Particle Separator

[just a tech]

I would like to know about Particle Separators on helicopters. Where I work we service at least one RPM Limit Detector that operates the Particle Separator, but no one is sure just exactly when a particle separator does its thing. I already know that they do as their name implies, and I understand the basics of how they remove particles from the air entering the engine's intake. What I'd like to know is when are they active and when are they bypassed, and during which mode the doors open.

Thanks in advance.

[Saint Jack]

First - put ALL helicopter-related questions etc. in the 'Rotorheads' Section where you're sure to get many authoritative answers.

There are a number of different particle separators available for helicopters, some are built into the basic design of the helicopter while others may be offered as an option. In addition, there are a number of OEM's offering different designs which are usually installed under the authority of an STC. The system you describe sounds like a Bell 212/412 where the particle separator door is controlled by the N1 RPM detector. In this case the particle separator is an integral part of the engine air management system and a short description of this can be found in the helicopter maintenance manual.

Briefly, when the particle separator door is closed all of the incoming air, and dirt particles, is directed into the engine intake plenum - but this is only during the start. However, when the door is open, i.e when the engine is running, the velocity of the exhaust gases cause air to be drawn into the ducting. This creates an airflow through the engine air management system (above the power section) which accelerates the incoming air and this in turn imparts momentum to any dirt particles (you'll need to know the principles of convergent and divergent ducts to fully understand this). The result is that the dirt continues in a straight line through the system and exits with the exhaust gases while the 'clean' air is drawn down into the engine intake.

So why not leave the door open all the time or even remove it completely? Another function of the engine air management system is to cool the combustion area of the power section and this is achieved by creating a low pressure in the rear portion of the engine air management system that continually draws ambient air through the vents of the cowling into the combustion section area and exiting through the gap between the exhaust duct and the air management system.

Now, imagine a scenario where there is an engine fire warning and the pilot discharges the fire extinguisher to fight it. The cooling airflow through the combustion section area would simply carry the extinguishant out into the air management system and it would have no affect on the fire. So, one of the functions of pulling the fire 'T' handle is to close the particle separator door and this, in turn, prevents airflow through the system which immediately nullifies the low pressure area which would otherwise draw out the fire extinguishant.

Sorry, it's been a long time since I worked on these helicopters so I can't quote you the actual N1 speeds at which the particle separator door operates. Perhaps someone else can.

[NutLoose]

Also ask in the military as the Puma and Chinook use them in Afghan.

I remember when the RAF first fitted them to a Puma, they were designed for dust etc, but they used them in icing conditions in Norway, one iced up, this was detected and the bullet opened at the front to let air in, it then hoovered up all the ice that had formed blocking it LOL.

[helicopterray]

The one in the Bell 206 series did not have an on/off function, it was on all the time when the engine was running.
P2 air ran through the unit to keep it operating.

The AS350 has an on/off switch in the cockpit, for whenever you're in 'sandy' conditions. They call it a sand filter. It shuts off the p2 air supply.

There are no bypass doors on those 2 helicopters.

[B1-3]

PM sent...........................

[ShyTorque]

IIRC (it's been 20 years since I instructed on the OCU), The first RAF Pumas to have Polyvalent Intake Protection System (PIPS) retro-fitted to the front of the engine intakes had it done primarily as an anti-icing mod. They were first fitted to 33 Sqn aircraft in the late 1970s/early 1980s (they held the Norway commitment), then later to those of 230 Sqn as more modules became available and the whole fleet was converted.

The intake modules were double skinned. They had large diameter "bullets" with an inflatable sealing ring, which could be motored forward on hollow tubes, under the influence of P2 air pressure. When fully forward, a sealing ring expanded, closing off the open intakes, so that all engine intake air went in through the sides of the PIPS modules, where the debris was separated before being blown overboard.

The 3 position cockpit switch was marked "OFF / ON / AUTO". The first two positions are obvious. The AUTO position was coupled to the landing gear position, i.e. with the gear down, the PIPS automatically motored to the ON position. When fully forward, the modules were supplied with P2 air, supposedly blowing the accumulated debris overboard (not always fully effective in severe conditions).

I understand that in later years the aircraft had "dustbin lid" blanks fitted over the engine intakes, making the PIPS work permanently.

Great for dusty conditions, not so good for the already less than mediocre fuel endurance of the old girl!