Section 6  Closure

Aerodynamic formulae published in model magazines often contain constants based on a standard atmosphere at sea level. In general the density and Reynolds number predicted using these constants will be too high for model aircraft flight.

If you live at any appreciable altitude above sea level, fly in the summer, and/or in humid air the predictions using sea level conditions can be significantly in error. If you want to make some approximate calculations and you don’t have the pressure, approximate it by the pressure in the standard atmosphere at your altitude but use the temperature you expect to fly at. Try two other cases with the high/low percentage pressure variation.

Look at the graphs for dry air again and remember that humidity can reduce density by 1% or 2%. This reduction also applies directly to Reynolds number. Engine power - internal combustion engines at least - will be affected even more. If you fly electric powered aircraft the hit won’t be so bad.

If you like to look at numbers or use computers the program ATMO99 written for the Microsoft Windows(R) operating system, described in the section ATMO99 Computer Program may be of interest.

The Atmosphere you fly in plays an important part in the performance of your model, but even though high altitude degrades performance some models have gone far beyond the range of variables discussed here. Maynard Hill’s radio control model aircraft world record, set in 1970, is 26,900 feet. The outer edge of performance. You need a government lab to measure the altitude and to help see the model to fly it. Only model designing, building and flying skills, however, will help fly in the zero or sub zero temperatures and density at that altitude is only 42% of sea level standard. Even the free flight helicopter word record set in 1963 by Stefan Purice of Romania is 12,300 feet! You might like to guess how he did it.

Significant increases in altitude above the current record may not come this century not only because the because the cost and technical challenges are legion but also interest in setting a record when you can’t see your model fly is certainly low.

How hard it is might be realized by examining well-financed efforts of somewhat larger aircraft. For instance, consider the slightly amusing but, as Wayne would say, “Bogus and also sad”, experience of the 1.5 million dollar NASA Perseus project: Perseus, somewhere in size between a small man-carrying glider and a giant scale model, was called a “pathbreaking but controversial effort to use inexpensive robot aircraft ...” ( Los Angeles Times, December 4, 1994). although it was promised to fly to 98,000 feet, so far 35,000 feet is the actual achievement, but it broke apart in midair there and records require the aircraft to return. They shoulda asked Maynard for advice. (Meanwhile we need to redefine “inexpensive”). In terms of dollars per foot, Maynard Hill’s achievement, on view at the AMA museum, is all the more impressive.

Aside from world records, what would be interesting is to hear from modelers who fly in extreme conditions. I have flown in Yellowstone Park at 7000 feet (I lived there at the time and flew gliders. Not advisable for power models!). In Alaska, doing time in the Air Force during the Korean war, I gave up flying when winter really set in, but surely there are many who fly on under such daunting circumstances. Estimate the density altitude at your flying site during the climactic extremes of the year. If you fly very small models or use wind tunnel data, check the variation in R_e.