Posted: Thu Apr 26, 2012 11:07 am Post subject: retreating blade stall
Hello there.
Back again with another theoretical question. I´ve 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".
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?
Joined: Feb 14, 2008 Posts: 879 Location: Stavanger, Norway
Posted: Fri Apr 27, 2012 10:00 am Post subject:
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.
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 I´ve found the explanation. Im looking for this: "Ah there it is" feeling
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
Joined: Sep 05, 2009 Posts: 211 Location: the dog house
Posted: Sat Apr 28, 2012 6:21 pm Post subject:
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
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.
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?
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 I´m generally very satisfied with his book. And also, there is a great possibility that he is in fact right on this matter, and I´ve 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.
Im waiting for the invention of the co.axial multi rotor rotor
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.
Joined: Sep 05, 2009 Posts: 211 Location: the dog house
Posted: Sun Apr 29, 2012 6:51 pm Post subject:
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?
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.
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
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