Reingestion!

[validator]

Hello fellow rotorheads;

I have a question about engine placement on helicopters. Recent studies i have been doing resulted in my finding of the following problem:

At Ng above 85% when hovering IGE with a strong tailwind the exhaust gases deflected to the engine intake may be recycled in the engine resulting in a partial stall of the axial compressor also in a downwind hovering situation there is a tendency for hot exhaust gases to re-circulate into the engine intakes. This has the effect of reducing the mass flow (less dense air) and increase the temperature in the intake which can result into surge. This is commonly known as reingestion.

I have searched if some kind of engine placement (angle, direction etc.) could solve this problem, but i couldnt find any papers or anything at all for that kind of matter. (except for 2 papers written in 1982, which is sold for 30$ each, almost never cited, so i couldnt trust them enough to buy =) )

What are the solutions to this problem? Is there some precautions for pilots when doing HIGE or strong-tailwind situations to overcome the reingestion problem, or is this a complete engineering problem that can be solved with different air intake or compressor designs? Have you ever seen such documents? What is the most effective solution to overcome this, engine placement , inlet/compressor design or rotor design?

I have heard that the engine control system also arranges the engine such that the reingestion problem does not affect the engine very much. Is that true?

Lastly, can i ignore the reingestion effect in small helicopters (which are about 5000kg)?

Thanks for your help

[oleary]

To all of your questions: NONE/NO.

[Hilife]

Exhaust gas re-ingestion. What are the solutions to this problem?

In my experience, the best solution to alleviate hot gas re-ingestion has been achieved through engine exhaust duct design, as this is usually the best way to eject the engine exhaust gases away from the aircraft, thereby reducing re-ingestion issues.

[validator]

To all of your questions: NONE/NO.

so should i just accept the problem and don't talk about it in my designs? =)

there should be something that can be done! Increased inlet cooling, different inlet and exhaust locations on the helicopter or something.

Or is this problem too insignificant to care about?

[[email protected]]

Avoid hovering directly downwind for extended periods.:ok:

[Thomas coupling]

About as important as having windscreen wipers on the side screens;)

A complete waste of a designers time. Doesn't affect anything except a slight rise in Ng and T4.

[industry insider]

Not quite true TC. Just ask Sikorsky why it took so long to certify the D model......whoops, hush my mouth.

[Hilife]

Maybe not so much a problem for older design engines, but hot gas re-ingestion on a modern ECU/FADEC equipped engine could be sensed by the T1 probe which in turn will cause fuel scheduling and surging issues, so as noted by II, it can be a very big problem.

[Fareastdriver]

The Makila has a manual or automatic system that utilises a bleed valve in the engine to prevent surging when hovering a 332 downwind. The pilot selects the bleeds when he is turning to or hovering downwind. It is also useful when operating off turbulent helidecks. On some versions, designed for thick pilots, the bleeds are on whilst the undercarriage is down.

[Gemini Twin]

During the certification testing it must be demonstarted that the helicopter is controlable in 17 kts winds from all directions. Direct tail wind is not always the worst case. During this test the engine charcteristics requirements i.e. free from stalls and surges, must also be complied with. If reingestion is a problem it must be rectified, usually it takes slight tweaks to the shape of the engine cowling and or exhaust stack(s).

So operating at high power with quartering tail winds over 17 kts MAY result in high T4's or possible a stall in some helicopters.

[Aesir]

About as important as having windscreen wipers on the side screens

:E It´s something to be aware of but really not a big problem and not worth it to worry to much about.

[Fareastdriver]

On the Puma with Turmo IIIC4 in the seventies the recirculation on a downwind approach and landing would quite often trip the fire warnings on both engines. They would continue to stay on even after landing.

You would casually reassure some observer sitting in the jumpseat having kittens as you lifted it into the hover, turned into wind and watched the lights go out as you passed 50 knots.

[MightyGem]

Often in downwind hovers in our 135. Can't say I've ever noticed a problem.

[oleary]

Most modern helicopters have turbine engines.

Turbine engines eject lots of hot air.

All turbine engine inhale lots of air - from the immediate area - to state the obvious.

And helicopter main rotor blades agitate everybody's air.

Conclusion:

Depending on wind azmith, no matter how you design it, SOME of the ejected turbine engine air is gonna get into the engine inlet some of the time - which will cause higher Ng (N1) and T5 (T4.5, TOT, ITT, and FADEC problems, etc)

Get over it.

[Sir Korsky]

The 76Bs with their PT6s are prohibited from any kind of left or right quartering tailwind, whether caused naturally or from downwash when the EAPS is selected on with under 15% torque set. Engines have been cooked rapidly due to minor distractions!:{

[Shawn Coyle]

While testing of FADEC engines under development (with the airframer, engine manufacturer and fuel system developers), we had an interesting experience.
While hovering downwind with 20 knot tailwind, watched the TOT on the right engine start to climb, followed shortly thereafter by one single bang from right engine. I commented that this must have been a compressor stall - flight test engineer from airframer said - yes, but on this helicopter with earlier, non-FADEC engine, it would have been continuous compressor stalls until aircraft exited this situation.
Was later given photos of non-FADEC model with flames coming from both intake and exhaust in downwind hover.
FADEC computer would note condition of compressor stall, and prevent it for duration of flight. Very neat!!!