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AoA indicator question!


Maverick Su-35S

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Hello. Is the AoA indexer showing the real angle of attack of the MIG-21, or it's just measured in unit? If these are units of AoA, do they linearly vary with real angle of attack, or non-linearly? From what I could find in this unique post about the real 21's high alpha capabilities ([ATTACH]105667[/ATTACH]), these units don't seem to be linear (meaning that it would be hard to express a general or constant relation between the indicated units and the actual/real angle of attack of the plane).

 

If the real mig-21 completely stalls the wings (generated wing sweep angle vortexes also break up and detach) above ~20 deg. real AoA, which would be ~33 units on the AoA indexer if they are units indeed, you have achieved a great and detailed flight model simulation that very few might actually have learned about.

 

Great work!

Making the best out of MIG-21.pdf

When you can't prove something with words, let the maths do the talking.

I have an insatiable passion for helping simulated aircraft fly realistically!

Sincerely, your correct flight model simulation advisor!

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How have you defined your use of the word "units"? I know that in the F-15, a unit of AoA as shown on the indicator is (AoA)_units == (AoA)_deg + 10.

 

Well, as far as it's usually found in areas regarding the real angle of attack in relation to the indicated angle onboard the aircraft, the latter is said to be measured in units, not degrees, so it's a common definition for AoA indicators. I've also read the topic for the F-15, and as GGTharos said there, that indexer for the Eagle is always showing you 10 degrees more than the real angle of attack. I don't know whether it's the same story for the MIG-21 as well or different, but this we must find out. I've also shared a document about it in the first post. Read it (it's quite useful and interesting) and you'll see the relation between degrees of angle of attack and units indicated (for the AoA indicator). So regardless to the Eagle where you only substract 10 from the HUD's AoA indexer to get the REAL angle of attack, for the MIG-21 this might not be the case because for each plus unit on the AoA indicator, the real AoA in degrees will increase less and less (example: for 10 units you have 4 deg. of AoA, but for 20 units you'll have like 6~7 degrees) so it's not varying constantly.


Edited by Maverick Su-35S

When you can't prove something with words, let the maths do the talking.

I have an insatiable passion for helping simulated aircraft fly realistically!

Sincerely, your correct flight model simulation advisor!

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The real-world flight manual for the MiG-21bis says this gauge indicates the "local angle of attack" in degrees, e.g. the degrees of deflection of the AoA measuring vane. It also says that stall = 33 degrees.

 

Wait a minute, are you sure that the AoA indicator on the right upper right corner of the front cockpit panel indicates the angle in degrees? Then there's a problem somewhere because the MIG-21 in game stalls at exactly 20 REAL AoA not 33, which is also what the pdf that i've provided tells. You must keep in mind that the angle that the vane measures is almost always higher than the angle at which the undisturbed air meets the wing's airfoil chord IF the vane is mounted sideways and close to the fuselage, as in this picture:

 

558671751_MIG-21AoAvane.jpg.540f10624958f4d85378b0bee1043a4e.jpg

 

Now i don't know whether the 21 measures the alpha (AoA) and beta (yaw/sideslip) angles from that nose spike (which indeed should measure the correct and real AoA/Beta) or only the alpha from a side mounted pivoting vane (which will always show you an exaggerated angle due to the airflow that wants to go to the lower pressure of the fuselage)

 

If you want to see at what angle of attack any airplane stalls, do this simple test:

 

Fly your aircraft straight and leveled trying to maintain 1G all the time (so, always try to hold the vertical speed indicator near 0) and watch your pitch attitude indicator which doesn't lie..., and as you gradually reduce your airspeed and carefully holding 1G you can see at what pitch angle your plane starts to stall, and that's your REAL angle of attack for stall not the indication on the AoA which might be wrong! For short, when you fly straight (your real path is aligned with the horizontal) and with no roll angle, your pitch attitude indicator is also your real angle of attack indicator.

 

I'm waiting for you answer. Good day!

When you can't prove something with words, let the maths do the talking.

I have an insatiable passion for helping simulated aircraft fly realistically!

Sincerely, your correct flight model simulation advisor!

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Wait a minute, are you sure that the AoA indicator on the right upper right corner of the front cockpit panel indicates the angle in degrees? Then there's a problem somewhere because the MIG-21 in game stalls at exactly 20 REAL AoA not 33, which is also what the pdf that i've provided tells. You must keep in mind that the angle that the vane measures is almost always higher than the angle at which the undisturbed air meets the wing's airfoil chord IF the vane is mounted sideways and close to the fuselage, as in this picture:

 

Here's what it says in the flight manual:

 

"Angle-of-attack indicator YYA-1 shows the current angle of attack to the pilot. The scale of the YYA-1 indicator is calibrated in degrees of local angle of attack (corresponding to the deflection angles of the vane of transmitter UYA-3) with a scale increment of 1 degree; the scale is numbered every 10 degrees."

 

The key words there are "local angle of attack", e.g. the angle of the AoA vane, uncorrected for effects of local airflow around the vane. So it is calibrated in degrees but does not read the actual angle of attack.

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Maverick, I don't quite understand what your point is.

 

Yes, UUA-1 indicates AoA in degrees. Yes, it is operated by the vane, and thus, yes, it doesn't show actual AoA of the whole airplane, but only local AoA of the vane. Yes, it changes quite a lot, compared to indications of gyro-operated KPP, depending on speed, flap position, BLC system operation etc. There's a whole chapter about utilizing this device in "MiG-21 Bis Pilot's Flight Operating Instruction".

 

So what? Whatever plane one flies, whatever the construction of the indicator mechanism is and whatever angular units it uses, its main job is to inform the pilot how hard he should pull the stick during certain maneuvers. And UUA-1 does just that, nothing less, nothing more. It won't always tell the pilot what the actual AoA of the plane is but that's irrelevant as long as he keeps out of "red range" on the indicator.

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Maverick, I don't quite understand what your point is.

 

Yes, UUA-1 indicates AoA in degrees. Yes, it is operated by the vane, and thus, yes, it doesn't show actual AoA of the whole airplane, but only local AoA of the vane. Yes, it changes quite a lot, compared to indications of gyro-operated KPP, depending on speed, flap position, BLC system operation etc. There's a whole chapter about utilizing this device in "MiG-21 Bis Pilot's Flight Operating Instruction".

 

So what? Whatever plane one flies, whatever the construction of the indicator mechanism is and whatever angular units it uses, its main job is to inform the pilot how hard he should pull the stick during certain maneuvers. And UUA-1 does just that, nothing less, nothing more. It won't always tell the pilot what the actual AoA of the plane is but that's irrelevant as long as he keeps out of "red range" on the indicator.

 

Sorry I didn't understand the idea of "local AoA", just confused it with the real one. Thanks!


Edited by Maverick Su-35S

When you can't prove something with words, let the maths do the talking.

I have an insatiable passion for helping simulated aircraft fly realistically!

Sincerely, your correct flight model simulation advisor!

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Here's what it says in the flight manual:

 

"Angle-of-attack indicator YYA-1 shows the current angle of attack to the pilot. The scale of the YYA-1 indicator is calibrated in degrees of local angle of attack (corresponding to the deflection angles of the vane of transmitter UYA-3) with a scale increment of 1 degree; the scale is numbered every 10 degrees."

 

The key words there are "local angle of attack", e.g. the angle of the AoA vane, uncorrected for effects of local airflow around the vane. So it is calibrated in degrees but does not read the actual angle of attack.

 

Thanks you "FishBike", so the indicated AoA onboard the aircraft is not the real angle of attack between the wing's chord and the freestream air. The conclusion is that the real angle of attack after which the plane stalls is 20.


Edited by Maverick Su-35S

When you can't prove something with words, let the maths do the talking.

I have an insatiable passion for helping simulated aircraft fly realistically!

Sincerely, your correct flight model simulation advisor!

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Anyone know exactly how much drag you are getting depending on AOA at given altitude/speed?

 

Good question, but probably only the devs who worked on the aerodynamic model could tell.

 

It is possible to also extract the drag force for any given AoA as long as you know the glide ratio (that can be determined experimentally in the sim) and the G load (from where you extract the lift force, knowing the weight of the plane also) at that AoA, but this requires some passion to do!

 

A very general formula (usually gives underestimated total drag values because it doesn't take friction, form and parasite drag into account) which tells the drag coefficient at a given lift coefficient (at a given AoA of course), which is also known as induced drag, is this one noted with CDi:

 

http://www.grc.nasa.gov/WWW/k-12/VirtualAero/BottleRocket/airplane/Images/induced.gif


Edited by Maverick Su-35S

When you can't prove something with words, let the maths do the talking.

I have an insatiable passion for helping simulated aircraft fly realistically!

Sincerely, your correct flight model simulation advisor!

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Easiest way to reckon the real AOA vs indicator, I would figure, would be to simply fly at specific AOA at 0 vertical airspeed as fast as you need and recording HSI angle above horizon and comparing to the AOA indicator.

 

Thanks you, but i've already mentioned the same in my earlier posts, when, although i've referred only to the critical or stall AoA, it was logic that the AoA travels with airspeed for the same 1G flight and you can build a diagram for real values (reading the pitch attitude indicator) and those shown on the AoA indicator.

When you can't prove something with words, let the maths do the talking.

I have an insatiable passion for helping simulated aircraft fly realistically!

Sincerely, your correct flight model simulation advisor!

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