Question on Retreating Blade Stall

[Eric Hunt]

There has never been a problem with the following simple premise:

 

When there is a puff of wind across a disc, one blade will become an advancing blade, with more relative tangential flow (same plane as rotational airflow), and on the other side of the disc will be a retreating blade with less tangential flow.

 

This change in relative airflow will create a difference in the lift between the two sides. The advancing side will have more lift, and will want to climb. The retreating blade will have less lift, and will want to descend. This is an unbalanced force, and the disc will start to accelerate up on the right and down on the left.

 

This acceleration causes an increase in the induced flow on the advancing side, reducing the lift generated. On the retreating side, the induced flow is reduced, giving an increase of lift. If we are really lucky, the blades will now find that they are back to where they were previously, with the lift on the advancing side being equal to the retreating side.

 

Balance has been restored. This is called "Flapping to Equality."

 

Alright so far?

 

But now we find that the disc is tilted backwards. Doesn't matter if we are sitting on the ground. DOES matter if we want to fly, or even stay in the hover in one place.

 

What do you do next?

 

Step up gentlemen, and we will walk through this one step at a time.

[eagle5]

This retreating blade has stalled. Please, please, please, just chop the throttle, yank up hard on the collective and put it out of its misery!

[Guest pokey]

http://www.decodedscience.com/how-helicopters-fly-without-turning-over-and-crashing/20707

 

apparently the term "flapping to equality" really does exist, i just never heard that term in my 30 years of helicopter experience

 

i think we are making progress with this post now tho

 

 

OH ! and btw? notice that the autogyro was given credit

 

now if i could only find a reference to this new term:

"Stick Fixed Dynamic Instability" ---from post #61

Edited August 9, 2014 by pokey

[aeroscout]

This retreating blade has stalled. Please, please, please, just chop the throttle, yank up hard on the collective and put it out of its misery!

Maybe we could merge it with "Fudging the Logbook" ?

 

Other than that it has been a fun and informative if not refreshing thread.

[Guest pokey]

 

 

 

But now we find that the disc is tilted backwards. Doesn't matter if we are sitting on the ground. DOES matter if we want to fly, or even stay in the hover in one place.

 

What do you do next?

 

Step up gentlemen, and we will walk through this one step at a time.

 

this is the point where the wheels come off, the disc is NOT tilted backwards, the lift across both sides of the disc has been equalized, now the cyclic feathering can 'take over' and continue in forward flight

 

if what you are trying to make us believe?--why did the early gyrocopters, and helicopters roll over on the retreating side, rather than on their backs?

Edited August 9, 2014 by pokey

[Eric Hunt]

 

 

this is the point where the wheels come off, the disc is NOT tilted backwards, the lift across both sides of the disc has been equalized, now the cyclic feathering can 'take over' and continue in forward flight

Geez, Pokey, first you say that the advancing blade HAS to flap up to equalise the lift, and now you say that it doesn't?

[Guest pokey]

Geez, Pokey, first you say that the advancing blade HAS to flap up to equalise the lift, and now you say that it doesn't?

 

i would like you to show me where i ever said that the advancing blade does anything BUT flap up?

 

just when i think you my be on the up&up?? you start playing this game again

Edited August 9, 2014 by pokey

[eagle5]

Maybe we could merge it with "Fudging the Logbook" ?

 

Other than that it has been a fun and informative if not refreshing thread.

That one was epic! Though I don't think these guys have enough material to take this one that far.

[iChris]

 

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.

 

 

OH ! and btw? notice that the autogyro was given credit

 

now if i could only find a reference to this new term:

"Stick Fixed Dynamic Instability" ---from post #61

 

That change in pitching moment due to the change in forward speed is known as Speed Stability or Static Longitudinal Stability. All good helicopters have it (§27.173). In free flight the change in flapping with increasing forward speed is stabilizing since it produces a nose-up moment causing the helicopter to pitch up and slow down to its original speed.

 

We know our helicopter has Positive Speed Stability since it requires us to move the cyclic stick forward to keep the helicopter trimmed as we increase speed.

 

The Cierva’s invention of the flapping hinge did not cure all rotor riddles. Moreover, Cierva’s rotor only provided lift for his small autogyro, the helicopter’s rotor needs to provide both lift and thrust, in addition to, providing a means of controlling and directing the lift and thrust. A means of control called cyclic pitch was developed.

 

Unlike the autogyro, in our system the pilot cyclically changes the pitch of the blades about the feathering axes by tilting a mechanism known as a swashplate. When the swashplate is perpendicular to the rotor shaft, the blade angle is constant; however, when the swashplate is tilted via cyclic, the blades pitch will go through one complete feathering cycle during each revolution.

 

The problem is most of your training books for simplicity; limit their discussion to only blade flapping as a result of forward speed in isolation. The Helicopter Flying Handbook on page 2-19 states; “ In reality, the main rotor blades flap and feather automatically to equalize lift across the rotor disk.”

 

The following statement, “The retreating blade is flapping down and the advancing blade is flapping up in an attempt to equalize the lift across the rotor disc,” was used for simplicity to explain how blade flapping changes AOA. Don’t lock yourself into insisting that the advancing blade is flapping up in forward flight when we must add forward cyclic to cause the disk to flap forward to trim the helicopter in equilibrium. Blade flapping and cyclic feathering (pitch variations) correct the lift asymmetry in forward flight as shown in the figure below.

 

The truth is a little more complex as you can see from the AOA distribution across the rotor disk.

 

 

 

Edited August 11, 2014 by iChris

[Eric Hunt]

Longitudinal stability is only part of the equation.

 

To even consider dynamic stability, one must first have static stability - does an object try to return to its original position when it is disturbed? If it does try to return, that is static stability - think of a pyramid on its base. If it diverges away from its undisturbed position, it is statically unstable, like pyramid balanced on its point. If it just settles into a new position, like a ball rolled on the floor, it is neutrally stable.

 

To have any kind of static stability, a helicopter must be "stick fixed", not "stick free" - if you let the cyclic go, it will flop around the cockpit, but at least with the stick fixed it will have static stability.

 

Add some dynamic airflow (flight) and this tendency to return to its original position becomes an unstable oscillation, it overshoots its position each time and diverges to a crash.

 

Pokey, you said:

 

 

the disc is NOT tilted backwards,

How can the advancing blade flap up without making the disc tilt backwards? If the disc doesn't tilt, then the blade isn't flapping.

[CharyouTree]

There has never been a problem with the following simple premise:

 

When there is a puff of wind across a disc, one blade will become an advancing blade, with more relative tangential flow (same plane as rotational airflow), and on the other side of the disc will be a retreating blade with less tangential flow.

 

So what if that "puff of wind" is coming straight off the nose, at 120 kts (or whatever your forward IAS is)?

 

 

This change in relative airflow will create a difference in the lift between the two sides. The advancing side will have more lift, and will want to climb. The retreating blade will have less lift, and will want to descend. This is an unbalanced force, and the disc will start to accelerate up on the right and down on the left.

 

I can generally accept this. I understand it as "as the blade begins advancing (comes over the tail) it begins to encounter that tangential flow (another term I've never heard before this thread, but I assume we're using it as a vector to add our forward airspeed to our rotor velocity?) and that equates to a rise in V (from the lift equation). A rise in V results in increased lift, which effectively causes the blade to flap up. Flapping up increases the induced flow velocity, reducing the AOA, and thus lift. Additionally, due to the conservation of angular momentum, on a fully articulated rotor system, the blade leads forward.

 

The blade then comes across the nose, having an effective zero net gain/loss from forward airspeed, and now it starts becoming the retreating blade. Forward airspeed is now subtracted from rotational velocity, giving us our 3 no lift areas (reverse flow, negative stall and negative lift) towards the hub, we then get our positive lift, and (bringing it full circle) potentially a positive stall at the tip due to too high of AOA. Anyway.... that forward airspeed subtracted from rV gives us a reduced V, causing the blade to flap down, giving us a *reduced* induced flow velocity, increasing AOA and helping to equalize lift. As the blade becomes "straighter" the conservation of angular momentum comes back into play, and the blade lags behind."

 

Seems like about the same thing, but with a different order...

How'd I do? Have I been taught right, or did every one of my instructors and examiners do a poor job?

 

 

This acceleration causes an increase in the induced flow on the advancing side, reducing the lift generated. On the retreating side, the induced flow is reduced, giving an increase of lift. If we are really lucky, the blades will now find that they are back to where they were previously, with the lift on the advancing side being equal to the retreating side.

 

Balance has been restored. This is called "Flapping to Equality."

 

Alright so far?

 

Seems like it... but many posts ago, you specifically said that the retreating side flaps up, and the advancing side flaps down...which is where the confusion has been coming in.

 

But now we find that the disc is tilted backwards. Doesn't matter if we are sitting on the ground. DOES matter if we want to fly, or even stay in the hover in one place.

 

What do you do next?

 

Step up gentlemen, and we will walk through this one step at a time.

But the disc isn't tilted backwards. Just because the blade flaps down, doesn't mean we can't push it back forwards again. This is (how I understand it) why it takes increasing amounts of cyclic to go at an increasingly higher airspeed.

 

The way I taught it, going back to my last post (blade flapping and cyclic feathering being the "cure" to dissymmetry of lift) is that aerodynamics does its thing using flapping to correct, and we correct the flapping with cyclic feathering.

I think we just ran into some confusion of the use of the word flapping. I guess we'll find out, depending on how this gets torn apart.

[Eric Hunt]

You are so hung up on flapping to equality that you ignore the elephant in the room.

 

In forward flight (and you can only get into forward flight by pushing the cyclic forward) the disc is low at the nose and high at the tail.

 

But you continually say that on the advancing side the disc is flapping up, and on the retreating side it is flapping down, when even the elephant can see it is the opposite.

 

And the reason why the disc is low at the front?

 

Cyclic. The feathering action reduces the AoA on the advancing side, and increases it on the retreating side - look at the swash plate, look at iChris diagrams, look at the Huey video, they all show the same story. AoA on advancing side is lowest, due to forward cyclic, not "flapping to equality", and highest on the retreating side, causing blade stall if you go fast enough.

[Guest pokey]

You are so hung up on flapping to equality that you ignore the elephant in the room.

 

 

 

 

 

we said we never heard of that term "flapping to equality" so how can we be hung up on it?

[brian74]

1-33. The up and down movement of rotor blades about a hinge is called flapping (figures 1-18 through 1-22 ). It occurs in response to changes in lift due to changing velocity or cyclic feathering (figure 1-18 ). No flapping occurs when the tip-path plane is perpendicular to the mast. The flapping action alone, or along with cyclic feathering, controls dissymmetry of lift (section V ). Flapping is the primary means of compensating for dissymmetry of lift.

 

Figure 1‑18. Flapping in directional flight

1-34. Flapping also allows the rotor system to tilt in the desired direction in response to cyclic input. See figures 1-19 and 1-20, Figures 1-21 and 1-22, for depictions of flapping as it occurs throughout the rotor disk.

Figure 1‑19. Flapping (advancing blade 3 o’clock position)

 

 

Figure 1‑20. Flapping (retreating blade 9-o’clock position)

Figure 1‑21. Flapping (blade over the aircraft nose)

 

Figure 1‑22. Flapping (blade over the aircraft tail)

 

1-35. In the semirigid rotor system, a blade is not free to flap independently of the other blades because they are affixed through the hub. The blades form one continuous unit moving together on a common teetering hinge. This hinge allows one blade to flap up as the opposite blade flaps down, although blade flex limits the amount of blade flapping. In the fully articulated rotor system, blades flap individually about a horizontal hinge pin. Therefore, each blade is free to move up and down independently from all of the other blades. Aircraft design can reduce excessive flapping in several ways; for example, a forward tilt of the transmission and mast helps minimize flapping and installation of a synchronized elevator or stabilator (UH-60 and AH-64) helps maintain the desired fuselage attitude to reduce flapping.

[iChris]

I can generally accept this. I understand it as "as the blade begins advancing (comes over the tail) it begins to encounter that tangential flow (another term I've never heard before this thread, but I assume we're using it as a vector to add our forward airspeed to our rotor velocity?)

 

Tangental Flow / Tangental Velocity

 

http://youtu.be/ljuNEv2ofHo

[Eric Hunt]

For Pokey, who says "the disc is not tilted back".

 

Over the Aircraft Nose and Tail

1-109. Blade flapping over the nose and tail of the helicopter are essentially equal. The net result is an equalization, or symmetry, of lift across the rotor system. Up flapping and down flapping do not change the total amount of lift produced by the rotor blades. When blade flapping has compensated for dissymmetry of lift, the rotor disk is tilted to the rear, called blowback. The maximum upflap occurring over the nose and the maximum downflap occurring over the tail cause blowback. This would cause airspeed to decrease. The aviator uses cyclic feathering to compensate for dissymmetry of lift allowing him to control the attitude of the rotor disk.

[Eric Hunt]

For those who say the disc is NOT flapped down at the nose:

 

Cyclic Feathering

1-110. Cyclic feathering compensates for dissymmetry of lift (changes the AOA) in the following way. At a hover, equal lift is produced around the rotor system with equal pitch and AOA on all the blades and at all points in the rotor system (disregarding compensation for translating tendency). The rotor disk is parallel to the horizon. To develop a thrust force, the rotor system must be tilted in the desired direction of movement. Cyclic feathering changes the angle of incidence differentially around the rotor system. Forward cyclic movements decrease the angle of incidence at one part on the rotor system while increasing the angle in another part. Maximum down flapping of the blade over the nose and maximum up flapping over the tail tilt the rotor disk and thrust vector forward. To prevent blowback from occurring, the aviator must continually move the cyclic forward as velocity of the helicopter increases. Figure 1-56 illustrates the changes in pitch angle as the cyclic is moved forward at increased airspeeds. At a hover, the cyclic is centered and the pitch angle on the advancing and retreating blades is the same. At low forward speeds, moving the cyclic forward reduces pitch angle on the advancing blade and increases pitch angle on the retreating blade. This causes a slight rotor tilt. At higher forward speeds, the aviator must continue to move the cyclic forward. This further reduces pitch angle on the advancing blade and further increases pitch angle on the retreating blade. As a result, there is even more tilt to the rotor than at lower speeds.

This is all in your books, girls, not from my imagination.

Edited August 10, 2014 by Eric Hunt

[CharyouTree]

For Pokey, who says "the disc is not tilted back".

 

That's exactly what I said.

 

 

 

When blade flapping has compensated for dissymmetry of lift

 

Lest you forget that that part is in there... blade flapping is compensating for dissymmetry of lift, and we compensate for blade flapping with cyclic feathering.

 

You can't get blowback if the retreating blade isn't flapping down. You (seem) to be implying that the retreating blade flaps up... In fact, when you say it's "opposite" of flapping down, I can't see how you can take that any other way.

 

Yes, the blade flaps down, causing blowback, yes, the pilot pushes it forward again. If I'm not mistaken, this happens as we go through ETL and Transverse Flow (along with a right roll, that's corrected with left cyclic).

 

 

The end state of the tip path plane being lower in the front doesn't mean that the blade never flapped down on the retreating side.

 

It seems like we're all saying basically the same thing, except you're saying we're wrong about it.

[Eric Hunt]

Mr Tree,

 

You can't get blowback if the retreating blade isn't flapping down. You (seem) to be implying that the retreating blade flaps up... In fact, when you say it's "opposite" of flapping down, I can't see how you can take that any other way.

I am talking about the end state. The pilot has compensated for all the effects, and the aircraft is in steady unaccelerated level flight. The advancing blade, for whatever reason, is flapping down, and the retreating blade is flapping up. There is no other way that the disc can still be tilted forward and the aircraft flying forward.

 

I have always been discussing the end state, but people keep harking back to the initial transient state where flapback starts and the pilot hasn't done anything about it.

[Eric Hunt]

For those who have not heard of stick fixed stability, it has been around for a long time:

 

http://naca.central.cranfield.ac.uk/reports/arc/rm/2505.pdf

If it has never been demonstrated to you, your instructor is apparently inexperienced. Ask to fly with somebody with a bit more time up.

[helonorth]

For those who say the disc is NOT flapped down at the nose:

 

This is all in your books, girls, not from my imagination.

 

The guy that wrote that seems almost confused as you are. Flapping and feathering seem to be the same thing to you folks.

[Eric Hunt]

Helonorth, tell me then how the pilot can make the disc tilt in any direction without it flapping?

 

Remember that the definition of flapping is movement up and down about its flapping/teetering hinge.

 

Feathering puts pitch on the blade, which makes it flap. Even in nil wind.

[iChris]

 

I understand it as "as the blade begins advancing (comes over the tail) it begins to encounter that tangential flow (another term I've never heard before this thread, but I assume we're using it as a vector to add our forward airspeed to our rotor velocity?) and that equates to a rise in V (from the lift equation). A rise in V results in increased lift, which effectively causes the blade to flap up. Flapping up increases the induced flow velocity, reducing the AOA, and thus lift. Additionally, due to the conservation of angular momentum, on a fully articulated rotor system, the blade leads forward.

 

The blade then comes across the nose, having an effective zero net gain/loss from forward airspeed, and now it starts becoming the retreating blade. Forward airspeed is now subtracted from rotational velocity, giving us our 3 no lift areas (reverse flow, negative stall and negative lift) towards the hub, we then get our positive lift, and (bringing it full circle) potentially a positive stall at the tip due to too high of AOA. Anyway.... that forward airspeed subtracted from rV gives us a reduced V, causing the blade to flap down, giving us a *reduced* induced flow velocity, increasing AOA and helping to equalize lift. As the blade becomes "straighter" the conservation of angular momentum comes back into play, and the blade lags behind."

 

Have I been taught right, or did every one of my instructors and examiners do a poor job?

 

Seems like it... but many posts ago, you specifically said that the retreating side flaps up, and the advancing side flaps down...which is where the confusion has been coming in.

 

 

What you have described is what most have been taught, a rotor working in isolation experiencing a relative airflow and the effects of that airflow on the flapping action. This is the basic way of explaining flapping and how it can change the blades AOA.

 

The discussion is limited only to blade flapping as a result of forward speed. It’s not the complete package, it neglects that we have a swashplate, cyclic pitch, and feathering. We need it all in order to initiate forward flight and maintain equilibrium.

 

In our system the pilot cyclically changes the pitch of the blades about the feathering axes by tilting a mechanism known as a swashplate. When the rotor disk (Tip-path plane) is perpendicular to the rotor shaft, the blade angle is constant; however, when the Tip-path plane is tilted via cyclic, the blades pitch will go through one complete feathering cycle during each revolution causing the blades to flap.

 

In order to even initiate forward flight, we must first tilt the thrust vector forward. To do that, we move the cyclic forward and through inputs via the swashplate the rotor disk (Tip-path plane) tilts forward and flapping begins at the onset (advancing blade flapping down and retreating blade flapping up). Thereafter, for each knot of airspeed the cyclic must move further forward and laterally to maintain equilibrium. The complete package, Swashplate, Cyclic pitch, Flapping, and Tip-path plane all in motion simultaneously.

 

Also take a look at §27.173; §29.173; CAR 6.123 on how your helicopter was designed to operate.

 

§27.173 Static longitudinal stability.

 

(a) The longitudinal control must be designed so that a rearward movement of the control is necessary to obtain an airspeed less than the trim speed, and a forward movement of the control is necessary to obtain an airspeed more than the trim speed. (So, if you're trimmed at 0 airspeed and want to move into forward flight, the cyclic must be able to initiate flapping)

 

Confusion, take a look at a few terms from the glossary of one of Ray Prouty’s aerodynamics books:

 

Flapping – The up-and-down blade motion about the horizontal hinge at the blade root or about a corresponding flexible portion of the hub.

 

Feathering – Rotation of the blades about its span-wise axis to change its pitch.

 

Cyclic pitch – Changing pitch of one blade in one direction and the pitch of the opposite blade in the other direction.

 

Swashplate – The element used to transmit inputs from the nonrotating aircraft control system to the rotating rotor control system.

 

Tip-path plane – A plane containing the path of the rotor-blade tips.

 

 

 

 

Note the position of the blades over the nose vs. the blades position over the tail. Blade flapping is from a low point over the nose to a high point over the tail.

http://youtu.be/2eni0y5tosA

 

http://youtu.be/aGNz0NOs7VQ

Edited August 11, 2014 by iChris

[CharyouTree]

 

What you have described is what most have been taught, a rotor working in isolation experiencing a relative airflow and the effects of that airflow on the flapping action. This is the basic way of explaining flapping and how it can change the blades AOA.

 

The discussion is limited only to blade flapping as a result of forward speed. It’s not the complete package, it neglects that we have a swashplate, cyclic pitch, and feathering. We need it all in order to initiate forward flight and maintain equilibrium.

 

In our system the pilot cyclically changes the pitch of the blades about the feathering axes by tilting a mechanism known as a swashplate. When the rotor disk (Tip-path plane) is perpendicular to the rotor shaft, the blade angle is constant; however, when the Tip-path plane is tilted via cyclic, the blades pitch will go through one complete feathering cycle during each revolution causing the blades to flap.

 

In order to even initiate forward flight, we must first tilt the thrust vector forward. To do that, we move the cyclic forward and through inputs via the swashplate the rotor disk (Tip-path plane) tilts forward and flapping begins at the onset (advancing blade flapping down and retreating blade flapping up). Thereafter, for each knot of airspeed the cyclic must move further forward and laterally to maintain equilibrium. The complete package, Swashplate, Cyclic pitch, Flapping, and Tip-path plane all in motion simultaneously.

 

Also take a look at §27.173; §29.173; CAR 6.123 on how your helicopter was designed to operate.

 

§27.173 Static longitudinal stability.

 

(a) The longitudinal control must be designed so that a rearward movement of the control is necessary to obtain an airspeed less than the trim speed, and a forward movement of the control is necessary to obtain an airspeed more than the trim speed. (So, if you're trimmed at 0 airspeed and want to move into forward flight, the cyclic must be able to initiate flapping)

 

Confusion, take a look at a few terms from the glossary of one of Ray Prouty’s aerodynamics books:

 

Flapping – The up-and-down blade motion about the horizontal hinge at the blade root or about a corresponding flexible portion of the hub.

 

Feathering – Rotation of the blades about its span-wise axis to change its pitch.

 

Cyclic pitch – Changing pitch of one blade in one direction and the pitch of the opposite blade in the other direction.

 

Swashplate – The element used to transmit inputs from the nonrotating aircraft control system to the rotating rotor control system.

 

Tip-path plane – A plane containing the path of the rotor-blade tips.

 

I'm with you on all that. I understand that it doesn't work in a vacuum (heh) but the basics don't change.

 

In my head, to make it all make sense, I imagine it like vectors. Forward airspeed is causing flapping of x amount, and cyclic pitch is causing flapping of -x (to compensate) plus y. But in order to not confuse things, I don't imagine flapping as something I directly control. I understand that in the grand scheme of things, in order to change the tip path plane, I AM controlling flapping, but I personally think that it just muddies the waters to combine them. (Edit: I feather, it flaps in response. But it can also flap without me feathering. Which I guess means I don't *directly* control it, but I can affect it.)

 

I think the biggest thing to me, is that we can't ignore the fact that the retreating blade is flapping down. We're just forcing it back up. Personally, that's what got my attention, and would be like saying "Transverse flow effect doesn't happen, because I put in left cyclic and didn't roll right." ("The retreating blade doesn't flap down, because I put in forward cyclic, and didn't start flying backwards.")

 

Does any of that make sense? Am I inherently wrong with any of my assumptions/thoughts/understandings?

Edited August 11, 2014 by CharyouTree

[Guest pokey]

 

 

Does any of that make sense? Am I inherently wrong with any of my assumptions/thoughts/understandings?

 

all makes perfect sense to me, however ? i did learn a couple of new terms from this thread. Isn't that what a "rational" discussion is all about? for the most part it has been rational and tactfull. but? i still wanna learn the moonwalk, no sarcasm meant, but? it is cool