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While looking at the Boeing Chinook CH-47 for this question, I noticed the screens in front of the engines, and found similar devices in other photos:

enter image description here
Source: left -- right

  • What are these devices used for?

A recurrent question here is about protecting turbine engines against bird/FOD ingestion. It seems that a screen can be used, at least at low speed.

  • It is the case? Is it used for this purpose?
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The first photo shows the engine air intake screen. Its basically a screen to prevent FOD ingestion, as you noted. From CH-47 Theory of Operations:

Engine Inlet Screens

An engine inlet screen which maximizes foreign object damage (FOD) is installed on each engine. The reduction in engine power available with screens installed is negligible. The engine inlet screens have bypass panels. These two panels are on the aft end of each screen. Quick release fasteners attach the panels to the screens. They are removed if the ambient temperature is 4°C or below and there is visible moisture

  • I'm pretty sure it is supposed to be minimizes foreign object damage.

It is an All Weather Screen (AWS) intended primarily to prevent ingestion of large items that would cause foreign object damage to the compressor blades.

Chinook engine

By Mr.Z-man - Own work, CC BY-SA 3.0, Link

The second photo shows the Engine Air Particle Separator(EAPS) unit, which in this case is the Pall PUREair System

PUREair

Pall PUREair system; image from Pall aerospace

The EAPS is there primarily to protect against ingestion of sand and dust, which, according to Why an Engine Air Particle Separator (EAPS)? can cause multiple problems like,

  • erosion of compressor blades

  • clogging turbine cooling passages

  • cause sulphidation, corrosion and overheating

As the article notes,

A CH-47 in the desert with only the AWS could achieve only 12 landings before both engines were changed. With EAPS, it did 13 landings with no change in horsepower. in another desert test CH-47 engines with AWS deteriorated to an unacceptable level in only 30 hours. With EAPS, there was no degradation.

In case of desert environments, EAPS is a must if you want your engine to survive any more than double digit hours. This particular type of EAPS belongs to the Vortex tube separators and uses a vortex generator to separate the sand and dust from air.

Contaminated air entering the ... air cleaner tube assembly is given a swirling motion induced by the vortex generator. This swirling motion causes the heavier dirt particle and water droplets to be thrown radially outward by centrifugal force toward the wall of the tube.

EAPS operation

Principle of EAPS operation, image from Pall aerospace.

You can find about other types and their operation in A Comparative Study of Helicopter Engine Particle Separators

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  • $\begingroup$ I suppose you must be right about the typo for the screens :-) Any idea about what prevents their use in turbofan hot flow and fixed wing turbo/jet/prop? $\endgroup$ – mins Oct 21 '17 at 13:25
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    $\begingroup$ @mins The operational envelope of helicopters and aircraft are quite different. Simply put, the cost in design, weight, drag, maintenance etc of the screens are not enough to offset their supposed benefits in FOD ingestion in the normal aircraft operating environment. Also, note the point about the reduction in engine power due to them. $\endgroup$ – aeroalias Oct 21 '17 at 13:43
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    $\begingroup$ @mins I would surmise that the big difference between helicopters and fixed wing in this situation is the speed. You can make a screen that can stand up to bird strikes at helicopter speeds but at fixed wing speed a large bird would penetrate and take the screen with it. They’re also less needed for sand, etc. The debris thrown up by a fixed wing is mostly behind the engine whereas a helicopter is right in the middle of its downwash. $\endgroup$ – TomMcW Oct 21 '17 at 20:15
  • $\begingroup$ @mins, turboprops designed for use on unpaved runways, e.g. dhc-6, do use “intake deflectors”, which is a simpler kind of EAPS. The simpler version is sufficient because in fixed-wing aircraft the engine is not blowing up debris ahead of itself, so less sand and dust gets ingested. $\endgroup$ – Jan Hudec Oct 21 '17 at 20:33
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    $\begingroup$ @mins in a high bypass ratio turbofan, the fan itself acts as a flow separator by centrifuging small pieces of debris away from the engine core and into the bypass duct, where they can't do much damage. $\endgroup$ – alephzero Oct 21 '17 at 20:45

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