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Tip path plane, forward flight


heliflyknow

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This question might seem kind of basic but I'm having a hard time understanding what the tip path plane will be in forward flight.

 

Here are the definitions I have, so correct me if I am wrong.

 

Axis of rotation- always perpendicular (90degrees) to mast

Plane of rotation- plane the rotor hub turns in

Tip path plane- path the outer edges of the blades take( parallel to POR)

 

What confuses me is the concept of blade flapping correcting for dissymmetry of lift. How does this change the tip path plane?

 

In my head I imagine the tip path plane/rotor disk just like it sounds, like a dinner plate. I understand there is coning that would make it more bowl shaped but I'm just imagining the outer edge.

 

When you use the cyclic to tilt the rotor disk the thrust is always perpendicular to the rotor disk, so the aircraft moves in the direction that the tip path plane/rotor disk is the lowest. Correct?

 

To gain forward airspeed you use the cyclic to take pitch from the advancing/right side(ccw rotation) and put more on the retreating side in order to tilt the disk forward via phase lag/precession.

 

My confusion comes from the helicopter flying handbook. It says the cyclic makes the blades flap, greater angle of incidence(blade pitch) = up flap, and vise versa.

 

Then later it says in forward flight dissymmetry of lift causes the advancing(right side) to flap up. How can the tip path plane be tipped both forward and to the left(right side high)? Or is it the difference of those two, lowest at the 10/11 o'clock position?

 

The way I see it there are three options to explain what is happening

1. I am overthinking it and it's just low in the front left 10:30 position

2. The cyclic compensates for this dissymmetry by lessening pitch on the advancing size and increasing pitch on the retreating side equalizing the AoA on both sides. This seems the most logical since in every dissymmetry of lift picture/drawing/explanation the advancing/retreating blades have the same pitch angle and I know that's not how the cyclic works.

3. I am oversimplifying the tip path, and it's much more dynamic than I can imagine. I am thinking it's a perfect disk, and it's more like a pickle slice with numerous oscillations during its rotation and it's constantly slightly flapping up, reaching equilibrium, flapping down, and repeating. Maybe the helicopter flying handbook is oversimplifying what is happening during blade flap just to better understand it.

 

P.s. Flapping is a mechanical action right? Meaning it is measurable? It can be expressed in degrees between the blade root/hub and mast

90 degrees = zero flap?

Does that mean cyclic indirectly causes blade flap?

On the ground any deflection of the rotor disk is flapping of the blades?

Is there a difference between rotor deflection/tilt and flapping or is it the same thing?

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Hoo boy! Here we go again.

 

First, read the last few pages of the "Retreating blade stall" thread, to work out that to get the disc to tilt forward, the advancing blade is flapping down, and the retreating blade is flapping up. This is because the cyclic has been used to stop flap-back.

 

Your comments:

 

Axis of rotation- always perpendicular (90degrees) to mast
Plane of rotation- plane the rotor hub turns in
Tip path plane- path the outer edges of the blades take( parallel to POR)

No, the axis is not perpendicular to the mast, it IS (generally)the mast, it is what the blades rotate about. But when the disc tilts forward, the axis moves away from the mast.

 

Tip path plane and plane of rotation can be the same thing when you have flat pitch.

 

Add some collective, the blades cone up, and the TPP is now displaced from the POR, but parallel to it.

 

Add some cyclic, the TPP tilts in the chosen direction, and away you go. Dissymmetry of lift tries to make the advancing blade flap up, the pilot says "I don't want the nose to pitch up" and adds some more forward cyclic, and totally compensates for that lift problem.

 

Flapping, by definition, is any up/down movement of the blade. It can be caused by a wind gust, or collective application, or cyclic. Cyclic DIRECTLY causes flap, as does the collective.

 

You are fairly close to the money when you say that the helicopter handcrank.. err..handbook... is simplifying things. They are aiming at the lowest possible level of knowledge - you appear to be well above that level - well done.

Edited by Eric Hunt
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This question might seem kind of basic but I'm having a hard time understanding what the tip path plane will be in forward flight.

 

Here are the definitions I have, so correct me if I am wrong.

 

Axis of rotation- always perpendicular (90degrees) to mast

Plane of rotation- plane the rotor hub turns in

Tip path plane- path the outer edges of the blades take( parallel to POR)

 

What confuses me is the concept of blade flapping correcting for dissymmetry of lift. How does this change the tip path plane?

 

In my head I imagine the tip path plane/rotor disk just like it sounds, like a dinner plate. I understand there is coning that would make it more bowl shaped but I'm just imagining the outer edge.

 

When you use the cyclic to tilt the rotor disk the thrust is always perpendicular to the rotor disk, so the aircraft moves in the direction that the tip path plane/rotor disk is the lowest. Correct?

 

To gain forward airspeed you use the cyclic to take pitch from the advancing/right side(ccw rotation) and put more on the retreating side in order to tilt the disk forward via phase lag/precession.

 

My confusion comes from the helicopter flying handbook. It says the cyclic makes the blades flap, greater angle of incidence(blade pitch) = up flap, and vise versa.

 

Then later it says in forward flight dissymmetry of lift causes the advancing(right side) to flap up. How can the tip path plane be tipped both forward and to the left(right side high)? Or is it the difference of those two, lowest at the 10/11 o'clock position?

 

The way I see it there are three options to explain what is happening

1. I am overthinking it and it's just low in the front left 10:30 position

2. The cyclic compensates for this dissymmetry by lessening pitch on the advancing size and increasing pitch on the retreating side equalizing the AoA on both sides. This seems the most logical since in every dissymmetry of lift picture/drawing/explanation the advancing/retreating blades have the same pitch angle and I know that's not how the cyclic works.

3. I am oversimplifying the tip path, and it's much more dynamic than I can imagine. I am thinking it's a perfect disk, and it's more like a pickle slice with numerous oscillations during its rotation and it's constantly slightly flapping up, reaching equilibrium, flapping down, and repeating. Maybe the helicopter flying handbook is oversimplifying what is happening during blade flap just to better understand it.

 

P.s. Flapping is a mechanical action right? Meaning it is measurable? It can be expressed in degrees between the blade root/hub and mast

90 degrees = zero flap?

Does that mean cyclic indirectly causes blade flap?

On the ground any deflection of the rotor disk is flapping of the blades?

Is there a difference between rotor deflection/tilt and flapping or is it the same thing?

 

Yes, we did cover those questions in the following links:

 

Question on Retreating Blade Stall & Blade Flapping in forward flight

 

Plane of Rotation vs. Tip Path Plane

Edited by iChris
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I have read over the "retreating blade...." post and it seems like the 9 pages of argument mirror what I have experienced. Everyone thinks I'm crazy when I say the advancing side flaps down in forward flight.

 

It seems the disconnect is from what flapping is. There seems to be a consensus that flapping is only caused by dissymmetry of lift, which is true to some extent. Cyclic input causes a dissymmetry of lift by changing the pitch at two opposite points in the POR. Somehow this rotor disk tilt isn't flapping in their eyes.

 

Is there any publication that specifically says that dissymmetry of lift in a helicopter is compensated for by cyclic input? Most seem to explain gyroscopic precession, which accounts for how and why the rotor tilts and how that effects the thrust vector. Then they seem to contradict themselves when they say DoL causes the advancing side to flap up, and in their diagrams show the advancing and retreating having the same pitch angle.

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You can see the tip path plane in flight. Look at it, and you can plainly see where it is, at least forward and on the sides. A little hard to see at the rear, but I've always assumed it wasn't doing anything really outlandish back there, because I've flown in very close formation with other helicopters, and none of them did. The tip path plane isn't really a plane, it has bumps and wiggles, but it's always higher in the rear and lower in the front, in forward flight. I've been watching them for more than 40 years, and I've never observed anything different. The advancing blade is always descending, and the retreating blade is always rising in forward flight. Whether you call any of that flapping or not, that's what happens. It has to, in order to direct thrust to the rear, and move the fuselage forward.

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I think one thing that has many people confused is the mistaken idea that the rotor tip path plane is always perpendicular to the mast. That's the theory, but it doesn't work in practice. If you look at a Bell 206 in cruise flight, as Gomer has observed the tip-path plane is most certainly tilted "down in front" and "up in rear." That's because the horizontal stabilizer is pulling the tail down so the passengers have a more comfortable ride. In a 206L flown 80% power by a light pilot, the cyclic will be nearly on the forward stop while the cabin rides along in a "fairly" level attitude. If that doesn't give you some idea of where the tip-path plane is in relation to the mast, I don't know what will.

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Yes, you'll get higher airspeed and lower fuel consumption with an aft CG, at least in a 206, because the fuselage is more level, and thus lower drag. Interestingly, at least to me, the rotor plane in a fully articulated system, such as the S76, is well off of plane. The path in the front is close to level, not nearly as forward tilted as a two-bladed system, but the rear part is tilted very much up, and the path there is higher than what you would usually see in a two-bladed system. The fuselage stays very level, and it goes very fast. The first limit for an S76A++ is often Vne. You have to back off on the power to keep from exceeding Vne in level flight. This isn't all due to the rotor system, of course, but that's certainly part of the equation.

 

One of the more eye-opening instruction periods in Army flight school was a movie made by mounting a high-speed camera on the mast of a UH-1, looking down one rotor blade. That thing flexed like a wet noodle, and made one think at least twice about even being in a flying helicopter. No matter what make or model, there is no 'plane' in which the blades rotate, it's very much three-dimensional. Calling it a plane is a gross over-simplification, but it's often good enough for the purpose.

Edited by Gomer Pylot
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Is there any publication that specifically says that dissymmetry of lift in a helicopter is compensated for by cyclic input?

 

Most seem to explain gyroscopic precession, which accounts for how and why the rotor tilts and how that effects the thrust vector. Then they seem to contradict themselves when they say DoL causes the advancing side to flap up, and in their diagrams show the advancing and retreating having the same pitch angle.

 

U.S. Army Field Manual: Fundamentals of Flight, FM 1-203

U.S. Army Field Manual: Fundamentals of Flight, FM 3-04.203

Rotorcraft Aeromechanics, Wayne Johnson, NASA

Principles of Helicopter Aerodynamics, J. Gordon Leishman

Helicopter Performance Stability, and Control, Raymond W. Prouty

Helicopter Aerodynamics Vol 1 & 2, Raymond W. Prouty

The Art of the Helicopter, John Watkinson

Cyclic & Collective Art and Science of Flying Helicopters, Shawn Coyle *

 

*Cyclic & Collective Art and Science is an excellent short read without the technical jargon pages 34 - 40 and pages 211 -212

 

Moreover, Ray answered most all of your questions in the quote below. you need the read it carefully a few times, word by word, and think it over a while.

 

 

Cyclic Pitch can be used for two purposes: to trim the tip path plane with respect to the mast, and to produce control moments for maneuvering.

In the first case, the pilot can mechanically change the angle of attack of the blades by the same amount, as the flapping motion would have, thus eliminating the flapping.

This can be used to eliminate all of the flapping or leave just enough to balance pitching or rolling moments on the aircraft such as those due to an offset center of gravity.

In the second case, the pilot deliberately introduces an unbalanced lift distribution in order to make the rotor tilt for maneuvering. For example, if the helicopter is hovering and the pilot wishes to tilt the nose down, he pushes the stick forward, which tilts the swashplate down in front. The pitch of the blade at Ψ = 90º (advancing blade) is decreased and that at Ψ = 270º (retreating blade) is increased.

The resultant imbalance accelerates the right-hand blade down and the left-hand blade up. The rotor flaps down over the nose and up over the tail, tilting the rotor thrust vector forward to produce a nose down pitching moment about the center of gravity. The procedure is similar if the pilot wishes to pitch nose up or to roll in either direction.

Whether being used for trim or for control, the cyclic pitch is equivalent to flapping in that the changes in rotor conditions due to one degree of cyclic pitch are the same as those due to a one-degree change in flapping.

Ref: Helicopter Performance, Stability, and Control (page 154); Raymond W. Prouty

Edited by iChris
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Here's a pretty great video that should certainly help.

 

Another thing to keep in mind is that dissymmetry of lift is certainly counteracted by blade flapping, although depending on whether your helicopter has Fully Articulated or Semi Rigid rotor system, flapping is done differently.

 

Semi Rigid performs flapping by the rotor hub teetering as a unit (like a see saw). Since the rotor hub changes angle, so would the axis of rotation.

 

Fully Articulated is accomplished by the use of flapping hinges (the rotor hub does not flap or change angle) therefore neither does the axis of rotation.

 

The video shows a heli with a Fully Articulated rotor system. Notice that the Axis of Rotation does not change. If it were Semi Rigid, the rotor hub would literally teeter to allow for flapping, which would change AOR.

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Here's a pretty great video that should certainly help.

 

 

 

This too is a very poor representation of how dissymmetry of lift is corrected in forward flight, with respect to a helicopter. We're back to the old flap-up over the nose, flap down over the tail.

 

This over simplification from the Autogiro applied to the helicopter has fostered all this confusion.

 

 

 

 

AutogirovsHelicopter_zps4313f2aa.jpg

 

 

 

After 161 replies in the following posts we still have this confusion:

 

Question on Retreating Blade Stall & Blade Flapping in forward flight

 

Plane of Rotation vs. Tip Path Plane

Edited by iChris
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Couple of points, Pond Jumper.

 

Firstly, the video is only relevant to an aircraft sitting in the hover or on the ground with cyclic neutral. There is absolutely no feathering going on. A wind gust has given a relative wind from the front, and this video neatly shows what happens - the disc flaps away from the wind.

 

What would happen next is that (if hovering) the aircraft will start to move backwards. The pilot doesn't want this, feeds in forward cyclic to introduce feathering, and the video is no longer relevant.

 

Second thing - the axis of rotation is 90 degrees to the tip path plane - it is only along the mast when the disc is flat. So, when the disc is tilted forward, the axis of rotation, especially on rigid or fully articulated heads, moves away from the mast to stay at 90 degrees to the tip path plane, same as on a teetering head.

 

The difference, though, is that the forces from the teetering head have no moment on the mast, they just pull it in the direction of the disc tilt. Fully articulated heads have a moment applied which is equal to the force from the blades times the distance of the hinge from the mast. On a rigid head, like the BO105 or BK 117, where there is no flapping hinge, the moment can be very considerable, so they instal a Mast Moment Indicator to show when you are approaching the bending limits of the mast.

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Couple of points, Pond Jumper.

 

Firstly, the video is only relevant to an aircraft sitting in the hover or on the ground with cyclic neutral. There is absolutely no feathering going on. A wind gust has given a relative wind from the front, and this video neatly shows what happens - the disc flaps away from the wind.

 

What would happen next is that (if hovering) the aircraft will start to move backwards. The pilot doesn't want this, feeds in forward cyclic to introduce feathering, and the video is no longer relevant.

 

Second thing - the axis of rotation is 90 degrees to the tip path plane - it is only along the mast when the disc is flat. So, when the disc is tilted forward, the axis of rotation, especially on rigid or fully articulated heads, moves away from the mast to stay at 90 degrees to the tip path plane, same as on a teetering head.

 

The difference, though, is that the forces from the teetering head have no moment on the mast, they just pull it in the direction of the disc tilt. Fully articulated heads have a moment applied which is equal to the force from the blades times the distance of the hinge from the mast. On a rigid head, like the BO105 or BK 117, where there is no flapping hinge, the moment can be very considerable, so they instal a Mast Moment Indicator to show when you are approaching the bending limits of the mast.

I can't agree with a few of your points Erik.

 

First, the blade flapping video that I posted applies to any condition that would exist where the rotor system has a horizontal component of RW (other than rotational). A helicopter's rotor blades can't tell the difference between horizontal RW caused by forward airspeed or a gusting headwind while in a stationary hover, as long as there is a change in RW, the retreating blade flaps down and the advancing blade flaps up. There is no way around that fact. The point of this video is to give the most basic explanation of blade flapping without involving cyclic feathering.

 

If I understand the original question of this thread, it is how can our rotor disk display max downflap in the 6 oclock position(for dissymmetry of lift), while at the same time be tilted forward to achieve forward flight?

 

I think you answered this pretty well when you mentioned 'blowback.' In order to keep our rotor disk tilted forward, we simply use cyclic feathering. Our retreating blade still experiences a loss of lift, therefore must flap downward, we just push forward cyclic to tilt the 'disk' far enough forward to 'push through it.' If we didn't, it would happen exactly how you said. We wouldn't be able to move forward.

 

This is kind of getting off topic but I'd like to make sure I understand what you were saying about a semi-rigid rotor system. It seemed like you said the semi-rigid rotor system doesn't apply a moment to the rotor mast (please correct me if I'm wrong). I'm not sure I would agree with that either. As long as the main rotor system is supporting the weight of the helicopter, it is indeed applying a moment to the helicopter, and the rotor mast is the arm.

 

The only time this doesn't apply would be in a low G condition, which is how we get mast bumping.

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Think of a toy helicopter, with a teetering head, sitting on the floor. You can tilt the blades up and down in any direction, and the fuselage is not affected, as the hub is allowing the blades to move without applying a force to the mast.

 

Now let that hub get rusty, and there is some friction resisting the flapping of the blade. You lift up one blade, and you see that the near side skid is trying to lift up too, because of the force applied to the mast - a moment. This is more like an articulated head.

 

Take it all the way, fix the blade rigidly to the mast, and as soon as you try to lift the blade, the skid comes off the floor. BIG moment. Rigid head.

 

Obviously this is an over-simplified way of describing what is happening to the rotor head, but you get the drift.

 

 

As for the flapping due relative airflow,

 

 

First, the blade flapping video that I posted applies to any condition that would exist where the rotor system has a horizontal component of RW (other than rotational). A helicopter's rotor blades can't tell the difference between horizontal RW caused by forward airspeed or a gusting headwind while in a stationary hover, as long as there is a change in RW, the retreating blade flaps down and the advancing blade flaps up. There is no way around that fact.

Yes there is a way around it, and you already stated it. Cyclic stops it.

 

Go back thirty pages or whatever and read the bits from Chris and others who clearly state that when the advancing blade is flapping down in forward flight, there is NO COMPONENT of flapping up for Dissymmetry of Lift. That would be the same as saying that when a car slows down a bit from a certain speed, there is a component of driving in reverse.

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Eric, nothing wrong with using a toy helicopter as an example at all. I do it all the time. Although in your example, the reason you are able to move the semi-rigid rotor hub of the toy helicopter without the toy helicopter moving is simple. In your example, the lift of the rotor system isn't supporting the weight of the helicopter (the disk isn't "loaded"). This is why in my previous post I was clear to say "as long as the main rotor system is supporting the weight of the helicopter."

 

If this were not true, any attempt of directional flight in a helicopter with a semi-rigid system would result in Mast Bumping.

 

It also seems that you are saying cyclic feathering eliminates blade flapping. If that is the case, it just isn't accurate. No amount of cyclic input will keep blade flapping from happening. As long as there is an advancing blade and a retreating blade (as there is in forward flight or hover with a head wind), there will be dissymmetry of lift hence blade flapping.

 

Think to retreating blade stall and how it occurs. Retreating blade stall is caused by exceeding VNE> Retreating Blade Exceeds Critical Angle (due to flapping b/c of lift dissymmetry)> Retreating Blade Stalls

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Cyclic feathering eliminates blade flapping. If that is the case, it just isn't accurate. No amount of cyclic input will keep blade flapping from happening.

As long as there is an advancing blade and a retreating blade (as there is in forward flight or hover with a head wind), there will be dissymmetry of lift hence blade flapping.

 

Cyclic Pitch can be used for two purposes: to trim the tip path plane with respect to the mast, and to produce control moments for maneuvering.

In the first case, the pilot can mechanically change the angle of attack of the blades by the same amount, as the flapping motion would have, thus eliminating the flapping.

This can be used to eliminate all of the flapping or leave just enough to balance pitching or rolling moments on the aircraft such as those due to an offset center of gravity.

Whether being used for trim or for control, the cyclic pitch is equivalent to flapping in that the changes in rotor conditions due to one degree of cyclic pitch are the same as those due to a one-degree change in flapping.

 

This is one of Raymond Prouty's quotes (full quote below) that has cleared up lots of this confusion over blade flapping and what controls it. It's been posted many times and we still get these misleading post.

 

You can also get good information on this issue from Shawn Coyle's book:

Cyclic & Collective Art and Science is an excellent short read without the technical jargon pages 34 - 40 and pages 211 -212

 

 

Cyclic Pitch can be used for two purposes: to trim the tip path plane with respect to the mast, and to produce control moments for maneuvering.

In the first case, the pilot can mechanically change the angle of attack of the blades by the same amount, as the flapping motion would have, thus eliminating the flapping.

This can be used to eliminate all of the flapping or leave just enough to balance pitching or rolling moments on the aircraft such as those due to an offset center of gravity.

In the second case, the pilot deliberately introduces an unbalanced lift distribution in order to make the rotor tilt for maneuvering. For example, if the helicopter is hovering and the pilot wishes to tilt the nose down, he pushes the stick forward, which tilts the swashplate down in front. The pitch of the blade at Ψ = 90º (advancing blade) is decreased and that at Ψ = 270º (retreating blade) is increased.

The resultant imbalance accelerates the right-hand blade down and the left-hand blade up. The rotor flaps down over the nose and up over the tail, tilting the rotor thrust vector forward to produce a nose down pitching moment about the center of gravity. The procedure is similar if the pilot wishes to pitch nose up or to roll in either direction.

Whether being used for trim or for control, the cyclic pitch is equivalent to flapping in that the changes in rotor conditions due to one degree of cyclic pitch are the same as those due to a one-degree change in flapping.

Ref: Helicopter Performance, Stability, and Control (page 154); Raymond W. Prouty

Edited by iChris
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OK, lift the toy off the floor. The blades are supporting the toy.

 

With a teetering head, it is possible to hold the blades at an angle to the horizontal, but the toy still hangs below the pivot point at the top of the mast. There is no moment applied around the pivot point.

 

When a REAL teetering helicopter is on the ground, flat pitch (no lifting force) the disc can be tilted right until it reaches the flapping restraints, but there is NO moment applied to the mast up to that point.

When it flies, the cyclic tilts the disc, the total reaction pulls the fuselage, via the single pivot point at the top of the mast, and it follows the disc.

 

Now we have a rigid rotor machine sitting on the ground with flat pitch. If the pilot feeds in cyclic to tilt the disc, he will see a LARGE increase in mast moment, because to make the disc tilt, the blades have to flap by bending, and this passes a force on to the rigid hub.

 

You also have the wrong concept of blade flapping. Flapping is defined as the up and down movement of the blade. It can be caused by dissymetry of lift (no feathering), or by collective application (both feather the same amount and flap up, causing coning) or by cyclic application, with different feathering for each blade, so one flaps up and the other flaps down.

 

In stable forward flight, there is no dissymetry of lift. Each side has exactly the same lifting force. Otherwise it is not stable and the machine will roll until the force is balanced. This is a basic fact of life. But a lot of junior students and their almost-equally junior instructors are stuck on the idea that the advancing blade is flapping up in forward flight. It ain't.

 

The pilot balances the force by using cyclic to reduce the pitch angle on the advancing side, and increase the pitch angle on the retreating side. The advancing side generates around 7 times the lift that can be generated by the retreating side, so it has to be disposed of somehow - feathering. It is only allowed to generate as much lift as the poor old retreating blade can muster up.

 

 

 

Think to retreating blade stall and how it occurs. Retreating blade stall is caused by exceeding VNE> Retreating Blade Exceeds Critical Angle (due to flapping b/c of lift dissymmetry)> Retreating Blade Stalls

You need to go back to school and read your books on this, and then look at the thread on retreating blade stall to see how many people are equally as confused as you are.

 

Retreating blade stall is NOT connected to Vne. (Vne is determined by multiple other factors such as stress on the rotor head from parasite drag, and is usually well below the speed at which you get RBS. I have flown an S76 at 180kt when the Vne is 155kt, no RBS evident)

 

Yes, the retreating blade reaches its critical angle of attack, but NOT because of lift dissymetry (as explained above, lift is equal on both sides) but simply because there is not much of the retreating blade that is generating lift, and the bit that IS doing it has reached the stall angle.

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  • 3 weeks later...

The best conclusion I can come to is that the HFH(helicopter flying handbook) considers only the plane of rotation when it says the advancing side flaps up. In its diagrams it shows advancing and retreating blades having the same pitch(incidence) angle. If you view the blades from this plane the increased lift would make the blades flap up on the advancing side, making the highest point in the front, and the lowest in the rear. In reality the blades are just not flapping down as much... If that makes sense. Instead of the front of the disk at an 80degree angle (compared to the mast) it changes to an 85degree angle. So- viewed from mast=Flapping down, viewed from POR= flapping up. End result=mass confusion, both sides saying the same thing.

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In its diagrams it shows advancing and retreating blades having the same pitch(incidence) angle

Yes, this is only stage 1 of anything happening - THE BLADES HAVE THE SAME PITCH ANGLE BECAUSE THE PILOT HAS NOT YET MOVED THE CYCLIC. he is in a hover with no wind, but then a gust has appeared and made the blades flap.

 

Once he sees that the blades have flapped back, and that the aircraft is starting to move backwards from the hover, he decides he doesn't want that, and pokes the cyclic forward. The blades now have a continuous change in pitch, and the diagram in your HFH has been left way behind.

 

If the pilot wants to fly forward from the hover, he has to tilt the lift vector forward, by tilting the disc down at the front with more cyclic. The advancing blade FLAPS DOWN and the retreating blade FLAPS UP. No confusion, just reality.

 

The confusion only arises because the HFH only looks at the situation before the pilot has moved the cyclic, but students and some instructors think that this situation is still in play. It ain't.

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