Sponsor Message:
Aviation Technical / Operations Forum
My Starred Topics | Profile | New Topic | Forum Index | Help | Search 
Engine Start Up Noise  
User currently offlineSul From , joined Dec 1969, posts, RR:
Posted (9 years 2 months 3 weeks 4 days 7 hours ago) and read 3983 times:

Hello all,

Ive noticed that when some older jet engines are being started (specifically PIA 747-300s!), they rev up with a high pitched noise and then a deep drone starts to happen. Its also apparent at the point where a bit of white smoke is exhausted. There also seems to be hot air coming out the back from the moment the front fan spins. I was under the impression that fuel only goes in once the fan is spinning fast enough, so where does the heat come from?

So my question is what is the deep drone?

and

What is the white smoke?



Cheers.

12 replies: All unread, jump to last
 
User currently offlineUSAFMXOfficer From United States of America, joined Nov 2004, 174 posts, RR: 0
Reply 1, posted (9 years 2 months 3 weeks 4 days 6 hours ago) and read 3882 times:

Quoting Sul (Thread starter):
they rev up with a high pitched noise

I believe this would be the starter just beginning to come up to speed

Quoting Sul (Thread starter):
a deep drone starts to happen

This would be the inertia to overcome rotating the core of the engine from a standing start.

Quoting Sul (Thread starter):
a bit of white smoke is exhausted.

Just a guess but I think this would be initial ignition burning off some residual fuel....

Quoting Sul (Thread starter):
I was under the impression that fuel only goes in once the fan is spinning fast enough

Once the engine is rotating at a certain RPM (15% on the old TF33's I used to be engine run qualified on) the ignition sequence starts, so that is when the "hot air" you see coming out. We used to just "motor" an engine without ignition by pulling the ignition circuit breakers and using the starter to motor it---could leak check certain components that way.



44th Fighter Squadron Vampire Bats - 63 years of history
User currently offlineAir2gxs From , joined Dec 1969, posts, RR:
Reply 2, posted (9 years 2 months 3 weeks 4 days 6 hours ago) and read 3830 times:

Quoting Sul (Thread starter):
There also seems to be hot air coming out the back from the moment the front fan spins. I was under the impression that fuel only goes in once the fan is spinning fast enough, so where does the heat come from?

How long has the engine been shut-down? What is the ambient temp? Engines do not cool down uniformly. Ambient conditions such as temperature, sunlight and wind factor into how fast the core assumes ambient temperature. You will sometimes see heat distortion on start-up because the core engine has not reached ambient yet. Fuel isn't introduced until N2 (N3 on RR) reaches the appropiate speed, usually between 20% & 25% but max motor is common. Fan speed is not factored into ignition on a normal start.

Quoting Sul (Thread starter):
Its also apparent at the point where a bit of white smoke is exhausted.

If you are hearing the drone when the "smoke" appears, you are hearing the start of the combustion process. It's kind of hard to explain, but different types of fuel nozzles and/or injectors produce a different sound when the fuel is accelerated and ignited. The difference is pronounced if you ever listen to a CFM equipped DC8. Engine with fuel nozzles are quiet when they start. Engines with injectors have a deep rumble that you can feel when they start. (Or the opposite, I could never keep it straight.) As the engine increases in speed and the airflow through and around the core increases the sound disappears.

There is debate as to what the white smoke is, some say water vapor, but it is fuel that has not been involved in the combustion process. The fuel is not ignited all at once. There are only 2 ignitors in the combustion chamber. Some engines/airframes only use one at a time. The combustion at the point of ignition is instantaneous and complete (hence no black smoke), but the flame takes a few seconds to propagate around the combustor. The white smoke you see is unburned fuel from that process. I'd wager that the more heat distortion you see out the exhaust at starter rotation, the less fuel vapor you see at light-up. A warm core engine facilitates propagation. Cold weather starts are notorious for fuel vapor out the exhaust.


The high pitched noise is the starter.


User currently offlineSulman From United States of America, joined Mar 2004, 2035 posts, RR: 32
Reply 3, posted (9 years 2 months 3 weeks 4 days 5 hours ago) and read 3805 times:

This is particularly distinctive on CFM56 engines on the 733 and A320, not just from startup but also when power is set from idle - for a brief moment you do get that raspy groaning sound. I think it's great!


It takes a big man to admit they are wrong, and I am not a big man.
User currently offlineAirplanepics From United Kingdom, joined Jun 2003, 2730 posts, RR: 41
Reply 4, posted (9 years 2 months 3 weeks 4 days 5 hours ago) and read 3773 times:

Quoting Sulman (Reply 3):
This is particularly distinctive on CFM56 engines on the 733 and A320, not just from startup but also when power is set from idle - for a brief moment you do get that raspy groaning sound. I think it's great!

When you get right next to them, the earth shakes and you get a weird feeling in your chest. I agree. a fantastic experiance!  Silly



Simon - London-Aviation.com
User currently offlineTinPusher007 From United States of America, joined Feb 2005, 970 posts, RR: 1
Reply 5, posted (9 years 2 months 3 weeks 4 days 2 hours ago) and read 3564 times:

On the MD-88's that I work around, the JT8D's produce an almost muffled explosion sound right at "light off". If inside the terminal, it actually does sound as if something exploded outside....just the engines getting ready to handle business. I love that sound.


"Flying isn't inherently dangerous...but very unforgiving of carelessness, incapacity or neglect."
User currently offlineGoldenshield From United States of America, joined Jan 2001, 5970 posts, RR: 14
Reply 6, posted (9 years 2 months 3 weeks 4 days 2 hours ago) and read 3558 times:

Tin --

Turboprops sound the same way from the inside, except that once the prop gets up to speed, it tends to drown out the exhuast.



Two all beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun.
User currently offlineCCA From Hong Kong, joined Oct 2002, 827 posts, RR: 14
Reply 7, posted (9 years 2 months 3 weeks 3 days 20 hours ago) and read 3489 times:

Quoting Air2gxs (Reply 2):
The white smoke you see is unburned fuel from that process. I'd wager that the more heat distortion you see out the exhaust at starter rotation, the less fuel vapor you see at light-up. A warm core engine facilitates propagation. Cold weather starts are notorious for fuel vapor out the exhaust.


View Large View Medium
Click here for bigger photo!

Photo © Shotaro Shimizu
View Large View Medium
Click here for bigger photo!

Photo © Sven De Bevere



View Large View Medium
Click here for bigger photo!

Photo © Zhang Yun Zhe
View Large View Medium
Click here for bigger photo!

Photo © Jussi Kettunen



I couldn't agree with Air2gxs more, just to add that deep drone is also very particular to the Rolls Royce RB211-524 which are on the ex CX 743s of PIA.

I must say it's a great sound.

Rgds CCA



C152 G115 TB10 CAP10 SR-22 Be76 PA-34 NDN-1T C500 A330-300 A340-300 -600 B747-200F -200SF -400 -400F -400BCF -400ERF -8F
User currently offlineTwinCommander From United States of America, joined Apr 2005, 155 posts, RR: 0
Reply 8, posted (9 years 2 months 3 weeks 2 days 21 hours ago) and read 3349 times:

Quoting Goldenshield (Reply 6):
Turboprops sound the same way from the inside, except that once the prop gets up to speed, it tends to drown out the exhuast.

that depends on what turboprop engine your running.


Garret and Pratts dont have any growl to them on start up, all you hear is the ignitors clicking and the low "pop" when combustion begins. Any other noise is the intake with the incoming airflow, or on a garret, the internal gears whining.


User currently offlineIFIXCF6 From United States of America, joined Sep 2004, 108 posts, RR: 0
Reply 9, posted (9 years 2 months 3 weeks 2 days 18 hours ago) and read 3312 times:

Try an L-1011 on a cold day for hum and smoke!!!

Mike


User currently offlineDAirbus From United States of America, joined Nov 2003, 593 posts, RR: 2
Reply 10, posted (9 years 2 months 3 weeks 2 days 17 hours ago) and read 3300 times:
Support Airliners.net - become a First Class Member!

Quoting TinPusher007 (Reply 5):
On the MD-88's that I work around, the JT8D's produce an almost muffled explosion sound right at "light off". If inside the terminal, it actually does sound as if something exploded outside....just the engines getting ready to handle business. I love that sound.

I love that sound on the MD88's as well. I also love the fact that you can hear the engine spooling up to a certain rpm and then seems to just hang there for 3-4 seconds before your hear the "poof" of ignition and the engine starts accelerating again.



"I love mankind. It's people I can't stand." - Charles Shultz
User currently offlineHAWK21M From India, joined Jan 2001, 31667 posts, RR: 56
Reply 11, posted (9 years 2 months 3 weeks 2 days 14 hours ago) and read 3279 times:

Quoting TinPusher007 (Reply 5):
the JT8D's produce an almost muffled explosion sound right at "light off". If inside the terminal, it actually does sound as if something exploded outside....just the engines getting ready to handle business.

The B732 driven JT8Ds also have that particular interesting Small Explosion at light up,followed by the Acceleration to self sustaining speed.Sounds good.
regds
MEL



Think of the brighter side!
User currently offlineC133 From United States of America, joined Jan 2005, 225 posts, RR: 0
Reply 12, posted (9 years 2 months 3 weeks 2 days 7 hours ago) and read 3245 times:
Support Airliners.net - become a First Class Member!

Quoting IFIXCF6 (Reply 9):
Try an L-1011 on a cold day for hum and smoke!!!

Yes! A frosty cold RB211 smokes like crazy for a minute or so, and they really have a hum to them at initial idle all the time. It's just the nature of the beast.



Fine: Tax for doing wrong. Tax: Fine for doing well.
Top Of Page
Forum Index

Reply To This Topic Engine Start Up Noise
Username:
No username? Sign up now!
Password: 


Forgot Password? Be reminded.
Remember me on this computer (uses cookies)
  • Tech/Ops related posts only!
  • Not Tech/Ops related? Use the other forums
  • No adverts of any kind. This includes web pages.
  • No hostile language or criticizing of others.
  • Do not post copyright protected material.
  • Use relevant and describing topics.
  • Check if your post already been discussed.
  • Check your spelling!
  • DETAILED RULES
Add Images Add SmiliesPosting Help

Please check your spelling (press "Check Spelling" above)


Similar topics:More similar topics...
Engine Start-up Durring De-icing! posted Mon Mar 6 2006 03:27:13 by CYEGsTankers
WHOOP-WHOOP Sound On Engine Start UP posted Tue Nov 22 2005 14:53:56 by THVGJP
Why Different Engine Start-up Sequences? posted Thu Jun 17 2004 18:00:44 by Thrust
747 Correct Engine Start-Up Order posted Sat Mar 13 2004 00:43:00 by Cmckeithen
AE EMB 135/140/145 Engine Start Up posted Tue Dec 30 2003 16:46:50 by Flyingbronco05
Wind And A Jet Engine Start Up? posted Wed Apr 2 2003 15:37:06 by Atcboy73
Engine Start-up? posted Sun Apr 22 2001 03:07:32 by Bryan Becker
Engine Start-up posted Thu Nov 16 2000 04:09:40 by Turboprop
747-400 Engine Start Up posted Sat May 27 2000 12:44:48 by Future_747_f/o
Exhaust Gas Speed At Start-up posted Sat Aug 19 2006 00:25:26 by Hugh3306

Sponsor Message:
Printer friendly format