Jump to content

rossmum

Members
  • Posts

    693
  • Joined

  • Last visited

1 Follower

Personal Information

  • Flight Simulators
    Il-2 '46, Il-2 CloD, Il-2 BoX, DCS
  • Occupation
    Soviet aviation fanatic, DCS F-5 spare parts distributor

Recent Profile Visitors

3790 profile views
  1. IRL I believe the mach limit is a stability limit (like 23, 25, 31). Breakdown of directional stability worsens to the point the aircraft becomes unsafe and some of the reasons can be seen if you read the flight manual's description of potential quirks of the aircraft's behaviour as it approaches M 2.0. The original limit was 2.05, later models with the larger side area from the full-length spine and increased vertical stabiliser chord were upped to 2.1 practically but manuals continued to print 2.05. As far as I can tell, the engine flameout is an artificial constraint added to prevent people abusing the aircraft's excess thrust to fly at speeds that were considered too unsafe IRL, but where the DCS module does not/cannot simulate worsening stability or canopy failure due to friction heating.
  2. Gordon's stuff is a mixed bag. He's very good at telling the story of the aircraft but regularly mixes up or misinterprets technical data, so take assertions about systems capabilities or flight performance with a few grains of salt - but at least in English, you will not find a better source for info on development, operational history, aircraft operators, etc., and the photos and diagrams are hard to equal. Markovskiy is generally a very good source if you can read Russian, though I don't know what he's written on the 21 - just that he's the usual go-to for the 23 and has some other stuff out there as well.
  3. The rudder is unboosted and was known to be extremely heavy on control forces, to the point of being almost useless at fighting speeds. The 21 was a similar story and lack of rudder authority (or rather inability to deflect it sufficiently) is mentioned in the flight manual. There's no gearing system/ARU equivalent/additional channel limiting the rudder, it's just that you're trying to get a quite large surface moving against very high speed airflow, and the 19's incredibly fat tail probably just makes the matter even worse.
  4. You're overspeeding the aircraft. Do not exceed 1350km/h IAS (technically the limit is 1300, but ~1350-1380 is where the engine dies in DCS). Do not fly level below 4000m with the emergency afterburner active - either switch it off, or only advance the throttle until the first afterburner status light comes on.
  5. Checking in to say I'm not dead, just overworked Right now I have maybe 1-2 days a week I can even be online around server prime time and that's subject to other commitments as well right now. Hopefully I can shift some hours around and get back into it, I'm so rusted up I can barely fly in a straight li... ok, I can fly even less straight than I already did.
  6. Revised figures for the T/D equilibrium - group 3 (now using correct KB-RA-XT-RA-RA) 1391km/h IAS or M ~1.14; group 4 (now SB-SB-XT-24-SB) 1325km/h IAS or M ~1.08. This means that group 4 is also too fast by a fair margin. AJS 0km drag equilibrium GRUPP 4.trk AJS 0km drag equilibrium GRUPP 3.trk
  7. Good catch - I'll run those again over the weekend and see what the difference works out to be. My eyes go square when looking at tables.
  8. I would've liked to draw up curves for fuel burn and distance covered as well, just haven't got time at the moment. I figure fuel burn in particular might help solve what's going on. Do you know if there's much hope for actual notes from flight testing to double check the numbers against? I ask because in the case of the 21 it turned out the charts were quite conservative, though not by a large enough margin to explain what's going on here. More numbers is always better, even if it means more work
  9. All charts are sourced from SFI AJ 37 del 3, 1979 print. Test flights were flown in ISA, using weights as close as possible to those described for each loadout (remembering that the AJS is somewhat heavier than the AJ), either start weight or flight weight depending on what the chart in question described. LEVEL ACCELERATION Tests were run at 0km, 6km, and 11km in ISA conditions only. Each test included two runs (one by myself, one by Airhunter) with clean config/rent FPL. In addition, I ran a second set of 0km tests with a group 3 loadout (4x ARAK + XT), and 6km tests with a group 4 loadout (4x SB + XT). Two tests were run at 11km with the loadout stipulated in the manual, one by myself and one by Airhunter. Airhunter provided me with tacview files and graphs which I then charted against the manual (obviously only for the clean config 0km and 6km tests, and the 11km test). On my own end I forgot to save tacviews, but did save trackfiles, which are attached as well. The results show what I'd found myself: At sea level, mil thrust is maybe a touch slow to accelerate to M 0.9 but is still relatively close, and zone 3 is tuned almost on the dot. Zone 1 and 2 show discrepancies, particularly zone 1, although they aren't very large at this altitude. Excuse the rather unscientific graphs, but unfortunately just overlaying the ones from tacview wasn't really workable and I'm working with what I've got here: At 6km alt, mil thrust gradually diverges from the chart, ending up a little slower than expected. Zone 1 and 2 show large discrepancies. Zone 1 follows zone 2's curve, zone 2 follows zone 3's curve, and zone 3 is doing something else entirely. At 11km alt, only zone 3 was tested. The results were very far off the expectation: according to the chart, level acceleration from M 0.9 to M 1.6 should take about 5 minutes and 45 seconds. In the sim, this was achieved in just one minute (!), while acceleration from M 0.9 to M ~1.63 (as far as the line is drawn) should take about 7 minutes and 15 seconds, while in DCS it took 1 minute and 15 seconds. This is a very large discrepancy even compared to the aircraft's own charts, but also compared to acceleration profiles of other aircraft known to have phenomenal performance in this area (F-104, MiG-23, MiG-29). Something is clearly causing an issue here but I don't know what. Big thanks to Æck for spotting this one during a MP session and bringing it to light - I wouldn't have thought to test it otherwise. Finally for now, THRUST/DRAG EQUILIBRIUM SPEEDS AT SEA LEVEL This has been something I've been meaning to report for a while, but invariably by the time I got trackfiles, DCS would update and break them. As a result I'll just post the results of my testing now and drum up the trackfiles when I get time. Please note that this is separate to, but was compounded by, the now-fixed drag issues with Sidewinder launch rails. Each test was run using the circled loadout group from the AJ 37 loadout tables - so rent FPL was clean, group 1 was KA-24-XT-24-KB, group 2 was KA-24-XT-blank-04, group 3 was RA-RA-XT-RA-RA, group 4 was SB-SB-XT-SB-SB. All loadouts except group 4 (bombs) reach thrust/drag equilibrium above the intercept of MAX ZON 3 lines and thrust/drag lines, some significantly so. Group 4 is slightly slower than expected, the clean airframe flies beyond even SAAB's estimate for a world speed record using a stripped and polished modified aircraft, groups 1 and 2 are beyond the aircraft's Vne and fall off the chart, and group 3 (which should be the slowest according to the chart - just barely subsonic, likely due to the rocket pods generating enormous amounts of transonic drag compared to anything else tested) is sitting slightly above the aircraft's Vne. It seems drag values need looking at. I don't have trackfiles handy for these tests (the ones I did have are now several DCS patches old), but I do have tacviews for them. I can get tracks again if necessary. I haven't checked climb performance yet, but hopefully there won't be anything to add for that. OWN TRACKFILES.zip Tacviews from Airhunter.zip drag tests.zip
  10. I'm not sure what you're expecting of the jet, here. Its nose will follow anything an F-5 can do right up until the departure (which is typically happening because the wing is shadowing the tail at a high enough AoA - like Koty said, just because the wing isn't fully stalled, it doesn't mean the jet doesn't have other reasons to depart controlled flight) and the F-5 can only avoid a similar fate because it has specific design features meant to impart stability at high angles of attack. In most aircraft you'd drop a wing or spin, in the 21 you get the tail wag and maybe a rollover and tumble if you monkey the stick hard enough. I dunno dude, plenty of us seem to have few to no issues fighting in it and it's entirely possible to outfight less experienced Hornets, despite the latter's clear (and realistic) superiority in the AoA department. Being able to fly one direction while pointing your nose isn't everything, and it's not even very good BFM, honestly. I would also again like to draw attention that you tried to assert that the official documentation says it stalls at 28 and 33. First of all, no - as addressed by Koty. Second of all - I'm going to assume you can't read Cyrillic, but for anyone who can, it plain as day says "UUA-1" alongside each angle, aside from two marked "SUA-1" (the blinker-light warning system). Again, all values in the manual are given in the same measures the aircraft indicates. It is neither here nor there to the pilot what angle his plane is actually pointing, because all the charts are worked out for what the UUA shows, as it is the aircraft's only method of measuring angle of attack and thus has to be relied on regardless whether you find its relationship to true AoA (which as FrederF pointed out, is not strictly double) objectionable or not. If you don't understand what you're reading in the manuals, and you're willing to ignore the effects of that AoA on somewhat important things like airflow over other surfaces of the aircraft, then I don't know what to tell you because it looks like it'll just get thrown back anyway.
  11. It doesn't really matter much what the case he's making is, if the entire basis of his assertions is completely incorrect. Bringing up the 28 and 33 degree curves from the documentation, without noticing the literal "alphaUUA-1=" part of the labels for each, is not going to help his cause even if he is correct... which Frederf has just addressed. Even if the gauge is misreading, the aircraft is performing about as it should. I suspect this is the case with a few modules to be honest, but all it means is that the body angle of the aircraft appears too high or too low ingame. As for that paper - certainly someone in the thread has read it, if not OP... there has been quite a bit of magical thinking going on in here.
  12. Actually, while I'm here - if you want to see some of the magical nose authority you guys want, make a hard, sharp pull at reasonably low speed and full power or afterburner. Make sure the aircraft is wings level and the slip ball centred, or if you're in a turn, that you have it perfectly balanced. The aircraft will either rock its wings briefly and then stabilise, or it will not even wing rock at all, and will enter an extremely stable state where it's basically hanging in the air on its thrust. Maybe that approaches what you're expecting, but it's debatable if the aircraft should even do it, considering that at such a high AoA the wing is fully shadowing the vertical tail surface and the ventral strake is no longer enough to maintain directional stability. Now perhaps you've got a better idea of why the MiG-21 departs where it does. It's not a Mirage 2000 with vortex generators, dogtoothed leading edge slats, and fly by wire... it's a 1950s aircraft. You cannot expect modern performance out of it just because a single academic paper told you (with a bunch of qualifiers, I should add) that the 21 still has some relevance in the modern age due to manoeuvring potential.
  13. The "actual documents" you mean are, I'm going to guess, the original flight manuals - which explicitly state 28 and 33 degrees as indicated by the UUA-1, which is the AoA gauge, which tells you what the DUA sensor reads, which is on the side of the fuselage and not in clean air. Every single chart I have seen for the 21 across flight manuals and memoranda references the UUA. I have never seen one that references true AoA, as that would be absolutely useless to the pilot as he has no point of reference except UUA-1. What it says is law as far as he is concerned. Where do you guys think the Americans got all those AoA values from when they test flew the aircraft? Magic? A bolted-on F/A-18 air data computer reconfigured to derive true AoA from the UUA reading? No, they referenced the gauge in the aircraft, just like US manuals reference units of AoA rather than true angles. The pilot has no use for units of measurement he doesn't have access to in flight. I already know which paper you guys have read, which is somehow leading you to draw the conclusion that our 21 has been unfairly treated and should be able to turn with modern fighters. You should not read too much into a single paper, especially when it leads you to conclusions that fly directly in the face of documented Soviet and American flight testing.
  14. The explanation myself, Torbernite, and Frederf gave are hardly wild - they're exactly what you'll find if you read the aircraft's flight manual, or for that matter any aircraft's flight or technical manuals from that era, or if you ask mechanics, or aerodynamicists. You are trying to derive angle of attack of the wing from the aircraft's pitch angle which makes absolutely no sense, even before considering the position of the AoA vane and the effect of the airflow off the aircraft itself.
  15. Clearly. The reason the AoA is not a direct relationship (and also why the fuselage vane - which is the one UUA uses, the others on the probe are for the gunsight) is because the airflow over the vane is already disturbed by the aircraft. This is also why older American aircraft use "units" of AoA, not degrees, with "units" being an arbitrary measure that takes this effect into account. If you make the assumption pitch angle = AoA, you're very quickly going to come unstuck when dealing with a lot of aircraft, not only because they lack flight computers that can run the equation to get true AoA from the local AoA, but also because not everything has a 0 degree angle of incidence wing.
×
×
  • Create New...