Perry Mick wrote:

It takes 2200 RPM to taxi with the duct on, 2800 with duct off.
With the duct on and a 10 knot headwind, it took 1000' to reach 70 knots
airspeed, was airborne at 1500'. It usually takes me 1800' with no headwind.
Without the duct, acceleration was sooooo sluggish, I thought I must
have been running on one rotor.

Marc de Piolenc wrote:

Quick addendum.

Perry's experiment does not prove that it is impossible to get
acceptable performance with an unducted fan. The drastic loss in
performance with the duct removed shows that his propeller is optimized
for use in the duct - as it SHOULD be. A propeller designed ab initio to
work in the open should do better - how much better, I couldn't say.

There's an enormous difference in flow conditions with and without a
duct. That's why you can't just wrap a shroud around a conventional prop
and be sure of better performance. But take the duct away from a prop
designed to work with it and you are sure to lose big.

Marc-- 

 
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