Question on Retreating Blade Stall

[Sonic04GT]

Where does the stall begin on the retreating blade? At the root, or at the tip?

 

Then, the golden question is: Why?

[eagle5]

Well it used to be called retreating tip stall, so I'm going to say tip. As to why, I don't know but perhaps the tip has the highest angle of attack compared to the rest of the blade?

[Whiteshadow]

I'd like to take a crack at this one:

 

It does begin at the tip, and moves inboard from there. The reason as to why is because the retreating blade is flapping down in an attempt to equalize the lift across the rotor disc. It can only flap so far (and thereby increase the AOA), before it "exceeds the critical angle of attack", which is when the stall actually occurs.

 

I'd like to hear what others have to say about it. The "why" questions are really tough sometimes and I'm really trying to make sure I can answer them as thoroughly and correctly as possible.

[Sonic04GT]

A DPE asked me this question on my CFI checkride last year and his answer was the tip. It has resurfaced and I was curious to see what you guys have to say. Thanks for the replies!

 

The flapping kind of makes sense. Are you implying that the maximum down flap is at the tip?

 

It's just an interesting question because it defies common aerodynamics, where the root feels less relative wind, and blade twist causes a lower angle at the tip.

 

Helicopters should not fly.

[Whiteshadow]

Yes, the tip does have the maximum down flap, and that is just because of it's distance from the hub. Obviously, the blade tip is more "free" to move up and down due to it's distance from the hub. That's the best way I can think to explain it, but I'm a brand spankin' new CFI, so take it for what it's worth……….which might not be much right now!

[iChris]

 

It does begin at the tip, and moves inboard from there. The reason as to why is because the retreating blade is flapping down in an attempt to equalize the lift across the rotor disc. It can only flap so far (and thereby increase the AOA), before it "exceeds the critical angle of attack", which is when the stall actually occurs.

 

 

Excellent, it's as simple as that. Flight and wind tunnel test agree. Variation in tangential velocity across the rotor disk, blade flapping, and cyclic feathering generally account for the angle of attack distribution.

 

The figure shows angle of attack distribution during retreating blade stall.

Edited August 4, 2014 by iChris

[Eric Hunt]

Flapping DOWN on the retreating side???

 

Have any of you people actually looked at a disc while a helicopter is travelling at high speed? The disc is down at the front, and high at the back. To get high, it has to flap UP while it is travelling from the front to the back. The maximum angle of attack, and maximum rate of flapping UP is at 90 degrees on the retreating side. Look at the diagram.

 

This misguided idea of flapping to equality only happens if the cyclic is not moved - for example, a flat disc meets a puff of wind from the front. Advancing side sees more relative airflow, gets more lift, starts to climb. Retreating side sees less RAF, gets less lift, starts to drop.

 

The advancing blade, as it climbs, now sees more induced flow, which reduces the angle of attack and it is back to where it was before the puff of breeze. The retreating blade, falling down, sees less induced flow, gets more lift, and is back to where it was before the puff. The blade has flapped to equality. BUT!!! The disc is now tilted up at the front and down at the back!

 

What will happen next? The aircraft will start to move backwards, but then the left side is now seeing more RAF, the right side sees less, the reverse starts to happen, the disc flaps forward, and the helicopter is in a dynamically unstable situation, it will continue these oscillations until it crashes - OR - the pilot uses cyclic to stop it.

 

Using the cyclic cancels out "flapping to equality".

 

So, in forward flight, the advancing side is flapping DOWN because the cyclic told it to, and to shed some lift, and the retreating side is flapping up like crazy to try to generate some lift. But it is fighting the reverse flow, the stalled region and only a small part of the blade is generating lift.

 

The advancing blade has the ability to produce LOTS of lift, but it has to dump it down to what the poor retreating blade can generate. This is why the Advancing Blade Concept (2 contra-rotating sets of blades) is so efficient, because it does not rely on the retreating blade to generate lift, it has an advancing blade on each side.

 

Seriously, gentlemen, a qualified flight instructor who doesn't understand that a retreating blade is flapping up, needs to get his head back into the books.

[Pohi]

Flapping DOWN on the retreating side???

 

Have any of you people actually looked at a disc while a helicopter is travelling at high speed? The disc is down at the front, and high at the back. To get high, it has to flap UP while it is travelling from the front to the back. The maximum angle of attack, and maximum rate of flapping UP is at 90 degrees on the retreating side. Look at the diagram.

 

This misguided idea of flapping to equality only happens if the cyclic is not moved - for example, a flat disc meets a puff of wind from the front. Advancing side sees more relative airflow, gets more lift, starts to climb. Retreating side sees less RAF, gets less lift, starts to drop.

 

The advancing blade, as it climbs, now sees more induced flow, which reduces the angle of attack and it is back to where it was before the puff of breeze. The retreating blade, falling down, sees less induced flow, gets more lift, and is back to where it was before the puff. The blade has flapped to equality. BUT!!! The disc is now tilted up at the front and down at the back!

 

What will happen next? The aircraft will start to move backwards, but then the left side is now seeing more RAF, the right side sees less, the reverse starts to happen, the disc flaps forward, and the helicopter is in a dynamically unstable situation, it will continue these oscillations until it crashes - OR - the pilot uses cyclic to stop it.

 

Using the cyclic cancels out "flapping to equality".

 

So, in forward flight, the advancing side is flapping DOWN because the cyclic told it to, and to shed some lift, and the retreating side is flapping up like crazy to try to generate some lift. But it is fighting the reverse flow, the stalled region and only a small part of the blade is generating lift.

 

The advancing blade has the ability to produce LOTS of lift, but it has to dump it down to what the poor retreating blade can generate. This is why the Advancing Blade Concept (2 contra-rotating sets of blades) is so efficient, because it does not rely on the retreating blade to generate lift, it has an advancing blade on each side.

 

Seriously, gentlemen, a qualified flight instructor who doesn't understand that a retreating blade is flapping up, needs to get his head back into the books.

I guess I need to get my head back into the books.

[Guest pokey]

as i recall, the flapping hinge was invented by Juan de la Cierva, because it stopped his auto gyro from rolling over in flight. It allowed the advancing blade to flap up, rather than roll the aircraft on its side. As the advancing blade moves faster, it produces more lift, and this is how it "dumps" its additional lift to the retreating blade.

If viewed from above, a rigid counter clockwise rotating rotor, in forward flight will roll in which direction? (ignore any blade elasticity)----------i bet my $$ it will roll to the left.

 

now add a flapping hinge? ,,, well you can see my point

 

and BTW? the cyclic does NOT tell the blade how to flap--unless the cyclic/flap hinge is a delta 3 configuration, which i know of no modern helicopters that use this, nor even any older ones.

Edited August 4, 2014 by pokey

[helonorth]

Flapping DOWN on the retreating side???

 

Have any of you people actually looked at a disc while a helicopter is travelling at high speed? The disc is down at the front, and high at the back. To get high, it has to flap UP while it is travelling from the front to the back. The maximum angle of attack, and maximum rate of flapping UP is at 90 degrees on the retreating side. Look at the diagram.

 

This misguided idea of flapping to equality only happens if the cyclic is not moved - for example, a flat disc meets a puff of wind from the front. Advancing side sees more relative airflow, gets more lift, starts to climb. Retreating side sees less RAF, gets less lift, starts to drop.

 

The advancing blade, as it climbs, now sees more induced flow, which reduces the angle of attack and it is back to where it was before the puff of breeze. The retreating blade, falling down, sees less induced flow, gets more lift, and is back to where it was before the puff. The blade has flapped to equality. BUT!!! The disc is now tilted up at the front and down at the back!

 

What will happen next? The aircraft will start to move backwards, but then the left side is now seeing more RAF, the right side sees less, the reverse starts to happen, the disc flaps forward, and the helicopter is in a dynamically unstable situation, it will continue these oscillations until it crashes - OR - the pilot uses cyclic to stop it.

 

Using the cyclic cancels out "flapping to equality".

 

So, in forward flight, the advancing side is flapping DOWN because the cyclic told it to, and to shed some lift, and the retreating side is flapping up like crazy to try to generate some lift. But it is fighting the reverse flow, the stalled region and only a small part of the blade is generating lift.

 

The advancing blade has the ability to produce LOTS of lift, but it has to dump it down to what the poor retreating blade can generate. This is why the Advancing Blade Concept (2 contra-rotating sets of blades) is so efficient, because it does not rely on the retreating blade to generate lift, it has an advancing blade on each side.

 

Seriously, gentlemen, a qualified flight instructor who doesn't understand that a retreating blade is flapping up, needs to get his head back into the books.

I haven't read any books that say what you just said.

[iChris]

As I stated in my post above, variation in velocity across the rotor disk, blade flapping, and cyclic feathering generally account for the angle of attack distribution.

 

We need to take all three in combination, tangential velocity, flapping, and cyclic feathering. When you move from the hover into forward flight things start to happen. The tangential velocity variations across the rotor disk cause the blades to flap, which requires you to make corrective cyclic inputs to counter unwanted rotor flap-back and roll effects.

 

The problem is most of your training books for simplicity; limit their discussion to only blade flapping. 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.” However, they don’t say much more about that feathering (cyclic pitch variation).

 

The figures below show the individual contributors during forward flight. When combined they generally account for the angle of attack distribution shown in my above post.

 

 

 

Edited August 18, 2014 by iChris

[Eric Hunt]

Nice diagrams, Chris.

 

The top one shows the most basic "gust of wind" story, with cyclic neutral, no feathering, and it shows the blade at its highest point over the nose and lowest point over the tail. This is right before the aircraft starts to move backwards, away from the initial gust of wind.

 

To STOP this flapback and subsequent dynamic instability, the pilot uses cyclic forward. The blades feather, the angles of attack are modified, and the disc comes back to level.

 

To move forward, he pokes the cyclic even further forward to tilt the disc down at the front - and tilt that thrust vector forward - and he flies forwards.

 

This is the second diagram, cyclic full forward, with the blade having its lowest pitch angle at 90 right (and flapping down) to reach the lowest point at the front, and then having its maximum pitch at 90 left, flapping up, to reach the high point at the back.

 

The advancing blade is not allowed to have any more lift that the retreating blade can generate - or else the machine will roll left. And the feathering of the cyclic overcomes any "flapping to equality" tendency.

[Eric Hunt]

 

 

and BTW? the cyclic does NOT tell the blade how to flap--unless the cyclic/flap hinge is a delta 3 configuration, which i know of no modern helicopters that use this, nor even any older ones.

Pokey, the cyclic pits pitch on the blade, the blade's lift changes, and the lift causes the blade to rise or fall - flapping. Result - the cyclic controls the blade's flapping. So does the collective.

 

If you couldn't control the flapping with cyclic, the machine would not fly in a predictable manner.

[helonorth]

Nice diagrams, Chris.

 

The top one shows the most basic "gust of wind" story, with cyclic neutral, no feathering, and it shows the blade at its highest point over the nose and lowest point over the tail. This is right before the aircraft starts to move backwards, away from the initial gust of wind.

 

To STOP this flapback and subsequent dynamic instability, the pilot uses cyclic forward. The blades feather, the angles of attack are modified, and the disc comes back to level.

 

To move forward, he pokes the cyclic even further forward to tilt the disc down at the front - and tilt that thrust vector forward - and he flies forwards.

 

This is the second diagram, cyclic full forward, with the blade having its lowest pitch angle at 90 right (and flapping down) to reach the lowest point at the front, and then having its maximum pitch at 90 left, flapping up, to reach the high point at the back.

 

The advancing blade is not allowed to have any more lift that the retreating blade can generate - or else the machine will roll left. And the feathering of the cyclic overcomes any "flapping to equality" tendency.

 

 

 

What you are talking about is dissymmetry of lift (which you have not even so much as mentioned).

 

The feathering of the blades creating thrust has little or nothing to do with retreating blade stall. I have read nothing that mentions it. You also seem to use flapping and feathering interchangeably.

 

My explanation.

 

The blades flap up on the advancing side due to increased velocity. The change in the angle of the relative wind creates a smaller AOA. The reverse is true for the retreating side. Without blade flapping, we have the dissymmetry of lift problem.

 

The best way to visualize this is to simply look at the lift formula: the velocity is higher on the advancing side than the retreating side. Since we know increasing velocity increases lift (and the reverse for the retreating side) something has to compensate for this. By creating larger and smaller angles of attack through blade flapping, lift is equalized across the disc. If you would like to cite your sources for your explanation, I'm all ears. Dissymmetry of lift, retreating blade stall and (to a small degree) feathering are related but should be discussed separately or this is the result.

 

As for the OP's question, it seems as though the stalling of the blade at the outer portion is what leads to loss of control. I believe the roots are stalled before loss of control due to their higher angles of attack in addition their low velocity due to reverse flow.

Edited August 5, 2014 by helonorth

[Whiteshadow]

The portion on the discussion that I contributed was assuming that the OP knew the factors leading up to RBS, which I'm sure he does. I think my answer was still satisfactory for the question asked.

[eagle5]

Just how technical can you before you lose your students attention?

[Ross_Cristiano]

I was always told and know it as the blade would stall first at the root. Because of the blade twist. Giving a higher angle of attack at the root and a lower at the tip. The Critical AOA would occur first at the root then work its way outward (At least on the 22 and 44)....

[eagle5]

The glossery of the Helicopter Flying Handbook has the answer. Safety Notice 24 in the Robbie POH also touches on it a bit. They both say tip!

[Eric Hunt]

I am flabbergasted at how little some of you understand about what is happening on the disc.

 

Look at the disc in forward flight. Is it tilted down at the front and up at the back? Yes! Then how can you say "the advancing blade is flapping UP???"

 

Helonorth says:

 

 

The feathering of the blades creating thrust has little or nothing to do with retreating blade stall.

So you are saying that the swash plate, tilted forwards, and applying maximum pitch to the blades at the 90 left position, has nothing to do with the angle of attack reaching the stall? Feathering is what applies pitch to the blades. Added to the relative airflow, we get an angle of attack, which produces lift, which makes the blade flap up or down relative to its opposite number.

 

The lift is the same on both sides of the disc. There is no dissymmetry of lift - because we have used cyclic to stop it. On the advancing side, the swash plate is driving the pitch down to its minimum figure at 90 right, and driving it up to its maximum figure at 90 left. If there was any dissymmetry of lift, the aircraft would roll.

 

It is called feathering. It has EVERYTHING to do with retreating blade stall.

 

Ross says:

 

 

I was always told and know it as the blade would stall first at the root

Sorry, it is not stalled at the root.

 

Draw yourself the vector diagrams of the retreating blade - you know, the stuff you had to draw to prove to the examiner that you knew enough about rotational airflow, induced airflow, chord line, pitch angle, relative airflow and angle of attack - to be granted your instructor's certificate.

 

There is reverse flow over the root in forward flight, there is negative lift, then as we move outboard the blade has a relative airflow coming from above the blade, giving a negative angle of attack and negative lift. Then as we move further out, for the same IF, the rotational flow is getting bigger and the relative airflow moves downwards to be right along the chord line. The lift is zero, then further out the RAF starts to come from below the chord line - we now have a small amount of positive lift.

 

Move further out, the IF is the same, but now the RAF is getting longer and the difference between the pitch angle and the RAF (the angle of attack) is getting bigger, generating more lift. It is now usefully supporting the helicopter.

 

Further out, bigger rotational flow, bigger angle of attack, more lift.

 

Nearing the tip, MUCH bigger rotational flow, same induced flow, much bigger angle of attack - oops... here comes the stall. At the TIP. And some people think that to cure the tip stall, they should increase RRPM. That will make it worse - more rotational flow, bigger angle of attack, worse stall.

 

Draw the diagrams, look at a helicopter in flight, and have a think about it.

[iChris]

Click Photo To enlarge

 

 

 

 

REF: U.S. Army Field Manual

FM 3-04.203

Edited August 5, 2014 by iChris

[iChris]

 

“Dissymmetry of lift is the differential (unequal) lift between advancing and retreating halves of the rotor disk caused by the different wind flow velocity across each half.

 

This difference in lift would cause the helicopter to be uncontrollable in any situation other than hovering in a calm wind. There must be a means of compensating, correcting, or eliminating this unequal lift to attain symmetry of lift.

 

In forward flight, two factors in the lift equation, blade area and air density, are the same for the advancing and retreating blades. Airfoil shape is fixed for a given blade, and air density cannot be affected.

 

The only remaining variables are blade speed and angle of attack. Rotor RPM controls blade speed. Because rotor RPM must remain relatively constant, blade speed also remains relatively constant. This leaves angle of attack as the one variable remaining that can compensate for dissymmetry of lift. This is accomplished through blade flapping and/or cyclic feathering.”

 

REF: U.S. Army Field Manual FM 3-04.203

 

 

“An important concept in rotor analysis is the equivalence of feathering and flapping. As far as the blade is concerned its angle of attack with respect to the tip-path plane is the only thing of importance.”

 

Ray Prouty, Helicopter Aerodynamics Volume I, Blade Flapping and Feathering

 

Again, as I stated in my post above, variation in velocity across the rotor disk, blade flapping, and cyclic feathering generally account for the angle of attack distribution.

 

We need to take all three in combination, tangential velocity, flapping, and cyclic feathering. When you move from the hover into forward flight things start to happen. The tangential velocity variations across the rotor disk cause the blades to flap, which requires you to make corrective cyclic inputs to counter unwanted rotor flap-back and roll effects.

 

The problem is most of your training books for simplicity; limit their discussion to only blade flapping. 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.”

 

Once you move into forward flight the angle of attack distribution is effected by tangential velocity variations, flapping, cyclic feathering, coning, and inflow.

 

Edited August 18, 2014 by iChris

[Guest pokey]

Pokey, the cyclic pits pitch on the blade, the blade's lift changes, and the lift causes the blade to rise or fall - flapping. Result - the cyclic controls the blade's flapping. So does the collective.

 

If you couldn't control the flapping with cyclic, the machine would not fly in a predictable manner.

 

the cyclic controls the feathering, take a look at chris's diagrams, max pitch is on the retreating blade side (which has the minimum flap), you will also notice that minimum pitch has the maximum flap (the advancing blade side).

 

If what you are saying were true? why isn't the max flap and max pitch on the same side?

Edited August 5, 2014 by pokey

[helonorth]

Lets not forget about phase lag. The swash plate control inputs manifest 90 degrees later in the direction of rotation. In forward flight, maximum feathering angles would be at the rear of the disk, tilting the disk forward. To me, that would make the blade AOA's about the same on the retreating and advancing sides. Flapping now compensates for the change in velocity.

 

Mr. Hunt, you shouldn't be too terribly surprised by our lack of knowledge as to what you are saying since you are the only one saying it.

[Eric Hunt]

Helonorth, look at Chris's diagrams showing angles of attack around the disc. You might notice that they are different all around, the AoA is NOT the same on the advancing and retreating sides. The swash plate is continually changing the pitch through every cycle of rotation (Hey! THAT might be why it is called CYCLIC!)

 

Pokey, you didn't say which diagram you were referring to. The maximum RATE of flap is 90 degrees ahead of the maximum (or minimum) POSITION of flap. For the disc to tilt forward (Min flap at front) the min pitch angle, and thus the max rate of flapping down, is at 90 right. Then further around the disc, to get the max UP position of the disc, over the tail, the max pitch angle and max rate of up flap is 90 left. And that is where the retreating blade will stall, 90 left.

[Guest pokey]

 

Pokey, you didn't say which diagram you were referring to.

 

they all pretty much say the same thing as far as how much pitch and flap. Except for 2-33, which is caused by transverse flow effect