Originally Posted by slowrotor
Interesting replies.
My plan is to build an ultralight with an aspect ratio 2.5 wing.
AR 2 or 3 is very common in RC models and the aircraft have amazing performance.
As an RC model builder with 30 years experience perhaps I can comment...
It depends how you define performance? Most of the low aspect ratio RC models are either deltas or similar. They are mainly highly aerobatic models with power to weight ratios approaching 1:1 or better. They have very high roll rates - sometimes able to roll 360 degrees in less than 1 second. Some are able to loop in just a few plane lengths. Part of their amazing performance is due to the prop wash - even the ailerons are in the prop wash!
The problem comes when the motor quits... You loose the prop wash over the controls (so some of your control authority vanishes) but the roll stability/instability is unchanged. It can also be tricky to get them to slow down without tip stalling.