Complex sentence: The RC-3 Seabee was designed by Percival Hopkins "Spence" Spencer, an aviation pioneer who built his first hang glider in April 1911, just two weeks after his 17th birthday, using plans he found in a "Popular Mechanics" magazine.
Split into simpler sentences: The RC-3 Seabee was designed by Percival Hopkins "Spence" Spencer, an aviation pioneer who built his first hang glider in April 1911. At the time he was 17 years old and constructed it from plans he found in a "Popular Mechanics" magazine.