É essencialmente duas hélices empilhadas umas sobre as outras. Você pode vê-lo nesta foto de um Lazair .
Quanto a por que eles foram escolhidos para o Lazair ,
Many have asked why Ultraflight opted for double propellers stacked in a biplane fashion. This is a very unique setup and draws lots of attention. Apparently when Ultraflight changed over to the Rotax 185 they tried several different propellers. The problem was the 185’s had a woodruff key slot in the crankshaft and would snap the crank whenever the engine coughed or kicked back on start up. This was quite a serious problem. After much deliberation it was determined that the rotational inertia of the propellers was too much for the crank to handle.
Dale could not locate a two bladed prop light enough to fit the need. However, since they had been producing a lot of the early lazairs with Pioneer engines fitted with a single plastic prop, he had a ton left over that fit the pioneers. He found that these could be “doubled up” and help to produce the extra thrust that the Rotax 185 was capable of producing over the 5.5 hp Pioneer engines.
Parece ser uma maneira rápida de ganhar mais impulso dos adereços sem aumentar a massa prop para muito.