Viggen still won't pull max G at certain airspeeds at max full stick aft at specific fuel load

[renhanxue]

2 hours ago, TOViper said:

Guys, before I do, we have to make sure the module is following this graph. You know, the manual describes the module, not the real Viggen.
On the other hand, it would be good to know that HB implemented this exactly...
I would use this graph as well ... erm ... Myse1234 ... erm ... would you please?

Big brain time: if you document the behavior of the real aircraft then people will file bugs complaining it doesn't work as documented and so the module will become more realistic. Everybody wins!

[TOViper]

I am aware of that, indeed. My concern is that HB might not implement things in a timely manner, so if I document now, the manual is wrong for a very long time.
IMHO it would be better if e.g. Machalot puts that one on the bug list, and HB eventually implements the changes, then I will change the doc as well. Sounds like a plan, does it?

But first we have to know if the aircraft behaves like described (I mean in the very detail). Maybe @Machalot can ask the devs before we do hundreds of flights ...

Edited March 5, 2023 by TOViper

[Machalot]

Question submitted. Let's see what they say. 

[D4n]

On 3/2/2023 at 8:29 PM, TOViper said:

The torque of the actuators acting on the elevon surfaces. It is just not enough power to deflect the elevon surfaces for full travel ...
Welcome in the world of flight engineering

I just saw your reply just now, sorry. Not enough power to move the elevons to the max. pitch up position???  Did the Viggen engineers not realize the insane importance of full 9G capability at low altitude at any high airspeed, for most successful defensive maneuvers (or even for case of a surprising offensive merge with a bandit)? And shouldn't it be way easier to reproduce this issue in DCS with weapons loaded instead of clean jet? (Afaik heavier jet = higher G at fast turns, compared to a lighter jet at turns of equal speed? )

Edited March 6, 2023 by D4n

[renhanxue]

2 hours ago, D4n said:

I just saw your reply just now, sorry. Not enough power to move the elevons to the max. pitch up position???  Did the Viggen engineers not realize the insane importance of full 9G capability at low altitude at any high airspeed, for most successful defensive maneuvers (or even for case of a surprising offensive merge with a bandit)? And shouldn't it be way easier to reproduce this issue in DCS with weapons loaded instead of clean jet? (Afaik heavier jet = higher G at fast turns, compared to a lighter jet at turns of equal speed? )

 

The elevons do not deflect anywhere near their max angles at high airspeeds, but no, the hydraulics are not strong enough to overcome the force of the airflow over the elevons at very high dynamic pressure. The full extent of these problems was only discovered fairly late in the development process of the aircraft, specifically in 1968 (the first flight was in February of 1967). In order to save weight the aircraft had been shortened compared to the configuration originally studied in the early 1960's, but this led to some very undesirable aerodynamic characteristics in the transonic region, and these problems weren't caught during wind tunnel testing because of the limited capacity and scale of Swedish wind tunnels. The full story of this covers almost an entire chapter of Krister Karling's book on the chief aerodynamicists's perspective of the development* so I won't try to summarize it here, but suffice to say that the elevon servos were already strengthened once at this late stage of development, and that was deemed to be sufficient for a strike aircraft - and keep in mind that's what the AJ 37 is; it's not a fighter. On the JA 37 (the fighter version) that came along 10 years later, they added a whole another set of hydraulic servo arms and resolved the issue.

* Karling, Krister: Saab 37 Viggen: Utvecklingen av ett nytt enhetsflygplan för det svenska flygvapnet 1952 - 1971 : Sett ur en aerodynamikers perspektiv. Svenska Mekanisters Riksförening 2002. Can be ordered for 100 SEK here.

Edited March 6, 2023 by renhanxue

[D4n]

12 hours ago, renhanxue said:

The elevons do not deflect anywhere near their max angles at high airspeeds, but no, the hydraulics are not strong enough to overcome the force of the airflow over the elevons at very high dynamic pressure. The full extent of these problems was only discovered fairly late in the development process of the aircraft, specifically in 1968 (the first flight was in February of 1967). In order to save weight the aircraft had been shortened compared to the configuration originally studied in the early 1960's, but this led to some very undesirable aerodynamic characteristics in the transonic region, and these problems weren't caught during wind tunnel testing because of the limited capacity and scale of Swedish wind tunnels. The full story of this covers almost an entire chapter of Krister Karling's book on the chief aerodynamicists's perspective of the development* so I won't try to summarize it here, but suffice to say that the elevon servos were already strengthened once at this late stage of development, and that was deemed to be sufficient for a strike aircraft - and keep in mind that's what the AJ 37 is; it's not a fighter. On the JA 37 (the fighter version) that came along 10 years later, they added a whole another set of hydraulic servo arms and resolved the issue.

* Karling, Krister: Saab 37 Viggen: Utvecklingen av ett nytt enhetsflygplan för det svenska flygvapnet 1952 - 1971 : Sett ur en aerodynamikers perspektiv. Svenska Mekanisters Riksförening 2002. Can be ordered for 100 SEK here.

 

Shortened, wow, okay! Do you know by chance which parts of the fuselage were shortened? And indeed, then, JA 37 would be a great DCS module, for all these amazing dynamic PvP campaign airquake servers. (unless that one can't carry RB-05 anymore, then idc ^^)

[renhanxue]

5 hours ago, D4n said:

Shortened, wow, okay! Do you know by chance which parts of the fuselage were shortened? And indeed, then, JA 37 would be a great DCS module, for all these amazing dynamic PvP campaign airquake servers. (unless that one can't carry RB-05 anymore, then idc ^^)

Compared to the "TK1" (typkonfiguration 1) design from late 1962 the production aircraft was shortened by a bit over half a meter, and most of it was taken from the fuselage between the canards and the main wing. On the AJ 37 the aft end of the canards coincide with the front of the main wing; on TK1 they had a gap of several decimeters here. Interestingly on the JA37 they actually lengthened the fuselage in this section again by maybe two decimeters (usually the reason cited for this is that they wanted to fit an extra compressor stage to the engine, but I think it also had some aerodynamic benefits). So, if you compare AJ 37 and JA 37 planform sketches you can see the JA 37 has a gap between the canards and the main wing, although it's not as big as on the original TK1 configuration.

JA 37 cannot carry rb 05, unfortunately. It looks like an AJ 37 but it's almost an entirely new aircraft - the engine is different, the airframe is different, the FCS is different, the radar is different, the computer is different, it shares no weapons with the AJ 37 except the rocket pods and the Sidewinders, and so on and so forth.

Edited March 6, 2023 by renhanxue

[D4n]

Thanks a lot!! Might the fuselage-shortening thing also have caused the altitude-hold autopilot to fail everytime we pass through transonic speed?

[D4n]

Btw, any idea how Viggen pilots would make a pitch-trim reset after landing and fully rearming again? How would pilots know in which position the trim is reset?

Edited March 7, 2023 by D4n

[razo+r]

1 hour ago, D4n said:

Btw, any idea how Viggen pilots would make a pitch-trim reset after landing and fully rearming again? How would pilots know in which position the trim is reset?

 

Theres a pitch trim gauge in the cockpit showing you the current pitch trim... 

[LazyBoot]

17 hours ago, D4n said:

Btw, any idea how Viggen pilots would make a pitch-trim reset after landing and fully rearming again? How would pilots know in which position the trim is reset?

 

you'd look at the indicator gauge hiding down here

Edited March 8, 2023 by LazyBoot

[TOViper]

On 3/7/2023 at 10:41 AM, D4n said:

Btw, any idea how Viggen pilots would make a pitch-trim reset after landing and fully rearming again? How would pilots know in which position the trim is reset?

 

You might use 5° nose-up for normal take-off with attached x-tank, and 4° without it (despite the manual says something about 3° with x-tank and 0° without x-tank).
This makes a very nice and smooth rotation without having to pull much on the stick...

 

Edited March 8, 2023 by TOViper

[D4n]

On 3/8/2023 at 4:19 AM, LazyBoot said:

you'd look at the indicator gauge hiding down here

 

Thanks! Btw. it seems now that the no-max-G is a DCS trim-glitch / Heatblur trim-glitch!!!!!! I JUST tested it!!!!! I once AGAIN was in such a turn, MAX stick aft but it pulling only 7-8 G instead of 9, so I quickly leveled off, quickly engaged attitude hold AP to trim out the pitch and instantly disengage attitude hold again, and BAM, right after that it DID pull 9G with full stick aft!!!!!

[razo+r]

37 minutes ago, D4n said:

Thanks! Btw. it seems now that the no-max-G is a DCS trim-glitch / Heatblur trim-glitch!!!!!! I JUST tested it!!!!! I once AGAIN was in such a turn, MAX stick aft but it pulling only 7-8 G instead of 9, so I quickly leveled off, quickly engaged attitude hold AP to trim out the pitch and instantly disengage attitude hold again, and BAM, right after that it DID pull 9G with full stick aft!!!!!

How badly trimmed and what direction was the trim at? 

Did you do both cases at the exact same speed and altitude? 

[TOViper]

On 3/5/2023 at 5:24 PM, Machalot said:

Question submitted. Let's see what they say. 

Hello Machalot!
Long time no read ..., hm ... do we have any new information regarding this question?

Edited April 26, 2023 by TOViper

[Machalot]

On 4/25/2023 at 2:40 PM, TOViper said:

Hello Machalot!
Long time no read ..., hm ... do we have any new information regarding this question?

 

There was no discussion on my question, actually.  The day I posted it happened to have a whole bunch of activity about multithreading so it got lost.  I bumped it.

Edited May 1, 2023 by Machalot