Eric Greenwell wrote:

Also, the thermal fliers regularly exceed the 400 feet, don't they?

Absolutely! When I flew thermal R/C sailplanes the winch launch height
was typically 400'+. After that the height was limited only by the
thermals and your ability to see the model. I'm sure 1000'+ AGL is
common, especially with a large span model. My open class model had a
146" span and was not hard to see "way up there". I recall several
cases where low flying GA aircraft flew *under* our models.

Regards,

-Doug