Good question, Jim. This discussion has passed over some of the basics.

To make a model visually appear to be travel at a realistic speed, you have to scale the relationship of size and speed. As you suggested, this means that the model would travel the same number of fuselage lengths per second as does the real plane. Then it looks right to the eye as it flies by. To accomplish this requires that the model weight be scaled by a definite ratio to the real plane. Without going into the math, it works out that "fourth power scaling" will get you the appearance of scale speed. As an example:

If we want to make a DC-3 model that is 1/12 of full size, the weight needs to be 1/12 X 1/12 X 1/12 X 1/12, or 1/20,736 of the full size airplane's weight. Since the weight of a full size DC-3 is around 35,000 lbs, the model would need to weigh 35,000 / 20,736 = 1.7 lbs to fly at scale speed with a similar stall margin.

That's why we don't see many models flying at scale speeds.

There is a further complication in this weight scaling business. If a model is "fourth power scaled", it only looks right in straight flight. A model scaled to fly at that speed will make non-scale maneuvers. For instance, a turn at any given bank angle will have an unrealistically small radius. If you were to scale a fighter or aerobatic plane, the loops would look impossibly small compared to the size of the model.

In order to make the size of your maneuvers look right, it is necessary to do "third power scaling". That is, the model weight would be 1/12 X 1/12 X 1/12, or 1/1,728 of full scale. That would be 35,000 / 1,728 = 20 lbs. But then the speed looks too fast.

The bottom line is that there is only one scale ratio which gives a perfect match of both speed and maneuver size. Unfortunately, that's full size.

The best we can do with scale models is somewhat of a compromise between visual speed and maneuver size. That would be about "three point five power scaling". If you have a scientific calculator, you can do that. In the case of the 1/12 scale DC-3, the weight ratio would be 1/5985, or 5.8 lbs.

I'm sure there will be folks who will disagree with this analysis, but "the physics don't lie". There has been a lot of pretty good work done in this area over the years. An example of where serious scaling analysis is used would be flight testing of spin models by NACA and NASA.

Dick