
* Abrupt maneuvering
* Improper rigging or badly designed rotor system

In post #32, cburg posted some pictures that are the same exact ones I have seen for decades. The text with these pictures draws a clear distinction between the inner stalled portion of the retreating half of the disk and the outer portion of the disk stalling. The inner stalled portion, which grows outward as airspeed increases IS NOT included in the definition of retreating blade stall. Retreating Blade stall has not occurred until THE TIP OF THE RETREATING BLADE STALLS.

I have been flamed on here several times for arguing that higher forward speeds need more right cyclic, even in gyros. It looks like to me, that even in gyros, the inner stalled portion of the retreating half of the disk grows when airspeed increases. If no right cyclic is needed, the teter hinge is well designed so that the hinge does the job instead of cyclic input.

The bottom line, as I understand it, is...retreating blade stall HAS NOT HAPPENED UNTIL THE TIP OF THE RETREATING BLADE STALLS. As I have been taught, it is easy to recognize because a very noticeable vibration/shake/buffet happens and is followed by a significant pitch-up/roll left. Also, as I understand it, a gyro rotor with its massive amount of drag, and the fact that this drag increases as a SQUARE of Airspeed, I would not think retreating blade stall is likely for an autorotating gyro rotor?

For the ones who was thinking the retreating half flaps upward...That is not true because the retreating half must make equal lift with the advancing half. To do that, since the retreating half experiences a slower relative wind, it must increase AOA. To do that, it must flap DOWNWARD.

________________
Bryan Cobb, Boiler/Presure Vessel Designer, AutoCAD Draftsman
Commercial Pilot, Rotorcraft/Helicopter, US Army Trained, Royal Blue Flight, Class 87-17, TH-55, UH-1H, CH-47C
US Air Force Munitions Maintenance 46170 1979-1986
Commercial Instrument Pilot, Fixed Wing/SEL
CFI, Sport, Fixed Wing, Tailwheel

[helonorth]

I would say the confusion for the flat earthers around here is they don't understand the angle of incidence vs. angle of attack change with flapping. If they did, there would be no argument.

[HighCountry]

i have no idea, but i do know i can make the blades make a helluva noise if i want to.

 

maybe you know?

Again, they have nothing to do with each other. Sorry if I misunderstood your post #42. I thought you were relating flap and slap. Up until then I was just enjoying the entertainment of the debate but that thought lost me.

 

Let the entertainment continue but let's differentiate between the aerodynamic tendency to return to equilibrium when disturbed versus pilot control inputs. Most of what I'm reading both sides are correct but your talking about 2 different things.

[Eric Hunt]

That re-post from Cburg says:

 

 

For the ones who was thinking the retreating half flaps upward...That is not true because the retreating half must make equal lift with the advancing half. To do that, since the retreating half experiences a slower relative wind, it must increase AOA. To do that, it must flap DOWNWARD.

Of course the retreating side has to make lift - that is why the cyclic is putting the maximum pitch onto the blade to get that lift. And all it takes is one look at a rotor in flight to see that the retreating blade is CLIMBING to reach its highest point over the tail. It is also why the advancing blade has the minimum pitch, to reduce the lift that its extra relative airflow generates. It is flapping down towards the front.

 

It sure as heck isn't flapping down. It's flapping up at its greatest rate at 90 left. It is not a case of "It must flap downward", it is a case of "it follows the swash plate, increases pitch, and flaps upward."

 

A lot of people here are taking one part of aerodynamics - that a gust of wind causes the advancing blade to flap up and the lift is equalised by the changes in angles of attack - and then conveniently forgetting that this action would leave the disc tilted backwards, so the aircraft will fly backwards.

 

The pilot uses cyclic to overcome flapping, to tilt the disc forward, and fly forward.

 

On the Huey / B206/ B47 the flapping hinge is the teetering hinge. One flaps up, the other flaps down. The R22 has a slightly different setup but is almost the same.

[UH60L-IP]

Wow, I believe I am dumber for having read this thread. I award each of you no points, and may God have mercy on your souls.

 

This is basic aerodynamics yet you are confusing literally everything - you can't even seem to separate dissymmetry of lift from retreating blade stall. I realize this is a discussion forum, but I don't think giving a correct answer now will help any of you. I can hear Ray Prouty crying.

 

You each need to look up plane of rotation, relative wind, resultant relative wind, angle of incidence, angle of attack, and total aerodynamic force for starters. But mostly, you must determine how induced flow modifies your angle of attack (especially considering the errant dissymmetry of lift excursion). If you were to learn nothing more than how induced flow modifies your angle of attack and extract from the lift equation that lift moment must be equalized over the advancing/retreating sides (in level flight), you would realize that the advancing blade indeed flaps up, and the retreating blade flaps down. It is impossible for it to be otherwise without rolling.

 

I must go take a shower now. While I do, you must each write the lift equation 50 times.

[Whiteshadow]

I can't believe what this has turned into. Every answer to every question doesn't have to go as deep as possible, and start as far back as possible. We all know that there are several things that lead up to RBS, but the question asked was quite a simple one, and has a simple answer. I think the answer has been given…………..on page one.

[eagle5]

I can't believe what this has turned into. Every answer to every question doesn't have to go as deep as possible, and start as far back as possible...

Awh come on! I was just about to explain what the atoms in retreating blade are doing at the quantum level.

[Eric Hunt]

A blackhawk pilot says:

 

 

you would realize that the advancing blade indeed flaps up, and the retreating blade flaps down. It is impossible for it to be otherwise without rolling.

You must be having a Michael Jackson Moonwalk moment running through your brain.

 

You know, the bit where it looks like his feet are making him walk forwards, but in fact he is going backwards.

 

So you still think that the advancing blade is flapping up, when it is painfully obvious that it is flapping down.

 

Hey, let's say "The advancing blade is flapping up." Therefore, the highest part of its travel would be at the front of the disc, right? Which way is the lift vector pointing now? Backwards.

 

To get forward flight, the disc tilts forward, the lift vector points forward, and for the disc to tilt forward, the advancing blade has to flap down, and the retreating blade has to flap up.

 

How about we shift the focus a little bit here. How do we turn right? Lean the cyclic to the right, the disc tilts right, the blade reaches its lowest point of travel on the right side, the lift vector points right, and we fly to the right. Has the blade on the right side moved DOWN to make that happen, or did it move UP?

 

Over to you, Michael Jackson...

[Guest pokey]

Eric, i see the confusion here, you are thinking disc tilting as flapping, cyclic does control the tilt of the disc and the highest pitch is on the retreating side--which in turn tilts the rear portion of the disc up. This is not the flapping that some of us others are talking about, the flapping we are talking about is due to the differential velocity across the advancing and retreating blades that will control dissemetry of lift.

[Whiteshadow]

Thank you pokey. I was trying to come up with a way to say that. I believe Mr. Hunt is referring to the tip-path plane of the rotor disc.

[Eric Hunt]

It is all part of the same thing, girls!

 

When, in your mind, a blade flaps up, does it actually move up? From what your Michael Jackson statements have shown so far, you say that the advancing blade is "flapping up to equality" even though it is actually flapping down.

 

Yes, when I say "the disc" it is the same as the tip path plane, which is parallel to the plane of rotation.

 

When the disc or TPP of a teetering head tilts forward, or sideways, it is flapping - because it only has a flapping hinge, also known as a teetering hinge (yes it has a feathering hinge as well).

 

What you find impossible to understand is that the cyclic is used to overcome the flapping which is due to differing tangential velocities. Cyclic lets the pilot overcome dissymmetry of lift. If he didn't use it, if he just locked the cyclic and didn't move it, he enters Stick Fixed Dynamic Instability and will eventually crash. To stay alive, and to make the aircraft continue forwards (instead of continually flapping away from the relative airflow) he puts in forward cyclic which adjusts the pitch and the angles of attack and Bob is his uncle.

 

Am I dealing with trolls here, or are you Wannabes, or Students, or Licenced Pilots, or even flight instructors? At a guess, UH60L-IP might be a military instructor, in which case I am sadly disappointed that he is passing on such a poor level of understanding to students.

[Guest pokey]

my mistake Eric, you really don't get it, take a good look at Chris's diagrams, especially the 2 that deal with cyclic pitch and the other that deals with flapping. And uh60's little quote of yours is absolutely correct. (from your post number 58)

 

edit to add post number

Edited August 7, 2014 by pokey

[Guest pokey]

It is all part of the same thing, girls!

 

Bob is his uncle.

 

Am I dealing with trolls here, or are you Wannabes, or Students, or Licenced Pilots, or even flight instructors? At a guess, UH60L-IP might be a military instructor, in which case I am sadly disappointed that he is passing on such a poor level of understanding to students.

 

i am fairly sure you are just F'n with us Eric. ok ok we all had our laugh now

Edited August 7, 2014 by pokey

[Eric Hunt]

Pokey says:

 

 

Eric, i see the confusion here, you are thinking disc tilting as flapping,

OK, Pokey, what are the three axes about which the blade can move?

 

Up and down, called FLAPPING

Fore and aft, called Lead/Lag or Dragging

Rotation, called Feathering.

 

So when a blade moves up or down, it is flapping.............by definition.

 

Doesn't matter a rat's patootie whether it is caused by a wind gust or a cyclic or collective input, it is still flapping.

 

This is a pretty basic definition.

 

Yeah, you must be right, I will let you think I am just teasing you. When you eventually get some time up, your eyes might open a bit.

Edited August 7, 2014 by Eric Hunt

[Guest pokey]

:

OK, Pokey, what are the three axes about which the blade can move?

 

Up and down, called FLAPPING

Fore and aft, called Lead/Lag or Dragging

Rotation, called Feathering.

 

 

 

yes, those are the 3, now? can you tell me why bell and similar 2 bladed rotor systems do not have a lead/lag hinge?

[Pohi]

I think you are just feeding a troll, pokey. Apparently it's really hungry.

[Guest pokey]

I think you are just feeding a troll, pokey. Apparently it's really hungry.

 

i'm not sure Pohi.

 

i really think it is a communication problem,,,,,,,,,,at least i hope that is all it is

[Eric Hunt]

You can't be serious, asking an incredibly basic question like that. You really are fishing for a bite, I will let this one go.

[CO2WO]

Alright Gents, I've read the entire thread and I honestly think this boils down to 1. A miscommunication and 2. A fundamental flaw in helicopter aerodynamics training.

 

You guys have beaten home the following facts:

1. The cyclic controls intentional blade flapping.

2. The flapping hinge also allows for unintentional flapping (wind, etc...)

3. A rotor disc at speed will create dissymmetry of lift

 

The disconnect here is that the ONSET of RBS is taught to us as a physical change to the rotor disk when in actuality it's more or less a theoretical change since cyclic adjustments negate any actual TPP change(s)*** UNTIL the retreating blade actually stalls THEN AND ONLY THEN does the rotor disk flap upwards due to the loss of lift on the retreating side.

 

***The advancing blade may cone a bit more, but the rotor disk is still ultimately flapping down to the position the cyclic tells it to.

 

Eric and Pokey are both right respectively; however, at different points in the RBS spectrum I.e., Onset of RBS versus Actual Stall.

[CharyouTree]

Eric: A blade doesn't flap due to cyclic movement. It feathers due to cyclic movement. (It cyclically feathers, as a matter of fact.)

 

I see that this thread has gotten off track from retreating blade stall a bit, and I'm not even going to touch it. However, I really feel the need to try and help figure out the issues with blade flapping going on here.

 

Some questions:

 

1) In forward flight, would you agree that a rotor disk experiences a dissymmetry of lift (that is corrected for...)?

 

2) If so, what corrects this dissymmetry of lift?

 

3) Which half of the blade (retreating, or advancing) experiences a loss of lift due to the forward flight, and (bonus) why?

 

4) When a blade flaps up, what happens to the induced flow velocity, and what does that do to the angle of attack?

 

5) Conversely, when a blade flaps down, what happens with the induced flow velocity, and angle of attack?

 

 

 

That's all I've got for now. I'll be back for more when these are answered.

[Eric Hunt]

"Eric: A blade doesn't flap due to cyclic movement. It feathers due to cyclic movement. (It cyclically feathers, as a matter of fact.)"

****OK, Mr Tree, absolutely I agree that cyclic makes the blades feather. But what happens then? Say you are on the ground at 100%RRPM, disc level, with absolutely no wind, no advancing or retreating blade. You poke the cyclic forward to feather the blades. The disc tilts forward. The blade on the right is flapping down, and the blade on the left is flapping up. How does your theory of "only forward speed makes a disc flap" deal with this? Yes, you feathered the blades, but the result is : the disc flaps.

Go back to your first ever lesson, Effects of Controls, and the board brief that preceded it, and the instructor showed you that the Primary Effect of Cyclic is pitch and roll (Attitude), with the desired performance being Airspeed and Direction. The disc flaps, the fuselage follows.

"I see that this thread has gotten off track from retreating blade stall a bit, and I'm not even going to touch it. However, I really feel the need to try and help figure out the issues with blade flapping going on here.

Some questions:

1) In forward flight, would you agree that a rotor disk experiences a dissymmetry of lift (that is corrected for...)?"

***** There is only a dissymmetry if you don't stop it.

If you don't stop it, the machine will experience flapback, it will climb, lose speed, yaw as it slows, then go back the way it came, but then flap back again, each time getting worse, until it either crashes, or THE PILOT DOES SOMETHING WITH THE CYCLIC to stop it. Remember the inflight demonstrations for Effects of Controls, lesson number 1? Lock the cyclic, you die.

"2) If so, what corrects this dissymmetry of lift?"

*****The pilot uses cyclic to ensure that the aircraft keeps going the way he wants it to.

Yeah, yeah, the answer you are looking for is "flapping to equality", but that only happens if you allow it to. If you let it "flap to equality", your disc is now pointing backwards, so you will not be in forward flight much longer.

"3) Which half of the blade (retreating, or advancing) experiences a loss of lift due to the forward flight, and (bonus) why?"

****How did you get into forward flight anyway?? You poked the cyclic forward to tilt the disc forward, and forward flight developed. If it started to roll, you stopped it WITH CYCLIC, if it started to pitch up, you stopped it WITH CYCLIC. So the simple answer is:

There is no lift differential - each side has exactly the same amount of lift. Because the pilot does not allow it to change. Otherwise, you have an unbalanced force, and the disc will roll or pitch. And the reason there is no differential of lift is that the cyclic feathering has lowered the angle of attack on the advancing side, which is flapping down, (to make up for the greater relative airflow) and increased the angle of attack on the retreating side, which is flapping up, to allow for the reduced relative airflow. You are still in forward flight, with the disc tilted forward, and the advancing blade is now flapping down, and the retreating blade is now flapping up.

Open your eyes and look.

"4) When a blade flaps up, what happens to the induced flow velocity, and what does that do to the angle of attack?"

***No debate here, and that is why the cyclic is putting such big pitch angles on the retreating blade to overcome this reduction in angle of attack from the extra IF while it is flapping up to get to the high point over the tail, that we get to the original question - RETREATING BLADE STALL. How the heck does YOUR theory account for the pitch angle reaching such a high figure?

"5) Conversely, when a blade flaps down, what happens with the induced flow velocity, and angle of attack?"

***Same story.

You can come back now.

If your instructors didn't show you all this in Lesson 1, then he/she was a poor instructor.

Edited August 9, 2014 by Eric Hunt

[Pohi]

I really hope you made a bet with somebody to see how many different people you could get to explain basic aerodynamics to you. That's the only logical explanation, nobody who is a pilot could be as clueless as you are pretending to be. I'm impressed with the numbers you are putting on the board.

[Eric Hunt]

Pohi, how about YOU put your answers up to the same questions?

 

They will come out as flappingtoequalityflappingtoequalityflappingtoequalityflappingtoequalityflappingtoequalityflappingtoequalityflappingtoequalityflappingtoequalityflappingtoequalityflappingtoequalityflappingtoequality with no room to consider what is really happening.

 

Did anybody other than iChris and Nearly Retired actually LEARN aerodynamics?

[CharyouTree]

 

"Eric: A blade doesn't flap due to cyclic movement. It feathers due to cyclic movement. (It cyclically feathers, as a matter of fact.)"

****OK, Mr Tree, absolutely I agree that cyclic makes the blades feather. But what happens then? Say you are on the ground at 100%RRPM, disc level, with absolutely no wind, no advancing or retreating blade. You poke the cyclic forward to feather the blades. The disc tilts forward. The blade on the right is flapping down, and the blade on the left is flapping up. How does your theory of "only forward speed makes a disc flap" deal with this? Yes, you feathered the blades, but the result is : the disc flaps.

Go back to your first ever lesson, Effects of Controls, and the board brief that preceded it, and the instructor showed you that the Primary Effect of Cyclic is pitch and roll (Attitude), with the desired performance being Airspeed and Direction. The disc flaps, the fuselage follows.

"I see that this thread has gotten off track from retreating blade stall a bit, and I'm not even going to touch it. However, I really feel the need to try and help figure out the issues with blade flapping going on here.

Some questions:

1) In forward flight, would you agree that a rotor disk experiences a dissymmetry of lift (that is corrected for...)?"

***** There is only a dissymmetry if you don't stop it.

If you don't stop it, the machine will experience flapback, it will climb, lose speed, yaw as it slows, then go back the way it came, but then flap back again, each time getting worse, until it either crashes, or THE PILOT DOES SOMETHING WITH THE CYCLIC to stop it. Remember the inflight demonstrations for Effects of Controls, lesson number 1? Lock the cyclic, you die.

"2) If so, what corrects this dissymmetry of lift?"

*****The pilot uses cyclic to ensure that the aircraft keeps going the way he wants it to.

Yeah, yeah, the answer you are looking for is "flapping to equality", but that only happens if you allow it to. If you let it "flap to equality", your disc is now pointing backwards, so you will not be in forward flight much longer.

"3) Which half of the blade (retreating, or advancing) experiences a loss of lift due to the forward flight, and (bonus) why?"

****How did you get into forward flight anyway?? You poked the cyclic forward to tilt the disc forward, and forward flight developed. If it started to roll, you stopped it WITH CYCLIC, if it started to pitch up, you stopped it WITH CYCLIC. So the simple answer is:

There is no lift differential - each side has exactly the same amount of lift. Because the pilot does not allow it to change. Otherwise, you have an unbalanced force, and the disc will roll or pitch. And the reason there is no differential of lift is that the cyclic feathering has lowered the angle of attack on the advancing side, which is flapping down, (to make up for the greater relative airflow) and increased the angle of attack on the retreating side, which is flapping up, to allow for the reduced relative airflow. You are still in forward flight, with the disc tilted forward, and the advancing blade is now flapping down, and the retreating blade is now flapping up.

Open your eyes and look.

"4) When a blade flaps up, what happens to the induced flow velocity, and what does that do to the angle of attack?"

***No debate here, and that is why the cyclic is putting such big pitch angles on the retreating blade to overcome this reduction in angle of attack from the extra IF while it is flapping up to get to the high point over the tail, that we get to the original question - RETREATING BLADE STALL. How the heck does YOUR theory account for the pitch angle reaching such a high figure?

"5) Conversely, when a blade flaps down, what happens with the induced flow velocity, and angle of attack?"

***Same story.

You can come back now.

If your instructors didn't show you all this in Lesson 1, then he/she was a poor instructor.

 

 

There seems to be some confusion with terms here, and that's what I'm trying to solve. Either that, or you really don't understand basic aerodynamics. (Or, as a few people have posited, you're playing a game.)

 

Please, try to play along, and just answer the (what I hope are) simple questions with simple answers. Particularly 3, 4, and 5. I'm trying to find our "aha moment" here, and sort out if we're even talking about the same thing. Maybe if we're lucky, one or both of us can learn something, which would be the ideal situation.

 

1) I agree that there's only dissymmetry if you don't stop it. But there has to be something for you to stop, and I tried to make sure my question covered that "that is corrected for". So... does dissymmetry of lift exist. i.e. with no corrective action by the rotor system, or the pilot, does a rotating wing, that has air flowing over it from one direction to the other (and therefore has a retreating, and advancing side) experience a loss of lift on one side, due to a change in wind velocity? Put another way, "is there a differential of velocities between the retreating and advancing halves of the blade, due to rotational velocities and relative wind due to forward flight"?

 

2) Actually, I was looking for "blade flapping and cyclic feathering", but I would have accepted many variations of the same. I've never heard the phrase "flapping to equality".

 

3) See Number 1. If uncorrected due to flapping, feathering, or PFM, which half has more, and which half has less lift? Advancing, or retreating? What causes that?

 

4 and 5, you didn't even bother answering. Simple questions, with simple answers. What does flapping up do to the induced flow and angle of attack, and what does flapping down do to induced flow/angle of attack?

 

Every instructor I've ever had (a dozen or so officially, off the top of my head? Many more that I've studied and worked with in one way or another) has instructed me in this way, and as an instructor, I've done the same.

Edited August 9, 2014 by CharyouTree

[Guest pokey]

good luck Tree,,,we all tried, he's YOUR baby now,,, and BTW? flapping to equality was a 1953 movie starring Burt Lancaster, Montgomery Clift and was based upon a James Jones novel,,,,

 

errr no no,, that was flapping to eternity----my mistake