Jump to content

The critical angle of attack is wrong, and its not because the F2 view shows the pitch angle.


KenobiOrder

Recommended Posts

On 5/24/2022 at 1:10 PM, m4ti140 said:

What do you mean "lower discrepancy"? The discrepancy is by a factor of 2, just like in MiG-21bis! The zero point is just moved. It's not the same aircraft, the way it's mounted can be different even. And at this discrepancy you would still already be dangerously close to a tailstrike if you followed the real takeoff procedure. Please show me a piece of documentation showing the real critical AoA is higher.

The flight characteristics book for the 21 refers to loss of conventional lift as a stall, because aircraft would tumble at this point, vortex lift regime is not even mentioned there. I don't know why the aircraft becomes uncontrollable by this point (and couldn't find anything in documentation), but it does.

In DCS it doesn't actually even stall at all at this point, if you can keep wings level and pitch further keep flying like that. In fact you can keep flying way past reasonable angles of attack into region when the aircraft would surely tumble due to vortex shedding.

Except its not by a factor of two. Not sure why you bothered pointing out that they are different planes, since I already stated that. But since you want to use the Mig-23 error equations as an example, I am simply continuing that example. And they do not show the AoA being off by a factor of two. Those error equations yield errors of about 40%. If you plug them into the Mig-21, you get stall angles of attack of around 20 degrees. 

And no, the planes stalls when it hits 33 degrees in game, except at very very very low speeds. The stall wing rock is clearly some kind of scripted behavior. It also doesn't make sense that the critical AoA changes based on speed, because that is not how wings work. 

Link to comment
Share on other sites

15 hours ago, Frederf said:

"Stall" means maximum CL. That's the definition. It is independent of controllability or instrument value. The argument put forth was that the freestream AOA and the UUA-1 indication should be or more closely be 1:1 i.e. 16 freestream AOA = 16 on the gauge. Clearly in the landing regime that's impossible because the flight manual describes UUA-11 readings of 15 or so. The idea of 1:1 degrees:degrees is interesting but I haven't seen any evidence for that.

The other notion is that a stall landing would be at essentially 28-32° body angle which can't be true either. Back in the day the AI flew "UUA-1" AOA according to the F2 value and it was silly. They were at double-triple-quadruple body angle in level cruise compared to pilot. Notice that the flight manual says that indication can depend if on 24V battery or 28V ground/engine power by about that ratio.

At 360 km/h approach speed (0.3M), Cy max is about 1.2 assuming it can even be reached. What CL is needed for approach path? Assuming 73.5kN of lift, 360km/h 23m2, 1.225kg/m3 it's about 0.52. Assuming (again) the UUA lines are linear spacing I would put the UUA-1 reading at 18°. Is that what we get?

The original main point was simply that the explanation given by the developer was not correct, as F2 view does not show pitch angle. It shows true AoA. 

The reason that the stalling AoA of 15-16 is wrong is because a delta of the Mig's type does not stall anywhere close to this. It simply does not. 

The issue with the landing AoA being off assumes either 1:1 or that the error relationship should be linear. The error of the wing mounted gauge in the test was not linear. Landing angles of attack vs read out on the UUA could be much smaller. Although it isnt obvious to me why one would land at critical angle of attack. 

In fact what might be wrong here is that the relationship in the game does in fact seem linear, as it appears to be two to one at all angles of attack. 


Edited by KenobiOrder
Link to comment
Share on other sites

Eh? That's Tsagi S-12 alpha sweep.

image.png

UUA-1 indication is according to this formula in DCS: (Y = 1.96X + 1.41) It's not "double". I agree that this is almost certainly a weak simulation of the actual instrument. The relationship between UUA-1 indication and free stream AOA is certainly not this simple linear equation at all Mach. However, put a Post-It over the gauge and check the airplane performance for reasonableness. It looks much better in these cases.

Link to comment
Share on other sites

45 minutes ago, Frederf said:

Eh? That's Tsagi S-12 alpha sweep.

image.png

UUA-1 indication is according to this formula in DCS: (Y = 1.96X + 1.41) It's not "double". I agree that this is almost certainly a weak simulation of the actual instrument. The relationship between UUA-1 indication and free stream AOA is certainly not this simple linear equation at all Mach. However, put a Post-It over the gauge and check the airplane performance for reasonableness. It looks much better in these cases.

Yes that's for the airfoil, not the entire wing. If you reference the chart from the U.S manual it shows the actual stall at a much higher aoa than 33 indicated. There is no mention of wing rock except at speeds at 250kph and below, making me wonder if the dcs mig 21 is modeled backwards by accident.

Link to comment
Share on other sites

15 hours ago, Frederf said:

Have Donut? MiG-21US PFOI? Which diagram number? "Wing rock" is not a stall condition. That's controllability or stability.

Wing rock is a stall condition. It generally occurs because one wing stalls first, then the other, and then oscillates. In any case the US manual mentions no wing rock whatsoever until you get below something like 250kph, at which point it reports +-50 degree wing rock. The only pre-stall condition reported is buffet, which itself only occurs in certain conditions. As can be clearly see in the stall diagram, on both Russian and American documents, the only warning is for buffet. There is no wing rock warning on the chart, and no mention of it at higher speeds in the expanded description that I could find.  What can also be seen in that the 28 and 33 degree limits are NOT the stall limits, unlike in DCS Mig-21. The instructions call the 33 degee limit the stall limit, but thats contradictory to the chart....except at high mach numbers. I sort of get the impression that the 33 degree limit is placarded as a safety limit because the stall AoA appears to decrease substantially with mach number.  

nullnullnullnullnullnull

image.png

image.png

image.png

image.png

image.png

image.png

Link to comment
Share on other sites

You don't have to post the manual at me. I've read them dozens since the module was first released and experienced every public flight model change over that time.

What is your assertion? There are three (to keep it simple) variables here: A. UUA-1 B. freestream "F2" AOA C. Cy.

All the charts in the world showing BC relationship tells nothing about AB relationship. The only factoid I can think of is that takeoff pitch of 5° gives some 11-13 UUA-1.

Which relationship AB BC or AC do think is wrong. I definitely think something isn't right since AB is Mach independent in DCS but I want you to draw those graphs how you think this airplane is so we can evaluate them.

Link to comment
Share on other sites

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...