Unstable flight model?

[Fred00]

I did a quick test flight of the Viggen yesterday (tough to find the time with a one week old baby in the house). However, I found the flight model a bit strange. Particularly it feels very sensitive to pitch input. It's easy to get an up-and-down bouncing right after take-off, but also in flight it feels very sensitive. I tried to apply a curvature of 25 and a little dead zone, but it still feels extremely sensitive. Switching over to the Mig-21 or M-2000C the difference is night and day. Has anyone else noticed this? Like with the Mig-21 I assume that the FM is a work in progress and may change drastically (like the Mig has several times).

[Almaril]

Are you sure you had SPAK on? Any stores?

[Fred00]

Actually I don't know. I set up a runway start in the mission editor to just do a free flight. Is SPAK on by default when doing a runway start?

[Almaril]

Yeah, it should be. Starts automatically on engine start if I recall correctly.

[renhanxue]

Yep. There's a big green light near the top of the instrument panel on the left side that's lit up when it's on (should always be lit, basically - there's no reason to ever turn SPAK off unless it's actively malfunctioning).

[BachoGrande]

I'm noticing this with he pitch especially, it's very sensitive even w/ high curves.

[RagnarDa]

Be aware that there is a differential pitch and roll gearing that makes the pitch and roll a lot more sensitive when below 350 km/h. Also, the SAS/SPAK tries to give pitch inputs so a full deflection of the stick gives 8 G's, and in slow speed this would be a very fast movement of the nose. Newer aircrafts, like the Gripen, also considers max alpha when translating pilot input to max elevon deflection and not just max G.

[JorgeIII]

I did a quick test flight of the Viggen yesterday (tough to find the time with a one week old baby in the house). However, I found the flight model a bit strange. Particularly it feels very sensitive to pitch input. It's easy to get an up-and-down bouncing right after take-off, but also in flight it feels very sensitive. I tried to apply a curvature of 25 and a little dead zone, but it still feels extremely sensitive. Switching over to the Mig-21 or M-2000C the difference is night and day. Has anyone else noticed this? Like with the Mig-21 I assume that the FM is a work in progress and may change drastically (like the Mig has several times).

 

 

I got the same feeling here. Something doesn't feel real on how the plane reacts. I cant be specific right now but it is my first impression that it doesn't feel right. Its something related to instability (nose moving to freely? But even instability feels artificial) and something related to how fast the aircraft reacts and countereacts to inputs (might react too fast and it don't seems well associated with IAS). I'm not talking about performance here, just about the subjective feeling of flying a plane.

 

I will test it a bit more. As reference I do have the experience of how a plane feels and reacts to inputs, weight, IAS, wind, gusts, thermals, etc. I usually fly in real life single engine planes (private pilot) and I have been behind the stick of planes ranging from seaplanes and tail draggers to a Learjet (sadly never a supersonic deltawing). I have used PC flight simulators for 20+ years. In my experience the Viggens FM needs more work to feel real.

 

Just to comment: I got the same "fake" feeling with the A-10C when it was in beta (2010, improved a lot) and with the recently released Spitfire, I just didn't like it at all (pitches and rolls without initial inercia, etc) . For me the best flying sensation (closest to reality) in DCS are experienced with the P51D (excellent all around physics) and while hovering or slow flying the UH-1H. The Mig-21 changed over time but I feel it real too.

Edited January 28, 2017 by JorgeIII

[Fred00]

Be aware that there is a differential pitch and roll gearing that makes the pitch and roll a lot more sensitive when below 350 km/h. Also, the SAS/SPAK tries to give pitch inputs so a full deflection of the stick gives 8 G's, and in slow speed this would be a very fast movement of the nose. Newer aircrafts, like the Gripen, also considers max alpha when translating pilot input to max elevon deflection and not just max G.

Thanks for the reply. The sensitivity is indeed greater below 350 km/h, but it still feels way too sensitive above 350 km/h too. Obviously I haven't been in a real Viggen, but it feels a bit artificial and does not exhibit the precision that I thought it should have. Maybe the input response is technically correct, but has to be tuned to the short joystick throw that we have compared to the real thing?

 

I have to say though that the Mig-21 is the best feeling module in DCS flight model wise, so that is why I'm a little surprised by the feel of flying the Viggen. But again, I may be completely wrong about all this.

[Jonmarshall]

I remember in the mig-21 FM there was a flight controls setup guide for a standard joystick dictating you how curves, saturation and dead zones would match the real thing. I was expecting the same with the Viggen, let's hope they come with something similar since the sensibility between their two aircrafts are like night and day.

 

I know both aircrafts are not the same, its just too sensitive for my taste. I must say I got used to fly the fishbed, guess I need some time to learn the new beast.

 

I always wish to get the 2 and 3 positions switches to work with their aircrafts and the warthog too. It would be nice to hear if a setting plan is in the works for the official release??

 

Other than that, congrats on this bird, it is a piece of art. Well done.

[Tango777]

The recreation is an amazing simulation of the Viggen! Well done developers.

 

But I do agree that something is wrong. I have the F-15C, M-2000, A-10C, Sabre, Su-27 and as long as the stores and fuel tanks on both sides are equal, these planes always maintain a fairly straight and level flight if I let go of the stick if I am manually flying in straight and level configuration.

 

I always have the SPAK on as usual in the Viggen, and I noticed that in the majority of the cases it always enters a hard right bank angle accompanied by a ever dropping nose pitch the moment I let go of the stick when my stick is centered. Therefore as soon as the altitude and heading is where I want it, I have to immediately engage the attitude and altitude hold modes. I always check all trims and they are at 0 degrees at all times, and this happens no matter what the wind component is. The stores are balanced or I am flying a clean Viggen with only a center fuel tank. The right bank happens regardless. The flight controls are dead center which makes this even more strange.

Edited January 28, 2017 by Tango777

[Pocket Sized]

I got the same feeling here. Something doesn't feel real on how the plane reacts. I cant be specific right now but it is my first impression that it doesn't feel right. Its something related to instability (nose moving to freely? But even instability feels artificial) and something related to how fast the aircraft reacts and countereacts to inputs (might react too fast and it don't seems well associated with IAS). I'm not talking about performance here, just about the subjective feeling of flying a plane.

 

I have a small obsession for flight models and the Viggen flies exactly as I expected it would, I am really impressed.

 

The pitch/roll/yaw is heavily augmented by the autopilot to make the plane feel more snappy. At low speeds especially the outer elevons and rudder are working like mad to keep you stable. Sometimes they don't have enough authority to damp large movements, which leads to some unexpected behavior at times.

 

Turn off the SPAK to feel how the base aircraft flies, its certainly a handful!

[Hummingbird]

Havent had the time to test out the module yet, but I have a feeling this is what I noticed in the first trailer videos, i.e. a marked instability in pitch - Cobra put that down to poor flying, but I guess it wasnt so much a pilot error after-all.

[NeilWillis]

The rudder also feels far too sensitive in the latter phase of the take off roll.

 

Rather than moving the pedals, just applying slight pressure seems enough to make pilot induced oscillations a real issue.

 

I suspect both axes require a little toning down to give a decent feel.

[Corrigan]

Rudder is a known and internally fixed bug (or several actually).

[Fred00]

Okay, I've just come to the conclusion that the curvature settings don't seem to do anything for the Viggen at the moment. I have created a new thread about this. Now I understand why the plane still felt nervous with +35 in curvature...

 

Having this fixed will mean a lot for us who feel that the Viggen is too sensitive right now.

[Pocket Sized]

Havent had the time to test out the module yet, but I have a feeling this is what I noticed in the first trailer videos, i.e. a marked instability in pitch - Cobra put that down to poor flying, but I guess it wasnt so much a pilot error after-all.

 

The pitch "instability" isn't a bug, and the Viggen certainly isn't the only aircraft in DCS to exhibit it.

 

It's due to low stability margins at low AoA. When disturbed, the plane will try to return to its trimmed AoA because it's positively stable, but there's a small deadzone near zero AoA in which the nose will wander around even with extremely small inputs.

 

That's where the SPAK system comes in. It dampens those wandering tendencies and makes control response more linear. Same goes for the dampers in the F-5, and the Control Augmentation System in the F-15.

[Tango777]

My problem is roll instability at any speed which leads to immediate nose pitch instability. I take my hand off the stick or disable autopilot and it rolls right every time, and it rolls right pretty fast.

[drPhibes]

With the exception of the over-sensitive rudder, I'm not seeing any of the symptoms described here. TM Warthog, no curves and 100% Y-axis saturation.

[xxJohnxx]

My problem is roll instability at any speed which leads to immediate nose pitch instability. I take my hand off the stick or disable autopilot and it rolls right every time, and it rolls right pretty fast.

 

Trim.

[Tango777]

Trim.

 

My trims are at 0.

[tranquillity]

My trims are at 0.

 

I had this happen twice to me so far. Once, when I only fired one of the two x15 that I carried.

Second time i was in full autopilot and then manually rolled out of it but somehow the autopilots input must have gotten stuck because the plane kept rolling and rolling. I think I fixed it by toggling autopilots on and off again multiple times

[00Donkey]

So i just recently started learning the Viggen, (BTW, THANKS SO MUCH FOR THE TUTORIAL VIDEOS xxJohnxx! (im a recent subscriber but ive been watching your videos since you started making them, also thnks for the stability lesson on making DCS run smoother!)

 

that being said, im having issues with the Viggen, when flying it, Just traveling in a straight line at about 250 feet off the ground, as i change speed, im constantly fighting the aircraft's urge to nose HARD into the ground, or if i speed up, im suddenly fighting its urge to climb VERTICAL, and i MEAN VERTICALLY at 90 degrees of pitch if i take my hand off the stick or allow my hand to relax on the stick in a central position,.. it Just doesn't feel right,... is this how the viggen is supposed to behave? i've tried with and without the "SPAK" enabled, and still the same, how's the viggen supposed to do any sort of high speed engagements at low altitude if when you speed up it climbs like crazy, (even with the trim centered at 0 and later trying Just to leave it at the +3 degrees suggested for take off)

[Pocket Sized]

Because the Viggen, like most aircraft, lacks auto trim, you must re trim the aircraft with changes in airspeed. (Also stay below 0.9M, trim gets weird above that)

 

Here's a little trick I figured out to get around the very slow pitch trim hat:

 

When you level off, click the ATT hold autopilot on for a few seconds and center your stick. Once the plane stabilizes, disengage ATT hold and presto! Perfectly trimmed for 1G flight at that airspeed.

[Csar]

Interesting, I wouldn't really expect the Viggen to "feel" like other aircrafts based on the canards. Would it be correct to assume they would move the nose around more than other aircrafts?