RE: DUCTED FANS
Computer fans have low pitch and a very large hub, so small actual working blades to allow high air flow. The Axiflow for 1/2A(.049/.051)gave reasonable airflow but the Cox engine couldn't produce enough rpm for real high performance compared to what electric can achieve now with 40000+ rpm. DF is still possible for very light free flight models as was done in the 50's in the UK.