In regard of loop formation and loop shapes and sizes I several times met fly casters who indeed tried to convince me that they "can cast a much smaller loop when side casting, because the upper loop leg does not fall that easy on the lower one". Well, they obviously had no clue what loop size actually is about. The loop size is defined by the distance of the upper to the lower leg in a vertical position. If you cast sideways it is more or less the distance from the rod tip to the upper leg measured in a 90° angle.
I call this false attempt of casting a smaller loop nothing else but casting a cheated loop.
And YouTube is full of them! Such a loop might look quite small if you look at it from some distance from a 90° angle to the casting direction but if you place yourself in some distance in front or behind the caster and look at his loops in a line with the casting direction you will see their real size. Such pretty-looking loops that the inexperienced caster would eventually call sexy can turn out to be easily up to 2 m in size if you look at their real size from a point that enables you to look right into the loop.
When teaching well-advanced fly casters I always force them to shape down the loop to the minimum by casting vertically (!). By doing this the loops turn into real loops (in regard to their size) so you can easily see your progress.
If one wants to move on to get closer to master like fly casting one has to reduce the input by still getting the same result. This means a perfect presentation to a target with the least possible input that would finally result in significantly fewer spooked fish. To reach this goal the casting speed has to be reduced and the loop has to be tightened. If one performs that exercise correctly, small U-shaped waves will show up at the end of the fly line. I have been often asked where they come from. I tried not only to explain but also proof my explanations with an experiment that we made in a laboratory specialized in aerodynamics in 1999.
The experimental arrangement should reproduce the wanted effect continuously. Therefore two pieces of well-visible knitting wool were put through an iron ring and knotted end to end. That endless loop was blown at by a very pointed air beam positioned behind the center of the iron ring so the loop was speeding up. The higher the speed the smaller the loop got. Suddenly the wanted little waves showed up (pictures). The shape of the loop could be changed by altering the pressure of the air beam or creating a wall from the front side with your hand.
Experimenting with Aerodynamics
What is the reason for these waves?
The upper leg of the loop of an enrolling fly line always needs to move faster than the lower one in order to keep the line enrolling. This is in my opinion one of the most important fly casting principles. If you now go down with the casting speed and keep the loop as tight as possible it gets more and more difficult for the parts of the fly line to move around "the invisible wheel" that keeps the line unrolling. The waves that travel through the line show up again near the end of the fly line and the tractive forces start fighting with the stiffness of the coating. The speeds of the two legs finally reach a critical point and the upper line starts pushing against the parts in front of it. The last meter of the fly line starts to crimp. I call this the accordion effect.
To get rid of these little waves one just needs to widen the loop a bit or to speed up a little. Both helps. Especially triangulated fly line tapers and such with very thin ends flatter more than others if you perform this exercise.
You might ask now for the reason to cast such extremely small loops. Well it is a good training exercise on one hand which helps you to cut a front wind better if necessary. On the other hand it is a perfect one to get better line control, too. You will also find out about the minimum loop size for your fly line or if the front taper of the fly line was not cut correctly and is a bit too long.
There is only one negative point: You will work down your fly line much faster as it gets more and more micro cracks. Smallest loops are line killers, but they also help you to make your line lose memory. It gets dead -as we used to say- much faster. So the exercise has its good but also has its bad to say it in Bono Vox’s words but in any way the little waves at the very end of the fly line of a extremely narrow vertical(not side cast) loop point to the fact that you are on a good track to become a superb fly caster.