can a helicopter not have a rotor brake?

Human rotor brake:pilotfly:

Literally every turbine helicopter on the planet operates this way. If the rotor doesn't spin, neither does the N2 turbine in the engine.

 

(EDIT: Now that I've made such an absolute statement, someone will find a way to prove me wrong)

If by N2 you mean free-power turbine of course it spins , Exhaust gases pass over the fan blades and turn the High speed shaft which drives the main rotor gearbox.

Twin engine Helicopters have what is called an accessory drive where the drive from the free power turbine goes into the main rotor gearbox but doesn't drive the rotors it runs all accessories IE Hydraulics, Generators.

Pre start check Acc drive selected Start No 1 Eng and advance up to speed (seaking was 104%)

When the pilot starts the second Engine which has no accessory drive he will preload the torque (rotor brake holding the drive) up to about 30% then release the rotor brake. Eng 2 Will drive the rotor up to speed. The Pilot will then retard No 1 Eng Back to ground idle so that the acc drive is unloaded and then can be put into Flight Drive (where both engines share the load of turning the rotors (Matching torque)

Literally every turbine helicopter on the planet operates this way. If the rotor doesn't spin, neither does the N2 turbine in the engine.

 

(EDIT: Now that I've made such an absolute statement, someone will find a way to prove me wrong)

You’re in a way correct but N2 is only valid for free turbine engines. :-)

N1 is the speed of the compressor/compressor driving turbine.

N2 is the speed of the free turbine, driving the main gearbox/rotor.

Some(mostly older) turbine engines is single spool where there is only one shaft combining both the compressor and the turbine. In these engines theres really no N2 rpm, as all parts of the engine is spinning with the same speed.

To be able to perform the engine start there is a clutch. For the Gazelle engine there is, as Ramsay say, a centrifugal clutch. Theres also (of course) a freewheeling unit to be able to autorotate without driving the engine with the rotor.

Twin engine Helicopters have what is called an accessory drive where the drive from the free power turbine goes into the main rotor gearbox but doesn't drive the rotors it runs all accessories IE Hydraulics, Generators.

Pre start check Acc drive selected Start No 1 Eng and advance up to speed (seaking was 104%)

When the pilot starts the second Engine which has no accessory drive he will preload the torque (rotor brake holding the drive) up to about 30% then release the rotor brake. Eng 2 Will drive the rotor up to speed. The Pilot will then retard No 1 Eng Back to ground idle so that the acc drive is unloaded and then can be put into Flight Drive (where both engines share the load of turning the rotors (Matching torque)

This doesnt apply to all twin engine helos, actually it is not that common at all. I would say it is rare. Seaking is very old, so I’d say thats the reason for having this.

We had an extra option on the superpumas I flew, where we could disconnect one engine via a clutch to be able to start one engine and run it for extended time without turning the rotor. This option was disconnected early after the went into service, so IRL never used.

Most twin engine helos doesnt have a possibility to disconnect any engine from the MGB/rotor drive.

For the rotor braking:

I never heard of any helo where it is allowed to use the collective to increase drag to stop the rotor. All types I have flown have had a note ”not allowed to use the collective to brake the rotor.

In smaller helos the rotor brake is an option that you have to pay extra when buying.

These helos, in most cases only have a brake to stop the rotor after engines shut down, and not for keeping the rotor stopped during engine start.

The ones I haw flown that has the ability to use the rotor brake for engine start has two modes, one for braking and one for starting the engine. There is a big difference in hydraulic power for braking vs keeping it stopped (somewhere around 5 times more hydraulic pressure for keeping it stopped). The normal braking power is not enough to keep it still during engine start, and if it starts to slip this could very quick end up in a overheated rotor brake or fire.

These rotorbrakes also is built to be able to be left in braking position to stop the rotor from turnning by wind or when towing the helos. The super puma we always left in braking position and there was added a funktion for inhibitation of start if the brake not was in released position.

in the book chickenhawk the company pilot instructor is ridiculed because he tried to stop the spinning rotors with a tie down, and instead he himself got swung around the helicopter.

it was a very noobish thing for the most senior pilot to do. in front of everyone..

so I bet he wished the huey had a rotor brake :)