[FIXED] Mach 1.85 ...

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.

As I posted above, the physics of supersonic flight are too complicated to just use "cause physics" as a justification for whether something behaves correctly or not.

Now I cannot fly faster than M1.715 :(

Now I cannot fly faster than M1.715 :(

Try colder temperatures?

Try colder temperatures?

I was on ISA. Try it out, the "wall" now occours on M1.715, not on 1.85 as before.

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

That is quite possible, but my problem is, that if the engine inlet parameters would change as rapidly as the thrust change suggests, then there would be an engine surge or stall. Or the very least there would be a visible fluctuation of engine parameters. I've seen none of those.

That is quite possible, but my problem is, that if the engine inlet parameters would change as rapidly as the thrust change suggests, then there would be an engine surge or stall. Or the very least there would be a visible fluctuation of engine parameters. I've seen none of those.

true

I was on ISA. Try it out, the "wall" now occours on M1.715, not on 1.85 as before.

I did some tests today, OATC was -3.1, and yep, maximum Mach number I was able to reach was 1.72, regardless of the altitude. It's quite interesting how the aircraft accelerates extremely quickly but all of the sudden stops at M 1.72.

But I agree on the statement above, something is choking the engine.

(It's been a few months since I flew the Viggen, so much fun flying it!)

"Chocked" is a term used in compressible aerodynamics when talking about convergent ducts in the exhaust. Chocked names the effect that supersonic speed is reached, but whatever we do before the narrowest diameter (e.g. increase p, T, mass flow) there is no acceleration beyond M1 possible, unless we attach a divergent duct (Laval) to accelerate the air further beyond M1.

IMHO the inlet might be the limiting factor. According to my thinking about programming (I might be wrong), the code could take the inlet geometry into considerations. The inlet appears as a normal Pitot super sonic inlet, thus the re-compression after the normal shock is not as efficient than it would be when using ramps and braking it down stepwise. Also, the shock might move at different speeds, leading to complex airflows along the inlet lip and the duct behind the lips.

What happened in the recent one is unknown to me, maybe Cobra has some news on this issue to bring light into darkness.

@TOViper

Thanks, I am aware of its meaning in the fluid dynamics. I was using the 'choke' as a typical/general term, not the one explained by the physics. Choke in the sense of "choke, stifle, asphyxiate, suffocate...", i.e; something is preventing the engine from doing its natural work of propelling the aircraft forward at Y or X speeds.

Perhaps I shouldn't have used that word in this discussion unless I was using its technical meaning... :D

@TOViper

 

Thanks, I am aware of its meaning in the fluid dynamics. I was using the 'choke' as a typical/general term, not the one explained by the physics. Choke in the sense of "choke, stifle, asphyxiate, suffocate...", i.e; something is preventing the engine from doing its natural work of propelling the aircraft forward at Y or X speeds.

 

Perhaps I shouldn't have used that word in this discussion unless I was using its technical meaning... :D

talking on a higher level is fun, isn't it :)

Anyway, we are thinking, approximating, believing, assuming, guessing ...

Nothing a technician likes to do :smartass:

Mathematically speaking, we are approaching M0.0.

Not tomorrow, not the day after tomorrow ... but ... who knows how many updates will follow.

The gradient is negative ... what will happen when we arrive at M-0.8? Ok, that one was really bad ... ;)

We should go flying instead of this, and Cobra is invited to polish our inlet ducts. :P

Looks like there’s some mixing of ”choked” and ”shocked” flow here...

https://en.m.wikipedia.org/wiki/Choked_flow

A shock wave can choke a duct, since supersonic flow causes a shockwave, where local airflow is reduced to subsonic speed, thus choking the duct.

Looks like there’s some mixing of ”choked” and ”shocked” flow here...

 

https://en.m.wikipedia.org/wiki/Choked_flow

 

A shock wave can choke a duct, since supersonic flow causes a shockwave, where local airflow is reduced to subsonic speed, thus choking the duct.

Yeah, that was my understanding.

I think chocolate is something that we should consider "using" when reading the books about chocked nozzles due to their normal shocks ... :D

What do you think of it?

What do you think of it?

That you are contributing to the confusion by spelling ”shocked” as ”chocked”. :)

I hope you don’t choke on my reply ;)

I was testing this out again and I noticed the Viggen doesn't have RPM lockup, this isn't simulated apparently, any idea how this works in the RL Viggen?

Kinda relevant to the topic. I measured the acceleration of various aircraft in DCS at 40'000 ft from Mach 1 to Mach 1.5, clean, 60% fuel:

[b]Viggen: 26 seconds[/b]
M2000: 55 seconds
 F-15: 67 seconds
MiG-29: 72 seconds
Hornet: 93 secods
Su-27: 96 seconds
MiG-21: 108 seconds

Low altitude performance seems believable, and Viggen is one of the slowest of the bunch at sea level, but at high altitude it's way off.

A bit too fast... :)

A little derail i know, but try dogfighting an AI viggen. most surprising. He will fly circles around any A/C.

Yes, high speed-high altitude performances are weird. HB should have a look at it

Kinda relevant to the topic. I measured the acceleration of various aircraft in DCS at 40'000 ft from Mach 1 to Mach 1.5, clean, 60% fuel:

 

[b]Viggen: 26 seconds[/b]
M2000: 55 seconds
 F-15: 67 seconds
MiG-29: 72 seconds
Hornet: 93 secods
Su-27: 96 seconds
MiG-21: 108 seconds

Low altitude performance seems believable, and Viggen is one of the slowest of the bunch at sea level, but at high altitude it's way off.

:shocking:

There's definitely something at odds!

Kinda relevant to the topic. I measured the acceleration of various aircraft in DCS at 40'000 ft from Mach 1 to Mach 1.5, clean, 60% fuel:

 

[b]Viggen: 26 seconds[/b]
M2000: 55 seconds
 F-15: 67 seconds
MiG-29: 72 seconds
Hornet: 93 secods
Su-27: 96 seconds
MiG-21: 108 seconds

 

Low altitude performance seems believable, and Viggen is one of the slowest of the bunch at sea level, but at high altitude it's way off.

There is data for 20k feet for the Viggen and a clean ac will accelerate from 1.0 to 1.5 in approximately 80sec.

The Viggen can go up to mach 1.34-1.35 when clean down on the deck in stage 3 burner. It wasnt really designed for high altitudes and going high mach's (at least the AJS variant).

Kinda relevant to the topic. I measured the acceleration of various aircraft in DCS at 40'000 ft from Mach 1 to Mach 1.5, clean, 60% fuel:

 

[b]Viggen: 26 seconds[/b]
M2000: 55 seconds
 F-15: 67 seconds
MiG-29: 72 seconds
Hornet: 93 secods
Su-27: 96 seconds
MiG-21: 108 seconds

 

Low altitude performance seems believable, and Viggen is one of the slowest of the bunch at sea level, but at high altitude it's way off.

F-14 takes 68,4 seconds by the way. :) Looks like HB got it right this time.

So, where are we at with this issue? How does this

Kinda relevant to the topic. I measured the acceleration of various aircraft in DCS at 40'000 ft from Mach 1 to Mach 1.5, clean, 60% fuel:

 

[b]Viggen: 26 seconds[/b]
M2000: 55 seconds
 F-15: 67 seconds
MiG-29: 72 seconds
Hornet: 93 secods
Su-27: 96 seconds
MiG-21: 108 seconds

Low altitude performance seems believable, and Viggen is one of the slowest of the bunch at sea level, but at high altitude it's way off.

fit to this:

Heatblur? Anyone? :dunno:

Also, why does the Viggen's acceleration gets faster with increasing ambient temperature?

Put together some numbers, after testing I think the F14-B might the fastest Jet in DCS on the deck. Please feel free to double check and correct.

 

Alt under 1kft

clean air frames

speed taken from F10 map

(knots ground speed)

 

 

Temps @ -12c/10F below,

 

F14B- 956

Viggen (rb24 glitch)- 924

F18C- 885

SU27- 851

Viggen- 830

F16C- 829

F15C- 827

MIG29A- 798

M2000C- 751

AV8- 560

SU25T- 545

A10C- 360

TFP51-318

UH1- 124

 

 

Temps @ 15c/59f below,

 

Viggen (rb24 glitch)-970

F14B- 895

F16C- 878

Viggen- 866

MIG29A- 825

F15C- 822

SU27- 813

M2000C- 797

F18C- 780

AV8- 560

SU25T- 558

A10C- 350

TFP51= 326

UH1 =131

 

 

Temps @ 50c/122f below,

 

Viggen (rb24 glitch)- 1017

Viggen- 905

F-16C- 863

M2000C- 853

F15C- 815

F14B- 813

SU27- 752

MIG29A- 718

F18C- 706

F86F- 622

AV8- 593

SU25T- 574

A10C- 317

TFP51- 309

UH1- 118

Clean, stage 3 A/B 46000ft straight and level, I can only get to M1.71

2-3months ago I was doing M1.81

What happened?

RagnarDa:

With the last updates this issue got fixed, thus I suggest to mark the thread as [FIXED].