Originally Posted by
JPerrone
For HarryC and others more technically inclined:
I usually try to avoid getting to technical as there's a very wide range of knowledge in these forums, easy to get lose in it. Yes, there is only one set of laws of physics. But when we compare models of objects, even if they are "exact scale" models, they don' behave the same.
Dynamic similitude requires that kinematic, dynamic, and geometric similarity all exist simultaneously. When you take a model into a wind tunnel, or a tank, you can get one set of dimensionless parameters to match (eg, Reynolds number), but then another one (mach number) is not representative. The smaller the scale, the worse it gets. That is one of the challenges in wind tunnels, getting a model big enough to be close to similitude, without being prohibitively expensive.
My memory is fuzzy on this as it's been a number of year, but I recall reading about the difficulties in doing hypersonic testing (mach 20); eventually they went to full scale testing rather than rely on scale models. Results on that weren't encouraging.
But what I said was, and I quote
"The laws of physics do not recognise "model" or "full-size". There are differences due to size but the difference between a large model and a full size Cessna 150 is a lot smaller than the difference between a full size C150 and a full size Boeing 747, so the phrase that models are different to full-size which is often trotted out, is just plain wrong."
I said there are differences due to size but that is all, the laws of physics do not recognise "model" or "full-size", show me where such a parameter exists in any equation. A large model is very similar in character to a full-size C150, much closer than the full-size C150 is to a full-size B747, hence the phrase that "models are different to full size" is nonsense. The fundamentals of piloting a model are the same as full-size.
To do a turn you bank with aileron and hold the height with elevator. If the plane is travelling a bit side on to the direction of travel you use rudder to keep it
straight on to the direction of travel, you don't use the rudder to turn it like a boat which is a popular misconception amongst model fliers.