The new critical angle of attack might be too low! - Page 8 - ED Forums
 


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Old 05-01-2017, 02:03 PM   #71
Maverick Su-35S
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Quote:
Originally Posted by BadHabit View Post
...No matter the speed or flaps usage whilst in F2 view other ac's lets say F-5 can read 20++ AoA. Su-25 has show max AoA 15 before stall. I'm asking is the mig 5 degrees and more less capable on AoA? It also feels it cannot pull anymore as it looks it should...
Speaking of witch, I've read somewhere about the F-5E's in flight performance and was amazed to notice that although it's wings start stalling above 25 true AoA when droops are fully deployed, the flow that remains attached to the wing surface along the path of the strong vortexes created by the LERX (leading edge root extension) only separates above +70 AoA where the vortexes break up (supercritical AoA). Although the F-5's wings have a higher wing aspect and lower wing sweep than those of a MIG-21, which mandatorily reduces the critical AoA, the early F-5's wings which were fitted directly to the fuselage without any LERX or apexes were stalling above 18..19 AoA when the droops and flaps were fully retracted. The droops usually increase the AoA with around 4..7 AoA depending on their design. Slats (fowler leading edge devices) however can increase the AoA with as much as 8..12 AoA (737s slats do so). So even like that (without droops and flaps) the F-5's wings had higher AoA than what those on the MIG-21 right now.

The Su-25s indeed start to encounter aerodynamic buffet when the AoA indexer reaches critical and the true AoA is around 15, but the lift to AoA slope is still positive (although curved) between 15 and 18 AoA. Only above 18 AoA the wings of the Su-25s physically start to develop stall (flow separation). The Su-25s, A-10s and L-39s with their straight and high aspect ratio wings still have a critical AoA above 17..18 AoA, so it makes no sense for the 21's wings to stall even earlier than that. The maximum thickness and camber of airfoils indeed affect the critical AoA (higher thickness and camber give higher critical AoA), but only by 2..4 degrees of AoA (so not that much) between a very thin and straight and very thick and cambered one. The strong impact on critical AoA is controlled by wing aspect ratio and sweep. Although both the airfoil shapes and wing aspect ratio and sweep (combined) govern the maximum achievable critical AoA, the airfoils used only affect about 25..30% while the aspect ratio and sweep affect 70..75% of it. So, once more, through every example the MIG-21's wings should provide a much higher AoA before the flow separates.

I'm not making this up nor wanting to waste my time, but what I'm saying is based on years of research/experience and can be found on the internet (if one has the proper patience and knows what to search) and technical reports and LN or other 3rd party members can start researching to make sure that people are right and not trolling or trying to create confusion.

Here's an interesting article regarding the MIG-21's in flight performance:

http://www.military-quotes.com/forum...is-t86206.html

Exactly what I'm saying about the critical AoA is also stated here. Just scroll down to about half of the discussion and you'll find this:

"Aircraft’s stall speed (speed at which dynamic directional stability breakdown occurs) is function of Mach number, because directional and lateral static stability usually decreases with speed. Stall angle of attack decreases from above 30º (far beyond indicated α) at Mach 0.2 to 20º (i.e. 33 units local angle of attack on indicator) at Mach 0.95.

In those days when MiG-21 was designed, electronic flight controls to limit the angle of attack in function of Mach number didn’t exist. A fighter was built primarily for high speeds, high altitude interceptions. At slower speeds previous generations MiG-19/17 were better.

Designers put the angle of attack indicator, calibrated in local angle of attack, to warn the pilot of approaching stall limit. At recommended and allowed limit 28 units (about 17º true angle of attack) safety margin to stall is from 13º at Mach 0.2 to 3º at Mach 0.95."

"Just before stall α, aircraft nose would start wandering accompanied by more noticeable wing rocking (roll oscillations that intensify thru the stall), symptoms of dynamic directional instability.
Stalling proceeds more vigorously with fewer signs at higher subsonic speeds."

The statements tell important (and nice) effects that need to be considered for more accurate/correct simulation. The fact that near stall AoA (18.20) the MIG-21 should experience reduced lateral (roll) and directional (yaw) stability which allows the wings to start rocking more and more as the AoA passes stall (above 20). The rocking effects will normally dissipate a couple of more degrees of AoA above stall (some 2..3 more) while the lift would still remain high (perhaps with slight drops). If not asking too much, we're expecting to see this simulated as well (if possible) in the future updates.


Regards.
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Last edited by Maverick Su-35S; 05-03-2017 at 05:51 AM.
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Old 09-11-2017, 05:46 AM   #72
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and nothing...
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Old 01-20-2018, 01:35 AM   #73
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Again..., I don't even know what to do to convince these guys who made the MIG-21 in DCS that THE SIMULATED CRITICAL ANGLE OF ATTACK IS TOO LOW FOR THIS PLANE'S WINGS! All the facts are gathered around and still they won't review this problem?

There are 3 modifications that need to be done inside the aerodynamic input data tables:

1. Reduce the CL0 - the AoA 0 lift coefficient. It is too high and for this reason this aircraft would be flying supersonic (through shockwaves) at a negative AoA at 1G, which is only possible if this plane has a highly cambered airfoil section, which is not true! The MIG-21 has an almost symmetrical wing airfoil from root to tip (Tsagi S12 has a very low camber), therefore, the CL0 must be very low (quite close to zero). Some data on the internet says it is in fact purely symmetrical, but actual airfoil coordinates show that it is very lightly cambered. Still, the CL0 value must be very low as compared to what it is right now.

2. Increase the critical AoA from 15 to 20..20.5, which corresponds to this wing, NOT airfoil, but whole wing.

3. Reduce the lift versus AoA slope according such that the maximum CL (or lift coefficient) would not be altered after correcting the critical AoA.

3 steps and everyone's happy again. What makes it so hard? You don't trust what I'm saying, right? You need "proof" as always. OK! But one thing's for sure: Even if we don't have access to real wind tunnel data, we should at least agree to the following logic:

If the A-10's higher AR wing, zero wing sweep, very high camber as compared to MIG-21's wing, has a critical AoA of 16, the MIG-15, a higher AR, relative sweep, non delta, has a critical AoA of 18, the Su-25, higher AR, very low sweep, has a critical AoA of around 17, how come someone forced a critical AoA on the MIG-21's wing to just 15? Keep in mind that between 2 wings with the same AR and sweep for example, one being a delta and the other a non-delta, the delta wing will always have a higher critical alpha if the airfoils don't differ too drastically (ie: if one is being symmetrical and the other highly cambered)!

Just by these contradictions one should understand that even if the true critical AoA isn't exactly known, it should definitely be higher than those of the mentioned aircraft. This is common sense for anyone with a good knowledge and experience in aerodynamics.
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Last edited by Maverick Su-35S; 01-20-2018 at 01:40 AM.
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Old 01-20-2018, 07:05 AM   #74
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May I suggest you post these in their Bug Tracker? I really feel your disappointment maybe a post there will trigger them?

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Old 01-20-2018, 07:53 AM   #75
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I think there is a slight angle between the fuselage datum axis and the wing chord line. And so the "AOA" shown in the software is the angle between the fuselage datum and the air flow. The AOA between the chord line and air flow might have a small fixed offset (~1°?) by construction. It should not be totally assumed that "F2" info bar AOA is showing actual chord AOA.

Everything else makes sense.
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Old 01-30-2018, 12:45 PM   #76
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Quote:
Originally Posted by Frederf View Post
I think there is a slight angle between the fuselage datum axis and the wing chord line. And so the "AOA" shown in the software is the angle between the fuselage datum and the air flow. The AOA between the chord line and air flow might have a small fixed offset (~1°?) by construction. It should not be totally assumed that "F2" info bar AOA is showing actual chord AOA.

Everything else makes sense.
Hello Frederf,

I know what you say and by documentation the Mig-21 has 0 wing incidence and yes the AoA shown in outside view is grossly taken as the angle between the undisturbed airflow upstream of the plane and the plane's X-axis. It's not correct giving the definition of the angle of attack, but anyways, the difference between what this wing should have as a maximum AoA and what it would be even if it would be 16 (as you'd suggest with that 1 degree of positive incidence) is unacceptably great.

I'll relate this on the bugs forum then.

Kind regards!
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Old 03-04-2018, 04:33 PM   #77
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Will this issue ever be fixed?
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Old 03-05-2018, 06:24 AM   #78
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I think we're pissing up a rope, development seems to have effectively stopped from LN/M3, apart from Rudel doing a bit of texture work it seems the lights are on, but no-one is home!
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Old 03-05-2018, 09:01 AM   #79
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I'm so sad about Mig 21 but I see the reality , if I want to fly soviet planes I will take a ride in Mig15...and wait for the Mig 19 will be much more complete in early acces then 4-5 years of development for the fishbed.
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Old 03-05-2018, 05:59 PM   #80
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If Magnitude can't get it done then I hope someone else can step in, anything else would be a real shame.
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