Effect of Humidity on Turbine Engine Perf

[windmill brake state]

Guys,

I know that Helicopter Flight Manuals do not take into account humidity on performance charts however, I was wondering in any of your performance testing if you could give me a rough idea what effect high humidity did have. The company I work for are looking at whether allowance will need to be made for some of the offshore work we do in tropical areas where aircraft are already operating at MTOW and minimal power margins. Any help would be greatly appreciated.

Cheers,

WBS

[nodrama]

I was taught in engineering school (mechanic school for our USA cousins) that because roughly 70% of mass airflow in a GT engine is used for cooling, humidity effects aren't an issue. Moisture in the air has negligible effects on combustion.

Common sense says that if humidity is, for example 80%, that that amount of moisture in the air must have some sort of effect, so I would be interested in hearing a more technical/ experienced answer also.

[NickLappos]

Reduces performance of engines and rotors. Water vapor is gassified H2O (18 grams per mole), which is almost half the density of air which is mostly N2 (28 grams per mole). Thus all wind machines (rotors and compressors) have higher Density Altitude when they have more moisture in the air.

An engine also suffers on the combustion end when moisture rises, since that H2O has no free oxygen, while air has about 20% free O2. Thus, higher moisture blocks out some of the oxygen needed for burning.

Engines are rated with some moisture assumed in the air, so spec power is not changed much. Also, helos ar tested in normal air, and the performance is not backed down for dry air, so the "normal" performance charts include some humidity.

The net effect is that high humidity reduces performance. The good news is, the moisture content of air is not very high, so the effect is not tremendous. Count on a max of 2% power loss in the highest moisture environment relative to completely dry air, and maybe less than 1% for "normal" air vs fog.

[Shawn Coyle]

The only thing I could add to Nick's answer is that the Russians actually have taken this into account on their flight manuals (unfortunately, no graphs available). The hover charts have a section for high humidity in high temperatures that shows the effect.
Sorry, can't remember any rules of thumb.
Anyone out there with Russian flight manuals???

[Gomer Pylot]

Remember that 80% humidity means that the air has 80% of the moisture it can hold at that temperature, not that 80% of the air is water. At 80% humidity, the air may be only 1% water, depending on the temperature. Even at 100% humidity in the tropics, the percentage of water in the air is still relatively small.

[Graviman]

This page has some useful info

http://en.wikipedia.org/wiki/Relative_humidity

http://upload.wikimedia.org/wikipedia/en/4/41/Relative_Humidity.png

I'll do some digging on engine performance effects.

[Darkhorse30]

Shawn,
I have several at home. I'll have a look over the weekend and see what I can find in the way of humidity effects. It has been several years since I have flown one but I don't recall seeing hover charts with humidity as a variable.

[ShyTorque]

Remember that 80% humidity means that the air has 80% of the moisture it can hold at that temperature, not that 80% of the air is water.

Strewth no, that would be sparkling water, not air! :p

As a newly qualified Puma pilot in the late 1970s I was asked to do some research into this; we had to fly in Belize, in 100 degrees Fahrenheit, 100% humidity. The answer was that high humidiity did have a detrimental effect on DA but nowhere near as much as that of the high OAT. Unfortunately, I no longer have the written info, I think it went into a file on 230 Sqn and has probably gone into "file 13" now.

[Rich Lee]

There is really not enough information in your question to provide a good answer, but for the purpose of discussion I submit the following observation.

There can be no doubt that water injection, at the appropriate percentages, into the combustion of a turbine engine improves performance by reducing temperatures. Nick, Shawn and I have probably all conducted testing on various engines using water injection and water/methanol (or other combustive additive) injection to improve performance for take-off and or landing. Usually the injection is limited because the water or water/methanol is limited so cannot be used throughout the flight.

One can argue that this intentional water injection creates an artificially higher humidity and it might then be argued that high humidity may improve perfromance. Many pilots believe that engine performance is improved in high humdity (e.g. rain, over-water operations). This does not argue against Nick's statement. In terms of pure air density he is correct, but because the lower engine temperatures during combustion at high engine power settings when water is injected into the engine have a greater impact than lower density during compression in the overall power equation, water injection (high humidity) can improve the performance of some engines.

[arismount]

Atmospheric humidity has a net decrease effect on engine power output. But fluid injection has a net positive effect. Here's the difference:
Atmospheric humidity results in less mass flow of the working fluid through the engine heat cycle. Also, fraction of combustible oxygen is reduced in the working fluid.
On the other hand, water/alcohol injection significantly increases mass flow of the working fluid, AND heat is required to vaporize the injected fluid (while atmospheric humidity is already vaporized), thus reducing engine temperature significantly and thus allowing more fuel input, thus more power.

[Shawn Coyle]

Arismount:
To add to your valid point, I remember some old RAF Hunter pilots saying that when it rained, they got better performance out of their jet engines on takeoff than when it was just very humid at the same temperatures...

[windmill brake state]

Thanks very much for all the replies. It seems that it is not really something that we will have to worry too much about.

WBS

[JimEli]

Maybe an issue, maybe not, consider a few points:

The molecular weight of water is considerably less dense than dry air, and the humidity transforms moist air into a slightly different type of gas with a lower molecular weight.

The comparison of density altitude between completely dry air and 100% saturated air on an ISA sea level day is 0 vs. 221’. However, consider the same comparison done at 4000’ and 35 degrees, results in 7121 vs. 7923’ (about a 2.5% decrease in density ratio).

There is an additional factor to consider for engine performance, due to moist air displacing some O2 content. By my calculations, using the above examples causes the engine to experience (or “see”) equivalent densities of 582’ and 9268’.

The above comparisons are for academic purposes.

See “Performance of Light Aircraft,” John T. Lowry, and if interested I could give you the mathematical derivations of the above.

[windmill brake state]

To be more specific I guess what I was after was the effect of humidity on power available and power required and whether the humidity effect was linear ie twice the effect for 100% humidity vs 50% humidity all other conditions being equal.

JimEli I am interested in the mathematical derivations of your figures please.

Thanks.

WBS

[Shawn Coyle]

From the FARS (part 27)
Regarding Performance Requirements:
(d) For reciprocating engine-powered rotorcraft, the performance, as affected by engine power, must be based on a relative humidity of 80 percent in a standard atmosphere.
(e) For turbine engine-powered rotorcraft, the performance, as affected by engine power, must be based on a relative humidity of--
(1) 80 percent, at and below standard temperature; and
(2) 34 percent, at and above standard temperature plus 50 degrees F. Between these two temperatures, the relative humidity must vary linearly.

So, they don't take it into account for anything above ISA+50°F. Pity, because it does have an affect.
And remember that power available for a turbine depends on pressure altitude and temperature and power required depends on density altitude. And they are different!