On the models, the aerodynamic effect can be significant.
I built a 4*40, and after a while decided to home-make some wheel pants for appearance. installing the wheel pants changed my elevator trim 3 clicks. (had to give it down trim because of reduced drag below the wing. These were SIMPLE teardrop profile, flat ply wrapped wheelpants too... (the draggiest design there is in wheelpants)
Later, I clipped a rock landing and had to take off one wheelpant that was rubbing the wheel. (was too lazy to take off the other) suddenly I needed 4 clicks rudder trim and 2 clicks up trim. There were no other changes to the aircraft... just lost one wheelpant.
If it can make that much difference on the slow flying 4*40... think what better wheelpants wll do on faster models.
You will also note that Pylon racers with fixed gear ALL use wheelpants. These things add weight. Weight is an enemy of a racer. if there was no aerodynamic reason to use the wheelpants you can bet they wouldn't be there.
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even on the full scale stuff going back to the 1930-/40 etc., airshow daredevils- the wheelpants were among the first things to go -They simply were extra weight .
The Bucker designs - the best in the world for aerobatics for many years - used only tiny mud catcher fenders.
When SPEED is not a consideration, then the weight will be, as in the aerobatics example. For low speed aerobatics the weight is a bigger factor than the drag reduction. But for anythig except low speed aerobatics, the wheel pants are highly desired.