Section 2 Overview

The atmosphere is the natural element of all things that fly. This article covers the physical properties of the atmosphere affecting model aircraft flight characteristics; for example stalling speed, turning radius, engine power required and developed, and the bonndary layer and flow properties that depend on the boundary layer.

This article will interpret these effects in terms of atmospheric density, Reynolds number, and humidity. Meteorology, which includes dynamic properties like winds will not be covered, but an understanding of meteorology requires a knowledge of physical properties of the atmosphere.

The dry atmosphere is composed primarily of Nitrogen (78.08% N₂) and Oxygen (20.95% O₂). The less than 1% remaining is mostly a Argon. The CO₂ component (which may produce global warming) is only 0.03%.

This composition, amazingly, is essentially constant over the whole earth from sea level to an altitude of about 250,000 feet! Although the molecular composition, except for water vapor, is constant, the density, temperature, and pressure are not.

As a reference point, at sea level a cubic yard of air weighs a little more than two pounds. Helium under the same conditions weighs about 0.27 pounds which accounts for the buoyant lift of balloons, blimps, and dirigibles. The ’real’ atmosphere contains a small proportion of water vapor, which will be discussed later.

Many articles in model aircraft-related publications contain formulae to compute aerodynamic forces; drag and lift for example. Usually these mathematical relations have been stated in what one might call “model builder units", such as inches or miles per hour, and involve several numerical constants.

Most of these “constants" are not really constant, but refer to sea level conditions in something called a ’standard atmosphere’. The conditions in which you fly models will vary from this standard, sometimes by a significant amount. Here you’ll find out what the ’standard’ is, how to determine the right numbers for your flying conditions, and what the differences mean.

In the early years of aeronautics aircraft performance was difficult to pin down due to a combination of promoter’s optimism and variations in mechanical and atmospheric conditions when different tests were made. By the 1920’s aircraft performance predictions in were made more useful by referencing them to a ’standard atmosphere’ - a specification for pressure, temperature and density versus altitude.

This standard has been extended, revised, and adopted internationally. It has profited from a great deal of research, culminating in the current international standard as specified in the US Standard Atmosphere 1976. The model for the atmosphere below 80000 feet is identical to the ICAO standard, an international standard used by all the airlines and aeronautical services in the world.

The standard atmosphere, however, is based on a night-and-day average of conditions and a Northern U.S. - European model for temperature. The gravity model is based on a North latitude of 45 degrees 32 minutes;. The standard sea level temperature of 59 ^oF was chosen, in part, because it converts exactly to 15 ^oC.

The standard atmosphere provides a model for altitudes from below sea level to 1000 km (i.e. about 3.3 million feet), but sea level conditions are usually quoted. The standard temperature decreases steadily from sea level 59 ^oF to a temperature of -69.7 ^oF in the stratosphere, which starts at about 36000 feet.

For example the approximate altitude of Colorado Springs is 6000 feet. The standard temperature at that altiotude is about 38 ^oF. So the standard atmosphere is both too cold and too dry to represent an average flying day for modelers.

The ’standard’ pressures for a given altitude can be higher than the true average at many locations. It is useful as a comparison reference and that is how it will be used here.

The effects of the atmosphere on model flight (subsonic speeds are assumed) will be discussed below in terms of density, viscosity (Reynolds number), and humidity.