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Old 04-16-2018, 07:57 PM   #1
Catah
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Default Question about engine sound

After listening to the startups on Wags' livestreams, I was wondering if anyone knew if ED made the engine "groan" sound variable during startup. By that I mean, does the sound length and frequency change based on conditions like temperature, humidity, or maybe anything else that could effect the amount of time it takes to get the engine cranked up to idle?

Just asking out of curiosity and because I have heard in videos of F/A-18s starting up that the "groaning" from the F404s is sometimes longer and sometimes shorter.
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Old 04-17-2018, 01:30 PM   #2
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Without getting too crazy complicated.... The groan happens when the compressor blades near the stall condition. You'll notice that it occurs after fuel is introduced, as the compressor starts to spin up rapidly, the little airfoils are moving faster in relation to the flow of air, and this can lead to a stall. There are variable guide vanes that re-direct the flow of air to prevent compressor stall automatically. Early versions of turbo-jet engines did not have these and were much more prone to compressor stalls, especially when airflow as disturbed, or when really hammering the throttle.

Anyway, what you're hearing is the disturbance of the airflow over compressor blades... while the engine is redirecting the flow of air to avoid a compressor stall as the compressor accellerates rapidly during startup.

Check out AgentJayZ on youtube. He's a jet engine mechanic, works on a lot of J-79's, and has really great videos about every part of jet engines. https://www.youtube.com/user/AgentJayZ

Last edited by Banzaiib; 04-17-2018 at 01:35 PM.
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Old 04-17-2018, 05:57 PM   #3
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As Banzaiib said its the compressor close to stall, sometimes when the throttles are rigged a little low on the ground idle it will stay like it , most pilots will advance the throttle a bit to make it go away because its loud as hell and very annoying, new pilots don't always do this. I don't know if you seen this video https://youtu.be/JCrnRLV5slc but I have been meaning to make a version of this with the hornet , bit of a niche market though.
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Old 04-17-2018, 06:59 PM   #4
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Thanks for the info guys, glad to learn the cause of this sound. Now I'm just wondering if it will be modeled into our Hornet module that the sound becomes prolonged when the throttles aren't up all the way into idle.

Quote:
Originally Posted by aceofspades9963 View Post
... I don't know if you seen this video https://youtu.be/JCrnRLV5slc but I have been meaning to make a version of this with the hornet , bit of a niche market though.
No, I haven't seen it before but that would be pretty funny haha.

EDIT: Now that I think about it, I guess that wouldn't be modeled in because the throttle going from cutoff to idle is a key bind so there's no in between the two states.
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Old 04-21-2018, 04:00 PM   #5
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Quote:
Originally Posted by aceofspades9963 View Post
As Banzaiib said its the compressor close to stall, sometimes when the throttles are rigged a little low on the ground idle it will stay like it , most pilots will advance the throttle a bit to make it go away because its loud as hell and very annoying, new pilots don't always do this. I don't know if you seen this video https://youtu.be/JCrnRLV5slc but I have been meaning to make a version of this with the hornet , bit of a niche market though.
I was leery of touching the throttles during startup because I didn't want to increase the chance of an engine fire.

Interesting, I never heard of this throttle trick to shorten the startup noise.

I also assumed that during this phase of the startup the annoying sound was influenced by the sudden change of air temps during ignition.

In any case the sound was very unflattering of the jet.

ace I have question for you, assuming you are/were a pilot and not aircrew: Do you remember when the apu shuts off after first engine startup? My memory is fading, so far the sim videos shows the apu staying on during both engine starts and I don't remember this.
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Old 04-21-2018, 04:08 PM   #6
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Quote:
Originally Posted by Catah View Post
After listening to the startups on Wags' livestreams, I was wondering if anyone knew if ED made the engine "groan" sound variable during startup. By that I mean, does the sound length and frequency change based on conditions like temperature, humidity, or maybe anything else that could effect the amount of time it takes to get the engine cranked up to idle?

Just asking out of curiosity and because I have heard in videos of F/A-18s starting up that the "groaning" from the F404s is sometimes longer and sometimes shorter.
In the hundreds of startups I did in the hornet I never noticed a major change of duration during startups. Sure there could have been a couple seconds variance between aircraft but I assume it was due to the slight variances of the engine builds since I flew lot 10 to lot 20s.

The variances could also be due to how the engines were tuned by the groundcrew.

I seriously doubt the air properties would change anything drastically during startup to be noticeable.
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Old 04-21-2018, 07:19 PM   #7
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Quote:
Originally Posted by neofightr View Post
I was leery of touching the throttles during startup because I didn't want to increase the chance of an engine fire.

Interesting, I never heard of this throttle trick to shorten the startup noise.

I also assumed that during this phase of the startup the annoying sound was influenced by the sudden change of air temps during ignition.

In any case the sound was very unflattering of the jet.

ace I have question for you, assuming you are/were a pilot and not aircrew: Do you remember when the apu shuts off after first engine startup? My memory is fading, so far the sim videos shows the apu staying on during both engine starts and I don't remember this.
After both engines are running should shut down close to a minute.
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Old 04-22-2018, 12:45 AM   #8
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odd, I don't remember doing this for startups. I manually (I think it was auto) turned off the apu after the first engine start and let the 1st engine startup the second one with the crossfeed procedures. This was to reduce wear on the apu.

It was standard practice in my day not sure if they changed this after my flying days.
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Old 04-22-2018, 02:03 AM   #9
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Quote:
Originally Posted by neofightr View Post
odd, I don't remember doing this for startups. I manually (I think it was auto) turned off the apu after the first engine start and let the 1st engine startup the second one with the crossfeed procedures. This was to reduce wear on the apu.

It was standard practice in my day not sure if they changed this after my flying days.

Can there be squadron specific instructions? or are checklists updated for everyone generally across the squadrons aircraft type etc?


If APU or crossbleed start -

b. Bleed air knob - CYCLE
THRU OFF TO NORM
The bleed air shutoff valves
close during the fire warning
test and the bleed air knob
must be cycled thru OFF to
NORM with ac power on to
reset the valves.
9. Warning and caution lights -TEST
For a crossbleed start insure APU switch is OFF and a minimum of 80% rpm and 1,900 pph fuel flow.
10. Engine crank switch - L
11. Left throttle - IDLE (15%RPM minimum)
12. Engine crank switch - CHECK OFF

.
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Last edited by David OC; 04-22-2018 at 03:39 AM.
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Old 04-22-2018, 03:50 PM   #10
neofightr
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Quote:
Originally Posted by David OC View Post
Can there be squadron specific instructions? or are checklists updated for everyone generally across the squadrons aircraft type etc?


If APU or crossbleed start -

b. Bleed air knob - CYCLE
THRU OFF TO NORM
The bleed air shutoff valves
close during the fire warning
test and the bleed air knob
must be cycled thru OFF to
NORM with ac power on to
reset the valves.
9. Warning and caution lights -TEST
For a crossbleed start insure APU switch is OFF and a minimum of 80% rpm and 1,900 pph fuel flow.
10. Engine crank switch - L
11. Left throttle - IDLE (15%RPM minimum)
12. Engine crank switch - CHECK OFF

.
Yea this was the standard across the fleet. I also remember the APU turning off automatically 20 seconds or so after the first engine was online. All I did was verify it was off before initiating the crossbleed start. I don't remember a 80 percent minimum requirement.
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