N1 Rise in rain

[VH-ABC]

Could someone please set me straight on the theory behind why a Jet engine N1 rises when encountering rain.

Thanks in advance

[FCS Explorer]

:confused: :confused: :confused: eergh? says who?
explanation *might* be: when encountering icing conditions in descent anti-ice systems are switched on. they use bleed air and then the idle value goes up to sustain the engine.
i don't think the water drops in rain can really raise the air density to explain higher N1 values.......

[Clarence Oveur]

Water is heavier than air, so higher mass flow.

Water evaporates in the compressor, cools the air. Same effect.

[FCS Explorer]

well, sure, water is heavier then air. but if you take a m³ of rainy air, how much water does it contain?

[IFixPlanes]

When we do performance tests on engines the AMM does not mention any influence by rain. Only temperature and atmospheric pressure are important.

[Piper19]

I think the EEC holds thrust at the throttle commanded level, whatever the ambient conditions are? I know some planes have an idle reset valve, which turns on high idle (approach idle) when engine anti ice is set, or slats not fully up.

[barit1]

1) Even though the AMM engine checks neglect the effect of humidity / liquid water content. the test cell corrections include humidity effects (most of the liquid rain gets removed by the large low-velocity inlet area of the test cell).

2) I'd expect the N1/N2/(N3) relationship to change as the rain changes state to water vapor in the compression process.

But whether N1 changes will depend on the design of the control system. Since modern engines often control N1 directly, the rain should result in N1 remaining steady, and N2 varying. Older engines (early CF6's e.g.) have no N1 control, only N2, so N1 can indeed "float".

[FlexibleResponse]

Engine inlet temperature is measured by the T1 Probe. The T1 probe is heated to prevent icing. When in icing conditions or rain, the cooling effect (or latent heat transfer or whatever the scientists want to call it) on the probe causes an error in the measurement of temperature, possibly due to the cooling effect overpowering the anti-ice heating capability. This will cause the Max N1 limit or Max EPR limit to change.

I demonstrated this effect to a Rolls Royce engine representative in flight on an A330 with Trent engines through the tops of tropical cloud about 10 years ago. His eyebrows bounced off the overhead panel! RR eventually said that the EEC computed max thrust change caused by the incorrect temperature reading was safe. So don't expect anything from Mr Rosemount's deficient probes to change soon!

[barit1]

Very good point, FR!

:D

[VH-ABC]

Just to clarify...

I was asking directly about a fairly old engine, the TFE 731-3-1
g, used on Lear 35, Westwinds etc...

We commonly see N1's climb up to 1 percent when enterering heavy rain, with nothing else added, for instance, anti-ice.

[Re-entry]

Water ingestion up to the saturation point reduces the compressor inlet temp. to near 'wet bulb' temp. The droplets then evaporate within the blade path, the adiabatic process causing the temp. to drop further. Since it takes less energy to compress cooler air, compressor work is saved. This translates to less load on the turbine, so the N1 increases.

This 'wet compression' process is used in some power stations to increase the efficiency of the combustion turbines.

Water ingestion beyond the saturation point (very heavy rain) results in water entering and evaporating in the N2 compressor and combustion sections. This results in reduction of the cycle temp. of the engine, reducing the work available to drive the N1 fan. Consequently, N1 decreases in this condition.

[VH-ABC]

Thankyou all for your thoughts.

Cheers.

[Fully Articulated]

Moist Air is less dense than Dry Air. Density Altitude directly affects N1 RPM. If the selected powersetting is not changed the N1 RPM at 5000Ft DA will be lower than at 5500Ft DA. This being said, the Relative Humidity increase seen in heavy rain will decrease the Density Altitude and increase the N1 RPM.:8

[Old Smokey]

Ever wondered why for years, many engine manufacturers deliberately injected water into their engines in the form of Water-Methanol?:confused:

Aha! Because it increases power:ok: (For all the good reasons that earlier posters have indicated).

I have memories from my earlier flying career with RR Darts with Water injection, a "Wet" takeoff on an Australian 45°C day, upon turning the W/M off at 400 feet, it seemed like the engines had stopped, RPM would initially drop considerably, until the prop CSU restored set RPM. A jet, of course, has no such speed governor, and the RPM would stay dropped.

Regards,

Old Smokey

[john_tullamarine]

I have memories from my earlier flying career with RR Darts ..

.. likewise .. out of Tennant .. wet on two .... and about 200 ft/min for ever and a day ... not impressed.