I'll want to learn more about Reynolds numbers (I expect I can do that without troubling you), but thank you for the info.

Wow! You sure seem to know your stuff!

From : Don Stackhouse

Well, at least enough to be dangerous!

If I remember correctly there's some articles in the "Ask Joe and Don" section on our website that discuss it. Try the design section, look for an article about why scale models don't fly well at scale weights, or something like that.

Reynolds numbers ("Re" in engineering jargon) are the numerical form of what modelers often refer to as "scale effect". What this means is that when you make the model smaller, you aren't also making the air molecules smaller. A molecule that feels proportionately like a ping pong ball to a full scale aircraft will feel more like a basketball to a model. The job of burrowing through a pile of ping pong balls is totally different from the job of burrowing through a pile of basketballs.

To calculate Re, multiply air density times speed times some characteristic size (typically the chord of the wing, although we can also figure different local Re's along the span, or at different points along the chord), divided by air viscosity. It's the ratio of inertia effects (the forces involved in moving the mass of the air out of the way as you go by) to viscosity effects (skin friction). I saw a dramatic demonstration of the effects of Re in a movie in one of my fluids classes in college. They took a teardrop shape (streamlined, which reduces inertia drag) and a sphere of the same diameter (which has less surface area than the teardrop) and dropped them simultaneously into water. Since the Re's in water are relatively high (its density is relatively high compared to its viscosity), the teardrop fell to the bottom much faster than the sphere. When they tried the same experiment in glycerine ( similar density but far higher viscosity, therefore much lower Re's), the sphere (due to its lower surface area and therefore lower skin friction) reached the bottom long before the teardrop!

For Re of your model at sea level standard conditions, multiply speed in MPH times chord in inches times 778. At higher altitudes the Re goes down, because although the air is less dense at higher altitudes, the viscosity stays about the same. This is part of the reason why a model that flies well in San Bernadino might not fly as well in Denver.

Just as a curious observation, it seems odd that the slower you go and the thinner air you fly through, the more drag there is. Seems like that would mean that the faster you go, the easier it is to go even faster. And if you really wanted to go fast, you would find the densest possible air to plow through. I'm sure I'm missing something here and I will take it upon myself to figure it out.

That's sort of correct from a certain point of view. The proportions of things change. For example, denser air makes more lift AND more drag, but the ratios between them (L/D) will be better than in thinner air. Of course the denser air means you need to fly at a lower airspeed to make the same amount of lift, but since lift is proportional to the square of airspeed, while Re is only linearly proportional to airspeed, the Re at your reduced airspeed will still be higher than in the thinner air at the higher speed. Your max lift coefficient will therefore probably be better at the lower altitude, as well as your L/D.

For most larger models it's usually not a huge difference, but it becomes an increasingly huge factor at Re's below about 100,000. This just happens to be the vicinity where most 1.5 meter R/C HLG's operate, and Mosquito class models (with their thermalling Re's as low as the 20K or 30K range) are even worse. Also, since most of the research (even by model aircraft specialists) has been done at Re's well above this, much of what you read in the books doesn't work anymore at very low Re's. It tends to make life very interesting. Some of the toughest model designs I've ever attempted have been for the Mosquito class. It's very easy to fool yourself, but very difficult to get it right.

Don Stackhouse
DJ Aerotech