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HeliTorque Forum Index » Flight Dynamics

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retreating blade stall
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sen
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PostPosted: Thu Apr 26, 2012 11:07 am    Post subject: retreating blade stall Reply with quote

Hello there.
Back again with another theoretical question. Ive been trying to understand a certain aspect of retreating blade stall = the fact that the blade stalls at the tip first.
Now if I read my wagtendonk book it says on page 138 (2006 edition), and I qoute:
"Even though the retreating blade has the aircraft forward speed deducted from its rotational velocity, its tip speed are still in the 300 to 350 knot region. The stall angles of a blade with these kinds of tip speeds are small. This is one of the main reasons why the retreating blade stall originates at the tip and works inward towards the hub."
So according to Wagtendonk, it stalls at the tip first because stall angles are smaller with a higher TAS. In a way that makes sense in my head - It makes sense to make that higher air speed causes the air to more readily separate from the airfoil. However, when I think back on my ATPL principles of flight lessons, a basic saying was: "a blade always stall at the same angle of attack, no matter speed".

I did find another explanation on the web:
http://www.ultraligero.net/Cursos/helicoptero/Introduccion_a_la_aerodinamica_del%20_helicoptero.pdf

page 69.

This one has a better explanation I think, that the stall angle remains the same thoughout the blade, and only the AoA varies due to twist, speed, induced flow etc.
qoute: "Figure 5-2 shows a rotor disk that reached a stall condition on the retreating side. It is assumed the stall AOA for this rotor system is 14. Distribution of AOA along the blade is shown at eight positions in the rotor disk. Although the blades are twisted and have less pitch at the tip than at the root, AOA is higher at the tip because of less induced flow or flow coming from below due to flapping."

So what are your thoughts? Is Wagtendonk wrong? Or are both explanations valid?

good day
/Sen
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haggishunter
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PostPosted: Fri Apr 27, 2012 10:00 am    Post subject: Reply with quote

Hi Sen,

The root of the blade can spent most if its time stalled during forward flight due to its slow rotational speed compared to the tip.

The rotational airflow over the blade is significantly reduced by the opposing airflow caused by the aircraft moving forwards at high speed. With the lower blade pitch at the tip and the reduced airflow over it, the blade will gradually produce less lift from the tip inwards. Until it meets the stalled region at the root.

As this is happening the aircraft will begin to roll towards the retreating side, this will cause an increase in airflow from below increasing the AoA. The tip will see the largest downward movement causing the largest change in AoA, eventually stalling it if not recovered.

Adding opposing cyclic will not help, as you are causing the blade pitch to increase on the retreating side, further increasing the AoA. Reducing collective and slowing down is the only way to begin recovery from retreating blade stall, providing it is still in a recoverable state.

Two ways of getting retreating blade stall:

- Sharp, abrupt rolls towards the retreating side;

- High speed forward flight around or above Vne.

A combination of both would also greatly aggravate the situation. High density altitude also has a big influence, hence reduced Vne at higher altitudes. There are several reason Vne reduces with altitude, RBS is one of them.

I have never been the best at explaining aerodynamics, but I hope this helps alittle.

HH
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sen
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PostPosted: Fri Apr 27, 2012 2:50 pm    Post subject: Reply with quote

Hi HaggisHunter, thank you for your answer. However, Im not entirely happy with it.

HaggisHunter wrote:
The root of the blade can spent most if its time stalled during forward flight due to its slow rotational speed compared to the tip.

The rotational airflow over the blade is significantly reduced by the opposing airflow caused by the aircraft moving forwards at high speed. With the lower blade pitch at the tip and the reduced airflow over it, the blade will gradually produce less lift from the tip inwards. Until it meets the stalled region at the root.


Yes, so the root doesnt produce lift due to lack of airflow, however Im not sure its stalled in the traditional sense. Its just "reverse airflow area"?
In all cases, what every textbook says, as far as Im concerned, and what every ATPL question says, questionbank or not, is that it stalls at the tip first, and that is what Im trying to find the reasons as to why, because to some extent I agree with you - it should lowspeed stall at the root first, we have very low airspeed there. Nevertheless, everywhere its said: "nonono it stalls at the tip first". Why, I ask? What is the exact explanation to that?

HaggisHunter wrote:
As this is happening the aircraft will begin to roll towards the retreating side, this will cause an increase in airflow from below increasing the AoA. The tip will see the largest downward movement causing the largest change in AoA, eventually stalling it if not recovered.


Here I guess we somewhat agree. The second explanation I found in my first post, the one that says it stalls at the tip first due to flapping, however I dont believe it stalls due to excessive flapping coming from a roll towards the retreating side, I believe the flapping already taking place due to dissymetry of lift, is responsible, as this flapping increases with increasing forward speed, as Im sure you are well aware of.

HaggisHunter wrote:
Adding opposing cyclic will not help, as you are causing the blade pitch to increase on the retreating side, further increasing the AoA. Reducing collective and slowing down is the only way to begin recovery from retreating blade stall, providing it is still in a recoverable state.

Two ways of getting retreating blade stall:

- Sharp, abrupt rolls towards the retreating side;

- High speed forward flight around or above Vne.

A combination of both would also greatly aggravate the situation. High density altitude also has a big influence, hence reduced Vne at higher altitudes. There are several reason Vne reduces with altitude, RBS is one of them.


I agree with you on how to get it, and how to recover.
Im not trying to shoot your theories to the ground, Im just not quite convinced Ive found the explanation. Im looking for this: "Ah there it is" feeling Wink

I would love to get this debate rolling with some more inputs. Perhaps Veeany or some other guy, I understand has a thing or two to say about aerodynamics, would enlighten us Smile
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flip2
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PostPosted: Sat Apr 28, 2012 6:21 pm    Post subject: Reply with quote

My simple understanding:

The first thing to happen is the retreating blade encounters airflow reversal. This spreads out from the root.

This means that a progressively smaller section of the retreating blade is producing lift.

This is countered by the retreating blade operating at increasing angles of attack. The tips will reach a point where the critical angle of attack is exceeded, and the stall then spreads inboard. At this point, it gets given the label of retreating blade stall.

Almost certainly an oversimplification with aerodynamic half-truths, but it works for me Smile

Please remember that retreating blade stall is a term given to a specific aerodynamic phenomenon. To ask about "retreating blade stall" is not the same as asking "which bit of the retreating blade stalls first"... because sections of the retreating blade are in a stalled condition even when the "retreating blade stall" phenomenon has not been encountered.

Perhaps semantics is the cause of your confusion?
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PostPosted: Sun Apr 29, 2012 6:09 pm    Post subject: Reply with quote

flip2 wrote:
Please remember that retreating blade stall is a term given to a specific aerodynamic phenomenon. To ask about "retreating blade stall" is not the same as asking "which bit of the retreating blade stalls first"... because sections of the retreating blade are in a stalled condition even when the "retreating blade stall" phenomenon has not been encountered.

Perhaps semantics is the cause of your confusion?


So we can agree that for some time before the phenomenon retreating blade stall occurs, the root of the blade is already stalled due to airflow reversal.

I still have questions:

Is it true then, by definition, that the phenomenon retreating blade stall occurs the moment we no longer just have root stall, but also tip stall?

If the answer to that question is yes, then:

What causes the tip stall? Why doesnt the stall just spread progressively from the root to the tip? Is it flapping, as suggested in the link on my first post, or is Wagtendonk right in his explanation about "increasing airspeed reduces stall angle"? Are they both right? Am I making too big a deal out of something I should not? Smile

flip2 wrote:
The first thing to happen is the retreating blade encounters airflow reversal. This spreads out from the root.

This means that a progressively smaller section of the retreating blade is producing lift.

This is countered by the retreating blade operating at increasing angles of attack. The tips will reach a point where the critical angle of attack is exceeded, and the stall then spreads inboard. At this point, it gets given the label of retreating blade stall.


From what I can read from your previous post, Flip2, you believe its due to flapping? I agree with you on that, but what are your thoughts on Wagtendonks theory?

Let me just remind you the reason I started this thread. It was, and still is, because I feel Wagtendonk is wrong when it comes to his explanation on why it stalls at the tip first. No disrespect to the man, I dont doubt his knowledge, and Im generally very satisfied with his book. And also, there is a great possibility that he is in fact right on this matter, and Ive just not quite understood it yet.

I hope I make myself clear, else dont hesitate to mention. I have a habit of thinking too much while typing, and thus not always make myself clear. It all boils down to the question, why does it stall at the tip first. What is the cause/causes.
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PostPosted: Sun Apr 29, 2012 6:44 pm    Post subject: Reply with quote

As if it wasnt already complicated enough: have a look at this.

http://www.youtube.com/watch?feature=player_detailpage&v=tbBET56H8wU
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PostPosted: Sun Apr 29, 2012 6:45 pm    Post subject: Reply with quote

Im waiting for the invention of the co.axial multi rotor rotor Laughing
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PostPosted: Sun Apr 29, 2012 6:50 pm    Post subject: Reply with quote

paddywak wrote:
Im waiting for the invention of the co.axial multi rotor rotor Laughing


hehe good one. Also, I cant imagine what those engineers at Sikorsky and Eurcopter have gone though, and are going through developing x2 and x3 respectively.
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PostPosted: Sun Apr 29, 2012 6:51 pm    Post subject: Reply with quote

sen wrote:
Is it true then, by definition, that the phenomenon retreating blade stall occurs the moment we no longer just have root stall, but also tip stall?

That I can't answer. The point could be considered when the stall first develops at the tips... or it could be when the effects are first felt. Or something else entirely... I guess it depends on whoever coined the phrase! (not a good answer - I just don't have enough knowledge to answer it)

sen wrote:
Am I making too big a deal out of something I should not? Smile

For operational knowledge as a regular pilot, I'd say yes. But as a point of discussion, or as an aerodynamicist, I'd say it's a good discussion.

sen wrote:
It all boils down to the question, why does it stall at the tip first. What is the cause/causes.


I can't answer definitively. To my knowledge, the stalls occurs when the critical angle of attack is exceeded. There are various factors which change the angle of attack, and you'd probably have to consider all of them as contributing. Flapping is most pronounced at the tips (see this video http://www.youtube.com/watch?v=Ug6W7_tafnc). But I imagine you also have to consider the faster speed at the tips, blade twist and other aerodynamic considerations at various Mu ratios.

This topic is beyond my level of knowledge, so I am merely giving suppositions. You may find Nick Lappos or Shawn Coyle on other forums could give you a good clear answer.

I'm sorry I can't be of more help.
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PostPosted: Sun Apr 29, 2012 6:59 pm    Post subject: Reply with quote

flip2 wrote:
I'm sorry I can't be of more help.


Dont say that, Im very grateful of your replies. They, and all other post, add up to my understanding of what is going on. That definite answer to what exactly happens might not even exist with the level of technology, knowledge and whatnot present in the year 2012 Smile
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PostPosted: Fri Aug 30, 2013 12:38 am    Post subject: Reply with quote

Due to the retreating blade flapping down to try and equalise the lift across the disc, the tip of the retreating blade, although generally having a low pitch angle, has a relatively large rotational flow and a small induced flow. With increaing aurspeed, the reduction of induced flow due to more flapping down really increases the local AoA and causes the retreating blade to stall from the tip. The AoA can be as high as 21 degrees (could be as low as 5 to get the same lift on the advancing blade) which is much higher than the normal stall angle. However some tips (BERP) can still produce lift at the tip on the retreating side by producing a vortex to energise the boundary layer and have a bigger fineness ratio that keeps the laminar flow for longer. The secondary effect of the notch on a BERP blade also stops the spanwise (tip to root) spreading of any stall. I hope this helps.
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PostPosted: Fri Aug 30, 2013 6:56 am    Post subject: Reply with quote

Why do people persist in saying that the retreating blade is flapping down?

As soon as you put some cyclic in to prevent flapback, the "flapping to equality" is overcome by the blade's feathering action. The blade is in fact flapping UP, otherwise why is it at its highest position over the tail?

To answer the original question:

Draw a rotor blade segment, with a bit of pitch angle on it, and an arrow coming into it from the plane of rotation - this is your rotational airflow.
At the origin of this airflow arrow, draw the induced flow arrow pointing downwards at 90 degrees, with the induced arrow head touching the tail of the rotational flow arrow.

Now you have a combination of 2 vectors, and if you draw another arrow starting from the top of the induced flow arrow and going to the head of the rotational arrow, you get a resultant vector, the relative air flow.

The angle between the RAF and the chordline of your blade segment will give you the angle of attack. It might not be very big.

Now, make the rotational flow bigger (longer) but leave the induced flow arrow the same - you have to drag the arrow away from the blade to keep it touching the end of the rotational flow. Redraw the RAF, and now you can see that the angle of attack has got bigger. More rotational airflow, bigger angle of attack.

Now, in forward flight at high weights and speeds, the pitch angle on the blade is much bigger, and the angle of attack approaches the stall. If you look at the part of the blade where the angle of attack is right at the stall, and then move a little further out, the extra rotational flow adds enough to the angle of attack to cause a stall.

The stall happens due to rotational airflow getting bigger and pitch angles being relatively the same - the washout towards the tip isn't enough to stop this from happening. Thus, if you get into retreating blade stall, the cure for it is NOT increasing rotor RPM, because that will cause the stall to progress inboard and make it all worse.
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