Aluminum Donkey Posted September 7, 2018 Share Posted September 7, 2018 I'm sure this has been brought up, I just don't remember much about it... The AoA gauge in the cockpit reads twice the value of the info bar. So, external view says 10 degrees, cockpit gauge says 20 deg. Any idea why it was done this way? AD Kit: B550 Aorus Elite AX V2, Ryzen 7 5800X w/ Thermalright Phantom Spirit 120 SE, 2 x 16GB Kingston Fury DDR4 @3600MHz C16, Gigabyte RTX 3070 Windforce 8GB, EVGA SuperNova 750 G2 PSU, HP Omen 32" 2560x1440, Thrustmaster Cougar HOTAS fitted with Leo Bodnar's BU0836A controller. --Flying is the art of throwing yourself at the ground, and having all the rules and regulations get in the way! If man was meant to fly, he would have been born with a lot more money! Link to comment Share on other sites More sharing options...
corn322 Posted September 7, 2018 Share Posted September 7, 2018 Examine this post: https://forums.eagle.ru/showpost.php?p=3429374&postcount=144 Link to comment Share on other sites More sharing options...
Dahlbeck Posted September 7, 2018 Share Posted September 7, 2018 (edited) I may be confusing the Mig-21 with some other aircraft, but it could be that the AOA indicator in the cockpit isn't showing AOA in degrees but in some arbitrary "AOA units". Edit: If so, that value should be lower than the one in the info bar though (the point with the non-degree units is probably to have easier-to-read numbers) . Edited September 7, 2018 by Dahlbeck Link to comment Share on other sites More sharing options...
Frederf Posted September 7, 2018 Share Posted September 7, 2018 It's degrees. The air flow around the sensor is significantly different than the relative angle between the airplane body and the motion through the air mass in general. If it's exactly the relationship as demonstrated in the module, who knows. Link to comment Share on other sites More sharing options...
Aluminum Donkey Posted September 7, 2018 Author Share Posted September 7, 2018 Examine this post: https://forums.eagle.ru/showpost.php?p=3429374&postcount=144 That certainly sheds some light! So, the data bar at the bottom in external views shows the real, airflow-over-wings AoA, and the cockpit gauge shows an arbitrary, wildly inaccurate and somewhat useless reading that at least still varies directly with actual air-over-wings AoA :) Thanks for the posted link... makes sense to me now. Still, considering how much time and effort (and bread!!) they pour into developing a fighter jet, you'd think the cockpit AoA gauge would be calibrated/corrected to show a usefully accurate value. It's not like it would have been difficult or anything. But, if this is the way the real MiG-21 AoA indicator behaves, then it's certainly the way the simulated gauge in DCS should behave too. Imagine if the speedometer in your car showed 100 km/h when you're actually going 50 :) AD Kit: B550 Aorus Elite AX V2, Ryzen 7 5800X w/ Thermalright Phantom Spirit 120 SE, 2 x 16GB Kingston Fury DDR4 @3600MHz C16, Gigabyte RTX 3070 Windforce 8GB, EVGA SuperNova 750 G2 PSU, HP Omen 32" 2560x1440, Thrustmaster Cougar HOTAS fitted with Leo Bodnar's BU0836A controller. --Flying is the art of throwing yourself at the ground, and having all the rules and regulations get in the way! If man was meant to fly, he would have been born with a lot more money! Link to comment Share on other sites More sharing options...
Frederf Posted September 8, 2018 Share Posted September 8, 2018 The question is accurate to what? Imagine if different parts of your car went different speeds. Which part would you attach the speed sensor to? The reason for DUA-3 sensor placement is because that's where the leading edge of the wing is. And this is a sensible choice as the wings are the "tires of the sky," where the metal meets the wind. The purpose of AOA is predominately to monitor lift characteristics of the main wing. If the wing stalls at X angle you don't really care if those happen at different body angles in different situations. What you want is "when this gauge reads X, wing is doing this." By looking at the DCS module you may wonder why bother caring about the AOA at DUA-3 location because it's a fixed relationship to the body or true AOA. You could just as easily have a scale from 0-14 instead of 0-28 and stall would happen at 14 instead of 28. And you'd be correct in thinking this way. There are two reasons why in reality the AOA is measured that isn't the true AOA. First, true AOA is hard to measure. Experimental test airframes put very long probes on the front of the nose to sample air flow very far in front of the disturbance of the body. Original MiG-21 did this (and bis still retains it for gunsight AOA input) but when you put long boom it flexes and wobbles the input. bis (and I think later F-13) changed over from pitot boom to DUA-3 on fuselage. I don't know the exact reason, perhaps it has something to do with the autopilot. The second reason is you get an AOA that is representative of what the wing sees. Maybe true AOA is wildly different for stall at different Mach but local wing AOA is consistent. Then you measure local wing AOA and your safety system can reference a consistent value as it relates to the practical limits of lift. And while I'm sure that was their goal, real MiG-21bis doesn't reach peak lift at 33 by UUA-1 everywhere. There is a small zone around M0.9 in which 33 units is maximum lift (definition, stall). At other (lower) places you can go much higher UUA-1 value before maximum lift. I'm guessing however that the compression at high subsonic causes the most deviation which is why that region is the most restrictive. Link to comment Share on other sites More sharing options...
corn322 Posted September 8, 2018 Share Posted September 8, 2018 Imagine if the speedometer in your car showed 100 km/h when you're actually going 50 :) I don't have to imagine much, 100kph = 62mph.:smilewink: Link to comment Share on other sites More sharing options...
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