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

[D4n]

Has anybody else noticed this in multiplayer that sometimes, even with full stick aft, it only pulls 7.5-8G ? I have tried reproducing in singleplayer several times, so far was unable. It might even be related to a certain pull-ratio on the stick (how slowly or fast one enters a turn), what do you think? Am very surprised that nobody reported this awful issue after all these years.

(loadout: 2x AIM-9B, 2x CM pods)

I have 52 MB multiplayer track, but the past has proven that DCS-devs (incl. ED) don't bother checking such massive tracks.

[razo+r]

What are the conditions when that happens? Whats the specific fuel load? Whats the airspeed? Altitude?

[Rudel_chw]

1 hour ago, D4n said:

it only pulls 7.5-8G ?

 

Those figures seem reasonable to me, were you expecting 9G?  Here is a nice analysis based on theorical data, it may be interesting to you:

 

http://www.temporal.com.au/viggen_final.pdf

 

Also, check here:

 

Edited March 2, 2023 by Rudel_chw

[TOViper]

Hey D4n!
Maybe you are running into the following issue, which is described in RC2 (or RC2.1beta) page 269:

nullnull

Edited March 2, 2023 by TOViper
figure

[D4n]

1 hour ago, TOViper said:

Hey D4n!
Maybe you are running into the following issue, which is described in RC2 (or RC2.1beta) page 269:

nullnull

 

Wow thanks!!!!! But, "torque"? limited by what kind of maximum available "torque", torque of what?

Quote

the control surfaces authority may be insufficient to fully
deflect at excessively high loads.

How can this even be, that the Viggen is the only DCS module affected by such a fatal limitation? European military aircraft industry has always been one of the best aircraft industries in the world, I'm shocked!

Edited March 2, 2023 by D4n

[TOViper]

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

Edited March 2, 2023 by TOViper

[razo+r]

23 minutes ago, D4n said:

 

How can this even be, that the Viggen is the only DCS module affected by such a fatal limitation?

 

Its not. Other modules also have decreasing effectiveness of control surfaces at high speed modeled, like the Bf-109, F-86 and MiG-15 to name a few.

[D4n]

18 hours ago, razo+r said:

Its not. Other modules also have decreasing effectiveness of control surfaces at high speed modeled, like the Bf-109, F-86 and MiG-15 to name a few.

Well, MiG-15 lacks G indicator, so no way of realizing that there anyways. Never noticed it in F-86 At which speeds does it occur, and what is the max. G there in the worst case?

[razo+r]

9 minutes ago, D4n said:

Well, MiG-15 lacks G indicator, so no way of realizing that there anyways. Never noticed it in F-86 At which speeds does it occur, and what is the max. G there in the worst case?

You dont need a G indicator to see the effect. The closer you are getting to the critical Mach number, the stiffer the controls become up to a point where they either become unresponsive due to aerodynamic reasons or because the weak meat in the cockpit does not have enough strength to move the stick. 

In the worst case the G limit will be 1G because you cannot move controls anymore. 

[renhanxue]

Here is a chart of the maximum attainable load factor ("G's") for a clean AJ 37 in SPAK mode (disabling SPAK makes it worse, so don't do that):

 

This chart is not particularly easy to read, so I'll try to explain a bit. On the vertical axis is altitude in kilometers and on the horizontal axis is the Mach number. The solid lines marked with numbers indicate the load factor. You should read them like height curves on a map, and "higher up" = higher load factor. Or, in other words, go outside (or "below") a solid line and you are limited to the number of G's that line is marked with. Then the chart is also shaded to show the reasons for these limits, with a key on the right. It's only mildly cryptic but from top to bottom the reasons are:

• 18° alpha (angle of attack) limit
• -22° elevon deflection limit (in other words, load factor is limited by how far the elevons can physically move. technically the elevons can deflect to -27° but part of the deflection range is reserved for roll inputs - this is not a fly-by-wire aircraft so it cannot use the full deflection range for a pure pitch input)
• "pitch gearing", the continuously variable gearbox that attempts to maintain a reasonably linear relationship between stick deflection force and G loading throughout the envelope (see this post for more details about how it works and why it limits the load factor)
• elevon torque limit (in this area the load factor is limited by how much force the hydraulics are capable of exerting on the elevons)

From this we can see that at reasonably low altitudes, 8 G should be attainable almost up to M1.0, but above that it drops off sharply and especially so at medium to high altitude.

The AJ 37 has a really kind of awkward flight control system; it has a bunch of electronics and clever mechanical gearboxes in the control loop to make it more stable and easier to fly, but it's not fly-by-wire, and they were prepared to accept some tradeoffs in order to get a system that would fail safe (if you lose all of your electronics you still have a basic pulleys-and-linkages-and-hydraulics mechanical flight control system to fall back on). On the JA 37 they fixed pretty much all of the issues though (since maneuverability is much more important on an air superiority fighter than it is on a strike aircraft) and the max rated load factor is available throughout the envelope.

Edited March 4, 2023 by renhanxue

[Spirale]

Thx Renhanxue, a very clear explanation

[Machalot]

19 hours ago, D4n said:

Well, MiG-15 lacks G indicator, so no way of realizing that there anyways. Never noticed it in F-86 At which speeds does it occur, and what is the max. G there in the worst case?

When the F-86 is transonic, above Mach 0.85 or 0.9, the roll authority drops off severely.

[TOViper]

12 hours ago, renhanxue said:

Here is a chart of the maximum attainable load factor ("G's") for a clean AJ 37 in SPAK mode (disabling SPAK makes it worse, so don't do that):

 

This chart is not particularly easy to read, so I'll try to explain a bit. On the vertical axis is altitude in kilometers and on the horizontal axis is the Mach number. The solid lines marked with numbers indicate the load factor. You should read them like height curves on a map, and "higher up" = higher load factor. Or, in other words, go outside (or "below") a solid line and you are limited to the number of G's that line is marked with. Then the chart is also shaded to show the reasons for these limits, with a key on the right. It's only mildly cryptic but from top to bottom the reasons are:

• 18° alpha (angle of attack) limit
• -22° elevon deflection limit (in other words, load factor is limited by how far the elevons can physically move. technically the elevons can deflect to -27° but part of the deflection range is reserved for roll inputs - this is not a fly-by-wire aircraft so it cannot use the full deflection range for a pure pitch input)
• "pitch gearing", the continuously variable gearbox that attempts to maintain a reasonably linear relationship between stick deflection and G loading throughout the envelope (see this post for more details about how it works and why it limits the load factor)
• elevon torque limit (in this area the load factor is limited by how much force the hydraulics are capable of exerting on the elevons)

From this we can see that at reasonably low altitudes, 8 G should be attainable almost up to M1.0, but above that it drops off sharply and especially so at medium to high altitude.

The AJ 37 has a really kind of awkward flight control system; it has a bunch of electronics and clever mechanical gearboxes in the control loop to make it more stable and easier to fly, but it's not fly-by-wire, and they were prepared to accept some tradeoffs in order to get a system that would fail safe (if you lose all of your electronics you still have a basic pulleys-and-linkages-and-hydraulics mechanical flight control system to fall back on). On the JA 37 they fixed pretty much all of the issues though (since maneuverability is much more important on an air superiority fighter than it is on a strike aircraft) and the max rated load factor is available throughout the envelope.

 

Hey both renhanxue + Machalot!

Do you think this chart was worth putting into the RC2.1, including some of your explanations?
To be honest, I personally think it would perfectly fit into the chapter 6.
Following this, maybe someone out there is interested in pimping this chart a bit (straighten the axes and make the numbers better readable), since I am not the world number one specialist for graphics software... 

[MYSE1234]

25 minutes ago, TOViper said:

....

Following this, maybe someone out there is interested in pimping this chart a bit (straighten the axes and make the numbers better readable), since I am not the world number one specialist for graphics software... 

I'll see what I can do with the chart. 

[TOViper]

3ThumbsUp for you!

[Machalot]

Looking at that chart, I really wonder about some of those complex curves that wiggle back and forth, especially in the TIPPVAXEL and SERVOMOMENT regions. They must have either a ton of test data, or done a huge amount of analysis, to have enough confidence in all those little wiggles to put them in the manual. 

[MYSE1234]

@TOViper How does this look?
I decided to color code it instead of using patterns like in the original, as we have the technical means to do so these days... I made sure to make it fairly colorblindness friendly too. If that website is to be believed. 

[TOViper]

Oh damn, you are a genius! I looks awesome!
So finally, and if you agree, I will integrate this into the chapter 6.

Edited March 4, 2023 by TOViper

[MYSE1234]

5 minutes ago, TOViper said:

Oh damn, you are a genius! I looks awesome!
So finally, and if you agree, I will integrate this into the chapter 6.

 

Thank you 
I made it for it, so.. Feel free to add it your manual.

Edited March 4, 2023 by MYSE1234

[TOViper]

A quick question: Are you sure about that the AJS uses the same diagram? Not even minor changes/modifications in the nineties which we know about?

[MYSE1234]

Should be the same as far as I know.

edit: There are some differences but it's largely the same. I'm not sure if it's because the aircraft was actually changed (probably not I think), their data was refined or just that the chart was remade more correctly. The AJS chart is covering a much smaller regime than the AJ's one, so I'd say the one above is accurate enough.
It's unlikely the DCS AJS is that accurate anyway tbh 
 

Edited March 4, 2023 by MYSE1234

[TOViper]

Yes, I was thinking about the same. I now anticipate guys flying and testing if the module is like shown on the charts ... I fear it is not following it, is it?

Edited March 4, 2023 by TOViper

[renhanxue]

My chart is from the aerodynamics compendium, which reads like it's written by and for engineering nerds. I'd bet the flight manual version is cleaned up and simplified to avoid giving the pilots information overload. AJ and AJS should have identical flight performance for all practical purposes.

Very nice job on the cleanup by the way! I think an extended explanation would be a good idea for the manual with or without the chart, but who doesn't like charts?

Edited March 4, 2023 by renhanxue

[TOViper]

3 hours ago, MYSE1234 said:

Should be the same as far as I know.

edit: There are some differences but it's largely the same. I'm not sure if it's because the aircraft was actually changed (probably not I think), their data was refined or just that the chart was remade more correctly. The AJS chart is covering a much smaller regime than the AJ's one, so I'd say the one above is accurate enough.
It's unlikely the DCS AJS is that accurate anyway tbh 
 

 

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?

[renhanxue]

8 hours ago, Machalot said:

Looking at that chart, I really wonder about some of those complex curves that wiggle back and forth, especially in the TIPPVAXEL and SERVOMOMENT regions. They must have either a ton of test data, or done a huge amount of analysis, to have enough confidence in all those little wiggles to put them in the manual. 

It seems to me to be simultaneously too detailed and too smooth to be based on empirical test data, so I think it's mostly analytical in origin. I'd bet it was made by basically layering four separate charts (one for each reason) on top of each other, and that's why it looks so messy. Also, part of the odd squiggliness in the pitch gearing-limited region probably originates with the "series trim" function which is supposed to alleviate trim changes in the transonic region; it starts kicking in at M0.93.

Edited March 5, 2023 by renhanxue