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[REPORTED]EGT behaviour on Start-Up


Gianlc
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Hi

 

I am happy to look into this if you have information that shows me it does not at startup.

 

Even a real world video showing the F-5E EGT at startup would help

 

thanks

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Hi

 

I am happy to look into this if you have information that shows me it does not at startup.

 

Even a real world video showing the F-5E EGT at startup would help

 

thanks

 

Would a video of a T-38 engine start suffice? The F-5 is a much less flown aircraft these days and it could be more difficult to find such a video, whereas the T-38 uses the same type of engine.

 

If not, there’s an F-5N pilot I follow on Instagram who I could ask to take a video of an engine start, but no idea if he’d be comfortable having one of his hands occupied during a critical ground op.

 

Besides that, I have time in 4 different turbine engine aircraft and can safely say EGT/ITT/TOT should never reach 800 during a normal start. Usually they peak anywhere between 550 and 650.


Edited by Chuck_Henry
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Sure we can take a look at the video, but we should never ask a pilot to compromise safety for a video.

 

If there are documents out there that state otherwise I am happy to look at them also, if you need to PM me.

 

thanks

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Besides that, I have time in 4 different turbine engine aircraft and can safely say EGT/ITT/TOT should never reach 800 during a normal start. Usually they peak anywhere between 550 and 650.

 

Why? Different manufacturers have different ITT probe placement relative to the burner can so comparing ITTs across makes or really even different models by the same manufacturer isn't particularly useful. Per 1F-5E-1 there is no reason to abort a start until EGT reaches 845 C, so no limitations are being exceeded. I'm not saying you are wrong that most J85s might start a little cooler, and I too would be very interested to see a startup video, but if you were going to squawk "our" F5 with maintenance, what would you tell them? It is performing within book parameters, so how could you really argue a change is needed even if video from another J85 shows a cooler start? Lots of factors are at play here and as you know, even the two engines on the same aircraft will have different indications during start, but as long as they stay within limitations it isn't a concern.

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Sure we can take a look at the video, but we should never ask a pilot to compromise safety for a video.

 

If there are documents out there that state otherwise I am happy to look at them also, if you need to PM me.

 

thanks

 

Here's what I have right now.

 

 

 

The first video, starting at 19:48, shows the engine start procedure for a real life T-38. At about 21:53, you see the EGT steadily rise, peak around 760 degrees Celsius for less than 1 second, and then immediately decrease and stabilize between 450 and 500 degrees.

 

The second video, starting at 1:23, shows the engine start procedure for the DCS F-5. After pressing the engine start button and advancing the throttle to idle, the EGT rapidly rises and holds at about 780 degrees Celsius for nearly 15 seconds before dropping to about 200 degrees and holding there.

 

First point: The DCS F-5's EGT technically falls within limits during engine start, but the rate of increase and time spent at the "peak" is extremely inaccurate compared to the real J85 engine, which it shares with the T-38.

 

Second point: Until now I hadn't paid much attention to the idle EGT indication, but 200 degrees is unrealistically low, especially with such a relatively warm start. Not only is 200 outside the green arc (normal continuous range) of 325-650 degrees (per the current F-5 NATOPS manual), but it is hundreds of degrees lower than the real life J85 engine idle EGT shown in the first video.

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The first video, ...

 

Cool video (not sarcasm, I actually really enjoy any of the old training films from the various services) but I have a question when I watch it. They only show instruments on super tight shots, with usually just one whole gauge visible. Also, they demonstrate several abnormal start conditions. Do you really believe they actually induced a hot start and dealt with it so expertly on the first try (because you won't get a second take if you don't) that they captured the needle at exactly 845 and it slammed right back down?

 

Because personally, if I were going to make such a video where I didn't need to show a wide shot of the whole process, it wasn't live, and modern CGI wasn't an option, I would just have maintenance pull the EGT harness apart at one of the cannon plugs back by the engines. Then they could hook up a test set and induce a voltage to show the readings I need to capture. That seems much safer than potentially burning up a perfectly good engine just for a training film. And once they had that done and the camera was already perfectly framing the EGT gauge, I would probably just capture footage of the other "starting abnormalities" and a "good start" as well. This would be much safer and has the benefit of not putting extra cycles on the engines, and it would be easy to dub over whatever sound was required. Of course I can't prove that was the method used to generate that footage any more than I or any other viewer can prove those were actual shots of an actual EGT gauge in a real T38 in real time. But regardless, as you said, the DCS start violates no limitations set forth in the manual.

 

Perhaps more importantly, at least on their website, the USAF says the T38 uses a -5 engine. Our DCS F5 uses a -21, so I'm not sure trying to match numbers to a T38 is a valid comparison.

 

Second point: Until now I hadn't paid much attention to the idle EGT indication, but 200 degrees is unrealistically low, especially with such a relatively warm start. Not only is 200 outside the green arc (normal continuous range) of 325-650 degrees (per the current F-5 NATOPS manual), but it is hundreds of degrees lower than the real life J85 engine idle EGT shown in the first video.

 

There is a difference in published engine limitations between the state "continuous" and the state "idle." By your logic, since the DCS J85 idles in the mid 50s, that is also incorrect. Should it idle at 80% as denoted by the bottom of the green arc? Because right now in DCS as I begin a smooth spool up for takeoff my EGT and N1 both enter the green arc at almost exactly the same times, which is perfectly logical.

 

This whole thread seems to lend some credence to the design philosophy of removing numbers entirely from engine instruments and just using colored arcs, since from the pilot's perspective the numbers are basically arbitrary anyway. This world has plenty jet engines that have higher temperature / longer duration EGT starting limits than the J85. There is nothing strange about it, it is just the difference between holding a thermometer 1 foot away from an acetylene blowtorch to holding it 5 feet away. The torch temp stays the same, you are just reading the temp at a different location.

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@tom_19d

 

Honestly, man, I have no idea how they produced those clips of the engine instruments during various normal and abnormal starts. I would like to think that if they had the ability to manipulate the gauges like that, they would make the rate of EGT, FF, RPM increase the way they would for real. Otherwise these training films would be somewhat misleading to the student pilots.

 

I know the F-5 uses a more powerful variant of the J85, but that shouldn't have anything to do with how the engine behaves to the point that the one (the real life T-38) would peak for less than a second and the another in the same family (the DCS F-5) would hold that peak for 15 seconds. You'll unfortunately have to take my word for it since I don't film while flying (or doing ground ops), but turbine engines just don't work like that (the latter). I base that on my former experience with the Allison 250, 2 variants of the PT6, and currently the Rolls-Royce T406.

 

As for the idle EGT indications, my point isn't so much about them being outside the green arc, but indicating strangely in relation to the peak EGT during the start. T-38 engine peaks at 760 and stabilizes at ~475 at idle, but the F-5 engine peaks, holds at 780, and stabilizes at 200 at idle? Sorry, but I don't buy that for a second.

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I know the F-5 uses a more powerful variant of the J85, but that shouldn't have anything to do with how the engine behaves to the point that the one (the real life T-38) would peak for less than a second and the another in the same family (the DCS F-5) would hold that peak for 15 seconds. You'll unfortunately have to take my word for it since I don't film while flying (or doing ground ops), but turbine engines just don't work like that (the latter). I base that on my former experience with the Allison 250, 2 variants of the PT6, and currently the Rolls-Royce T406.

 

The J85-GE-5 fitted to the T-38 used a 8 stage compressor, while the -21 is a 9 stage engine.

 

As for the idle EGT indications, my point isn't so much about them being outside the green arc, but indicating strangely in relation to the peak EGT during the start. T-38 engine peaks at 760 and stabilizes at ~475 at idle, but the F-5 engine peaks, holds at 780, and stabilizes at 200 at idle? Sorry, but I don't buy that for a second.

 

While what you say is likely true, the update from -5 to -21 improves/changes a number of other engine components, so unfortunately the T-38 training video can't be used as direct proof that DCS's F-5 EGT startup values or timing are incorrect.

 

There's more detail on the differences between the -5 and -21 in "J85 Rejuvenation Through Technology, p010433.pdf"

 

"DISTRIBUTION: Approved for public release, distribution unlimited"

 

https://apps.dtic.mil/dtic/tr/fulltext/u2/p010433.pdf

 

... but I haven't found a cockpit startup video for the -21

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The J85-GE-5 fitted to the T-38 used a 8 stage compressor, while the -21 is a 9 stage engine.

While what you say is likely true, the update from -5 to -21 improves/changes a number of other engine components, so unfortunately the T-38 training video can't be used as direct proof that DCS's F-5 EGT startup values or timing are incorrect.

Nevertheless an idle EGT of 200°C is very unrealistic for any jet engine.

 

As Chuck_Henry wrote (can be easily verified by various F-5 cockpit videos), the idle EGT should be well above 400°C.

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You'll unfortunately have to take my word for it since I don't film while flying (or doing ground ops), but turbine engines just don't work like that (the latter).

 

It‘s almost as if you think you are the only person on this forum who has ever started an engine. Regardless, I haven’t been disagreeing with you so much in principal as trying to point out that trying to get ED to change anything on this module that doesn’t violate manual limitations is awful tough. Getting them to do it without incontrovertible evidence is a lost cause. Good luck though (again, not sarcasm, more realism is always better in my book).

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It‘s almost as if you think you are the only person on this forum who has ever started an engine. Regardless, I haven’t been disagreeing with you so much in principal as trying to point out that trying to get ED to change anything on this module that doesn’t violate manual limitations is awful tough. Getting them to do it without incontrovertible evidence is a lost cause. Good luck though (again, not sarcasm, more realism is always better in my book).

 

Sorry, that probably came off more condescending than I meant to be.

 

In any case, you're right, but even the most current F-5N NATOPS manual doesn't really go into detail about what the engine start should look like in terms of the rate or magnitude of EGT increase (or any of the other gauges for that matter), other than the minimum and maximum limits.

 

Short of reaching out to that F-5N pilot I mentioned in my first reply to this thread, this is really the best I can do. Even if the T-38 and the F-5's engines are not the exact same, I'm hoping someone up at ED realizes that the F-5's shouldn't act that differently.

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Can you ask your F-5N friend to shoot this video and put it to rest?

Or at least give his comments (I doubt a RL pilot/operator comments would serve any purpose seeing how they have been routinely dismissed in the past...but worth a shot).

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Nevertheless an idle EGT of 200°C is very unrealistic for any jet engine.

 

As Chuck_Henry wrote (can be easily verified by various F-5 cockpit videos), the idle EGT should be well above 400°C.

 

I agree though I can't find the "various F-5 cockpit videos" you cite.

 

There is a 1961 J85-5 evaluation chart which shows EGT's near 215°C but it looks "off" (perhaps because it's taken at M0.9 @ 40,000 ft ?)

 

 

 

... compared to later 1973 test charts for the J85-13 which show an idle EGT of 400-415°C in a test cell.

 

 

 

IMHO given that 200°C is the minimum value the EGT gauges can display, the low DCS idle temp. is almost certainly a bug and perhaps related to the "[REPORTED]F-5 Engine Behavior" thread ?

 

https://forums.eagle.ru/showthread.php?t=260974


Edited by Ramsay

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Can you ask your F-5N friend to shoot this video and put it to rest?

Or at least give his comments (I doubt a RL pilot/operator comments would serve any purpose seeing how they have been routinely dismissed in the past...but worth a shot).

 

He’s not a friend; just a guy I follow on IG who happens to be relatively public with his in-flight photography. Nevertheless, I’ll send him a message and see what he says.

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That was faster than I thought. F-5N engine start, straight from a real-life source.

 

 

Being a real life video, it obviosly shows the RL behaviour. At least the Brazilian F-5EM behaves like that, usually doesn't get to 600C. That's also because the temperature shown on the instrument does not represent the actual EGT, each engine has it's own operating temperatures, but their maximum operating limit is set to 675C on the instrument.

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Hi all

 

I have reported this to the team for review.

 

If I find anything out I will share it with you, I have no timeline for this currently.

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