Humidity Factor
Thread Starter
Humidity Factor
Does anyone have some sort of reference as to how much power is lost in humid conditions...
Tables of humity vs temperature vs altitude etc would be a winner.
Just had a scrape off take off in Angola with a 120 with very little on board...
Tables of humity vs temperature vs altitude etc would be a winner.
Just had a scrape off take off in Angola with a 120 with very little on board...
Avoid imitations
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Try this link:
http://ntrs.nasa.gov/search.jsp?N=0&...idity%2Bengine
Hours of reading, if not exactly what you're looking for.
http://ntrs.nasa.gov/search.jsp?N=0&...idity%2Bengine
Hours of reading, if not exactly what you're looking for.
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Thread Starter
Need Graphs
Not exactly what I'm looking for.....
I'm putting together a checklist for our helicopters for pilots in the field for quick referencing before quick decisions on lifts and need some sort of graph or numbers that I can put into a graph...
Any one seen or have something like that???
I'm putting together a checklist for our helicopters for pilots in the field for quick referencing before quick decisions on lifts and need some sort of graph or numbers that I can put into a graph...
Any one seen or have something like that???
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You might consider this
I suppose you are familiar with the field method of computing Density Altitude, namely: multiply the difference from standard temp by 120 & add to PA (or what altitude is showing on the meter...it's close enough). e.g.: 1000' showing on the meter & 35 C; that's 20 C above standard; 20 x 120 = 2400; 2400 + 1000 = DA of 3400 ft.
Well, humidity has an effect on DA and a "field" method is to add 100 feet for each 10% of relative humidity, on top of the DA already computed; e.g.: 3400 ft DA already computed, with 80% relative humidity, makes another 800 ft of DA, or: 4200 ft.
If it feels really "sticky" outside, just assume 100% humidity & add a thousand feet to your temp-computed DA; this is a very inexact "science," after all.
This trick may prevent your being "surprised" in the future. Best to you.
Well, humidity has an effect on DA and a "field" method is to add 100 feet for each 10% of relative humidity, on top of the DA already computed; e.g.: 3400 ft DA already computed, with 80% relative humidity, makes another 800 ft of DA, or: 4200 ft.
If it feels really "sticky" outside, just assume 100% humidity & add a thousand feet to your temp-computed DA; this is a very inexact "science," after all.
This trick may prevent your being "surprised" in the future. Best to you.
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The rules of thumb are in that fantastic little calculator, if you just ask it properly!
Here is what I got:
0% humidity = baseline
25% humidity = +100 feet DA, and minus 1% power (therefore -1% gross weight to hover.)
50% humidity = 200 feet DA, and minus 2% power
75% humidity = +300 feet DA and -3% power
100% humidity = +400 feet DA and -4% power.
So, for each 25% humidity above 0, subtract 1% from your hover weight.
Why is humidity a problem? Because the water in the air is a gas that displaces the air (mainly the oxygen) and also the water vapor is less dense than air. These two factors mean that the rotor has less mass of air to act on, and the engine eats less oxygen with each cylinder full, so it can burn less gas and produce less power.
Here is what I got:
0% humidity = baseline
25% humidity = +100 feet DA, and minus 1% power (therefore -1% gross weight to hover.)
50% humidity = 200 feet DA, and minus 2% power
75% humidity = +300 feet DA and -3% power
100% humidity = +400 feet DA and -4% power.
So, for each 25% humidity above 0, subtract 1% from your hover weight.
Why is humidity a problem? Because the water in the air is a gas that displaces the air (mainly the oxygen) and also the water vapor is less dense than air. These two factors mean that the rotor has less mass of air to act on, and the engine eats less oxygen with each cylinder full, so it can burn less gas and produce less power.
Last edited by NickLappos; 2nd Apr 2007 at 23:23.
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If you want to try to make your own graphs (say in Excel) then Ed Williams' Aviation Formulae is a great source (In fact a goldmine). I've used it often.
Aviation Formulary V1.43
By Ed Williams
With full respect for his rights, I here's a taster of what you can find there.
Specifically, he has this formula to calculate RH. Of course multiply by 100 for a percentage.
f= exp(17.27(Td/(Td+237.3)-T/(T+237.3)))
or inverted to find dewpoint.
Td=237.3/(1/(ln(f)/17.27+T/(T+237.3))-1)
A related formula gives the increase in effective density altitude due to humidity. It only addresses the reduction of air density, and not the effect on engine power output:
Increase(ft)=0.267*RH*(T+273)*exp(17.3*T/(T+237))*(1-0.00000688*H)^(-5.26)
RH (f above) is the relative humidity expressed as a fraction, T is the temperature in Celsius and H is the pressure altitude in feet.
Examples are:
SL/30C/100% -> 565' increase in DA
10000/5C/80% -> 124' increase in DA
5000/40C/80% -> 977' increase in DA.
In terms of the dewpoint, Td the formula is:
Increase(ft)=0.267*(T+273)*exp(17.3*Td/(Td+237))*(1-0.00000688*H)^(-5.26)
which clearly agrees with the above when T=Td and RH=1.
Have fun.
cl12pv2s
Aviation Formulary V1.43
By Ed Williams
With full respect for his rights, I here's a taster of what you can find there.
Specifically, he has this formula to calculate RH. Of course multiply by 100 for a percentage.
f= exp(17.27(Td/(Td+237.3)-T/(T+237.3)))
or inverted to find dewpoint.
Td=237.3/(1/(ln(f)/17.27+T/(T+237.3))-1)
A related formula gives the increase in effective density altitude due to humidity. It only addresses the reduction of air density, and not the effect on engine power output:
Increase(ft)=0.267*RH*(T+273)*exp(17.3*T/(T+237))*(1-0.00000688*H)^(-5.26)
RH (f above) is the relative humidity expressed as a fraction, T is the temperature in Celsius and H is the pressure altitude in feet.
Examples are:
SL/30C/100% -> 565' increase in DA
10000/5C/80% -> 124' increase in DA
5000/40C/80% -> 977' increase in DA.
In terms of the dewpoint, Td the formula is:
Increase(ft)=0.267*(T+273)*exp(17.3*Td/(Td+237))*(1-0.00000688*H)^(-5.26)
which clearly agrees with the above when T=Td and RH=1.
Have fun.
cl12pv2s
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But I dont think it makes much difference
When I was a flying instructor, I used to produce Bell 47 charts to illustrate this phenomenon. As you say, a very inexact science.
Later I flew IFR on the North Sea, and noted carefully when I flew from a clear blue sky into a cloud. I was careful to not move the collective.
Speed stayed the same. Height remained the same.Torque and all power parameters remained the same.
So I think that it is the aeronautical equivalent of that mediaeval debate of how many angels can stand on the head of a pin.
TK
Later I flew IFR on the North Sea, and noted carefully when I flew from a clear blue sky into a cloud. I was careful to not move the collective.
Speed stayed the same. Height remained the same.Torque and all power parameters remained the same.
So I think that it is the aeronautical equivalent of that mediaeval debate of how many angels can stand on the head of a pin.
TK
From 14 CFR Part 27.45 (and similarly worded in 29.45):
(a) Unless otherwise prescribed, the performance requirements of this subpart must be met for still air and a standard atmosphere.
(b) The performance must correspond to the engine power available under the particular ambient atmospheric conditions, the particular flight condition, and the relative humidity specified in paragraphs (d) or (e) of this section, as appropriate.
(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.
My first question is, how is this accomplished? Paragraphs (d) and (e) state "as affected by engine power." Does this mean the engines are run in a test cell in these conditions? Or is their test cell performance corrected to these conditions. Either way, it seems that the performance flight testing would then be conducted by adjusting the power available to meet the RH requirements.
Second question: Why the different RHs for recip vs. turbine?
It would first seem to me that the performance reduction due to RH has already been accounted for (up to the RHs defined above). But the "engine power" verbiage now leads me to believe that aero perf reduction may not be.
(a) Unless otherwise prescribed, the performance requirements of this subpart must be met for still air and a standard atmosphere.
(b) The performance must correspond to the engine power available under the particular ambient atmospheric conditions, the particular flight condition, and the relative humidity specified in paragraphs (d) or (e) of this section, as appropriate.
(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.
My first question is, how is this accomplished? Paragraphs (d) and (e) state "as affected by engine power." Does this mean the engines are run in a test cell in these conditions? Or is their test cell performance corrected to these conditions. Either way, it seems that the performance flight testing would then be conducted by adjusting the power available to meet the RH requirements.
Second question: Why the different RHs for recip vs. turbine?
It would first seem to me that the performance reduction due to RH has already been accounted for (up to the RHs defined above). But the "engine power" verbiage now leads me to believe that aero perf reduction may not be.
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Tweedles,
The way the FAA/JAA do performance is to divorce the engine power output from the helicopter power needs, and do the two tasks separately. The helo is flown with very carefully measured power inputs against carefully measured speed, altitude, ROC and weights. This tells us what the helo will do, given the power. Then the engine is measured in a test cell against a bunch of atmospheres, humidities and such and its power output is determined. Then the engine inlets are flight tested on the actual aircraft, using pressure rigs to to see what effect the aircraft has on "choking" the engine (causing some need for the engine to pull harder to get its air.) This corrects the test cell engine data for the actual engine installation.
The two data sets are blended when the flight manual is published.
Why recips have different humidities than turbines? I really don't know, but would hazard a guess that the early recips were held to the simpler 80% constant rule, and the turbines came out when more was known and therefore have a more reasonable rule to test to. Later recips could have been held to the old rule just for simplicity of comparison(?).
Note that the change in DA for humidity is nearly in the mud, at a total of only 400 feet from 0 to 100% so the effect on the rotor is nearly nil.
The way the FAA/JAA do performance is to divorce the engine power output from the helicopter power needs, and do the two tasks separately. The helo is flown with very carefully measured power inputs against carefully measured speed, altitude, ROC and weights. This tells us what the helo will do, given the power. Then the engine is measured in a test cell against a bunch of atmospheres, humidities and such and its power output is determined. Then the engine inlets are flight tested on the actual aircraft, using pressure rigs to to see what effect the aircraft has on "choking" the engine (causing some need for the engine to pull harder to get its air.) This corrects the test cell engine data for the actual engine installation.
The two data sets are blended when the flight manual is published.
Why recips have different humidities than turbines? I really don't know, but would hazard a guess that the early recips were held to the simpler 80% constant rule, and the turbines came out when more was known and therefore have a more reasonable rule to test to. Later recips could have been held to the old rule just for simplicity of comparison(?).
Note that the change in DA for humidity is nearly in the mud, at a total of only 400 feet from 0 to 100% so the effect on the rotor is nearly nil.
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I think the difference between turbine and recip revolves around the fact that compression is not a factor with a turbine, and most of the airflow is used for cooling anyway.
However, you would be wise not to try to lift too much after a heavy shower has passed through!
Phil
However, you would be wise not to try to lift too much after a heavy shower has passed through!
Phil
However, you would be wise not to try to lift too much after a heavy shower has passed through!
To be realistic 0-100% makes a difference of 400' as Nick L sez........working backwards via my RFM on the current type, when NOT limited by engine output, that makes a difference of about 20 kg to 25 kg or ~ 50 to 60 lbs (.8% of MTOW)
If this is stopping you from getting airborne I would be worried! In other words if I pulled T/O power and waited about 5 mins the effect would be negated.
Easy fix - stop listening to the wife or better still, throw her out!
Last edited by RVDT; 3rd Apr 2007 at 17:12.
Thanks Nick and paco.
So, if I'm getting this right -
On a nearly saturated 59°F day, I can estimate a 1% reduction in power available, piston or turbine.
On a nearly saturated 109° day, this would more likely be a 3-4% hit on power in a turbine. The piston engine was not required to be tested to anything above standard temperature, but I should be prepared for a similar effect.
These examples assume you are engine limited as RVDT pointed out (adding MAP as a limitation for the piston).
Hot air can handle more water vapor before saturation, so it makes sense that the performance will be degraded to a greater degree.
So, if I'm getting this right -
On a nearly saturated 59°F day, I can estimate a 1% reduction in power available, piston or turbine.
On a nearly saturated 109° day, this would more likely be a 3-4% hit on power in a turbine. The piston engine was not required to be tested to anything above standard temperature, but I should be prepared for a similar effect.
These examples assume you are engine limited as RVDT pointed out (adding MAP as a limitation for the piston).
Hot air can handle more water vapor before saturation, so it makes sense that the performance will be degraded to a greater degree.
