robban75 Posted January 29, 2017 Posted January 29, 2017 I love flying the LNS Viggen. While it is generally easy to fly, a great deal of care is also needed. IMO, much more so than any other (jet propelled) aircraft in the sim. Now, I'm aware that deltas are supposed to be draggy. I'm just curious as to whether the induced drag on the Viggen really made such an impact on performance to the degree we see in the sim. The RM8A is very powerful, and in a horizontal low AoA acceleration the power is noticable. However during take off, shortly after rotation at an IAS of 290 km/h, AoA around 12 the aircraft almost stops accelerating. Is it correct? Or is it excessive? Anyone care to educate me here? :) Video below. pMwSivqnzmg
renhanxue Posted January 29, 2017 Posted January 29, 2017 There is an extensive discussion of this exact problem in the aerodynamics compendium, see page 38 and onwards in the PDF (printed page number 42) and also note fig. V.11 on page 42 (printed page number 46). That last figure is for landing but shows just how limited maneuvers you can do at high alpha and low speed, even with a very light aircraft. My understanding is that yes, this is fairly realistic modeling. At low speeds and high alpha you're not only seeing induced drag but also less thrust because of losses in the intakes. So, yeah, don't over-rotate.
robban75 Posted January 29, 2017 Author Posted January 29, 2017 Thanks renhanxue! :) Yeah, LNS did a good job! The Viggen requires a different flying style than other jets in the sim. It's a new experience for everyone I guess? After all, the sim-market hasn't excactly been flooded by tailless deltawinged aircrafts. Makes you appreciate the advantages offered by relaxed stability and FBW. :)
howie87 Posted January 30, 2017 Posted January 30, 2017 Have to admit I was surprised at how draggy the Viggen is during takeoff considering it's supposed STOL capabilities. It requires a huge amount of nose up trim and a steady hand. I thought the canards would help more but it seems trickier to takeoff in than the MiG-21.
Grundar Posted January 30, 2017 Posted January 30, 2017 I love her flight characteristics - she gets very draggy at low speed, she does warn you before hand though with some gentle yawing as well as descent. The engine is powerful but it is not instantaneous power so in other jet aircraft you could slam the throttle and get that safety back very quickly, in the Viggen it is much more dicey - which is great for learning how to manage/avoid low speed and too much alpha for the speed. I imagine it is also great practice for the F-14A. She flies wonderfully down low at high speed and she is a joy to throw around - I've been terrain hugging everywhere I go, I should spend more time with her up high to see how she handles. I can really appreciate after a short time of flying her, why it was reported that some Viggen pilots would of liked a second engine!
renhanxue Posted January 30, 2017 Posted January 30, 2017 (edited) Well, short takeoff is an actual separate procedure. If you just do a rolling start with full fuel and four rocket pods in zone 2, it's gonna feel like a school bus. It'll get you off the ground, sure, but it's not going to "wow" you, at all. If you want to do an actual short takeoff, first of all you need to a) go from military power to max zone 3 while the brakes are still applied (release brakes as soon as aircraft starts to skid) and b) ignore the HUD's idea of when you're supposed to rotate. Instead you follow this diagram (takeoff weight in tons on the left side of each polygon, air temperature in °C on the right; rotation speed in km/h IAS on the horizontal axis): Minimum permitted rotation speed is 190 km/h IAS. Rotate in time, but do not over-rotate! You must maintain exactly the alpha you need for sufficient lift while not exceeding it so you get too much drag. The HUD tells you roughly the attitude you should maintain; just align the flight path marker with the tops of the outermost "posts" and you'll get 13° above the horizon, which is usually what you want at max zone 3. If you do it right (no tailwind or other weird things going on) you should have a ground roll <700m and reach Mach 0.55 within 45-50 seconds after brakes release (assuming you level out at the lowest safe altitude) on a standard day, even with the heaviest possible loadouts. Edited January 30, 2017 by renhanxue 1
BlackLion213 Posted January 30, 2017 Posted January 30, 2017 At low speeds and high alpha you're not only seeing induced drag but also less thrust because of losses in the intakes. So, yeah, don't over-rotate. Yeah....I did that today. It didn't end well (compressor stall and tumble with no chance for recovery). But I love aircraft that demand you fly properly - otherwise there are no rewards for the effort of learning. Easy aircraft are boring aircraft (I'm looking at you F-15C...:smilewink:). -Nick
Nerd1000 Posted January 30, 2017 Posted January 30, 2017 A little while back I built an R/C model of the Viggen powered by a electric ducted fan (flying weight 1kg, static thrust circa 490 grams), and I can confirm that high AoA causes massive speed loss on that sort of wing planform- mine needed full throttle to maintain height in turns, but easily built up a nice speed in level flight even at around 60% throttle. It also has interesting characteristics near the stall- the plane doesn't really stall as such and will happily remain controllable at high AoA while warning you of the near stall condition by rocking its wings back and forth (the drag makes you lose altitude fast). I've got another tailless model that uses a straight wing with LERX, and it has nowhere near the same level of speed loss in turns even though it's aerodynamically much dirtier than the Viggen- of course it's propelled by a prop and has a better TWR, so some of that may be masked by raw thrust. Unsatisfied with the Viggen's amount of thrust (I wanted to do aerobatics) I switched from the original 3 cell 11.1V battery to a 4 cell 14.8V battery- this gave me the thrust I wanted (closer to 700g I think), with nice quick climbs and enough power to do loops. Unfortunately it was a bit much for the electric motor and it self-destructed around 4 minutes into the first flight with that setup, so my Viggen is now confined to the hangar while she awaits a heart transplant :(
hilmerby Posted March 19, 2017 Posted March 19, 2017 IRL the AJ37 was not at all that sensitive in pitch. We normally used Afterburner Z2 for takeoff even with full load and got airborne with 8-10 degree pitch. If you do that in DCS you will be airborne with 15-18 degree and almost no accelleration. You can also make a test. This is what we sometimes did, just for fun. Fuel 30-40%. Take off with Z3, retract the gear and stay at max 10 meter above the RWY. With a RWY of 2100m the speed at the other and should be around 750km/h. In DCS you will get 550km/h probably due to the high pitch after rotaion, it creates a lot of drag. 1
renhanxue Posted March 19, 2017 Posted March 19, 2017 IRL the AJ37 was not at all that sensitive in pitch. We normally used Afterburner Z2 for takeoff even with full load and got airborne with 8-10 degree pitch. If you do that in DCS you will be airborne with 15-18 degree and almost no accelleration. You can also make a test. This is what we sometimes did, just for fun. Fuel 30-40%. Take off with Z3, retract the gear and stay at max 10 meter above the RWY. With a RWY of 2100m the speed at the other and should be around 750km/h. In DCS you will get 550km/h probably due to the high pitch after rotaion, it creates a lot of drag. Very interesting to hear, thank you!
TOViper Posted March 19, 2017 Posted March 19, 2017 (edited) You can also make a test. This is what we sometimes did, just for fun. Fuel 30-40%. Take off with Z3, retract the gear and stay at max 10 meter above the RWY. With a RWY of 2100m the speed at the other and should be around 750km/h. In DCS you will get 550km/h probably due to the high pitch after rotaion, it creates a lot of drag. Following your test configuration, I managed to have 600 km/h at 2100 meters, with a ambient air temperature of 0°C and having 35% of fuel on board. I am still working on some optimization, but I am far away from 750. Is it me? :cry: BTW: here is the mission file on caucasus map, if you wanna try it out. Viggen_Speed_Test_Sochi_2100m.miz .) Taxi to the soldier standing in front of you (about 400 meters away), and stop there .) Set correct pitch trim, power up to zone 3 .) Release brakes .) Take-off and accelerate as best as you can .) Read speed at end of runway, and finally .) Report your speed here if you like :book:. Edited March 19, 2017 by TOViper upload of mission file Visit https://www.viggen.training ...Viggen... what more can you ask for? my computer: AMD Ryzen 5600G 4.4 GHz | NVIDIA RTX 3080 10GB | 32 GB 3.2 GHz DDR4 DUAL | SSD 980 256 GB SYS + SSD 2TB DCS | TM Warthog Stick + Throttle + TRP | Rift CV1
Hummingbird Posted March 19, 2017 Posted March 19, 2017 No it isn't you, it's already confirmed that the aircraft suffers from excessive induced drag atm, with the max sustainable load factor being a massive 1 G below real life values in stage three AB.
p1t1o Posted April 7, 2017 Posted April 7, 2017 Heck I have trouble keeping the dang thing from stalling without some level of AB, whether flying straight or turning, and I mean turning leisurely. Can anyone say if the Viggen IRL was flown almost exclusively on AB, or is that just me? In youtubes I have seen, the pilot (DCS pilots that is) hardly ever left AB.
OnlyforDCS Posted April 7, 2017 Posted April 7, 2017 It's not you. It's most definitely a bug. Check this thread: https://forums.eagle.ru/showthread.php?p=3082830#post3082830 RagnarDa said they are aware of the issue, but it isn't a priority fix just yet. Current specs: Windows 10 Home 64bit, i5-9600K @ 3.7 Ghz, 32GB DDR4 RAM, 1TB Samsung EVO 860 M.2 SSD, GAINWARD RTX2060 6GB, Oculus Rift S, MS FFB2 Sidewinder + Warthog Throttle Quadrant, Saitek Pro rudder pedals.
hilmerby Posted April 8, 2017 Posted April 8, 2017 IRL we used AB Z2 or Z3 at takeoff but not during the rest of the mission unless we wanted high speed after veapon release. We could cruise at M.85 at 9000m on main engine with no problem. I have also seen some videos where people use AF during attackrun with RB04, we never did that. No problem to fly M.08 and still not use max main engine. We had to give our wingman some slack! /Stellan
SpeedStick Posted April 8, 2017 Posted April 8, 2017 I guess it's pretty obvious but I take it you are a viggen pilot, Stellan? If you are, keep the information coming! It is invaluable to us all. Thank you! "Hard to imagine bigger engine. its got a beautiful face and an arse built like sputnik." - Pikey AKA The Poet, on 37 Viggen.
hilmerby Posted April 9, 2017 Posted April 9, 2017 Well, I was. 1968 I started my basic training at F5 Ljungbyhed. From1970 I flew the A32 and transferred to AJ37 1973. Left the Airforce for SAS in 1979 but continued to fly AJ37 6 weeks every year until 1987. Retired from SAS 20009.
Pocket Sized Posted April 10, 2017 Posted April 10, 2017 We could cruise at M.85 at 9000m on main engine with no problem. Carrying weapons or clean airframe? DCS modules are built up to a spec, not down to a schedule. In order to utilize a system to your advantage, you must know how it works.
hilmerby Posted April 10, 2017 Posted April 10, 2017 Clean, only external fuel. Sweden is a long country and sometimes we had to change base from south to north.
impeller83 Posted April 10, 2017 Posted April 10, 2017 Well, I was. 1968 I started my basic training at F5 Ljungbyhed. From1970 I flew the A32 and transferred to AJ37 1973. Left the Airforce for SAS in 1979 but continued to fly AJ37 6 weeks every year until 1987. Retired from SAS 20009. Thats awesom, please keep sharing
mattebubben Posted April 10, 2017 Posted April 10, 2017 (edited) Well, I was. 1968 I started my basic training at F5 Ljungbyhed. From1970 I flew the A32 and transferred to AJ37 1973. Left the Airforce for SAS in 1979 but continued to fly AJ37 6 weeks every year until 1987. Retired from SAS 20009. From those dates im Guessing that you belonged to F7? Gustav Blå?. And its nice to see a AJ 37 Pilot on the Forum=). Edited April 10, 2017 by mattebubben
p1t1o Posted April 12, 2017 Posted April 12, 2017 Well, I was. 1968 I started my basic training at F5 Ljungbyhed. From1970 I flew the A32 and transferred to AJ37 1973. Left the Airforce for SAS in 1979 but continued to fly AJ37 6 weeks every year until 1987. Retired from SAS 20009. AH! I just twigged! At first I was all "Jeeze, this guy must be BAD. A$$. First a fighter pilot, then joins the SAS!?" Until just now when I realised you were more likely referring to Scandinavian Airlines lol! Still impressive! Military pilot is my dream job, and followed by a stint in the airlines is the ideal. A bout of mumps at uni took care of my hearing unfortunately. Not that I was guaranteed to get in otherwise, of course.
SpeedStick Posted May 13, 2017 Posted May 13, 2017 Well todays update does not seem to have fixed it. According to Herr Stellans info "We could cruise at M.85 at 9000m on main engine with no problem". Just did a test, climbed up to 9000m and dropped from abz2 to mil and watch it go from M.82ish to M.63 (and continued sinking). This was with only emtpy external and about 70% fuel. Same thing happened at 8000m and 7000m just at slower at rates. It just seems to keep dropping too. "Hard to imagine bigger engine. its got a beautiful face and an arse built like sputnik." - Pikey AKA The Poet, on 37 Viggen.
Pocket Sized Posted May 15, 2017 Posted May 15, 2017 Well todays update does not seem to have fixed it. According to Herr Stellans info "We could cruise at M.85 at 9000m on main engine with no problem". Just did a test, climbed up to 9000m and dropped from abz2 to mil and watch it go from M.82ish to M.63 (and continued sinking). This was with only emtpy external and about 70% fuel. Same thing happened at 8000m and 7000m just at slower at rates. It just seems to keep dropping too. Turn on manual fuel regulation and the cruise performance will be much closer to reality. In automatic (default mode) the engine throttles back significantly as altitude increases. DCS modules are built up to a spec, not down to a schedule. In order to utilize a system to your advantage, you must know how it works.
CoBlue Posted May 15, 2017 Posted May 15, 2017 Turn on manual fuel regulation and the cruise performance will be much closer to reality. In automatic (default mode) the engine throttles back significantly as altitude increases. That's only a temporary solution. That switch was never used IRL. https://forums.eagle.ru/showpost.php?p=3078865&postcount=3 i7 8700k@4.7, 1080ti, DDR4 32GB, 2x SSD , HD 2TB, W10, ASUS 27", TrackIr5, TMWH, X-56, GProR.
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