prepositions - Meaning and usage of “be of”

As I'm preparing my GMAT test, I see the "be of" structure very frequently.
for example

By 1940, the pilot Jacqueline Cochran held seventeen official national and international speed records, earned at a time when aviation was still so new that many of the planes she flew were of dangerously experimental design.

This really bothers me as it contradicts the conclusion from the post
Meaning and usage of "be of", because in GMAT writings the "be of" looks very flexible. I find it's very difficult to understand sometime, but I know these are really fine and efficient writings.

I guess my question was, in this case, can you get rid of the "of" and what the usage of it here?
If I take off the "of" here, does it modify the meaning of this sentence?

By 1940, the pilot Jacqueline Cochran held seventeen official national and international speed records, earned at a time when aviation was still so new that many of the planes she flew were dangerously experimental design.

Answer

This is a different phemomenon from the one discussed in the "Meaning and usage of "be of" post. That one describes a set of idiomatic predicate prepositional phrases -- be of assistance/service/use/help -- that have special pragmatic uses.

This phenomenon is a headless relative clause that happens to have a prepositional phrase. If you put back all the stuff that has been left out and unwind the transformations you get something like

• ... the planes that she flew were planes that were of dangerously experimental design.

That-deletion results in

• ... the planes she flew ...

and Whiz-deletion results in

• ... the planes she flew were planes of dangerously experimental design.

And, since planes just occurred a few words back, it gets deleted here, producing a headless relative clause meaning "[ones that are] of dangerously experimental design".
These are all optional, and unordered, and independent, like most syntax.

English deletes a lot of stuff from relative clauses, producing sentences that look like other sentences with very different uses and conventions.