Sport Pilot Talk

The results would be the same if the field level pressure was 29.92. If it's lower or higher, then the takeoff distance will be longer or shorter as a result. Lower than 29.92 means less dense air and longer takeoff distance. Higher than 29.92 means more dense air and shorter takeoff distance.

Non-standard pressure would be accounted for in the AWOS’s reported Density Altitude, right?

FastEddieB wrote:Non-standard pressure would be accounted for in the AWOS’s reported Density Altitude, right?

Yes, as well as non-standard temperature.

On further reflection of your Denver example, you couldn't find 8,400' density altitude on the ISA line the best you could do would be 8,400' pressure altitude. No?

FastEddieB wrote:Non-standard pressure would be accounted for in the AWOS’s reported Density Altitude, right?

Yes. I think density altitude is pressure altitude plus or minus factor for non-standard temperature. So, hotter than standard would be even longer take-off distance and cooler than standard would be shorter take-off. You could back calculate from density altitude to pressure altitude (if you know how), by factoring in the non-standard temperature. Also, humidity factors in there somewhere. More humid, longer takeoff. The way I think about that is a molecule of water (H2O) has molecular weight of 18 and a molecule of oxygen (O2) has molecular weight of 32, so moist air is less dense than dry air.

The charts he posted start with pressure altitude, then factor in temperature. This, in effect, calculates density altitude. They don't tell you or show you the density altitude, but that's what it is.

EDIT: I guess his charts are constructed that way because you might not always know the density altitude if you don't have weather reporting at your airport. However, you could always determine pressure altitude by setting your altimeter to 29.92 and you would always know the temperature.

JJ: Thanks for posting this. This has been a refresher for many of us who might have forgotten most of this stuff by now.

TimTaylor wrote: The way I think about that is a molecule of water (H2O) has molecular weight of 18 and a molecule of oxygen (O2) has molecular weight of 32, so moist air is less dense than dry air.

That's true in principle, but the relevant comparison is of H2O to Nitrogen (which comprises about 78% of the atmosphere). Since water vapor is less dense than gaseous nitrogen, the higher the relative humidity, the less dense the air, and the higher the density altitude. (The molecular weight of oxygen has nothing to do with it, since there's so little oxygen in the atmosphere.)

You're right, but O2 is 21 percent of our air so it does have something to do with it.

https://whyfiles.org/2010/the-weather-g ... index.html

How about this? "Oxygen helps, but Nitrogen rules!"

drseti wrote:How about this? "Oxygen helps, but Nitrogen rules!"

Too bad NASA didn't figure that out before Apollo 1. And yes, I'll give you 4:1 odds on that.

To further confuse everybody: the molecular weight of water vapor (H2O), as Tim stated, is 18 grams per mole. That of molecular nitrogen (N2) is 28 grams per mole. This just might be on the final.
Class dismissed.

drseti wrote:To further confuse everybody: the molecular weight of water vapor (H2O), as Tim stated, is 18 grams per mole. That of molecular nitrogen (N2) is 28 grams per mole. This just might be on the final.
Class dismissed.

And O2 is 32, so air is approximately ((.78 X 28) + (.21 X 32)) / .99 = (21.84 + 6.72) / .99 = 28.56 / .99 = 28.85

Divided by .99 because air contains 1 percent other gases.

This was for extra credit. PS: I hated chemistry, but loved physics.

That was good for 10 bonus points.

PS: I loved teaching physics.