------ Actually downthrust does come into play in engineering equations in terms of pitching moment. All wings produce a forward pitching moment downwards because of the reaction to downthurst from the back of the wing. This is one of the reasons you need a tail stuck out on a long boom behind you to fly straight. Or a canard out front to keep the nose of the plane from dropping. This is the reason some of the early homebuilt canards ran into trouble in heavy rain. The rain stalled the canards (too small in area) and the nose pitched down in an uncontrollable dive. ----------

Well the pitching moment is due to the integrated pressures above and below the wing. As a result of those pressure fields the air as it leaves the wing will flow down and we have downwash, but, the wing is reacting to the forces that are applied on it. Those forces (as banktoturn indicated) can come from only one thing, the pressure differential. What happens first is you get differential pressures and as a result you get downwash. Just follow the time history of a molecule from front to back.

------ So the question is then: How does a flying wing work? No tail out back, no canard up front. Stability and control in a flying wing is a real design challenge. It has a great deal to do with airfoil design. --------

Actually it is quite easy, put the CG at 20 percent and reflex the elevons. I have several little electrics that are flat plate and follow that advice. Stability is only the relationship of the CG to the Neutral Point of the thing flying, in a flying wing it is located at 25 percent of the mean aerodynamic chord. The pressure differential does give a nose down moment about the CG which is at 20 percent. Counter that with some reflex in the airfoil or elevons and it is a done deal. Nothing harder than that.

------ I think the practical solution to my question is exactly as described by banktoturn. Just pick an airfoil that works. I am just curious about scaling. Wish I could find that missing article. ------

I try to push the concept that the vast majority of model designs that have been flying over the last 40 years are a vast source of knowledge. It is better than having a wind tunnel test data set. Pick something that is working very well and copy it whether a flying wing, canard or pattern ship. The evolution into today's RC airplanes has produced a fine bunch of flying machines. Copy and enjoy.

In terms of Reynolds number effect on airfoils its effects can be kinda summarized by looking at what flies. Dicks (and mine) flat airfoil which work really fine at small sizes and lots of power, symmetrical airfoils of 10-18 percent thickness for a 60 size model. The big 30-40 percent scale monsters can use the full scale airfoil. In general the only downside of not using a perfect airfoil for the given Reynolds number is that the airplane might be draggier and the stall might be off. In our models - who cares, as Dick said, just add a little power and power through it. A lot of the 3D maneuvers are done with a fully separated wing.

The Selig airfoils are a good example of airfoils optimized for model use. Think about it, we go to dentists and doctors that get really picky about what they do and the medicine they give you. They don't just stand back and throw a hand full of different pills and say that it doesn't matter, just take the ones that hit you. Aerodynamics is a science which includes the very low speed through the multi-sonic speeds - including the speeds our models fly at. The science can and will predict airfoil behavior and performance if you have the right tools. There is no need to dumb down the science by saying that some airfoils or model sizes don't fly according to the "Laws of Aerodynamics". Not true, what is true is that we may not understand the laws governing the flow dynamics in question.