[FIXED] Mach 1.85 ...

[TOViper]

Hi folks,

 

i recently noticed that Mach 1.85 is the limit (I am on version 1.5.7.).

Some time before (dont know which version) it was Mach 2.0+.

 

IMHO the behaviour when reaching M1.85 is not very realistic, since it immediately and hardly stops there after reaching this value. Following maths the acceleration should become lower and lower when reaching the range of maximum Mach. The dynamic pressure should then more and more prevent futher acceleration.

 

What happend to the flight model?

 

Kind regards,

TOViper

Edited December 19, 2020 by RagnarDa

[mattebubben]

And this is flying clean? (no Weapons or External tank?)

[TOViper]

And this is flying clean? (no Weapons or External tank?)

 

Clean aircraft, no weapons, no tank, no brakes, nothing.

Rest please see attached cockpit and outside view:

 

The acceleration is decreasing before 1.85, but it is still fast at 1.84, but then comes to full stop.

The final Mach oscillates a little around 1.85.

[microvax]

Yeah clean it doesnt go beyond 1.85 anymore

[TOViper]

.... , it's a bit odd that it would manifest itself as some sort of wall though. .....

 

That was the point I initially went for! Not focusing on the final maximum speed, but focusing on the gradient before reaching it! :smilewink:

Any news on this somewhere?

[Eldur]

She accelerates well, but suddenly at ~1.84ish it feels like she's entering some super-dense atmosphere which instantly kills the acceleration. But still, she can climb insanely, just pull her up 30° and she'd go into orbit if the AB would die up at ~14km. After that, she still has thrust, but the AB is broken. The RM8B still eats fuel like nothing at Z1-3, but the flame and thrust won't come back until after a repair.

The AJS is not made for high alt jobs, but still it would be great to have realistic behaviour up there and if it's just for some high alt high speed ferry flights from a remote base to the place of action.

[Vitormouraa]

She accelerates well, but suddenly at ~1.84ish it feels like she's entering some super-dense atmosphere which instantly kills the acceleration. But still, she can climb insanely, just pull her up 30° and she'd go into orbit if the AB would die up at ~14km. After that, she still has thrust, but the AB is broken. The RM8B still eats fuel like nothing at Z1-3, but the flame and thrust won't come back until after a repair.

The AJS is not made for high alt jobs, but still it would be great to have realistic behaviour up there and if it's just for some high alt high speed ferry flights from a remote base to the place of action.

 

We have the A model and not the B actually. The issue with AB at high altitudes was fixed in the B model though.

[TOViper]

Thanks guys for your answers so far, and also the interesting infos on some topics! :thumbup:

 

In fact, my initial question hasn't been discussed in detail.

The flight model suddenly stops acceleration, so if I assume this point (under the conditions written in first post), the acceleration BEFORE reaching the Mach limit of 1.85 is too high.

 

Mathematically the Mach limit should be reached in infinity, but here we have a kind of "wall" (nice description with the super-dense athmosphere Eldur! :) ) limiting the speed abrubtly.

I would love to have that a little more realisticly modelled, that' all :smilewink:

 

But still, she can climb insanely, just pull her up 30° and she'd go into orbit if the AB would die up at ~14km.

If the RM8A feels high dynamic pressure it starts smiling and works like a booster rocket. :P

Following this it might not only be an option to escape low from a mission, but also high.

 

 

Would be interesting to compare the acceleration of the MiG-21 and the AJS-37 in the following situation:

.) Clean, fuel at 100%, ISA.

.) Start on ground at 0 km/h and 0 m MSL (airbase near the sea would be good)

.) Accelerate to M0.9 flying low

.) Climb at an angle of 25 to 35°.

.) Reach 12000m MSL

Who would reach 12km first?

Since I don't have the MiG-21, I only can try it out with my Viggen and post the time here later on ...

It is not only interesting to climb fast, but also to FLY fast when reaching the altitude:)

 

Viggen_Climb_Test.miz

Edited April 7, 2018 by TOViper

[TOViper]

Would be interesting to compare the acceleration of the MiG-21 and the AJS-37 in the following situation:

.) Clean, fuel at 100%, ISA.

.) Start on ground at 0 km/h and 0 m MSL (airbase near the sea would be good)

.) Accelerate to M0.9 flying low

.) Climb at an angle of 25°.

.) Reach 12000m MSL

Who would reach 12km first?

Since I don't have the MiG-21, I only can try it out with my Viggen and post the time here later on ...

 

My first time: 2m06s @672 km/h IAS

Edited April 7, 2018 by TOViper

[Vitormouraa]

Mach 1.85 ...

 

TOViper this is a known bug. It was fixed sometime ago iirc, but its back.

 

So I don't know why we should discuss this knowing it's a bug and it's not supposed to be like that.

 

But the RM8A produces around 164kN at Mach 1.1 sea level. That's why it climbs like a rocket

[TOViper]

TOViper this is a known bug. It was fixed sometime ago iirc, but its back. So I don't know why we should discuss this knowing it's a bug and it's not supposed to be like that.

 

I didn't know that, but If you say so that is OK for me. I just wanted to bring this on the table again after a long period of reading nothing about it, since - IMHO - this issue is worth not to be forgotten. :music_whistling:

 

But the RM8A produces around 164kN at Mach 1.1 sea level. That's why it climbs like a rocket

 

True, and I really love it. :joystick:

Another reason for falling in love with this sweet aircraft. :love:

[Akula67]

I agree, the speed at high altitude is one major advantage the Viggen has over the Harrier, so I guess it is worth fixing !

 

I am fully aware the fixing bugs takes time but it would be nice if HB could acknowledge that the issue is being worked on ;)

 

Otherwise it is a wonderful aircraft, very different from other CAS airplanes :pilotfly:

[Eldur]

Just thought I'd give it a try again, but still crashing into that M 1.85 barrier

[Goblin]

It's a Mach 2 aircraft... it must be able to do that in the sim...

 

Although no Viggen pilot I ever talked to, ever reached Mach 2...

The fastest recorded speed was 1.97, I believe.

But hitting a «wall» @1,85 doesn’t sound right.

[renhanxue]

I'm not so sure it's unrealistic. These graphs are for a clean JA 37, which actually has a more powerful engine:

 

 

 

 

 

 

As you can see, the real aircraft does hit a "wall" around Mach 1.8 at 12 km MSL. Mach 2.0 in level flight is only achievable on a cold day, and only at a narrow range of altitudes around 11 km MSL.

Edited July 15, 2018 by renhanxue

[Goblin]

the real aircraft does hit a "wall" around Mach 1.8

 

It sure does! :huh:

Good find. :thumbup:

[Cobra847]

We did some testing on this and our findings match renhanxue. (We use automated testing versus tables to verify).

 

At some point there was a bug of overspeeding in the Viggen due to incorrect engine parameters. That was fixed without putting in artificial speed "walls".

[renhanxue]

It sure does! :huh:

Good find. :thumbup:

I have no idea why it behaves like that. It looks really weird in the graphs. Maybe it's the intakes that are the limiting factor?

[TLTeo]

We did some testing on this and our findings match renhanxue. (We use automated testing versus tables to verify).

 

At some point there was a bug of overspeeding in the Viggen due to incorrect engine parameters. That was fixed without putting in artificial speed "walls".

 

 

Adding on to this, predicting the way an object moves at supersonic speed is really really complicated because you would need to actually solve the equations of hydrodynamics, which in the supersonic regime become even more messy and complicated., and all sorts of weird stuff happens (like, indeed, objects hitting "speed walls").

 

 

Claiming that something is a bug "because it clearly makes no sense" is incredibly over-simplistic and mostly shows a deep misunderstanding of hydrodynamics.

[Goblin]

Very likely a propulsion issue, maybe because of inlet choking, considering the performance increase with colder temperatures.

[theOden]

While showing graphs on internet is cool and all that trying this ingame you'll notice this "wall" people talk about comes after a 24 second period going from mach 1.6 to 1.85 (check that in the graph btw).

 

 

Also this "rubberband" effect when speed almost reach 1.86 and jumps back to 1.85 to stay there could most surely add to folks preception of a wall.

 

 

Good job there adding engine parameters to make this happen, I can only imagine what had to be done to manage that.

[Vitormouraa]

I do remember Nick saying something like "This seems to be a bug, we are investigating it" a long time ago.

 

So I assumed it was, in fact, a bug. Looks like I was wrong? Anyway, I'll take down what I said about it.

 

But I'm glad to see that they were able to implement this feature without using scripts (from my understanding).

 

So good job! Now all we have to do is found out why that happens. :D

[TOViper]

I'm not so sure it's unrealistic. These graphs are for a clean JA 37, which actually has a more powerful engine:

As you can see, the real aircraft does hit a "wall" around Mach 1.8 at 12 km MSL. Mach 2.0 in level flight is only achievable on a cold day, and only at a narrow range of altitudes around 11 km MSL.

 

I agree that there is a limit (this was clear to me, but I didn't knew about the M1.85, always thought it was M2), and obviously it is definietely somewhat of M1.8 to M1.9, depending on conditions.

 

What I also wanted to point out in the whole discussion was at what rate the maximum Machnumer is reached. The rate should get lower and lower the nearer to the maximum Machnumber. I didn't recognize this in the flights before.

 

But today I really have some time (waiting for THE update ...;)) to test this in detail. After flying a few minutes ago, I have to admit that:

- The rate really slows down a bit when reaching somewhat of M1.75

- The rate it is still high at M1.84

- When reaching the M1.85 there is a "bump" and the aircraft is somehow bungie-chord-decelerated-accelerated around M1.85 for a few times.

 

 

Here is the video:

[Eldur]

As you can see, the real aircraft does hit a "wall" around Mach 1.8 at 12 km MSL. Mach 2.0 in level flight is only achievable on a cold day, and only at a narrow range of altitudes around 11 km MSL.

 

It would definately not hit a "wall" as described in this very thread. Yes, the top speed is OK. But it accelerates like a SR-71 on steroids even from 1.8 to 1.85 and then suddenly the acceleration is gone completely. No real airplane would do that, 'cause physics. It should gradually accelerate until it won't do so anymore roughly at ~ 1.85 - 1.86 in ISA conditions. So the real problem is just in acceleration, it should decrease and going from 1.8 to 1.85 shouldn't last just like half a second, but rather a minute or even minutes.

[amalahama]

It would definately not hit a "wall" as described in this very thread. Yes, the top speed is OK. But it accelerates like a SR-71 on steroids even from 1.8 to 1.85 and then suddenly the acceleration is gone completely. No real airplane would do that, 'cause physics. It should gradually accelerate until it won't do so anymore roughly at ~ 1.85 - 1.86 in ISA conditions. So the real problem is just in acceleration, it should decrease and going from 1.8 to 1.85 shouldn't last just like half a second, but rather a minute or even minutes.

 

If the aircraft had complex adaptative inlets to manage inbound shock waves then yes, but Viggen has fixed ones that can choke, so it might be possible that the Viggen experiments such a wall IRL

 

Regards

Edited July 19, 2018 by amalahama