Bell Jet Ranger helicopter startup sounds?

Factual Questions
1

Here’s a video of a Bell 206 Jet Ranger helicopter starting up. At :06 is when the sounds begin. I assume the first 9 seconds or so are just a battery-powered motor spinning up the turbine, along with the clicks of the igniter(s). Then there are a few new sounds:

:15 sounds like a burner being lit
:16 adds a peculiar howling component
:25 sounds like the turbine really spooling up fast all of a sudden

So what are all these sounds? Why is the turbine sudden spinning up so rapidly at :25?

2

NM

3

I was going to ask if this helicopter had dual engines, and it looks like some variants of these do. Would that explain the “wind-up” sound at :25 (second engine start up)?

4

The first 15 seconds are the starter spinning the compressor turbine, and the clicking is the ignitor. At 15 seconds, compressor turbine speed (N1) is up to (IIRC) 15% and the pilot rolls on throttle, and the engine ignites. The starter is kept on until I believe ~60% N1 and then the engine continues to accelerate to idle. The rotors start turning after engine ignition because the engine is a dual shaft - the power turbine is free wheeling. Once air starts flowing across it, it starts turning the rotors.

5

I suppose that’s possible - although why would one wait for 9 seconds (after firing the first engine) before starting the second engine?

Also, for gas turbine engines, I’ve read that part of the start procedure is to watch exhaust temp for the first ?? seconds to ensure it’s not roasting the turbine blades. If it’s too hot you quickly shut the engine down, but if it’s not too hot you can go ahead and add throttle. What kinds of things can cause excessive EGT on startup?

6

Too much fuel with/or too little turbine speed, meaning too little airflow. Either the N1 hung up and the fuel control is putting too much in the burner, or maybe the fuel control itself is in need of repair.

Also it can happen if your last aborted start, or the last shutdown, left raw fuel in the combustor can. If that lights off during start you can get EGT exceedences very fast! Like spitting turbine blades out the back fast. After aborted starts the procedure usually is to motor the engine without ignitors on to blow out any fuel puddles.

7

here’s the engine which powers this:

educated guess is the "howling" or moaning sound is a resonance in the inlet or exhaust ducts at a particular gas flow rate. Cars have the same problem which is why you'll see one or more Helmholtz resonators hanging off of the intake duct.

eta: here’s another novel application for that engine: Jet Boat With Rolls Royce- Allison Gas Turbine - YouTube

8

Interesting.

Back when I was in college, the ANG wing at the nearby airport flew F-4 phantoms. Often times when they were on final approach for a landing, they’d emit an eerie loud howling noise, like that Allison turbine, except higher pitched. It’s apparently a known feature of the phantom’s J79 engines:

It was often intermittent, semi-repetitive; not sure if that characteristic was because of local turbulence or because the pilot was getting jiggy with the throttle, but it was always interesting to hear, and I missed it when they upgraded to F-16s.