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What is the loud noise when a helicopter is ready to start flying, but the rotor is not yet rotating?

For example at Trump's "chopper talk", when he is intercepted by reporters on the way to Marine One? The helicopter nearby makes really loud noise already, which makes it hard to communicate between reporters an Trump - so it seems to be something important. It's not the rotor itself, as it's not rotating.

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The sound you're hearing is the APU (Auxiliary Power Unit) running.

Turbine (jet) engines turn at a much higher RPM than reciprocating engines. This means that an electric starter motor would have to be bigger and heavier in order to produce enough torque to get the engine moving fast enough. The bigger the engine, the bigger the starter (and the batteries to drive it). At some point, it becomes impractical to have an electric starter on a big turbine engine.

That's where an APU comes in. It's essentially a small jet engine. Small enough, in fact, to be started electrically. Once it's running, it feeds high-pressure air to spin the main engine fast enough for it to start. It can also provide electricity, run the A/C, etc.

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    $\begingroup$ It could also be the main engines idling with the rotor brake set, but watching the vids I agree it sounds more like an APU running. $\endgroup$ – John K Aug 25 at 3:03
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    $\begingroup$ It's mainly the starter batteries that would be way too heavy. See the 787, which uses electric starters (even though the Trent certainly is "big"), but still needs an APU to feed them. $\endgroup$ – TooTea Aug 25 at 19:47
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    $\begingroup$ “Turbine (jet) engines turn at a much higher RPM than reciprocating engines” – that would seem to be quite irrelevant here, since it is actually easier to build high-RPM electric motors than low-RPM ones of the same power, especially at small size&mass. As TooTea says, the problem isn't in electric motors but electric power supply – which raises the question why helicopters aren't just started with an electric motor supplied by an umbilical cord, to avoid wear and fuel demand of an APU. $\endgroup$ – leftaroundabout Aug 26 at 15:30
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    $\begingroup$ @leftaroundabout Probably because there are a lot more places to land a helicopter that won't have any ground support than there are to land airplanes. $\endgroup$ – T.J.L. Aug 26 at 17:53
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    $\begingroup$ @T.J.L., even large fixed-wing aircraft often use APU because ground power is either too much hassle, not available or not sufficient. There is a comment by Terry (retired 747 pilot) somewhere around how it was problem to scramble a start cart capable of starting a 747 somewhere in Harare—they simply didn't have a large enough one at all, so two had to be hooked, which the 747 fortunately supports. I've also read mention by an ATR-42 pilot how in some places in India they simply were not used to bring the power cart around (ATR-42 has no APU and has to use the engine with propeller brake). $\endgroup$ – Jan Hudec Aug 26 at 18:37
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You may be expecting the rotor to spin up in lock-sync with the engine.

That's not quite what happens on a turbine helicopter.

Turbine helicopter engines (turboshaft engines) have a turbojet engine inside them that makes thrust. This engine has a shaft of its own - so that its compressor blades are connected to its own turbine blades. The turbojet's thrust acts on a second "stage" of turbine blades, which are on their own shaft. Those connect to the helicopter rotor.

So the "internal" turbojet engine can be started and idled, without making enough thrust to spin up the second stage. Turboprop engines work the same way, and you'll see where they are "barely turning over" at times. That is the idle thrust working on their rather free-turning mechanism.

Some helicopters have a rotor brake so they can keep the rotor from spinning slowly like that.

Closer to home, that same effect occurs in your car's automatic transmission. The engine is able to turn at idle speeds while in gear. That's why you have to hold the brake constantly, the engine's "thrust" will creep the car up to about 5 mph if you don't.

Both of these are a hydraulic coupling - transmission fluid in the car, and jet engine exhaust air in the helicopter/turboprop/turbofan.

Once you advance the throttle to meaningful amounts of power, this "slip" becomes insignificant, and power is transmitted through just fine.

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