AA5BY

I completed a 96" 16.5lb plane powered with a 35cc running a Vess 20A at the end of April and have flown it now for two months. It performs fairly well and yields large loops and will climb vertical as high as I normally go for a hammerhead or spin. Well, yesterday it performed better than ever... yielding nearly unlimited vertical climbs and this very shortly after takeoff carrying the full fuel load (16 oz).

The most notable difference was a lower humidity yesterday, though the temperature was about ten degrees fh warmer.

Is it typical to get better performance with lower humidity? Is the variation seen yesterday an indication that the engine was not tuned optimally for the more humid conditions or is poorer performance in humid conditions normal?

RCVFR

My Feedback: (7)

Higher humidity is less dense air than lower humidity. Water vapor is less dense than dry air, thus the dryer air can hold more oxygen than humid air. So, aircraft benefit from lower humidity two ways: more power from the engine (more oxygen) and more dense air to support flight.

Zeeb

My Feedback: (41)

Hmm.....

It's called "density altitude" and combines a field elevation, ambient temp, barometric pressure and although not normally figured a lot, humidity.

On full scales, they are rated for performance based on a "standard day" which is 59 degrees F, barometer at 29.92 and Sea Level. Change those parameters and the performance changes. Generally speaking; raising the temp decreases performance. Raising the field elevation decreases performance, raising the amount of water in the air (humidity) decreases performance because it displaces air molecules.

So it would seem you were mis-tuned if your model flew better with it being a hotter ambient temp and a lower humidity.

Please remember that you may not see a lot of difference in the model as the weather changes but it can make a HUGE difference in a full scale operating in mountainous areas in the summer...

raptureboy

Yep. I flew choppers in Colo in the Army and our lifting capacity was greatly reduced on hot days at 5,000 ft base elv. It's the same for glow also, tuning changes in hot or cold weather high/low humidity

earlwb

Out here in Texas we get a lot of performance loss out of the engines, heat sag, especially glow engines. In the summer heat with temperatures going well over 100 degrees F and humidity exceeding 50%, the engines just won't run well at all. Some days you wind up running the engines extra rich and they just don't have much power. If you tried to tune them more lean to run better, they just overheat on you. So if you hadn't overpowered your plane to start with, you may not be able to fly it.

Electric planes can have problems too. The electronic speed controls can get hot enough to trip their temperature overheat circuits or logic and the ESCs will refuse to run the motors.

Gliders would have been a good way to go, but when it gets hot like it does, the thermals all but disappear on you too. But I remember one day having a glider stuck in the thermal zone about three feet off the ground. I could fly large lazy circles as long as I wanted to as long as the plane stayed about three feet above the ground. I had even watched some of the birds above in the sky and they couldn't even find a thermal either.

One other thought is the higher temperatures tend to reduce the lift as well, so a plane with a higher wing loading may exhibit wing stalling more when it is really hot outside.

Truckracer

My Feedback: (19)

Density altitude and all the things that go into that equation can have a huge affect on airplane performance. Anyone that has ever tried to get off the ground in a heavily loaded J-3 on a hot summer day would give witness to that for sure!

But removing most or all of the variables, humidity by itself can play a huge roll in engine performance regardless of what that engine powers. If you ever race anything, you soon learn that with all else being equal, water in the air can cause huge changes to engine tune. Add water to the air and the engine goes rich as there is less oxygen to burn. Remove water and engines go lean as you don't have enough fuel for the available oxygen level. High performance race car engines can go so lean as to cause damage such as burned pistons. Our RC gas engines are no different and major changes in humidity levels will require some needle adjustments. Performance changes are normal in this case.

AA5BY

Though I've sensed in the past that cooler temps offer better flight performance, I guess I'd never flown the same plane enough in varying weather to make the kind of comparison seen yesterday. Though it was a few degrees warmer, it was significantly dryer and the performance was notably better on my gas engine. I asked one of the glow profile guys who is a very good pilot and a retired airline pilot if he sensed much difference yesterday and he hadn't.

After reading the explanations... I'm now wondering if the barometric pressure played a part along with the dryer air and together they produced better performance. The glow guy wouldn't sense any change in barometric pressure because of no metering circuit in carb that depends upon it.

If the pressure was greater (most likely given our weather) the carb mixture would have gone slightly richer due to the influence on the metering circuit. At any rater, the engine was tuned back at the first of May when temps were cooler and perhaps should have been adjusted richer after temps moved from low seventies to upper 90s. Perhaps I've been running a touch too lean the last month.

AA5BY

Ooooops... forgot to express appreciation for those who offered insight... much thanks!

pe reivers

like truckracer said. The dry air caused your engine to lean out. This can cause a very noticeable power increase if your engine was tuned a couple 100 rpm rich off peak power.
General rule:
More humidity makes your engine run richer, reduces power (less oxygen)
More heat makes your engine run richer, reduces power (thinner air, less oxygen to burn)
more altitude makes your engine run richer. (thinner air, less oxygen)

and vice versa.

MTK

To supplement these thoughts, in hot, humid or high altitude environments that require leaner settings, it doesn't mean you will get more power out of the engine. That's especially when compared to same engine running in cooler, drier air or lower altitudes. Why? Less oxygen AND less fuel (stoichiometric) mean less power. The engine must move fuel mass and air mass through to create the necessary torque with the appropriate heat generated by "full" combustion. The more mass per rpm an engine can move through itself at stoichiometric conditions, the greater the power it can generate.

That's why superchargers work. Or other materials that bring in more oxygen (nitrous oxide means more fuel). It's more complex than that but that's the basic concept

Truckracer

My Feedback: (19)

The key phrase that allows dryer air to produce more power is "all else being equal or nearly equal". When you change several variables, all bets are off and dryer air may not necessarily produce more power.ather would be ideal.

Bass1

or when taking off from an extremely short, muddy field on an oppressively hot and humid day in an old 1850 SMOH Cessna 140 with your overweight girlfriend full of fried chicken, potato salad, bisquits and apple pie ala mode with huge pine trees to clear at the end of the runway[X(]

captinjohn

http://www.csgnetwork.com/vaporpressurecalc.html ....kinda interesting calculator.

http://www.ringbell.co.uk/info/humid.htm......another calculator

http://www.omega.com/ppt/pptsc.asp?ref=RH87&Nav=temhu02 .....a neat device

karolh

The difference that altitude and humidity makes is no stranger to me as I fly from two fields, one at sea level and the other at approx. 3,000', and the carbs require leaning at the lower altitude and richening at the higher one. Needless to say that at sea level the engine makes about 300 more rpm.

Karol

RCVFR

My Feedback: (7)

That's unusual. Full scale take off at richer setting and lean it at altitude.

av8tor1977

My Feedback: (6)

He's got that backwards. It should be "richen at low altitude and lean at high altitude". Most full size engine manufacturers recommend leaning for take off at density altitudes above 5000 feet. Below that it is full rich for take off and lean out after power reduction and/or upon reaching altitude.

AV8TOR

AA5BY

Not so sure Karol has it backwards as it seems to me that the higher the altitude, the less force on the metering diaphragm to open the metering needle. That means the metering circuit is producing less pressure/flow so the HS needle would need opening to produce the same fuel to air mixture. In this scenario I'm not factoring in any altitude variance in fuel/air mixture however.

av8tor1977

My Feedback: (6)

TOM once said that the Walbro carbs were self compensating due to the very fact you mention I believe, up to about a 6000 foot elevation variance. I can't verify this personally, but TOM certainly knows his stuff....

As a norm however, an engine will need leaned out as altitude increases, and enrichened as altitude decreases.

AV8TOR

captinjohn

Karolh.... is talking 2 cycle engines....full size airplanes have 4 cycle engines. Another thing to think about is air temperture. It could be a lot warmer where he flys at sea level & a lot cooler at 3000 feet where his other RC flying is done. Capt,n

av8tor1977

My Feedback: (6)

2 cycle, 4 cycle, it doesn't matter. The effects of density altitude exist and affect carburetion. There's lots of density altitude calculators on the web to experiment with temps and altitudes. It is an eye opener for sure...

AV8TOR

RCVFR

My Feedback: (7)

TOM is an authority whom I greatly respect. Do you suppose he was talking about self compensating for atmospheric pressure changes? Seems like a valid observation to me. Do you want to also sweep changes in temperature and humidity in that "quote"? I'll believe that when I hear it from TOM.

av8tor1977

My Feedback: (6)

Doesn't matter if you go up "X" thousand feet, or if the temperature thins the air out to be the same as that "X" thousand feet, the result will be the same on the engine. It's called density altitude, and that is what affects engines, and props/airframes as well. All full size aircraft manuals fully document this and give performance and fuel consumption figures for varying temps, altitudes, etc.

Just play with one of the density altitude calculators and you will see what I mean.

AV8TOR

Q500RC

Hey Capt! This is a bit off topic but could you PM me please I would like to ask you a few questions on ST90 gas conversions and when I try to PM you it won't let me! Thanks

AA5BY

If it matters that one must be careful to avoid a cowled engine where the pressure goes high, then atmospheric pressure changes matter on the tuning. IIRC, a high pressure in the cowl requires leaning the engine at full throttle when the pressure goes up in the cowl but then when coming off the throttle and the pressure drops, the engine is too lean. That would mean that lowering the pressure such as at greater altitude would require richening.

Factoring in the needled changes needed for air density/fuel ratio... it might then become a plus and minus thing with what issue is more dominant and thus what is the net effect. One cannot ignore that walbro carbs use atmospheric pressure to provide a reference to which the needles are set.

pe reivers

The fact that you gain 300 rpm means other factors than just altitude play a role here. All other things equal, rpm should be the same at sea level or at altitude.