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Old 12-07-2018, 11:57 AM   #31
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Quote:
Originally Posted by amalahama View Post
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.
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Old 12-07-2018, 12:23 PM   #32
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Originally Posted by HWasp View Post
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
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Old 12-07-2018, 01:20 PM   #33
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Quote:
Originally Posted by TOViper View Post
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!)
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Old 12-07-2018, 02:24 PM   #34
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"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.
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Old 12-07-2018, 03:23 PM   #35
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@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...
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Old 12-07-2018, 03:59 PM   #36
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Quote:
Originally Posted by Vitormouraa View Post
@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...

talking on a higher level is fun, isn't it
Anyway, we are thinking, approximating, believing, assuming, guessing ...
Nothing a technician likes to do
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.
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Last edited by TOViper; 12-07-2018 at 04:03 PM.
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Old 12-10-2018, 03:47 PM   #37
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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.
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Old 12-10-2018, 06:47 PM   #38
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Originally Posted by Goblin View Post
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 ...


What do you think of it?
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Old 12-10-2018, 08:38 PM   #39
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Quote:
Originally Posted by TOViper View Post
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
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