Sorry if this is a dumb question, but could someone please explain reynolds numbers to me? Or a link to a good website? Thanks, Hans

Reynolds numbers are a reference for describing the viscosity of air, relative to our models. Ollie or someone can give a more precise explanation, but it basically boils down to this:

We can make our airplanes any scale we want, but air molecules don't oblige by changing their size. At a model size, the air molecules are effectively "larger".

So...
What is true, in terms of airfoil performance on a full-scale plane is not true for a model-size airfoil. Andy Lennon's book "The Basics of Model Aircraft Design" has some good information that will help you apply this fact. (Model Airplane News has the book.) When you choose an airfoil for a model, it's good to know how it performs at different Reynolds numbers. I don't try and memorize these things... Just look them up when I need them.

Reynolds number is density times velocity times length divided by viscosity. Reynolds number is useful in comparing flow conditions. If the Reynolds numbers are the same then the flow will be the same around similar shapes. In a standard atmosphere at sea level the air density and viscosity are fixed and that leaves only the length and velocity to be compared. A particular airfoil will have exactly the same flow conditions as anothe example of itself of a different size has the product of chord times airspeed the same. For example, if a 10 inch chord NACA0009 at 50 MPH is compared to a 20 inch chord NACA0009 at 25 MPH, the flow conditions will be the same. that means that the lift coefficients and drag coefficients will be the same for the same angle of attack. It also means that the boundary layer thickness distribution along the chord will be the same and the location of the stagnation point will be the same. Also, the downwash and upwash patterns will be similar. The transition from laminar to turbulent flow will occur at the same percentage of chord and the seperation of flow will be at the same percent of chord.

However, if two examples of the same airfoil are operated at different Reynolds numbers the flow conditions may be somewhat different and the greater the difference between the reynolds numbers, the greater will be the differences in flow conditions. At model Reynolds numbers the rate of change of flow conditions with changes in Reynolds number is much greater than it is at full scale.

Perhaps a bit of overview is in order here. Reynolds Number belongs to a class of dimensionless numbers of Fluid Mechanics. These unitless numbers are used for studying and comparing physical phenomenons at various scale factors. They are useful because one can build a scale model, which despite being physically smaller has similar dimensionless numbers as the full size counterpart.

An example is that one can study the aerodynamics of a scale model aircraft and relate it to the full size airplane by adjusting the parameters to achieve the same Renolds Number, Re, as the full size plane. Since:

Re = Length x Velocity / Viscocity

if we have a 1/8 scale model in a wind tunnel, we can crank up the airspeed to 8 times the normal flight regime of the full-scale aircraft and achieve the same Reynolds Number. Then the tests will fairly accurately predict full scale results.

Likewise, we can keep the airspeed the same and use fluid that has 1/8th the viscocity, and the resultant Reynolds Number will still be the same.

I remember when I took my Fluid Mechanics class, Reynolds number was the big one, since it's applicable to constrained (pipe) and general fluid flow. Others D-less numbers mentioned as commonly used are Weber's Number and Rayleigh's Number, which are useful for surface ship design, and Euler number. Drag coefficient, Cd, is also another common D-less number.

Thank you for the replies. Now I'm thirsty for more. Have any suggestions for good websites? I have already read quite a bit in the Glenn Learning Research Center at NASA's site. Very informative, but quite basic. Any others? I am 17 now and have been fascinated by airplanes since elementary school. I will probably take related classes in college, but would like to learn what I can now. Thanks Hans

Doing a search on the web for "dimensionless number fluid mechanics" will net you plenty of material to chew on.

Truthfully though, a lot of the material you'll likely encounter will require a basic foundation of engineering curriculum to digest fully. It is great that you are very interested in taking college classes. Go for it! It'll surely lead to an exciting career.

That is, if you don't end up designed a single turbine blade for 15yrs.