Airstrip: Newsletter Challenge (06/07/05)

Watch out for wind shear!

Cold-dense good, humid-dense bad. Although air may be heavier and denser, it will also have more drag. This on top of diminished engine power in warm air adds up to a thumbs down for take-off.

Atomic weight of water is about 18 (16 for oxygen and 1 for each hydrogen). Air is mostly diatomic nitrogen with an atomic weight of 28 (2 x 14) plus diatomic oxygen, atomic weight 32. Air laden with humidity therefore is less dense than dry air and for the same temperature will give slightly less lift. Humid air only feels heavy because if provides less evaporative cooling and we sweat more easily.

Air density is more a function of temperature than humidity. Lower temperature=higher density. Further, higher humidity usually=higher temperature=lower density.

Because the factors used to determine the takeoff distance required for this aircraft are missing; Pressure Altitude, Temperature, and what the conditions were in the past etc.. We must assume that the only thing that has changed is the humidity. If we do this then the performance will not be affected because the performance is dependent on the pressure altitude and the temperature, and humidity plays no role in the performance, with the exception that the extra humidity may in reality increase the performance of the engine, because the additional water in the air will produce steam during the combustion cycle "if" it is a piston type engine. This was used in some of the old radial engines to increase takeoff power and it was rightly named "water injection".

The density of air is also a function of the composition of air. Humid air at 70 degrees F can be as much as 2% less dense than dry air at the same temperature and pressure. If you recall Avogadro's number, the Ideal Gas Law etc. the density of air is dependent on the atomic weight of the molecules it is composed of. Which is why helium balloons float.

Also, water injection was used for temporary boosts of power by both radial and inline engines in WWII that were supercharged (or turbo-supercharged). I think the effect was comparable to increasing the intercooler size allowing more fuel and air to flow through the engine. If the engine wasn't supercharged, I don't think water injection would apply. In any case, in humid air, the water is already a vapor (steam) so it would not boil converting thermal energy to mechanical.

As written in the 6/14 issue of Specs & Techs from GlobalSpec:

You say the forecasted humidity is exactly why you are concerned. Most people know that water is heavy and moisture is water, concluding that humid air is denser than dry air. But think about it: air is primarily nitrogen (N2) and Oxygen (O2) and if you introduce a lot of water (H2O), you can see you are introducing water molecules that are of considerably less molecular weight. So humid air is materially less dense than dry air. Therefore, your friend should wait until conditions improve — ideally when a dry, cool air mass comes into town.