MiG-21 bis AoA flight problems

[mpdugas]

Hello, Mr. Dackard;

 

 

After I read through the whole post concerning the MiG-21's negative angle of attack at low altitude and supersonic speeds (http://forums.eagle.ru/showthread.php?t=135653&highlight=angle+attack) I was left with an uneasy suspicion that the answer, i.e. that a slight negative AoA was normal under such conditions, was really not correct.

 

 

I have several old MiG-21 references that show more detail about the construction of the MiG-21 bis, and they further suggest that perhaps your flight model may be affected, among other issues, by a discrepancy in the construction of the alignment of your MiG's nose-cone and inlet ring assembly.

 

 

I do not have your 3D model to examine, but my visual examination of this area of the 3D model of the aircraft, from close-ups of it in the game, suggest that the nose-cone center-line (i.e. the line drawn from the tip of the nose-cone perpendicular to the inlet ring) is parallel to the fuselage center-line. It also appears that the inlet ring itself is also constructed to be perpendicular to the fuselage center-line.

 

If so, and if your flight model takes this alignment into account, for purposes of considering the presence of disturbed airflow affecting available AoA, then the whole dynamic flight model may be adversely affected by this choice.

 

 

I say that because, in the real MiG-21 bis, the nose-cone and inlet ring center-lines are not aligned parallel to the fuselage center-line, but rather, they both have a 3 degree negative angle relative to aircraft center-line.

 

I confirmed this by corresponding with a Finnish Air Force pilot who has access to a MiG-21 bis in the museum at Kymi airbase near Kotka. Please take a look at the illustration that he forwarded to me.

 

This off-central alignment was done to make the airflow over the nose-cone, at supersonic speed, create the proper shock wave at the inlet ring. If the nose-cone and inlet ring are parallel to the fuselage center-line, then the shock-wave from the airflow over the MiG-21, at high speed AoA, would strike the inlet ring off-center, because the MiG-21 bis typically flies supersonic at a 3 degree positive AoA.

 

 

The real MiG 21 bis construction compensates for the 3 degree positive AoA at supersonic speed by aligning the tip of the cone and face of the inlet ring to be perpendicular to the airflow over the fuselage by canting them downwards by 3 degrees.

 

 

 

The normal MiG-21 bis positive cruise AoA is very much at odds with your present flight model which requires the plane be flown at a negative AoA under conditions described in the forum thread; in those circumstances, the shock wave would strike the inlet ring at an angle that would be 5.5 degrees off-center.

 

That would be the result if your cone and inlet were constructed at right angles to the fuselage center-line, which is what I suspect is the case in your 3D model.

 

 

 

I really doubt that your flight model, or DCS world for that matter, is this precise, but I offer this to you because the 3D model alignment of the MiG-21 nose-cone and inlet of your version of the MiG-21 appears to be wrong.

 

I really do believe that the negative AoA at low altitude and high speed is also wrong, but until the 3D model construction is confirmed as correct, then this can't be measured.

 

 

My last contact with the Finnish Air Force pilot tells me this:

 

"The whole MiG-21 nose intake including the lip and and the cone is really tilted slightly downwards to adjust the intake perpendicular to the average high speed flight airflow to ensure symmetrical airflow into the intake. This is specially important at supersonic speeds due to the cone produced shock wave formation. The angle to the fuselage reference line is 3 degrees (see the attached pictures). The angle is close to the wing angle of attack at high speeds.

 

The angle of attack of the MiG-21 is always positive when not flying inverted. The symmetrical airfoil wing does not produce positive lift at negative angle of attacks. At high speeds like 500 kt the lift coefficient at 8800 kg is about 0,15 which corresponds to an angle of attack of some 2-3 degrees.

 

I have flown the MiG-21BIS at at sea level (100 meters) at an indicated airspeed of 1300 km/h (1.06 Mach), which is the maximum allowed indicated airspeed (Vmo). It can go even faster, but this is the limit. At altitude you are limited to 2.05 Mach (Mmo) due to directional stability. It can go even faster, but this is the limit."

 

 

I hate to be the bearer of difficult news, but I think that there are some fundamental problems with the design of your 3D model and with the way the flight model of the MiG-21 itself is represented in DCS World.

[CoBlue]

Hello, Mr. Dackard;

After I read through the whole post concerning the MiG-21's negative angle of attack at low altitude and supersonic speeds (http://forums.eagle.ru/showthread.php?t=135653&highlight=angle+attack) I was left with an uneasy suspicion that the answer, i.e. that a slight negative AoA was normal under such conditions, was really not correct.

I did a test, see picture :smilewink:

[mpdugas]

I did a test, see picture :smilewink:

 

I'm not quite sure what your test illustration is supposed to show, since the conditions of flight are different than proposed in the original thread on this topic; you are higher and faster than the special conditions under which this negative AoA was supposed to occur.

 

Can you elaborate on what you mean to show by your illustration?

[CoBlue]

I'm not quite sure what your test illustration is supposed to show, since the conditions of flight are different than proposed in the original thread on this topic; you are higher and faster than the special conditions under which this negative AoA was supposed to occur.

Can you elaborate on what you mean to show by your illustration?

The thread you are referring to begins 12-12-2014 & has the last post in 08-31-2015. lt's been many patch-fixes since then & the problem is fixed....otherwise the topic would still be alive!

I can't replicate the negative AoA scenario under any conditions at all.

From 20m & above at supersonic speeds, my AoA is always positive.

In the pictures I'm flying at ISA, RA 60m, M1.12, AoA +1°, Flight Path ang. 0.0°.

[mpdugas]

The thread you are referring to begins 12-12-2014 & has the last post in 08-31-2015. lt's been many patch-fixes since then & the problem is fixed....otherwise the topic would still be alive!

I can't replicate the negative AoA scenario under any conditions at all.

From 20m & above at supersonic speeds, my AoA is always positive.

In the pictures I'm flying at ISA, RA 60m, M1.12, AoA +1°, Flight Path ang. 0.0°.

 

Since the thread, whether it is old or not, is marked "solved", and the issue of the negative AoA was unchanged, then I gathered that the possibility of flight under negative AoA was possible and was an accepted notion.

 

How would you read that otherwise?

 

Thanks you for your efforts to illustrate the solution here, but your screenshots are not taken at a steady-state and that is what is of concern here.

 

They appear to be taken at transitory moments, since 50% power (DCS screenshot) at that altitude will not yield supersonic speed.

 

The flight panel screenshot shows 100% power, which is improbable, too.

 

The Tacview screen shows a positive rate of climb of .6 m/min, and a speed that is significantly over-limit.

 

Still, your AoA of +1 degree under these conditions is incorrect; you would be descending if that AoA is true, since the MiG-21 bis cruises, supersonic, at an AoA of 3 degrees, yet the screenshot shows a slight climb.

 

None of these screenshots illustrate the original area of concern, nor do they illustrate that the flight model is correct yet.

 

I appreciate your defense of LNS, but I doubt that they require us to reinforce their already-amazing performance with the MiG-21 bis model. I would rather just leave the solution in their hands, since everyone here admires their concern for accuracy and detail.

 

Perhaps LNS can address the question about the construction of the nose-cone and inlet ring at their leisure. I turned it in as a minor bug, because I sincerely doubt that LNS or DCS actually use the physical shape of their models to interact with the environment; I would suppose that they use look-up tables or the like to address flight modelling.

 

Thanks for explaining your initial screenshot and offering these additional ones to further explain your position.

 

I am content that LNS will, eventually, address the matter.

[Farlander]

I hope this gets looked at :)

[Cobra847]

The 3D model has nothing to do with the way the FM operates. I could rotate the aircraft to a 90 degree vertical angle and the aircraft would perform the same.

 

If you have any flight test data (demos) we need to look at; please feel free to record one and attach it.

The negative AoA bug was fixed a long time ago now.

 

The nosecone model has been looked at and is correct.

https://leatherneck-sim.mantishub.com/view.php?id=287

[BadHabit]

Always in flights at cruise speeds I see my aoa being 5+ but I still need to trim elevator up to keep my nose up so I can keep level flight even at low and medium alts. Is this normal?

 

Mig-21 Headbutter