Reality check

[Peter Connan]

These are both radio-controlled models. The intention was to make the larger one look real.

Any tips on how to do it better next time?

[Jakov Minić]

Peter, the image is cool as is!

Since you asked, here it goes:

Perhaps you could improve if you were to put some air between the two planes so that they are more distinct.
Perhaps you could move back a km or two and use you 500/4 to blur the background.
Perhaps you could move in closer with a fish-eye to make it funkier.
Perhaps you can stand in between the two planes and create a 360 panorama with a fish-eye.
Perhaps you can use that 360 degree panorama to create a planet

I cannot think of any other tips at the moment 

[Sash]

You tricked me. I did think one was real.

[Peter Connan]

  Thanks Jakov

And Sash

[Jan Anne]

Just shoot the bigger model with a wide-angle, get as close as possible from a low angle perspective and have real looking people do something in the background, the wide-angle will make the people appear smaller than they really are making the plane look bigger.

[Mike G]

I'm with Sash you fooled me, I too thought the large one was real

[pluton]

Except for the presence of the smaller, more obviously fake model, I'd have 'bought it'...at least in a quick look.
Aside from getting rid of the smaller model, I like Jan Anne's idea of using false perspective.  A few people standing 50 meters back might appear to under the wing of the plane.  Give them flight suits or other paraphernalia.
Also, in post production you could add some aerial perspective(distant haze) to the the background.

[Peter Connan]

Thank you Jan, Mike and Pluton. Some great ideas there!

[elsa hoffmann]

Boet you fooled me too

Agree in the breathing space between the two models.

[armando_m]

 it does look like a runway

and the one in the back looks like the real thing

+1 on a bit more separation

[Peter Connan]

Thanks Elsa and Armando

[Gary]

I'll join the fooled lot.

[Peter Connan]

Thanks Gary

It's incredible the time and money these guys spend. Almost like photographers!

[pluton]

For a complete illusion, you'd have to change the material that shows in the foreground...finer sand, maybe.
You might as well focus stack to get the foreground sharper as well, though I don't know how that works.

[Akira]

Yet another fooled guy here.  Well done, Peter!