Engine startup sound different before second leg

[Gábor]

There is a great sound effect of startup engine when you add fuel for an engine. I think this is the sound when the fuel start to burn. In that case the ITT was zero.

I noticed that before second leg when I start the engines this sound effect is not there .

Could be the reason that the engine is hot when I start it up?

Edited January 22, 2022 by Gábor

[Graeme_77]

Yes indeed, that's expected behaviour with a hot engine.

The reason is the sound is a resonance caused by combustor & flame aerodynamics. When the combustor is hot, the aerodynamics change ever so slightly that it simply doesn't induce that resonance anymore.

[Gábor]

1 minute ago, Graeme_77 said:

Yes indeed, that's expected behaviour with a hot engine.

The reason is the sound is a resonance caused by combustor & flame aerodynamics. When the combustor is hot, the aerodynamics change ever so slightly that it simply doesn't induce that resonance anymore.

Guys, what you achieved is unbelievable. I feel extremely lucky to fly this beauty. Wow, just wow and wow!

[JRBarrett]

@Graeme_77Actually, most of the “moan” that is heard from the CF34 engine as it is coming up to speed during start is caused by the change in position of the variable inlet guide vanes in the LP compressor section. These vanes optimize the airflow through the compressor for the current N1 speed to prevent blade stalls. The sound should be the same on every start, whether the engine is hot or cold.

I am a CRJ-200 mechanic and that aircraft uses the same model CF34 as the 650

There is also a noticeable “whoomp” at the moment the fuel ignites in the combuster. 

Edited January 22, 2022 by JRBarrett

[Epikk]

7 hours ago, Graeme_77 said:

Yes indeed, that's expected behaviour with a hot engine.

The reason is the sound is a resonance caused by combustor & flame aerodynamics. When the combustor is hot, the aerodynamics change ever so slightly that it simply doesn't induce that resonance anymore.

Unbelievable. 

[Gábor]

10 hours ago, JRBarrett said:

@Graeme_77Actually, most of the “moan” that is heard from the CF34 engine as it is coming up to speed during start is caused by the change in position of the variable inlet guide vanes in the LP compressor section. These vanes optimize the airflow through the compressor for the current N1 speed to prevent blade stalls. The sound should be the same on every start, whether the engine is hot or cold.

I am a CRJ-200 mechanic and that aircraft uses the same model CF34 as the 650

There is also a noticeable “whoomp” at the moment the fuel ignites in the combuster. 

 @Graeme_77So I guess it will be corrected, yes?

[Graeme_77]

The sound in the simulation is recorded directly from the real aircraft, and the pilots and ground crew on the testing team verified the sound environment in the product. The "whoomp" effect you initially asked about is dependant on engine temperature, where a hot engine does not make the same sound for the reasons explained.

Based on all the information available, there is no "fix" required.

[Gábor]

51 minutes ago, Graeme_77 said:

The sound in the simulation is recorded directly from the real aircraft, and the pilots and ground crew on the testing team verified the sound environment in the product. The "whoomp" effect you initially asked about is dependant on engine temperature, where a hot engine does not make the same sound for the reasons explained.

Based on all the information available, there is no "fix" required.

I meant the "moan" effect initially and also the crj200 technician meant this.