Our current A10 is 62" Span and weighs about 96 ozs flying weight with fixed U/C. The frame is about 32 oz and the hardware and batteries are the remaining 64 oz. The wing area is 550 sq ins. which means a wing loading of 25 oz/sq ft. This is ideal for a model which can fly fast or slow, take off with short run on grass, and land slow.

Now lets see what happens when it is scaled down to 40" which is a factor of about 30%. The frame will defintely go down by 30% to 22.4 oz. The problem is the hardware. Yes you can use smaller fans, smaller motors, smaller batteries, etc. but no matter what you do, you cannot save 30%, maybe 10% if you are lucky. So that would be 57.6 oz.

Add these two, and we get a flying weight of 80 oz, which is a 17% savings for a plane which is 30% smaller in wing area i.e. 385 sq in. That means a wing loading of almost 30 oz/sq ft.

At 30 oz/sq ft you have to run very fast to take off, fly the model really fast to stay up, and land fast to prevent stall. Flying fast means shorter flight times, which means more battery, which means more weight, which means higher wing loading, which means you have to fly it even faster......and on and on. We are in a vicious cycle by now.

I hope you see why smaller planes are harder to fly, especially if the hardware weighs what it weighs.