Can someone please help me. I was informed that since I live at the Equator the air is more dense (moisture) hence my plane will require a bigger engine. Is there any thruth in this? I know that altitude makes a difference but does moisture make that much? Thanks!

Denser air means you need LESS power to stay airborne at the same weight.

Hot air is less dense than cold air.

Moisture adds some density.

elevation above sea level has a large effect on density (higher above sea level... less dense.)

Generally... if you are close to sea level the minimum size of the recommended engines by the airplane's instructions will be just fine. (Yes I did say that!)

If you are well above sea level then you will tend to need more runway for takeoff and landing. or some way to get more thrust, or you have to lose some weight. usually just shifting to a longer lower pitch prop helps a lot. At elevatons above 2500 ft... you usually need a bit more power to swing an even bigger prop (the max size engine listed). By the time you get to 8,000 ft above sea level you are wanting more nitro... and a bgger prop and an even bigger engine. (if max says .46... you may actually want about a .60)

REally though... even at the 8000 ft level the plane WILL fly with the minimum recomended engine... it just may have very poor performance.

You CAN get passable performance just by using a different flying style. (Unless you want to hover that 30% Extra 330 L you are dreaming of...) Then you really do need more engine at high altitudes.

****

You notice I didn't say anything about temperatures related to engine choices above? Well... the density altitude problems that come from higher temperatures are almost always associated with flying from higher altitude airfields, on HOT summer days. (some airliners can't get out of Denver more than half full on a "warm" day.... but have no problem full loaded at the same temperature taking off from Hawaii fully loaded.

Huber ... I know about the density with sea levels and hot days ... so from what you are saying, I am like not high from the sea level, maybe just about 200' - 300' so I really do not need a bigger engine. However, it gets totally hot here, say about 98F so maybe that is why the guy who told me I needed a bigger engine due to the moisture was experiencing loss of lift due to heated air. I am sure what I said makes sense! Heheeheee ... thanks dude.

Gets to over 100 here in the summer... and a 4*40 with a .40 will do inside-outside vertical 8's as long as it has fuel. (if the pilot knows how to do it... best string I got was 8 in a row.)

[quote]ORIGINAL: FHHuber

Denser air means you need LESS power to stay airborne at the same weight.

Hot air is less dense than cold air.

Moisture adds some density.

elevation above sea level has a large effect on density (higher above sea level... less dense.)


Dear FH....
One of the above statements is not correct. (Sorry to sound so "authorative"). Air with moisture content actually is less dense than dry air.

The average molecular weight of air is about 29 units. The molecular weight of water vapor is 18. Thus if any of the air is displaced with water vapor, the average density of the combination will be less than dry air. Many folks will say the air feels "heavy" on a humid day. Actually the air mass is less dense, but feels less comfortable.

I agree with everything else you have mentioned regarding engine power at various altitudes and air temperatures. Full scale pilots concern themselves with "Density Altitude", which is a number calculated from the actual height above sea level, corrected for temperature, humidity, etc. It is not uncommon for density altitude at sea level to be over several thousand feet on a hot day.

Bill

Moist air, (higher humidity), is less dense than than dry air. Although liquid water is heavier than air, water vapor is lighter than air. At a given temperature and pressure a given volume of any gas can contain only a certain number of molecules. If the air is moist some of the oxygen, nitrogen and carbon dioxide molecules are displaced by the lighter water molecules. Since the prop produces thrust and the wing produces lift by moving these molecules the result is less thrust and lift. The engine also has less power on a humid day because some of the oxygen molecules needed by the fuel are replaced by the water molecules. In flying full size aircraft pilots are cautioned to be aware of the 4-H; hot, high, humid and heavy.

szdon there's some bad information (as usual) in the answers posted above yours..
For an extensive explanation of the reality...
http://www.usatoday.com/weather/wdensity.htm

Thanks guys ... guess I should have paid more attention in my Physics classes!!!

FHHuber
In your above statement you said "If you are well above sea level then you will tend to need more runway for takeoff and landing. or some way to get more thrust, or you have to lose some weight. usually just shifting to a longer lower pitch prop helps a lot" My problem is with shifting to a longer lower pitch prop. At 8000' above sea level a internal combustion engine loses 30% + of it's power, so you need to prop it for a engine 30% smaller to get the same performance as at sea level. I would say less pitch and maybe a shorter prop to maintain engine performance. I run a Zenoah G62 with a 20X8 and get max. about 7400 RPM at 7500 feet above sea level, the engine is in great shape. Just wanted to add my 2 cents.


Thane

FWIW, humidity does play a part in airplane performance (both FS and model scale), but its effect is marginal compared to the effect of pressure altitude (elevation) and temperature....

In FS airplanes, pressure altitude and temperature are factored into performance calculations, but humidity is ignored, other than on a theoretical level... If it had a significant contribution, I am sure some bureaucrat in the FAA would require its inclusion in performance data.... I never met a bureaucrat that could resist expanding his empire...

Cheers!

Jim