[RESOLVED] Angle of attack

[ttaylor0024]

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

 

Correct, just in the wrong direction in terms of negative AOA.

[Golo]

Correct, just in the wrong direction in terms of negative AOA.

 

No I ment positive lift at negative AOA up to like lets say -0,5° AOA, but Im just guessing here.

Man I would like to see real profile of that airfoil and its polar diagrams.

[ttaylor0024]

No I ment positive lift at negative AOA up to like lets say -0,5° AOA, but Im just guessing here.

Man I would like to see real profile of that airfoil and its polar diagrams.

 

AOA is the angle between the chord line and the relative wind. How I perceive it, it is impossible for a wing to generate positive lift with a negative AOA, it's just math at that point.

[PiedDroit]

AOA is the angle between the chord line and the relative wind. How I perceive it, it is impossible for a wing to generate positive lift with a negative AOA, it's just math at that point.

Yes it's possible because the "chord line" is not actually the geometric center of the wing profile.

At zero AoA (relative to the chord), the wing will generate some lift, it's therefore possible to go in the negative a little until you meet the real "zero lift line".

 

EDIT: In other words, as soon as you're above that line (which is at negative AoA), you generate some lift. If you go fast enough this lift is sufficient for level flight.

 

Check this diagram:

http://amasci.com/wing/airfoil2.gif

http://amasci.com/wing/airfoil.html

The whole website is great ressource to understand how an airfoil really works.

Edited December 16, 2014 by PiedDroit

[ttaylor0024]

Yes it's possible because the "chord line" is not actually the geometric center of the wing profile.

At zero AoA (relative to the chord), the wing will generate some lift, it's therefore possible to go in the negative a little until you meet the real "zero lift line".

 

EDIT: In other words, as soon as you're above that line (which is at negative AoA), you generate some lift. If you go fast enough this lift is sufficient for level flight.

 

Check this diagram:

http://amasci.com/wing/airfoil2.gif

http://amasci.com/wing/airfoil.html

The whole website is great ressource to understand how an airfoil really works.

 

Ah, I see what you mean. If the MiG-21 has a symmetrical wing however, then this cannot be the case.

 

[Art-J]

That's the big "if", though. We don't have any proper sources. If it's symmetrical, I'd agree with You, but if it's not, like in Golo's post at the top of the page, then I can see negative AoA being required for positive lift beyond certain velocity treshold, 'cause that's how every cambered airfoil works.

 

Without lift and drag polars for the whole airplane, we're going nowhere.

[ttaylor0024]

That's the big "if", though. We don't have any proper sources. If it's symmetrical, I'd agree with You, but if it's not, like in Golo's post at the top of the page, then I can see negative AoA being required for positive lift beyond certain velocity treshold, 'cause that's how every cambered airfoil works.

 

Without lift and drag polars for the whole airplane, we're going nowhere.

 

Yeah and you guys must be correct if Dolphin is saying that it's normal as well. I would like it if he would elaborate for us, I'm curious. :music_whistling:

[JorgeIII]

Symetrical or not a 2 degrees of negative AoA for level flight at this speed looks and probably is wrong.

This is, most likely, an EFM error, maybe a consecuence of trying to correct the high positive AoA needed for level flight at high altitude (over 10000 m).

 

With same weight, loadout and IAS: AoA needed for a level flight should be independent of altitude. In DCS Mig21 AoA for level flight at SAME IAS increases with altitude.

 

I cant imagine what a nightmare must have been (and fun, of course) to create the EFM for the mig, to try to correct some values without breaking others, etc. Dolphin you have a lot of patient, good work.

Edited December 16, 2014 by JorgeIII

[PiedDroit]

I agree, I would expect a -0.5° to be normal at high speed.

[ttaylor0024]

With same weight, loadout and IAS: AoA needed for a level flight should be independent of altitude. In DCS Mig21 AoA for level flight at SAME IAS increases with altitude.

 

AOA should increase with altitude (barring all other factors equal) because your airframe is requiring a higher AOA in order to generate the same amount of lift because the air molecules are less dense, which is why if you try to fly the MiG-21 at 50,000ft you're at an insane AOA.

 

From how I think about it, AOA should be the same for the same config and same speed at any altitude because that same less dense air is effecting the pitot system as well, which is the same reason you fly the same IAS when on approach to land at high altitudes in GA aircraft.

 

I agree, I would expect a -0.5° to be normal at high speed.

 

This goes back to the debate on whether it's a symmetrical wing or not. A negative AOA on a symmetrical wing will cause you to go down. Until we have definitive proof of the wing type, we cannot accurately assess the question.

[Hadwell]

I think you can tell just by looking at the plane that the wings are not symmetrical, there's definitly more weight above the wings and forwards of the wings than underneath the wings, you will need some downwards aoa at high speeds to keep it level...

 

that being said, their so close to being symmetrical, might as well call them that...

 

as for exact numbers, well I'm sure knowing what they are doesn't allow you to change them, will still need to fly the plane the same way...

 

to rephrase... the center of mass on the mig-21... theres no way in hell its also the center of lift.

Edited December 17, 2014 by Hadwell

[ttaylor0024]

I think you can tell just by looking at the plane that the wings are not symmetrical, there's definitly more weight above the wings and forwards of the wings than underneath the wings, you will need some downwards aoa at high speeds to keep it level...

 

that being said, their so close to being symmetrical, might as well call them that...

 

as for exact numbers, well I'm sure knowing what they are doesn't allow you to change them, will still need to fly the plane the same way...

 

to rephrase... the center of mass on the mig-21... theres no way in hell its also the center of lift.

 

Weight above the wing doesn't effect where the front of the leading edge is, thus has no effect on the AOA (angle between the chord line, which is an imaginary line connecting the furthest aft part of the trailing edge with the furthest forward part of the leading edge, and the relative wind). Symmetrical wing purely refers to the shape of the wing.

 

 

AOA that we're discussing has no use when flying. 0.

 

Cg and Cl are different, and with aircraft that have a tail (all except aircraft with big canards like the Eurofighter) the Cg simply needs to be in front of the Cl.

[Hadwell]

oh, in that case... the mig-21 doesn't have any wing camber, but the thickness of the wing does change...

[Hadwell]

http://airfoiltools.com/airfoil/details?airfoil=tsagi12-il

 

so now you can see what i mean when i say so close to symmetrical it might as well be...

 

and where i got the info for the airfoil type:

 

http://m-selig.ae.illinois.edu/ads/aircraft.html

Edited December 17, 2014 by Hadwell

[PiedDroit]

Let me rephrase: "I agree, I would expect a maximum of -0.5° to be normal at high speed depending on the camber of the wing." is closer to what I had in mind.

 

@ttaylor0024,Hadwell: Maybe there is some confusion here? I think he meant "thickness", not "weight".

 

Weight above the wing doesn't effect where the front of the leading edge is, thus has no effect on the AOA (angle between the chord line, which is an imaginary line connecting the furthest aft part of the trailing edge with the furthest forward part of the leading edge, and the relative wind).

"Weight" above the wing doesn't affect where the chord is but it will definitely affect the AoA.

Just compare a pure flat wing - a wooden board (which can't fly level without AoA > 0) and the cambered wing of a Cessna (flies fine with AoA = 0).

[Hadwell]

the fuselage is also a lifting body, and where its mass/center of lift is relative to the lift of the wings also changes the AOA required for level flight.

 

 

so like i said before... trying to figure out what the exact numbers for why the mig-21 flies the way it does is pretty useless...

 

but i will also add... unless you're a genius at fluid dynamics... pretty darn hard to find out...

Edited December 17, 2014 by Hadwell

[ttaylor0024]

"Weight" above the wing doesn't affect where the chord is but it will definitely affect the AoA.

Just compare a pure flat wing - a wooden board (which can't fly level without AoA > 0) and the cambered wing of a Cessna (flies fine with AoA = 0).

 

Again, AOA is the angle between the chord line and the relative wind. The only way weight effects the AOA is, with all things equal, weight is changed. Let's not confuse the conversation with weight, as it's an entirely different variable that is not immediately important to our purposes.

 

I 100% know the aerodynamics of aircraft under Mach 1 inside and out, it's my job to know it. I understand the point that you're trying to make, but the Skyhawk absolutely does not stay aloft with an AOA of 0. Wings need a positive AOA to stay aloft, there's no way around it. Every wing needs positive AOA.

 

the fuselage is also a lifting body, and where its mass/center of lift is relative to the lift of the wings also changes the AOA required for level flight.

 

so like i said before... trying to figure out what the exact numbers for why the mig-21 flies the way it does is pretty useless...

 

but i will also add... unless you're a genius at fluid dynamics... pretty darn hard to find out...

 

It's not extremely difficult to understand, but again, leave mass out of it all, it's not relevant to this discussion.

 

Fluid dynamics for flight is really about Bernoulli's Principle and Newton's 3rd law, both easy topics to understand. The wing would be a combination of both, the fuselage would be described with Newton's 3rd law, etc.

Edited December 17, 2014 by ttaylor0024

[Random]

A thought: The AOA indicator is displaying a measured AOA from a vane on the side of the nose. Like all measurements errors are possible.

 

The aircrafts 3d model is just a representation of the plane and isn't really important as far as the flight model is concerned. Anyone remember the early problemd with the 109 flying sideways when RRG were developing it?

 

Could this be a situation where the FM is right but the instrument and 3d representation is wrong? We need to know what figures the FM is using before we know whats really going on!

Edited December 17, 2014 by Random

[Hadwell]

Again, AOA is the angle between the chord line and the relative wind. The only way weight effects the AOA is, with all things equal, weight is changed. Let's not confuse the conversation with weight, as it's an entirely different variable that is not immediately important to our purposes.

 

I 100% know the aerodynamics of aircraft under Mach 1 inside and out, it's my job to know it. I understand the point that you're trying to make, but the Skyhawk absolutely does not stay aloft with an AOA of 0. Wings need a positive AOA to stay aloft, there's no way around it. Every wing needs positive AOA.

 

 

 

It's not extremely difficult to understand, but again, leave mass out of it all, it's not relevant to this discussion.

 

Fluid dynamics for flight is really about Bernoulli's Principle and Newton's 3rd law, both easy topics to understand. The wing would be a combination of both, the fuselage would be described with Newton's 3rd law, etc.

 

 

but airplanes often consist of 2 or more different lifting bodies...

 

you need to take into account that it's 2+ flying things attached to each other that create a stable platform that an engine and person can sit in...

 

the fueslage, wings and tail section share a symbiotic relationship, and just knowing the wing type won't give you any indication of what AOA is needed

 

and the reason i say weight is because you will need to use greater forces on the tail to keep the nose up... if its nose heavy... creating more drag, in turn needing a greater AOA to keep level at any given speed

[ttaylor0024]

A thought: The AOA indicator is displaying a measured AOA from a vane on the side of the nose. Like all measurements errors are possible.

 

The aircrafts 3d model is just a representation of the plane and isn't really important as far as the flight model is concerned.

 

Could this be a situation where the FM is right but the instrument and 3d representation is wrong? We need to know what figures the FM is using before we know whats really going on!

 

This is true, and again, I do not fully understand the instrument error of an AOA vane during transonic flight. One reason it vary well could be a normal indication.

 

 

 

but airplanes often consist of 2 or more different lifting bodies...

 

you need to take into account that it's 2+ flying things attached to each other that create a stable platform that an engine and person can sit in...

 

the fueslage, wings and tail section share a symbiotic relationship, and just knowing the wing type won't give you any indication of what AOA is needed

 

and the reason i say weight is because you will need to use greater forces on the tail to keep the nose up... if its nose heavy... creating more drag, in turn needing a greater AOA to keep level at any given speed

 

For our purposes of discussion this is not relevant, as a matter of fact, the situation you described hurts your case.

[PiedDroit]

Again, AOA is the angle between the chord line and the relative wind. The only way weight effects the AOA is, with all things equal, weight is changed. Let's not confuse the conversation with weight, as it's an entirely different variable that is not immediately important to our purposes.

There is some misunderstanding here... I'm talking about wing thickness. Camber.. "Stuff" above the chord.. Non-geometrically-symetrical chord.

Choose your term.

But please stop dismissing the comments based on that only ;)

[iFoxRomeo]

...

 

I 100% know the aerodynamics of aircraft under Mach 1 inside and out, it's my job to know it.... Wings need a positive AOA to stay aloft, there's no way around it. Every wing needs positive AOA.

...

Then you should change the wikipedia article

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

Especially the part with "Relation between angle of attack and lift"

 

Fox

[ttaylor0024]

There is some misunderstanding here... I'm talking about wing thickness. Camber.. "Stuff" above the chord.. Non-geometrically-symetrical chord.

Choose your term.

But please stop dismissing the comments based on that only ;)

 

Camber and weight are two separate things. I don't want anyone confused with improper terminology, especially when that improper term has been used (incorrectly) in this discussion before.

 

Then you should change the wikipedia article

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

Especially the part with "Relation between angle of attack and lift"

 

Fox

 

What's your point, exactly?

 

Taken from your same article, "zero angle of attack corresponds to zero coefficient of lift." " A symmetrical wing has zero lift at 0 degrees angle of attack."

 

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. The wing actually still produces lift during a stall, however it's not sufficient to keep the aircraft aloft. This is the exact reason a spin occurs, because one wing is producing slightly more lift than the other.

 

 

 

 

 

Anyway, this discussion has got extremely off-topic. 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. If you'd like to discuss AOA or anything else related to flight, please shoot me a PM.

Edited December 17, 2014 by ttaylor0024

[OnlyforDCS]

Why is this discussion still ongoing?? The developer of the flight model himself came into the thread and stated that the behaviour is correct. He is a real life Mig21 pilot as well as the developer of this sim.

[OnlyforDCS]

"zero angle of attack corresponds to zero coefficient of lift."

 

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