[RESOLVED] Angle of attack

[PiedDroit]

I apologize for using the incorrect terminology and making things more confusing.

 

I agree that there's not much to say on the topic now, we need facts.

 

On a Wing. A theoretical wing. The Mig21 is a lot more then its delta wing.

Regarding the quote from the article on wikipedia, the complete sentence was not taken, twisting the meaning and making the topic even more confused.

The full sentence is: "Some authors[3][4] do not use an arbitrary chord line, but use the zero lift axis instead — zero angle of attack corresponds to zero coefficient of lift."

So, the AoA in this sentence is completely different from the AoA discussed in this thread.

[iFoxRomeo]

Taylor, you claim to know it 100%, but from my point of view: What you write doesn´t confirm this claim.

 

First you say

...

I 100% know the aerodynamics of aircraft under Mach 1 inside and out,... Every wing needs positive AOA.

...

 

And now

...

The wing will still product lift at those negative AOAs in an asymmetrical cambered wing, but as mentioned before, it's not enough to keep it in the air.

And in the same post

...As mentioned before, if the flight model is representative of the real aircraft (as Dolphin said it was), then it is, and that's that.

Thats my point.

 

Anyway, as long as there is no hard data that the FM is wrong I consider it right. Even if I didn´t expect that behaviour of the MiG. Now it seems plausible.

 

For everyone who wants to discuss aerodynamics, open a separate thread. Thanks.

 

 

Fox

[ttaylor0024]

Taylor, you claim to know it 100%, but from my point of view: What you write doesn´t confirm this claim.

 

First you say

 

And now

 

And in the same post

 

Thats my point.

 

Anyway, as long as there is no hard data that the FM is wrong I consider it right. Even if I didn´t expect that behaviour of the MiG. Now it seems plausible.

 

For everyone who wants to discuss aerodynamics, open a separate thread. Thanks.

 

 

Fox

 

Again, my disclaimers were left out:

1. Level flight

2. Aerodynamics below transonic flight

 

But yes, this thread has run its course.

[JorgeIII]

Very interesting theoretical discussion here about Alfa and wing lift.

But the bug about the angle of attack got diluted with it. Real Mig-21 wont fly level with negative angle of attack and this error is still present in DCS-Mig-21 Bis current version.

[JorgeIII]

From real MIG-21 BIS English manual:

 

 

 

[Dirty Rotten Flieger]

I think there is a bug when respawning or rearming/refueling and maybe the engine doesn't make full power.

 

I respawned and tried to fly over 10k at 2.2mach at 1 or 2 degrees aoa as I have many times but couldn't get even close. Needed about 8 degrees aoa to hold level flight at 7 km altitude.

 

Loaded up another mission and easily got to 12km altitude, 2.2 mach, 2 degrees aoa.

[JorgeIII]

The thread is tagged as [Resolved] but in 1.2.15 build:

 

In game AoA is positive, not "almost" horizontal, at 500 km/h IAS and it descends with speed to the point of reaching negevative values at max speed (around -2,5 Alfa degrees at level flight).

 

As linked in a previous post, from Mig-21 Bis real manual:

 

"In level flight, at speeds of 500 km/h and higher, the angle of attack indicator pointer is in an almost horizontal position. ...will remain practically constant while...passing the transonic speed range, irrespective of the altitude or external loads "

AoA indicatior is not only important to control high G maneuvers, it is used "to promote the aircraft handling accuracy", is the reference instrument for approaching and landing.

 

 

Maybe the AoA of the EFM is to hard to fix without braking other EFM parameters or the manual is inaccurate or I just misinterpreted it (not being sarcastic here, the manual has a bad translation to english and it's hard to read/interpret sometimes).

Edited March 13, 2015 by JorgeIII

[Maverick Su-35S]

...

 

Airfoil of a wing is symmetrical, i.e. it uses only AoA and speed to create lift, not camber, because it doesn't have any. So it's a clear-cut case, positive AoA - positive lift, negative AoA - negative lift. That is for isolated wing, however and one must remember, that some lift is also created on non-symmetrical fuselage for most of the airplanes. Nevertheless, what we can see above would suggest that MiG's fuselage produces so much lift it offsets negative lift created by wings and that just... smells fishy :D.

 

Labels on the pitot switch in English pit are marked correctly compared to Russian one and the position shown on OP's screenshots indicate "main" operation indeed.

 

1. Tsagi S-12 is not symmetrical, it still produced minor CL (lift coeffiecient) at exactly 0 AoA.

 

2. The MIG-21's fuselage shape, especially due to it's very low planform projected area in relation to that of the wings, shouldn't produce even 20% of the wing's lift. The MIG-21's wings have zero degrees of incidence (angle between wing's root chord and fuselage's axis), so if the fuselage is at exactly 0 degrees on the artificial horizon so should the wing be at 0 deg AoA in also. That's the answer, not that the fuselage creates opposite lift to the wing, cause that won't happen, except when the wing's incidence would be drastically high positive or high negative...!

Edited January 20, 2017 by Maverick Su-35S
MIG-21's wing doesn't have 1 degree of incidence, but 0, so I had to re-formulate!

[Maverick Su-35S]

My instrument interpretations:

 

The KPP-1 shows slight angle of descent, nose down and below the artificial horizon.

 

HUD shows attitude pointing slightly below horizon; fixed net shows aircraft center-line pointing slightly below horizon.

 

AoA shows slight downward pitch.

 

External picture shows engine thrust pointing slightly downward, toward ground.

 

Ok, that's right!

 

Although the MiG-21BIS Pitot installation is not prone to freezing due to speed dynamic heating (air-friction heating effect), freezing might still happen.'

 

So, yes, freezing is possible.

My brain hurts...! What would the icing, or not, of the pitot tubes ever have to do with what you clearly see? The icing affects pressure sensors such as altimeter, variometer (vertical airspeed) and forward indicated airspeed and AoA vanes which you clearly know, but NOT EVER IN THIS WORLD it can affect gyroscopes (artificial horizon indicator) and what you can see! Why are you lying to yourself that way? There is no descent even in the video! Can't you see that even in the next video at high altitude where the density is almost a quarter than it is at sea level and so would the dynamic pressure and IAS, the nose is still pointing downwards?

You have probably entered the little-known "third flight regime" (also known as the twilight zone).

 

Well, if you mentioned this..., tell us about this magic zone where the lift goes against the laws of physics!

 

When any aircraft reaches a critical Mach number know as "Mach Drag Rise", where the drag starts rising rapidly (as Mach continues to increase towards 1) on par due to shock resistance (which besides the modification of important thermodynamic parameters, is nothing more than an abrupt deceleration of the upcoming air making that air almost move with the aircraft) and to "shock stall" (here's a link:http://scilib.narod.ru/Avia/DAC/dac.htm#1_1_3) where the LIFT suffers a massive degrade as Mach continues to increase up to a given number, from where the lift starts rising again with Mach number until the wing alone becomes supersonic (Mach 1-1,1...) and then will starts degrading again as the Mach number continues to increase towards hypersonic.

 

The "shock stall" which is inevitably going to happen to any man made aircraft once the wing of that aircraft has 3 zones of airflow on it (2 subsonic and one supersonic) has never been seen in DCS neither in SFM, nor in AFM/PFM up till this day, at least not by me. This wasn't such a great concern for me cause I haven't seen it here..., but seeing such aberrant values and seeing people trying to give all sorts of explanations to themselves of what is happening, explanations which ultimately tend to mislead from the initial facts and which will NEVER help us evolve and find the truth..., is a bit disappointing!;(

[Maverick Su-35S]

The 21 behaviour is normal.

 

With increase of the speed, distribution of forces changes, and there is significant lift generated which need to be compensated with negative AoA in order to keep the aircraft in level flight.

 

Wing is not the only surface that creates forces, although it is the most important.

 

Are you sure?...

 

If the real MIG-21 would fly straight and level at the so high negative angle of attack in our sim, please show me a real life footage of that or some relevant real life data which to make me believe that, otherwise this is only what ED's CFD or their third party members CFD or a form of aerodynamic analysis that would suggest such a thing and it's not the first and will not be the last time when those kind of analysis, not supported by real data, could be wrong.

 

Now, this happens to be my domain and I'm curious about the results that can come up...!

 

Let's presume that the MIG-21's lift produces a maximum lift coefficient of 0.9 (from some real aerodynamic tests with no flaps it almost reached 0.9 at very low subsonic Mach numbers for 1:1 scale model) at a critical or stall AoA of about 20 and a 0.016 lift coefficient at 0 AoA. Later I should turn the AoA from degrees to radians in order to use it in calculations to determine the lift slope of the MIG-21's wing for low subsonic speed.

 

I've talked early in this year to a RL and active MIG-21 pilot in order to have the maximum (critical) AoA and 0G AoA which I desperately wanted to know because I debated the real AoA vs indicated AoA of the MIG-21 on a thread that I've started on the subject much earlier before.

 

Now let's calculate the lift to AoA derivative or slope (which is otherwise important to predict the lift coef. at any given AoA) by dividing the lift difference to AoA difference on the linear lift/AoA margin only (at least 2 degrees AoA before stall): 0.9 - 0.06 / (18*pi/180) = 0.9 / 0.3142 => lift slope of MIG-21's wings alone = 2.6738

 

We must know that the CL of an aerodynamic component like wing, fuselage, fin, etc., for some AoA (already knowing the slope and the CL at 0 AoA) is:

 

CL = CL at 0 AoA + lift slope * AoA in radians

 

By knowing from experimental results that the mig-21's wings produce about 0.06 lift coef. at 0 AoA, then for -2.5 deg. AoA which is now equivalent to the -2.5 pitch down attitude that the fuselage's axis has in straight horizontal flight, we should get a positive wings lift coefficient at which the plane should be flying with, but sadly this is what we get:

 

0.06 + 2.6738 * (-2.5*pi/180) = -0.0567

 

So, there you go, -0.0567 lift coef. at -2.5 deg. The plane should in NO WAY keep flying straight and leveled as it seems to do in the sim, but rather plunge at some negative G-load instead. The wings should produce such negative lift coefficient alone at that negative AoA.

 

In order to produce no lift, the wings should be flying at this AoA:

 

-0.06 * 180 / (pi * 2.6738) = -1.2857

 

That's the 0 lift angle of attack -1.2857

 

Now let's calculate the lift slope for fuselage also (for the sake of discussion) where the fuselage would have a CL (lift coefficient) of 0.024 at 10 deg. AoA (obtained this one using a CFD). As you can see, the fuselage would produce mostly 0.048 lift coefficient at 20 deg. AoA, so it's very small in comparison to the wings..., and if you want a relation between them: 0.048/0.9 = 0.0533 (so 5.3% of the total lift is produced by the fuselage itself), but let's see the fuselage's lift slope now...:

 

0.024 / (10 * pi/180) = 0.138. THAT'S your lift slope for fuselage only!

 

As a contradiction to some who believe that the fuselage produces upward lift by being pointed downwards like that, guess what..., the fuselage's lift coef. at 0 AoA is about 0.0005 and at -2.5 deg. AoA where the game tells that the plane still has 1G, the lift coef. for fuselage only is:

 

0.0005 + 0.138 * (-2.5 * pi/180) = -0.0055

 

Now as a final conclusion, let's calculate the total LIFT (careful, not lift coefficient or CL) for the MIG-21 at near sea level altitude using the following well known formula:

 

Lift = q * A * CL

 

MIG-21's wing planform area (A) = 23 m^2

 

MIG-21's fuselage planform area (Af) = 7.25 m^2

 

q, which stands for dynamic pressure, where "rho" is the local air density and near sea level it can be taken as 1.2:

q = 0.5 * 1.2 * (1300/3.6)^2 * 23 * -0.0567 = -102033.75 Newtons of lift force produced by the wings alone.

 

Now let's calculate the lift force from the fuselage (which is ALSO NEGATIVE and not positive as everyone started thinking)

 

q = 0.5 * 1.2 * (1300/3.6)^2 * 7.25 * -0.0055 = -3119.85

 

The TOTAL lift would now be:

 

-102033.75 + (-3119.85) = -105153.6 Newtons..., which is as negative as nothing can stop it.

 

The weight of a MIG-21bis (took from mission editor) with 50% fuel, having it's mass multiplied by the gravity acceleration: (7480kg + pilot) * 9.81 = (7480 + 90) * 9.81 = 74236.34 Newtons of weight force.

 

Now if we divide the lift force to weight force we'll get the G-load:

 

-105153.6 / 74236.34 = -1.4165.

 

There you go now..., the G-load should be -1.4165. Even with some error from my part let's say..., the G load still can't be even 0, with no room for saying it's above 0 or equal to +1.

 

Let's stop calculating for the alt of 10000 meters and presume that for the same "q" (dynamic pressure) that occurs at much higher true airspeed and Mach number but the density drops much also (the dynamic pressure is what matters), the negative lift would provide almost the same negative G-load as for near sea level, which is about the same -1.2 Gs!

 

Keep in mind that these calculations took the presumption of having the CL constant with Mach number. This CL only varied with AoA here and was taken for a very low Mach number, but in reality, shock stall phenomena would make things even worse and would require the aircraft to stand at an even higher AoA in order to maintain a straight and level flight..., and there's nothing like that seen yet in DCS, but I can only wish for better days when ED will actually take into account that variation too. It would definitely make a much greater difference and realistic challenging experience.

 

Some can also check this for MIG-21's lift variation with Mach at constant AoA:

http://forum.keypublishing.com/attachment.php?attachmentid=180128&stc=1&d=1262166833

 

Indeed this is the variation of lift coefficient (NOT DIRECT LIFT, learn what a lift coef. is) with Mach number when a critical angle of attack is held constant, but the general shape of the curve will remain almost the same (slightly flattened so the variation would be a bit lower) even at lower AoA where the wing still produces a non-zero lift coefficient.

 

That drop of lift coefficient and then rises again (somewhere at the middle) is the so called "shock stall" which no aircraft escapes from, sooner or later as Mach increases.

 

And as a matter of fact, I didn't even need to tell any of this..., it could've been obvious and simple enough for anyone to just try and compare how the MIG-21 with it's very poor lift slope wings and almost symmetrical airfoil which produces very little lift near 0 AoA flies at such negative AoA in relation to the F-15 and Su-27 under same conditions, cause they all have PFM. Someone should try and fly the F-15 and Su-27 in level flight at supersonic and find out that they always have positive AoA at any height and even with empty weight (almost no fuel left) when flying supersonic. If there was an aircraft to have a nose down attitude at supersonic (even regardless of shock stall that I discussed about and doesn't exist yet...) that should've been the F-15 which has a drastically cambered airfoil which produces a lot of lift at null AoA, and even so it keeps a positive alpha or near 0 in such circumstances...!

 

Try it:

F-15 & Su-27 supersonic AoA in level flight.trk

Edited January 21, 2017 by Maverick Su-35S
Corrected values for wing incidence and NULL AoA lift coefficient with more correct data.

[Maverick Su-35S]

... Wings need a positive AOA to stay aloft, there's no way around it. Every wing needs positive AOA...

 

You know 100% inside and out of subsonic + compressible aerodynamics?

 

Now don't get me wrong, but I'm an aerodynamicist and I don't make such claims although I know and have learned a lot having a decade of experience in this domain including supersonic airflow and even so..., I know that I'll never know it all even after I die, but hearing you saying that no wing should ever produce lift if it doesn't have positive AoA..., just leaves me speechless! What about cambered airfoils, I guess you heard about them, right?

 

Even car aerodynamics take the lateral shape of the car as an airfoil and it NEVER had been symmetrical...!

 

I honestly bet you meant something else by what you told!

[Exorcet]

Now let's calculate the lift slope for fuselage also (for the sake of discussion) where the fuselage would have a CL (lift coefficient) of 0.024 at 10 deg. AoA (obtained this one using a CFD).

Out of curiosity, was is just the fuselage? That won't give an accurate result, you'd need to simulate the wings and fuselage together to get the fuselage CL. Disregard if that's what you did.

 

If there was an aircraft to have a nose down attitude at supersonic (even regardless of shock stall that I discussed about and doesn't exist yet...) that should've been the F-15 which has a drastically cambered airfoil which produces a lot of lift at null AoA, and even so it keeps a positive alpha or near 0 in such circumstances...!

The F-15 has a symmetrical airfoil on the inboard wing if I recall. The tips have some washout.

 

In any case, negative AoA level flight is absolutely reasonable, though I do wonder about it in the case of supersonic flight as the shockwaves on the wing could have a big impact on pressure distributions.

[Maverick Su-35S]

Out of curiosity, was is just the fuselage? That won't give an accurate result, you'd need to simulate the wings and fuselage together to get the fuselage CL. Disregard if that's what you did.

 

Hi Exorcet,

 

I know you're a car aerodynamicist if I remember correctly and I'm glad to talk with you.

 

You're right, I did not have them together and I've also mentioned myself it would normally be needed to do so, this time I only took the fuselage separately for those who believe so much in having the fuselage generate so much lift and opposed to that of the wings. I wanted to show the aberrancy of their theory by making this compromise. The initial CL.max of 0.9 was for the whole aircraft together with the elevator at 0 deflection and normally I didn't do quite a right thing adding the fuselage as a standalone body to something that already had an overall lift distribution, but I say again, I had to find a way to dismiss some ideas that the fuselage (even if the wings would have 1 deg. of positive incidence) which should create an even higher tendency for downward/negative lift, can do such things as we see.

 

The F-15 has a symmetrical airfoil on the inboard wing if I recall. The tips have some washout.

 

Indeed, you're right, but the MAC on which the whole wing's performances (in some areas) can be reduced to make calculations simpler and not affect reasoning, is overall cambered and has a slight washout (negative incidence gradient across the spanwise for a smoother lift distribution) as well.

 

Either way, the F-15 (50% fuel, no weapons) has a lower wing loading AND higher CL slope than the MIG-21 (50% fuel, no weapons) and even with a cambered MAC it still flies with both the longitudinal axis and MAC's chord with a slight positive attitude angle and positive angle of attack respectively but higher than 0 (in level flight). The same thing does apply for the Su-27. Only a combination of a highly cambered airfoil and very low wing loading could possibly allow for an aircraft traveling in transonic-supersonic to fly at a negative real AoA at +1G, especially in the presence of shock stalls (which greatly degrade the lift slope and stall AoA values).

 

I've attached a track where you can select either the F-15 or the Su-27 where you can test them or watch how they perform.

 

In any case, negative AoA level flight is absolutely reasonable, though I do wonder about it in the case of supersonic flight as the shockwaves on the wing could have a big impact on pressure distributions.

 

I'm not saying that negative angles of attack aren't possible (talking about wing only, no need to discuss about fuselage which has very little effect anyway). In the right circumstances of camber, indicated airspeed (dependent on dynamic pressure) and wing loading, it is possible to obtain negative angles of attack flights, at positive G-loads. One good example for such situations would be the gliders which are having just the kind of high cambered MAC and very low wing loading which allows them to fly with negative AoA at or even above 1G at low subsonic Mach numbers, but I say again..., there is nowhere near that case for the MIG-21...!

 

There is no explanation for why the MIG-21 does such a thing in our simulator, even if there were no shockwaves to separate flows (stall), there is no explanation than the fact that there is a mistake or error somewhere in the aerodynamic analysis. I'm feel sorry, but this is how it is. It might not be a huge error at all to most of us and we're all pleased with how it flies in general, but in this particular situation something shows up as not being physically right whatever we might think!

Edited January 21, 2017 by Maverick Su-35S

[Exorcet]

Hi Exorcet,

 

I know you're a car aerodynamicist if I remember correctly and I'm glad to talk with you.

 

Yes, so supersonic aero isn't my area of expertise, I only know the general trends.

 

 

 

I'm not saying that negative angles of attack aren't possible (talking about wing only, no need to discuss about fuselage which has very little effect anyway). In the right circumstances of camber, indicated airspeed (dependent on dynamic pressure) and wing loading, it is possible to obtain negative angles of attack flights, at positive G-loads.

 

The last part of my post was only in general, it wasn't directed at you, I was pretty sure that you understood where negative AoA flight could happen.

 

I do agree that it seems strange, but I don't have MiG specific documentation or CFD to back up that feeling.

[Maverick Su-35S]

Is the issue regarding the lift produced by the MIG-21 at all angles of attack resolved? That would be great, cause nothing was solved for at least 2 years!

Edited January 23, 2017 by Maverick Su-35S