[RESOLVED] Angle of attack

I made a few flights with the latest release(v1.2.12u1) and stumbled over something.

If you fly at full mil power or better with full reheat @ SL or up to 250m, the AoA will go negative down to -2,5°.

So I re-checked it with a earlier version. I still have 1.2.10u3 on my harddrive.

In the same situation as mentioned above you will get -1.5° AoA.

It doesn´t sound like it its a lot. But it looks strange to me that you get a negative AoA on a symmetrical airfoil in strait and level flight.

All pictures are strait and level with recovery mode on.

Also compare the attitude indicator between both versions

But I´m not sure if this is a bug at all.

1.2.12u1

For comparision the older version V1.2.10u3

Fox

Maybe a reading instrument error because supersonic speed?

The manual says something about instrument errors because speed.

Maybe a reading instrument error because supersonic speed?

 

The manual says something about instrument errors because speed.

Look at the first image also look at the hud, the aircraft is definitely at an angle to the horizon. Strange....

Could you post tracks?

Negative AoA makes perfect sense at (very) high speed. Say that at AoA=0 speed is at 800kph. More speed at the same AoA = more lift. So increase speed to say 1200kph, your AoA HAS to decrease to keep level.

Maybe a reading instrument error because supersonic speed?

 

The manual says something about instrument errors because speed.

Altimeter and Indicated Airspeed, yes. But the attitude indicator and AoA indicator are not affected.

Look at the first image also look at the hud, the aircraft is definitely at an angle to the horizon. Strange....

 

Could you post tracks?

Try it for yourself. Then we know if it is a isolated problem with my dcs installation. But I will post a track tomorrow.

Negative AoA makes perfect sense at (very) high speed. Say that at AoA=0 speed is at 800kph. More speed at the same AoA = more lift. So increase speed to say 1200kph, your AoA HAS to decrease to keep level.

Yes it does make sense, when the airfoil isn´t symmetrical. But what is the shape of the MiG´s wings? The only information I could find was "TsAGI S-12".

But no picture of this airfoil.

Fox

edit: Track added

I'm still getting AOA spikes on the ground for some reason.

Negative AoA makes perfect sense at (very) high speed. Say that at AoA=0 speed is at 800kph. More speed at the same AoA = more lift. So increase speed to say 1200kph, your AoA HAS to decrease to keep level.

Increased lift from speed would cause the nose to rise until equilibrium at a new altitude is reached. Some slight forward pressure to offset that is helpful, to remain at a desired flight level, but I wonder if negative angles of attack would be called for; that would normally cause altitude to drop, defeating the purpose.

His screenshots show the plane appears to be descending, which would be expected with a negative angle of attack.

Some of his instruments also seem to show that he is not flying straight-and-level.

He looks like he has just passed through the trans-sonic range, where instruments sometimes temporarily abandon their senses

He is beyond it, but its effect may still be a culprit in this mystery.

The only fly-in-the-ointment is the variometer, whose indications are (unusually) pegged at zero-zero.

The variometer is sensitive to pitot performance in this function.

It appears to me that he has selected his standby pitot; I can't see whether the pitot heat switches are on.

It may be that the variometer is in its default position due to a frozen main pitot.

It, too, should probably be showing a descent. It is the only instrument out-of-step with all other indicators, and as such, is suspect.

I'm still getting AOA spikes on the ground for some reason.

Yes, me too. It depends somehow on the airfield I´m taxiing.

...

 

His screenshots show the plane appears to be descending, which would be expected with a negative angle of attack.

Negative, no descend. As I wrote in the first post: straight and level.

 

Some of his instruments also seem to show that he is not flying straight-and-level.

You misinterpret the instruments.

 

He looks like he has just passed through the trans-sonic range, where instruments sometimes temporarily abandon their senses

Attitude indicator and AoA indicator are not affected by this.

 

He is beyond it, but its effect may still be a culprit in this mystery.

 

The only fly-in-the-ointment is the variometer, whose indications are (unusually) pegged at zero-zero.

Autopilot recovery mode is the key

 

The variometer is sensitive to pitot performance in this function.

 

It appears to me that he has selected his standby pitot; I can't see whether the pitot heat switches are on.

Pitot selector is in the standard position. Both pitot heaters are on.

 

It may be that the variometer is in its default position due to a frozen main pitot.

You expect pitot tube freezing at 1300km/h IAS? I didn´t mention it in the first post: OAT is 15°C at SL, QNH 760mmHg.

 

It, too, should probably be showing a descent. It is the only instrument out-of-step with all other indicators, and as such, is suspect.

He uploaded a video

Fox

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.

In my MiG-21 module, I can only move the 'pointer' of the pitot selection switch between what appears to be the main and standby pitot tubes. I'm using the shorter end as pointer, and longer end as handle to turn with.

I took another look at the Pilot's manual, trying to see whether I have interpreted this switch position incorrectly, but I do not find any mention of it other than when to turn it to standby.

I am reading the labels in your screenshot as (left) 'stby' and (right) 'main'. The switch, as I interpret it's position, is pointing to standby.

However, the manual lists the positions of the switch as 'main/aux'. Maybe the English cockpit version has the position reversed by label? I searched the forums to see if this has been mentioned, without any results.

The effect of transonic flight depends on where the shock-wave forms, creating the separation between areas of supersonic airflow and subsonic flow; I only suggested that this may have caused pitot freezing, which, if it occurs, can take up to two minutes to be corrected by the heater.

The pilot manual mentions 'Transonic Pitot system error', but you did not mention how long you were in that flight region. If so, and if pitot freezing did occur, then the variometer is affected.

The local temperature and pressure were not available when I wrote my reply, but these do not necessarily reflect local aircraft conditions, which are a product of local variables and aircraft variables, such as mach number, etc.

You mentioned "You expect pitot tube freezing at 1300km/h IAS? I didn't mention it in the first post: OAT is 15°C at SL, QNH 760mmHg."

Regarding your reply, the pilot's manual states:

'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.

However, I am convinced by the video that 'straight-and-level' is the flight attitude. The flight instruments depict the aircraft attitude properly, but such flight performance at that attitude, as shown in your video, is improbable at best.

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

Or, the autopilot has sense of humor after all.

Maybe the latest DCS patch corrected this; check it out.

Edit:

I can't even say that. Standby while I look for Russian sources. <.<

Edit edit:

How are diagrams of an airfoil so hard to find?

My instrument interpretations:

 

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

No, it shows the angle of pitch in relation to the horizon. Only the PRMG can show you the angle of descent in relation to a runway. Sorry for being picky.

 

...

 

I took another look at the Pilot's manual, trying to see whether I have interpreted this switch position incorrectly, but I do not find any mention of it other than when to turn it to standby.

We obviously had the same thought. I went through the same "drama" with the pitot tube selector switch. I cannot see any difference if I switch from one system to the other. Thats why I leave it in the default position.

And btw.: If you have one of the pitot tube heaters off, both will freeze, when the freezing conditions are met, switching to the other pitotsystem doesn´t bring the pressure instruments back to life. I should report that in a separate thread.

 

...

 

The effect of transonic flight depends on where the shock-wave forms, creating the separation between areas of supersonic airflow and subsonic flow; I only suggested that this may have caused pitot freezing, which, if it occurs, can take up to two minutes to be corrected by the heater.

If you had it switched off before the icing occured, then the up to 2 minutes apply for the deicing process.

 

...

 

You mentioned "You expect pitot tube freezing at 1300km/h IAS? I didn't mention it in the first post: OAT is 15°C at SL, QNH 760mmHg."

 

Regarding your reply, the pilot's manual states:

'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.

 

Okay, freezing can of course occur. But at 1300km/h IAS, Mach 1.02 @ SL improbable. Well @ around -35°C OAT it might occur even at this speed and altitude if you can find enough humidity in the air... If you want to get the idea why, check these links, especially the .pdf file:

 

http://en.wikipedia.org/wiki/Aerodynamic_heating

 

http://spaceagecontrol.com/AD-InFlightTemperatureMeasurement.pdf

Pages(of the pdf doc) 133 fig.57, 135 fig.60 and 155 fig. 97,98a,98b are interesting.

 

The good old Concorde.

http://www.nap.edu/openbook.php?record_id=9251&page=7

 

However, I am convinced by the video that 'straight-and-level' is the flight attitude. The flight instruments depict the aircraft attitude properly, but such flight performance at that attitude, as shown in your video, is improbable at best.

Finally... that is what this thread is about.

 

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

Tell me more about this.

 

Maybe the latest DCS patch corrected this; check it out.

Check what I wrote in the opening post, there you will see what version I used.

Edit:

 

I can't even say that. Standby while I look for Russian sources. <.<

 

Edit edit:

 

How are diagrams of an airfoil so hard to find?

Say what?:huh:...:D

And another video. This time at 10km.

Fox

I'm running 1.2.12.35194, latest patch of DCS. Have you applied this and run the situation again?

The attitude indicator, the AoA sensor, the fixed net, the external view all suggest a slight nose-down attitude, which you proposed and I agree with.

You suggested, originally, that the plane was flying straight-and-level; my sense of the original presentation was that you were eventually going to hit the ground, due to error the in the variometer reading leading you to believe that you were straight-and-level.

Given the evidence that you presented ab initio, I think my analysis is thoughtful and on-point. I am always content, however, to have a gang of cruel facts destroy my theory; I have no vested interest in the outcome.

I accept that the video shows a different scenario.

Oh, as for the third regime, Rod Serling is probably your autopilot; that's a Good Thing.

Happy hunting!

p.s. If icing occurs, it will also take time to de-ice; I do not know (nor has anyone spoken to this) if icing of the pitot (s) can occur in the MiG-21 when the heaters are on. I suspect that it can, under the right conditions.

I do know that carburetors can ice over, even if heat is applied and re-applied, under certain conditions.

p.p.s It appears that the English labels for CU44 may be switched. Perhaps someone who is conversant in Russian can help there.

Attitude indicator and AoA sensor work based on info from: a) "blade" vanes on pitot tube; b) vane of AoA sensor on the left side of the nose; c) both sets of gyroscopes. I'm not sure how severe the icing should be for a) and b) to stop working correctly (or whether DCS models icing so thoroughly, which I doubt), but I'm sure it won't affect c). So maybe let's drop the icing.

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.

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.

...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.

The MiG-21 does not have exactly symetrical wing, it uses TsAGI S-12 (4.2%) airfoil at the root and TsAGI S-12 (5%) airfoil at the tip.

It would still generate lift at 0° AOA. And at that altitude and that speed and the fact that not only wings generate lift as Dolphin887 said that would be a lot of lift, so you actualy need to fly at negative AOA.

so you actualy need to fly at negative AOA

So this behaviour is the opposite of the autopilot increasing pitch as speed decays, then.

Hi,

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.

Seriously? Even the thrust vector is working to descend the aircraft. I've heard about the "upside-down ice-cream cone theory" but even the XB-70 didn't fly with negative wing angle (relative to horizon) at Mach 3.0 (in this phase of flight 85% of the lift was generated by the fuselage).

If you look, not only the airframe and thrust vector are negative, but so too the wing angle. Everything is pointing at a descent (literally).

To me, it appears the angle of incidence is wrong.

Best regards,

Tango.

After a long and careful read of this topic, I did learn that the long arm of the pitot switch CU44 (the one below the 'main/aux' pitot choice labels) is supposed to be the one pointing to the correct label, not the shorter arm, which is closest to the label.

So when the shorter, closer one is positioned directly below and pointing at the "stby" label, the switch is actually set for the "main" pitot.

After absorbing that ergonomic mystery, the problems that I had encountered trying to understand that the MiG-21 flight model is correct, when the airplane has a comprehensive descending attitude and simultaneously steady level flight, simply pale in comparison.

I think this newly-documented flight form must be an 'shallow inverted Cobra', which is only found in the third flight regime, a term which I coined myself.

Of course, consulting with Rod Serling helped me a lot in this regard.

Alternatively, it is probably just a reportable bug. Not even string theory can explain this.

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.

Thank you Novak. It does look strange you know. But I thought about it. Who has ever been perpendicular to a MiG-21 flying at Mach 1.02 @ sea level? I guess not many jetpilots did that. There are many reasons why noone has been in such a position. And even if someone was, I guess he wouldn´t have the awareness to recognize the nose down attitude.

If MiG added negative angle of incidence to the 21, the landing would be even more a drama.

The MiG-21 does not have exactly symetrical wing, it uses TsAGI S-12 (4.2%) airfoil at the root and TsAGI S-12 (5%) airfoil at the tip. ...

I found this information too. But how does this airfoil look like?

Hi,

 

 

Seriously? Even the thrust vector is working to descend the aircraft. I've heard about the "upside-down ice-cream cone theory" but even the XB-70 didn't fly with negative wing angle (relative to horizon) at Mach 3.0 (in this phase of flight 85% of the lift was generated by the fuselage).

 

If you look, not only the airframe and thrust vector are negative, but so too the wing angle. Everything is pointing at a descent (literally).

 

To me, it appears the angle of incidence is wrong.

 

Best regards,

Tango.

The question is, what angle of incidence does the XB-70 or MiG21 have? It appears to me, that the 21 has nearly 0°. And mind that the XB-70 is made for Mach 3 and it´s wingtips fold down which should reduce the total amount of lift. I would like to throw the F-16 into the round. At least it´s wingtips have negative angle of incidence.

Tango, tell me more about the "upside-down ice-cream cone theory". Haven´t heard of that before.

Btw.: Tango and Novak, how about making a XB-70 :D:D. I love this bird..

Fox

Hi,

 

 

Seriously? Even the thrust vector is working to descend the aircraft. I've heard about the "upside-down ice-cream cone theory" but even the XB-70 didn't fly with negative wing angle (relative to horizon) at Mach 3.0 (in this phase of flight 85% of the lift was generated by the fuselage).

 

If you look, not only the airframe and thrust vector are negative, but so too the wing angle. Everything is pointing at a descent (literally).

 

To me, it appears the angle of incidence is wrong.

 

Best regards,

Tango.

Well since Dolphin actually flew the Mig21Bis, I would assume that he is intimately familiar with the behaviour of the plane in all the different flight envelopes and is speaking from experience. :thumbup:

 

I found this information too. But how does this airfoil look like?

 

The question is, what angle of incidence does the XB-70 or MiG21 have?

Angle of incidence on MiG-21 is 0°.

I could not find the exact airfoil of MiG-21, but it should look like this. (If im not mistaken)

Airfoil (4.2).bmp

Angle of incidence on MiG-21 is 0°.

I could not find the exact airfoil of MiG-21, but it should look like this. (If im not mistaken)

Thanks.

Angle of incidence on MiG-21 is 0°.

I could not find the exact airfoil of MiG-21, but it should look like this. (If im not mistaken)

EDIT: nvm, read Dolphin's post

The problem is not so much the wing but the whole thing.

* Nose attitude = negative

* Wing attitude = negative (not talking AoA here)

* Thrust vector = negative

Everything says it should be descending. I doubt that the fuselage generates *that* much lift...

Best regards,

Tango.

Yea I know what you geting at, but to realy know whats going on we would have to take in account forces and their momentum on the airframe from all aerodynamic surfices with given center of gravity.

I dont have enough data on that, if you do lets get on it.

All I know is that that wing can still create lift at 0° AOA or very small negative AOA.