Dynamic stall discussion

[ACE-OF-ACES]

Other than that there is a debate about Dynamic Stall ...
Got Link? Maybe someone overthere can explain the 'key' differences between an acc and dynamic stall?

[Insuber]

Got Link? Maybe someone overthere can explain the 'key' differences between an acc and dynamic stall?

it's not relevant, and rather off topic. The fact is that the dynamic stall is not modeled in CloD.

[ACE-OF-ACES]

it's not relevant, and rather off topic. The fact is that the dynamic stall is not modeled in CloD.
Hard to say since I have not seen the topic.. Just curious as to what they think the difference is between the two if any

[Verhängnis]

From my understanding a dynamic stall is when the Wing (Airfoil) changes Angle of attack so dramatically that the leading edge creates turbulent, fast moving airflows (Vortex's) which travel over the top of the wing created an increased lower pressure momentarily increasing lift cooeficient but once the vortex has passed the wing incurs a normal stall where there is not enough airflow over the airfoil to generate lift.

So essentially a delayed stall, and IMO this effect would even really be that noticeble. This is precisely why the Bf-109 has leading edge slats, aswell as the Me-262. It slows the airspeed and reduces/delays the effects of a stall by reducing pressure and turbulent airflow at the leading edge.

...

I don't see why they can't disclose any information about the sequel or it's compatibility with BoB, it's seriously seems quite childish. It's not like it's a National secret or matter of security, nobody is out to get their ideas for a WW2 Battle of Moscow Combat Flight Simulator.

[ACE-OF-ACES]

From my understanding a dynamic stall is when the Wing (Airfoil) changes Angle of attack so dramatically that the leading edge creates turbulent, fast moving airflows (Vortex's) which travel over the top of the wing created an increased lower pressure momentarily increasing lift cooeficient but once the vortex has passed the wing incurs a normal stall where there is not enough airflow over the airfoil to generate lift.
That follows the wiki definition I provided a few pages back.. but the wiki also included a reference for that [22] which expanded on that definition to point out that a lot of the dynamic part of it had to do with the dynamic nature of the atmosphere (wind vector)..

So it appears that an accelerated and dynamic stall share the following

1) load factor > 1g
2) stalls at higher speeds than normal stall speed (1g LF)

And what sets them apart is simply the rate of change in the AoA.. Which is probably why they call it dynamic?

But both will stall at the same critical AoA.. Just the dynamic will experience a brief increase in lift just before the stall.. due to the vortex movement.

So the question is how brief is brief?

Are we talking tens of seconds or tens of Millie seconds? If the later, than it is not worth adding the additional math to simulate the brief increase in lift because it will go un-noticed to the sim pilot. That is to say, in the real world an instrumented plane under test might notice (log the data) the Millie second spike of increased lift.. But the human won't.. So no need to simulate it IMHO.

[Verhängnis]

Given the normal airspeed over at wing in a stall, if this is accelerated, then that probably means it twice as fast, enough to generate lift, the duration would be so short, and definitely, as a fighter pilot trying to survive, it's the last thing I'm worrying about. Just don't stall! ;)

[VO101_Tom]

Dynamic stall. Three things must be understood:
- If you pull the stick, you can not raise nose of the machine, but the tail press. This is an important thing, though normal flight conditions, do not even notice the difference (if you start climb with huge speed, the air flow AoA remain in small angle, no matter what is the a/c climb angle).
- If a large mass, high-speed flying, it will be great inertia.
- If the angle of the wing is above the critical value, it will stall. Speed ​​may depend on how much value, and when, but it certainly happens.
http://www.youtube.com/watch?v=xW63SZ1LAqo&feature=related

Now imagine what happens when a high speed, sudden you pull hard on the stick. The tail pushed down, but the inertia does not allow the aircraft to rise immediately. The wing angle of attack instant above the critical value, and it will stall. That's the dynamic stall. High speed, high G.

The best example of a dynamic stall: Pugachev's Cobra
http://www.youtube.com/watch?v=1EpJ3KoUNmI

[Insuber]

Are we talking tens of seconds or tens of Millie seconds? If the later, than it is not worth adding the additional math to simulate the brief increase in lift because it will go un-noticed to the sim pilot. That is to say, in the real world an instrumented plane under test might notice (log the data) the Millie second spike of increased lift.. But the human won't.. So no need to simulate it IMHO.

This series of funny statements illustrates perfectly how ignorance tries stubbornly to rule the knowledge.
Dynamic stall is a violent phenomenon, leading to a huge loss of speed / energy. Just for sake of simplicity: pull strongly the stick at high speed in real life, even in straight flight, and you will stall. In this game you can pull strongly the stick at high speed, and your plane climbs. The flight envelope is false. So: big need to simulate the dynamic stall, in my educated opinion.

Cheers!

[Skoshi Tiger]

Aircraft can do loops and depending on how much energy and or power and or altitude you have on hand you can do multiple loops one after the other as long as your angle of attack does not exceed the stall angle. (ie you still have to fly the aircraft within it's flight envelope)

I expect this would effect the 'going vertical' manuvers where the pilot just yanks back on the stick and expects all their aircrafts energy to be converted to height or violent break turns.

I found (what i thought) reasonable description of a dynamic stall at
http://psasir.upm.edu.my/3610/1/An_Examination_of_Dynamic_Stall_Vortex_Inception.p df


When an aerofoil is pitched through and beyond the incidence of static stall at a
sufficiently high pitch rate, the resulting series of events is often termed dynamic stall.
It is characterised by a significant lift overshoot, followed by a sudden loss of lift and a
major surge in pitching moment. Carr, (1977),

Interesting reading.

cheers!

[klem]

I'm no aerodynamicist but here's an explanation of Static, Dynamic and Accelerated stalls
http://www.futurecam.com/stalls.html
Yes, its written about parachuting but the principles should be the same: angle of attack; rate of change of angle of attack; the added effect of increased G loading.

Whether CoD models Dynamic and Accelerated stalls I'm not sure. I'd be surprised and disappointed if it doesn't. I guess we'd need some a/c data to test against to find out.

[winny]

I have a question...

Is a dynamic stall the same thing as what the British refered to as a 'high speed stall'?

[Skoshi Tiger]

I have a question...

Is a dynamic stall the same thing as what the British refered to as a 'high speed stall'?

I was just wondering the same thing.

After reading the Spitfire MKI manual, I was doing a few stalls. From 250mph I was pulling back on the stick into the climb before stalling at about 160mph, drops a wing violently and then enters a spin.

Unfortuately it's hard to work oput if this is due to a dynamic stall or an accelerated stall.

If we knew what G's we were pulling be able to calculate the stall speed. If that corresponded to the stall speed in the test, then that would indicate a accelerated stall and not a dynamic stall, Or we could ask a question to the developers (which Insubar already has) and wait for a response. :)

Cheers!

[bongodriver]

at 2 g's its about 1.4 times the static stall speed, I seem to recall the formula having something to do with the square root of the G's

[Insuber]

Hard to say since I have not seen the topic.. Just curious as to what they think the difference is between the two if any

It was so off-topic that Uther created another topic for this.

Back on *this* topic: as anyone hopefully understands now, and as some clever guy pointed out already few posts above, the lack of Dynamic Stall in this sim allows one to convert all his kinetic energy (speed) into positional energy (height), at any AoA (Angle of Attack) and speed. This of course is based on my own essays, mostly on 109, Spit and Hurricane, and I will be glad to be proven wrong.
This is why I called out this subject as "very important" with respect to the FM.

Cheers,
Insuber

[IvanK]

I have a question...

Is a dynamic stall the same thing as what the British refered to as a 'high speed stall'?


No A High speed stall is the same as an accelerated stall. i.e. A stall occurring at greater than 1G therefore at a faster speed than the 1G stall ... but occurring at the same AOA. If your 1G Stall speed is say 100Kmh your 4G stall speed would be 200kmh (1G Stall speed X SQR of the Load Factor).

A Dynamic stall is another condition that involves Stall AOA and rate of change of AOA.

[Insuber]

... Or we could ask a question to the developers (which Insubar already has) and wait for a response. :)

Cheers!

Yes it is what I did, despite the desperate efforts of my friend Ace to entropyze the discussion ... :)

[Insuber]

Best answer to this thread, just published by BlackSix:


Will the dynamic stall be implemented in the Flight Model?

4.
Yes, it’s one of the things the new FM programmer will work on.

[ACE-OF-ACES]

This series of funny statements illustrates perfectly how ignorance tries stubbornly to rule the knowledge.
No don't be so hard onyourself! I realise you were un-able to explain the difference which is why I ask you for the link in the russian forum

Dynamic stall is a violent phenomenon, leading to a huge loss of speed / energy.
Ah good so you agree with me and the link I posted a few pages back, i.e.

Advanced Topics in Aerodynamics (http://web.archive.org/web/20070914040546/http://www.aerodyn.org/Dstall/dstall.html#below)

Where it defines a dynamics stall as..

Dynamic stall is a phenomenon that affects airfoils, wings and rotors in unsteady flows. It is due to changes, periodic or not, in the inflow conditions and/or angle of attack. In some cases, such helicopter rotors in advancing flight, dynamic stall is intrinsic to their state of operation.

Just for sake of simplicity: pull strongly the stick at high speed in real life, even in straight flight, and you will stall. In this game you can pull strongly the stick at high speed, and your plane climbs. The flight envelope is false.
What your forgeting is there is a chance that CoD does simulates stick inputs the same way IL-2 did.. Where the stick input is not a position offset as much as a force input.. That is to say just because you move the still all the way 'back' does not mean your elevator moved all the way 'up'.. What is actully going on is when you move your stick all the way ack, your have appled the max force input.. Thus if your max force input is limited to 20lb and due to the airspeed it would require 60lb to fully deflect the elevator.. Well you can see your not going to fully deflect the elevator

So: big need to simulate the dynamic stall, in my educated opinion.
And your welcome to it! S!

[ACE-OF-ACES]

I found (what i thought) reasonable description of a dynamic stall at
When an aerofoil is pitched through and beyond the incidence of static stall at a sufficiently high pitch rate, the resulting series of events is often termed dynamic stall. It is characterised by a significant lift overshoot, followed by a sudden loss of lift and a major surge in pitching moment. Carr, (1977),
That is the best definition thus far IMHO..

It highlights that the difference between a acc stall and a dynamic stall as being the rate of change in AoA.. And makes note of the 'brief' increase lift (lift overshoot) due to the high rate of change in AoA.. The point I think some are missing is the critical AoA must be reached in both cases to stall.. Just that the dynamic has a brief overshoot past that point with a brief increase in lift.. Thus if CoD is not stalling now due to large/fast stick movements.. Than it is not simulating acc stall either.. Or as I noted the stick force input is limited to such a low value that you can not move the elevator far or fast enough to cause it to happen.. To be honest, I dont fly like that (jerk the stick) so I have not noticed the acc stall missing if it is missing

Thanks for the link Tiger

[TomcatViP]

Dyn Stall =

A stall that has its root in a dynamic phenomena. A stall is essentially linear : the AOA increase (or the speed decrease at a given aoa). In the dyn stall the flow ard the wing is disturbed briefly inducing non linear effect

The source can be mechanical (aileron reversing), aerodynamic (temp vortex) or thermal (critical temp inducing critical external pressure change - eg turbulences)

An example will be the rotor blade turning around it's axis. The blade AoA hve to be raised when the blade travel backward in order to compensate for the loss of speed and equilibrate the lift generated by the rotor.

At certain speed the AoA variation can be critical to the airflow generating vortex on top of the blade that prevent the normal airflow circulation. The flow is separated at a higher than normal speed and the blade is stalled (generating a high amount of drag).

But this has more to do in the FM thread, no ? ;)

An example here in a short vid http://www.aerobuzz.fr/spip.php?article866


PS: all the ppl that hve some sort of knowledge were among the "ignorants" before.

[Insuber]

No don't be so hard onyourself! I realise you were un-able to explain the difference which is why I ask you for the link in the russian forum


Ah good so you agree with me and the link I posted a few pages back, i.e.

Advanced Topics in Aerodynamics (http://web.archive.org/web/20070914040546/http://www.aerodyn.org/Dstall/dstall.html#below)

Where it defines a dynamics stall as..




What your forgeting is there is a chance that CoD does simulates stick inputs the same way IL-2 did.. Where the stick input is not a position offset as much as a force input.. That is to say just because you move the still all the way 'back' does not mean your elevator moved all the way 'up'.. What is actully going on is when you move your stick all the way ack, your have appled the max force input.. Thus if your max force input is limited to 20lb and due to the airspeed it would require 60lb to fully deflect the elevator.. Well you can see your not going to fully deflect the elevator


And your welcome to it! S!

You are welcome my friend! I'm glad that you evolved from your yesterday's position of "what's a dynamic stall", and then "no need to waste time to model the dynamic stall", IIRC ... Only fools never change their mind, and you're not one of them luckily.
Regarding the topic, I've found the issue of , I've asked the question to B6 and I've got my answer. And I'm happy that someone learned some lil' bits of aerodynamics in the process.

[ACE-OF-ACES]

Dyn Stall =

A stall that has its root in a dynamic phenomena. A stall is essentially linear : the AOA increase (or the speed decrease at a given aoa). In the dyn stall the flow ard the wing is disturbed briefly inducing non linear effect

The source can be mechanical (aileron reversing), aerodynamic (temp vortex) or thermal (critical temp inducing critical external pressure change - eg turbulences)

An example will be the rotor blade turning around it's axis. The blade AoA hve to be raised when the blade travel backward in order to compensate for the loss of speed and equilibrate the lift generated by the rotor.

At certain speed the AoA variation can be critical to the airflow generating vortex on top of the blade that prevent the normal airflow circulation. The flow is separated at a higher than normal speed and the blade is stalled (generating a high amount of drag).
Agreed in that your definition agrees with the links I have posted thus far..

S!

[ACE-OF-ACES]

You are welcome my friend!
Ditto!

I'm glad that you evolved from your yesterday's position of "what's a dynamic stall"
Ah, now I see why you were so confused!! I wasn't asking what a dynamic stall is as much as I was asking what 'they' (you being one of the they) were saying what a dynamic stall is.. Does that help you?

, and then "no need to waste time to model the dynamic stall",
That still depends on how brief is brief.. As I noted, if CoD is not currently simulating accelerated stalls, than yes that should be added.. But the difference between an accelerated stall and a dynamic stall could be so small as to go un-noticed, and thus not needed to simulate. For example, if your telling me that in a dive in CoD, if I pull the stick hard & fast and the plane never stalls.. Than CoD is not even simulating an accelerated stall and it should.. But assuming it does stall.. Than what your talking about adding is the 'brief' increase in lift just prior to the stall to the accelerated stall code to simulate the dynamic stall.. My point an others is that 'brief' spike in lift may go un-noticed if it only lasts for a Millie second.. Does that help you?

IIRC ... Only fools never change their mind, and you're not one of them luckily.
This is true! I am very open minded.. So open minded that I ask people to explain what it is they are referring to as oposed to assuming what it is they are referring too.. Problem is there are some here he don't really know what they are talking about and thus can not explain what it is they are referring to without showing everyone they really don't know what they are talking about

Regarding the topic, I've found the issue of , I've asked the question to B6 and I've got my answer. And I'm happy that someone learned some lil' bits of aerodynamics in the process.
And only time will tell.. Guess we will have to wait for the CoD update/patch to see who really learned anything here today! S!

[Insuber]

ACE, I think that being in a forum you must be more straightforward and direct, rhetoric questions and elliptic thesis get easily misinterpreted. I thought indeed that you ignored a simple concept such as the now famous DS, and denying the need to add it to the FM. Moreover I thought erroneously that you were trolling around trying to hijack the discussion and confuse B6. I'm glad to discover that you are the finest and deepest expert of aerodynamics of this forum. S!

Cheers,
Ins