Imagine that you are on the beach in 1903 with the Wrights. They have to test to the thrust of their propellers. They won't fly without a test, and without the right result. A test like this has never been done before, because there's never been propellers like these ever made before! The materials for the test are what's at hand in the camp... including all the sand. The illustration below shows an idea of what the Wrights actually did: